Biotransformation of Citrus By-Products into Value Added Products
REVIEW
Biotransformation of Citrus By-Products into Value Added Products
Diomi Mamma ?Paul Christakopoulos
Received:27February 2013/Accepted:4June 2013/Published online:18June 2013óSpringer Science+Business Media Dordrecht 2013
Abstract Citrus by-products are the processing wastes generated after citrus juice extraction and constitute about 50%of fresh fruit weight.This solid residue comprised of peel (?avedo and albedo),pulp (juice sac residue),rag (membranes and cores)and seeds.The disposal of the fresh peels is becoming a major problem to many https://www.wendangku.net/doc/ff3237641.html,ually,citrus juice industries dry the residue and it is either sold as raw material for pectin extraction or pellet-ized for animal feeding,though none of these processes is very pro?table.This residual material is a poor animal feed supplement because of its extremely low protein content and high amount of sugar.The application of agroindustrial by-products in bioprocesses offers a wide range of alter-native substrates,thus helping solve pollution problems related to their disposal.This article reviews attempts that have been made to use citrus by-products to generate several value-added products,such as essential oils,pectin,enzymes,single cell protein,natural antioxidants,ethanol,organic acids,and prebiotics.
Keywords Citrus wastes áGreen productivity áBioconversion áUpgrading áEnzymes áBiofuels áAntioxidants áReview
Abbreviations AG-I Arabinogalactan I BOD Biological oxygen demand BPF By-product feedstuffs COD Chemical oxygen demand CPW Citrus peels waste CW Citrus waste D.I.C.Instantaneous controlled pressure drop DCP Dried citrus pulp DF Dietary ?bre DM Dry matter GHG Greenhouse gas HG Homogalacturonan HR Hairy regions IDF Insoluble dietary ?bre MHG Microwave hydrodiffusion and
gravity
MP Mandarin peel MSDf Microwave steam diffusion OLR Organic loading rate OPP Orange-pulp pellets POS Pectic oligosaccharides RG-I Rhamnogalacturonan-I RG-II Rhamnogalacturonan-II SCP Single cell protein SDF Soluble dietary ?bre SFME Solvent free microwave extraction SHF Separate hydrolysis and fermentation SmF Submerged fermentation SSF Simultaneous sacchari?cation and
fermentation
SWE Subcritical water extraction TOC Total organic carbon
VMHD Vacuum microwave hydro distillation VS Volatile solids
D.Mamma
Biotechnology Laboratory,School of Chemical Engineering,National Technical University of Athens,9Iroon Polytechniou Str.,Zografou Campus,Zografou,15780Athens,Greece P.Christakopoulos (&)
Biochemical and Chemical Process Engineering,Division of Sustainable Process Engineering,Department of Civil,Environmental and Natural Resources Engineering,
Lulea
?University of Technology,97187Lulea ?,Sweden e-mail:paul.christakopoulos@ltu.se
Waste Biomass Valor (2014)5:529–549DOI 10.1007/s12649-013-9250-y
Introduction
The genus Citrus includes several important fruits[1],with the most important on a worldwide basis being sweet orange(C.sinensis:61.1%of world citrus production), tangerine(C.reticulata:19.9%),lemon and lime(C.limon and C.aurantifolia:12.1%)and grapefruit(C.paradisi: 5.0%).Minor citrus genuses that comprise the bulk of the remaining 2.0%include sour orange(C.quarantium), shaddock(C.grandis),citron(C.medica).
Citrus fruit is segmented into two sections,peel and ?esh,as illustrated in Fig.1.The peel consists of?avedo and albedo.Flavedo(or exocarp)is the outermost layer of the fruit,which bears oil glands and pigments.Flavedo is mostly composed of cellulosic material but also contains other components,such as essential oils,paraf?n waxes, steroids and triterpenoids,fatty acids,pigments(carote-noids,chlorophylls,?avonoids),bitter principles(limo-nene),and enzymes.When ripe,the?avedo cells contain carotenoids(mostly xanthophyll)inside chromoplasts, which,in a previous developmental stage,contained chlorophyll.This hormonally controlled progression in development is responsible for the fruit’s change of color from green to yellow upon ripening.The internal region of the?avedo is rich in multicellular bodies with spherical or pyriform shapes,which are full of essential oils.The albedo is located next to?avedo.It is the inner part of the peel which is commonly removed before eating and it is rich in pectin.The endocarp is separated into sections, which are most commonly called segments.The juicy pulp ?lling the segments is usually referred to as juice sac (vesicles)[2].
According to FAO estimates the world citrus production for the year2009/2010was109,338.9thousand tons (Table1)[3].About19%of world production of citrus is in the Mediterranean countries of Spain,Italy,Greece, Egypt,Turkey and Morocco,with Brazil(16%),China (22%)and the USA(9%)being major individual citrus producing countries(Table1).Approximately,23,733.2 thousand tons of the total citrus production for the year 2009/2010(22%of the world’s citrus production and 29%of orange production)were processed to yield juice, essential oils and other by-products(Table2)[3],and results large quantities of by-products.
Citrus by-products are the principal solid derivatives of the citrus processing industry and constitute about50%of fresh fruit weight[4](corresponding to approximately 11,866.6thousand tons of citrus wastes according to FAO estimates for the year2009/2010,Table2).They contain the peel(60–65%),internal tissues(30–35%)and seeds (0–10%).
Until comparatively recently these waste products led to signi?cant disposal problems,since there was no satisfactory means of disposal other than dumping on land adjacent to the production sites.This led in some regions to the generation of large tracts of land containing signi?cant quantities of putrefying waste which presents a signi?cant risk to local water courses and in some cases leads to uncontrolled methane production.The major environmental problem associated with citrus peel is its highly fermentable carbo-hydrate content which accelerates its degradation when not carefully managed[5].
Traditionally,the only other disposal routes for citrus waste were to utilize it as raw material in the manufacture of cattle feed or simply to burn it[6,7].Citrus wastes can be employed as cattle feed,upon drying,but with a protein content of only6%,this not a high protein source feed-stock.Additionally,decreasing its moisture content from 80to10%is highly energy-intensive and costly,rendering feed applications for waste citrus peels only marginally pro?table[8].Combustion of citrus waste would also generate global warming gases,although some of the car-bon dioxide generated from combustion would not con-tribute signi?cantly to a net increase,since it would be subsequently assimilated by plants[6,7].
This disposal problem has stimulated signi?cant recent interest in developing more responsible ways of dealing with waste products,ideally with the added bene?t of yielding high value products and establishing more envi-ronmentally responsible approaches.
This article reviews developments regarding processes and products that employed citrus peels as a substrate. Valorization Strategies for Citrus Wastes
In general,citrus residues have no economic value,even though their composition is rich in soluble sugars,cellulose, hemicellulose,pectin and essential oils that could form the basis of several industrial processes.Strategies for citrus waste utilization include:extraction of essential oil and pectin,production of enzymes,bioethanol,methane,organic acids,single cell protein,natural antioxidants,prebiotics, dietary?bers and use for adsorption of heavy
metals. Fig.1Structure of the citrus fruit(adapted from[20])
Production and Composition of Citrus By-Products Citrus fruits are consumed in high quantities all over the world in the natural and peeled forms and as a juice.A generalized?ow scheme of orange juice production is depicted in Fig.2.
