ABSTRACT
Activity of endogenous enzymes may cause browning of fruits and vegetables. These enzymes can be inactivated, for example by heat treatment, but the response of enzymes to heat treatment depends on many factors. Foods are very complex systems and the stability of enzymes in such an environment can be influenced by other components of the system potentially leading to enhancement of unwanted reactions or reduction of wanted ones.

Caseins can potentially enhance the stability of other food proteins, as they were
shown to exhibit chaperone-like activity. In this study influence of β-casein on the enzymatic activity of three targets was tested by monitoring enzyme activity after heat treatment and by measuring the intensity of scattered light during and after heat treatment.

β-Casein was shown to interact at elevated temperatures with three selected targets:horseradish peroxidase, tyrosinase from Agaricus bisporus and equine
cytochrome c. Only for the first target β-casein was acting as a molecular chaperone i.e. its presence resulted in higher residual activity (higher degree of the function preservation). β-Casein did not have any influence on the residual activity of tyrosinase. Surprisingly, peroxidase activity of cytochrome c was increasing after heat treatment with β-casein (up to 518±9%). This indicates loss of the cytochrome c native conformation.

Presence of bovine serum albumin (BSA) during heat treatment did not affect residual activity of tyrosinase and cytochrome c. Surprisingly, residual activity of horseradish peroxidase was lower in samples containing BSA than in samples without any addition. Horseradish peroxidase heated with BSA did not regain activity within one hour after treatment. BSA is often added to enzyme solutions to prevent enzyme adhesion to vial surfaces. The negative effect of BSA on enzyme was not observed before.

The residual activity of horseradish peroxidase was also improved by the
reconstituted skim milk: addition of reconstituted skim milk prior to heat treatment resulted in higher residual activity of HRP compared to no addition (58±3% and 30±1%, respectively)

The findings of this study show that β-casein can influence the response of food
enzymes to heat treatment. β-Casein is not a universal chaperone and its effect on different targets needs to be evaluated on a case-by-case basis. This study also shows that proteins as e.g. BSA may affect the stability of food enzymes