Ssf Processes For Organic Acids

Production of organic acids such as citric acid for food application by SSF has been employed since olden times. For example, citric acid production by SSF (the Koji process) was first developed in Japan, and is the simplest production method. SSF can be carried out using several raw materials. Generally, the substrate is moistened to about 70% depending on the water-holding capacity of the substrate. The initial pH is normally adjusted to 4.5-6.0 and the temperature of incubation can vary from 28 to 30°C. One of the important advantages of the SSF process is that the presence of trace elements does not affect citric acid production negatively as it does in SmF (113,119).

Commercially, citric acid is produced mainly using the filamentous fungus A. niger, although Candida sp. has also been used, employing both molasses- and starch-based media. In SSF, production has been obtained using crops and crop residues such as apple pomace (114), grape pomace (115), coffee husk (116), cassava bagasse, beet molasses (117) pineapple waste (110), and carrot waste (113) as substrates by A. niger. Citric acid production is largely dependent on the microorganisms, production techniques, and substrates employed. Generally the addition of methanol increased citric acid production in SSF (118). It has been proposed that overproduction of citric acid was related to an increased glucose flux through glycolysis. At low glucose fluxes, oxalic acid could accumulate. Several studies with different strains of A. niger have been made to compare citric acid production in flasks and in different kinds of bioreactors such as trays, packed bed bioreactors (single layered and multilayered), and rotating drums, and varied results have been obtained (106-113,119,120). For example, Tran et al. (109) reported the best production in flasks, and lower yields in tray and rotating drum bioreactors. Lu et al. (111,112) found that a multilayer packed bioreactor improved the mass transfer considerably compared with a single layer packed bed operated under similar conditions. Higher yields were obtained in packed bed than in flasks. In packed bed bioreactors using inert support, heat

Table 4.2

SSF for citric acid production using agro-industrial residues by different strains of Aspergillus niger

Raw Material Citric Acid

Apple pomace 766-883 g/kga

Grape pomace 413-600 g/kga

Kiwifruit peel 100 g/kga

Cellulose hydrolysate and sugar cane 29 g/kg

Orange waste 46 g/kg

Beet molasses (ca-alginate gel) 35 g/L

Saccharose (sugar cane bagasse) 174 g/kgb

Coffee husk 150 g/kgb

Carrot waste 29 g/kga

Okara (soy residue) 96 g/kga

Pineapple waste 132-194 g/kgb

Glucose (sugarcane bagasse) 21.24 g/L

Kumara (starch containing) 103 g/kgb

Mussel processing wastes (polyurethane foams) 300 g/kg

Cassava bagasse 347 g/kgb a based on sugar consumed; b based on dry matter removal by conductive mechanism was the least efficient (8.65%) when compared with convective (26.65%) and evaporative (64.7%) (119). Table 4.2 shows SSF for citric acid production using agro-industrial residues by different strains of A. niger.

Another important organic acid required for food applications is lactic acid, which has been produced in SSF using fungal as well as bacterial cultures. The commonly employed cultures belong to Rhizopus sp. and Lactobacillus sp., e.g., R. oryzae, L. paracasei, L. helveticus, and L. casei (121-123). Different crops such as cassava and sweet sorghum, and crop residues such as sugarcane bagasse, sugarcane press-mud, (122) and carrot-processing waste (123) served as the substrate. A comparative study involving fungal strains of R. oryzae to evaluate l-(+) lactic acid production in SmF and SSF showed that SSF was superior in production level and productivity. Fermentation yields were 77% (irrespective of media) and yields were 93.8 and 137.0 g/l in SmF and SSF, respectively (121). The productivity was 1.38 g/l per hour in SmF and 1.43 g/l per hour in SSF. In another comparison using L. paracasei, lactate concentrations and yields were 88-106 g/l and 91-95% for SmF, and 90g/kg and 91-95% for SSF, respectively, but the time required for SSF was 120-200 h in comparison to 24-32 h in SmF (123).

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