Evaluation of qPCR and plate counting for quantifying thermophilic starters in cheese

The respective inputs of plate counting and qPCR for the quantification of starters in cheese were evaluated using hard-cooked cheeses made with various starter combinations. Five starter strains were quantified at their different growth phases, from 0.5 h to day 214 of manufacture: one strain of Streptococcus thermophilus (ST) and two strains each of Lactobacillus delbrueckii (LD) and Lactobacillus helveticus (LH). Numbers of colony-forming units (CFU) were obtained by plate counting (PC) and qPCR (GN_CFU). The qPCR standard curves require a special attention since GN_CFU depends on the degree of culturability of the standard culture. Discrepancies were evidenced from the vat milk to the end of ripening. During cheese making, GN_CFU were lower than PC at the inoculation for all ST and LD samples and 83% of the LH samples, and during both the exponential and stationary phases for many of the ST and LD samples. During ripening which corresponds to the decline phase, GN_CFU were higher than PC for 90% of the ST, 78% of the LD and 69% of the LH samples. Hypotheses are discussed to explain those discrepancies. The data provided by GN_CFU and PC complement each other, providing a better description of starter growth in cheese.