After juice is extracted from the fruit,there remains a residue consisting of peel(?avedo and albedo),pulp(juice sac residue),rag(membranes and cores)and seeds.These components,either individually or in various combina-tions,are the source materials from which citrus by-product feedstuffs(BPF)are produced[9,10].The major citrus
World109,338.964,931.425,710.412,141.66,555.4 Northern Hemisphere83,776.144,697.123,722.59,475.05,881.6 USA9,978.77,479.0577.2799.81,122.7 Mediterranean region21,112.012,257.65,339.22,733.3781.9 Greece1,134.8996.0100.033.0 5.8 Italy3,671.62,349.8826.0486.29.6 Spain5,347.62,621.82,000.7673.152.0 Israel553.7121.8171.558.9201.5 Algeria571.0415.0111.044.0
Morocco1,521.1827.0651.042.0 1.1 Tunisia348.8188.137.037.386.4 Egypt3,512.02,401.0731.0320.060.0 Cyprus211.770.982.010.848.0 Lebanon392.0230.035.0115.0
Turkey2,780.01,230.0500.0750.0300.0 Others1,067.7806.294.0163.017.5 Portugal280.2185.864.412.1
Japan1,157.062.01,088.0
Costa Rica370.0370.0
Mexico6,793.04,051.0450.01,891.0401.0 Belize266.1225.3
Cuba345.0178.3137.7 Iran Islamic Rep.of2,541.41,860.8
China23,850.06,500.014,200.0524.02,900.0 India7,966.55,201.02,598.8194.0 Pakistan2,203.01,542.1
Indonesia2,032.72,032.7
Korea Rep740.0740.0
Thailand876.6452.3
Others3,263.92,299.31,263.7916.1344.3 Southern Hemisphere25,562.820,234.41,988.02,666.6673.8 Argentina2,270.0770.0360.01,000.0140.0 Brazil17,483.215,422.01,094.5899.862.8 Chile289.0155.0
Paraguay324.2229.945.6 Peru853.1394.6221.3233.0 4.2 Uruguay268.8130.093.037.0
Venezuela386.2350.9
Australia524.0380.036.511.0 South Africa1,989.01,428.0218.0343.0 Others1,175.31,129.0219.187.267.3
BPF from citrus processing are fresh citrus pulp which is the main residue after extraction of juice,representing between 492and 692g/kg of fresh citrus fruit with 600–650g dry matter (DM)/kg peel,300–350g/kg pulp and 0–100g/kg seeds,and dried citrus pulp (DCP)which is formed by shedding,liming,pressing and drying the peel,pulp and
World
23,733.219,039.31,651.52,012.41,046.5Northern Hemisphere 10,994.28,014.81,260.8960.9757.7USA
6,586.05,570.0142.0370.0504.0Mediterranean region 1,924.21,127.8348.8
293.9153.7Greece 198.8197.00.8
Italy 602.0400.070.0130.0Spain 615.1264.0207.0142.1 2.0Israel 181.046.027.0 2.0106.0Morocco 35.032.0 3.0
2.6Egypt 56.948.0Cyprus 59.624.515.0 1.318.8Turkey 135.080.020.015.020.0Others 40.836.3
6.80.9 6.1Japan 128.0126.0Mexico 1,275.0880.0295.0
100.0China 722.0202.0520.0Korea Rep 124.0124.0Others
359.0235.00.00.00.0Southern Hemisphere 12,739.011,024.5
390.71,051.5288.8Argentina 944.084.091.0698.071.0Brazil 11,187.9
10,567.0
283.3
277.160.6Uruguay 19.519.5Australia 124.6105.08.2
14.4 5.2South Africa 463.0249.062.0152.0Others
0.0
0.0
0.0
0.0Fig.2Generalized orange juice ?ow scheme
seed residues to about80g/kg moisture,and citrus meal and ?nes which is formed and separated during the drying process[11].A typical processing plant produces these BPF in a ratio of about850g/kg DCP,140g/kg citrus meal and 10g/kg citrus?nes.Other citrus BPF include citrus molasses,made by concentrating the press liquor from the citrus peel residue,which has a bitter taste and con-tains about100–150g/kg of soluble matters of which 500–700g/kg consisting of sugars[10],citrus peel liquor, which is similar to citrus molasses,but not as concentrated, and citrus activated sludge which is produced from liquid wastes from citrus processing plants.Other minor BPF from citrus include cull or excess fruit[12].
The composition of citrus fruit is affected by factors such as growing conditions,maturity,rootstock,variety and climate[1].Citrus fruits contain N(1–2g/kg on a wet basis),lipids(oleic,linoleic,linolenic,palmitic,stearic acids,glycerol,and a phytosterol),sugars(glucose,fruc-tose,sucrose),acids(primarily citric and malic,but also tartaric,benzoic,oxalic,and succinic),insoluble carbo-hydrates(cellulose,pectin),enzymes(pectinesterase,phosphatase,peroxidase),?avonoids(hesperidin,narin-gin),bitter principles(limonin,isolimonin),peel oil(d-limonene),volatile constituents(alcohols,aldehydes, ketones,esters,hydrocarbons,acids),pigments(carotenes, xanthophylls),vitamins(ascorbic acid,Vitamin B com-plex,carotenoids),and minerals(primarily calcium and potassium)[8,13,14].The nutrient content of citrus BPF is in?uenced by factors that include the source of the fruit and type of processing[15].Bampidis and Robinson[8] investigated the composition of several citrus by-products (Table3).It is noteworthy that the pH of orange peel can be as low as3.64.This means neutralization of pH may be required for some applications.The carbohydrate content of different citrus by-products is presented on Table4.
Extraction of Essentials Oils
Citrus peel contains essential oils which are well-known antimicrobial agents[16].Essential oils are contained in oil sacs or glands that range in diameter from0.4to0.6mm.
Table3Chemical composition of several citrus by-products(g/kg DM)[8]
Orange peels(fresh)Lemon peels(fresh)Bergamot peels Grapefruit pulp(fresh)Mandarin pulp fresh
DM233202163197197
OM975967957962960
CP5866–7180
Crude Fat–16136361
NDF200330101––
ADF129–73168174
Lignin––271921
Sugar–191–––
ADF acid detergent?bre,CP crude protein,DM dry matter,NDF neutral detergent?bre,NE net energy,OM organic matter
Table4Composition of differente citrus wastes(%of dry weight)
Ash Sugar Fat Protein Pectin Lignin Cellulose Hemicellulose
Lemon peels a 2.5 6.5 1.57.013.07.623.18.09 Lemon pulp a 2.59.0 3.18.722.57.636.211.1 Orange peels a 2.69.6 4.09.123.07.537.111.0 Orange pulp a 2.6 6.0 1.5 6.612.17.524.57.6 Orange peels b 1.741.4 3.97.914.4 1.016.213.8
Kinnow mandarin waste c 3.231.6n.d.f 5.822.60.610.1 4.3 Mandarin peel waste d 3.021.6n.d.n.d.14.28.920.817.2 Grapeftuit Peels e 3.3n.d.n.d.n.d.16.18.219.818.3
a Data taken from[139]for citrus wastes from Spanish citrus juice factories
b Data taken from[78]for orange peels from a fruit processing facility in Greece
c Data taken from[102]for kinnow waste(compose
d of peel and pulp)from a fruit processing facility in India
d Data taken from[103]for mandarin citrus peel waste(MCPW),from citrus juic
e factories in Spain
e Data taken from[104]
f n.d.not determined
The sacs or glands are located at irregular depths in the ?avedo,which is located in the outer coat of the fruit. Orange peel typically contains5.436kg of oil per1,000kg of oranges,of which approximately90%is D-limonene [16,17];a hydrocarbon classi?ed as a cyclic terpene.The production of essential oils from orange peel is economi-cally viable,since this by-product has high added value [18].
As the main odorous constituent of citrus fruit,D-limo-nene is employed in the manufacture of food and medicines as a?avoring agent and has many applications in the chemical industry as well as in cosmetics and domestic household products[19].Given these features,it is essen-tial to remove D-limonene from the peel prior to other uses.
The physical methods used are distillation(steam, steam/water and water),expression(also known as‘cold pressing’for citrus peel oils),or dry distillation of natural materials.Following distillation or expression,the essential oil is physically separated from the water phase[20].The traditional way of isolating volatile compounds as essential oils from Citrus is mostly by cold pressing the Citrus peels. The oils are removed mechanically by cold pressing and, since cold pressing yields a watery emulsion,this emulsion is then subjected to centrifugation to separate out the essential oil from the water.
Distillation is also used in some countries as an eco-nomical way to recover Citrus essential oils from Citrus by-products.During distillation,Citrus peels exposed to boiling water or steam release their essential oils through evaporation.Recovery of the essential oil is facilitated by distillation of two immiscible liquids,viz.water and essential oil,based on the principle that,at the boiling temperature,the combined vapour pressure equals the ambient pressure.Thus,the essential oil ingredients,whose boiling points are normally in the range200–300°C,are evaporated at a temperature close to that of water.The essential oil-laden steam rises and enters narrow tubing that is cooled by an outside source.As steam and essential oil vapours are condensed,both are collected and separated in a vessel traditionally called the‘Florentine?ask’[20].The essential oil,being lighter than water,?oats at the top, while water goes to the bottom and can be easily separated. The amount of essential oil produced depends on four main criteria:the duration length of the distillation time;the temperature;the operating pressure;and,most importantly, the type and quality of the plant material.Depending on the variety of the fruit,peel oils yield an essential oil content of 0.05–5.0%.
The conventional methods for the extraction of Citrus essential oils have some disadvantages.For steam distil-lation and hydrodistillation,the elevated temperatures and prolonged extraction time can cause chemical modi?ca-tions of the oil components and often a loss of the most volatile molecules.When using cold pressing,Citrus essential oil is agitated vigorously with water and a gradual diminution in citral and terpene alcohols contents occurs. Furthermore,during agitation,air is thrashed into the liquid,thereby creating conditions favourable for hydro-lysis,oxidation and resini?cation[21–23].
