Properties of rice flour and effects of sucrose and coconut milk on properties of rice flour

Abstract

This research consists of three parts. The first part aimed to investigate the physicochemical (chemical composition, amylose content, % damaged starch, flour particle size, and % crystallinity), functional (water absorption index, water solubility index, and swelling power), thermal, pasting and gel textural properties of flours from 6 varieties of non-waxy Thai rice, i.e. Pathum Thani 1 (P1), Hawm Suphan Buri (HS), Suphan Buri 1 (S1), Suphan Buri 2 (S2), Suphan Buri 3 (S3), and Suphan Buri 60 (S60). Second part aimed to study the effects of sugar and coconut milk on thermal, pasting, gel textural properties, and retrogradation of P1 flour. Third part involved the study of the effect of sucrose (0-20%, w/w) on the gelatinization of rice flour treated under the high hydrostatic pressure (HHP). The results showed that flours from 6 varieties had different properties, i.e. amylose content 18.64-34.19%, damaged starch 2.52-6.38%, onset temperature (Tonset) 70.48-77.72°C, % crystallinity 23.14-31.30, and hardness 24.87-61.15 gf. Based on these properties the flour samples could be divided into 2 groups: flours from P1, HS, S2, and S60 having amylose content of 18-22% had Tonset 70°C, % retrogradation 32-40, and low final viscosity and setback; second group was flours from S1 and S3 having amylose content of 28-33% had Tonset 77°C, % retrogradation 55-59, and high final viscosity and setback. In the second part it was found that adding sucrose increased thermal properties (Tonset, Tpeak, Tend) of flour suspensions in both water and coconut milk systems. From pasting measurement it showed that sucrose and coconut milk increased setback from 893 cP (flour in water system) to 1796 cP and 1359 cP (flours with 50% sucrose in coconut milk and in water). After storage the gels at 4°C for 30 days, it was found that coconut milk retarded retrogradation while sucrose accelerated the process. The rate constants (k) of rice gel in coconut milk and gel in water with 50% sucrose were 0.308 and 0.419 day-1 which were different from k of flour in water (0.355 day-1). The SEM micrographs of the gels stored at 4°C for 15 days showed that there was noticeable difference between the surfaces of the gels in water and coconut milk. Lastly, HHP at 600 MPa and 40°C for 30 minutes could gelatinize 20% (w/w) flour suspensions completely. Addition of sucrose would retard the gelatinization process. From the polarized light micrographs it showed that HHP gelatinized the flour without destroying starch structure.