EFFECTS OF PH ON GELATION AND EMULSIFICATION OF A BETA-LACTOGLOBULIN CONCENTRATE

The thermodynamic parameters of heat denaturation of 5-lactoglobulin solutions (2.5 less-than-or-equal-to pH less-than-or-equal-to 8) are determined by differential scanning calorimetry and the rheological properties of heat induced gels are studied in compression with an Instron Universal Testing machine. The emulsifying activity index is determined by a turbidimetric method and by calculation from the globule size distribution measured by laser diffraction. The thermal stability of the protein, evaluated from the denaturation temperature, is maximum at the isoelectric pH in citric acid/NaOH buffer and at pH 3.5 in distilled water. The incipient gelation temperatures is compared to the denaturation temperatures T(S), determined from the onset of increase of the heat flow which are slightly lower than the gelation temperatures, except at pH 4.8 and at pH 8 where they are higher or equal, respectively. The emulsifying activity index is relatively low at pHs less-than-or-equal-to 4.8 and it increases with higher pHs. The results are discussed in terms of free thiol groups oxidation and of SH/S-S interchange reactions occuring during heating.