These shortcomings have led to consideration of the use of new techniques in essential oil and aroma extraction, which typically enhance the quantity of essential oil,pre-serve its quality and use less energy,such as supercritical ?uid extraction,ultrasound extraction,subcritical water extraction,a controlled pressure drop process and micro-wave extraction.A brief description of the above men-tioned techniques is given below.
Supercritical?uid extraction is a technique,developed since the1970s that involves the use of a gas(i.e.CO2) above its critical temperature and pressure.It is employed for dif?cult separation processes based on low quantity of valuable products,while its main advantage is the low solvent consumption and the absence of corresponding residues in the extract.Other advantages include rapid extraction,concentration and high selectivity.CO2is a non-?ammable odourless gas produced during the burning of fossil fuels,by alcoholic fermentation and also through human and animal respiration.The technique of extraction with supercritical CO2uses compressed gas at a pressure of up to300MPa at a moderate temperature(30–40°C),to replace organic solvents such as hexane.Supercritical CO2 extraction is being applied in several sectors such as food, cosmetics and pharmaceutical industry.It is used for weakly polar compounds of low molecular weight such as carotenoids,triglycerides,fatty acids,aromas,etc.The main drawbacks remain its high initial investment and dif?culties to perform continuous extractions[24,25].Supercritical ?uid extraction is suitable for fragrance extraction,while it has been industrially applied for the recovery of hydroxy-tyrosol from olive mill wastewater[26].
Ultrasound waves have been successfully utilized for the extraction of essential oils,as their cavitational effect accelerates heat and mass transfer by disrupting the plant cell walls and facilitates the release of extractable https://www.wendangku.net/doc/ff3237641.html,ing ultrasounds,extraction can be completed in minutes with high reproducibility and low solvent con-sumption,although an additional?ltration step is still required.Pulsed electric?eld is a non-thermal food pro-cessing technology of high potentiality.An external elec-tric?eld can induce critical electrical potential across the cell membrane that leads to pores development,breakdown and increased permeability[27–29].Phenolic compounds are also extractable from citrus peels by using ultrasound [30].
Subcritical water extraction(SWE)is a technique based on the use of water as extractant,at temperatures between
100and374°C and pressure high enough to maintain the liquid state state(critical point of water,22.4MPa and 374°C)[31].SWE has unique characteristics such as high density,high reactivity,and good solubility for a series of organic compounds having relatively low molecular weights and it is emerging as a powerful alternative for solid sample extraction[32].SWE was used to extract nutraceuticals with antioxidant activities from citrus pom-ace[33]as well as from other bio-resources.
The instantaneous controlled pressure drop process also known as‘‘D.I.C.’’,was developed and patented by Allaf et al.[34]initially for use in the?eld of drying-texturation of various food products by improving hydration capacity [35].This process is based on a thermomechanical pro-cessing induced by subjecting the product to a fast transi-tion from high steam pressure to vacuum and it was used for the separation of volatile oil from the solid material. This extraction process,successfully tested on isolation of essential oil from rosemary leaves[36,37]and orange peels[38]represents an interesting alternative not only to classical processes such as extraction by solvent and steam distillation,but also to more effective processes such as extraction with supercritical?uids.This extraction process does not require using of any solvent and induced cooling when the plant is rapidly transferred from a high steam pressure to vacuum stops all thermal degradation of oil components.Due to the‘‘?ash’’evaporation of bulk water present in peels,the processing by instantaneous controlled pressure drop increases the global diffusivity of the product and improves availability of the liquid in plant.Moreover, compared to steam distillation,the short time contact(few minutes)between plant and heat avoids the loss and deg-radation of volatile and thermolabile compounds[38].
Microwave extraction is now recognized as ef?cient technique that dramatically reduces extraction time, increases the yield and the quality of the extract[39].
Vacuum Microwave Hydro Distillation(VMHD)was elaborated and patented by Archimex in1994[40].This technique is based on selective heating by microwaves combined with application of sequential vacuum.The plant material is placed in a microwave cavity with water to refresh the dry material.The plant material is afterward exposed to microwave radiation to free the natural extract. Reducing the pressure to between100and200mbar allowed the evaporation of the azeotropic water-volatile oil mixture from the biological matrix.The procedure is repeated in a stepwise fashion in order to extract all the volatile oil from the plant.
Solvent free microwave extraction(SFME)is a method patented in2004and speci?cally aimed at obtaining essential oil from plant material[41,42].SFME apparatus is an original combination of microwave heating and distillation at atmospheric pressure.This method involves placing plant material in a microwave reactor,without any added solvent or water.The internal heating of the in situ water within the plant material distends the plant cells and leads to rupture of the glads and oleifeous receptacles. Thus,this process frees the essential oil that is evaporated by the in situ water of the plant material.A cooling system outside the microwave oven condenses the distillate con-tinuously.The water excess is re?uxed to the extraction vessel in order to restore the in situ water to the plant material.This process was applied to several kinds of fresh and dry plants including citrus fruits[23,43].
A microwave solvent-free extraction technique called Microwave Hydrodiffusion and Gravity(MHG)has been used for extraction of essential oils,colours and antioxi-dants,was discoved at the Laboratory of Green Extractions (Universite d’Avignon France)in2008[44].It is an ori-ginal‘‘upside down’’microwave alembic combining microwave heating and earth gravity at atmospheric pres-sure.Based on a relatively simple principle,this method also involves placing the plant material in a microwave reactor,without adding any solvent or water.The internal heating of the in situ water within the plant material dis-tends the plant cells and leads to the rupture of glands and oleiferous receptacles.The heating action of microwaves thus frees essential oil and in situ water which are trans-ferred from the inside to the outside of the plant material, and drop by gravity out of the microwave reactor[45]. Bousbia et al.[46]have been successfully applied this technique for the extraction of essential oils from citrus peels.
Microwave steam diffusion(MSDf)is proposed as a novel method for green extraction of essential oils from different types of aromatic plants.It is a combination of microwave heating with saturated steam which favours the release of essential oils trapped,from the inside to the exterior of the cells of plant tissues and earth gravity to collect and separate[47].
Extraction of Pectin
Pectin is an important component of dicotyledonous plant cell walls,besides cellulose and hemicelluloses and it is probably the most complex macromolecule in nature,because it can be composed of as many as17different monosaccha-rides.Rather than making all possible combinations with these monosaccharides,mother nature has provided us a number of distinct polysaccharides which together form pectin.Three pectic polysaccharides[homogalacturonan (HG),rhamnogalacturonan-I(RG-I),and rhamnogalacturo-nan-II(RG-II)]have been isolated from primary cell walls
and structurally characterized[48].HG is a linear chain of 1,4-linked a-D-galactopyranosyluronic acid(Gal p A)resi-dues.The Gal p A residues can be methyl-esteri?ed at C-6, and carry acetyl groups on O-2and O-3.‘‘Smooth’’regions are mainly composed of HG.RG-I is a family of pectic polysaccharides that contain a backbone composed of as many as100repeats of the disaccharide[?4)-a-D-Gal p A-(1?2)-a-L-Rha p-(1?].The ramnosyl residues can be sustitued at O-4with neutral sugars.The side chains are mainly composed of galactosyl and/or arabinosyl residues. They can be single unit[b-D-Gal p-(1?4)],but also poly-meric such as arabinogalactan I(AG-I)and arabinan(50 glycosyl residues or more).AG-I is composed of a1,4-linked b-D-Gal p backbone;a-L-Ara f residues can be attached to the O-3of the galactosyl residues[48].The arabinans consist of a 1,5-linked a-L-Ara f backbone,which can be substitueted with a-L-Ara f-(1?2)-,a-L-Ara f-(1?3)-,and/or a-L-Ara f-(1?3)-a-L-Ara f-(1?3)-side chains[48].Com-plexes of RG-I and AG-I and arabinan are often referred to as pectic‘‘hairy regions’’(HR),in which AG-I and arabinan are the‘‘hairs’’.The abundance of HR can differ from species to species.RG-II is not structurally related to RG-I since its backbone is composed of1,4-linked a-D-galactopyrano-syluronic acid(Gal p A)residues rather than the repeating disaccharide[?4)-a-D-Gal p A-(1?2)-a-L-Rha p-(1?].A nonasaccharide and an octasaccharide are attached to C-2 of some of the backbone GalA residues and two structu-rally different disaccharides are attached to C-3of the backbone[48].
