Structural changes of corn starch during Saccharomyces cerevisiae fermentation

Native corn starch (NCS) dispersions (3%w/v) were inoculated with native (i.e., not genetically modified) Saccharomyces cerevisiae (2x10(7)cellsmL(-1)), fermented for 0, 12, 24, 48, 72, 96, 120, and 168h at 38 degrees C. It was found that glucoamylases, the amylolytic enzymes produced by native S. cerevisiae, produced morphological changes in the fermented corn starch (FCS) granules by attacking in a first stage the fractures around the Malta cross. In the long-term (96h and higher times), the starch granules were broken down into fragments of different sizes. The FCS dispersions were centrifuged and the precipitates were subjected to different analyses. XRD analysis indicated that while the crystallinity of the unfermented NCS was 32.82%, crystallinity progressively increased to a maximum of 40.94% for the FCS after 96h. The FTIR absorbance ratio 1047cm(-1)/1022cm(-1) remained nearly unchanged during the first 96h of fermentation, suggesting that the short-range crystallinity linked to double-helices was unaffected. Thermal analysis data showed that FCS exhibited gelatinization temperature range and enthalpy maxima after 96h of fermentation. The FCS obtained at different fermentation times, were subjected to stringent hydrolysis conditions. Longer fermentation times resulted in diminished hydrolysis percentage of FCS. Overall, the results of this work show that the fermentation of NCS with native S. cerevisiae for given specific times can induce structural changes in the fermented granules, leading to positive improvement of some properties, such as increased crystallinity, lower susceptibility to hydrolysis, and enhanced thermal properties.