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

Effect of processing on the composition of dietary fibre and starch in some legumes

The effect of processing on the total dietary fibre (TDF) insoluble (IDF) and water-soluble (SDF) fractions as well as total (TS), available (AS) and resistant (RS) starch were studied in three legumes, viz. bengalgram (Cicer arietinum L.), Cowpea (Vigna unguiculata) and greengram (Vigna radiata). The processes studied were fermentation, germination, pressure-cooking and roasting. The dietary fibre (DF) content and its components were determined using the enzymatic-gravimetric method. The TS content was determined by the enzymatic method after solubilization with KOH. The DF content ranged from 23.2 to 25.6 g/100 g in the raw and 16.0 to 31.5 g/100 g in the processed legumes. All the processing treatments significantly decreased the SDF content and increased the IDF content of all the three legumes. The mean TS, AS and RS content of the raw legumes were similar, 46.9, 36.7 and 10.2 g/100 g respectively. AS content of all the legumes was reduced by the processing treatments, except pressure cooking. Correspondingly, higher amounts of RS were observed in the processed legumes, except pressure cooked, resulting in an increase in the TDF content.

Hard-to-cook phenomenon in common beans--a review

Legumes are one of the world's most important sources of food supply, especially in developing countries, in terms of food energy as well as nutrients. Common beans are a good source of proteins, vitamins (thiamine, riboflavin, niacin, vitamin B6) and certain minerals (Ca, Fe, Cu, Zn, P, K, and Mg). They are an excellent source of complex carbohydrates and polyunsaturated free fatty acids (linoleic, linolenic). However, common beans have several undesirable attributes, such as long cooking times, being enzyme inhibitors, phytates, flatus factors, and phenolic compounds, having a "beany" flavor, and being lectins and allergens, which should be removed or eliminated for effective utilization. Grain quality of common beans is determined by factors such as acceptability by the consumer, soaking characteristics, cooking quality, and nutritive value. Acceptability characteristics include a wide variety of attributes, such as grain size, shape, color, appearance, stability under storage conditions, cooking properties, quality of the product obtained, and flavor. Storage of common beans under adverse conditions of high temperature and high humidity renders them susceptible to a hardening phenomenon, also known as the hard-to-cook (HTC) defect. Beans with this defect are characterized by extended cooking times for cotyledon softening, are less acceptable to the consumer, and are of lower nutritive value. Mechanisms involved in the HTC defect have not been elucidated satisfactorily. Attempts to provide a definitive explanation of this phenomenon have not been successful. The most important hypotheses that have been proposed to explain the cause of bean hardening are (1) lipid oxidation and/or polymerization, (2) formation of insoluble pectates, (3) lignification of middle lamella, and (4) multiple mechanisms. Most researchers have reported that the defect develops in the cotyledons. Recently, some authors have suggested that the seed coat plays a significant role in the process of common bean hardening. A better knowledge of cotyledon and seed coat microstructure may lead to a better understanding of the causes of seed hardness. In order to prevent the development of the HTC defect several procedures have been proposed: (1) appropriate storage, (2) controlled atmospheres, and (3) pretreatments. Probably, the most workable solution to the hardening phenomenon may be the development of materials less prone to HTC phenomenon. Decreasing cooking time, increasing nutritive value, and improving sensory properties of seeds with HTC defect would have great nutritional and economical impact.(ABSTRACT TRUNCATED AT 400 WORDS)

Contribution of the digestive tract microflora to amylomaize starch degradation in the rat

To study in vivo the contribution of the bacterial flora to amylomaize starch degradation in the rat, germ-free and conventional rats were fed on a diet containing either a normal maize starch or an amylomaize starch. In germ-free rats maize starch was almost totally digested in the small intestine, whereas 40% of the ingested amylomaize starch reached the caecum and 30% was excreted, despite the very high endogenous amylase activity. Study by transmission electron microscopy of germ-free caecal contents showed an endocorrosion of the starch granule. In conventional rats, as in germ-free rats, digestibility of maize starch reached 98% in the small intestine, whereas that of amylomaize starch was only 60%. In the caecum of these rats amylomaize starch was fermented, and this led to a decrease in caecal pH and to formation of short-chain fatty acids (SCFA), especially propionate. Comparison between conventional rats fed on maize starch or amylomaize starch showed that caecal SCFA concentrations during a circadian cycle varied in the same way whereas total SCFA and lactic acid concentrations were much higher in rats fed on amylomaize starch. Amylase (EC 3.2.1.1) activity was similar in the caecal contents of conventional rats whatever the ingested starch. It was lower in conventional than in germ-free rats, but no starch granule remained in the caecum of conventional rats. These results showed that bacterial amylase was more efficient at degrading resistant amylomaize starch than endogenous amylase.

