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 <i>Salmonella</i> from peanut products after different storage times was evaluated. The samples were inoculated with 5-6 log cfu g^-1 of <i>Salmonella</i> 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 (<i>p</i> < 0.05); however, it quantified both viable and non-viable cells. For roasted peanuts, a significant difference (<i>p</i> < 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 <i>paçoca</i> showed a significant difference (<i>p</i> < 0.05) among the three methods evaluated. In peanut brittle, qPCR-PMA detected 1.5 log cfu g^-1, while, in the culture method, <i>Salmonella</i> was recovered in 1 g. The pathogen was below the detection limit in <i>pé-de-moça</i> either by plate count or qPCR-PMA. Therefore, qPCR-PMA shows potential for use in quantifying <i>Salmonella</i> in peanut products.