Stability of bovine immunoglobulins to thermal treatment and processing

Thermal stability of bovine immunoglobulin (IgG) in model systems and in commercially processed milk products was investigated. Bovine serum IgG dissolved in either phosphate buffered saline (PBS), boiled milk or ultra high temperature (UHT) sterilized milk was heated at temperatures ranging from 62.7 °C to 80 °C. The D-values ranged from 90, 200 and 170 s at 80 °C to 25.5, 27.2 and 32.8 min at 72 °C for IgG in PBS, boiled milk and UHT milk, respectively. These results suggest slightly greater thermal stability of IgG in milk than in phosphate buffer. IgG content was not changed after 30 min at 62.7 °C, nor by holding for 24 h at either ambient temperature or 4 °C. Over the temperature range from 72 ° to 80 °C, z-values were 6.7, 8.9 and 8.5 °C, and energies of activation were 353.5, 258.2 and 298.5 kJ mol−1 for thermal destruction of bovine serum IgG in PBS, boiled milk and UHT milk, respectively. These findings in model systems suggest that commercial pasteurization processes should not result in complete destruction of IgG. Comparison of IgG content in raw milks and corresponding HTST-pasteurized milks of varying fat content indicated 59–76% retention after pasteurization, with no apparent effect by either homogenization, skimming or standardization to 1 or 2% fat levels. The IgG contents and specific antibody activity against lipopolysaccharide fractions of five bacteria were determined for several commercially processed milk products. HTST-pasteurized milks, reconstituted skim milk powder and whey from cheddar cheese production all showed high levels of IgG and specific antibody activity. However, canned evaporated milk and ultra high temperature (UHT) sterilized milk had little or no IgG. This study demonstrates the dependence of bovine IgG stability in milk products on severity of thermal treatment used in various commercial processes.