Milk is a valuable source of nutrients in human nutrition, but the composition could be altered as a result of physical interventions. Due to safety requirements, shelf life and consumer preferences, milk is normally pasteurised, homogenised and sometimes spray dried. Homogenisation is done using pressure and heat, and this causes changes in the location of proteins in the milk as well as protein modifications which can affect nutritional characteristics. The intention of the study cited was to investigate these proteomic changes further and to what extend they are variable.
Unprocessed milk was pasteurised at 72oC for 15 seconds and then, to separate the tested treatments for homogenisation, were heated at different pressures and temperatures: respectively heated at 45oC without pressure applied; heated at 45oC with 35MPa; 80oC without pressure applied; and, 80oC with 35MPa. Proteomic analysis was done after separating the milk into three fractions: whey, casein and cream. Protein modifications in each fraction were examined and characterised to compare protein modification sites and differences in proteins present in the fractions resulting from homogenisation and/or pasteurisation.
The results showed that both heat and pressure during homogenisation caused increases in protein modifications as a result of oxidation or the Maillard reaction, which differed in the milk fractions. The extreme homogenisation pressure and temperature (80oC with 35MPa) changed the structure of cream to a point where it could not be separated from milk. Total cysteine and total proline oxidation differed between treatments. While heat treatment resulted in increased modifications of cysteine, proline and total oxidation in both whey and cream fractions, homogenisation led to a decrease in cysteine oxidation for whey fractions, but an increase in cysteine and total oxidation for cream fractions. By examining α-S1-casein, lactadherin and β-lactoglobulin specifically, the results showed that protein modifications occurred in the same location in the protein sequence and not in random locations.
In conclusion, this work provided knowledge of where proteins are susceptible to modifications. By understanding the pressure and temperature effects and when they occur, treatment can be modified to reduce unwanted side effects. Since both temperature and pressure affected the degree of protein modification, low homogenisation pressures and temperatures are recommended.