S. Zakiuddin Ali

The fine structure of rice-starch amylopectin and its relation to the texture of cooked rice

Abstract Starches derived from 20 rice varieties containing from very low to very high total and hot-water-insoluble amylose-equivalent (AE) were fractionated by gelpermeation chromatography (GPC). Fraction I (amylopectin) and fraction II (amylose) correlated well with the insoluble-AE and soluble-AE, respectively, of the parent rice. Thus soluble-AE broadly represented the true rice amylose and insoluble-AE the iodine affinity of amylopectin. Amylopectins of eight representative varieties were therefore debranched and fractionated by GPC to study their chain profiles. Amylopectins from the highest-AE variety had the largest proportion of long B chains and the lowest proportion of short chains, while the reverse was true for waxy rice. Other varieties broadly followed this correlation between B-chain length and AE. In addition, when the eight amylopectins were first hydrolysed with β-amylase to remove the external chains and the β-limit dextrins were then debranched and fractionated, the greatest drop in the amount of long B chains occurred in the highest-insoluble-AE variety and the smallest drop (nil), in waxy rice. In other words, highest-insoluble-AE (i.e. high-iodine-affinity) amylopectin had not only the highest amount of long B chains, but the largest proportion of these chains was in the exterior region (carrying non-reducing ends), and vice versa. Difference in cooked rice texture seemed to be related to this difference in the fine structure of its amylopectin.

Structure of rice starch and its relation to cooked-rice texture ☆

Abstract Starch from seven varieties of rice, known to cook from very soft to very hard texture, was fractionated by gel-permeation chromatography on Sepharose CL-2B column. The high-molecular-weight fraction (Sepharose FRI) and the low-molecular-weight fraction (Sepharose FRII, further sub-divided into FRIIa, IIb and IIc) were debranched using isoamylase and fractionated on Biogel P-10. All the four fractions in all the different varieties of rice gave a similar trimodal chain profile, indicating the presence of branched molecules in all of them. Clearly, the branched component of starch (‘amylopectin’) is not necessarily big in size, but includes very small to very big molecules. The presence or absence of the largely linear, and relatively small molecule, ‘amylose’, could not be settled either way with the technique employed. However, based on certain assumptions, amylose content was calculated to be in the range of 7%–11% in the samples, much less than generally thought. The content of long-B chains of the branched molecule in the four Sepharose fractions individually and in aggregate, as well as the calculated amylose content, correlated well with the sensory tenderness of cooked rice. It was observed that the content of all long linear chains, including amylose if any, govern the texture of cooked rice.

MOLECULAR DEGRADATION OF STARCH DURING EXTRUSION COOKING OF RICE

ABSTRACT Effect of barrel temperature (80°–100°C) and amylose content (28.6 gkg−1, 22.3 gkg−1 and 5.0 gkg−1) of rice upon extrusion cooking on macromolecular profile of starch was studied by gel permeation chromatography (GPC) of the rice flour on Sepharose CL-2B. Starch in all rice samples was separated into two main fractions, viz. Fraction-I, a high molecular weight, was excluded by gel, amylopectin, and Fraction-II, a low molecular weight, that entered the gel, amylose. Extrusion cooking of rice led to the degradation of high molecular weight fraction of the starch, the extent of degradation increasing with increasing severity of extrusion conditions. The absorption maxima (λ max) of iodine complex of the fraction-I showed an increase after extrusion cooking and this increase was more in the non-waxy variety of rice than in waxy.

Studies on rheological properties of tamarind kernel powder, its derivatives and their blends with maize starch

Pasting and flow properties of cooked solutions of tamarind kernel powder (TKP), its carboxymethyl and hydroxypropyl derivatives, exhibited shear dependent and pseudoplastic flow behaviour characteristics. Chemical modification decreased the shear dependence of the solution viscosity as the degree of substitution increased. The solutions did not exhibit a yield stress at the concentrations studied. The effect of TKP and its carboxymethyl and hydroxypropyl derivatives, at different concentrations, on the rheological properties of maize starch dispersions was also studied. The seed gum associated with starch, and the effect was maximum at 10% concentration of the former in the blend. This was evident by the lowering of pasting temperature and the synergistic increase in pseudoplasticity and yield value of the blend pastes. The TKP derivatives also showed maximum association with starch at 10% concentration in the blend but, in comparison to TKP, the degree of association decreased. Thus, the pasting temperature of the blend increased while the pseudoplasticity and yield value decreased as a function of DS/MS. At higher levels of modification, the association was decreased. These results have been explained on the basis of molecular association of the seed gum and its derivatives with starch molecules.

Changes in physico‐chemical and rheological properties of rice during flaking

Abstract Hydration, solubility, pasting and rheological properties of roasted‐parboiled rice and flakes were examined in comparison to raw rice. There was an increase in the hydration capacity, swelling power and solubility during roasting‐parboiling of rice as a result of gelatinization of its starch. Flaking caused further increase due to damage of starch by application of mechanical force. The changes were higher in flakes from roller‐flaker as compared to those from edge‐runner. Pasting characteristics of flour slurries in Rapid Visco Analyzer showed a typical profile for pregelatinized products viz. a higher initial viscosity but a lower peak viscosity for roasted‐parboiled rice and flaked rice than for raw rice. The viscosity curves and flow curves of the products over a wide range of shear rate indicated a non‐Newtonian, pseudoplastic behaviour for all the samples. All samples showed typical hysteresis loop in their viscosity curves indicating their thixotropic nature. Flakes from roller‐flaker exhibited lower viscosity but more thixotrophy indicating higher starch breakdown in them than in edge‐runner flakes, which seemed to have more of heat damaged starch.