Swelling Pressure of Wet Rice Grains Estimated from Distribution Coefficients of Saccharides

Abstract

INTRODUCTIONSeasoning or flavoring of foods is an important cooking operation. The distribution equilibrium of a seasoning between the cooking ingredient and liquid preparation determines the final concentration or content of the seasoning within the cooking ingredient, and the seasoning rate is controlled by the diffusion of the seasoning compound into the ingredient.Food consists of a variety of polymers such as starches, proteins, and alginates, which are the main constituent polymers in rice, meat, and brown seaweed, respectively. 1) These polymers are not similar in their chemical or electrical properties. Starches are comprised of glucose and are electrically neutral, while proteins are comprised of amino acids and are amphoteric and alginates are comprised of uronic acids 2) and are anionic. Food polymers can form gel-like matrices during the cooking process after water addition or rehydration and then exhibit different properties. Starch, protein, and alginate gels are regarded as size exclusion resins, amphoteric-ion exchangers, and cation exchangers, respectively. On the other hand, although most substances used for seasoning food are low molecular-weight compounds, their chemical properties vary. Sucrose, sodium chloride, and acetic acid are typical seasonings used to enhance sweet, salty, and sour tastes, respectively. Sucrose is a non-electrolyte, sodium chloride is a strong electrolyte, and acetic acid is a weak electrolyte. Moreover, most bitter compounds are hydrophobic. 3) As a result of these varied chemical properties, various kinds of interactions, such as size exclusion, electrostatic, and hydrophobic interactions, occur between the food matrix and seasonings during the cooking process. In order to establish reasonable cooking processes, an understanding of these interactions is required. Among the possible interactions, distribution of non-electrolytes into an electrically neutral matrix such as starch gel seems to be the most straightforward, and this type of interaction is examined in this study.The swelling pressure of food matrix or gel plays an important role in the distribution of non-electrolytes into the matrix or gel. 4) Pressure during cooking would be also an important factor affecting the distribution of seasonings during preparation of flavored rice. Rice grains differently rehydrated and swelled 5,6) However, the swelling pressure of rehydrated rice grains has not been estimated. In this context, we estimated swelling pressures of two cultivars of non-glutinous rice and two cultivars of sticky rice based on our previous study where the swelling pressure of starch gel was estimated from the distribution coefficients of non-electrolytes having different molar volumes. 4) MATERIALS AND METHODS Materials.Non-glutinous rice cultivars (called Uruchi ), Koshihikari and Yumepirika, were harvested in Shiga and Hokkaido, respectively in 2013; sticky rice cultivars (called Mochi ), Habutaemochi and Kitayukimochi, were also harvested in Shiga and Hokkaido, respectively, in 2013. All rice ...