Fish cracker samples were prepared in three particle sizes (< 180 µm, 250–425 µm, and ~ 3000 µm) and tested at 25 and 35 ☌ for MSI using DDI. A novel model to fit the MSI data was also created to obtain 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. Further, an MSI curve without clear inflection point can make it difficult to extract the a w, crit. 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. 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.
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