Solidstate bioprocessing

Fermented foods have been consumed by humans all over the world for centuries. Most fermentation processes are conducted with liquid nutrient broths. Well known examples in the food industry are the production of yogurt, beer, wine, lactic acid, and many food flavors (213). However, partial fermentation and aerobic microbial growth based bioprocessing has also been used for processing food and food wastes. Here, instead of a nutrient broth, moist solid nutrients with minimal water are used as a substrate for microbial growth. This process is referred to as solid-state bioprocessing. Microbial fermentation and aerobic microbial growth on foods in solid state, for preservation of food and flavor enhancement, has been done for centuries and some of the common examples for these processes include manufacture of cheese and bread (214). Other wellknown examples are the production of microbe laced cheeses such as Roquefort, and the production of fermented sausages. In Asia, solidstate bioprocessing has been used for food fermentation for over 2000 years, for the production of fermented foods such as tempeh, natto, and soy sauce (193). The preservation of fish and meat by solid-state bioprocessing has also been reported to be carried by early human civilizations (193,214). Fruit wastes have been extensively used as substrates for solid-state bioprocessing. These wastes have mostly been used for the production of fertilizers, animal feed, as a growth substrate for mushrooms, ethanol production, production of organic acids such as citric acid, tartaric acid, and lactic acid, and for the production of various kinds enzymes such as pectinases (41,213). Solid-state bioprocessing of fruit substrates has also been carried out for several decades to produce compounds like gallic acid and vinegar (193,214). Recent research has also shown that consumption of fermented foods, especially solid-state bioprocessed foods is beneficial for health (215,216). Solid-state bioprocessing of fruit wastes such as cranberry pomace using the food grade fungi Rhizopus oligosporus and Lentinus edodes has been shown to enrich phenolic antioxidants and improve phytochemical consistency. These studies have shown that during the course of solid-state growth, the anti-oxidant activity and phenolic content of the pomace extracts increase several fold (52,53). The process resulted in enrichment of functional phenolics to a level found usually in fresh fruits and their juice products (217). The antimicrobial activity of the extracts against pathogens such as Escherichia coli O157H7, Listeria monocytogenes, Vibrio parahemolyticus and Helicobacter pylori of cranberry pomace was also enhanced by solid-state bioprocess-ing. It is suspected that the increase in phenolic and antioxidant activity could have been due to the production of various hydrolyzing enzymes by the fungi during the course of solidstate growth. These fungal hydrolases such as glucosidase and fructofuranosidases could possibly be hydrolyzing the glycosidic linkages between the phenolic moieties and sugars. A similar observation in the increase in phenolic aglycones was observed during the fermentation of soy milk for the production of tofu (218). The fungus, in adapting itself to utilize the fruit substrates, may produce various other types of hydrolases such as laccases and lig-nocellulases. The activity of these enzymes is suspected to responsible for the increase in the polymeric phenolics, and potentially to contribute to the enhanced bioavailability of such phenolic antioxidants. Enrichment of the solid-state bioprocessed fruit wastes such as cranberry pomace with ellagic acid after bioprocessing has been reported (52,53). This may have resulted due to the hydrolysis of ellagotannins by tannin hydrolyzing enzymes produced by the fungus. Further, it is suspected that phenolic enrichment could also occur through contribution from the growing fungal species. The endogenous phenolics present in the fruit wastes could be toxic to the growing fungus. In an attempt to adapt and utilize the substrate for growth, the fungus could be detoxifying the phenolics biochemically using a variety of enzymatic systems present in the fungus. The fungal detoxification can occur by a variety of mechanisms including methylating or demethylating the pheniolic ring, or by hydroxylation (219,220). Recent studies have shown methylated phenolic phytochemicals have excellent antibacterial properties against Gram-positive bacteria (221). Hydroxylation of the phenolic ring by the fungal system during its growth increases antioxidant properties (19) and therefore, phenolics resulting from biotransformation occurring during the solid-state bioprocess-ing may improve their functionality and be beneficial for human health. The advantage of this strategy is that the fungi, such as Rhizopus oligosporus and Lentinus edodes and other fungi used in this solid-state growth process are food grade and are generally recognized as safe (GRAS). This approach can easily be adapted to different substrates as well as be extended to liquid fermentation of juices to develop food and ingredients with enhanced functionality.

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