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The thermal and chemical resistance levels of four autochthonal bacteriophages of Lactococcus lactis subsp. lactis, isolated from cheese processes, was investigated. The times required to obtain 99% inactivation of phages (T99) at 63 and 72 degrees C in three suspension media (M17 broth, reconstituted commercial nonfat skim milk, and Tris magnesium gelatin buffer) were determined. Thermal resistance was dependent on the phage studied, and the results of this study demonstrate that pasteurization treatments used in dairy industries may leave viable viral particles in milk. It was possible to determine that M17 broth was generally the least protective medium, while phosphate buffer was the most protective one. Peracetic acid (0.15%, vol/vol) was the most effective viricidal agent, with exposures of 5 min being sufficient to inactivate high-titer phage suspensions (>10(6) PFU/ml). To achieve total inactivation (<10 PFU/ml) of viral suspensions, sodium hypochlorite was effective at 100 ppm for only two phages, while the other two phages needed concentrations of 200 and 300 ppm. Ethanol at concentrations of 100 and 75% proved to be very efficient in inactivating phages, but isopropanol was not effective against them.

The effect of several biocides and thermal treatments on the viability of four Lactobacillus helveticus phages was investigated. Times to achieve 99% inactivation of phages at 63 degrees C and 72 degrees C in three suspension media were calculated. The three suspension media were tris magnesium gelatin buffer (10 mM Tris-HCl, 10 mM MgSO4, and 0.1% wt/vol gelatin), reconstituted skim milk sterile reconstituted commercial nonfat dry skim milk, and Man Rogosa Sharpe broth. The thermal resistance depended on the phage considered, but a treatment of 5 min at 90 degrees C produced a total inactivation of high titer suspensions of all phages studied. The results obtained for the three tested media did not allow us to establish a clear difference among them, since some phages were more heat resistant in Man Rogosa Sharpe broth and others in tris magnesium gelatin buffer. From the investigation on biocides, we established that sodium hypochlorite at a concentration of 100 ppm was very effective in inactivating phages. The suitability of ethanol 75%, commonly used to disinfect utensils and laboratory equipment, was confirmed. Isopropanol turned out to be, in general, less effective than ethanol at the assayed concentrations. In contrast, peracetic acid (0.15%) was found to be an effective biocide for the complete inactivation of all phages studied after 5 min of exposure. The results allowed us to establish a basis for adopting the most effective thermal and chemical treatments for inactivating phages in dairy plant and laboratory environments.