Physicochemical properties and digestibility of eleven Vietnamese rice starches with varying amylose contents.

The physicochemical properties of eleven Vietnamese rice starches with apparent amylose contents ranging from 0.2% to 28.4% were investigated to identify the interplaying factors contributing to the in vitro digestibility of starch: in uncooked, cooked in excess water and under limited water conditions. The amylose content of rice starches governed the physicochemical properties of starch such as swelling power, pasting and gelatinization of starch which strongly correlated with raw starch digestibility. Amylopectin predominantly contributed to the crystallinity of starches both in short range and long range orders (observed by FT-IR and X-ray diffraction respectively). This study found that cooking destructively changed the molecular structure of starch that lead to an increase in the rate of starch digestion. Cooking in excess water resulted in a higher rate of starch digestion at least by 2 fold in all examined starches, suggesting that under these cooking conditions, factors that would normally limit the rate of digestion were eliminated. Cooking in limited water that was similar to domestic cooking conditions, seemed to allow rice starch granules to rearrange and interact together and limited the rate of digestion compared to cooking in excess water. It can be concluded that the physicochemical properties of starch can predict the digestibility of raw starches only but not cooked starches. The digestion of rice starch cooked in limited water cannot be predicted by just studying the uncooked starches or cooking in excess water.

Comparison of nutritional properties of Stinging nettle (Urtica dioica) flour with wheat and barley flours.

Stinging nettle (Urtica dioica. L) is a wild, unique herbaceous perennial flowering plant with Stinging hairs. It has a long history of use as a food sources as a soup or curries, and also used as a fiber as well as a medicinal herb. The current aim was to analyze the composition and bioactive compounds in Nepalese Stinging nettle. Chemical analysis showed the relatively higher level of crude protein (33.8%), crude fiber (9.1%), crude fat (3.6%), total ash (16.2%), carbohydrate (37.4%), and relatively lower energy value (307 kcal/100 g) as compared to wheat and barley flours. Analysis of nettle powder showed significantly higher level of bioactive compounds: phenolic compounds as 129 mg Gallic acid equivalent/g; carotenoid level 3497 µg/g; tannin 0.93 mg/100 g; anti-oxidant activity 66.3 DPPH inhibition (%), as compared to wheat and barley. This study further established that nettle plants as very good source of energy, proteins, high fiber, and a range of health benefitting bioactive compounds.

Quantification of sugars in breakfast cereals using capillary electrophoresis.

About 80% of the Australian population consumes breakfast cereal (BC) at least five days a week. With high prevalence rates of obesity and other diet-related diseases, improved methods for monitoring sugar levels in breakfast cereals would be useful in nutrition research. The heterogeneity of the complex matrix of BCs can make carbohydrate analysis challenging or necessitate tedious sample preparation leading to potential sugar loss or starch degradation into sugars. A recently established, simple and robust free solution capillary electrophoresis (CE) method was used in a new application to 13 BCs (in Australia) and compared with several established methods for quantification of carbohydrates. Carbohydrates identified in BCs by CE included sucrose, maltose, glucose and fructose. The CE method is simple requiring no sample preparation or derivatization and carbohydrates are detected by direct UV detection. CE was shown to be a more robust and accurate method for measuring carbohydrates than Fehling method, DNS (3,5-dinitrosalicylic acid) assay and HPLC (high performance liquid chromatography).

In vitro digestibility and physicochemical properties of milled rice.

Rice is a staple diet as well as a major ingredient in many processed foods. The physicochemical and supra-molecular structure of eight rice varieties with amylose content from 9% to 19% were studied to elucidate the factors responsible for variation in enzymatic digestibility of raw and cooked rice. Parboiled rice had a digestion rate coefficient almost 4.5 times higher than the least digestible Low GI rice. The rate coefficient was found to be independent of helical structure and long range molecular order, possibly attributed to the effect of rice flour architecture. Strong swelling and pasting behaviour and lower gelatinisation temperature were linked with apparently higher in vitro digestibility but the relationship was statistically insignificant. It is concluded that the enzymatic susceptibility of rice flours are independent of supra-molecular structure and are most likely controlled by external factors not limited to particle size, presence of intact cell wall and other non-starch polymers.

Molecular, mesoscopic and microscopic structure evolution during amylase digestion of extruded maize and high amylose maize starches.

Extrusion processing of cereal starch granules with high (>50%) amylose content is a promising approach to create nutritionally desirable resistant starch, i.e. starch that escapes digestion in the small intestine. Whilst high amylose content seems to be required, the structural features responsible for the slow digestion of extrudates are not fully understood. We report the effects of partial enzyme digestion of extruded maize starches on amylopectin branch length profiles, double and single helix contents, crystallinity and lamellar periodicity. Comparing results for three extruded maize starches (27, 57, and 84% apparent amylose) that differ in amylase-sensitivity allows conclusions to be drawn concerning the rate-determining features operating under the digestion conditions used. Enzyme resistance is shown to originate from a combination of molecular and mesoscopic factors, including both recrystallization and an increase in very short branches during the digestion process. This is in contrast to the behaviour of the same starches in the granular form (Shrestha et al., 2012) where molecular and mesoscopic factors are secondary to microscopic structures in determining enzyme susceptibility. Based on the structure of residual material after long-time digestion (>8h), a model for resistant starch from processed high amylose maize starches is proposed based on a fringed micelle structure with lateral aggregation and enzyme susceptibility both limited by attached clusters of branch points.

Heterogeneity in maize starch granule internal architecture deduced from diffusion of fluorescent dextran probes.

