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Puffed‐grain cereals are commonly used as ready‐to‐eat breakfast foods or as ingredients in snack formulations. They have characteristic lightness and crispness, qualities related to their cellular structure (Peleg 1997) and degree of expansion (Owusu‐Ansah et al. 1984). Moreover, the effect of the puffing treatment is strongly influenced by the morphology and composition of the kernel (Mariotti et al. 2006). Two technological processes are generally used in order to decrease the bulk density of puffed cereals. Oven puffing involves the sudden application of heat at atmospheric pressure to a prewetted cereal. The product expands due to the vaporization of water inside the kernel. Gun puffing is based on the sudden transfer of a piece containing superheated water to a lower pressure, thus allowing the water to suddenly vaporize (Delcour and Hoseney 2010b). The degree of expansion with gun‐puffing is 15–20 times, much greater than with oven puffing (2–5 times). Oven‐puffed cereals are made almost exclusively using whole grain rice or maize, while rice and wheat are the only types of grain used in gun‐puffed whole grain production (Fast 2000). Puffed products must keep less than 3% moisture to maintain crispness, but due to their high porosity, they absorb moisture rapidly. The greater the expansion, the more difficult is to maintain the moisture. In this way, gun‐puffed cereals require special packaging.

The ultrastructure of the grains is severely affected by puffing and is reflected in some physical properties such as bulk density and water uptake. Puffed rye and rice presents a very porous matrix made up of numerous cavities of increasing size from the center of the kernel outwards, whereas wheat and barley show a more compact and non‐homogeneous structure (Mariotti et al. 2006). The changes in the structure after compression of puffed cereals have also been studied (Roopa et al. 2009).