Pectin,from apple and citrus fruits,is widely used as functional food ingredient and it is listed among the ingredients of innumerable food products(or its EU code, E440):its worldwide annual consumption is estimated to be around45million kilograms,with a global market value of to be least400million euros.Its industrial utilisation is based mainly on its gelling properties for the production of jams and jellies,fruit juice,confectionary products and bakery?llings;nevertheless,pectin is also used for the stabilisation of acidi?ed milk drinks and yogurts.Pectins are used in the pharmaceutical sector as detoxifying agents,and are well known for their anti-diarrheal effects,as well as in cosmetics as gels and pastes[49].
The chief raw materials for the industrial production of pectin are the residues from the manufacture of fruit juices, apple pomace and citrus fruits[49].Pectins are industrially obtained in a chemical way with strong acids such as oxalic, hydrochloric,nitric,and sulphuric acids which are regarded as conventional acid extraction[50].Even though these chemical procedures have advantages from an ef?cient and economical point of view,they may cause environmental problems by producing hazardous contaminants that must be treated.
Nowadays,the consumer demand for‘‘green’’products stimulates the search for alternative means of extraction. Therefore,enzymes could represent,despite their potential cost,an alternative and environmentally friendly way to extract‘‘green labeled’’https://www.wendangku.net/doc/ff3237641.html,mercial enzyme preparations(usually multienzyme complexes)have been used by several researchers for the extraction of pectin from citrus by-products.Pectinolytic,cellulolytic,hemi-cellulolytic,proteolytic,mixtures of the above have been used so far[51–54].
Alternative treatments have been also taken into account for minimizing the use of detrimental chemicals during pectin extraction.Thus,several thermal and/or mechanical treatments have been applied to extract pectin including ultrasound[55],autoclaving[56],extrusion-cooking or microwave-assisted extraction[57–60]and subcritical water extraction[61].
Production of Enzymes
The most important area of citrus peels utilization is the production of enzymes,especially pectinolytic.Pectinases are a heterogeneous group of related enzymes that hydro-lyze the pectic substances.Pectinases are classi?ed under three headings according to the following criteria:whether pectin,pectic acid or oligo-D-galacturonate is the preferred substrate,whether pectinases act by trans-elimination or hydrolysis and whether the cleavage is random(endo-, liquefying of depolymerizing enzymes)or endwise(exo-or saccharifying enzymes).The three major types of pec-tinases are:pectinesterases,depolymerising enzymes (enzymes that either hydrolyze glycosidic linkages or cleave a-1,4-glycosidic linkages by trans-elimination)and protopectinase.Pectinolytic enzymes are of signi?cant importance in the current biotechnological era with their all embracing applications in fruit juice extraction and its clari?cation,scouring of cotton,degumming of plant?bers, waste water treatment,vegetable oil extraction,tea and coffee fermentations,bleaching of paper,in poultry feed additives and in the alcoholic beverages and food industries [62].
Due to the chemical composition of cirtus peels other enzymes could be produced resulted in multienzyme complexes.Cellulases and hemicellulases are among these enzymes.Cellulases are a complex enzyme system,com-prising endo-1,4-b-D-glucanase,exo-1,4-b-glucanase and b-D-glucosidase.These enzymes are employed in feed,fuel and chemical industries for the processing of lignocellu-losic materials.The hemicellulolytic system carrying out the xylan hydrolysis is usually composed of a repertoire of hydrolytic enzymes:b-1,4-endoxylanase,b-xylosidase,a-L-arabinofuranosidase,a-glucuronidase,acetyl xylan ester-ase,and phenolic acid(ferulic and p-coumaric acid)
esterase.Xylanolytic enzymes have attracted a great deal of attention in the last decade,particularly because of their biotechnological potential in various industrial processes such as food,feed,and pulp and paper industries[63,64]. Bacteria,yeasts,and fungi under both submerged(SmF) and solid state fermentation conditions are able to produce these enzymes.
Maldonado et al.[65]tested Aspergillus sp.from decaying lemons for extracellular pectinase production using differently pretreated lemon peel as the carbon source instead of pectin.It was found that the production of extracellular polygalacturonase was about the same and that of pectinesterase substantially higher when unwashed fresh lemon peel was used instead of pectin.
Pectinase production by Tubercularia vulgaris using orange-pulp pellets(OPP)or citrus pectin as carbohydrate sources was investigated by Fonseca and Said[66].The highest levels of extracellular polygalacturonase were detected with OPP as inducing substrate.High levels of endo-polygalacturonase were produced by Aureobasidiurn pulIulans on orange-peel waste[67].Polygalacturonase and pectin lyase production by P.viridicatum strain Rfc3 was carried out by means of solid state fermentation using orange bagasse,corn tegument,wheat bran and mango and banana peels as carbon sources.The maximal pectin lyase(2,000U/g)activity was obtained in medium com-posed of orange bagasse.The mixture of orange bagasse and wheat bran(50%)increased the production of polygalac-turonase and pectin lyase to55and3,540U/g respectively [68].
Larios et al.[69]studied the endo-polygalacturonase production by Aspergillus sp.CH-Y-1043using untreated lemon peel and citrus pectin as carbon sources.Untreated lemon peel proved to be a better substrate for endo-poly-galacturonase production.Optimization of the culture medium as far as the nitrogen source,concentration of phosphates and initial culture pH resulted65.2U/ml en-dopolygalactorunase activity.
The production of endo-polygalacturonase by the fungus A.niger MTCC281using powdered citrus peel was studied by Dhillon et al.[70].Maximum enzyme activity was observed with15%substrate semi-solid substrate, when incubated at30°C for120h,using5%inoculum.
Mahmood et al.[71]found that Bacillus species11089 was capable of growth in continuous culture on orange substrate as carbon-energy source in a mineral salts basal medium and produced a-amylase,neutral and alkaline proteases,and polygalacturonate-lyase.
Aspergillus foetidus ATCC16878when grown under solid-state fermentation using citrus waste as carbon source produced pectic enzyme activities up to1,600–1,700U/g after36h of culture,as reported by Garzo′n and Hours[4].Yield of pectinases was25%higher than that achieved with the same fungal strain and culture conditions using apple pomace as a substrate.
Bacillus sp.MG-cp-2,isolated from the outer covering of seeds of Celastrus paniculatus,produced140.1U/ml of an alkaline and thermostable polygalacturonase when grown on orange peels as carbon source[72].Martins et al.
[73]reported the pectin lyase(43U/g)and polygalactu-ronase(19,320U/g)production by the thermophilic fungus Thermoascus aurantiacus179-5under solid state fermen-tation using orange bagasse as carbon source.
Seyis and Aksoz[74]investigated the use of apple pomace,orange pomace,orange peel,lemon pomace, lemon peel,pear peel,banana peel,melon peel,and hazelnut shell as substrate for xylanase production using Trichoderma harzianum.The maximum enzyme activity was observed when melon peel was used as the substrate for solid state fermentation,followed by the apple pomace and hazelnut shell.Sporotrichum thermophile produces a thermostable polygalacturonase under submerged culture and citrus peel as carbon source[75].
Single cell protein(SCP)and crude pectinolytic enzymes production from citrus pulps was reported by De Gregorio et al.[76].Ismail[77]investigated the production of mul-tienzyme preparations containing pectinase,cellulase and xylanase using six fungal isolates,namely Aspergillus niger 2,A.niger A-20,A.oryzae1911,Memmoniella sp.6, Penicillium chrysogenum3486and P.oxalicum7,all grown on orange peels as the sole carbon source in submerged fermentation(SmF).Production of multienzyme preparations containing pectinolytic,cellulolytic,and xylanolytic enzymes,by the mesophilic fungi A.niger BTL,Fusarium oxysporum F3,Neurospora crassa DSM1129and P.de-cumbens,under solid state fermentation of dry orange peel was also investigated.The selected fungi were also compared for their abilities to secret speci?c enzymes when grown on orange peel.Under optimal conditions A.niger BTL was found to produce the greatest yield of polygalacturo-nase,pectate lyase,xylanase,b-xylosidase,and invertase. N.crassa DSM1129produced the highest yield of endoglu-canase[78].
Aspergillus giganteus grown on orange waste under SmF produces pectin lyase at26.0U/ml[79].Co-culture of A.niger GJ-2and Saccharomyces cerevisiae J-1using dry orange peels resulted in nearly twofold higher enzyme production than that of the culture without the presence of S.cerevisiae J-1[80].Lemon peel can be used as a sub-strate in submerged cultures to obtain high pectinase titres by A.?avipes FP-500and A.terreus FP-370[81].A.niger when grown on lemon peels pomace in a solid state bio-reactor produces approximately2,181.23U/L pectinase [82].
