Changes in water activity and some microbial groups during storage of pet feed

The effect of exposure of different pet feed samples to the typical temperature and moisture conditions of many months of the year (9 degrees C and 80% RH) on water activity (aw), pH and growth of various microbial groups (mesophilic aerobic bacteria, fungi and yeasts) was studied. The storage conditions were found to scarcely influence the samples pH, with no significant changes relative to the initial values (the standard deviation was 0.04 and the coefficient of variation 0.85%). By effect of exposure to the temperature and moisture conditions tested, the initially stable feeds reached moisture contents close to 20% and aw values near those of equilibrium with the ambient relative humidity during storage. Microbial counts increased with increasing aw, with final means of 8.44, 9.16 and 8.71 log CFU/g for mesophilic aerobic bacteria, fungi and yeasts, respectively. The risk of mycotoxin production at these aw values is discussed. Careful control of moisture is recommended in order to preserve the quality of pet feed throughout its scheduled shelf life.