N.-G. Asp

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.

On the digestibility of starch in wheat bread — studies in vitro and in vivo*

The digestibility of starch in white wheat bread was studied. Raw wheat starch and raw wheat flour were used as reference materials. The extent of digestion and absorption in vivo was evaluated through balance experiments in rats given test diets with and without addition of the antibiotic, Nebacitin, to reduce the fermentation in the hind-gut. As judged from a high faecal recovery of dietary fibre constituents in rats fed a wheat flour diet with Nebacitin, this drug significantly reduced the microbial activity in the hind-gut. Only minute amounts of starch could be detected in faeces of rats whether Nebacitin was present or not, indicating that pure wheat starch as well as starch in wheat bread and raw wheat flour was almost completely digested and absorbed in the rat small intestine. However, during baking, a fraction of the starch (0·6%–0·9%, dry basis) was rendered resistant to enzyme digestion in vitro unless solubilised in KOH. This modified starch fraction also remained undigested in vivo, but was readily metabolised by the hind-gut microorganisms, thus having physiological properties similar to those of dietary fibre. It is therefore recommended that resistant starch formed during baking (i.e. that rendered resistant to enzymic digestion as measured in vitro) should be regarded as dietary fibre rather than as dietary starch.

Formation of enzyme resistant starch during autoclaving of wheat starch: Studies in vitro and in vivo

The formation of in vitro resistant starch (RS) during autoclaving and freeze-drying of wheat starch suspension was evaluated. A substantial amount of starch, up to 8% (dry weight basis), was rendered resistant to amylases during heat-treatment unless solubilised in KOH, whereas freeze-drying had only marginal effects. Heat-treated wheat starch was incorporated into test diets to provide different levels of RS (0–5 %). The digestibility of starch was measured in vivo through balance experiments in normal rats and in rats treated with Nebacitin to suppress hind-gut fermentation. RS remained essentially undigested also in vivo. However, like certain types of dietary fibre, RS was readily metabolised (80–92 %) by the hind-gut microflora. Starch analysed with an enzymic method, in vitro accessible starch (AS), seemed to correspond to starch digestible in vivo. The in vivo digestibility of AS was close to 100 % whether Nebacitin was present or not. Energy utilisation decreased significantly with increasing level of RS in the diet. The biological value of dietary protein increased with increasing intake of RS in normal rats whereas no effect was noted in animals treated with Nebacitin. We conclude that RS formed during heat-treatment should be regarded as an easily fermentable dietary fibre component.

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

Influence of drum-drying and twin-screw extrusion cooking on wheat carbohydrates, II, effect of lipids on physical properties, degradation and complex formation of starch in wheat flour

Wheat flour was drum-dried or extruded with and without addition of 2% soya soil or 1% linoleic acid, otherwise maintaining similar processing conditions. The starches in the six resulting products were characterised in terms of their water solubilities, pasting properties, molecular weight distributions and in vitro digestibilities. Starchlipid interactions were investigated by measuring the efficiency of fat extraction, the amount of complexed starch calculated from iodine-binding capacities and the thermal behaviour of the flour in differential scanning calorimetry. All drum-dried flours exhibited less starch breakdown and higher initial Brabender viscosities than extruded flours. Lipid addition decreased viscosity and water solubility. The extent of starch breakdown was unaffected by linoleic acid but, surprisingly, increased upon addition of soya oil. Starches from all samples were equally available to glucoamylase, but pancreatic α-amylase digested drum-dried, and especially extruded, flours more slowly than an autoclaved control. Linoleic acid, but not soya oil, further decreased α-amylolysis rates. An additional autoclaving increased digestion rates and abolished differences between the two processes. A close relationship existed between the degree of α-amylolysis and the amount of complexed starch.

Controlling the nutritional properties of starch in foods — a challenge to the food industry

Abstract Several properties of starchy foods have been identified as influencing the rate and/or extent of starch absorption. By utilizing this new knowledge, nutritional characteristics of starch, such as the glycaemic index (GI) and the resistant-starch content, can be controlled by choice of raw material and processing method. Low-GI foods have favourable effects on the control of diabetes and on reducing risk factors for cardiovascular disease. Such foods may also be advantageous in relation to satiety and physical performance. Resistant starch has nutritional characteristics typical of dietary fibre. Thus, starchy foods with optimized nutritional properties are candidate ‘functional foods’.

Effects of processing on availability of starch for digestion in vitro and in vivo; I Extrusion cooking of wheat flours and starch

The effect of extrusion cooking on availability of starch for digestion in wheat products was tested in vitro and in vivo . Hydrolysis in vitro was measured using salivary amylase and the in vivo availability was evaluated by measuring the plasma levels of glucose and insulin after a gastric load in conscious rats. Ther fermentability of starch in human dental plaque in situ was also studied. Extrusion cooking of white flour and whole-grain wheat flour rendered the starch more susceptible to salivary α-amylase than did boiling for 20 min in water. Consequently, the pH drop in dental plaque was more pronounced with extruded materials. The rate of starch absorption in vivo , as judged from the plasma glucose pattern, was similar for boiled samples and for samples extruded under mild conditions. However, under more severe extrusion conditions, there was a significant increase in plasma glucose response. In the case of white flour, this enhanced glucose response was accompanied by an increased insulin response. Thus, processing conditions can influence both the glycaemic response to starch and its cariogenic properties.

Effects of processing on starch availability In vitro and In vivo. II. Drum-drying of wheat flour

The effect of drum-drying on starch availability in white wheat flour was studied and compared with boiling and pressure cooking. The starch in two drum-dried products was less susceptible to salivary amylase digestion in vitro than in the corresponding boiled or pressure-cooked materials. The rate of starch absorption in young rats, as reflected by the plasma glucose levels, was also lower with drum-dried products than with a boiled control. Similarly, the rise in plasma insulin was slower in drum-dried products. In addition to the conditions used during drum-drying, the method of preparation of the pre-cooked product affected starch availability, the rate of hydrolysis in vitro and, especially, the in vivo glucose response, which was higher in products that were drum-dried and then boiled. The fermentability of drum-dried and boiled wheat flour in human dental plaque was similar, resulting in approximately the same pH drop in situ . A novel type of amylose-lipid complex was formed during drum-drying. Under severe drying conditions, the apparent content of dietary fibre in wheat flour increased by about 20%, due to formation of enzyme-resistant starch.

Enzymic degradation of phytate (myo-inositol Hexaphosphate) in whole grain flour suspension and dough. A comparison between 31P NMR spectroscopy and a ferric ion method

The enzymatic hydrolysis of inositol hexaphosphate showed a very distinct pattern when the products were analysed using 31 P NMR spectroscopy. It seems likely that the hydrolysis proceeds in a stepwise manner and that each of the lower inositol phosphates is the dominant species present at different stages during the hydrolysis. The rate of hydrolysis of the different inositol phosphates decreases with decreasing degree of phosphorylation. In contrast to the 31 P NMR method, which distinguished inositol hexaphosphate from the intermediate hydrolysis products, the ferric ion method is not specific. Some of the lower inositol phosphates and maybe even inorganic phosphate appears to co-precipitate with phytate in this method which, thus, overestimates the phytate content.

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.