Bioethanol Production
The spiraling crude oil prices and increasing concerns over greenhouse gas(GHG)emissions in?icted by the use of gasoline and diesel have led to an increased global interest in alternative fuels.Ethanol production by microbial fer-mentation of starch-and sugar-based raw materials is being practiced(1st generation biofuels),and the recent turbu-lence in the oil market has triggered the worldwide demand for bioethanol to such an extent that the conventional starch and sugar sources are proving to be insuf?cient.Ligno-cellulose feedstocks,such as agricultural and forest resi-dues,industrial and municipal wastes,and dedicated energy crops,by virtue of their high carbohydrate content, hold tremendous potential for large-scale bioethanol pro-duction,and need to be exploited for commercial produc-tion(2nd generation biofuels)[83].
Overall fuel ethanol production from lignocellulosic biomass includes?ve main steps:biomass pretreatment, cellulose hydrolysis,fermentation of hexoses,separation and ef?uent treatment Furthermore,detoxi?cation and fermentation of pentoses released during the pre-treatment step can be carried out[84].The sequential con?guration employed to obtain cellulosic ethanol implies that the solid fraction of pretreated lignocellulosic material undergoes hydrolysis(sacchari?cation);this fraction contains the cellulose in an accessible to acids or enzymes form.Once hydrolysis is completed,the resulting cellulose hydrolyzate is fermented and converted into ethanol.This process is called separate hydrolysis and fermentation(SHF)[85]. The above mentioned two process steps can be performed together in so-called simultaneous sacchari?cation and fermentation(SSF),which has been shown to have several advantages over performing the steps separately[84,85]. The SSF process alleviates end-product inhibition of the enzymes,and is also less capital intensive than SHF.Fur-thermore,SSF has been shown to be superior to SHF in terms of overall ethanol yield[86].
Several researchers have successfully hydrolyzed both orange and grapefruit peel waste using commercial cellu-lase and pectinase enzymes to glucose,galactose,fructose, arabinose,xylose,rhamnose,and galacturonic acid[87–90].According to Grohmann et al.[88],glucose,fructose and galactose from hydrolyzed citrus peel waste can be fermented to ethanol by S.cerevisiae yeast.Galacturonic acid from pectin hydrolysis,arabinose,and xylose as well as the sugars mentioned above can be fermented by Escherichia coli K011to produce ethanol and acetic acid [91,92].E.coli KO11is a recombinant bacterial strain developed to ferment arabinose and xylose as well as hexoses to ethanol[93].However in order to ferment these sugars,orange peel oil concentration in the hydrolysate must be reduced prior to fermentation[88].The inhibitory effect on yeast growth due to orange peel oil and/or D-limonene,a monoterpene that makes up more than90%
of orange and grapefruit peel oils has been observed by several researchers[94,95].
Wilkins et al.[94]reported that ethanol produced by S.cerevisiae and Kluyveromyces mar xianus during fer-mentation of a solution modelling hydrolyzed orange peels waste was37.1and40.9g/l respectively(80and88.3% theoretical yield,respectively)in the absence of limonene, while in the presence of0.2%limonene ethanol produc-tion reduced at23.3and13.1g/l respectively(50.3and 28.3%theoretical yield,respectively).It also should be noted that limonene concentrations tested by Wilkins et al.
[94]were less than concentrations observed in commercial citrus peel,which have been reported as1.8%(w/w)for orange peel waste.Grohman et al.[91]reported that the recombinant bacterium E.coli KO11produced27.6g/l ethanol from approximately66.6g/l sugars in orange peels hydrolysate.The hydrolysate contained18.6g/l galact-uronic acid which could ef?ciently fermented to ethanol by the bacterium.
A two step acid hydrolysis was applied by Oberoi et al.
[96]for the hydrolysis of orange peels.Fermentation of the hydrolyzate by S.cerevisiae resulted in ethanol yield of 0.25g/g on a biomass basis(0.46g/g on a substrate-con-sumed basis).
The ethanologenic bacterium Zymomonas mobilis can achieve greater ethanol yields than S.cerevisiae and other yeasts with productivities that can be as much5times greater than yeast fermentation[97].Wilkins[98]investi-gated the inhibition caused by orange peel oil on ethanol production by Z.mobilis and reported that the bacterium is inhibited by concentrations similar to a previous study using S.cerevisiae and K.marxianus,but the ethanol yield was greater for Z.mobilis[94].
Since limonene inhibits ethanol production and keeping in mind that pretreatment is also required to improve the accessibility of substrates to cellulose-degrading enzymes, several researchers investigated the effect of different pretreatment methods on ethanol production from citrus wastes.
Choi et al.[99]applying popping pretreatment in man-darin peel(MP)waste,reduced limonene concentration from0.21%in raw MP to0.01%.Following enzymatic hydrolysis using commercial cellulase,xylanase,b-gluco-sidase and pectinase preparations,ethanol fermentation was conducted applying a modi?ed SHF process by the yeast S.cerevisiae KCTC7906.Ethanol production reached46.2g/L(0.462g/g sugar,90.6%of the theoret-ical)while the respected value for the raw MP was39.8g/ L(73%of the theoretical)while productivity was tripled.
Steam explosion was applying to citrus peels waste (CPW)followed by SSF of the pretreated material.Steam
explosion removed most of the limonene present in CPW. In the SSF commercial enzyme were used,while the fer-mented microorganism was S.cerevisiae.Ethanol con-centrations as high as39.03g/l(0.21g/g on biomass basis) (in the presence of0.08%,v/w,limonene)were achieved at high pectinase loading[95].
Sharma et al.[100]obtained an ethanol concentration of 26.84g/L(0.426g/g sugar,83.52%of the theoretical) using crude cellulase enzyme and a combination of hexose (S.cerevisiae)and pentose(Pacysolen tannophilus)fer-menting yeasts from a mixture of kinnow waste and banana peel pretreated by steam explosion.
In pilot plant scale Zhou et al.[101]applying steam pretreatment for the removal of limonene followed by SSF of the pretreated citrus peels waste(using commercial enzymes and the yeast S.cerevisiae)obtained40g/l eth-anol(approximately90%of the theoretical)with limonene as co-product.
Hydrothermally treated Kinnow mandarin waste was used for ethanol production applying SSF process.Aprox-imately89%of the initial D-limonene was removed during pretreatment while42g/l(0.28g/g on biomass basis)eth-anol under optimal conditions were produced[102].
Mandarin(Citrus reticulata L.)peel wastes and lemon (Citrus limon L.)peel wastes were evaluated for bioethanol production,obtaining also as co-products:limonene,galact-uronic acid,and citrus pulp pellets(CPP).Both wastes were pretreated by steam explosion.SSF was applied for ethanol production using commercially available cellulolytic and pectinolytic preparations and the yeast S.cerevisiae. Steam explosion reduces the concentration of the initial limonene content of both wastes to non-inhibitory levels [95].Ethanol production from pretreated mandarin wastes reached60L/1,000kg fresh mandarin wastes(approxi-mately0.22g/g on dry biomass basis)[103],while the respected value from the lemon waste was67.83 L/1,000kg fresh lemon peel waste(approximately0.25g/g on dry biomass basis)[104].
A process for production of ethanol and biogas was developed by Pourbafrani et al.[105].In this process,the citrus waste(CW)is mixed with dilute sulfuric acid and then steam-exploded to hydrolyze the CW and also get rid of limonene.The resultant slurry is then centrifuged,where the liquid part is fermented to ethanol and distilled.The stillage from the distillation and solids from the centrifuge were mixed and digested to biogas(methane).This biogas can partly be utilized to produce steam required for the process,such as steam explosion and distillation.
Zhou et al.[106]performed an economic analysis of ethanol production from citrus peels.They that concluded citrus ethanol has many advantages over starch or cellu-losic ethanol,including low feedstock price,lower main-tenance and equipment costs of the pretreatement facility and limonene which has a higher value than distiller’s dried grain or lignin.A summary of results on ethanol production from citrus residues is presented on Table5.
