Harmeet S. Guraya

Effect of Enzyme Concentration and Storage Temperature on the Formation of Slowly Digestible Starch from Cooked Debranched Rice Starch

Non-waxy and waxy starch suspensions were debranched with pullulanase followed by heating and cooling to form products with a mixture of rapidly digestible (RDS), slowly digestible (SDS) and resistant starches (RS). Products with a range of digestibility were formed by controlling the enzyme concentration (2 and 10 g of pullulanase per 100 g of starch), time of hydrolysis (2 to 24 h) and cooling temperature (1, 15, and 30 °C). Higher enzyme concentration and less time for debranching resulted in maximum formation of SDS while longer times increased RS. RDS decreased with increasing SDS and RS. Holding at 1 °C produced a product with highest proportion of SDS. Holding at 15 °C produced a starch that is relatively more resistant to digestion. Holding at 30 °C produced the smallest amount of SDS but the same amount of RS as with samples cooled at 1 °C. The most SDS was produced by debranching waxy starch with 10 g of pullulanase per 100 g of starch for 4 h and subsequent storage at 1 °C.Therefore, waxy starches would be more suitable to make SDS.

Effect of Cooling, and Freezing on the Digestibility of Debranched Rice Starch and Physical Properties of the Resulting Material

Ten percent non-waxy and waxy starch suspensions were debranched with pullulanase followed by heating and cooling (1 °C) to crystallize and/or gel. Products with a range of textures can be made depending on the type (waxy and non-waxy) of starch used. The water holding capacity was 35% and 84% for waxy and non-waxy cooled debranched starch, respectively, at 4 h of cooling and did not change. The hardness of the debranched waxy and non-waxy starch continued to increase beyond 24 h up to 45 g and 245 g of force, respectively. The particle size of precipitates of non-waxy and waxy debranched starch was 45 micrometer and 4 micrometer after 4 h of cooling and did not change. Cooling of debranched non-waxy starch at 1 °C for 12 h without agitation decreased digestibility by 59%; with stirring digestibility decreased by 42% after 24 h of cooling. Freezing of debranched cooled waxy and non-waxy starch does not effect the decreases in digestibility. Particle size of debranched, cooled/freeze-thawed, dried, and milled starch affects digestibility.

Digestibility and pasting properties of rice starch heat-moisture treated at the melting temperature (Tm)

Non-waxy and waxy rice starches adjusted to 20% moisture (wet based, w.b.) were heated in a differential scanning calorimeter to determine the optimum parameters for producing slowly digestible starch (SDS). Starches heated to the temperature of melting (T m ) and held for 60 min in the calorimeter showed a slow digestibility compared to unheated samples. Digestibility decreased by 25 and 10%, respectively, for non-waxy and waxy rice starches relative to non-treated starches. Heat-moisture treatment of waxy corn, non-waxy corn and wheat starches at the T m determined for non-waxy rice starch did not result in significant decreases in digestibility. For waxy rice starches heat-treated in microwave or conventional ovens at the T m , there were slight but significant increases in digestibility of the treated starches compared to non-treated starches at all incubation times. Digestibility was higher for starches heated for 30 min than for 60 min. Non-waxy rice starches did not show any significant changes in digestibility. Heat-moisture treatment at the T m and the holding time of sample at that temperature in a differential scanning calorimeter were found to be significant to the formation of slowly digestible heat-moisture treated starch.

Deagglomeration of Rice Starch-Protein Aggregates by High-Pressure Homogenization

Starch-protein agglomerates of rice are physically disrupted in presence of water by use of a high pressure homogenizer called microfluidizer® followed by density based separation. Rice flour slurry at concentrations of 22, 32 and 36 % was passed twice through the microfluidizer to determine optimum concentration and recycling conditions. It was determined that 32 % slurry and two passes were optimum but the optimum pressure was 10.0 x 10 4 kPa for non-waxy rice flour and 6.2 x 10 4 kPa for waxy flour. These conditions yielded low-protein starch with starch damage of 5.3%, 99.9% particles with size less than 10 μm, starch recovery of 72% and 2.7% protein in starch for non-waxy starch. The same parameters were 6.1%, 99.0%, 76% and 3.3% for waxy starch. The peak, minimum, breakdown, final and setback viscosity was 237.8, 115.2, 122.6, 145.1, 29.9 and 68.2 RVU for low-protein waxy rice starch and 223.4, 140.2, 83.2, 258.6 and 118.4 RVU for non-waxy low-protein rice starch, respectively. The pasting temperature was 68.7 °C for waxy and 81.33 °C for non-waxy low-protein rice starch. The solubility of protein increased with increasing concentration and number of passes, however, it decreased with increasing number of passes for waxy rice protein.

