J. Holm

Starch availability in vitro and in vivo after flaking, steam-cooking and popping of wheat

The availability for digestion of starch in vitro after flaking, steam cooking and popping of whole grain wheat was studied and compared with that of raw and boiled wheat. Some physical and morphological characteristics were also investigated and correlated with starch availability for amylolysis. In an in vitro assay using hog pancreatic α-amylase after preincubation with pepsin, starch in flaked wheat was less available than that in boiled, popped and steam-cooked wheat. The starch in raw wheat was digested only slightly. When pepsin was omitted, the availability to α-amylase of starch in raw and boiled wheat decreased substantially, indicating that a large fraction of the starch was encapsulated in a protein matrix. The starch in flaked wheat elicited lower plasma glucose and plasma insulin levels in rats in vivo than that in boiled wheat after a gastric load. With raw wheat, the plasma glucose peak was much delayed and of lower magnitude compared with that with the heat-treated samples. In order to obtain good agreement with the in vivo results, a pepsin step had to be included in the in vitro assay using α-amylase. Differential scanning calorimetry measurements indicated that starch in flaked wheat was not gelatinised completely and this fact probably accounted for the lower availability in flaked wheat compared with boiled, popped or steam-cooked wheat.

The effects of various thermal processes on dietary fibre and starch content of whole grain wheat and white flour

The content of dietary fibre (DF) and starch in raw and processed materials obtained from wheat was evaluated. The processes studied were extrusion cooking, drum-drying, autoclaving, popping and steam flaking. The processing conditions were chosen to represent the limits that could be used commercially. The DF content was detennined using an enzymic-gravimetric method for soluble and insoluble DF components and a gas-liquid chromatographic method for complete characterisation of the constituent monosaccharides. Values for the total DF obtained by the two methods for a range of samples were closely correlated ( r = 0⁗99; P

Effects of thermal processing of wheat on starch: II. Enzymic availability*

The availability to porcine pancreatic α-amylase of starch after extrusion-cooking, drum-drying, popping, dry-autoclaving and steam-flaking of wheat was studied. The processes were performed under conditions corresponding to the upper (u) and lower limit (l) of what is normally used commercially. Raw and boiled flour were used as reference. Steam-flaked (l) and dry-autoclaved (l) wheat showed the lowest availability to α-amylase, primarily due to low degrees of gelatinization. The availability to α-amylase of starch in popped (l) and steam-flaked (u) wheat was intermediate. Dry-autoclaved (u) wheat showed a low initial and a high late phase of α-amylolysis. Drum-drying resulted in a lower availability to α-amylase than extrusion cooking. Popped (u) wheat showed the highest rate of α-amylolysis, and its starch was slightly more available than in extruded flours. Preincubation with pepsin increased substantially the availability to α-amylase in raw, boiled and drum-dried flours, but did not affect the availability of starch in the other processed samples. Consequently, starch-protein interactions in drum-dried flours decreased the course of α-amylolysis. When both pepsin and α-amylase were used, the starch in extrusion-cooked and drum-dried materials was equally available, and the availability was independent of processing conditions used. With popping, steam-flaking and dry-autoclaving more severe conditions resulted in more rapid α-amylolysis.

Effects of thermal processing of wheat on starch: I. Physico-chemical and functional properties

Whole grain wheat and white flour were extrusion-cooked, drum-dried, popped, dry-autoclaved or steam-flaked under conditions corresponding to the upper (u) and lower limit (l) of what is normally used commercially. The starch in both steam-flaked and dry-autoclaved (l) wheat samples was gelatinized incompletely as measured by differential scanning calorimetry (DSC) or enzymically with glucoamylase. DSC-measurements also indicated an increased resistance to further gelatinization of starch in dry-autoclaved (l) wheat, as shown by an increase of 9 °C in the gelatinization temperature. Extrusion-cooking and popping led to macromolecular degradation of starch, as observed by gel permeation chromatography. Starch degradation was most extensive during popping (u), and an increase in reducing sugar content was observed. Popped (u) wheat and extruded flours also showed the lowest hot paste and final cooked paste amylograph viscosities, and the highest water solubilities of starch. Popped (u), drum-dried and extrusion-cooked samples showed a decrease in viscosity on heating to 95 °C. Popped (l), dry-autoclaved and steam-flaked samples showed low initial viscosities at low temperatures, which increased on heating, and low water solubilities of starch. The amylograms also indicated remaining intrinsic α-amylase activity in popped (l), dry-autoclaved and steam-flaked wheat.

Rye products in the diabetic diet: Postprandial glucose and hormonal responses in non-insulin-dependent diabetic patients as compared to starch availability in vitro and experiments in rats*

Rye flakes, rye bread and white wheat bread were given as suspensions to rats and in standardized breakfast meals to non-insulin-dependent diabetics. In both cases the postprandial glucose response was lower after rye bread than after wheat bread. A larger amount of starch remained in the stomach of the rats 15 min after ingesting rye bread compared to wheat bread, indicating that delayed gastric emptying may be one factor explaining the lower response after rye bread. Although the incremental postprandial glucose areas after rye flakes and wheat bread were similar, the rate of decrease of the glucose curve was slower after flaked rye. This would point to a prolonged absorption of some starch in the rye flakes, also indicated by higher late immunoreactive insulin (IRI) values after that product. In the rats the content of starch in the stomachs 15 min after feeding was higher after rye flakes compared to wheat bread. In vitro incubations with alpha-amylase showed lower availability of the starch in rye flakes than in the breads, indicating that several factors may contribute to the differential postprandial glucose response after the wheat and rye products. The levels of insulin, C-peptide, gastric inhibitory polypeptide (GIP), glucagon, somatostatin, triglyceride and glycerol were followed after the breakfast meals. No pronounced differences of these parameters were seen. However, wheat bread gave significantly higher glucagon and GIP responses than did rye flakes. In conclusion, the absorption pattern and metabolic response after rye bread seems preferable to that after wheat bread. The flaked rye on the other hand was not effective in reducing postprandial glycaemia despite a lower availability of starch in vitro.