Table5Citrus wastes as a substrate for bioethanol production
Substrate Process Microorganism Ethanol
concentration
(g/L)Ethanol
yield
References
Orange peel hydrolysate SHF S.cerevisiae40.0–45.0–[88] Orange peel hydrolysate SHF E.coli K01135.0–38.0–[92] Orange peel hydrolysate SHF S.cerevisiae and
Kluyveromyces marxianus
37.1–40.9–[94]
Citrus peel waste SSF S.cerevisiae39.00.210b[95] Kinnow waste and banana peel SSF S.cerevisiae26.80.426a[100] Orange peel Two stage hydrolysis followed by
fermentation
S.cerevisiae30.40.250a[96] Kinnow mandarin waste SSF S.cerevisiae42.00.280b[102] Pretreated mandarin peel SHF S.cerevisiae46.20.462a[99] Steam exploded manadarin
citrus peel waste
SSF S.cerevisiae59.3c0.220b[103]
Steam exploded lemon peel
waste
SSF S.cerevisiae67.8c0.250b[104]
Steam pretreated citrus peels
waste
SSF S.cerevisiae40.0–[101]
a Expressed in g of ethanol per g of total biomass
b Expressed in g of ethanol per g of consumed sugar
c Expresse
d in L of ethanol per1,000kg of fresh mandarin citrus peel or lemon peel waste
Methane Production
Anaerobic digestion,in which both pollution control and energy recovery can be achieved,is another possible way to treat and revalorize abundant orange peel waste.This process is de?ned as the biological conversion of organic material to a variety of end products including‘biogas’whose main constituents are methane(65–70%)and car-bon dioxide.The advantages of anaerobic digestion include low levels of biological sludge,low nutrient requirements, high ef?ciency and the production of methane,which can be used as an energy source for on-site heating and elec-tricity[107].The activity of the bacteria involved in the process varies with its age,morphology and temperature, with optimal temperature conditions at mesophilic(35°C) and thermophilic(55°C)range.Nevertheless,a wide variety of inhibitory substances are the primary cause of the upset or failure of thermophilic anaerobic digesters [108].Citrus peel contains essential oils which are well-known antimicrobial agents as mentioned earlier[16]so it is essential to extract D-limonene from the peel for anaerobic digestion processes to take place.
The anaerobic digestion of orange peel waste after a pre-treatmentmakes the addition of binder unnecessary,while preventing the generation of polluted wastewater.More-over,the digestate might be used as a high quality agri-cultural fertilizer[109].
Gunaseelan[110]evaluated different fruits and vegeta-ble solid wastes for methane production.Among them all the samples of CW tested gave methane yields greater than 0.3m3/kg volatile solids(VS)added thus represent an excellent choice for commercial methane production.
Kaparaju and Rintala[111]studied the thermophilic anaerobic digestion of industrial orange waste(pulp and peel)at laboratory scale.In anaerobic batch cultures,they obtained methane production rates of about0.49m3/kg VS (added peel waste).In semi-continuous anaerobic cultures, the loading of2.8kg VS/m3days and hydraulic retention times of26days generated a speci?c methane yield of 0.60m3/kg VS.
Anaerobic digestion of orange peel waste after a D-limonene extraction revealed higher methane production,
methane production rate and biodegradability in thermo-philic conditions.The highest methane yield coef?cient at pilot scale and thermophilic temperature was0.27–0.29m3 CH4/kg added COD(Chemical Oxygen Demand),obtained at an organic loading rate(OLR)of1.20–3.67kg COD/ m3days.Biodegradability was found to be84–90%, although a strong inhibition process was observed when the OLR was higher than4kg COD/m3days,reaching a normal manner again when the OLR was reduced[112].
Forga′cs et al.,[113]applied steam explosion pre-treat-ment to remove limonene from citrus wastes prior to anaerobic digestion.The methane potential reached0.537 m3/kg VS corresponding to an increase of426%compared with that of the untreated samples.
Organic Acid Production
Among the various products obtained through microbial cultivation on agroindustrial residues,organic acids are particularly important.The ratio of carboxylic acids man-ufactured microbiologically in the bulk of biotechnological products is very high.These compounds are valuable building blocks for chemical obtention,which can be used in several applications.
Citric acid is biodegradable,ecofriently,economical,safe and a versatile chemical for sequestering,buffering,wetting, cleaning and dispersing.The acid is mainly used in the preparation of medicinal citrates,confectionary,soft drinks and effervescent salts.Small quantities are employed in sil-vering and engraving and in dying and calico printing[114]. Due to its numerous applications and its generally recognized as safe(GRAS)nature,global production of citric acid has reached1.7million tonnes per year and is increasing at annual growth rate of5%[115].
Aravantinos-Za?ris et al.[116]reported citric acid fer-mentation from orange processing wastes having sugar con-tent of55g/l,in submerged culture,using A.niger and40g/l methanol.Yields obtained were citric acid concentration 30g/L,productivity[QP=0.104g/(L9h)]and yield YP/ S=0.63g/g.Rivas et al.[117]reported that a treatment of autohydrolysis at130°C and liquid/solid ratio of8.0g/g,had a bene?cial effect on its hydrolysis,producing liquors rich in soluble sugars,mainly sucrose,glucose,and fructose,which could be utilized for citric acid production by A.niger ATCC 9142.The highest values of citric acid concentration(9.2g/L), product yield on consumed sugars(YP/S=0.53g/g),and productivity[QP=0.128g/(L9h)]were achieved within 3days from initial sugar content of25.5g/l and in the presence of CaCO3and40ml/kg methanol.
Usually less methanol is required to stimulate citric acid release in solid-state fermentation.Hang et al.[118] reported optimal methanol concentration of only20ml/kg in solid-state fermentation of kiwifruit peel by A.niger ATCC9142,obtaining82g/L of citric acid after5days from168g/l of initial sugars[QP=0.683g/(L9h),YP/ S=0.60g/g].Similar results were reported by Zhang [119]for the solid residue of an orange juice factory and by Kang et al.[120]for tangerine peel.
Kuforiji et al.[121]applying solid state fermentation, reported that A.niger strains NRRL567and328produced 57.6and55.4%(g citric acid/100g of glucose used) respectively at a moisture content of38.9%in orange waste.The addition of methanol resulted in reduction of citric acid yields.
Dhillon et al.[115]reported citric acid concentration of 63.6±2.9g/kg dry substrate by A.niger NRRL2001in 72h incubation period using citrus waste as solid substrate.
Succinic acid is a dicarboxylic acid and is a potential precursor for the synthesis high value products of com-mercial importance including polymers,surfactants,sol-vents,detergents,?avors and fragrances[122].Presently it is manufactured by hydrogenation of maleic anhydride followed by its hydration to succinic acid and only small quantity is produced through microbial fermentation.The market for succinic acid is currently small and*16,000t per year.However,if the price becomes competitive, succinic acid could replace petroleum-derived maleic anhydride,which has a market volume of213,000t per year.An even higher market volume is conceivable for succinic acid as it is a versatile building-block chemical suitable for many uses[123].
Many different microorganisms such as fungi(Asper-gillus stc.,Byssochlamys nivea),yeast(S.cerevisiae),and some bacteria(Anaerobiospirillum succiniciproducens, Actinobacillus succinogenes,Mannheimia succiniciprodu-cens)have been screened and studied for succinic acid production using various sugar sources[124].
Production of succinate from waste orange peel and wheat straw by consolidated bioprocessing that combines cellulose hydrolysis and sugar fermentation,using a cellulolytic bac-terium,Fibrobacter succinogenes S85was reportrd by Li et al.[125].The succinate titres were1.9and2.0g/L for pre-treated orange peel and wheat straw,respectively.
Single Cell Protein
SCP is a microbial protein or biomass obtained from agricultural and industrial wastes as substrates to cover the protein need in animal and human nutrition.Citrus by-products are very high in cellulosic materials(cellulose and hemicellulose),but low in lignin,making them potentially good feed sources for ruminants and promising substrates for the production of microbial protein.In countries with inadequate supplies of conventional ruminant feeds the use of citrus waste can impact quite positively on the supply of feed for animal nutrition while reducing environmental pollution[126].
Unfortunately,citrus wastes have only minimal protein content which limits their value in animal nutrition.So, exploitation of these wastes in animal nutrition will depend on the deployment of processes for their protein enrich-ment by biotechnological means.Fermentative processes in both SSF and slurries employing both?lamentous and unicellular microorganisms have been employed for the protein enrichment of a citrus wastes.
Taiwo et al.[127]reported that unfermented citrus pulp (910g/kg DM)had relatively high levels of glucose and low levels of other nutrients.However,fermentation of citrus pulp without or with100g/kg molasses for61days resulted in production of primarily lactic and acetic acid [127],which enhanced citrus pulp ammonia holding capacity from0.1g NH3N/kg DM in unfermented citrus pulp to10.6and16.4g NH3N/kg DM in fermented citrus pulp without and with molasses,respectively.This suggests that the N content of citrus pulp can be enhanced by trapping excess ammonia generated from,for example, urea treated barley straw[127].
