Commercial breakfast cereals available in Mexican markets and their contribution in dietary fiber, ß-glucans and protein quality by rat bioassays.

The beneficial effect of dietary fiber (DF) consumption has long been recognized. The global economy and open market trade policies have increased the availability of food products in Mexican markets, resulting in a wide variety of ready-to-eat commercial breakfast cereals classified as 'high fiber'. This research was aimed to evaluate the total dietary fiber contents, its fractions (soluble and insoluble) and ß-glucan in 13 commercial 'high-fiber' breakfast cereals, as well as to evaluate their protein quality by rat bioassays. Commercial 'high-fiber' breakfast cereals had 7.42-39.82% insoluble dietary fiber, 2.53-12.85% soluble dietary fiber, and 0.45-4.96% ß-glucan. These ready-to-eat commercial 'high-fiber' breakfast cereals differed significantly in their total dietary fiber, their soluble and insoluble DF fractions, and also in their ß-glucan contents. When supplied as experimental diets, in 14-day rat feeding trials, the 'high-fiber' breakfast cereals showed an adverse effect on the % N digestibility but protein utilization, as measured as net protein ratio (NPR), was not significantly affected. The consumption of these commercial breakfast cereals, especially those made of oats as the basic ingredient, is highly recommended, since these products, being a concentrated source of dietary fiber, do not affect their protein quality.

Isolation and partial characterization of three isoamylases of Rhyzopertha dominica F. (Coleoptera: Bostrichidae).

Three isoamylases of Rhyzopertha dominica (termed RdA70, RdA79, and RdA90 according to their relative mobility in gel electrophoresis) were isolated by ammonium sulfate fractionation and hydrophobic interaction chromatography. RdA70 and RdA79 showed an optimal pH of 7.0, whereas for RdA90 the optimal pH was 6.5. The three isoamylases remained stable at 50 degrees C for 1 h, but at 60 degrees C, all lost 50% of their activity in 20 min and were completely inactivated in 1 h. RdA70 and RdA79 were inhibited by albumin extracts from wheat samples varying widely in amylase inhibitory activity; however, RdA90 was highly resistant to inhibition. beta-Mercaptoethanol up to 30 mM increased the activity of the three isoamylases by 2.5-fold. The action pattern of the three isoamylases was typical of endoamylases; however, differences were observed on the hydrolytic efficiency rates measured as V(max)/K(m) ratio on starch, amylopectin, and amylose. The hydrolyzing action of RdA90 on starch and amylopectin (V(max)/K(m)=90.4+/-2.3 and 78.9+/-6.6, respectively) was less efficient than that on amylose (V(max)/K(m)=214+/-23.2). RdA79 efficiently hydrolyzed both amylopectin and amylose (V(max)/K(m)=260.6+/-12.9 and 326.5+/-9.4, respectively). RdA70 hydrolyzed starch and amylose at similar rates (V(max)/K(m)=202.9+/-5.5 and 215.9+/-6.2, respectively), but amylopectin was a poor substrate (V(max)/K(m)=124.2+/-7.4). The overall results suggest that RdA70 and RdA79 appear to belong to a group of saccharifying isoamylases that breaks down long fragments of oligosaccharide chains produced by the hydrolytic action of RdA90. The simultaneous action of the three isoamylases on starch, aside from the high resistance of RdA90 to wheat amylase inhibitors, might allow R. dominica to feed and reproduce successfully on the wheat kernel.