Structural properties of dehydrated systems and the rate of non-enzymatic browning

NURIA ACEVEDO; CAROLINA SCHEBOR; MARÍA DEL PILAR BUERA

Puerto Vallarta, México

Congreso; CONGRESO IBEROAMERICANO DE INGENIERÍA DE ALIMENTOS; 2005

Introduction. During storage of dehydrated foods non enzymatic browning reactions (NEB) may develop, thus affecting their quality and limiting their shelf life. The kinetics of is known to be affected by several physico-chemical factors. In fluid liquid systems, NEB rate diminishes continuously as relative humidity (R.H.) increases, mainly due to the fact that water is a product of the reaction. However, in solid or quasi-solid systems, in which NEB reactants are submitted to mobility restrictions, a maximum rate of NEB is observed at a given intermediate R.H. value. The kinetics of NEB has also been related to the glass transition phenomenon (1). No systematic studies have been performed regarding the effect of physical and structural properties of the food system on the NEB rate in the relative humidity scale. The purpose of the present work was to relate the relative humidity for the maximum rate of NEB with structural characteristics, which are modified by thermal transitions taking place when dehydrated systems are submitted to thermal treatment.  Methodology. Food systems of different structural characteristics (apple, cabbage, chicken, potato, milk) and freeze-dried model systems (PVP, lactose) were equilibrated in a wide range of R.H. and incubated at 70°C. The glass transition temperature (Tg) was determined by DSC. NEB degree was determined photocolorimetrically.  Results and discussion. In order to analyze the effect of structural properties on the relative humidity for the maximum rate of NEB, three groups of systems with defined structural characteristics were selected: a) matrices containing water insoluble biopolymers (such as tissues); b) water soluble, non crystallizing and highly collapsing PVP polymeric matrices and c) water soluble, crystallizing lactose systems, showing low or intermediate structural collapse. In PVP systems the maximum NEB rate was observed at 33%R.H.. The Tg value corresponding to this R.H., was close to the storage temperature. Above 33% R.H. the PVP systems readily collapsed, and the NEB rate was relatively low. For lactose systems, the maximum rate of NEB occurred at 43% R.H. At this R.H. value complete lactose crystallization had occurred at the storage temperature (70°C). The maximum NEB rate in milk powder was located between 43 and 75% R.H.. The presence of proteins can be the cause for the higher R.H. for the maximum NEB rate observed in milk powder compared to pure lactose systems. Starch forms a high glass transition matrix which, when mixed with lactose delays crystallization of the sugar. Since lactose is a reducing sugar, NEB rate values were higher in the lactose systems than in the lactose-starch ones, due to a dilution effect of the reactants on one side and to mobility restrictions on the other. The R.H. values for the maximum NEB rate for tissues were: 52% for apple, 43-52% for chicken, 52-75% for cabbage and 75% for potato. It was generally accepted that the maximum NEB rate in food systems is in the range of R.H. between 60% and 80% (2). However, as observed in our results, the relative humidity values for the maximum NEB rate were located at different R.H. values, according to the structural characteristics of the systems. In both non-crystallizing systems forming a liquid-like structure (PVP), and those capable to form a crystalline structure (lactose and milk powder) the R.H. for the maximum NEB rate was in the range 33 to 43%, quite lower than the intermediate R.H. range, and coincident with the structural changes promoted by thermal treatment. The systems which have a stable structure, provided by insoluble components, like vegetable tissues or those containing polymers (such as starch), which can prevent crystallization and collapse, show the maximum NEB rate at high relative humidities (60-80%), in the supercooled (rubbery) state, at least 60°C above the Tg of each system. The maximum for the NEB rate is shifted in these systems towards high RH values due to mobility reasons caused by the presence of insoluble components and by compartmentalization.  Conclusions. The R.H. at which the browning reaction will be maximum is a combination of factors, and can be predicted on the basis of sorption, structural and thermal transition of the matrices where it takes place. The knowledge of the mechanisms involved in NEB reactions can offer the opportunity to develop strategies to control the kinetics of this deteriorative reaction.  Acknowledgment. The authors acknowledge financial support from Universidad de Buenos Aires (UBACYT X226) and CONICET (PIP 02734).  References. 1. Karmas, Buera & Karel, 1992b 2. Labuza & Saltmarch, 1981.