From raw cassava pulp to L-lactic acid via fermentation by Rhizopus oryzae

Currently the natural crude oil depletion has been brought into awareness. This affects many industries including petrochemical manufacturing. As a result, there are many attempts to develop novel processes to produce substitutes to such petrochemical products including plastics. Polylactic acid, made from polymerization of optically pure isomer of lactic acid, is one of such examples. In addition to the application in food and pharmaceutical industries, lactic acid can be used as a chemical building block for plastics and solvents. Nowadays lactic acid is produced via fermentation by lactic acid bacteria, which require complex media and produce a racemic mixture of lactic acid. In contrast, Rhizopus oryzae is capable of producing an optically pure L-lactic acid from simple and inexpensive media consisting of starchy materials and pentose sugars. Raw cassava pulp, containing approximately 50% starch (dry basis), is one of the starchy materials which are abundant in Thailand. Potentially, it can be used as a carbon substrate in L-lactic acid fermentation by R. oryzae. Therefore, in this study we investigated Llactic acid fermentation from raw cassava pulp. Raw cassava pulp was treated by acid hydrolysis before fermentation. The effects of hydrolysis conditions, including acid type and concentration, and time, on starch recovery and glucose production were observed. Among the conditions studied, the maximum starch recovery (88%) and the highest glucose production (98 g/L) were obtained when hydrolyzing cassava pulp at 121°C for 15 min with 1.0 M HCl (1 g dry pulp to 9 mL HCl solution). The kinetics of L-lactic acid fermentation using acid treated cassava pulp hydrolysates (ACPH) from different conditions as the carbon source was also studied. It was found that free ions present in the hydrolysates affected L-lactic acid fermentation. R. oryzae was capable of growing in the fermentation medium containing HCl treated cassava pulp hydrolysate (HACPH). However, instead of L-lactic acid, ethanol was produced as a major product in the fermentation. While no growth and product formation were observed in the fermentation of H2SO4 treated cassava pulp hydrolysate (SACPH) and H3PO4 treated cassava pulp hydrolysate (PACPH).