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The structure of cereal products such as muesli and porridge, just before their consumption, is composed of rolled whole grains or grain fragments in a continuous aqueous phase. The confinement of a great part of the starch inside the cellular structure of the grain fragments (which are semi‐intact inside the cells) is an efficient way of slowing down starch digestion, compared to other products with higher proportion of gelatinized starch such as bread (Poutanen et al. 2014). However, it has also been suggested that porridge, due to its semisolid nature, could be emptied faster from the gastric compartment. In cases where porridge is made from flour rather than flakes, this could result in faster intestinal hydrolysis of starch and release of glucose (Johansson et al. 2018).

Rolled cereal, such as oat, rye or wheat, is one of the main components of muesli. The elaboration of rolled cereal involves steaming, pressing between rollers and drying of the grains. Light microscopy images of rolled rye show that cell walls are partially disrupted after the rolling process, gluten bodies are deformed developing a more network‐like structure within the particulate fragments, and starch is partially gelatinized after the drying (Figure 1.3). In this way, the resulting rolled grains absorb water more quickly and, in the case of those consumed as porridge, the cooking time decreases as the thickness of the rolled grains decreases. There is also a relationship between the rolled grain size and its subsidiary effects on digestion and colonic microbiota fermentation. The thickness would determine the availability and types of dietary fibre that are able to avoid digestion, and this would in turn have a consequence in the fermentation profile (Connolly et al. 2010).

Porridge is a dish made by boiling rolled or crushed cereal, usually oats, in water, milk or both, usually served hot in a bowl or dish. From the structural point of view, porridge is a compact cereal product where insoluble fragments of grain are dispersed in viscous grain extract. The boiling process favors the partial release of amylose from the starch granules and their swelling which, together with the amounts of β‐glucan solubilized from the cell walls, directly affect the viscosity and the texture of the product (Source: Yiu et al. 1987; Vázquez‐Gutiérrez et al. 2015) (Figure 1.2B).

Figure 1.3 Microstructure of rolled rye grain. A: bright field micrograph with iodine and light green staining for observation of starch (purple) and protein (green), respectively; B: epifluorescence micrograph with Calcofluor and Acid Fuchsin staining for observation of cell walls (blue) and protein (red), respectively.