Investigations into practical applications for the critical water activity from dynamic dewpoint isotherms

Moisture plays an important role in the stability and storability of grain and grain based products. Stability factors such as microbial growth and texture are impacted by moisture and are well correlated to water activity, a measure of the energy status of water. The key to effectively using water activity as an indicator of product stability is to identify the critical water activity (RHc) where the rate of stability loss increases and shelf life ends. The RHc could be determined by interrogating pre-equilibrated samples at various water activities for quality performance and establishing the water activity where performance falls below acceptable levels. However, these types of studies tend to be labor and time intensive. A possible alternative that has shown promise is to identify the RHc by an inflection in the dynamic moisture sorption curve. Critical water activities determined using dynamic isotherms have been shown to be associated with glass transition in low molecular weight amorphous powders and could possibly be associated with other degradative change. The objective of this research project was to survey the dynamic isotherms of whole grain and grain-based products for an RHc that could be linked to a stability loss factor. Critical water activities were found in the dynamic isotherms of whole grain wheat and barley, oatmeal muffin premix, chicken gravy premix, graham crackers, and shortbread cookies. The RHc of whole grain was found to be associated with the ideal water activity for tempered grain to produce flour whose water activity was below critical levels for microbial spoilage. The RHc of the premix powders was found to be associated with the wetting of the instant powders and initiation of dissolution. Finally, the RHc of the grahams and shortbread cookies was related to an abrupt loss of crispness. Comparison to previous studies and general practices indicated that the RHc values determined in this study were similar to those previously identified. Therefore, this study provides evidence that the RHc for the stability of grain based products can be determined from a dynamic isotherm rather than requiring a time and labor intensive study.