The term "food waste" covers the wastes (and by­product streams) that are generated during the whole food supply chain starting from production of the raw material followed by the processing into edible products by the food industry and the final disposal by con­sumers, restaurants or catering services. Valorizing the waste derived from the food industry sector would result in the creation of novel biorefineries leading to restructured and advanced industrial plants that will not only satisfy the traditional market of food produc­tion but also other markets that are nowadays depen­dent on petroleum to provide the necessary feedstocks. Food processing waste streams constitute renewable re­sources enriched in carbohydrates, protein, oils and fats, phenolic compounds and various micronutrients.

PHA Production from Winery By-Products

Wine production constitutes an important industrial sector in many countries around the world, such as the South European countries, United States, Chile and Australia. Wine making generates both solid and liquid by-products. Residues from wine production involve mainly trimming wastes, grape stalk, grape pomace or marc, wine lees and winery wastewater. These by­products are currently supplied to ethanol distilleries (e. g. in the case of wine lees), used (if possible) as fertil­izers or processed as wastes in order to reduce the envi­ronmental impact caused by their disposal to the environment. However, given the fact that environmental policies are changing, new practices should be applied aiming at valorization of winery by-product streams.

Ongoing research focuses on valorization of residues from wine making. Trimming wastes are rich in cellu­lose, hemicellulose and lignin. Combined thermochem­ical treatment with enzymatic hydrolysis can be applied to convert cellulose and hemicelluloses into C5 and C6 sugars that can be assimilated by microorgan­isms. Delignification steps are usually required since the complex structure of lignin prevents hydrolysis of polysaccharide. Bustos et al. (2005) evaluated the use of trimming wastes and wine lees aiming at the produc­tion of lactic acid through simultaneous saccharification and fermentation carried out by Lactobacillus rhamnosus. Trimming wastes could be also used as solid support in solid-state fermentations for the production of various enzymes (Sanchez et al., 2002).

Grape pomace or marc is the solid fraction remaining after the extraction and it consists of skins, pulp, seeds and stems of grapes. Research has focused on efficientutilization of this waste stream, since it contains ligno — cellulosic fractions that can be hydrolyzed and further used in microbial bioconversions. Solid-state fermenta­tion for production of hydrolytic enzymes has also been reported using grape marc as solid support (Botella et al., 2005).

Wine lees is the remaining residue after the end of the fermentation stage. It is a rich source of ethanol, tartaric acid, phenolic compounds and yeast cells. Wine lees can be used for the production of potable alcohol (wine lees mainly produced by large wineries), as nutrient supple­ment for fermentation (Bustos et al., 2004; Salgado et al., 2010), for the production of tartaric acid (Versari et al., 2001; Rivas et al., 2006) and as raw material for compost­ing (Diaz et al., 2002; Nogales et al., 2005). A novel pro­cess has been developed at the Agricultural University of Athens targeting the creation of a novel biorefinery concept based on wine lees valorization (Figure 24.4). The process starts with centrifugation or filtration of wine lees in order to separate the liquid stream that can be used for ethanol production via distillation. The ethanol produced can be used as potable or fuel ethanol depending on the purity. Current processes produce potable ethanol. Ethanol could be also used as a plat­form chemical to supply the future sustainable chemical industry. Alternatively, ethanol could be also utilized as carbon source for microbial fermentation aiming to PHB production by the bacterial strain C. necator NCIMB 12080 (Senior et al., 1986). This, however, may not be a cost-competitive alternative when compared to the traditional potable ethanol market. The remaining liquid after ethanol extraction can be used in subsequent hydrolysis stages to increase the presence of nutrients.

The solid fraction that remains after centrifugation of wine lees contains phenolic compounds with antioxi­dant properties, tartrate salts and yeast cells. A phenolic-rich fraction can be easily isolated via sol­vent extraction. Tartrate salts can be subsequently sepa­rated from yeast cells via treatment with hydrochloric acid. Versari et al. (2001) extracted tartaric acid with pu­rity up to 99% from three different winery by-product streams, including wine lees. Moreover, Nurgel and Canbas (1998) investigated the production of tartaric acid from grape pomace. Use of tartaric acid is well established in wine making in order to adjust the pH of the must prior to fermentation. Tartaric acid could be also used as food additive.

After the extraction of phenolic compounds and tartrate salts, residual wine lees solids are subjected to enzymatic hydrolysis with the addition of crude en­zymes produced via solid-state fermentation of a fungal strain of A. oryzae on wheat bran. The ethanol-free me­dium that remains after the distillation step is used as liquid in the hydrolysis stage. In this stage, yeast cells are lysed and converted into a nutrient-rich supplement similar to yeast extract. This supplement is rich in various sources of nitrogen (e. g. amino acids and pep­tides), phosphorus and various trace elements. This nutrient supplement can be combined with a carbon source (e. g. crude glycerol from biodiesel industries) as fermentation media for the production of PHB with C. necator. Preliminary experiments with C. necator DSM 7237 and crude glycerol as carbon source showed that PHB production is feasible using wine lees hydroly­sates. However, supplementation with a low quantity of minerals is necessary showing that this nutrient supple­ment is deficient in some minerals. The wine lees hydro­lysate could be combined with a sugar-rich hydrolysate derived from treatment of lignocellulosic streams derived during wine production.

