Although direct disposal of fruit processing wastes in landfills has become environmentally unacceptable, some fruit processing wastes can be composted under anaerobic condition, and then used as fertilizer in landfills, because such wastes were readily degraded under anaerobic digestion conditions (57,58). Solid-state fermentation can also be used for composting of fruit and vegetable processing wastes such as apple waste (59) and tomato pomace (60). Composted apple pomace and other fruit and vegetable processing wastes can be used in nursery potting mixes and as field soil amendments (61). Composted grape pomace was used as an organic fertilizer in vineyards for growing grapes (62).
Apple pomace was directly used as a feed for cows and sheep, but its value as an animal feed is less than $7 per ton (47). Apple pomace is a poor animal feed supplement because of its low protein content. The nutritional value of apple pomace was improved by solid-state fermentation with a food yeast, Candida utilis (62). Yeast fermentation resulted in a 2.5-fold increase in protein, 3.4-fold increase in niacin, 10-fold increase in pantothenic acid, 1.5-fold increase in riboflavin, and 1.2-fold increase in thiamine (62). An improved stock feed was produced from apple pomace by solid-state fermentation with Kloeckera apiculate and C. utilis (63,64).
A solid-state fermentation of orange processing wastes with Aspergillus niger and Rhizopus species enriched the protein content by 300%, and the fermented product could be readily sold as animal feed (65). Banana wastes were also used for protein enrichment and protein and biomass production by solid-state fermentation using A. niger (66), yeast Pichia spartinae (67), and Saccharomyces uvarum (68).
As the world population increases, there is a greater demand for new sources of protein for formulating new types of food. For this reason, there is a great interest in the use of single-cell protein as a source of protein for animals or humans. Apple pomace is a potential substrate for producing single-cell protein. The microorganisms that can be used for single-cell protein production include yeast, filamentous fungi, bacteria, and algae. It is well recognized that these microorganisms have the ability to produce protein and other essential nutrients (49). Yeasts have been studied extensively and are already widely used as a source of protein for animals and humans. The production of "food yeast," C. utilis, has long been accepted for use in foods by the regulatory authorities. C. utilis has high nutritive value and is able to grow rapidly in high yields utilizing a variety of carbon and nitrogen sources, with high tolerance to low pH. A general review on the production of feed and food yeasts from various plant materials has been published (69). A single-cell protein of C. utilis was successfully produced from apple pomace (70).
Mushroom production is one of the few large-scale commercial applications of microbial technology profitable for bioconversion of cellulosic waste materials to valuable foods (11). Apple pomace was found to be a good substrate for the cultivation of edible mushrooms. Various oyster mushrooms (Pleurotus species) were growing very well on apple pomace with a biological efficiency ranging between 30 and 40% (71). The biological efficiency was a measure ment of the conversion rate, a ratio of the fruit body yield to the weight of the cultivation medium. In another experiment, a mixture of apple pomace and sawdust was used as a substrate for production of shiitake (Lentinula edodes) and oyster mushroom (Pleurotus ostreatus) on synthetic logs (72). Apple pomace is complex and has readily usable carbohydrates that complement the nutritional properties of sawdust. As a result, the mushroom mycelia grew faster and more densely in logs containing apple pomace than in sawdust alone (73).
In general, ethanol is produced from a liquid substrate by a culture of Saccharomyces cerevisiae. Because of its physical nature, apple pomace is not readily amenable to submerged yeast fermentation (74). A solid-state fermentation system for the production of ethanol from apple pomace with a strain of S. cerevisiae was devised (74-76). The yield of ethanol varied from about 29 g to more than 40 g per kilogram of apple pomace fermented at 30°C in 24 h, depending on the samples fermented; the fermentation efficiency of this process was approximately 89% (76). The spent pomace resulting from the separation of alcohol should be a better animal feed supplement, because its protein content was enriched by the yeast biomass. A novel process for concomitant production of ethanol and animal feed from apple pomace by solid-state fermentation was developed (77). Ethanol production from other fruit processing wastes such as corn fiber (78,79), carob (80,81), and banana waste (68) has also been reported.
Gupta et al. (80) further studied the effect of nutrition variables on solid-state alcoholic fermentation of apple pomace by several strains of yeasts and improved the fermentation efficiency by adding various phosphates, nitrogen sources, or trace elements. Ngadi and Correia (82) studied the effect of factors such as moisture and bioreac-tor mixing speed on the ethanol production from apple pomace. They even proposed a mathematical model to describe the kinetics of solid-state ethanol fermentation from apple pomace and suggested a logistic function of cell growth and ethanol production during solid-state fermentation (83).
Fruit and vegetable processing wastes were readily degraded under anaerobic digestion conditions (57,58,84). Jewell and Cummings (47) investigated the biodegrada-bility of apple pomace using long-term digestion experiments and by graphic analysis. A total of 12 steady-state loading conditions were examined, and kinetic analysis indicated that a hydraulic retention time of 45 days would result in nearly 90% conversion of the biodegradable or-ganics to biogas, and the methane content of the biogas was 60% (47). Under these conditions, the net energy yield from pomace would range from about 0.7 to 1.7 million kcal per wet metric ton, with dry matter contents varying between 20 and 40% of the wet weight, and the energy has a value of between $12 and $30 per wet metric ton of apple pomace (47).