The functional effectiveness of reprocessed rice bran as an ingredient in bakery products

Rice bran, as a coproduct of the rice milling industry, is yet to be efficiently utilized for human consumption. Despite its excellent nutrition, its hypoallergenicity and recently claimed nutraceutical properties, it is mainly utilized for animal feed or simply discharged. It is of interest to incorporate this healthy ingredient back into our diet. In these studies, rice bran was processed by drum-drying and pin-milling. This processing step increased hydration capacity and removed grittiness by decreasing mean particle size from 444 to 72 microns and producing a desirable monomodal size distribution. There are no reported studies addressing differences in rice bran composition in food applications and specifically their effect on bread quality. Thus, we were interested in examining the functional properties of bread made with processed full-fat (FFRB) and defatted (DFRB) bran from three cultivars (long, medium and short grain rice) and to compare them to a control. For 10% and 20% replacements of wheat flour, respectively, loaf volume increased 2% for FFRB and decreased 6% for DFRB and decreased by 6% for FFRB and 17% for DFRB. Loaf volume was highest with medium rice bran and this was attributed to its lowest fiber content and highest starch content among three varieties. Texture profile analysis showed no significant differences as far as cohesiveness and springiness, but bread hardness, gumminess and chewiness increased with increased levels of rice bran and was higher for DFRB bread than for FFRB. Measurements of texture determined that there was no detrimental effect in adding 10% FFRB to the bread and a very slight hardening of the loaves with the 20% FFRB, when compared to the control. It was found that FFRB gave better textural characteristics than DFRB overall and differences amongst different rice bran varieties were not significant.

Nutritionally Important Starch Fractions of Rice Cultivars Grown in Southern United States

Dietary starches can be classified into 3 major fractions according to in vitro digestibility as rapidly digestible (RDS), slowly digestible (SDS), and resistant starch (RS). Literature indicates that SDS and/or RS have significant implications on human health, particularly glucose metabolism, diabetes management, colon cancer prevention, mental performance, and satiety. In this study, the nutritionally important starch fractions (RDS, SDS, and RS) in cooked rice were assayed in vitro, making use of 16 cultivars grown in 5 southern U.S. rice growing locations (Arkansas, Louisiana, Mississippi, Missouri, and Texas). RDS, SDS, and RS were 52.4% to 69.4%, 10.3% to 26.6%, and 1.2% to 9.0%, respectively, of cooked rice dry weight. Cultivar, location, and cultivar-by-location interaction contributed to the variations in RDS, SDS, and RS contents. Means pooled across locations indicated that SDS was higher for the Louisiana samples than those from Texas, whereas RS was higher for the Texas samples than those from Arkansas, Louisiana, and Mississippi. Some cultivars were identified to possess high levels of RS (for example, Bowman and Rondo) or SDS (for example, Dixiebelle and Tesanai-2) and were also stable across growing locations. Apparent amylose content correlated positively with RS (n = 80, r = 0.54, P <or= 0.001), negatively with RDS (n = 80, r =-0.29, P <or= 0.05), and insignificantly with SDS (n = 80, r = 0.21, P > 0.05). RS and SDS were not collinear (n = 80, r =or-0.18, P > 0.05); it does not follow that a cultivar high in RS will also be high in SDS, and vice versa. The observed differences in RDS, SDS, and RS among the samples are indicative of wide genetic diversity in rice.

EVALUATION OF DEBRANCHED RICE STARCH SAMPLE PREPARATION METHODS FOR ANION-EXCHANGE CHROMATOGRAPHY WITH PULSED AMPEROMETRIC DETECTOR

Rice starch was debranched with the debranching enzyme pullulanase. Anion-exchange chromatography with pulsed amperometric detection (AEC-PAD) was used to study effect of time on the degree of enzyme catalyzed debranching. The stability of starch sample in different starch solvents during extended storage was evaluated. Lyphylized, non-debranched starch samples were prepared in 0.0375 M HCl, 0.0375 M NaOH and water, followed by heating at 121°C for 30 min. Samples were stored for 21 days. HCl hydrolyzed the starch. Starch samples prepared in water were also unstable with progressively increasing peak areas up to 21 days. Starch samples prepared in NaOH are most stable on storage. Lower concentration (0.009, 0.0046, 0.0023, and 0.0012 M) of HCl hydrolyzed freshly prepared non-debranched starch autoclaved at 121°C for 30 min in respective concentration of HCl to different degrees dependent on the concentration of HCl used. Starch samples prepared in water from freshly cooked or lypholized non-waxy and waxy starch samples were also unstable with progressively increasing peak areas up to 7 days of storage. To determine the effect of time on degree of enzyme catalyzed debranching, debranched starch samples were removed at various time intervals and analyzed immediately after sampling using water as the solvent.