| Summary: | 碩士 === 輔仁大學 === 食品科學系 === 96 === Tofu is rich in nutrient and is one of the important staple foods in Asia. Protein and water are two major compositions in tofu. Understanding the changes of the states of protein and water in tofu during storage would be able to provide information for quality improvement and new product development. Carrageenan is a hydrophilic colloid and, is usually used as a thickener, coagulant, and stabilizer to improve the texture of food. The objectives of this research were: to investigate the changes of texture and structure of calcium sulfate tofu during storage and to study the effect of different concentrations (0.5, 1.5, and 2.5 g/L) and different types of carrageenans (κ/ι-hybrid, κ/ι-mixture, and κ-mix with K+ types) on the yield, water-holding capacity, and the states of protein and water of calcium sulfate tofu. The results showed that, the state of protein in tofu could be classified into three fractions (λ1, λ2, λ3) using the relaxation times transferred by three-element Maxwell model from stress relaxation curve. λ1 state represents both the continuous phase (for example: water) and discrete phase (for example: protein and fat) located near the holes of the matrix; λ3 state represents both the continuous phase and discrete phase located in the matrix junction zones；λ2 state located in between λ1 and λ3 states. The value of λ3 in tofu increased during storage indicating the rearrangement of proteins in tofu matrix occurred and the heterogeneity of tofu increased. There were no obvious changes in the amount of expressible water and non-expressible water in tofu which were classified using centrifugation method during storage. Therefore, the distribution of water in different states might not be affected by the rearrangement of protein in tofu matrix. However, the rearrangement of protein during storage resulted in the increase of syneresis of tofu. The addition of carrageenans increased the yield and improved the texture of tofu. The moisture content increased but the value of λ2 decreased in tofu when the concentration of added carrageenan increased resulting in the decreased of the concentration and binding force of local proteins. Increased the concentration of carrageenan, increased the value of λ3 and the range of relaxation times in tofu. This result indicated that carrageenan might interact with the protein at λ3 state in tofu. The number of junction zones and the heterogeneity of tofu matrix increased with carrageenan addition. States of protein and water obtained by stress relaxation and centrifugation methods, respectively, could be used to study the mechanism of the changes in the texture of tofu during storage and the contribution of carrageenan to tofu matrix.
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