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Food structure is a key factor affecting starch digestion, and any disruption of the physical or botanical structure of cereal grains can increase the rate of digestion and postprandial glycemic response (Björck et al. 1994). Coarse particles of whole grains were found to reduce the rate of starch digestion more than fine whole grain flour (Liljeberg et al. 1992). Particle size of grains from wheat, corn and oat were likewise found to influence in vitro starch digestion rate, with larger particles being slower digesting (Heaton et al. 1988). Thus, the physical structure in whole grain foods is an important factor contributing to the starch digestion property and is likely related physiological effect. The botanical structure of grain kernels provides a nature‐produced physical barrier to protect the nutritive contents from environment influences. Starch, as the energy provider for seed germination, is mainly located in the endosperm cellular compartment that is embedded in a matrix formed by proteins and cell wall material (Kamal‐Eldin et al. 2009). Depending on the type of endosperm, a dense packing of starch granules in the protein matrix of the vitreous endosperm significantly decreases the rate of starch digestion (Lopes et al. 2009). The influence of the protein matrix, even after cooking, still affects starch digestibility with the report of SDS content of 20% in a cooked flour compared to 0–2% in cooked isolated starch (Zhang et al. 2008). Soluble fibre in the endosperm cell wall of oats was shown to affect starch digestion, where different degrees of β‐glucan solubilization using cooking methods caused that different degrees of starch digestion (Yiu et al. 1987). Whole grain kernels also have fibre‐rich multiple‐layered bran that may decrease the accessibility of hydrolytic enzyme to some starch granules. Accordingly, the ordered botanical structure of whole grain kernels is a natural way to produce slowly digestible or perhaps even some degree of resistant starches.

Although the whole grain botanical structure provides some degree of physical barrier to starch hydrolytic enzymes, most whole grain foods are further processed before consumption. An understudied area is how to effectively process whole grain foods to retain or minimize the loss of physical barrier function important to slow starch digestion properties and moderated postprandial glycaemia. Food processing with high temperatures and shear conditions may completely disrupt grain structure and disperse gelatinize starch, leading to a high content of RDS, which from the starch perspective differs little from processed refined grain products. On the other hand, moderate processing such as with rolled oats can lead to reduced rate of starch digestion due to a minimal disruption of the physical structure of the grain (Mishra and Monro 2009). Similarly, food processing to produce a dense packing of food form may create a physical barrier property to starch digestion, such as in pasta that can contain a significant SDS.