Fruits are processed to produce fresh-cut products, an industry increasing steadily in importance, and this generates large amounts of residues (peels, stones, husks, etc.). Fruits are also used for the production of juices, wines and oils by pressing technologies, leading to press-cake residues generally very rich in bioactive secondary metabolites.
Residues are mainly generated in the juice and cider production industries. The wastes are in the form of pomaces, and include peels and seeds in addition to other solid tissue parts. The composition of these residues is very variable and depends largely on the apple cultivar processed. The phenolic content of the pomaces is very variable depending on: the content of the cultivar used for processing; the inactivation of polyphenol oxidases that lead to phenolic compound degradation and formation of brown polymers; and the use of external enzymes (pectinases) for increasing juice yield. The main use of pomaces is for pectin extraction. Other potential uses would be the extraction of phenolic compounds. Flavan-3-ols (cat-echins and procyanidins), hydroxycinnamates, dihydrochalcones (phloretin glycosides) and flavonols (quercetin glycosides) are the main phenolic compounds present in apple wastes. Once the oxidative enzymes are inactivated an enhanced release of phenolics by enzymatic liquefaction with pectinases and cellulases represents an alternative that has been explored (Will et al., 2000). Recently, a method for the combined recovery of pectins and polyphenols from apple pomace has been established (Carle et al., 2001).
Grape pomace (from the wine and must industries) amounts to more than 9 million tons per year (Schieber et al., 2001). A great range of products such as ethanol, tartrates, citric acid, grape seed oil, hydrocolloids and dietary fibre are recovered from grape pomace. Polyphenol extracts are also produced and commercialised from residues. Anthocyanins, catechins, flavonol glycosides, and phenolic acids are the principal phenolic constituents of grape pomace. The content of the stilbenoid resveratrol, although small, is relevant as the price of the extracts often depends on the content of this minor constituent due to its demand and biological activity. Grape seed extracts rich in procyanidins with different degrees of polymerisation are traded, as well as fibre with antioxidant activity due to its polyphenols and white grape skin extracts rich in flavonols in addition to procyanidins.
These fruits are used for juice production and pomaces are produced but not yet used for phytochemicals extraction. The pomaces are rich in fibre and in polyphenols (mainly procyanidins and flavonols, and hydroxycinna-mates in smaller amounts), and in some cultivars they are rich in caroten-oids (P-carotene). Only fibre is industrially produced today and can be found in the market although phenolic and carotenoid extracts could be alternative extracts for the future.
The use of citrus by-products is a traditional industry and was reviewed 10 years ago by Braddock (1995). Essential oils are directly produced in the juice production plants as a co-product and have a market. The main wastes include the citrus peels and residues from segments and seeds after pressing. These residues are rich in pectins, and flavonoids (flavanones) and limonoids are present in minor amounts. Flavanones are generally extracted in alkaline water, the extracts are then acidified and the flavanones, which are slightly soluble in water, precipitate. There are differences in the flavo-noid content of different citrus species and this can be used for production of specific extracts enriched in compounds with specific properties. Thus, grapefruit is rich in bitter naringin that can be further chemically transformed into the intensely sweet dihydrochalcones. Lemon is rich in eriocitrin, a flavanone with higher in vitro antioxidant activity than flavanones from other citrus species; mandarins and tangerine peels contain polymethoxylated flavones (tangeretin, sinensetin, nobiletin, etc.) that have a relevant anticarcinogenic activity.
The residues include peels and stones. These are produced both in the juice and fresh-cut industries. The juice kernels are rich in gallic and ellagic acids as well as gallotannins and condensed tannins. The peels are rich in fla-vonols and carotenoids. These extracts are not produced as yet.
Processing to produce juice and fresh-cut pineapple leads to wastes that contain pulp residues and external parts of the fruit. Hydroxycinnamic acid derivatives (sinapyl glutathione etc.) can be produced from pulp residues, and processes for their recovery have been established (Wrolstad and Ling, 2001) although these extracts are not available in the market.
