Effects of solid‐state fermentation (Aspergillus oryzae var. oryzae) on the physicochemical properties of corn starch

The physicochemical transformations suffered by corn starch granules under the solid‐state fermentative action of Aspergillus oryzae var. oryzae were studied. Light microscopy images showed that the starch granules became increasingly fractured and broke‐down as fermentation time was longer. SEM micrographs showed that the mean particle diameter decreased linearly with the fermentation time, indicating a zeroth‐order hydrolysis kinetics. The crystallinity content of fermented corn starch achieved a maximum value (39.72 ± 1.02) after 100 h followed by a reduction at longer fermentation times. The short‐range ordering monitored by the absorbance ratio 1047/1022 from FTIR increased during the first 100 h, suggesting the formation of double‐helix structures as long chains was hydrolyzed. DSC analysis showed the deterioration of the internal ordering of starch granules for long fermentation times, reflecting an extensive disruption of the granule integrity. Overall, the results showed that starch granules underwent strong, although slow, physicochemical changes as the solid‐state fermentation evolved. Specifically, crystallinity increased about 25% and viscosity of 5% w/w gelatinized starch dispersion decreased about 85% in the first 100 h of fermentation. In addition, the gelatinization enthalpy decreased about 60.0% and the ζ‐potential doubled during the fermentation process.