Impact of almond moisture, almond cultivar and Salmonella Serovar on the desiccation, persistence and heat resistance of Salmonella in almonds.

Since 2001, there have been three outbreaks of salmonellosis associated with consumption of raw almonds. More is known about the ecology of Salmonella in almond production and processing environments than for other nuts, however, significant and important questions remain. Most of the research to date has been done with a single almond cultivar (Nonpareil) and a single strain of Salmonella (Enteritidis Phage Type (PT) 30) that was isolated from the initial outbreak in 2001. The assumption was made that this combination represented the “worst case” that the almond industry would face. However, since 2001 other types of Salmonella have been associated with outbreaks linked to a number of other low-moisture foods (e.g., peanut butter). Given the heightened awareness of the importance of Salmonella in low-moisture foods it is prudent to reevaluate the initial assumption. The objectives of the study are 1) to determine the impact of almond moisture and cultivar on the heat sensitivity of Salmonella Enteritidis PT30 inoculated onto almonds; and 2) to evaluate differences in drying, storage and heat sensitivity on almonds among different Salmonella isolates. The results of this study will provide a database of information on the storage, heat and drying resistance of Salmonella serovars on almonds. This information will enhance confidence in thermal and other validation studies for almonds that are based on S. Enteritidis PT 30.

Nuts and other low-moisture foods have generally been considered low-risks for foodborne illness because they are consumed in a dry state where water activity (available moisture) is too low to support microbial growth. However, it is increasingly recognized that many foodborne pathogens can cause illness at very low concentrations, such that microbial growth is not required. Since 2001, three outbreaks of salmonellosis associated with consumption of raw almonds have been documented. Since that time significant advances have been made in understanding the ecology of Salmonella in the almond production and processing environment. Much of the work has been done with Salmonella Enteritidis Phage Type 30, the strain associated with a 2000-2001 outbreak and with a single almond cultivar. Although moisture levels are known to impact survival of Salmonella during thermal processing, a systematic evaluation of raw almond moisture that occurs in practice (3 to 8%) and heat resistance of Salmonella has not been made. The objectives of the study are 1) to determine the impact of almond moisture and cultivar on the heat sensitivity of Salmonella Enteritidis PT30 inoculated onto almonds; and 2) to evaluate differences in desiccation, storage and heat sensitivity among different Salmonella isolates. The results of this study will provide a database of information on the storage, heat and drying resistance of Salmonella serovars on almonds. This information will enhance confidence in thermal validation studies and the risk assessment for almonds that are based on S. Enteritidis PT 30 and should provide a standardized protocol for evaluating potential surrogates for use in these types of studies.

Objective 1: To determine the impact of almond moisture and variety on the heat sensitivity of Salmonella Enteritidis PT30 inoculated onto almonds. 

Objective 2: To evaluate differences in desiccation, storage and heat sensitivity among difference Salmonella isolates.

Relative humidity rapidly impacts the percent moisture and water activity of almonds. In a short storage study (30 days) humidity (36 or 70%) did not impact the storage survival of Salmonella. However, moisture levels significantly impacted the efficacy of oil roasting with significantly greater reductions observed at higher moisture levels. Moisture levels should be routinely monitored in thermal validation studies after inoculation and drying of almonds. The current standard methods for preparation of inoculated almonds should be modified to include longer drying periods after inoculation and evaluation of moisture and water activity prior to initiating the study. A target moisture level should be specified in the protocol. At 121°C the heat sensitivity of four Salmonella isolates was similar in oil roasting. The sensitivity of these isolates to 127°C should also be compared. Salmonella Enteritidis PT30 is more desiccation tolerant than several other Salmonella isolates compared. Survival of a six-strain cocktail of Salmonella on almonds stored at room temperature was similar to survival observed for Salmonella Enteritidis PT30. No reductions in population of Salmonella were observed at 4 or -20°C. These data indicate that risk assessments for almonds that are based on data derived for Salmonella Enteritidis PT30 are more broadly applicable to Salmonella spp.