28.4 Techniques for Bioconversion of Food Waste Toward Circular Bioeconomy Approach

431

waste are in the form of macromolecules (such as starch and protein) which have

to be broken down into utilizable forms (glucose and free amino nitrogen) before

utilized by microorganisms for fermentative hydrogen production [37]. Although

some reported pretreatments were able to convert the macromolecules into utiliz-

able forms, various inhibitory products (such as furfural) for fermentative hydrogen

production could also be produced [38]. Enzymatic hydrolysis could release the

nutrients (glucose and free amino nitrogen) from food waste with advantage of

high hydrolysis speed which would be a promising way. However, there is little

information about fermentative hydrogen production from enzymatic hydrolysis of

food waste.

Biotechnological processes such as one-stage H2 fermentation, two-stage

H2 + CH4 fermentation, combining dark fermentation with anaerobic digestion,

and photofermentation with anaerobic digestion are for the production of hydro-

gen. Production of H2 from food waste depends on co-substrate such as animal

manure and sewage sludge, pH, temperature, and pretreatment. Food waste rich in

carbohydrate is most suitable for biohydrogen production than proteins and lipids

[39]. This hydrogen-producing bioprocess at the initial stage usually accompanied

with acidification. Hydrogen is generated from the food waste by the microflora

Clostridium and Enterobacter. To increase H2 production, pretreatment of food

waste by heat is necessary to suppress lactic acid bacteria. Food waste co-digested

with olive husk improved the yield of biohydrogen [40].

Lactic acid production from food waste is biodegradable and it is widely used in

food, pharma, cosmetic, and textile industries. Lactobacillus pentosus is used to pro-

duce lactic acid from wheat bran by fermentation. To control pH and inhibit the

accumulation of lactate, neutralizing agents such as calcium carbonate, sodium car-

bonate, and sodium hydroxide are added to fermentation medium [41]. Lactic acid is

produced by hydrolysis and acidogenesis, which are first two steps of fermentation.

To increase the lactic acid yield, the operating conditions such as inoculum temper-

ature, pH, and organic loading rate need to be optimized. The hydrolysis and acidi-

fication process enhanced at pH 4–5 for the food waste as a substrate. Temperature

influences the substrate conversion rate and microbial activity thereby lactic acid

production [42, 43]. Fruit and vegetable waste, mango peel waste, and potato peel are

found to be potential substrate for lactic acid fermentation. Thermal pretreatment

of sole food waste substrate for fermentation greatly reduces the fermentation time

and enhances the lactic acid yield [44]. The use of mixed cultures tolerates the vari-

ability of complex food waste due to metabolic flexibility. The complex food wastes

are efficiently converted into useful bioproducts from intermediate feed stock chem-

icals such as short-chain carboxylates produced by hydrolysis and fermentation with

undefined mixed cultures under anaerobic conditions. Propionate, lactate, acetate,

and n-butyrate are the intermediate chemicals obtained from carboxylate platform.

28.4.3

Enzymatic Treatment

European Union has established Directive 2008/98/EC, the hierarchy related

to reducing food waste. Accordingly, food waste must be valorized through (i)

the collection of biowaste, (ii) biowaste treatment, and (iii) the generation of