M.H. Norziah

Methods for the study of starch retrogradation

Abstract The wealth of current knowledge on starch retrogradation is due in large measure to the wide array of analytical methods at the disposal of food scientists. Since retrogradation is a complex process affected by many factors, it is unlikely that any single method would be able to give a complete picture of the retrogradation properties of starch gels at both the macroscopic and molecular levels. Independent evidence derived from two or more methods allows cross comparisons that can provide a fuller understanding of this phenomenon. For quantitative measurement of rates of retrogradation, the “ideal” method should be simple, rapid, non-destructive, precise, and inexpensive. Comparisons of kinetic data from different sources should be made with caution; various factors (thermal history, in particular) that can lead to unjustifiable comparisons and erroneous conclusions should be carefully considered first. This review covers the general principles, capabilities, advantages, and limitations of various methods available to study starch retrogradation.

Nutritional composition of edible seaweed Gracilaria changgi

Gracilaria changgi, an edible seaweed was analyzed to determine its proximate chemical composition, mineral elements, vitamin C, β-carotene, free fatty acid and amino acid contents. G. changgi showed vitamin A activity of 865 μg rectinol equivalents/100 g sample. It contained a higher composition of unsaturated fatty acids (74%), mainly the omega fatty acids and 26% of saturated fatty acids (mainly palmitic acid) and also relatively high levels of calcium and iron. Major amino acid components are glycine, arginine, alanine and glutamic acid. Among the essential amino acids assayed, lysine with a chemical score of 53% appeared to be the most limiting when compared with the essential amino acid pattern of egg protein. This study was conducted to create a nutritional data for G. changgi in order to popularize its consumption and utilization in Malaysia. Comparisons to corresponding nutrient values in several commonly consumed local vegetables were also made.

On the roles of protein and starch in the aging of non-waxy rice flour

Abstract Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during storage at 25, 35 and 45°C were compared. Aging had no apparent effect on the pasting behaviour of the isolated starch, but markedly affected that of the flour. Peak viscosity ( V p ) of rice flour pastes generally increased with both temperature and time of storage, but reached a plateau within 4 weeks of storage at 45°C. Fresh flour paste exhibited lower V p , a slower rise in apparent viscosity, and much better stability than an isolated starch paste. The addition of isolated oryzenin to isolated rice starch resulted in pasting behaviour which more closely approximated that of an extensively aged flour. DSC scans of fresh flour at ∼13% moisture revealed a weak heat-irreversible endothermic event over the temperature range from 47 to 66°C which was attributed to the denaturation of oryzenin. This transition was shifted to higher temperatures with increasing storage temperature and time. It also became increasingly skewed and broadened until it was no longer detectable after the flour had been stored at 45°C for 8 weeks. Starch gelatinization characteristics of both flour and isolated starch, as determined by DSC, were apparently unaffected by aging. However, rice flour exhibited significantly higher onset temperature of gelatinization, but lower gelatinization enthalpy, than isolated rice starch. Similarly, pulsed NMR studies showed no apparent effect of aging on retrogradation behaviour of rice flour or starch gels. These results suggest that modification of the protein component, rather than starch, was primarily responsible for rheological changes associated with aging of rice flour.

Effects of Na2CO3 and NaOH on DSC thermal profiles of selected native cereal starches

The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on the gelatinisation properties of 1:4 and 2:3 starch/water mixtures were studied using differential scanning calorimetry (DSC). The starches studied included wheat, rice, waxy rice, corn and waxy corn. The addition of Na2CO3 or NaOH resulted in significant increases in transition temperatures (onset, peak and conclusion), associated with starch gelatinisation, suggesting a stabilisation of the starch granules, probably through electrostatic interactions between Na+ ions and starch hydroxyl groups. The presence of Na2CO3 generally caused a significant reduction in gelatinisation enthalpy (ΔHG) of the higher moisture (1:4) systems, but had no effect in the less hydrated (2:3) systems. NaOH generally had no effect on ΔHG of 1:4 systems, but gave rise to inconsistent effects in the 2:3 systems. Heating of alkali-containing samples above the gelatinisation temperature range gave rise to an intriguing exothermic peak, the origin of which was attributed to repolymerisation of the products of alkaline hydrolysis of starch. A separate experiment, conducted to investigate the influence of increasing Na2CO3 concentration (up to 3 g/100 g starch) on thermal properties of 1:4 waxy rice starch/water systems, showed a gradual increase in gelatinisation transition temperatures, a general decline in ΔHG, and a marginal decrease in peak temperature of the repolymerisation exotherm.

Relationships of damaged starch granules and particle size distribution with pasting and thermal profiles of milled MR263 rice flour.

This research was conducted to investigate the effects of different grinding techniques (dry, semi-wet and wet) of milled rice grains on the damaged starch and particle size distribution of flour produced from a new variety, MR263, specifically related to the pasting and thermal profiles. The results indicated that grinding techniques significantly (p<0.05) affected starch damage content and particle size distribution of rice flour. Wet grinding process yields flour with lowest percentage of starch damage (7.37%) and finest average particle size (8.52μm). Pasting and gelatinization temperature was found in the range of 84.45-89.63°C and 59.86-75.31°C, respectively. Dry ground flour attained the lowest pasting and gelatinization temperature as shown by the thermal and pasting profiles. Correlation analysis revealed that percentage of damaged starch granules had a significant, negative relationship with pasting temperature while average particle size distribution had a significant, strong negative relationship with gelatinization temperature.

EXTRACTION AND CHARACTERISATION OF PECTIN FROM POMELO FRUIT PEELS

ABSTRACT Pectin was extracted from pomelo fruit (Citrus grandis) by three different extraction methods. Pomelo, a tropical fruit which belongs to the citrus family has a very thick skin which can be used as a source of pectin. It is currently widely cultivated in the country but little is known of the properties of pomelo pectin which could be extracted. This work describes the extraction of pectin from pomelo fruit peels using ethanol, aluminium salt and acetone precipitation extraction methods. The pectin obtained from these methods were compared in terms of yield, color and gelling characteristics. Acetone extraction using sodium hexametaphosphate was the best method which yielded 20.8% pectin (dry weight basis) from the raw material. The isolated pectin contained 12.7% moisture, 74.9% anhydrouronic acid (AUA), 5.3% methoxyl content, 40.5% degree of esterification and 4.5% ash. The content of AUA indicates the high degree of purity of the pectin isolated. According to the values for methoxyl content and degree of esterification, pectin isolated from pomelo peels can be classified as low methoxyl pectin and are of potential use in the manufacture of low sugar products such as low sugar jam and jellies. Gelling properties of pomelo pectin such as setting time, setting temperature and gel strength were studied and compared with those of a commercially available citrus pectin. The effects of temperature, time and pH of extraction on pectin yield and optimum conditions for pectin extraction using response surface methodology are also discussed.