How do biscuits and crackers get broken during storage even if they are not disturbed

The problem you describe is the one commonly referred to as ' checking' and is related to the distribution of moisture in the baked biscuit or cracker. It was first studied and the reasons for the problem reported by Dunn and Bailey (1928).

After leaving the oven the moisture remaining in biscuits and crackers is unevenly distributed. In particular, the edges and upper and lower surfaces have a much lower moisture content than the centre. During storage the moisture migrates from the higher moisture content centre to the drier areas in order to achieve equilibrium. This movement of moisture sets up a series of stresses and strains in the product which, because the product is inflexible, can be of sufficient force to crack the surface. In some severe cases the biscuit may completely break into a number of smaller pieces. The cracks develop along weaknesses in the product structure, many of which are microscopic in size.

The best means of avoiding this problem is to ensure a minimum of moisture gradient in the baked product. This commonly means baking at lower temperatures for longer times. Alternatively you can introduce immediate post-baking drying using radio-frequency or microwave heating. Ideally the moisture differential between surface and centre should be less than 1% and average biscuit moisture contents in the order of 2-3%.

It is possible for biscuits to absorb moisture from the atmosphere but this usually leads to softening of the biscuit rather than checking.

Reference

DUNN, J.A. and BAILEY, C.H. (1928) Factors affecting checking in biscuits. Cereal Chemistry, 5, 395-430.

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