Effect of milling and flour microstructure on in vitro starch digestibility in sweet biscuits

In 2021, type 2 diabetes mellitus (T2DM) accounted for nearly 2 million deaths, while obesity remained a key risk factor for ischemic heart disease, the world’s leading cause of mortality (1) . A major contributor to the prevalence of both T2DM and obesity is the long-term consumption of highly digestible carbohydrates, which trigger substantial post-prandial blood glucose excursions. Carbohydrate-rich staple foods, such as bakery products, are predominantly made from flour, in which starch is the primary carbohydrate source. We hypothesise that modifying the physicochemical properties of whole flours can lower starch digestibility and thus modulate the glycaemic impact of bakery products, providing a potential dietary strategy to help reduce the burden of metabolic disease (2) . In this study, we examine our hypothesis using whole-grain wheat flour in a sweet biscuit formulation. Whole wheat grains were milled using a beater mill to pass through a screen with either 2.0 or 0.1 mm perforations, producing coarse or fine flours, respectively. Biscuits were made from these whole flours, as well as refined wheat flour, using rotary-moulding technology. The in vitro starch digestibility of the biscuits was assessed using the Englyst method to obtain RDS, SDS and RS in g/100g of total starch, and an α-amylase assay (3) measuring the starch digested after 90 minutes using a fixed enzyme-to-substrate ratio. Significant differences were determined using one-way ANOVA. The mechanisms underlying the variations in starch digestibility among various biscuits were investigated by analysing the differences in the flours’ microstructure using bright-field and scanning electron microscopy. The milled flour which passed through the 2 mm screen was confirmed to have larger particle size (D50 = 559 ± 208 µm) than those that passed through the 0.1 mm screens (D50 = 35 ± 1µm). Biscuits made from fine whole flour and refined flour showed higher starch digestibility than those from coarse flour. After ninety minutes of amylolysis, starch digested was significantly greater in refined (44.4 ± 0.3 g/100g) and fine flour biscuits (50.6 ± 5.2 g/100g) compared to coarse flour (31.1 ± 2.2 g/100g). The Englyst method confirmed these findings, showing more SDS in coarse flour biscuits (53.7 ± 0.4 g/100g) than in refined (40.6 ± 0.3 g/100g) or fine flour biscuits (29.3 ± 0.3 g/100g). Microstructural analysis showed that coarse flours contained large endosperm fragments with some intact cells. In contrast, fine and refined flours mainly consisted of free, damaged starch granules and small endosperm fragments. The smaller particles in fine and refined flour increased surface area, enhancing enzyme access and starch digestibility in biscuits. Controlling particle size in milled whole wheat is helpful for developing low-moisture baked goods with slower starch digestion. This provides valuable insights for dietary interventions targeting populations that require improved glycaemic control.