3.1. Freezing effects
The effect of frozen storage at −18 °C on the survival of S.
Typhimurium DT12 and DTU292 in minced beef meat is illustrated in
Fig. 2 showing exponential phase cells to be noticeably more sensitive
than stationary phase cells (P b 0.05). For both strains in stationary
phase, the viable counts were only marginally affected by short- and
long-term freezing by approximately 0.1 log10 units of reduction. In
contrast, the reduction of exponential DTU292 cells during frozen storage
was significantly more sensitive than exponential DT12 cells
(P b 0.05) after 7 d of freezing. The average reductions were 0.6 log10
units and 0.9 log10 units of exponential phase DT12 and DTU292 cells,
respectively. However, there was no statistical difference in reduction
for the two strains after N35 d of storage (P = 0.18), with a reduction
of approximately 1 log10 unit. In comparison, there was also no difference
in reduction of the two S. Typhimurium strains in the stationary
phase after N35 d of frozen storage (P = 0.41) with a reduction of maximum
0.2 log10 units.
3.2. Fermentation parameters and LAB activity
Lactic acid bacteria (LAB) were enumerated and the pH was monitored
during sausage fermentation (Fig. 3). In sausages inoculated
with ~7 log10 cfu g−1 starter culture, LAB had increased to 9 log10 cfu g−1
approximately after 20 h of fermentation. At the tested concentration,
NaNO2 did not exert any antagonistic effect on the growth of this starter
culture. The high starter culture inoculum resulted in a fast pH decline
starting after 5–6 h of fermentation (Fig. 3A). During spontaneous fermentation,
indigenous LAB grew 4–5 log10 units, reaching levels of
8 log10 cfu g−1 after 25 h in the absence of NaNO2 and after 48 h in
the presence of NaNO2 (Fig. 3B).