The critical water activity from dynamic dewpoint isotherms as an indicator of crispness in low moisture cookies

Low moisture cookie snacks are expected to possess a crisp texture when consumed. If this crispness is lost, the product is deemed unacceptable to the consumer. The most important factors influencing the crispness of low moisture cookies are moisture and temperature. Studies have shown that there exists a critical water activity where desirable crispness will be lost. Typically, this critical water activity would be obtained through an extensive texture study. However, high resolution dynamic isotherm curves have recently been shown to identify critical water activity values (RHc) by sharp inflections in the adsorption curve. The purpose of this study was to determine if the dynamic isotherm curves for low water activity snack cookies could similarly be used to identify an RHc and if this RHc could be used as a stability indicator. Dynamic isotherms, developed for two low moisture cookies at three different temperatures, were used to determine the RHc. Then, samples preconditioned to various water activity values and temperatures were analyzed for crispness. The effect of water activity on crispness was more important than temperature. Cookie samples at water activity values less than RHc maintained their crispness, but suffered an abrupt loss in texture at water activity values higher than RHc, yielding a sigmoidal shaped response. Fermi’s equation for sigmoidal response was used to model the response of crispness to water activity and estimate a critical water activity for texture loss. Both the RHc and Fermi’s critical water activity were found in the range of texture loss, but the RHc corresponded with the initial loss in texture, while Fermi’s critical water activity was at the midpoint of the texture loss. Since the RHc can be obtained with much less labor and time than a texture study and provides an effective indication of the loss of crispness, it was concluded that the RHc provides a viable alternative for determining the critical water activity for crispness.