The critical water activity from dynamic dewpoint isotherms as an indicator of pre-mix powder stability

Abstract

Premix instant powders offer quick, easy alternatives for making common food products such as gravy, muffins, and beverages. To remain viable, premix instant powders must be free of caking while maintaining their wettability, solubility, and dissolution properties. The main extrinsic factors that influence the stability of powders are temperature and water activity. The critical water activity from dynamic isotherms has been demonstrated to be an effective moisture related specification to maintain the stability of glassy powders. Premix instant powders are often primarily wheat flour, which has a high glass transition, so it is questionable if a critical water activity associated with stability can be found. The purpose of this study was to determine if a critical water activity can be determined for premix instant powders using dynamic isotherms and if this critical water activity is associated with product stability. Instant oatmeal muffin powder and instant chicken gravy powder were found to have a critical water activity at approximately 0.70 aw. Samples of the premix powders were then equilibrated to water activities above and below the critical water activity and examined for changes in caking strength. The mean caking strengths at water activities less than the critical water activity were significantly higher than at higher water activities. The premix samples also experienced color and other appearance changes above the critical water activity. The changes in the premix powders’ matrices that occurred at the critical water activity resulted in degradation of the powder, indicating that the critical water activity provides an effective specification to prevent product loss without requiring an extensive stability study.