The swallowing threshold and starch hydrolysis of cooked rice with different moisture contents for human mastication

Swallowing threshold is a critical parameter that marks the condition of food after mastication in the human mouth being ready to be swallowed. In the current study, a new aspect was investigated in which the food boluses from cooked rice with three moisture contents (52%, 63% and 73% on a wet basis) were masticated and collected just before swallowing by a group of twelve voluntary participants. Both mechanical/textural properties and physicochemical characteristics of the cooked rice and their boluses were investigated. The results show that the oral duration of the cooked rice was positively associated with rice hardness, but negatively correlated with the moisture content of rice boluses. After chewing, the textural properties except cohesiveness and adhesiveness varied little among the rice bolus samples. A significant decrease in the textural parameters was shown for the cooked rice with a low (52%) and middle (63%) moisture content after mastication. In contrast, the decrease was insignificant for the cooked rice having the highest moisture content (73%). Consistently, a negative correlation between the hardness of cooked rice and the amount of soluble reducing sugar in the rice boluses was presented. The salivary impregnation and the particle size distribution of the rice boluses at the swallowing threshold were independent on rice physics (i.e., initial moisture content, texture) and oral physiology. In addition, due to the high efficiency of salivary α-amylase, the starch in cooked rice could be hydrolyzed as much as one-third during oral digestion. The present study suggests that the initial moisture content of cooked rice and mastication conditions such as the presence of α-amylase and salivary impregnation could influence the physicochemical properties of rice boluses at the swallowing threshold. These factors should be carefully considered in future in vitro digestion studies of carbohydrate-based food products.