Energy balance and energy values of alpha-amylase (EC 3.2.1.1)-resistant maize and pea (Pisum sativum) starches in the rat

Apparent and partial digestible energy values for alpha-amylase (EC 3.2.1.1)-resistant, retrograde starches, isolated from cooked maize and pea starches (RMS and RPS respectively), were determined in male Wistar rats (about 180 g) during a 28-29 d balance period with ten animals per treatment. The starches were provided as supplements (100 g/kg diet) to a semi-synthetic basal diet (B), and their effects on the apparent digestibilities of nitrogen and fat, and on gains of live weight, fat and lean tissue were examined. Diet B alone was the control; sucrose (Su) and Solka-floc cellulose (SFC) were also examined for reference. Apparent digestibilities for Su, SFC, RMS and RPS were 1.0, 0.16, 0.98 and 0.89 respectively. Whereas the apparent digestibilities of gross energy, N and fat in the diet were unaffected by supplementation with Su, each was decreased by supplementation with SFC, RMS and RPS. Partial digestible energy values calculated from the intakes and faecal losses of energy in the basal and supplemented diets were 15, 12.4 and 0.8 kJ/g for RMS, RPS and SFC respectively. These values were smaller than corresponding apparent digestible energy values calculated from the apparent digestibility of the supplement and its gross energy value. Only the Su and starch supplements increased the intake of apparent digestible energy and the gain of live weight. Both starches and Su increased total energy (and fat) deposition to almost similar extents. It is concluded that the resistant starches contribute significant dietary energy, enhance growth and elevate fat deposition to extents almost similar to Su.

Utilization of alpha-amylase (EC 3.2.1.1) resistant maize and pea (Pisum sativum) starch in the rat

1. The extent of utilization of alpha-amylase (EC 3.2.1.1)-resistant retrograded starches in vivo was assessed in male Wistar rats (about 100 g body-weight). All animals were given a fibre-free semi-synthetic basal diet (SS) containing sucrose as the only carbohydrate source, ad lib., for 13 d. On day 14, after an overnight fast, rats were allocated to one of five dietary treatments (n 30):1, fibre-free basal SS diet; 2-5, basal SS diet supplemented with 100 g sucrose, Solka floc (cellulose), resistant maize starch (RCS) or resistant pea (Pisum sativum) starch (RPS)/kg diet. Animals allocated to each dietary treatment were divided into three groups of ten rats which were given the appropriate diet for 8 or 9, 17 or 18 and 29 or 30 d (8/9, 17/18 or 29/30 d groups respectively). Rats were fed on 12 g diet/d (treatment 1) or 13.2 g diet/d (treatments 2-5) for the first 20 d, and 14 or 15.4 g/d respectively until the end of the experiment. Rats fed on the supplemented basal diets were thus given 10% more food to allow for the addition of the test carbohydrate. Faecal carbohydrate excretion was determined at intervals in the 17/18 d groups. At the end of each experimental period animals were killed after consuming their daily food ration and small intestinal length, weight of caecal and ileal contents and tissue, and pH of caecal contents measured. The amount of carbohydrate in the caecal and ileal contents from the 8/9 and 17/18 d groups was determined. 2. Weights of ileal and caecal contents, caecal tissue and faecal output were significantly greater at all time points for rats fed on the resistant starches compared with those fed on basal and sucrose-supplemented diets. Values were higher for RPS-fed rats than for RCS-fed rats. 3. The quantity of carbohydrate recovered from ileal and caecal contents showed that both RCS and RPS were partially digested and absorbed as carbohydrate, but that RPS was digested to a lesser extent. 4. The concentration of carbohydrate decreased between the ileal and caecal sites when RPS and RCS were given but was essentially unchanged when cellulose was given. This is consistent with rapid fermentation of a fraction of these starches. 5. Faecal carbohydrate elimination in the 17/18 d groups fed on RCS and RPS declined with time, which suggested an adaptive response resulting in increased utilization of the starches. This adaptive response was slower in the RPS-fed rats than the RCS-fed rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of starch absorption by dietary fibre. A comparative study of wheat bran, sugar-beet fibre, and pea fibre

