Glass transition and water eﬀects on sucrose inversion in noncrystalline carbohydrate food systems Kouame Kouassi a , Yrjo¨ H. Roos b, * a Department of Food Technology, PO Box 27, (Latokartanonkari 7), FI-00014 University of Helsinki, Helsinki, Finland b Department of Food Science, Food Technology and Nutrition, University College Cork, Cork, Ireland Received 29 November 2000; accepted 27 February 2001 Abstract The eﬀects of water and glass transition on the hydrolysis of sucrose by invertase in noncrystalline carbohydrate systems were investigated. Maltodextrin/sucrose (2:1) and maltodextrin/lactose/sucrose (1:1:1) were dissolved in distilled water. Invertase (10 mg/ 17.2 g) was added. Amorphous samples were produced by freeze-drying the solutions. Sorption isotherms were determined grav- imetrically at 24  C over the 0.113–0.763 a w , and over 0.239–0.764 a w , the glass transition, T g was determined using diﬀerential scanning calorimetry (DSC). DSC and water sorption results suggested that samples remained noncrystalline. Sucrose inversion was analysed by monitoring glucose content during storage. Sucrose hydrolysis occurred at signiﬁcant rates at 0.662 and 0.764 a w . The rate increase was not related to the apparent glass transition of the systems. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Crystallization; Glass transition; Hydrolysis; Invertase; Maltodextrin; Sucrose; Water 1. Introduction Since ancient time, dehydration has been one of the most common natural and reliable methods for food preservation. Although reaction rates are generally reduced by dehydration, undesirable changes due to enzymatic reactions may result in quality changes (Acker, 1969). Recently, possible relationships between the occurrence of such reactions and changes in the physical state of food systems have been suggested (Slade & Levine, 1991). An example of such physical change is the glass transition. The glass transition occurs over a temperature range and it has been recog- nized as a possible factor aﬀecting kinetics of enzymatic changes in low-moisture foods. The glass transition is often observed from changes in the mechanical proper- ties as the viscous, solid-like glass is transformed to a more ﬂowing, liquid-like state. The glassy state is accompanied by a reduction in translational and rota- tional motions of component molecules, supporting an assumption of a reduced rates of chemical and biologi- cal reactions in glassy systems (Cardona, Schebor, Buera, Karel, & Chirife, 1997; Le Meste, 1995). There- fore, in conditions resulting in the glass transition reac- tion, rates may occur signiﬁcantly faster, as the volume of the matrix and molecular mobility increase (Le Meste, 1995; Roos, 1995a–c; Roos & Karel, 1991). Most studies of the eﬀects of water and glass transi- tion on enzyme activity, in food systems were mainly focussed on the criterion of thermal stability (Cardona et al., 1997; Schebor, Buera, Chirife, & Karel, 1995). However, these studies indicated that the thermal stabi- lity was not attributable to T g . Chen, Aull, and Bell (1999) studied the eﬀect of water activity and T g on the thermal stability and reactivity of invertase in PVP sys- tems. They found that a w >0.62 was required for sucrose hydrolysis, and T g had no signiﬁcant eﬀect on the reaction. It may, however, be assumed that in PVP systems, sucrose is not fully miscible with the main matrix and the hydrolysis may occur locally in the material microstructure. In an earlier study, we (Kouassi & Roos, 2000) investigated the eﬀects of glass transition and water on the inversion of sucrose by invertase in a lactose-sucrose food model. In such mod- els, the substrate is fully miscible with the continuous, 0963-9969/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(01)00114-4 Food Research International 34 (2001) 895–901 www.elsevier.com/locate/foodres * Corresponding author. Tel.: +353-21-902386; fax: +353-211-27- 0213. E-mail addresses: kouame.kouassi@helsinki.ﬁ (K. Kouassi), yrjo.roos@ucc.ie (Y.H. Roos).