This fruit can be processed to obtain juice. The peels are around 30% of the fruit and constitute an interesting source of procyanidins (mainly polymeric). The polyphenol oxidase activity has to be inactivated before or during extraction to avoid browning and polyphenols degradation. Other agricultural wastes are the bracts which are especially rich in anthocyanins (delphinidin, cyanidin, pelargonidin, peonidin, petunidin and malvidin) (Pazmino-Duran et al., 2001). In addition, carotenoids (xanthophylls) ester-ified with myristate, laurate, palmitate and caprate have also been reported (Subagio et al., 1996).
Guava, papaya and passion fruit
These tropical fruits also provide a good source of wastes that could be used for extraction of bioactive compounds. Some seeds contain glucos-inolates. Especially relevant are the residues of passion fruit processing that constitute more than 75% of the harvested fruit (Schieber et al., 2001). Additional studies are necessary to evaluate the content of phytochemicals in these residues, and their biological activity and extraction and preparation methods.
This is processed to yield juice, leading to a waste that mainly contains peels and seeds. In addition, this fruit is also prepared as fresh-cut product, and in this case the residues are the peels with part of the flesh tissue. These residues would be a reasonably good source of phenolic acids (benzoic acid derivatives), flavanol monomers and oligomers, and flavonols (Dawes and Keene, 1999).
Pomegranate juice has been introduced in Western markets (especially in the USA) over the last few years, although this juice was traditionally produced in Mediterranean and Middle East countries where it is appreciated for its aphrodisiac and health-related properties. In the USA it is traded with a label indicating its high in vitro antioxidant capacity, which has been reported to be higher than that of red wine and green tea (Gil et al., 2000). During processing the husks, membranes and seeds constitute a residue that can account for more than 50% of the harvested fruits. This residue is rich in ellagitannins (mainly punicalagin isomers) that are water soluble and easily extracted from the residues (Gil et al., 2000). In addition, the membranes are a source of procyanidins, and the seeds contain interesting oils with interesting conjugated unsaturated fatty acids with relevant biological activity (Kohno et al., 2004).
Different berries are processed as juices, and the press-cake constitutes a relevant by-product rich in phytochemicals. In blueberry and bilberry (and other Vaccinium species) the residues are especially rich in anthocyanins and flavonols, and also procyanidins in smaller amounts, this being an evident source for extract preparation. The extracts are already traded and have a place in the dietetic and pharmaceutical markets. Raspberries and strawberries are other berries that produce phenolic-rich residues after juice production (Aaby et al., 2005). These residues also contain flavonols, anthocyanins, procyanidins, and ellagic acid and ellagitannins, all of which are compounds with biological activities and a potential position in the nutraceutical and functional food market. Currants (Ribes species, red and black) also produce phenolic-rich residues that could be used in the same way.
Vegetables are processed as fresh-cut products to produce salad mixes, as ready to cook preparations (spinach, potato, etc.) and also as juices (tomato and carrot), frozen vegetables, canned products, etc. In this case the residue generation in the packing houses that trade in fresh products is also relevant as many vegetables are trimmed before despatch, packed in trays covered with plastic films.
Tomato pomace consists of the dried and crushed skins and seeds of the fruit. This residue is rich in lycopene (from skins), and other carotenoids (P-carotene). Phenolic compounds are also present in relevant amounts, these being hydroxycinnamic acid derivatives, flavonols (quercetin derivatives), flavanones and naringenin-chalcone. Supercritical CO2 extraction of lycopene and P-carotene from tomato paste waste resulted in recoveries of up to 50% when ethanol was added (Baysal et al., 2000). Enzymatic treatment enhanced lycopene extractability (Schieber et al., 2001).
A pomace is generated after juice production. This is rich in P-carotene, but phenolic compounds are also present (hydroxycinnamates and coumarins) (Cinar, 2005).