Scerra et al.[128]found that colonization of bergamot fruit peel with10strains of Penicillium spp.improved its nutritional value by increasing levels of crude protein, crude fat and structural carbohydrates versus untreated bergamot fruit peel.Furthermore,solid-state fermentation with P.roqueforti Pr2could represent an economical sys-tem for recycling,as animal feed,orange and lemon pulps, increasing their protein and lipid content by microbial bioconversion[129].
De Gregorio et al.[76]showed that by utilizing A.niger, but preferably T.viride,it was possible to extensively hydrolyse the lemon pulps,producing useful amounts of SCP and a high-activity crude pectinases.The highest protein level was reached with A.niger after14days growth and later with T.viride,though the?nal amount of nitrogen was higher in T.viride(31.9%)than in A.niger (25.6%).
Productions from solid-state cultures of Geotrichum candidum using acid pretreated orange peel has been suc-cessfully demonstrated[130,131].This typically contains 35–40%crude protein characterized by a very high in vitro digestibility(73–88%).From the solid residue obtained after an acidic pretreatment of the peel,hesperi-din,a high-value by-product(vitamin P),was recovered, reaching yields ranging between3.7and4.5%DM[130]. This by-product thus increases the cost-ef?cacy of the whole process.
Citrus By-Products as Sources for Dietary Fibres Production
Dietary?bre(DF)is often classi?ed as soluble dietary?bre (SDF)and insoluble dietary?bre(IDF).It consists of a variety of non-starch polysaccharides which include cel-lulose,hemicellulose,pectin,b-glucans,gums and lignin. DF is composed mainly of remnants of edible plant cells; parenchymatous tissues are known to be the most impor-tant source of vegetable?bre[132].Cell walls of fruits, vegetables and cereals make up most of the dietary?bre intake.Increasing attention has been given to their bene-?cial physiological effects of DF on humans and animals. High DF diets are associated with the prevention,reduction and treatment of some diseases,such as diverticular and
coronary heart diseases[133].It is widely known that DF obtained by different methods and from different sources, behave differently during their transit through the gastro-intestinal tract,depending on their chemical composition and physicochemical characteristics and on the processing that food undergo[134].Currently,there is a great variety of raw materials,mainly processing by-products,from which DF powders are obtained[135].Residues from orange juice extraction are potentially an excellent source of DF compared to other alternative sources,such as cereals,due to its higher proportion of SDF and the availability in large quantities[136].This is important, considering that the requirement for DF intake must be balanced,i.e.the water-soluble fraction should represent between30and50%of the total DF.Moreover,?bers from citrus fruits have an additional advantage over DF from other sources due to the presence of associated bio-active compounds(i.e.?avonoids and vitamin C)with antioxidant properties[137–139].
Rezzadori et al.[140]described a process for the exploitation of orange wastes which comprises the removal of essential oil followed by utilization of the remaining cellulolytic?bers for DF production.They reported that from the total amount of solid residues generated in the production of orange juice(8,000t)and considering an essential oil extraction process with40%ef?ciency, approximately12.8t of essential oils can be obtained and the?ber yield is5,200t.Bortoluzzi and Marangoni[141] described a similar process,but the?ber was obtained directly from the dry residue after juice extraction,and obtained yields of47.9g100g-1from20.7g100g-1IDF and SDF.
Garau et al.[142]studied the antioxidant capacity of the DF of orange by-products and it exhibited high antioxidant capacity.Citrus?bre,added to meat products(bologna sausages,chicken hamburgers),are effective inhibitors of lipid oxidation,thereby improving their oxidative stability and prolonging their shelf life[137].Garcia et al.[143] showed that the addition of1.5%orange?bre,to dry fermented sausages gives an organoleptic characteristic similar to the conventional high fat product.
Enzymatic Production of Prebiotic Oligosaccharides Prebiotics have been de?ned as‘‘selectively fermented ingredients that allow speci?c changes,in both the com-position and/or activity in the gastrointestinal microbiota that confers bene?ts upon host well-being and health’’. Several oligosaccharides with prebiotic properties(such as fructooligosaccharides,galactooligosaccharides,or lactu-lose)are commercially available,but there is an increasing interest in the identi?cation and development of new pre-biotic ingredients with added functionality[144].
Pectic oligosaccharides(POS)are good candidates as prebiotics[145,146].Recent studies reported healthy effects for POS,including regulation of lipid and glucose metabolism with decreased glycemic response and blood cholesterol levels,anticancer and immunological proper-ties,anti-obesity effects,antibacterial and antioxidant properties[147].The prebiotic potential of POS is also under evaluation by means of in vitro fermentation assays using both individual cells and faecal inocula,and,even, some studies were also carried out in humans[148,149]. Experiments carried out using POS derived from high methoxy citrus pectin,low-methoxy apple pectin and orange peel enhanced the growth of bi?dobacteria and lactobacilli,while limiting the growth of pathogenic genera [144,150,151].The prebiotic effect of pectins and POS depend on the physicochemical characteristics of sub-strates,and particularly,on the molecular weight distribu-tion and degree of esteri?cation[147].For instance,it was demonstrated that POS with low degree of methylation show higher prebiotic index than high degree of methyla-tion POS[150].
Olano-Mart?′n et al.[152]reported on the continuous production of POS in an enzyme membrane reactor using pectin from citrus and apple.Pectinolytic enzymes were used to obtain POS from bergamot peel[144]and while pectinases and cellulases were used during POS production from orange peels waste[153].The enzymatic treatment of the solid residue,after extraction of‘‘green labeled pec-tins’’from citrus peels,led to the extraction of a soluble fraction,containing pectic oligosaccharides of speci?c structure,known as modi?ed hairy regions(MHR)[154].
Orange peel albedo(white part)was also a good source of pectic oligosaccharides with prebiotic properties pro-duced by a microwave and autoclave extraction[155]. Mart?′nez Sabajanes et al.[156]reported the manufacture of pectic oligosaccharides by nonisothermal processing with hot,compressed water(autohydrolysis or hydrothermal treatments)of the orange peel wastes.
Natural Antioxidants in Citrus By-Products
Phenolic compounds are important for their sensory and nutritional qualities that impart the colours,?avours and tastes of many plants.Of them,?avonoids have been found to have health-related properties,including anticancer, antiviral,and anti-in?ammatory activities[157].It is believed that they act as scavengers of free radicals,as well as modulate the activities of metabolic enzymes involved in the initiation of low-density lipoprotein oxidation(e.g. xanthine oxidase,glutathione reductase,lipoxygenase,and NADPH-oxidase)and inhibit cellular proliferation[158].
Citrus processing by-products potentially represent a rich source of natural?avonoids(e.g.,hesperidin,diosmin,
naringin,and tangeretin),owing to the large amount of peel produced,and that citrus peels contain a high concentration of phenolic compounds[157].Moreover,while?avonoids are abundant elsewhere in the plant kingdom,there are several compounds(e.g.?avanones,?avanone glycosides and polymethoxylated?avones)unique to citrus,which are relatively rare in other plants[159].
Up to now,several extraction techniques have been reported for the extraction of phenols from citrus peels like solvent extraction[18,160–162],hot water extraction [163],resin-based extraction[164],electron beam-and c-irradiation-based extractions[165],supercritical?uid extraction[166],ultrasound assisted extraction[167]and enzyme-assisted extraction[52,168].
Citrus By-Products as Biosorbens for Heavy Metal Removal
With the rapid development of industries such as metal plating facilities,mining operations,fertilizer industries, tanneries,batteries,paper industries and pesticides,etc., heavy metals wastewaters are directly or indirectly dis-charged into the environment increasingly,especially in developing countries.Unlike organic contaminants,heavy metals are not biodegradable and tend to accumulate in living organisms and many heavy metal ions are known to be toxic or carcinogenic.Toxic heavy metals of particular concern in treatment of industrial wastewaters include zinc, copper,nickel,mercury,cadmium,lead and chromium.
Faced with more and more stringent regulations,nowadays heavy metals are the environmental priority pollutants and are becoming one of the most serious environmental problems.So these toxic heavy metals should be removed from the waste-water to protect the people and the environment.Many methods that are being used to remove heavy metal ions include chemical precipitation,ion-exchange,adsorption, membrane?ltration,electrochemical treatment technologies, etc.[169].Scientists are focusing on the production of cheaper adsorbents to replace costly wastewater treatment methods such as such the above.
Adsorption is one the physico-chemical treatment pro-cesses found to be effective in removing heavy metals from aqueous solutions.The emerging process of‘biosorption’uses nonviable or viable biological materials to bind con-taminants via physico-chemical mechanisms,whereby factors like pH,size of biosorbent,ionic strength and temperature in?uence metal biosorption[170].Plant wastes are inexpensive as they have no or very low economic value and thereby most of the adsorption studies have been focused on untreated plant wastes.
