Abstract: This study was conducted to examine the effects of natural fermentation and pure culture fermentation (with the predominant strains of lactic acid bacteria and yeast isolated from naturally fermented millet) of millet on molecular structure and pasting properties of millet starch, aiming to provide a theoretical foundation for the analysis of the effect and mechanism of pure culture fermentation on the modification of millet starch. Starches were extracted from millet fermented by different starter cultures, and they were examined for differences in granular characteristics, crystallinity, functional groups, molecular mass, and pasting and retrogradation properties. Results were obtained as follows: 1) The starch granules showed marked surface erosion after fermentation with each isolate, but were only slightly affected by natural fermentation. 2) Lactic acid bacterial fermentation yielded a 1.49% increase and a 0.33% decrease in starch crystallinity as compared to natural fermentation and yeast fermentation, respectively. 3) Fermentation did not change the peak positions in the function group region of millet starch, but decreased the characteristic peak intensity; the fingerprint region disappeared after pure culture fermentation. 4). The weight average molecular mass of unfermented millet starch was between 1.5 × 104?5.9 × 105 g/mol, that of the naturally fermented one between 2.1 × 104 ? 5.4 × 105 g/mol, and that of the lactic acid bacteria fermented one between 1.6 × 104?5.3 × 105 g/mol, and that of the yeast fermented one between 1.6 × 104?4.7 × 105 g/mol. 5) After fermentation, the proportions of long-chain amylopectin and amylose decreased, whereas the proportion of short- and middle-chain amylopectin increased. Moreover, after fermentation by the lactic acid bacteria and yeast for 96 h, the gelatinization temperature decreased 0.84 and 1.13 ℃, the enthalpy increased by 1.00 and 0.78 J/g, the setback decreased by 743 and 471 mPa·s as compared with natural fermentation, respectively. It was concluded that the dominant bacteria lactic acid bacteria and yeast in natural fermentation played a major role in changing the molecular structure, and pasting and retrogradation characteristics of millet starch.

Key words: natural fermentation, lactic acid bacteria fermentation, yeast fermentation, starch, molecular structure, gelatinization, retrogradation