Oscillatory water sorption test for determining water uptake

Crust crispiness is an important driver of consumer appreciation for baked goods. Loss of crispiness is a relevant problem, poorly understood on a molecular level. This requires detailed knowledge of the behaviour of water. Unfortunately, interpretation in kinetic terms of results obtained by current sorption tests requires knowledge of the shape and surface area of the sample. We present a new sorption method where water uptake and the amount of exchanging water are followed while the relative air humidity (RH) oscillates between two set values. The obtained curves are analysed using a Langmuir-type adsorption model. Factors studied were the rate limiting step for water uptake, the effect of crust particle size on the Langmuir sorption parameters and the use of the Langmuir equation for the calculation of the number of sorption sites (deltam(max)). Different crust components were considered. Both the Langmuir sorption parameters and deltam(max) were found to depend on the crust particle size. Results showed no significant difference between oscillation times of 28 and 48 minutes. Based on these results we conclude that the water uptake kinetics over these times is dominated by one process. Likely this is water diffusion into the particles. Surface adsorption (wetting) will proceed faster while effects due to structural reorganisation of the solid crust matrix will take hours to days. The maximum RH level at which the test can be used is around 60%. At higher RH, large structural changes of the material occur. With this constraint, our test allows a quantitative and physically sound analysis of water uptake behaviour, without prior knowledge of shape and surface area of the sample.