**3. Production of volatile fatty acids (VFAs)**

The conversion of the organic content of waste into VFAs requires an acidogenic fermentation (AF). While soluble organics can be directly fermented into organic acids and other fermentation products, such alcohols and hydrogen, insoluble compounds need to be hydrolyzed prior to acidification, limiting the rate of VFA production [28, 29]. As such, the production of VFAs involves two steps: (1) hydrolysis, and (2) acidogenesis, commonly occurring in a single anaerobic reactor (**Figure 2**) [2]. In the hydrolysis step, enzymes excreted by hydrolytic microorganisms (e.g. *Clostridium* sp*.*, *Bacillus* sp., *Bifidobacterium* sp.) brake down complex organics (such as proteins, cellulose, lignin, and lipids) into simpler soluble monomers (such as amino acids, simple sugars, glycerol, and fatty acids), which lead to an increase in the soluble chemical oxygen demand (sCOD). Then, these monomers are mainly converted into VFAs (such as acetic, propionic, butyric, and valeric acids) by fermentative bacteria (e.g. *Acetovibrio cellulolytic*, *Butyrivibrio* sp., *Selenomonas* sp.) in the acidogenic fermentation step (acidogenesis) [2, 30–32].

The production of VFAs from acidogenic fermentation of FW involves a series of chemical reactions, where different metabolic pathways co-exist within the anaerobic digester. These pathways play a crucial role in the system performance and consequently in the FW conversion efficiency. Pyruvate is the primary intermediate

#### **Figure 2.**

*Overall anaerobic digestion process schematic representation.*

and can be converted into a wide range of products, such as VFAs, alcohols, hydrogen, and carbon dioxide. The type of substrate used, the environmental conditions and the microorganisms present in the reactor affects the proportions of pyruvate in each metabolic pathway and consequently the distribution of VFAs produced [5]. The acidogenic metabolic pathways can be classified in: acetate-ethanol type; propionate-type; butyrate-type; mixed-acid and lactate-type, depending on the main products produced during the acidogenic fermentation.

Acetate can be produced from acetyl-CoA pathway or from the syntrophic oxidation of ethanol or longer chain fatty acids. Ethanol can be produced from pyruvate in two or three steps, depending on the type of bacteria, and with acetyl-CoA and acetaldehyde as intermediates [5]. Propionate is produced by two distinct pathways: (1) pyruvate is reduced by the catalyzation of lactate dehydrogenase and then lactate is reduced to propionate through the propionate dehydrogenase; (2) propionate is produced by acidogenic bacteria (e.g., Corynebacteria, Propionibacterium and Bifidobacterium) via transcarboxylase cycle. Butyrate production from pyruvate comprises: (1) pyruvate conversion into acetyl-CoA by pyruvate dehydrogenase; (2) acetyl-CoA is converted into to butyryl-CoA with acetoacetyl-CoA, 3-hydroxybutyryl-CoA and crotonyl-CoA as intermediates sequentially by the catalysis of thiolase, 3- hydroxybutyryl-CoA dehydrogenase and butyryl-CoA dehydrogenase; (3) butyryl-CoA is converted into butyrate by phosphotransbutyrylase and butyrate-kinase enzymes or by the butyryl-CoA: acetate CoA-transferase [5]. For the lactate production, pyruvate is converted to lactate through lactate dehydrogenase and can be divide into two fermentation types: homolactate fermentation (one mole of glucose is converted into two moles of lactic acid) and heterolactate (lactic acid is produced with carbon dioxide and ethanol). In mixed fermentation, an equal amount of each acid is produced with a possible formation of carbon dioxide and hydrogen. This type of fermentation is common in FW fermentation with acetate and butyrate the main metabolites produced. Acetate can be also produced by homoacetogens, which are obligate anaerobes that can use hydrogen to reduce carbon dioxide to acetate. In autotrophic process, the homoacetogens consumed hydrogen and carbon dioxide producing acetate by Wood-Ljungdahl pathway [5].

*From Food Waste to Volatile Fatty Acids towards a Circular Economy DOI: http://dx.doi.org/10.5772/intechopen.96542*

Since acidogenesis is the second step of the anaerobic digestion of organic compounds into biogas (methane and carbon dioxide), high pH (above 8) or low pH (bellow 6), low temperature and/or low HRT are usually used to prevent methanogenic activity (**Figure 2**). Moreover, the operating parameters of the acidogenic fermentation, such as HRT, sludge retention time (SRT), organic loading rate (OLR), pH, temperature, and reactor configuration, must be optimized aiming at VFAs production yield maximization and at controlling the composition of the synthesized VFAs.

VFAs production from FW using mixed microbial cultures is mostly based on the use of suspended biomass. Thus, continuous stirred tank reactors (CSTR), stirred tank reactors (STR) and immersed membrane bioreactors are being applied for that purpose [1, 15, 21, 22, 33, 34]. Those reactors are usually operated in a continuous mode. However, for FW needing high retention times to be converted into VFAs, they are often converted into batch and semi-continuous (fed-batch) reactors [15, 19, 21, 33–38].
