Production of biostimulants from okara through enzymatic hydrolysis and fermentation with Bacillus licheniformis: comparative effect on soil biological properties.

In this work okara (OK), a by-product of soy milk manufacturing, is submitted to an enzymatic hydrolysis and a fermentative process to produce different soil biostimulants (BS): EH, hydrolysate obtained by the enzymatic process; FHEB, fermentation broth with Bacillus licheniformis and the enzymes secreted during the fermentation; FHE, fermentation broth without bacteria and FH, the FHE hydrolysate in which enzymes were denatured. Enzymatic hydrolysates showed a different chemical composition compared with fermented hydrolysates and OK. It had a higher protein concentration as well as C, P and K. The proteins of OK were converted into peptides with a lower molecular weight, the fermented hydrolysates being those with the lowest molecular weight profile. The influences of hydrolysates and OK were tested in soil, finding that β-glucosidase, phosphatase and dehydrogenase activities were stimulated by every treatment. However, it was observed that EH produced a greater stimulation of dehydrogenase and phosphatase than both OK and fermented BS. The bacterial and fungal phospholipid fatty acids were also higher in soils amended with BS than those of the control and soils with OK. It has also been found that β-glucosidase, phosphatase and microbial biomass were dose-dependent in every treatment, but dehydrogenase only was dose-dependent in EH and OK treatments.