Effect of powdering on critical water activity estimate from dynamic dewpoint isotherm of a crispy starch-based snack: a case study with fish cracker

Abstract

The moisture sorption isotherm (MSI) and critical water activity (a_w,crit) of a dry food determine the moisture barrier properties required of packaging for desired shelf life of starchy dry food. These can be obtained with the novel rapid dynamic dewpoint isotherm method (DDI), but with such rapid measurement incomplete equilibration is of concern, and this relates to suitable particle size of the sample tested. Further, an MSI curve without clear inflection point can make it difficult to extract the a_w,crit. The objective of this work was to investigate the effects of particle size of a starch based snack on its MSI and on a_w,crit. A novel model to fit the MSI data was also created to obtain the a_w,crit. Fish cracker samples were prepared in three particle sizes (< 180 µm, 250–425 µm, and ~ 3000 µm) and tested at 25 and 35 °C for MSI using DDI. The sample comminution by grinding had clear and consistent effects, with finer particles absorbing more moisture. For this crispy snack, the rapid dense measurements enabled estimating the critical point that was robust against a 10 °C temperature change with about 200 µm particle diameter. This suggests near complete equilibration by the lack of rate effects from temperature, which were seen with larger particles. The results indicate that powdering dry crispy foods enables proper DDI measurements, while without powdering the isotherms can be degraded by particle size effects. The broken-stick type model fit the data extremely well and showed the critical transition clearly as a jump discontinuity in the derivative (slope).