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

Interaction between starch and dietary compounds: New findings and perspectives to produce functional foods

Due to the increased prevalence of overweight, obesity, diabetes, colon cancer, cardiovascular diseases, and metabolic syndrome, dietary approaches to reduce starch digestion and regulate glucose homeostasis have gained attention. Starch is a polysaccharide in most daily food consumed as bakery products, snacks, breakfast cereals, and pasta, which are often vilified. However, it is also present in beans, lentils, and oatmeal, which are considered healthy food products. The difference relays on the food matrix and the thermal process that can produce interactions between starch and dietary compounds (protein, lipid, non-starch polysaccharide, and bioactive compounds) or among starch chains (retrogradation). Such interactions produce structural changes so the digestive enzymes cannot hydrolyze them; additionally, the physical barrier of some macromolecules (proteins, hydrocolloids) restricts starch gelatinization and accessibility of the digestive enzymes to hydrolyze the starch. The interactions mentioned above and the use of some macromolecules as physical barriers could be explored as a pathway to develop functional foods. This review analyzes the interactions between starch and dietary compounds influenced by the processing of some food matrices to better understand their potential for developing functional foods.

Effect of Bread Structure and In Vitro Oral Processing Methods in Bolus Disintegration and Glycemic Index

The growing interest in controlling the glycemic index of starchy-rich food has encouraged research about the role of the physical structure of food. The aim of this research was to understand the impact of the structure and the in vitro oral processing methods on bolus behavior and starch hydrolysis of wheat bread. Two different bread structures (loaf bread and bread roll) were obtained using different shaping methods. Starch hydrolysis during in vitro oro-gastro-intestinal digestion using the INFOGEST protocol was analyzed and oral processing was simulated by applying two different disintegration processes (basic homogenizer, crystal balls). The bread structure, and thus the shaping method during breadmaking, significantly affected the bolus particle size during all digestion stages. The different in vitro oral processing methods affected the bolus particle sizes after the oral phase in both breads, but they affected the particle size distribution after the gastric and intestinal phase only in the case of loaf bread. Aggregates were observed in the gastric phase, which were significantly reduced in the intestinal phase. When simulated oral processing with crystal balls led to bigger particle size distribution, bread rolls presented the highest in vitro starch hydrolysis. The type of in vitro oral processing allowed discrimination of the performance of the structures of the two breads during starch hydrolysis. Overall, crumb structure significantly affected texture properties, but also had a significant impact on particle size during digestion and starch digestibility.

Glycemic impact and health: new horizons in white bread formulations

The challenge of provision of a much wider range of foods of relatively low glycemic response than is currently available, especially in terms of cereal products, has been highlighted in recent years and this has particular relevance to bread consumption. Although there has been some transition to brown bread consumption, white bread remains a firm feature in the typical average western diet. This review first outlines the relationship between the glycemic impact of foods and health. What is important is that relatively small differences in glycemic potency of regularly consumed starch foods have been shown to have beneficial effects on health outcomes. Second, factors affecting glycemic response with particular application to white bread formulations are discussed. Novel ways of reformulating this highly favored carbohydrate staple, by using composite flours, with the aim of developing products of reduced glycemic response are highlighted in this review. Importantly, a new and significant focus on the role of unavailable carbohydrate in glycemic improvement is emerging. This has important application in increasing accessibility to health benefits by contributing to the prevention of and management of glucose intolerance, insulin resistance, and associated chronic disease to a wider range of consumers.

