Role of ß-glucan content, molecular weight and phytate in the bile acid binding of oat ß-glucan.

There is controversy about the role of viscosity and co-migrating molecules on the bile acid binding of beta-glucan. Thus, this study aimed to investigate the impact of ß-glucan molecular weight and the content of both ß-glucan and phytate on the mobility of bile acids by modelling intestinal conditions in vitro. Two approaches were used to evaluate factors underlying this binding effect. The first studied bile acid binding capacity of soluble ß-glucan using purified compounds. Viscosity of the ß-glucan solution governed mainly the mobility of bile acid since both a decrease in ß-glucan concentration and degradation of ß-glucan by enzyme hydrolysis resulted in decreased binding. The second approach investigated the trapping of bile acids in the oat bran matrix. Results suggested trapping of bile acids by the ß-glucan gel network. Additionally, hydrolysis of phytate was shown to increase bile acid binding, probably due to better extractability of ß-glucan in this sample.

In vitro study for investigating the impact of decreasing the molecular weight of oat bran dietary fibre components on the behaviour in small and large intestine.

The objective of this work was to evaluate the role of ß-glucan molecular weight (Mw) and the presence of other carbohydrates on the physiological functionality of oat bran via an in vitro digestion study. A complete approach using three different in vitro digestion models (viscosity of the small intestine digest, reduction of bile acids and on-line measurement of gas evolution) was used to predict the physiological functionality of enzymatically modified oat bran concentrate (OBC). OBC was enzymatically treated with two ß-glucanase preparations at three different levels in order to specifically decrease ß-glucan Mw (Pure: purified ß-glucanase) or ß-glucan and other cell wall polysaccharides (Mix: commercial food-grade cell wall degrading enzyme preparation). The Mw of ß-glucan in OBC was tailored to high (1000 kDa), medium (200-500 kDa) and low (<100 kDa) values. The amount of arabinoxylan-oligosaccharides varied from 0.3 to 4.7 g per 100 g of OBC when OBC was treated with the Mix enzyme at the highest dosage. When the enzymatically treated OBCs were studied in an upper gut model, a decrease in the viscosity of the digest simultaneously with the reduction of ß-glucan Mw was observed. At a similar ß-glucan Mw range, OBC samples treated with the Pure enzyme had lower viscosity than the samples treated with the Mix one, which also contained arabinoxylan-oligosaccharides. After enzymatic hydrolysis, the capacity of OBC to reduce bile acid was decreased regardless of the enzyme treatment used, and a positive correlation was found between ß-glucan Mw and bile acid reduction (r = 0.99**). The production of colonic gases by the enzymatically treated OBC samples in an in vitro colon model showed an inverse correlation between ß-glucan Mw and initial rate of gas formation (r = -0.9**), but no impact of arabinoxylan-oligosaccharides was observed. This study emphasised the complexity of factors affecting the functionality of oat components under physiological conditions and demonstrated the possibility to produce Mw-tailored oat fibre ingredients that could contribute to gut mediated health benefits.

Reaction pathways during oxidation of cereal ß-glucans.

Oxidation of cereal ß-glucans may affect their stability in food products. Generally, polysaccharides oxidise via different pathways leading to chain cleavage or formation of oxidised groups within the polymer chain. In this study, oxidation pathways of oat and barley ß-glucans were assessed with different concentrations of hydrogen peroxide (H2O2) or ascorbic acid (Asc) with ferrous iron (Fe2+) as a catalyst. Degradation of ß-glucans was evaluated using high performance size exclusion chromatography and formation of carbonyl groups using carbazole-9-carbonyloxyamine labelling. Furthermore, oxidative degradation of glucosyl residues was studied. Based on the results, the oxidation with Asc mainly resulted in glycosidic bond cleavage. With H2O2, both glycosidic bond cleavage and formation of carbonyl groups within the ß-glucan chain was found. Moreover, H2O2 oxidation led to production of formic acid, which was proposed to result from Ruff degradation where oxidised glucose (gluconic acid) is decarboxylated to form arabinose.

Lipid oxidation induced oxidative degradation of cereal beta-glucan.

In food systems, lipid oxidation can cause oxidation of other molecules. This research for the first time investigated oxidative degradation of ß-glucan induced by lipid oxidation using an oil-in-water emulsion system which simulated a multi-phased aqueous food system containing oil and ß-glucan. Lipid oxidation was monitored using peroxide value and hexanal production while ß-glucan degradation was evaluated by viscosity and molecular weight measurements. The study showed that while lipid oxidation proceeded, ß-glucan degradation occurred. Emulsions containing ß-glucan, oil and ferrous ion showed significant viscosity and molecular weight decrease after 1 week of oxidation at room temperature. Elevated temperature (40°C) enhanced the oxidation reactions causing higher viscosity drop. In addition, the presence of ß-glucan appeared to retard the hexanal production in lipid oxidation. The study revealed that lipid oxidation may induce the degradation of ß-glucan in aqueous food systems where ß-glucan and lipids co-exist.

The oxidative degradation of barley ß-glucan in the presence of ascorbic acid or hydrogen peroxide.

Cereal ß-glucans are polysaccharides with health benefits that have been linked to their ability to increase luminal viscosity. However, the functional properties of cereal ß-glucans may be diminished by the susceptibility of this polysaccharide to oxidative degradation. In this study, barley ß-glucan was oxidised with hydrogen peroxide or ascorbic acid and the oxidative degradation of ß-glucan was investigated using both asymmetrical flow field-flow fractionation (AsFlFFF) with aqueous solvent and high performance size exclusion chromatography (HPSEC) with LiBr in DMSO as the solvent. Oxidation was shown to cause degradation of ß-glucan, the reaction being faster when oxidised with hydrogen peroxide compared with ascorbic acid. Both HPSEC and AsFlFFF showed comparable results as long as aggregates (only observed in AsFlFFF) were not included in the integration. The compact aggregates observed in oxidised samples suggest oxidation driven interactions between ß-glucan molecules.