Mechanistic insights into the impact of thermally induced endogenous β-glucan structural modifications on the digestibility of highland barley flour

Thermal processing is inevitable during highland barley (HB) production, but few studies pay attention to the component changes and their impact on HB properties. This study explored the different thermal effects of HB on the molecular, structural and physicochemical properties of β-glucan, as well as the related ultrastructural properties of kernels and the nutritional properties of HB flour (HBF). Microstructure images illustrated that thermal treatment dramatically disrupted endosperm cell walls, roughening their surfaces and almost gelatinised the starch granules of the HB kernel. The molecular weight and polydispersity index of β-glucan decreased from 253.10 ± 0.05 × 10 4  g/mol to 9.68 ± 0.03 × 10 4  g/mol and 13.64 ± 0.08 to 1.66 ± 0.04, respectively. X-ray diffraction and infrared spectroscopy demonstrated that thermal processing resulted in the breaking of polymer chains in β-glucan, but its main functional groups remained unchanged. Structural modifications in β-glucan led to reduced thermal stability and lower G′ and G″ values, indicating weaker gel structures. Heat treatment increased the peak viscosity of 100-5-HBF and 150-5-HBF, while reducing it in 150-50-HBF and 200-5-HBF due to the effects of starch gelatinisation and depolymerisation. Meanwhile, higher pasting viscosity and much slower starch digestion were apparent in the HBF than in the HBF-non β-glucan system. The results indicated the structural disruption of endosperm cell walls enhanced component interactions, which was also evidenced by varying β-glucan release rates during initial digestion (20 min). Thus, controlled thermal processing that minimally disrupts cellular structures or forms starch-encapsulating complexes may produce β-glucan-rich products with reduced starch digestibility.