The effect of dietary fibre on starch absorption was investigated in 8 healthy subjects. Amounts of starch escaping small-bowel absorption was assessed by comparison of breath H2 excretion after test meals and after lactulose (10g). After ingestion of bread made from 100g of wheat flour increases in H2 excretion occurred in all subjects; the calculated fractions of unabsorbed starch ranged from 4% to 17% (median, 8%). Concurrent ingestion of this bread with either wheat bran, sugar-beet fibre, or pea fibre increased the fraction of unabsorbed starch to 12.5% (5-22%) (p less than 0.05), 12.5% (5-20%) (p less than 0.01), and 12% (5-27%) (p less than 0.01), respectively. Bread made from 100 g of low-gluten wheat flour only escaped small-bowel absorption in three subjects with a maximal fraction of 6% (p less than 0.05). All three fibres decreased mouth-to-caecum transit time. We conclude that the dietary fibres used in this impaired the absorption of wheat starch and thereby increased the amount of starch-derived carbohydrate available for colonic fermentation.

Influence of starches of low digestibility on the rat caecal microflora

1. Male Sprague-Dawley rats were fed on either a purified, fibre-free diet or a diet in which half the maize starch was replaced with uncooked amylomaize or potato starch (equivalent to 100 or 200 g amylase-resistant starch (ARS)/kg diet respectively). Changes in short-chain fatty acids (SCFA), pH, ammonia and a number of bacterial variables in caecal contents were then assessed. 2. Both ARS supplements decreased caecal content pH by approximately 1-2 units, with an associated reduction in ammonia concentration. Potato starch significantly decreased the concentration of SCFA in the hindgut, while amylomaize supplementation increased propionic and butyric acids but decreased the occurrence of minor, branched-chain fatty acids. 3. Caecal bacterial biotransformation activities (beta-glucosidase (EC 3.2.1.21), beta-glucuronidase (EC 3.2.1.31), reduction of p-nitrobenzoic acid, apparent ammonia formation) were consistently decreased by both ARS sources. 4. The results demonstrate that amylase-resistant carbohydrate altered toxicologically important functions in the large-intestinal flora of the rat.

Effect of cooking, pH and polyphenol level on carbohydrate composition and nutritional quality of a sorghum (Sorghum bicolor (L.) Moench) food, ugali

1. The present work was undertaken to study the effects of cooking, pH and polyphenol level on carbohydrate composition and nutritional quality of sorghum (Sorghum bicolor (L.) Moench). Three different sorghum varieties; Dabar, Feterita and Argentine containing zero, intermediate to low and high levels of polyphenols respectively were used in the study. From these varieties uncooked, uncooked acidified, cooked, and cooked acidified diets were prepared. Diets were characterized with regard to resistant starch (RS), dietary fibre (DF), acid-detergent fibre (ADF) and amino acid content. Raw materials were further analysed for content and composition of non-starch polysaccharides and Klason lignin. The nutritional properties were studied in balance trials with rats. True protein digestibility (TD), biological value (BV), net protein utilization, digestible amino acids, digestible energy (DE) and digestible DF were used as criteria in the nutritional study. 2. Cooking at neutral and acid pH resulted in significantly higher assayed values for DF. Increase in DF could be accounted for by formation of RS. Approximately 50% of RS was recovered in the faeces. 3. In vitro values for protein associated with ADF and in vivo balance values using rats suggest that an endosperm protein fraction, kafirins, was made unavailable during cooking. This resulted in reduced TD and increased BV. It is assumed that unavailable kafirins serve as a nitrogen source for microflora in the hind-gut. 4. Dietary polyphenols changed the excretory route for N from urine to faeces. This resulted in lower TD and higher BV in Argentine (high in polyphenols) than in Dabar and Feterita (low in polyphenols), although dietary lysine (first limiting amino acid) was the same in the three varieties. 5. Variation in DE of the diets was attributed to DF, RS and the amount of faecal protein, which in turn were influenced by undigested kafirins and polyphenols.

