Effects of high hydrostatic pressure in the microbiological, microscopical and physicochemical properties of milk

As consumers demand more minimally processed and highly nutritious products, emerging technologies are being applied as alternative techniques to improve specific characteristics in food products. Such is the case with the application of high hydrostatic pressure an alternative to thermal pasteurization in milk. Previous research has mainly focused on the microbiological effects of high pressure treatments, such as the inactivation of pathogenic bacteria and other microorganisms. More recently, the focus has shifted to the effects on the physicochemical and functional properties of milk and other dairy products. In the present study, inoculated milk with Escherichia coli, Salmonella enterica, Staphylococcus aureus, Listeria monocytogenes and Listeria innocua were subjected to high pressure treatments at 400, 500 and 600 MPa, from the come-up time up to 15 min. The obtained inactivation results were analyzed through a mathematical model and the implementation of a new parameter, F, which depict the time required to achieve a specific number of logarithmic (log10) inactivation was proposed. Staphylococcus aureus was the most baroresistant bacteria and, in order to obtain seven log10 reductions for all five microorganisms at 25°C a 600 MPa, 5.77 min treatment is required. The experimental data adjusted more appropriately to the Weibull primary model and the Bigelow and Le Chatelier secondary models were also applied. A microscopical analysis of casein micelles through scanning electron microscopy and of milk fat globules though optical microscopes was conducted. Casein micelle size, shape and 7 dispersion were greatly affected by high pressure processing, agglomeration and fragmentation were also observed. A downwards trend in casein micelle diameter was detected as pressure treatments grew in intensity. Milk fat globules also presented agglomeration in treated samples, globules with increased diameters were present in treated samples. The effects combining high pressure and heat were also analyzed. After subjecting previously homogenized, raw whole and skim milk to a 600 MPa, 35°C, 5 min, the effect on the previously mentioned five microorganisms, native microorganisms, pH values, acidity, color parameters and emulsion stability were studied. All five microorganisms had more than seven log10 reductions, the native microorganisms were completely inactivated, pH and acidity values were not statistically different, color parameters presented significant differences and the emulsion stability was unchanged.