The interactions between water content, matrix structures and chemical and physical events in complex foods are usually regarded in terms of water activity and glass transition. In this work, the water content and water activity values that define important changes in the textural and optical properties of freeze-dried banana and apple slices have been related to the critical water content and water activity that cause the glass transition in both products. From the obtained results, it may be concluded that both approaches, glass transition and water activity, may be considered to prevent changes in the mechanical properties, while only the concept of water activity is needed to control the browning reactions that take place in the fruits. (C) 2011 Elsevier Ltd. All rights reserved.