Densely packed matrices as rate determining features in starch hydrolysis

To aid in the design of starch-containing foods with slow and/or incomplete digestion in the upper gastrointestinal tract, the starch structural factors which control the rate of action of alpha-amylase are reviewed. It is concluded that local starch molecular density has the major influence on amylase digestion kinetics, and that density sufficient to either limit enzyme binding and/or slow down catalysis can be achieved by either crystallization or dense amorphous packing

Mastication effects on carotenoid bioaccessibility from mango fruit tissue

The release of carotenoids from fresh fruits or vegetables is determined by the encapsulating plant tissue matrix, intracellular carotenoid location within the cell, and the mastication process. The objectives of this study were to assess the particle sizes obtained after mastication of mango fruit tissue, and how the resulting degree of plant tissue rupture affects carotenoid bioaccessibility. A fine and a coarse chewer were selected after screening 20 healthy volunteers for in vivo human mastication, and the collected chewed boluses were subjected to wet sieving fractionation, followed by an in vitro gastric and small intestinal digestion model. Confocal micrographs show that the smallest particle size fraction (0.075 mm) consists mostly of fragmented cells and the largest size fraction (2.8 mm) contains bulky clusters of whole cells and vascular fibers. Higher amounts of total carotenoids (211–320 μg/100 g) were observed in the larger particle size fraction (2.8 mm) relative to the 1 mm (192–249 μg/100 g) and 0.075 mm fractions (136–199 μg/100 g). Smaller particles showed a greater % release of total carotenoids after in vitro digestion. Xanthophyll derivatives are more bioaccessible than β-carotene for all particle sizes. The effects of particle size or degree of fine vs coarse chewing are unexpectedly small (p > 0.05), but the process of chewing substantially reduced the release of β-carotene and xanthophylls by 34% and 18%, respectively. While there is a (small) particle size effect, this appears to not be the primary factor controlling bioaccessibility for soft tissues such as mango, in contrast to previous reports that a single cell wall appears to be enough to prevent bioaccessibility of carotenoids in more robust carrot tissues

Influence of extrusion on expansion, functional and digestibility properties of whole sweetpotato flour

Beerwah Gold, Northern Star, Snow White, and L49 cultivars of sweetpotato from Australia and Papua New Guinea, were studied for their extrusion behaviours in a co-rotating twin-screw extruder at three moisture (30, 35, 40 g/100 g) and screw speed (150, 220, 300 rpm) levels with a slit die. Low moisture increased the die pressure (2-6 bar) and specific mechanical energy (280-600 kJ/kg) of the extruder. Expansion, functional and digestibility properties of the extrudates were extrusion-dependent and cultivar-specific. Extrusion moisture increased the longitudinal expansion (15-30 m/kg) of the extrudates, which were almost completely gelatinised (100 g/100 g degree of gelatinisation). In-vitro starch digestion revealed that salivary-gastric digestion in the extrudates ranged from 8 to 18 g/100 g dry starch, while the rate of starch digestion was 3.0-3.7 min-1. Salivary-gastric digestion in the non-extrudates was from 2 to 11 g/100 g dry starch, with the rate of starch digestion being 0.1-0.8 min-1. Estimated glyceamic index of the extrudates ranged from 87 to 124 g/100 g, higher than in the non-extrudates and dependent on extrusion moisture and screw speed. This is the first study on extrusion-property relationships of the cultivars to guide global utilisation of sweetpotato

Amylase binding to starch granules under hydrolysing and non-hydrolysing conditions

Although considerable information is available about amylolysis rate, extent and pattern of granular starches, the underlying mechanisms of enzyme action and interactions are not fully understood, partly due to the lack of direct visualisation of enzyme binding and subsequent hydrolysis of starch granules. In the present study, α-amylase (AA) from porcine pancreas was labelled with either fluorescein isothiocyanate (FITC) or tetramethylrhodamine isothiocyanate (TRITC) fluorescent dye with maintenance of significant enzyme activity. The binding of FITC/TRITC-AA conjugate to the surface and interior of granules was studied under both non-hydrolysing (0 °C) and hydrolysing (37 °C) conditions with confocal microscopy. It was observed that enzyme binding to maize starch granules under both conditions was more homogenous compared with potato starch. Enzyme molecules appear to preferentially bind to the granules or part of granules that are more susceptible to enzymic degradation. The specificity is such that fresh enzyme added after a certain time of incubation binds at the same location as previously bound enzyme. By visualising the enzyme location during binding and hydrolysis, detailed information is provided regarding the heterogeneity of granular starch digestion

The interplay of α-amylase and amyloglucosidase activities on the digestion of starch in in vitro enzymic systems

In vitro hydrolysis assays are a key tool in understanding differences in rate and extent of digestion of starchy foods. They offer a greater degree of simplicity and flexibility than dynamic in vitro models orin vivo experiments for quantifiable, mechanistic exploration of starch digestion. In the present work the influence of α-amylase and amyloglucosidase activities on the digestion of maize and potato starchgranules was measured using both glucose and reducing sugar assays. Data were analysed through initialrates of digestion, and by 1st order kinetics, utilising logarithm of slope (LOS) plots. The rate and extent of starch digestion was dependent on the activities of both enzymes and the type of starch used. Potatorequired more enzyme than maize to achieve logarithmic reaction curves, and complete digestion. The results allow targeted design of starch digestion experiments through a thorough understanding of the contributions of α-amylase and amyloglucosidase to digestion rates

Characterisation of soluble and insoluble cell wall fractions from rye, wheat and hull-less barley endosperm flours

