Cooking qualities of parboiled rices produced at low and high temperatures

Abstract

The cooking characteristics of parboiled rice are related to (i) its hydration behaviour at temperatures above and below the gelatinisation point; (ii) to kernel elongation on cooking; and (iii) to the extent of amylose solubility. These properties differ among samples, depending on the parboiling conditions. Samples were prepared by parboiling paddy at 70,80,90, 100,110 and 120°C. The kernel elongation on cooking and the amount of soluble amylose in the gruel were then determined. The water uptake values for raw and parboiled rice samples were determined by hydrating them at room temperature (25-30), 60 and 98°C (boiling temperature) for optimal cooking times. The rate of hydration at temperatures below the gelatinisation point increased on parboiling and, conversely, a reverse pattern above this point. Close correlations existed between the temperature of parboiling and the properties studied. The different properties studied also correlated well. The temperature of parboiling influenced the linear elongation of the kernel after cooking. The soluble amylose content was negatively correlated with the temperature of parboiling. Though the hydration properties of different parboiled samples differed among themselves, depending on the degree of parboiling, they fell into two distinct classes, viz. the samples parboiled at a temperature close to the gelatinisation point having cooking qualities similar to raw rice, and above this point qualities differing from raw rice. The water uptake values at room temperature and at 60°C, and the ratio of water uptake at 98°C and optimum cooking time to that at 60°C were found to be useful in differentiating the parboiled rices into the two classes.

Quality Evaluation of Parboiled Rice with Physical Properties.

“…It was found that final hardness value for the steaming temperature of 105 and 110˚C (0.2 and 0.4 kg/cm 2 gage pressure) was almost the same (94.72 and 94.26 N); this might be due to a lower temperatures difference, but higher final hardness value (106.21 N) was obtained for the higher temperature (120˚C). Although the reaction rate constant values for different steaming temperatures did not show a distinct pattern, this might be due to lack of data for the transient points; higher final hardness value for the higher steaming temperature, however, reconfirms the general understanding, supported by other researchers, that the hardness of parboiled rice increases with the severity of the parboiling process (Raghavendra Rao & Juliano, 1970;Pillaiyar & Mohandoss, 1981;Kimura, 1991). The final hardness and reaction rate constant value can also be treated as the index of quality and the rate of change of quality for the respective steaming temperature.…”

“…The hardness depends on the severity of the parboiling treatment. Many researchers reported that it is greatly affected by parboiling conditions, moisture content after drying, balance of starch gelatinisation and retrogradation and other factors (Ali & Bhattacharya, 1976;Pillaiyar & Mohandoss, 1981;Bhattacharya, 1985;Itoh & Kawamura, 1985;Kimura, 1991). Figure 2B shows the effect of steaming on the hardness of brown rice.…”

“…It is generally understood that cooked parboiled rice is harder and less sticky than raw cooked rice. The tenderness of cooked parboiled rice, determined by extrusion value (Mohandoss & Pillaiyar, 1980) or pressing device value (Pillaiyar & Mohandoss, 1981) was increased due to the severity of heat treatment during parboiling. Kato et al (1983) reported that hardness and cohesiveness of cooked parboiled rice is increased.…”

Effect of Processing Conditions on Physical Properties of Parboiled Rice.

“…The hardness of parboiled rice depends on the severity of the parboiling treatment. Many researchers reported hardness is greatly affected by parboiling conditions, moisture content after drying, elapsed time, the balance of starch gelatinization and retrogradation and other factors (Ali & Bhattacharya, 1976;Pillaiyar & Mohandoss, 1981;Bhattacharya, 1985;Itoh & Kawamura, 1985;Kimura, 1991;Islam et al, 2001). Figure 6 shows the effect of parboiling treatment on the hardness of brown rice for different steaming conditions and sample sizes.…”

“…This is a negative effect of the parboiling process, because dark colored parboiled rice loses market value and lowers consumer acceptability in most countries (Bhattacharya, 1985). Many researchers measured the color intensity value following parboiling treatment (Jayanarayanan, 1964;Bhattacharya & Subba Rao, 1966; Pillai- yar & Mohandoss, 1981;Kimura et al, 1993;Bhattacharya, 1996), and reported that discoloration was mainly caused by the Maillard type of non-enzymatic browning reaction, and that the processing conditions determine the intensity of color during parboiling. Figure 10 shows the effect of parboiling treatment on the color intensity value for different steaming conditions and sample sizes.…”