Changes in Selected Nutrients and Microstructure of White Starch Quality Maize and Common Maize During Tortilla Preparation and Storage

Raw maize, masa, and fresh and stored tortillas of white starch quality maize (Costeno) and common maize were analysed for chemical composition, dietary fibre, digestible (DS) and resistant (RS) starches. Changes in starch crystallinity and microstructure were also evaluated. Protein content was lower in masa and tortillas than in the original maize samples. Insoluble (IDF) and soluble (SDF) dietary fibres were higher in Costeno than in common maize, but stored tortillas showed the highest value in both samples. DS decreased and RS increased during tortillas storage, made with both starches. The development of RS explained the observed decrease in DS in masa (enthalpy of gelatinisation). Differential scanning calorimetry studies showed peak gelatinisation temperatures (Tp) of 75.1 and 69.4 C for Costeno and common maize respectively. Masa of common maize showed a second transition endotherm (Tp, 108.9 C) which corresponded to amylose–lipid complexation. Tortillas with 5 and 10 days of storage showed an endothermic event at 53 and 55.7, and 54.5 and 59.9 C for Costeno and common maize respectively, due to starch retrogradation. These values were in accordance with the higher total RS contents recorded after prolonged storage. Microscopic structures for masa and fresh tortillas showed that the high temperature imposed on the masa and the influence of mechanical stress during tortillas making were great enough to disrupt starch granules. In general, structural changes were in agreement with changes in starch.

Measurement of rapidly available glucose (RAG) in plant foods: a potential in vitro predictor of the glycaemic response

The glycaemic index (GI) is an in vivo measurement based on the glycaemic response to carbohydrate-containing foods, and allows foods to be ranked on the basis of the rate of digestion and absorption of the carbohydrates that they contain. GI values are normalized to a reference amount of available carbohydrate and do no reflect the amounts of carbohydrate normally present in foods; for example, a food with a low content of carbohydrates will have a high GI value if that carbohydrate is digested and absorbed rapidly in the human small intestine. This is potentially confusing for a person wishing to control his or her blood glucose levels by the choice of foods. The rate and extent of starch digestion in vitro has been measured using a technique that classifies starch into three major fractions: rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). In addition, this technique gives a value for rapidly available glucose (RAG), which includes RDS, free glucose and the glucose moiety of sucrose. When the values for thirty-nine foods were expressed on the basis of the available carbohydrate content of these foods, highly significant (P < 0.001) positive correlations were observed between GI and both RDS and RAG. The measurement of RAG in vitro provides values for direct calculation of the amount of glucose likely to be rapidly absorbed in the human small intestine and, thus, to influence blood glucose and insulin levels. These values can be used to compare foods, as eaten, on an equal-weight basis. Food-table RAG values would allow simple calculation of the total amount of RAG provided by single foods, by whole meals and by whole diets. Studies are planned in which RAG and the glycaemic response in man will be measured for identical food products.