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28 Bioconversion of Food Waste to Wealth – Circular Bioeconomy Approach

solid-state fermentation method [30]. Several kinds of pretreatments including

mechanical milling, steam explosion, acid treatment, and organic solvents are

reported to increase the yield of bioethanol by enhancing the hydrolysis and micro-

bial fermentation of sugars into chemical substances. The yeast Saccharomyces

cerevisiae is mostly used for production of bioethanol due to its high yield and

tolerance to accumulation of inhibitory compounds during industrial fermentation

[31]. As compared with bioethanol, biobutanol has more energy content, and chem-

ical components like butyl acetate and acrylate can be obtained as co-products.

However, butanol concentration above 13–20 g/l will have inhibitory effect on

microbial growth and to avoid this, the produced butanol is removed from the broth

during fermentation. Adopting liquid–liquid extraction, adsorption, gas stripping,

or butanol tolerant strains may overcome the limitations.

The main constituents of the food waste are carbohydrates, proteins, and lipids

which can be anaerobically fermented by association of hydrolytic, acetogenic,

hydrogen producing, and acetate forming microbes to produce methane, hydrogen,

and volatile fatty acids. Hydrolysis is the first step in anaerobic fermentation

followed by acidogenesis, acetogenesis, dehydrogenation, and methanogenesis [32].

Volatile fatty acids are intermediate products recovered during acidogenesis and are

widely used in food, pharma, textile, leather, and plastic industries. Optimization

of acidogenic metabolic pathway is important for efficient recovery of volatile

fatty acids and their derivatives. During hydrolysis treatment, sugars like glucose,

fructose, galactose, and ribose are mostly extracted, and the composition of sugars

vary with the food waste substrate composition.

Hydrogen is regarded as the most promising renewable source of energy mainly

due to its high energy content (energy yield of hydrogen is 122 kJ/g which is 2.75

times higher than that of fossil fuel) [33]. Generally, biological hydrogen production

can be divided into two categories: photosynthesis and dark fermentation [29]. Dark

fermentation is seemed to be a more feasible biotechnology for hydrogen production

than the photosynthesis due to less energy consumption and no light limitation [34].

Dark fermentation method is now being widely researched globally by scientists in

an attempt to produce hydrogen from food waste more efficiently as this method

requires only less chemicals and low energy in its application when compared to

other processes. As this method depends on food waste as the raw material, when

implemented globally, this can successfully decrease the issues arising with respect

to food waste management. Although currently there are still researches going on

regarding this process for hydrogen production to establish a clear knowledge for

global implementation, the idea of this process is clear and has been limited only

in a laboratory scale. However, low hydrogen production rate and high cost are the

dominant obstacles for large-scale dark fermentative hydrogen production [35].

Food wastes are cheap carbon and nitrogen source for microbial fermentation

to produce numerous bioproducts including enzymes, proteins, antioxidants, and

pigments. Bioconversion of food waste into valuable bioproducts can reduce the

environmental pollution by eliminating the waste. Lactic acid and ethanol are the

common end products in food waste fermentation [36]. Proteins and starch are

the two main components essentially present in the food waste that are suitable

economic source for the production of biofuels. However, nutrients stored in food