Advances in dietary fibre characterisation. 2. Consumption, chemistry, physiology and measurement of resistant starch; implications for health and food labelling

Resistant starch (RS) is defined as ‘the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals’. This basic definition includes different types of starches that (1) are physically inaccessible, usually due to an encapsulation in intact cell walls, or (2) are naturally highly resistant to mammalian α-amylase, or (3) have been modified by hydrothermic treatments then retrograded. Interest in RS has increased significantly during the last two decades, mostly due to its capacity to produce a large amount of butyrate all along the colon. Butyrate has been observed to have a range of effects on cell metabolism, differentiation and cell growth as well as inhibition of a variety of factors that underlie the initiation, progression and growth of colon tumours. The physiological definition of RS, which seems to be nearly consensual, raises a difficulty in proper analytical quantification of RS. A number of methods have, however, been proposed and provide similar values for the RS content in most of the starch types and starchy foods. It seems, however, that some starches, proven to be partly resistant according toin vivoinvestigations on ileostomy subjects, could not be quantified by most of these methods. This may be due to a widespread use of glucoamylase during the first steps of these methods. Accordingly, there is an international debate on health aspects of RS and on how to quantify the RS content of food products. The present review describes aspects of classification of RS, past and current consumption, physiological effects and analytical aspects, and concludes with impacts on food and product labelling.

Fermentation of non-starch polysaccharides in mixed diets and single fibre sources: comparative studies in human subjects andin vitro

The present study investigated whether the extent of fermentation of NSP in human subjects could be predicted by anin vitrobatch system. Fibre sources studied were five mixed diets containing different amounts and types of fibre and three single fibre sources (citrus fibre concentrate, coarse and fine wholemeal rye bread). Fermentation in human subjects was determined in balance experiments in women who were also donors of the faecal inocula.In vitrofermentations were performed with fibre residues prepared from duplicates of the fibre-containing foods consumed during the balance trials. Fermentation of total NSPin vivowas between 65.8 and 88.6% for the mixed diets and 54.4, 58.0 and 96.9 % for the coarse and fine wholemeal rye breads and the citrus fibre concentrate respectively. For the mixed diets and the citrus fibre concentrate, mean differences between the extent of NSP degradation after 24 hin vitroincubation and thatin vivowere between −0.7 and 5.0 %. Differences were significant for one diet (P< 0.05). For the wholemeal rye breads, the fermentationin vitroexceeded thatin vivosignificantly, but the magnitude of the difference in each case was small and without physiological importance. Particle size of breads had no influence on the extent of NSP degradation. These results indicate that thein vitrobatch system used could provide quantitative data on the fermentationin vivoof NSP in mixed diets and some single fibre sources. Anin vitroincubation time of 24 h was sufficient to mimic the NSP degradationin vivo.

Determination of the differences in 2-h plasma glucose values after ingestion of carbohydrate foods and oral glucose in Caribbean non-diabetic subjects

It has been reported that mixed meals are used in clinics in developing and developed countries in screening and diagnosis of diabetes. Thus, we aimed to determine the differences in 2-h plasma glucose values after non-diabetic subjects ingested 75 g pure glucose and its equivalent content in frequently consumed carbohydrate foods in Caribbean subjects. Twenty-seven apparently healthy non-diabetic subjects (nine males, 18 females) consumed 75 g pure glucose and its carbohydrate equivalent in three ethnic test foods (bread, rice and roti) at 7 days apart. Plasma glucose and insulin levels were determined in blood samples collected before and after 60, 90, 120 and 150 min of ingestion of these foods. In comparison with each of the test foods, the postprandial 1-h and 2-h plasma glucose values and the 60, 90, 120 and 150 min incremental glucose concentrations after oral glucose load were significantly higher than the corresponding values for each of the test foods (all P<0.01). In spite of these higher postprandial glucose concentrations, the postprandial insulin responses following the oral glucose load and the test foods did not significantly differ at any time point (all P>0.05). However, the test food, roti, tended to stimulate higher absolute and incremental insulin secretions than pure glucose or any other test food (all P>0.05). Generally, the correlation between 2-h plasma glucose value after the ingestion of the pure glucose and each of the test foods was significant (all correlation coefficients were greater than 0.70, P<0.01). In conclusion, different ethnic mixed meals could serve as an alternative to glucose in routine screening and diagnosis of diabetes if its available carbohydrate content is known and quantified.

