Biotechnological Conversion of Sugar and Starchy Crops into Lactic Acid

Abstract

Abstract Lactic acid can be used not only as a key substance in chemical synthesis, but also as a special agent in agriculture. For microbial production, original products of agriculture such as sweet sorghum stalks and rye grains can be used as raw material. In laboratory experiments, sweet sorghum stalks were milled, steam-treated and pressed. The sugar of the aqueous extract, which amounted to 89% of the total sugar of stalks, was completely converted into lactic acid by fermentation. The yield amounted to 94% (94 g of lactic acid/100 g of sugar consumed). Also in laboratory experiments, rye grains were milled, fractionated and hydrolyzed in a two-step process using commercial enzymes. The optimum temperature and pH value were 82·5°C and 5·8 for starch liquefaction and 51·6°C and 4·0 for saccharification of liquefied starch, respectively. Starch liquefaction was also influenced by particle size, the optimum value of which was 3 mm. For optimum starch saccharification, a process duration of 2 h was necessary. Of the solid starch, 99·6% was liquefied in the first hydrolysis step. In the second step, 97·9% of the liquefied starch was converted into glucose. Bioconversion of the glucose formed by starch hydrolysis into lactic acid also gave a maximum yield of 94%. Various possibilities of process improvement in order to make the whole operation less expensive are discussed, and a flow-sheet of an improved process for manufacturing lactic acid from cereals is presented.