Some of the advantages of using plant wastes for wastewater treatment include simple technique,requires little processing,good adsorption capacity,selective adsorption of heavy metal ions,low cost,free availability and easy regeneration.However,the application of untreated plant wastes as adsorbents can also bring several problems such as low adsorption capacity,high COD and biological chemical demand(BOD)as well as total organic carbon(TOC)due to release of soluble organic compounds contained in the plant materials.The increase of the COD, BOD and TOC can cause depletion of oxygen content in water and can threaten the aquatic life.Therefore,plant wastes need to be modi?ed or treated before being applied for the decontamination of heavy metals[171].
Citrus peel principally consists of cellulose,pectin, hemicellulose,lignin,chlorophyll pigments and other low molecular weight hydro-carbons.These components con-tain various functional groups,such as carboxyl and hydroxyl,which make citrus peel to be a potential adsor-bent material for removing metal ions from aqueous solu-tions.Several studies have been proposed in literatures about using modi?ed or raw citrus waste in relation to heavy metal adsorption from water[172–181]. Integrated Approaches for Citrus Waste Valorization
A biore?nery is most commonly de?ned as a facility that integrates biomass conversion processes and equipment to produce fuels,power,and chemicals ideally from waste biomass.The biore?nery concept is analogous to today’s petroleum re?neries,which produce multiple fuels and products from petroleum and importantly utilizing the entire gross starting product.By producing multiple prod-ucts,a biore?nery can take advantage of the differences in biomass components and intermediates,and maximize the value derived from the biomass feedstock while leaving little waste.
Lo′pez et al.[182]proposed a general scheme for in-tergrated utilization of orange peels.The?rst step would consist of the removal of essential oils.This would be followed by an assessment of how the impact of oil removal in?uenced subsequent ethanol and methane pro-duction.Other high value products such as pectin,indus-trial enzymes,and single cell protein would also been removed before the?nal step of utilizing the remaining lignin as an energy source.
Pourbafrani et al.[105]presented a method for commer-cial treatment of CWs to yield different value-added prod-ucts.The process focused on ethanol,biogas and limonene production.Depending on the market and pro?tability of the process,pectin can be recovered as a by-product from the process.Brie?y,citrus wastes were hydrolyzed by dilute-acid process in a reactor equipped with an explosive drain-age.At this stage,high solubilization of pectin present in the
CWs was obtained,and77.6%of total pectin content of CWs could be recovered by solvent recovery.The limonene of the CWs was effectively removed through?ashing of the hydrolyzates into an expansion tank.The sugars present in the hydrolyzates were converted to ethanol using baker’s yeast while the stillage and the remaining solid materials of the hydrolyzed CWs were anaerobically digested to obtain biogas.By applying this process,39.64l ethanol,almost 45m3pure methane,8.9l limonene,and up to maximum 38.8kg pectin can be produced per ton of the wet CW.
Furthermore the same authors were performed an eco-nomic analysis of the proposed process and concluded that the total cost of ethanol for base case process with100,000t/year CW capacity was calculated as0.91USD/L,assuming10 USD/ton handling and transportation cost of CW to the plant. This price is sensitive to the plant capacity.With constant price of methane and limonene,changing the plant capacity from25,000to400,000t CW per year results in reducing ethanol costs from2.55to0.46USD/L in an economically feasible process.In addition,the ethanol production cost is sensitive to the transportation cost of CW.Increasing this cost from10to30USD/ton for the base case results in increasing the ethanol costs from0.91to1.42USD/L[183].
Balu et al.[184]have also recently developed a microwave assisted approach to transform orange peel residues into a range of valuable products ranging from chemicals(D-limonene and a-terpineol)and polysaccha-rides(pectin)to a novel and most unique form of meso-porous cellulose.
Recentlty Rezzadori et al.[140]proposed six integrated systems for the valorization of citrus wastes based on cal-culations and production data relating to a large orange juice plant with a daily processing rate of approximately 16,000t generated approximately8,000t of wastes and the concepts of clean technologies.According to authors pos-sible applications of citrus waste include animal feed (System1,7,200t of bran can be obtained daily and used as an ingredient for animal feed)bio-oil and charcoal pro-duction(System2,4,800t of bio-oil and864t of charcoal can be obtained daily)extraction of essential oils and die-tary?bers(System3,12.7t of essential oils and5.200t of dietary?bers can be obtained daily),pectin extraction (System4,yielding342t of pectin daily),adsorption of heavy metals and chemical compounds from wastewaters (System5)and production of limonene,biogas and ethanol (System6,which is the integrated process proposed by Pourbafrani et al.[105]).
Conclusions
Citrus fruits are grown around the globe in the region of the equator,and even though the harvesting season is?xed in speci?c locations around,they spread around the globe throughout the year,ensuring a constant supply of citrus and citrus waste for valorisation purposes.
Advanced citrus waste valorization practices aiming to achieve sustainable development and a circular economy should focus on innovative low environmental impact legislation compliant technologies able to convert waste into value-added products.These include anaerobic digestion,low environmental impact chemical technologies (including smart chemical separation technologies),inte-grated bio-chemical processing approaches(e.g.fermen-tation and chemical transformations of converted platform molecules to high-added value chemicals and biofuel precursors),extractive processes for the recovery of valu-able compounds(e.g.antioxidants,terpenes)using benign methodologies.
Transformation of citrus peel residues into higher value products would allow companies to increase competitive-ness by generating additional pro?ts and reducing disposal costs together with improving the resource ef?ciency of the citrus supply chain.
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如何写先进个人事迹
如何写先进个人事迹 篇一：如何写先进事迹材料 如何写先进事迹材料 一般有两种情况：一是先进个人，如先进工作者、优秀党员、劳动模范等；一是先进集体或先进单位，如先进党支部、先进车间或科室，抗洪抢险先进集体等。无论是先进个人还是先进集体，他们的先进事迹，内容各不相同，因此要整理材料，不可能固定一个模式。一般来说，可大体从以下方面进行整理。 (1)要拟定恰当的标题。先进事迹材料的标题，有两部分内容必不可少，一是要写明先进个人姓名和先进集体的名称，使人一眼便看出是哪个人或哪个集体、哪个单位的先进事迹。二是要概括标明先进事迹的主要内容或材料的用途。例如《王鬃同志端正党风的先进事迹》、《关于评选张鬃同志为全国新长征突击手的材料》、《关于评选鬃处党支部为省直机关先进党支部的材料》等。 (2)正文。正文的开头，要写明先进个人的简要情况，包括：姓名、性别、年龄、工作单位、职务、是否党团员等。此外，还要写明有关单位准备授予他(她)什么荣誉称号，或给予哪种形式的奖励。对先进集体、先进单位，要根据其先进事迹的主要内容，寥寥数语即应写明，不须用更多的文字。 然后，要写先进人物或先进集体的主要事迹。这部分内容是全篇材料
的主体，要下功夫写好，关键是要写得既具体，又不繁琐；既概括，又不抽象；既生动形象，又很实在。总之，就是要写得很有说服力，让人一看便可得出够得上先进的结论。比如，写一位端正党风先进人物的事迹材料，就应当着重写这位同志在发扬党的优良传统和作风方面都有哪些突出的先进事迹，在同不正之风作斗争中有哪些突出的表现。又如，写一位搞改革的先进人物的事迹材料，就应当着力写这位同志是从哪些方面进行改革的，已经取得了哪些突出的成果，特别是改革前后的．经济效益或社会效益都有了哪些明显的变化。在写这些先进事迹时，无论是先进个人还是先进集体的，都应选取那些具有代表性的具体事实来说明。必要时还可运用一些数字，以增强先进事迹材料的说服力。 为了使先进事迹的内容眉目清晰、更加条理化，在文字表述上还可分成若干自然段来写，特别是对那些涉及较多方面的先进事迹材料，采取这种写法尤为必要。如果将各方面内容材料都混在一起，是不易写明的。在分段写时，最好在每段之前根据内容标出小标题，或以明确的观点加以概括，使标题或观点与内容浑然一体。 最后，是先进事迹材料的署名。一般说，整理先进个人和先进集体的材料，都是以本级组织或上级组织的名义；是代表组织意见的。因此，材料整理完后，应经有关领导同志审定，以相应一级组织正式署名上报。这类材料不宜以个人名义署名。 写作典型经验材料-般包括以下几部分： (1)标题。有多种写法，通常是把典型经验高度集中地概括出来，一
关于时间管理的英语作文 manage time
How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.
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