The rat as a model for pigs: comparative values for the digestibility of NSP and other macronutrients

The present investigation was undertaken to study whether conventional male Wistar rats could be used as a model for pigs with regard to total tract digestibilities of NSP and macronutrients and whether nebacitin-treated rats could be used as a model for small intestinal digestibility in pigs. Nineteen experimental diets prepared from different fractions of wheat and oats, and which all had been evaluated in experiments with ileal cannulated pigs, were used for the present study. There was a close relationship between the total tract digestibilities of organic matter in the two species. The same was the case with regard to the digestibility of total NSP and arabinoxylans, but the values were on average 6 % lower in rats than in pigs. On average, there were no significant differences between rats and pigs with regard to faecal protein digestibility. However, protein in oat-based diets was significantly better digested in the rat than the pig. The digestibility of fat was consistently higher in rats than in pigs, with the biggest difference being found in oat-based diets, in which most of the fat was locked in cell structures. For the wheat-based diets, in which a large proportion of the fat was present as added fat, there was a greater similarity between the two species. In nebacitin-treated rats the digestibility of organic matter, starch, protein and fat was negatively related to the dietary level of NSP, but this model could not be used to predict the small intestinal digestibility of NSP and macronutrients in ileal-cannulated pigs.

Estimates of starch digestion in the rat small intestine differ from those obtained using in vitro time-sensitive starch fractionation assays

The objectives of this study were as follows: 1) to determine the rate and extent of starch disappearance from the small intestine of the rat fed selected starch sources, 2) to determine the ratios of the major starch fractions [rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS)] in those starch sources using two in vitro methods and 3) to compare the two data sets to determine the accuracy of the in vitro methods. Diets were prepared using cornstarch, potato starch, amylomaize, maltodextrin, modified maltodextrin or pullulan. Starch sources and diets were analyzed for starch fractions by two in vitro methods. Diets were fed to rats, intestinal contents were collected and the ethanol-induced precipitate from the contents was analyzed to obtain a digestion curve that was mathematically modeled for comparison to results obtained using the two in vitro methods. Only the cornstarch diet had a defined amount of RDS, SDS and RS. The RDS concentration obtained from the intestinal contents of the rats fed the cornstarch diet differed (P < 0.05) from that determined by one in vitro method but was consistent with the value obtained using the other in vitro method. All other digestible starch values obtained differed (P < 0.05) among methods except for that of amylomaize. Starch fractions in starch sources obtained using in vitro procedures differed (P < 0.05) from values obtained for diets. The rate of disappearance differed (P < 0.05) between in vivo and in vitro procedures. There was minimal agreement between in vitro methods tested, and there was also minimal agreement between in vitro and in vivo results. Classification of starch into RDS and SDS components cannot be accomplished for a variety of starch sources, with cornstarch being the major exception.

Importance of processing for physico-chemical and physiological properties of dietary fibre

There is considerable loss of DM during wet heat treatment of vegetables, leading to an increase in dietary fibre. Correction for the loss of DM indicates that the effects on total dietary fibre are minor. There is, however, depolymerization of the dietary fibre polysaccharides. The degradation is related to the severity of the heat treatment. Souring, freezing and mild microwave treatment have no effects. The viscosity is in general related to the extent of polymerisation. Microwave treatment has different effects on various cultivars of green beans, and the addition of salt (NaCl and CaCl2) to the boiling water changes the physico-chemical properties of soluble fibre in carrots, depending on the cation. The higher viscosity of the soluble fibre in raw carrots may partly explain the lower glucose and hormonal responses observed in healthy subjects when compared with blanched and microwave-cooked carrots. In studies on rats the amount of butyric acid in the distal colon has been shown to be higher with dietary components containing high amounts of resistant starch. Further, the fermentability is lower and the butyric acid concentration higher with composite foods than with the corresponding purified fibre fractions. In human studies the faecal concentration of butyric acid has been shown to increase in patients with ulcerative colitis when [beta-glucan-enriched oat bran (20 g fibre) is added to the diet for 12 weeks. Also, an improvement of symptoms was reported.

