Abstract
Milk is one of the most widely consumed foods throughout the world. Thermal pasteurisation of milk is practiced in dairy plants to ensure safety of consumers by killing harmful bacteria that may be present in raw milk. The impact of heating milk has been widely studied and found to adversely affect some aspects of quality and functional properties such as drop in pH, denaturation of proteins, and flavour changes. Concerns about heat-induced changes in raw milk have led to the development of non-thermal preservation technologies. Pulsed electric field (PEF) processing is a promising approach to kill harmful bacteria to the same extent as thermal pasteurisation. Most PEF studies have been carried out using skim milk or model suspensions containing proteins and/or fat globules in simulated milk ultrafiltrate or buffers. The effectiveness of PEF to inactivate bacteria is affected by the composition of the treatment media. Further work is warranted to show the efficacy of PEF to inactivate bacteria in more complex matrices than skim milk or model solutions, such as whole milk, where the presence of fat or proteins can provide protection against bacterial inactivation.
In the present study, the PEF pasteurisation was determined by assessing the effectiveness of PEF treatment against the inactivation of Gram-negative (Pseudomonas aeruginosa and Escherichia coli) and Gram-positive (Staphylococcus aureus and Listeria innocua) bacteria inoculated into whole milk (8.3 log cfu mL-1). Milk was pre-conditioned at different temperatures 4 to 55°C for 24 s and PEF-treated in continuous mode with intermediate cooling at electric field intensities ranging from 16 to 26 kV cm-1 for treatment times of 17-101 μs. Gram-negative bacteria were less resistant to PEF treatments than Gram-positive bacteria. PEF treatments in combination with the pre-heating of milk to 55°C were found to be effective at reducing bacterial numbers to below the detection limit (2 log cfu mL-1). This pasteurisation of PEF was later validated in whole milk by evaluating the reduction in numbers of E. coli and L. innocua, as well as the natural micro-flora present in raw milk. The comparable inactivation of bovine alkaline phosphatase after PEF pasteurisation to that of thermal treatments at 63°C for 30 min or 73°C for 15 s was defined as a pasteurisation indicator. The indigenous milk enzymes of technological importance, such as plasmin, and, xanthine oxidase, were partially inactivated by 12%, and, 32%, respectively, whereas lipolysable was fat reduced by 82%.
Milk treated by PEF pasteurisation processing was examined for physico-chemical, microbiological, and enzymatic stability over 21 days at 4°C. By the end of this storage period, the total bacterial counts in milk were much lower (2.8 log cfu mL-1) than the regulations require for drinking milk with good microbiological quality, and the pH was lower; however, enzymatic activity was comparable to that in raw milk. The impact of PEF pasteurisation on whole milk components, such as the milk fat globule membrane (MFGM), was examined. PEF treatment resulted in a decrease in the size of fat globules and a corresponding increase in the surface ζ-potential and specific surface area, which indicated adsorption of milk proteins from the serum phase. The probable adsorption of serum proteins onto the MFGM surface was observed by transmission electron microscopy, and further confirmed by identification using gel electrophoresis and confocal laser scanning microscopy, showing that caseins were primarily adsorbed along with whey proteins. Moreover, endothermic thermograms showed that PEF pasteurisation did not considerably affect protein denaturation temperatures but resulted in ~13% less protein denaturation compared to thermal treatments. A surface hydrophobicity analysis also suggested adsorption of milk proteins from the serum phase, probably due to formation of complexes between denatured whey proteins and caseins through cysteine-hydrophobic sites followed by adsorption to the MFGM surface through disulphide bonds.
This study revealed that PEF treatment in combination with pre-heating has the potential to pasteurise whole milk using the well-defined indicator of alkaline phosphatase inactivation. The activities of plasmin and xanthine oxidase, as well as lipolysable fat, were partially reduced suggesting the suitability of milk treated with pulsed electric fields for making dairy products such as cheese where the presence of enzymes is desirable for flavour development. PEF-induced changes led to less damage to the MFGM and changes to the protein composition compared to the thermal treatments of whole milk.