Within cereal endosperm flours, arabinoxylan and β-glucan molecules exist in either a soluble or an insoluble form. From a nutritional functionality viewpoint, soluble and insoluble forms offer different potential health advantages, so it is important to define both the features controlling solubilisation and the properties of each of the soluble and insoluble fractions. Factors known to affect the stability of arabinoxylan (AX) and β-glucan (BG) solutions include AX branching extent and type, and the ratio of cellotriose to cellotetraose units (DP3/DP4) in BG. Through studying the solubilisation of AX and BG from wheat, rye, and hull less barley endosperm under conditions that avoid the use of alkali or ethanol during the solubilisation process, we report (a) similar A/X ratios and fine structures for extracted soluble arabinoxylan and the corresponding insoluble AX within the cell walls for rye and wheat endosperm flours, (b) comparable DP3/DP4 ratios for soluble β-glucan, flour and insoluble β-glucan within the endosperm cell wall of hull less barley, and (c) evidence for enrichment of β-glucan at the exterior of residual insoluble cell walls. Therefore, the factors determining solubilisation of AX and BG from endosperm cell walls are different to those that determine the stability of aqueous solutions of the same polymers, and β-glucan may show limited solubilisation by being trapped within restraining cross-linked arabinoxylans in the cell wall

Compact structure and proteins of pasta retard in vitro digestive evolution of branched starch molecular structure

The roles that the compact structure and proteins in pasta play in retarding evolution of starch molecular structure during in vitro digestion are explored, using four types of cooked samples: whole pasta, pasta powder, semolina (with proteins) and extracted starch without proteins. These were subjected to in vitro digestion with porcine alpha-amylase, collecting samples at different times and characterizing the weight distribution of branched starch molecules using size-exclusion chromatography. Measurement of alpha-amylase activity showed that a protein (or proteins) from semolina or pasta powder interacted with alpha-amylase, causing reduced enzymatic activity and retarding digestion of branched starch molecules with hydrodynamic radius (R-h) 100 nm. (C) 2016 Elsevier Ltd. All rights reserved

Functional categorisation of dietary fibre in foods: Beyond 'soluble’ vs ‘insoluble’

© 2018 Elsevier Ltd Background: Diets rich in dietary fibre are associated with multiple health benefits, but there is often only a restricted understanding of the mechanisms underlying these associations. This limits the ability to select or design foods for specific nutritional purposes. Traditionally, the diverse physical and chemical forms of dietary fibre have only been categorised as either soluble or insoluble. Scope and approach: In this review, the physicochemical properties that have been proposed to be responsible for the biological functionality of dietary fibres in the digestive tract are summarised and classified. The extent to which these properties follow naturally from categorisation into soluble vs insoluble forms are then assessed. Based on this analysis, a new approach to functional categorisation of dietary fibres is proposed. Key findings and conclusions: The physicochemical properties of dietary fibre components that are relevant to digestive tract functionality can be classified under the headings of binding, structuring, and transport barriers. Major nutritional outcomes such as control of macronutrient digestion or the nature of residual digesta that are available for fermentation by the large intestinal microbiota depend on combinations of these physicochemical properties in ways which are not readily reflected by a soluble vs insoluble fibre definition. An alternative approach is proposed based on 2D mapping of dietary fibre materials as a function of molecule/particle size and local density. This effectively separates diverse fibre materials and can be linked semi-quantitatively with functionally-important properties

Efforts to capture high amylose in rice

Screening of wild and cultivated rice in IRRI germplasm collection revealed that majority have intermediate apparent amylose content. It appears that ancient farmers selected rice based on texture of the lower amylose varieties, considering that the majority of rice consumers today prefer intermediate to soft-textured rice. Furthermore, 30% seems to be the natural upper natural limit of amylose levels in wild-type rice. If this is the case, the rich biodiversity of rice has been subjected to the bottleneck of domestication to select for grains that have superior cooking and eating but not nutritional or satiating qualities considering that the majority of rice consumers today eat rice three times a day. On the other hand, the amylose content of available rice mutants with deficient SBEIIb or an over-expressed GBSSI also revealed amylose levels of around 35% which is significantly lower by comparison with other high amylose cereals, whose amylose content ranges from 70–90%. Hence, to produce the high amylose phenotype in rice, one might need to target different sets of enzymes or regulatory pathways. Since increasing the amylose levels in rice might mean a concomitant increase in its resistant starch content and in its levels of satiety, and a decrease in its glycemic response, developing high amylose rice by biotechnology is imperative. This type of rice will be important not only in addressing the growing obesity epidemic which now also affects the developing countries but also as a basis of novel degradable biopolymers and for further elucidating the mechanisms of starch synthesis in the cereal endosperm. In this paper, we also present the status of our research project which aims to silence the expression of SBEIIa, SBEIIb and SSIIa singly or in combination using microRNA and RNAi silencing technologies with the aim of increasing the amylose levels in rice beyond its natural limits

Extrusion induced low-order starch matrices: enzymic hydrolysis and structure

Waxy, normal and highwaymen maize starches were extruded with water as sole plasticizer to achieve low-order starch matrices. Of the three starches, we found that only high-amylose extrudate showed lower digestion rate/extent than starches cooked in excess water. The ordered structure of high-amylose starches in cooked and extruded forms was similar, as judged by NMR, XRD and DSC techniques, but enzyme resistance was much greater for extruded forms. Size exclusion chromatography suggested that longer chains were involved in enzyme resistance. We propose that the local molecular density of packing of amylose chains can control the digestion kinetics rather than just crystallinity, with the principle being that density sufficient to either prevent/limit binding and/or slow down catalysis can be achieved by dense amorphous packing