Production of 13C labelled pea flour for use in human digestion and fermentation studies

Stable isotope breath tests offer a new approach to the study of digestion and fermentation of carbohydrates in man. In this study, 13C labelled peas were grown by pulsing 250 ml 13CO2 into a sealed growth chamber. A second pulse was added to a portion of the peas to increase the 13C enrichment. This generated pea flour with an enrichment of 2.36 at.% excess (range 2.09-2.71 n = 3) and 8.64 atom % excess (range 7.37-9.78 n = 3) respectively. This represented incorporation of an absolute yield of 3.8% of the 13CO2 into peas in the 'once-labelled' treatment and 7.5% in the 'twice-labelled' treatment. Ingestion of a mixture of the labelled pea flour (300 mg) by two volunteers generated measurable 13CO2 excretion for breath test analysis. The profile of breath 13CO2 enrichment increased to a maximum within three hours after consuming the pea flour followed by a decrease almost back to baseline by 13 hours. Breath 13CO2 appeared to rise again after this apparent nadir at 13 hours until the end of the sampling period. Mathematical analysis of the data suggested that two peaks best described the profile of breath 13CO2 up to 13 hours. A third peak was necessary to describe the late rise in breath 13CO2 enrichment. This use of 13C enriched pea flour may provide a useful non invasive method for measurement of digestion and fermentation in vivo.

Modifications to physicochemical and nutritional properties of hard-To-cook beans (Phaseolus vulgaris L.) by extrusion cooking

The objective of this work was to evaluate extrusion cooking as a means to improve the nutritional properties of Phaseolus vulgaris L. that had been stored either at 42 degrees C and 80% relative humidity for 6 weeks or for periods >1 year in cereal stores in tropical conditions. Storage under these conditions resulted in an increase in cooking time increased (7.7- and 12-fold, respectively) as a result of development of the hard-to-cook (HTC) defect. Single-screw extrusion of the milled beans was carried out at four barrel temperatures and two moisture contents. The extrudate bulk density and water solubility index decreased with increasing temperature, whereas the water absorption index increased due to the higher proportion of gelatinized starch in the extruded samples. Both fresh and HTC beans contained nutritionally significant amounts of lectins, trypsin, and alpha-amylase inhibitors, which were mostly inactivated by extrusion. Extrusion also caused a considerable redistribution of insoluble dietary fiber to soluble, although the total dietary fiber content was not affected. Changes in solubility involved pectic polysaccharides, arabinose and uronic acids being the main sugars involved. Stored beans subjected to extrusion cooking showed physical and chemical characteristics similar to those of extrudates from fresh beans.

Resistant starch--an update on its physiological effects

Resistant starch (RS) has emerged as one of the main substrates for colonic fermentation, together with other undigestible polysaccharides and oligosaccharides. There are indications that RS may be a good source of butyrate, and that the rate and site of fermentation can be varied and optimized. This makes RS potentially important for colonic health, and production of food products containing RS challenging. The present RS content in most Western diets is probably low, but can be increased by foods high in RS. The physiological effects of RS are reviewed, as well as the formation of RS in foods and its analysis.

Fermentation in human subjects of nonstarch polysaccharides in mixed diets, but not in a barley fiber concentrate, could be predicted by in vitro fermentation using human fecal inocula

