RESUMO
Water-soluble carbohydrates commonly exist in an amorphous state in foods and undergo glass-rubber transition (glass transition) at the glass transition temperature (Tg). The critical water content (Wc) and critical water activity (awc) are the water content and water activity (aw) at which the glass transition occurs at 298 K (typical ambient temperature), respectively. For amorphous water-soluble carbohydrates, Wc can be predicted from the Tg of anhydrous solid (Tgs) using previously reported equations. However, an approach for predicting awc is still lacking. This study aimed to establish an awc-predictive approach for amorphous water-soluble carbohydrates based on Tgs. First, the water sorption isotherms of four hydrogenated starch hydrolysates were investigated, and the results were analyzed using the Guggenheim-Anderson-de Boer (GAB) model. Second, the effect of Tgs on the GAB parameters (C, K, and Wm) was evaluated using the Tgs values reported in previous literatures. C and Wm decreased and increased logarithmically, respectively, with increasing 1/Tgs. K was fixed to 1 (constant), as it showed little variation. These results enabled the prediction of the GAB parameters from Tgs. The GAB model could then predict awc from Wc, which was determined using the previously established equations. The predicted awc values were in good agreement with the experimentally determined awc. Additionally, we demonstrated that this awc-prediction approach is also applicable to amorphous water-soluble electrolytes and partially water-insoluble carbohydrates. Thus, this approach can be used for the quality control of amorphous water-soluble carbohydrates and carbohydrate-based foods.
RESUMO
Pre-harvest sprouting (PHS) of wheat (Triticum aestivum L.) grains induces hydrolyzing enzymes such as α-amylase, which considerably decreases wheat product quality. PHS occurs when cool and wet weather conditions before harvest break dormancy and induce grain germination. In this study, we used PHS-tolerant varieties, Gifu-komugi (Gifu) and OS38, to characterize the mechanisms of both dormancy breakage and dormancy maintenance at low temperatures. Physiologically mature Gifu grains exhibited dormancy after imbibition at 20°C, but germinated at 15°C. In contrast, OS38 grains remained dormant even at temperatures as low as 5°C. Embryo half-grains cut out from the dormant Gifu grains germinated by imbibition at 20°C, similar to conventional varieties worldwide. However, OS38 embryo half-grains were still dormant. Hormonome and pharmacological analyses suggested that ABA and gibberellin metabolism are important for temperature-dependent dormancy maintenance and breakage. Imbibition at 15°C decreased ABA levels but increased gibberellin levels in embryos of freshly harvested Gifu grains. Additionally, low temperatures induced expression of the ABA catabolism genes,TaABA8' OH1 and TaABA8' OH2, and the gibberellin biosynthesis gene,TaGA3ox2, in the embryos. However, in embryos of freshly harvested OS38 grains, ABA levels were increased while gibberellin levels were suppressed at 15°C. In these dormant embryos, low temperatures induced the TaNCED ABA biosynthesis genes, but suppressed TaABA8' OH2 and TaGA3ox2.These results show that the regulatory mechanism influencing the expression of ABA and gibberellin metabolism genes may be critical for dormancy maintenance and breakage at low temperatures. Our findings should help improve PHS-resistant wheat breeding programs.