Enzyme resistant starch fractions and dietary fibre

Starch fractions that are more or less enzyme resistant may behave like dietary fibre, both physiologically and analytically. Ungelatinized granules from potatoes, high amylose maize and green bananas are poorly digested. Starch made resistant to amylase due to new covalent bindings, formed at heat treatment or present in starch derivatives used as food additives, may also be more or less undigestible. "Resistant starch" present in bread and corn flakes is probably retrograded amylose. It is undigestible in the small intestine, but readily degraded by the large bowel microflora. Amylose-lipid complexes seem to be completely absorbed in spite of their resistance to amylase degradation in vitro. Since undigestible starch fractions behave physiologically like non-starch polysaccharides, they should be included in the dietary fibre concept. "Resistant starch" is analysed as glucose based fibre with all current methods except one, which includes an initial DMSO solubilization step.

Dietary fibre--definition, chemistry and analytical determination

Dietary fibre includes non-starch polysaccharides and lignin that are not digested or absorbed in the human small intestine. It contains a mixture of chemically complex polysaccharides. Lignin is a highly cross-linked complex polymer of phenylpropane units. The plant cell wall is the main source of dietary fibre and its structure is reviewed briefly. The structure of the main dietary fibre polysaccharides is then summarized. The demarcation between starch--the main digestible polysaccharide--and dietary fibre presents some problems due to more or less enzyme resistant starch fractions that occur naturally or are formed with processing. "Resistant starch" formed during baking passes through the small intestine in the rat and, probably, in man and is fermented in the colon. It should therefore also be regarded as dietary fibre. Methods for dietary fibre determination fall into two categories: gravimetric methods, weighing the dietary fibre after removal of other components; component analysis methods, determining monomeric composition of fibre polysaccharides (preferably by gas-liquid chromatography) supplemented with a gravimetric lignin determination and separate assay of uronic acid components (pectin). Recently developed enzymatic gravimetric methods are most convenient for the assay of total dietary fibre or water soluble and insoluble fibre separately, whereas component analysis is required for determining the dietary fibre composition.

Fermentation of dietary fibre in the intestinal tract: comparison between man and rat

1. The breakdown and faecal bulking capacity of dietary fibre preparations from wheat bran, apple, cabbage, carrot, and guar gum were compared in man and rat. 2. The degradation of the fibre showed good correlation between man and rat (r 0.99, regression coefficient 0.86). Wheat bran was the least well-digested, 66 and 59% of the neutral sugars being excreted in faeces of man and rat respectively. The breakdown of the fibre in apple, cabbage, carrot and guar gum was more complete and 4-29% of the neutral sugars were recovered in faeces. 3. The main dietary fibre constituents in each preparation were degraded to a similar extent in man and rat. The main dietary fibre constituents of apple, carrot, cabbage and guar gum were almost completely degraded. Of the xylose in wheat bran 45% (man) and 48% (rat) were recovered in faeces. However, the percentage excretion of glucose and arabinose from bran was higher in man. 4. A faecal glucan other than cellulose was identified in human faeces after guar gum, and has been provisionally identified as starch. No such glucan occurred in rat faeces. 5. A good correlation between the faecal bulking capacity in man and rat was seen (r 0.97, regression coefficient 0.56). Wheat bran had the best bulking capacity, while that of apple, cabbage, carrot and guar gum was less pronounced. Faecal bulking was inversely related to the amount of fibre which was water-soluble in each preparation. 6. It is concluded that this rat experimental model is useful for the prediction of fermentative breakdown and bulking capacity of dietary fibre in man. However, more comparative studies are needed to evaluate animal experiments regarding other physiological effects of dietary fibre.