The fermentation of nonstarch polysaccharides (NSP) contained in a low fiber diet, two high fiber diets high or low in protein, and a barley fiber concentrate was determined in balance experiments in six women and in an in vitro batch system using fecal inocula obtained from these same women. In vitro fermentations were performed with fiber residues prepared from duplicates of the fiber-containing foods consumed during the balance trials. Fermentation of total NSP in humans was 83.8 +/- 0.9% (low fiber diet), 61.8 +/- 3.6% (high fiber diet high in protein), 59.2 +/- 3. 9% (high fiber diet low in protein) and 31.2 +/- 7.4% (barley fiber concentrate). Fermentation in vitro differed from fermentation in humans by -4.0 +/- 1.6% (low fiber diet, P < 0.05,), 4.9 +/- 3.7% (high fiber diet high in protein), 8.8 +/- 3.0% (high fiber diet low in protein, P < 0.01) and 19.7 +/- 8.0% (barley fiber concentrate, P < 0.05). Differences between in vivo and in vitro fermentation were most pronounced for NSP-glucose, i.e., cellulose. Production of short-chain fatty acids in vitro corresponded to the fermentability of NSP. The yield of short-chain fatty acids per gram of fermented NSP was similar for the diets (8.8-9.4 mmol) but lower for the barley fiber concentrate (7.4 mmol, P < 0.05). Although differences between the fermentation measured in humans and in vitro were significant for two diets, the magnitude of the differences was such that fermentation of NSP in mixed diets could be predicted with sufficient accuracy in vitro, whereas agreement between the fermentation in vivo and in vitro of NSP in the barley fiber concentrate was not satisfactory.

Resistant starch as energy

OBJECTIVE

This study was designed to compare the metabolizable energy of two starch sources, standard cornstarch and high amylose cornstarch.

METHODS

Diets containing 70% amylose (AM) or 70% amylopectin (AP) cornstarches were fed to 10 control and 14 hyperinsulinemic men for 14 weeks. During the last 4 weeks of each period, subjects were fed a controlled diet containing 34% of total energy from fat, 15% from protein and 51% from carbohydrate (55% of carbohydrate provided AM or AP). Duplicate food and all urine and feces were collected during the second week of the controlled diets for energy, nitrogen, fiber and starch determinations. Metabolizable energy (ME) was calculated as [energy intake minus (fecal plus urinary energy excretion)].

RESULTS

Total fiber uncorrected for resistant starch was 35.2 g and 48.8 g in the AP and AM diets, respectively. The AM diet contained an average of 29.7 g resistant starch (16% of total starch) while the AP diet averaged 0.8 g (less than 0.01%). ME was not significantly different between the AM and AP diets nor between the control and hyperinsulinemic subjects. Fecal energy and nitrogen was significantly higher after the AM compared to AP diet. Based on energy intake and fecal excretion from all subjects, the partial digestible energy value for the resistant starch averaged 11.7 kJ/g resistant starch which was 67.3% of the energy of standard cornstarch. Control and hyperinsulinemic subjects differed in their ability to digest resistant starch, averaging 81.8% and 53.2, respectively. The hyperinsulinemic, but not control, subjects had significantly higher breath hydrogen expirations (LS means, p > 0.05) in the fasting, 1-5 hours and 7 hour collections after consuming the AM when compared to the AP tolerance meal.

CONCLUSIONS

The type of starch consumed in the diet did not statistically affect metabolizable energy. Based on ME and breath hydrogen expiration, amylose and the resistant starch from amylose appears to be utilized as an energy source. Resistant starch averaged 2.8 kcal/g for all 24 subjects but only 2.2 kcal/g in the hyperinsulinemic subjects.

Content of starch and sugars and in vitro digestion of starch by alpha-amylase in five minor millets

Five varieties of minor millets were studied for their amylose, soluble amylose, amylopectin, soluble amylopectin, reducing sugar, total sugar and starch contents. Pure starch was isolated from each variety and the enzymic degradation of starch by porcine pancreatic alpha-amylase were examined with and without gelatinisation. Gelatinised sample of Echinochloa frumentacea (var. K2) showed minimal hydrolysis and gelatinised sample of Panicum miliaceum (var. CO3) showed maximum hydrolysis of starch by porcine pancreatic alpha-amylase. Gelatinised starch was highly susceptible to enzymic digestion when compared to ungelatinised starch. The extent of starch degradation varied from 71 to 85 percent in gelatinised samples and starch degradation in ungelatinised sample varied from 10 to 18 percent.