This is consumed as a vegetable, as juice and as a food colorant. Though still rich in betalains, the pomace from the juice industry is disposed of as feed or manure. The peel is the richest in phenolics and other compounds (betacyanins and betaxanthins); it also contains coumaric and ferulic acids as well as cyclodopa glucoside derivatives (Schieber et al., 2001).
Aqueous peel extracts are rich in phenolic acids, especially chlorogenic, gallic, protocatechuic and caffeic acids. Methods for complete separation of steroidal alkaloids from phenolic compounds prior to their use in foodstuff would be desirable to avoid any risk for human health.
The development of the fresh-cut lettuce industries during the last few years has led to increased waste production that includes external leaves, stems and whole low-quality lettuce heads. The greener lettuce tissues (external leaves) are richer in flavonoids and phenolic acids and could be used as a source for phytochemicals extraction. By-products from lettuce (Lactuca sativa L.) varieties (romaine, iceberg and baby) and one chicory (Cichorium endivia L.) variety 'escarole' have recently been evaluated for their polyphenolic content as well as their antioxidant capacity, with interesting results (Llorach et al., 2004). The phytochemical profile of lettuce by-products is composed of hydroxycinnamic acids (both caffeoylquinic and caffeoyltartaric acid derivatives) and flavonoids (both flavones and flavonols). The main hydroxycinnamic acid derivative identified was dicaffeoyltartaric acid (chicoric acid) followed by chlorogenic acid (5-O-caffeoylquinic acid). In addition, different isomers of isochlorogenic acid (3,5-O-dicaffeoylquinic acid) were identified. The flavone luteolin-7-O-glucuronide was identified, and - for the quercetin derivatives -quercetin-3-O-glucoside, quercetin 3-O-glucuronide and quercetin 3-O-(6-O-malonyl)-glucoside were identified. Regarding chicory byproducts, the high-performance liquid chromatography (HPLC) analyses of raw extracts showed a kaempferol 3-O-glucoside as the main flavonol and this compound has already been reported in chicory.
Lettuce by-products have shown an interesting antioxidant capacity with both free radical scavenging activity and the capacity to reduce Fe(III) to Fe(II) (Llorach et al., 2004); this has been applied to functionalise food products (Llorach et al., 2005).
460 Handbook of waste management and co-product recovery Artichoke
Artichoke heads are generally processed in the industry to produce artichoke hearts; this results in large quantities of waste including external bracts, stems and in some cases leaves. This waste can reach up to 70% of the harvested product depending on the processing method and the quality of the finished product. In addition, the blanching waters constitute a processing effluent that is rich in phenolic acids (hydroxycinnamic acid derivatives; cynarin, chlorogenic and isochlorogenic acids) and should be considered for its potential in the preparation of nutraceutical extracts (Tomás-Barberán et al., 2005).
These by-products have also been studied concerning their application for animal feedstuffs (Martínez-Teruel et al., 1998) and fibre production (Femenia et al., 1998; Goñi and Saura-Calixto, 1988). The artichoke byproducts are a very good source of antioxidant polyphenols with caffeic acid derivatives as the main phenolic compounds (Llorach et al., 2002). The antioxidant activity has been proven with different assays, showing a special capacity to prevent the peroxidation of linoleic acid (Llorach et al., 2002).
Onion handling activities produce wastes that include the external membranes. In the fresh-cut industry, scale tissues are also included in the residues. These external tissues are the richest in onion flavonoids, including different quercetin glucosides, the bioavailability of which has been demonstrated (Erlund, 2004). In addition, the residues can be a good source of organosulphur compounds that also have relevant biological activities (Tapiero et al., 2004).
During the handling of broccoli, cauliflower, cabbages and Brussel sprouts for the fresh market, for fresh-cut processing or for freezing, wastes are generated that include leaves, stems, low-quality florets, etc. The wastes are a good source of highly glycosylated flavonoids, hydroxycinnamates (mainly sinapic acid derivatives) and glucosinolates (Vallejo et al., 2004). The content of these metabolites is very variable depending on the nature of the tissue and the ripening stage.
