Enzymatic catalysis in a whey protein matrix at temperatures in the vicinity of the glass transition

- α-Glucosidase retained activity during spray drying of whey protein isolate.
- Below the mechanical Tg, enzyme activity was found to be low.
- Matrix changes at the Tg strongly affected enzyme activity.

The current study evaluates the effect of temperature on α-glucosidase activity, following incorporation of the enzyme into a whey protein matrix through spray drying. Thermomechanical characterization of the matrix was achieved using the techniques of modulated temperature differential scanning calorimetry and small-deformation dynamic mechanical analysis. As the concentration was raised from 75 to 94% (w/w), denaturation of the protein occurred at increasing temperatures. In contrast, denaturation was not observed in calorimetric scans after spray drying. The glass transition temperature (Tg) measured in the dried particles using dynamic mechanical analysis was approximately 40 °C. An optimized procedure was developed whereby α-glucosidase and its substrate p-nitrophenyl α-d-glucopyranoside were incorporated into the whey matrix. The effect of temperature on enzymatic catalysis was investigated and, below 40 °C, activity was low and relatively independent of temperature. However, the rates of product formation markedly accelerated as temperatures were increased beyond Tg. These novel observations strongly emphasize the pronounced effect of mechanical Tg of the protein matrix on enzymatic activity.