Effect of Water Activity and Temperature on Staphylococcus aureus Growth and Thermonuclease Production in Smoked Snoek

Abstract

Smoked snoek at different water activity (aw) values (0.966, 0.956 and 0.944) were inoculated with Staphylococcus aureus at a concentration of approximately log 4.7 colony forming units per gram (CFU/g) and incubated at different temperatures (4, 12, 24 and 37 C) for various periods. Samples of inoculated and control snoek were taken at different intervals to determine the microbial populations (S. aureus, total organisms and enterococci), pH and presence of thermonuclease. No growth of S. aureus nor thermonuclease production occurred at 4 C. At 12 C, notwithstanding high S. aureus log counts of 6.75/g, 9.74/g and 9.69/g in the inoculated 0.966, 0.956 and 0.944 aw samples, respectively, thermonuclease was absent after 21 days of incubation. At 24 and 37 C at all three aw levels, very high S. aureus counts were recorded and were accompanied by thermonuclease production before the end of the relative incubation periods (5 and 3 days, respectively). A reduction of the aw of the snoek resulted in a slightly slower S. aureus growth rate at 37 C and also delayed thermonuclease production at 24 and 37 C. The thermonuclease test was an effective means of detecting S. aureus growth except in samples incubated at 12 C. Enterococcus numbers increased in all control samples during storage, but none were found to produce thermonuclease.