Loss of crispness (staling) in RTE cereals is associated with moisture pickup. Fresh RTE cereals have a moisture content of 2-3% and are very crisp. A good indicator of sensory crispness is water activity (Aw), which is the ratio of the vapor pressure of water over the food material to the vapor pressure of pure water, both measured at the same temperature. Fresh cereals generally have an Aw of about 0.20. As the moisture content goes up, the Aw increases until it reaches a critical value; for most cereals, this is around 0.45 (16). Beyond the critical Aw, the cereal becomes stele and unacceptable.
In the case of fruited cereals such as raisin bran, the raisin, which contains up to 18% moisture, will transfer its moisture during storage to the cereal, which has about 23% moisture. As long as the critical Aw of the cereal is not exceeded, the cereal will remain crisp. But the raisin could become unacceptably hard.
Some remedies to keep fruit soft while retaining cereal crispness include controlled addition of moisture to the cereal to modulate moisture migration from the fruit. In some cases this could make the cereal lose crispness. Infusion of fruits with edible humectants such as glycerol will keep the fruits soft while preventing the cereal from becoming stale (16,17). Coating RTE cereals with hydrophobic ingredients such as fats and oils, or incorporation of ingredients like magnesium stearate in the formulation, can significantly improve the bowl life of the cereal (18,19).
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