Granule Size Heterogeneity in Barley Starch: Structural, Compositional, and Functional Differences between A- and B-type Granules and Their Industrial Implications

Abstract

Barley (Hordeum vulgare L.) is unique among cereal crops for the way its starch is organized not as a uniform population of granules, but as two structurally and functionally distinct classes. Large lenticular A-type granules (10–30 µm) and small, irregularly shaped B-type granules (≤6 µm) coexist in a bimodal size distribution that is rarely observed in other cereals. Numerically, B-type granules dominate (80–90% of total count), yet A-type granules account for 85–90% of total starch mass. This asymmetry is far more than a morphological curiosity: it reflects deep-seated differences in amylose content, lipid-complexed amylose concentrations, crystalline architecture, and surface porosity that collectively govern how barley starch behaves from the farm gate through to food processing, brewing, and bioethanol production. This review provides a comprehensive synthesis of current knowledge on the structural and functional differences between A- and B-type barley starch granules, considering three genotype backgrounds: normal, waxy, and high-amylose. The evidence consistently shows that B-type granules possess higher gelatinization temperatures, lower gelatinization enthalpies, greater enzyme hydrolysis rates under mild conditions, higher peak viscosities, and weaker gel network structures compared to A-type granules. Importantly, some properties including swelling factor in high-amylose genotypes and setback viscosity reverse direction depending on the genotype, underscoring the complex interplay between granule size and genetic background. The review also identifies significant research gaps in granule-size-specific chemical modification, freeze-thaw stability, and multi-scale structural characterization, and discusses how these gaps limit the full industrial exploitation of barley starch's inherent heterogeneity.