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of raw bovine milk, with a second homogenization cycle causing an additional inactivation ratio roughly equal to that achieved during the first cycle. Also, Wuytack et al. (2002) investigated the effect of cyclic high-pressure homogenization treatment on Y. enterocolitica and S. aureus, finding that the different rounds have an additive effect. Wong et al. (1997) previously claimed that it can be assumed the disruption of E. coli is a first-order process. In contrast, evidence of strong dependence on initial bacterial concentration was obtained by some authors. Moroni et al. (2002) reported that the effectiveness of HPH in inactivating lactococcal bacteriophages in PBS was affected by the initial concentration; with greater initial load, the treatment became less effective. Tahiri et al. (2006) also demonstrated that the effectiveness of the HPH increases at low initial bacterial concentration of different microbial strains (L. plantarum, Penicillium ssp., and S. cerevisiae). Analogously, Vachon et al. (2002) reported that the effectiveness of treatment of Listeria monocytogenes, Escherichia coli, and Salmonella enterica in PBS by HPH was dependent on the initial cell concentration, with a decreasing efficacy at increasing concentration. Recently our research group undertook a detailed investigation of HPH effects on Saccharomyces cerevisiae (Maresca et al., 2006) and E. coli (Dons覺` et al., 2006) in a Stansted homogenizer. Results showed that the extent of inactivation significantly depended on the initial cell concentration; the higher the inactivation the lower the concentration. Nevertheless, upon multiple steps of homogenization, an opposite effect was observed, showing reduced efficiency with increased number of passes. A possible explanation was attributed to the protective effect exerted by high cell concentration on one side, and the inefficiency of homogenization below a certain concentration level (Dons覺` et al., 2006; Maresca et al., 2006). A general conclusion can not be drawn, as the results reported in the literature on the subject are contradictory and cannot be applied to the different valve geometries of the homogenizers, since a different effect due to initial cell load was reported among homogenizers with the same class of valve geometry, and similitudes can be found among homogenizers with different disruption valves.
IV.D. Medium properties influencing microbial inactivation At a macroscopical level, the parameters that mostly affect microbial inactivation by HPH are fluid temperature, viscosity, water activity, and medium composition. IV.D.1. Temperature effect Temperature effects must be taken into account with HPH treatment, since upon homogenization, an important rise of temperature is observed in the