Cauliflower by-products (Brassica oleracea L. var. botrytis) mainly consist of leaves and, in lesser amounts, stems. Regarding the edible portion of the cauliflower, this is rather poor in phytochemicals and only small amounts of some hydroxycinnamic acid derivatives - such as caffeic, sinapic and ferulic acids - have been identified and quantified (Llorach et al., 2003a).
The HPLC analysis of cauliflower by-product extracts revealed the presence of both flavonoids and hydroxycinnamic acids (caffeic acid and sinapic acid). Different combinations of flavonols such as kaempferol and quercetin with sinapic acid and glucose have been identified with the main compounds being kaempferol-3-O-sophoroside-7-O-glucoside and its sinapoyl derivative (kaempferol-3-O-(sinapoylsophoroside)-7-O-glucoside). Moreover, some flavonoids with an unusually high grade of glycosylation (five sugar moieties) have been isolated and tentatively identified for the first time (Llorach et al., 2003a). To our knowledge, the characterisation of flavonoids with more than four sugars has not been previously reported.
The cauliflower by-products showed a relevant antioxidant capacity, estimated from their ability to reduce TPTZ(2,4,6-tripyridyl-5-triazine)-Fe(III) complex to TPTZ-Fe(II). In this way, 16 g (dry weight) of cauliflower by-products can provide the same antioxidant capacity as one cup of tea or one glass of red wine (Llorach et al., 2003b).
This is minimally processed to be marketed as fresh sticks. The residues are composed of leaves and greener and thinner stems. The residues are very rich in celery aroma compounds, flavones (apigenin derivatives) and hydroxycinnamates.
During the preparation of legumes, especially in the freezing industries, the external tissues of the pods are discarded. These residues are particularly rich in flavonoids (mainly flavonols, and special mention should be given to pea flavonols (Ferreres et al., 1995), beans and broad beans). The pods can also be a very good source of procyanidins and in some cases isofla-vones. More research is needed on this still relatively unexplored topic (e.g. lentils, chickpeas, peanuts, etc.).
Different industrial processes for plant food products yield large amounts of wastes that could also be used for phytochemicals extraction. Among them, oil extraction and nut preparation deserve a special mention due to the large volume of wastes generated and their phytochemical content. Olive-oil extraction wastes and nut hulls are of particular importance. In addition, the brines of pickles, capers and olives are also waste effluents that contain significant amounts of phytochemicals that could be used (Inocencio et al., 2000).
In the past, the by-products resulting from olive-oil extraction were the waters (vegetation water, black water or vegetable water) and the olive husk, including skins and stones. Recently both wastes have been mixed to produce a single by-product. These wastes are rich in antioxidant compounds and particularly in hydroxytyrosol and related derivatives, and oleuropein (Fernández-Bolaños et al., 2002). Flavones are also present as well as hydroxycinnamate derivatives. Hydroxytyrosol is probably the most relevant compound in terms of its biological activity, and extracts enriched in this compound have a very promising market. The price of the extracts obtained is generally related to their hydoxytyrosol content.
Nut shells and cereal bran are also produced as wastes and can be good sources of some phytochemicals. In this case the compounds can be directly extracted or extracted after chemical treatment to release them from the cell walls. Benzoic and hydroxycinnamic acids are the main compounds, but ellagitannins can also be extracted from some by-products.
The effluents from the preparation of olives and capers and other similar products are highly contaminant and difficult to handle due to their high salt content. These brines also contain flavonoids and other phenolic compounds. In the case of caper brines, a large amount of quercetin derivatives (quercetin rutinoside and rhamnosyl-rutinoside) could be recovered from the brines (Inocencio et al., 2000). In the case of olive brines, other compounds such as hydroxytyrosol, flavones and hydroxycinnamates could be recovered. Further research is needed to evaluate the potential of these by-products and to achieve salt recovery to avoid environmental contamination.
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