Not all Arabinoxylans are the same: let's find out the difference
Not all Arabinoxylans are the same: let’s find out the difference
Arabinoxylans are soluble fibres extracted from cereal grains, such as the wheat endosperm. However, not all arabinoxylans share the same structure, which affects not only the physicochemical properties, but also their health benefits. They vary in the degree of polymerization, solubility, degree of substitution or presence of antioxidants, such as ferulic acid.
What are arabinoxylans?
Arabinoxylans are soluble fibres extractable from cereal grains. Arabinoxylans have been identified in all major cereal grains, including wheat, barley, oats, rye, rice, sorghum, maize, and millet. In cereal grains, arabinoxylans are localized mainly in the cell walls of starchy endosperm and the aleurone layer, in the bran tissues, and in the husk of some cereals.
Arabinoxylans can be used as a dietary supplement due to their beneficial effects on gut health, glycaemic control and immune health.
Moreover, due to the physicochemical and technological properties of these molecules (e.g. water-binding capacity, gelation), arabinoxylans can also be used as a baking additive to improve dough consistency, increase loaf volumes and improve crumb structure.
The general structure of arabinoxylans
Arabinoxylans are very long molecules consisting of a copolymer of two pentose sugars: arabinose and xylose. The general structure of arabinoxylans is formed by a backbone of xylose with arabinose residues attached to xylose units in different positions.
The relative amount and the sequence of distribution of these structural elements vary depending on the source of arabinoxylans. The majority of arabinose residues in arabinoxylans are present as monomeric substituents; however, a small proportion of oligomeric side chains consisting of two or more arabinose residues have been reported.
Therefore, although arabinoxylans share a common general structure, one important distinction between them is chain length. Chain length is an important feature, as it influences the industrial applications and health benefits of arabinoxylans. For instance, while arabinoxylans from wheat endosperm have an average chain length higher than 60, arabinoxylan-oligosaccharides (AXOS) extracted from wheat have an average chain length of 2–10.
On the other hand, the molecular structure of arabinoxylans from rice, sorghum, finger millet, and maize bran is slightly different than the one from wheat, rye, and barley, since the side branches contain, in addition to arabinose residues, small amounts of other compounds, which can confer additional health benefits or physicochemical properties.
Not all arabinoxylans share the same structure
Arabinoxylans from various cereals and different plant tissues share the same general molecular structure. However, depending on the genus and species, the amount and structure of arabinoxylans in a particular tissue may differ drastically, which affects not only their physicochemical properties, but also their health benefits.
Arabinoxylans as part of dietary fibre have many potential physiological effects along the gastrointestinal tract. These effects are dependent on a complex mixture of molecular and physical properties of arabinoxylan preparations, as well as on the site, rate, and extent of their digestion and fermentation in the gut.
These differences are reflected, among others, in the degree of polymerization (chain length), solubility, degree of substitution or presence of other substituents, such as feruloyl groups.
• Degree of polymerization
It represents the length of the arabinoxylan chain, and it is related to a greater bifidogenic effect. [1]
For instance, native arabinoxylans from wheat endosperm have an average degree of polymerization higher than 60 and, often, higher than 100; while other fibres such as arabinoxylan-oligosaccharides (AXOS) and inulin typically have a degree of polymerization of 2–10. [2]
• Solubility
The wheat endosperm contains the highest amount of soluble arabinoxylans, which is associated with a greater bifidogenic effect.
Insoluble arabinoxylans are typically found in the bran, stalks or corn stover, and are not likely to be digested in the large intestine.
Moreover, the extraction of water-insoluble arabinoxylans requires the use of cell wall blasting enzymes or strong alkaline chemicals.
Such heavy enzymatic or chemical treatment has a negative impact on both chain length and ferulic acid bonds. [1] Often, these extracted molecules are no longer arabinoxylans, but have become AXOS.
• Degree of substitution
It represents the ratio of arabinose to xylose residues. The presence of arabinose substitution also affects the fermentation ability of microorganisms.
While arabinoxylans support the growth of some bacterial species, unsubstituted xylans are not fermented by any of the probiotic bacteria.
Unsubstituted xylans form insoluble aggregates and hinder the accessibility of the bacteria. [1]
• Presence of substituents
The beneficial role of arabinoxylans in the human diet may also be associated with the presence of other substituents, such as ferulic acid covalently bound to these polymers.
Ferulic acid is considered a natural antioxidant, food antimicrobial agent, anti-inflammatory agent, photoprotectant and food flavour precursor. [3]
In conclusion, although arabinoxylans from various cereals share the same general structure, the specific structure varies greatly. These differences in structure are characterized by different factors (degree of polymerization, solubility…), which determine the potential health benefits and physicochemical properties of arabinoxylans, thus affecting the industrial applications of these molecules.
