Comparison of the effect of heating on the thermal denaturation of nine different β-lactoglobulin preparations of genetic variants A, B or A/B, as measured by microcalorimetry

Three samples each of the pure A and B genetic variants together with three samples of the mixed A/B variants of beta-lactoglobulin were studied by differential scanning microcalorimetry. All the samples were characterised with respect to whey protein composition and shown to be almost pure beta-lactoglobulin. Other analyses showed only minor amounts of other contaminants but variable degrees of lactolation ranging from < 1.9 to 10.4 mol%. Thermograms were recorded at about 3 mg beta-lg mL(-1) in a 10 mM P-i buffer pH 7, with the ionic strength of milk, using a scan rate of 60 degrees C h(-1) on both Microcal MC-2 and MCS calorimeters and reasonable agreement was found for the denaturation temperature measured on the two instruments. The biggest source of variation in the thermograms of samples was due to the genetic variant. For the single variants, other differences were in the same order as the variability seen in repeat preparations of the same sample. A slightly greater variation was found among the A/B samples where the one with the lowest degree of lactolation was most stable. However, dialysis of the sample against the P-i buffer almost completely eliminated the difference. Qualitatively, all thermograms appear to be the sum of 4 processes centred at temperatures of approximately 55, 77, 105 and 125 degrees C. Compared to the A variant the thermogram of the B variant shows a less prominent shoulder at about 55 degrees C. Also, the broad feature at about 105 degrees C is reduced and the peak at 125 degrees C much enhanced. (C) 1998 Elsevier Science Ltd. All rights reserved.