Economical production of agricultural γ-polyglutamic acid using industrial wastes by Bacillus subtilis

Industrial wastes, such as monosodium glutamate waste liquor, chicken manure and soybean cake, were used for agricultural gamma-polyglutamic acid (gamma-PGA) production by Bacillus subtilis ZC-5. Crude extract of glutamic acid after isoelectric crystallization (CEGA) could be used instead of glutamate for gamma-PGA production. The optimal composition of medium was obtained by response surface methodology. Firstly, Plackett-Burman design was undertaken to assess the effects of eight factors. With statistic regression analysis, the significant factors affecting gamma-PGA production were determined as follows: chicken manure, soybean cake and CEGA. Then the Box-Behnken design was adopted for further optimization. The optimal addition amounts of factors were determined as: 62.35 g chicken manure, 25.15 g soybean cake and 15.09 g CEGA. Under such condition, the gamma-PGA yield was increased to 6.57 +/- 0.13%, which was 20.6% higher than the maximum value of the single factor tests. These investigations would lay a foundation for reducing the pollution of industrial wastes, exploring a late-model for agricultural gamma-PGA production.