Cell wall polysaccharides in cereals: chemical structures and functional properties

Abstract

β-Glucans and arabinoxylans are the two primary cell wall structural components in cereals, such as wheat, oat, barley and rye. The relative amounts of the two polysaccharides vary with species and growing environments. The cell walls of barley and oats are generally rich in β-glucan, whereas rye and wheat cell walls contain higher levels of arabinoxylans. Cereal β-glucan is a mix linked (1 → 3) (1 → 4)-β-d-glucan composed of two major building blocks: a trisaccharide and a tetrasaccharide unit: the combination of the two units is over 90%. The ratio of the two building blocks is used as a fingerprint for each β-glucans: it is 4.5, 3.3, 2.2 for wheat, barley and oat β-glucan, respectively. Of the two types of cell wall polysaccharides in cereals, β-glucan received greater attention due to its proved beneficial physiological effect as an excellent source of soluble dietary fibre for significantly attenuating blood glucose and insulin levels and its demonstrated ability to reduce low-density lipoprotein cholesterols (LDL) in serum. The ability of cereal β-glucan to attenuating blood glucose, insulin levels is linked to the viscosity produced by cereal β-glucans in a linear relationship. It is also demonstrated that the functional properties of cereal β-glucans are determined by their structural features and molecular weight: a higher trisaccharide to tetrasaccharide ratio favours gel formation and faster gelation process, and ultimately, gives stronger gels. Cereal β-glucan also demonstrated an unusual behaviour by forming gel faster and yielding stronger gels at lower molecular weight (above minimum gelation molecular weight). Such a structure–function relationship was established based on rheological, light scattering and computer modelling studies which cover both dilute and concentrated concentration regimes: high tri/tetra ratio gives high proportion of consecutive trisaccharide unit which favours the intermolecular association of β-glucan chains; on the other hand, low molecular weight chains have higher mobility that promotes intermolecular chain–chain interactions, hence, lead to faster gelation process and formation of stronger gels. Research also revealed that processing and storage conditions, such as temperature, pH, extrusion, baking and frozen before eating, have significant effects on the bioavailability of cereal β-glucans, hence, its ability to reduce blood glucose and cholesterol levels.