Orange processing waste was also used to produce biogas by solid-state fermentation. Srilatha et al. (85) reported that pretreatment of orange processing waste by solid-state fermentation using selected strains of Sporotri-chum, Aspergillus, Fusarium, and Penicillium improved overall productivity of biogas and methane.
Citric acid, a tricarboxylic acid, has a wide range of applications in the food, pharmaceutical, and beverage industries as an acidifying and flavor-enhancing agent. The use of apple pomace as a substrate for microbial production of citric acid under solid-state fermentation conditions has been reported (86,87). In the experiment, 40 g of apple pomace was introduced into 500-mL Erlenmeyer flasks and inoculated with A. niger spore suspension, and then incubated at 30°C for five days under stationary conditions. Methanol was added to the substrate before fermentation to enhance the yield of citric acid. Under the optimal fermentation conditions, this process yielded up to 90 g of citric acid per kilogram of wet pomace fermented, with a conversion rate of more than 88% based on the amount of sugar consumed (46). A process for leaching citric acid from apple pomace fermented with A. niger was also devised (88).
Pineapple waste could be a better substrate for citric acid production in solid-state fermentation than apple pomace (89), and A. niger was used in the process (90,91). The highest citric acid yield achieved on pineapple waste in four days was 161 g per kilogram of dried pineapple waste of 70% moisture content, and in the presence of 3% methanol, and the conversion rate was 62.4% based on the sugar consumed (89).
Kiwifruit peel is a by-product resulting from the manufacture of kiwifruit into nectars or slices and represents nearly 10 to 16% of the weight of the original fruit, depending on the peeling method used (52). Like apple or pineapple processing wastes, kiwifruit peel waste can be a good substrate for citric acid production by solid-state fermentation, and about 100 g citric acid per kilogram of kiwifruit peel was produced by A. niger in the presence of 2% methanol at 30°C in four days. The yield was more than 60% based on the amount of fermented sugar consumed (52).
Corn wastes were used as a substrate for citric acid production by A. niger with a maximum yield of 250 g/kg dry matter of corn cobs after 72 h of growth at 30°C (92). Carob kibble, a waste material from carob pod fruit was also an attractive substrate for citric acid production by solid-state fermentation (93,94). Orange processing wastes were also used as a substrate to produce citric acid by A. niger (95). During citric acid fermentation, the presence of methanol was a critical factor in increasing citric acid yield, and this appears to be a general phenomenon with strains of A. niger (52,95).
Other organic acids were also produced by solid-state fermentation from fruit processing wastes. Grape pomace could also be used for citric acid production by solid-state fermentation (96). The production of tartaric acid from grape pomace was also attempted (97), while banana wastes were targeted as a substrate for lactic acid production (98).
The acetone-butanol fermentation is a well-known process that is being looked at with renewed interest recently, owing to the increasing value of oil products. The classical substrates used for butanol production by fermentation are a variety of starchy materials and molasses, which are of relatively high cost. Apple pomace was used as the substrate for butanol production by solid-state fermentation with the strains of Clostridium acetobutylicum and C. bu-tylicum, and the yields of butanol from apple pomace were between 1.9 and 2.2% of fresh apple pomace (99).
During solid-state fermentation with edible fungus Rhi-zopus oryzae on apple pomace and cassave bagasse, substantial volatile compounds were produced in association with the fungal growth (100). The major volatile metabolites produced by R. oryzae from apple pomace were identified as acetaldehyde, 1-propanol, ethyl acetate, ethyl pro-pinioate and 3-methyl butanol (100). Ethyl acetate and ethyl propionate probably originated from the esterifica-tion between ethanol and acetic acid and propionic acid, respectively, which are the products of carbohydrate metabolism (100).
Pectic enzymes derived from Aspergillus species have been widely used to increase juice yields and clarify juices. The commercial enzyme preparations are produced by cultivating the mold in a synthetic medium under submerged fermentation conditions. Recently apple pomace has been reported to be an attractive raw material for the production of pectinases by Aspergillus foetidus in solid-state cultures (101). To obtain maximum enzyme yields, it is essential to supplement apple pomace with a sufficient amount of an organic nitrogen constituent such as corn steep, yeast extract, or peptone (102). Berovic and Ostroversnik (103) developed a solid-state bioprocess for production of pecto-lytic enzymes from apple pomace and the process parameters such as inoculation, influence of mixing, aeration, temperature, and moisture content on pectolytic enzymes production were studied. Under the optimal conditions, maximal amounts of 15 g of polygalacturonase and 200 mg of pectinesterase per kilogram of solid medium were obtained (103). Polygalacturonase is one of the most important pectinases and is widely used in juice processing industries (104). Apple pomace was used as the substrate of solid-state fermentation to produce polygalacturonase by A. niger, and the highest yield of25,000 units per kilogram of fermented apple pomace was achieved (105). In addition to apple pomace, citrus processing wastes were also alternative substrates for pectinase production in solid-state fermentation (106,107).
Amylases are enzymes catalyzing the hydrolysis of a-1,4-glucosidic linkages of polysaccharides and have wide applications in the food and beverage processing indus-
Fruit or vegetable processing wastes
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