Evaluation of Varietal Properties of Cooked Rice Grains by Rheological Measurement

Abstract

Dynamic viscoelasticities of cooked rice grains were measured using an instrument for evaluating dynamic viscoelasticity of irregularly shaped and small samples in the doubleside shear mode. Viscoelastic parameters, modulus G', dynamic loss G'' and loss tangent tanδ were obtained for five varieties of rice which had different amylose (17.5-30.0%) and protein (6.5-11.6%) contents. Treatments performed on these samples, included conductive heating by means of an electric rice cooker, steaming and microwave heating. For the coefficient of varience, these tanδ values were less than 5%, in contrast to both G' and G'' which were more than 10%. These tanδ values were not influenced very much by water content or polishing rate of grains, while they were substantially affected by the treatments and aging at room temperature. Each variety treated by conductive heating could be classified with respect to tanδ values which were between 0.175 and 0.335. In the case of equal value of tanδ, those varieties could be classified by their dynamic modulus. On the other hand, it was found that tanδ related amylose content and the breakdown viscosity by amylography from chemical and rheological analyses. As a result, the use of tanδ as the index of the breakdown properties of the starch granules upon heating was suggested.