Anna Pitotti

Polyphenoloxidase and peroxidase activity in partially frozen systems with different physical properties

Abstract Enzyme catalysed reactions can occur at significant rates at low temperatures, thus exerting a major control on the quality of frozen foods. The temperature-concentration effect in partially frozen systems can make enzyme reactions controlled by diffusion and thus rate limited by viscosity according to the Stokes–Einstein relation. Polyphenoloxidase (PPO) activity in sucrose, glycerol and fructose water media and (POD) activity in fructose and glycerol water media, were assayed spectrophotometrically from 20°C to −30°C. The media have at equal sub-freezing temperature marked differences in viscosities and others physical properties. For both enzymes, the activity data showed similar dependence on temperature and, above freezing, on the concentration of the media. A relation with the viscosity of the media at equal sub-freezing temperature was not evident. POD activity showed a “break” between −10°C and −20°C, suggesting a reversible conformational change. The temperature dependence of PPO rate constants were well described by both the Williams–Landel–Ferry ( r 2 =0.998) and the Arrhenius ( r 2=0.985) models. A hypothesis for the non conformity of the measured kinetics with the theoretical model is that in the temperature range considered the reactions are still activation controlled or in a transition zone, and should become diffusion limited only at lower temperatures and higher viscosities.

Improved activity in acidic media of immobilized lysozyme

Polyethylene glycols (PEGs) of various chain length were used tocrosslink lysozyme onto an insoluble support such as oxirane. A very high degree of modification and noinactivation of lysozyme were obtained with PEG 20000, but enzymatic activity increased up to 20 times at pH 3.0, at which point the activity of the native enzyme was lower when using Leuconos tok oenus as a macromolecular substrate.

Genetic regulation of trypsin inhibitory activity in soybean flour

Many biochemical and molecular details are available for soybean seed proteinase inhibitors, but little is known about the quantitative regulation of structural genes. In order to fill this gap, a complete set of diallel crosses was made between inbred lines characterised by the same inhibitor array coupled with a different inhibitor content in the raw flour. The Hayman analysis revealed the presence of different regulatory elements in the parental lines, giving rise to both additive and dominance variation. Dominant alleles were found to decrease the trypsin inhibitory activity (TI activity) shown by the raw flours and appeared to have higher frequencies than recessive alleles. Significant maternal effects were also detected, particularly in crosses of low-TI activity female x high-TI activity male; maternal effects strengthened the role of the regulatory genes transferred by the female parent to the hybrid. Data fitted the simple additive-dominance model with genes independent in both action and distribution. Narrow and broad heritability values were 54% and 82% respectively, thus indicating the feasibility of lowering TI activity through selection.

The effects of β-lactoglobulin genetic variants A and B on the functional properties of whey under different conditions

Abstract Partially delactosated and demineralized whey protein powders were purified from pooled milks of individual cows homozygous for the β -lactoglobulin ( β -Lg) genetic variants A and B ( whey A and whey B). The protein composition of the two samples was similar. The two whey protein powders were dissolved in water at different pH (2, 4.5, 6.8) and NaCl concentrations (150 and 400 mmol/dm 3 ), and emulsions were made utilizing grape-seed oil. Emulsion stability (ES), emulsion activity index (EAI) and emulsion capacity (EC) were examined. The results showed the influence of the two genetic variants of β -Lg on the functional behaviour of whey. Emulsion obtained from B powder solutions showed the higher stability at the different test conditions. The best results occurred at pH 6.8 with no significant differences between the two tested NaCl concentrations. At pH2 and 400 mmol/dm 3 NaCl, both samples did not demonstrate any stability. Data on emulsion capacity indicated that whey B is less affected by the different physicochemical conditions tested (pH, NaCl concentration, heat treatment at 90°Cl5 min), while whey A gave the highest and the lowest EC values depending on the experimental conditions.