Food stability determination by macro–micro region concept in the state diagram and by defining a critical temperature

In the 1950s the concept of water activity was proposed for determining food stability. This concept is now being used although it has some limitations. Indeed, these limitations mean that the concept is not universally applicable and in fact is invalid under certain conditions. In order to address the limitations of the water activity concept, the glass transition concept was proposed in the 1960s, although significant application of the concept only started in the 1980s. Recently, it has become evident that the glass transition concept is also not universally valid for stability determination in all types of foods when stored under different conditions. Currently in the literature the need is emphasized to combine the water activity and glass transition concepts since both concepts could complement each other. The glass transition concept was used to develop the state diagram by drawing another stability map using freezing curve and glass transition line. In this paper an attempt is made to review the published methods used to combine both concepts. These approaches are graphical plot of glass transition conditions and water content as a function of water activity, and macro–micro region concept in the state diagram. In addition, a new approach is proposed in this paper by defining a critical temperature for stability and then relating it with water content, and other hurdles affecting food stability. The water mobility concept is also reviewed to provide another dimension of food stability in order to determine a more complete picture.