澱粉質食品の調理科学的研究 澱粉の構造と糊化・老化性

Abstract

Starchy materials such as cereals, legumes, tubers, and rhizomes are served as foods through the appropriate processing of cooking, which causes the gelatinization of starch. The gelatinized starch is thermodynamically unstable and tends to retrograde during storage. The retrogradation generally decreases the quality of foods known as the staling of bread and the hardening of cooked rice. Therefore, how to control gelatinization and retrogradation is of prime concern of cereal chemists and technologists. I have been involved in this study for nearly 40 years. As early as 1961 we found that microwave heating was very efficient for the rapid cooking of rice, but the cooked rice rapidly hardened by the retrogradation of starch. This was improved by the addition of a small amount of Takaamylase in cooking. Root vegetables are well known for being unable to become soft through cooking even by continued heating after heating has been suspended for a while halfway through cooking. By a detailed analysis of the gelatinization of starch during cooking, this phenomenon was found to be caused by the enhancement of retrogradation of the partially gelatinized starch by cooling. Furthermore, it was found that the heat-stable materials, including various minerals, enhanced the retrogradation. The relationship between structures and functions was investigated on various kinds of starch such as nagaimo (yam), lotus, arrowhead, taro, bracken, and tapioca used in confectioneries. It was found that starches having a high tendency to retrograde had amylopectin with long chain-length and smaller amylose molecules. For example, tapioca starch showed the lowest retrogradation tendency, and this appeared to be due to the short chain length of amylopectin and large amylose molecules, besides thee low amylose content. The relatively higher retrogradation tendency of potato starch seemed to be due to the long chain length of amylopectin. A clear-cut relationship was found between the phosphate content and the pasting viscosity in potato starch that the higher level of phosphorus gave the higher-pasting viscosities. Kuzu amylopectin showed a relatively low retrogradation tendency because of the medium chain length of amylopectin, but kuzu amylose had a high retrograding tendency because of small molecules. Bracken amylopectin with a similar chain length showed a similar retrograding tendency to that of kuzu. Some taro starches showed similar structures and properties to tapioca starch, and they also had low levels of amylose. The starches found in vegetables used in cuisine on New Year's Day have low retrograding tendencies. Therefore they are suitable for use as preservable foods. Amylose molecules are slightly branched molecules on average, and the branches are mostly short chains, but the function of side chains has not been investigated. We attempted to clarify this function by means of a model experiment on molecules that had incorporated small amounts of maltosyl side chains into linear chains by the reverse action of isoamylase. The results clearly showed that these small amounts of maltose side chains affected a decrease in retrograding tendency.