Survival of Salmonella Tennessee, Salmonella Typhimurium DT104, and Enterococcus faecium in Peanut Paste Formulations at Two Different Levels of Water Activity and Fat

Abstract

Long-term survival of heat-stressed Salmonella Tennessee, Salmonella Typhimurium DT104, and Enterococcus faecium was evaluated in four model peanut paste formulations with a combination of two water activity (aw) levels (0.3 and 0.6) and two fat levels (47 and 56%) over 12 months at 20 ± 1°C. Prior to storage, the inoculated peanut paste formulations were heat treated at 75°C for up to 50 min to obtain an approximately 1.0-log reduction of each organism. The cell population of each organism in each formulation was monitored with tryptic soy agar plate counts, immediately after heat treatment, at 2 weeks for the first month, and then monthly for up to 1 year. The log reductions (log CFU per gram) following 12 months of storage were between 1.3 and 2.4 for Salmonella Tennessee, 1.8 and 2.8 for Salmonella Typhimurium, and 1.1 and 2.1 for E. faecium in four types of model peanut paste formulations. Enhanced survivability was observed in pastes with lower aw for all organisms, compared with those with higher aw (P < 0.05). In contrast, the effect of fat level (47 and 56%) on survival of all organisms was not statistically significant (P > 0.05). Whereas survivability of Salmonella Tennessee and Typhimurium DT104 did not differ significantly (P > 0.05), E. faecium demonstrated higher survivability than Salmonella (P < 0.05). Salmonella survived in the model peanut pastes well over 12 months, which is longer than the expected shelf life for peanut butter products. The information from this study can be used to design safer food processing and food safety plans for peanut butter processing.

Influence of Water Activity on Thermal Resistance of Microorganisms in Low‐Moisture Foods: A Review

“…The D ‐values of Salmonella strains at 71 °C ranged from 26.5 to 30.6 min while at 90 °C, the D ‐values ranged from 8.6 to 13.4 min (Ma and others ). Kataoka and others () reported greater survival of selected Salmonella strains and Enterococcus faecium at lower a w (0.3 compared with 0.6) in a peanut butter formulation stored for 12 mo at 20 °C after heat treatment at 75 °C for 25 to 50 min. However, a greater fat content (56% compared with 47%) in peanut butter formulation did not influence the survival of the selected bacteria, so that >47% may offer no further protective effect of fat on the survivability of bacterial cells (Kataoka and others ).…”

“…Kataoka and others () reported greater survival of selected Salmonella strains and Enterococcus faecium at lower a w (0.3 compared with 0.6) in a peanut butter formulation stored for 12 mo at 20 °C after heat treatment at 75 °C for 25 to 50 min. However, a greater fat content (56% compared with 47%) in peanut butter formulation did not influence the survival of the selected bacteria, so that >47% may offer no further protective effect of fat on the survivability of bacterial cells (Kataoka and others ). Ababouch and Busta () observed greater thermal resistance (higher D ‐ and z ‐values) of Bacillus cereus , Clostridium botulinum , and C. sporogenes spores suspended in oil (olive oil and commercial oil containing rapeseed oil and soy oil) compared to aqueous buffer (pH of 7.2).…”

Development of a Dry Inoculation Method for Thermal Challenge Studies in Low-Moisture Foods by Using Talc as a Carrier for Salmonella and a Surrogate (Enterococcus faecium)

“…Furthermore, E. faecium proved to be an appropriate surrogate for Salmonella in low-moisture food matrices consistently showing better survival and greater heat resistance than that of Salmonella, without being overly conservative. Previous reports (13,24) also indicated E. faecium was a suitable surrogate for long-term survival challenge studies for Salmonella in peanut butter products. More research on the characteristics of the dry inocula is warranted, for example, by investigating stability over longer periods of time and thermal resistance over a wider temperature range.…”

“…After drying the inoculated talc was sieved through a sterile stainless steel fine mesh (0.7 by 0.7 mm) strainer to break up small clumps. The inocula were stored in sterile plastic bottles with the lid tightly closed, and the lid seam was covered with parafilm and held at room temperature for the duration of the study (13,14,24). This point, after sieving, completes the preparation of the dry inocula and is considered day 1, and the means and standard deviations for inoculum levels and a" are presented in Table 2.…”

“…Although the aw value used to define low-moisture foods can vary, many references, including the Codex Alimentarius Commission, use the value of <0.85 (10). Although Salmonella cannot grow in these low-moisture foods, foodbome outbreaks and scientific studies have demonstrated that Salmonella can survive for lengthy periods of time (13,24). Salmonella outbreaks have been associated with a range of low-moisture food products such as spices, chocolate, and peanut butter, where cell levels may have been as low as one to three cells per gram (11,12,18,37).…”