Purpose: Two species of bacteriophages, identified for use as surrogates, were assessed for their effect of food safety control measures; pH and water activity (aW) on the infectivity of viruses.
Methods: In the pH studies, solutions of nutrient broth adjusted to various pH levels with hydrochloric and citric acid were prepared. Water activity was studied by adjusting solutions to aW 0.88 in water with sodium chloride, sucrose, or glycerol. Both pH and aW solutions were inoculated with a cocktail of both MS2 and φX174. The inoculated solutions were stored at 5⁰C for 6 months (pH broths) or 25°C for 2 months (aW broths). At set time points, the level of both bacteriophages were enumerated using the double layer agar plaque assay method.
Results: Data showed that the viruses were capable of surviving at pH values of 3-7 for at least 60 days. Both viruses had the lowest resistance to pH 2; with φX174 inactivated after 4days (6.8 log pfu/ml reduction) and MS2 after 18days (6.9 log pfu/ml reduction). With regards to aW, the bacteriophages showed greater stability in salt solution than in water. Sucrose, and then glycerol, had the largest effect on reducing infectivity for both surrogates. A one log pfu/ml reduction, in the sucrose solution, occurred in 4-5 days for φX174 and 1-2 days for MS2.
Significance: Trial results indicated that MS2 and φX174 were capable of persisting under low water activity conditions and at low pH; this demonstrates the dangers of assuming conventional antimicrobial processes are effective against viruses.