| Abstract: | The 13C nuclear magnetic resonance (n.m.r.) spectrum of sugars normally present in foods, viz. glucose, fructose, sucrose, maltose and raffinose contains resonances that are diagnostic of each sugar. The quantity of each sugar is determined by measurement of the ratios of the peak heights of the sugar resonances compared with peak height of an internal standard, present at constant concentration in all solutions. A linear calibration relationship between the peak height ratio and concentration is established for single resonances of glucose and fructose and for two resonances of each of sucrose, maltose and raffinose. Samples of sweet potato and taro flour dried at 40°C, were blended with 80% ethanol, the sugars were extracted and analysed by 13C n.m.r. and h.p.l.c. Both methods gave similar results and have similar degrees of reproducibility. The n.m.r. method is useful for quantitation of sugars and identification of other molecules extracted from the food, but requires more material than does h.p.l.c. Drying at 40°C causes a decrease in sucrose content and increases in glucose, fructose and maltose which partly compensate for the loss in sucrose. There is considerable variability of the sugar content between different sweet potato tubers and between different taro corms of the same cultivar as well as across different cultivars. For sweet potato the total sugar content is 0.6-3.6% fresh weight, with considerably more sucrose present than glucose, fructose and maltose. With taro total sugar content is 0.2-0.8% fresh weight. Raffinose is present in one sample only. |
