The bacterial contaminant Salmonella has challenged food scientists for over 100 years, but much has been learned in the past 30 years that has helped frame food-safety programs for industries such as California Almonds that are geared towards prevention and control of this potentially harmful bacteria.
In a presentation at the Almond Board of California Food Quality & Safety Symposium held in June, Dr. Linda J. Harris of the Department of Food Science and Technology at UC-Davis discussed some of the important developments in Salmonella research that are significant to food safety programs for both growers and processors. These are:
• Salmonella has a low infectious dose. Outbreaks of salmonellosis have been known to occur with only a few cells per serving.
• Salmonella survives a long time in dry conditions and in cold temperatures. In one of Dr. Harris’s trials, almost no loss of bacteria was noted after 450 days when almonds were stored at 4 C (39 F). She cited several outbreaks that were associated with low moisture foods with a long shelf life, such as nuts, seeds, and cereals, but noted that inadvertent moisture played a role in several of these outbreaks.
• Salmonella originates in animal feces, but can be transmitted or spread by several means — insects, birds, in water, by sewage and soil, and even in animal feed.
• Salmonella is very resistant to dry heat. The temperatures that rapidly kill the organism in moist foods have little or no impact on an almond. This is why it can be a challenge to validate some types of heat processes.
Moisture in the orchard or in the processing plant can encourage growth of bacteria, including Salmonella. In the orchard, almonds get wet from unseasonable rain or irrigation water applied too close to harvest. Using water in a cleaning and sanitation program presents a special challenge, as Salmonella grows in wet almond dust.
Dr. Harris did note that over several years of conducting pathogen surveys on almonds, the percent of positive samples and number of Salmonella present on almonds was low under ordinary circumstances.
These low levels are what the 4-log reduction rule was based on, and why it is important that growers continue to apply Good Agricultural Practices to ensure these levels are maintained.