Relationships between the maximum rate of non-enzymatic browning, relative humidity and structural changes

NURIA ACEVEDO; CAROLINA SCHEBOR; MARÍA DEL PILAR BUERA

Water properties of food, pharmaceutical and biological materials

CRC Press -Taylor and Francis Group

Lugar: Boca Raton; Año: 2006; p. 623 - 630

The rate and extent of non-enzymatic browning (NEB) are highly dependent on the availability of water. In solid systems NEB rate is expected to show a maximum value at intermediate relative humidities (R.H.), and in liquid systems the rate diminishes continuously while R.H. increases. The possibility to predict the R.H. range at which NEB rate is maximum, on the basis of specific properties of the systems, would allow improving food stability control. The purpose of the present work was to establish the relationship between the relative humidity for the maximum NEB rate and the occurrence of thermal transitions and structural changes taking place upon thermal treatment. Thus, analysed systems were selected in order to have different collapse and crystallisation behaviour in the RH scale when submitted to different temperatures. The analysed model systems consisted of freeze-dried aqueous suspensions containing 10% w/w polyvinylpyrrolidone (different molecular weights), lactose, native or gelatinized wheat starch and their mixtures. Maillard reactants (1% w/w glucose and glycine) were also added. Food systems consisted in freeze-dried cabbage and chicken meat. Water sorption data were determined for all the systems at 25°C. The samples equilibrated at several R.H. were incubated at various temperatures (43-70°C). The thermal transitions of the systems were determined by differential scanning calorimetry. NEB degree was determined by measuring absorbance at 445 nm or reflectance in a spectrocolorimeter. The degree of browning reached by the systems was more dependent on the water content in lactose and PVP systems than in starch containing systems or tissues. The R.H. for the maximum NEB rate was observed to be dependent both on the composition and on the physical state. In the polymeric collapsing systems the maximum NEB rate was located at R.H. values corresponding to systems of glass transition temperatures (Tg) close to the storage temperature, in non-collapsed samples. For the selected conditions the R.H. values were lower than 30%. In these systems the zone at which NEB rates decreased was located above Tg and could be related to the point at which the systems became fluid. In the lactose systems NEB rate increased when the sugar crystallised, due to the increase of reactant concentration in the non-crystallised parts of the matrix, or to the water released during lactose crystallisation. However, the presence of structural components affecting shrinkage and collapse modified the dependence of browning rate on the water content, making the maximum rate to shift towards higher R.H. values, as was observed for vegetable and animal tissues, or for those containing starch.