BACKGROUND: Lactic acid has many applications in the chemical industries and it can be produced economically by microorganisms using biomass raw materials of different origins. Sweet sorghum juice is a high sugar content raw material with potential for lactic acid production because after hydrolysis of its sucrose content the remaining glucose and fructose can supply the carbon demand of most lactic acid bacteria. However, satisfying the nitrogen and B-vitamin needs of the bacteria by supplementation with yeast extract and/or other alternative nitrogen-containing supplements can make the process too expensive.
Starch based lactic acid fermentation technology was examined and optimized using a non-amylolitic, mesophilic lactic acid bacterium. Comparing simultaneous saccharification and fermentation (SSF) and separate hydrolysis and fermentation (SHF) technologies several issues arose and applying the two techniques needed several compromises concerning the overall process time. A combined hydrolysis and fermentation method was developed which incorporate the advantages of SSF and SHF technologies: applying a time delay in inoculation and cutting down hydrolysis time before fermentation, an optimal inoculation and high efficiency was achieved by kinetic model aided experimental work. Experimental verification of the model gave an excellent productivity result: 4.32 g L −1 h −1 calculated with only the fermentation time, and 2.88 g L −1 h −1 calculated with the overall time of the two processes. With this method, hydrolysis and fermentation time was successfully reduced, enhancing lactic acid productivity and depressing production cost of this low-value chemical.
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