PHB Production from Confectionery and Bakery Industry Waste Streams

Significant quantities of waste streams are generated annually from confectionery industries and bakeries. The waste streams from the industrial sectors mentioned above produce flour-, starch — or sugar-rich waste streams generated either during processing or as end — of-date products returned from the market. Confection­ery waste streams are currently used as animal feed, for composting or are discarded to landfills. However, these low-cost materials constitute renewable feedstocks that could be used for the development of novel biorefinery schemes. Anaerobic digestion from various food waste streams and biodiesel production from cooking oils are predominant alternatives that have been proposed for the utilization of various food waste streams. Current research on confectionery waste streams and waste bread is rather limited, but in recent years research has started to focus on the valorization of such waste streams. Dorado et al. (2009) utilized hydrolysates derived from wheat milling by-products as fermentation media for the production of succinic acid (50.6 g/l). Leung et al. (2012) developed a two-stage bioprocess involving solid-state fermentation and enzymatic hy­drolysis of waste bread to produce a fermentation feed­stock for the production of succinic acid (47.3 g/l at a conversion yield of 0.55 g SA/g bread) using the bacte­rial strain Actinobacillus succinogenes.

A potential biorefining concept for the production of PHAs and biodiesel from confectionery industry waste streams is presented in Figure 24.5. In the case of confec­tionery wastes that contain high oil content, this could be removed via solvent extraction. The oil obtained from this step can be used for biodiesel production. Remaining fractions will be rich in directly assimilable sugars such as glucose, fructose, sucrose and lactose as well as starch and protein. Utilizing starch- and

protein-rich waste streams as sources of carbon and ni­trogen in fermentation processes demands the conver­sion of starch into glucose and protein into amino acids and peptides. The amylolytic and proteolytic en­zymes required for the hydrolysis of these macromole­cules could be produced via solid-state fermentation using the fungal strain Aspergillus awamori cultivated on wheat milling by-products. The fermented solids, rich in amylolytic and proteolytic enzymes, are subse­quently combined with confectionery waste to produce hydrolysates that can be used in fermentation processes for the production of platform chemicals, microbial oil or PHB. The production of PHB or PHAs from confec­tionery industry wastes could be employed for the pro­duction of biodegradable packaging materials for the same industry.

The proposed process is based on the results that were achieved for the production of PHB using wheat as the whole raw materials (Koutinas et al., 2007a, 2007b; Xu et al., 2010). In this biorefinery concept, wheat is fractionated into bran and gluten as value — added co-products, while remaining fractions are used for the production of fermentation media suitable for the production of PHB via fed-batch cultures using the microbial strain Wautersia eutropha NCIMB 11599. Xu et al. (2010) developed a fermentation process for the production of PHB from wheat-derived fermentation media during fed-batch cultures in a bioreactor. The highest PHB concentration achieved was 162.8 g/l. However, wheat is regarded a food resource and should not be used for chemical production. Starch — or flour- rich food wastes could be used, instead of wheat, as a renewable resource for the production of PHB.

PHB Production from Whey

Whey is the main by-product occurring from cheese manufacture and lactose is one of the primary compo­nents. Current whey valorization processes mainly focus on the production of whey powder, whey protein concentrate or whey protein isolate. Utilization of whey in fermentation processes has been widely investigated, given the fact that it is produced in many countries in significant quantities. Furthermore, whey valorization will also contribute to the improvement of the environ­mental impact of the cheese industry because whey disposal is a notorious environmental burden.

Future cheese industries could incorporate integrated processing schemes for the production of whey protein and PHAs. Koller et al. (2010) reviewed various biocon­versions that employed whey permeate as carbon source aiming at the production of PHAs. Different strategies were proposed concerning uses of whey permeate; direct conversion as substrate or hydrolysis of lactose to glucose and galactose were examined. Moreover, Wong and Lee (1998) presented PHB production from whey powder with recombinant E. coli in pH-stat cul­tures. In fed-batch cultures with additions of concen­trated whey solution, the corresponding dry cell weight and PHB concentrations were 87 and 69 g/l, respectively. The PHB content reached up to 80% (w/ w). These results established that PHB fermentation pro­cess from whey could be industrially employed, increasing the sustainability and market alternatives of traditional cheese producing plants.

Whey protein concentrate and isolate that could be extracted from whey by ultrafiltration and evaporation steps can be applied as food additives. Moreover, they are considered to possess therapeutic properties and for this reason, whey protein concentrate was applied for treatment of various clinical disorders. Furthermore, whey protein ingredients are added to food targeting to improve their functional or technological properties. Hence, keeping that in mind, biorefinery schemes based on whey utilization could be easily proposed.

CONCLUSIONS AND FUTURE
PERSPECTIVES

The necessity to eliminate our dependence on fossil resources will lead to an inevitable reconstruction of the current industry in order to introduce the utilization of renewable resources and produce chemicals, fuels and materials in a sustainable manner. Implementation of biorefinery concepts into existing industrial facilities provide an alternative processing option, taking into consideration that industrial by-products and waste streams are generated in significant quantities and currently, they are inadequately utilized. Consequently, production of value-added products from waste and by-product streams will enhance sustainability and diversify market opportunities. Furthermore, produc­tion of biofuels should coincide with chemical and biodegradable polymer production to enhance their sus­tainability. This study showed potential industries where biofuel and food production could coincide with PHA production. This research area is currently at the inception phase and significant effort is required in order to develop the technologies that will be imple­mented on industrial scale.