Lactic acid fermentation of proso millet (panicum miliaceum l.) starch

Abstract

Proso millet grain is a staple food in China, and its starch has a wide range of amylose content depending on genotype. Proso millet is commonly prepared by lactic acid fermentation, and the resulting Chinese traditional fermented proso millet gruel is called Suanzhou. Suanzhou is commonly home-made in the rural areas of Inner Mongolia province of China using the backslopping method. In natural fermentation, it is difficult to control quality because of variation in microbiomes during fermentation. Single lactic acid bacteria (LAB) strain fermentation was applied to ferment proso millet in this study. During fermentation, pH decreased due to the synthesis of organic acids as dominant metabolites of LAB. Sorghum was used as a control, and its fermented gruel also showed decreased pH and higher organic acid content. The free amino acid content was enhanced due to the hydrolysis of protein and the multiplication of LAB. The content of phenolic compounds increased because of degradation of cell walls; thus, the antioxidant capacities of fermented samples were altered. Sorghum showed similar changes in nutrient content after fermentation as for proso millet. Starch, as the main component of proso millet grain, has a vital role in the quality of proso millet foods. In this study, the changes in starch structure and physicochemical properties during fermentation were determined. Three amylolytic LAB (ALAB): Lactobacillus amylovorus, Lb. plantarum, and Lb. fermentum were chosen for inoculation. Starch samples isolated from fermented proso millet flour (FPS) and pure starch fermented samples (PS) were prepared. Proso millet flour has a more complex nutrient profile, which provides different substrates for ALAB leading to different physicochemical properties of starch. However, in the pure starch fermentation, starch was the only carbon source thus avoiding influence from other nutrients. The scanning electron microscopy of both FPS and PS showed more indentations and pores compared to the raw samples. But PS samples had more pores than FPS samples after the same period of fermentation. The crystallinities of FPS and PS decreased after fermentation. The gelatinization temperatures of fermented starch samples all decreased because of the lower crystallinities. The pasting viscosities of FPS and PS samples had different trends, and different LAB could lead to opposite changes. The fermented waxy samples tended to have higher shear stress compared to raw samples under the same shear rate, but the shear stress of non-waxy samples was decreased after fermentation. Salt was found to have effects on increasing the thermal and pasting properties of proso millet starch, but pH was found to have no significant influences. In summary, the effects of single strain LAB fermentation on the nutrient content of proso millet gruels were determined; the variation of starch structure and physicochemical properties with LAB fermentation was analyzed; and the effect of variation in pH and salt content was studied. These results can provide a basis to guide future development of novel fermented food products.