Heterogeneity in maize starch granules was investigated by studying the diffusion of fluorescent dextran probes (20, 70 and 150kDa) inside granules using fluorescence recovery after photobleaching combined with confocal microscopy. Access of probes to the interior of granules was greatly enhanced by limited (2.4%) amylolysis. The diffusion of probes within granules was found to be either 'fast' with diffusion coefficients in the order of 10(-6)cm(2)s(-1) or 'slow' with diffusion coefficients in the order of 10(-7)cm(2)s(-1), independent of the size of dextran probes or prior treatment of the granules by α-amylase. Results were compared with observations of pores and channels in granules by electron microscopy and by confocal microscopy after labelling with 8-amino-1,3,6-pyrenetrisulfonic acid. It is proposed that there is an inherent heterogeneity of internal architecture in maize starch granules due to the presence or absence in individual granules of (a) pores leading to a central cavity, resulting in 'fast' diffusion of dextran probes and (b) accessibility of the starch polymer matrix to dextran probes, leading to 'slow' diffusion behaviour. The observed heterogeneity of maize starch granule porosity has implications for chemical modification reactions and the kinetics of digestion with amylases.

Molecular, mesoscopic and microscopic structure evolution during amylase digestion of maize starch granules.

Cereal starch granules with high (>50%) amylose content are a promising source of nutritionally desirable resistant starch, i.e. starch that escapes digestion in the small intestine, but the structural features responsible are not fully understood. We report the effects of partial enzyme digestion of maize starch granules on amylopectin branch length profiles, double and single helix contents, gelatinisation properties, crystallinity and lamellar periodicity. Comparing results for three maize starches (27, 57, and 84% amylose) that differ in both structural features and amylase-sensitivity allows conclusions to be drawn concerning the rate-determining features operating under the digestion conditions used. All starches are found to be digested by a side-by-side mechanism in which there is no major preference during enzyme attack for amylopectin branch lengths, helix form, crystallinity or lamellar organisation. We conclude that the major factor controlling enzyme susceptibility is granule architecture, with shorter length scales not playing a major role as inferred from the largely invariant nature of numerous structural measures during the digestion process (XRD, NMR, SAXS, DSC, FACE). Results are consistent with digestion rates being controlled by restricted diffusion of enzymes within densely packed granular structures, with an effective surface area for enzyme attack determined by external dimensions (57 or 84% amylose - relatively slow) or internal channels and pores (27% amylose - relatively fast). Although the process of granule digestion is to a first approximation non-discriminatory with respect to structure at molecular and mesoscopic length scales, secondary effects noted include (i) partial crystallisation of V-type helices during digestion of 27% amylose starch, (ii) preferential hydrolysis of long amylopectin branches during the early stage hydrolysis of 27% and 57% but not 84% amylose starches, linked with disruption of lamellar repeating structure and (iii) partial B-type recrystallisation after prolonged enzyme incubation for 57% and 84% amylose starches but not 27% amylose starch.

Physicochemical and structural properties of maize and potato starches as a function of granule size.

Chemical composition, molecular structure and organization, and thermal and pasting properties of maize and potato starches fractionated on the basis of granule size were investigated to understand heterogeneity within granule populations. For both starches, lipid, protein, and mineral contents decreased and apparent amylose contents increased with granule size. Fully branched (whole) and debranched molecular size distributions in maize starch fractions were invariant with granule size. Higher amylose contents and amylopectin hydrodynamic sizes were found for larger potato starch granules, although debranched molecular size distributions did not vary. Larger granules had higher degrees of crystallinity and greater amounts of double and single helical structures. Systematic differences in pasting and thermal properties were observed with granule size. Results suggest that branch length distributions in both amylose and amylopectin fractions are under tighter biosynthetic control in potato starch than either molecular size or amylose/amylopectin ratio, whereas all three parameters are controlled during the biosynthesis of maize starch.

Molecular rearrangement of starch during in vitro digestion: toward a better understanding of enzyme resistant starch formation in processed starches.

Resistant starch (RS) is defined as the fraction of starch that escapes digestion in the small intestine, serving as a fermentation substrate for beneficial colonic bacteria. Several studies have been focused on the description of the RS fractions from different starch varieties, but little attention has been paid to the digestion process itself that, from the present work, seems to play a key role in the generation of enzyme-RS (ERS), as determined in vitro. High-amylose starch samples, extruded at two different processing conditions, have been characterized at different stages of in vitro digestion using scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), infrared spectroscopy (FT-IR), solid state (13)C NMR spectroscopy, and X-ray diffraction (XRD). Control samples kept for 18 h in the digestion solution without starch hydrolyzing enzymes (alpha-amylase and amyloglucosidase) were used for comparison purposes. An increase in molecular order was favored by the hydrolytic action of the enzymes, reflected in an increase in double helical order observed by NMR, higher crystallinity measured by XRD, and corresponding changes in FT-IR spectra. An increase in the intensity of the scattering objects was also observed by SAXS as a function of digestion. SAXS from the dry ERS fractions reveals the 001 reflection of crystallites formed during the digestion process, corresponding to a characteristic dimension of the resistant crystalline fraction of approximately 5 nm. The changes found suggest that enzyme resistant starch does not refer to a specific structure present in predigested starches, but may in fact be formed during the digestion process through the rearrangement of amylose chains into enzyme-resistant structures of higher crystallinity. Therefore, the resistance to enzyme digestion of a specific processed starch is the result of a competition between the kinetics of enzyme hydrolysis and the kinetics of amylose retrogradation.