The effects of surfactants on the solid substrate fermentation of potato starch (Aspergillus oryzae, Bacillus subtilis)

The potential of surfactants for improving the yields of α-amylase during the solid substrate fermentation (SSF) of potato starch using pure and mixed cultures was examined. The microorganisms used in this study were Aspergillus oryzae ATCC 1011, Bacillus subtilis ATCC 21556 and Bacillus subtilis ATCC 21332. The surfactants tested were Tween 20, Tween 80, SDS and surfactin. The fermentations were carried out in perforated trays after the addition of 10% (v/w) inoculum and with temperature and humidity controlled at 30°C and 90% RH respectively. Samples were taken and analyzed quantitatively for the production of α-amylase and biomass and qualitatively by scanning electron microscopy (SEM) using a JSM-840 A scanning microscope at 10 kV accelerating voltage.; It was possible to increase fungal α-amylase production by as much as 6 fold in the process with the addition of either synthetic surfactants or the biosurfactant surfactin. The bacterial α-amylase yields increased up to 11.5 fold in with the addition and/or the co-culture production of surfactants. The highest enzyme activity was found in the fermentation of a mixed culture of the two Bacillus strains with the addition of Tween 80. During the SSF with B. subtilis ATCC 21332 and ATCC 21556 as a mixed bacterial culture, there was also the production of surfactin in yields comparable to those obtained in a submerged fermentation. The biofilm formation as observed by SEM appeared to be associated with the presence of surfactants in the process and was not formed when no surfactants were present. The biofilm was observed as an entrapment of the bacteria in the substrate, resulting in improved access to the starch and higher production of α-amylase.