Quantification of Viable Salmonella by Propidium Monoazide Real-Time PCR After Long-Term Storage of Peanut Products

In this study, the performance of quantitative PCR, combined or not with propidium monoazide (PMA), to recover Salmonella from peanut products after different storage times was evaluated. The samples were inoculated with 5-6 log cfu g^-1 of Salmonella Typhimurium ATCC 14028 and stored at 28 °C for up to 540 d. The correlation between the threshold cycle number (Ct) and the colony-forming units (cfu) was obtained by a standard curve, which showed a linear correlation (R² = 0.97). The highest counts were recovered by qPCR (p < 0.05); however, it quantified both viable and non-viable cells. For roasted peanuts, a significant difference (p < 0.05) between qPCR-PMA and the culture method was verified only for samples stored for 30 d, i.e., 2.8 versus 4.0 log cfu g^-1. Further, there was no VBNC status in the roasted peanuts, even after long-term exposure to desiccation stress. For peanut-based products, after 540 d, only paçoca showed a significant difference (p < 0.05) among the three methods evaluated. In peanut brittle, qPCR-PMA detected 1.5 log cfu g^-1, while, in the culture method, Salmonella was recovered in 1 g. The pathogen was below the detection limit in pé-de-moça either by plate count or qPCR-PMA. Therefore, qPCR-PMA shows potential for use in quantifying Salmonella in peanut products.