High Pressure Inactivation of Microorganisms Inoculated into Ovine Milk of Different Fat Contents

¹ Unitat de Tecnologia dels Aliments (C.e.R.T.A.), Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain

High hydrostatic pressure inactivation of Escherichiacoli, Pseudomonas fluorescens, Listeria innocua, Staphylococcus aureus, and Lactobacillus helveticus were studied. These microorganisms were inoculated at a concentration between 10⁷ and 10⁸ cfu/ml in Ringer solution and in ovine milk adjusted to 0, 6, and 50% fat content to evaluate the baroprotective effect of fat content on inactivation of microorganisms. Treatments of pressurization consisted of combinations of pressure (100 to 500 MPa) and temperature (4, 25, and 50°C) for 15 min. Gram-negative microorganisms were more sensitive than were Gram-positive ones (more destruction P. fluorescens > E. coli List. innocua > Lb. helveticus > S. aureus). Pressurizations at low temperature (4°C) produced greater inactivation on P. fluorescens, List. innocua, and Lb. helveticus than at room temperature (25°C), whereas for E. coli and S. aureus the results were opposite. Ovine milk per se (0% fat) showed a baroprotective effect on all microorganisms, but percentage of fat (6 and 50%) did not show a progressive baroprotective effect in all pressurization conditions or for all microorganisms.

Key Words: high hydrostatic pressure • bacteria • ovine • milk • fat content

Submitted on April 28, 1999
Accepted on November 16, 1999

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