Sneaky Salmonella's ability to survive in dry foods challenges common perceptions of food risk

Salmonella. We’re all familiar with the bacterium that stops us from eating raw cookie dough and has us double- and triple-checking our meat thermometers. Common knowledge tells us that if we follow certain precautions, we can avoid a terrible bout of food poisoning. However, it is becoming increasingly evident that salmonella can also survive for several months in dry foods such as nuts, dry milk, crackers, and chocolate, revealing that it may be a greater threat to food safety than generally understood.

The public has been aware of the risks of salmonella for decades. Every year, salmonella is responsible for approximately one million cases of foodborne illness and three hundred eighty annual deaths in the United States, with risk of further complications in young children and the elderly. Usually, infected individuals experience symptoms such as fever, diarrhea, and abdominal cramps for four to seven days and recover without treatment.

Despite the associated threats, salmonella is not considered one of the more dangerous bacteria. Salmonella awareness literature, such as that presented by popular outlets such as foodsafety.net and webmd.com, commonly state that it is only a risk in raw meat and dairy, and so long as one cooks the food and cleans contacted surfaces, the risk is minimal.  In the past, it was often assumed that these precautions against salmonella were sufficient.

However, scientists studying salmonella have found that it can actually survive in a wide variety of host environments, including in the dry foods that sit on shelves. Most recently, Wonderful brand pistachios have been recalled due to salmonella outbreaks in several states, including Minnesota. These nuts are sold in many grocery stores and sit on shelves in countless households, thus demonstrating the need for greater awareness of salmonella in dry foods. Although it is not known exactly how these dry foods become contaminated, it is likely that salmonella capitalizes on the close quarters in industrial processing plants to travel from animal feces, to animal meat and dairy products, and eventually to dry foods.

Several recent studies have examined the extent to which salmonella can survive in a variety of dry environments; it was previously believed that a wet environment was required for survival. A current study from the lab of Dr. Irene Hanning at the Department of Food Science and Technology at the University of Tennessee examined the how different conditions- including acidity, temperature, and water content- affect salmonella survival.

Their primary finding was that salmonella is incredibly versatile and can adapt to a wide variety of environments, which explains how it can travel from a cow’s intestines, to soil, to a factory, to packaged food, and finally into humans. The study also found that not only could salmonella survive for long periods of time in dry foods, which was a relatively new discovery, but also that living in dryer environments actually makes the bacteria more resistant to the heat inactivation typically used to make foods safe during processing.[5] It is likely that this variable ability to survive at higher temperatures, which depends on the moistness of the environment, contributes to processing plants’ inability to fully eradicate salmonella from drier foods.

A second study done by the team of Dr. Larry Beuchat at the Center for Food Science and Technology at the University of Georgia looked into the ability of salmonella to survive in dry snack foods. Researchers in Beuchat’s lab exposed different varieties of peanut butter and cheese based cream-filled crackers to salmonella.  They found that the bacteria was able to colonize and survive in all of the food products, living the longest in cheese-filled crackers, likely due to the dryer environment. These studies show that salmonella is no longer something that we can combat with thorough baking and cleaning practices alone.

Since the beginning of 2016, two salmonella outbreaks have spread across the United States in dry food products, both reaching Minnesota. The first contamination discovered was in RAW Meal Organic Shake and Meal Products, causing illness for thirty-three people in twenty-three states. The second was in Wonderful pistachios, causing illness for eleven people in nine states.

This widespread contamination of dry food products is concerning for a number of reasons. First, unlike meat, consumers often buy products like nuts and consume them without heating to temperatures that would kill the bacteria. As a result, if a dry food product is contaminated, illness is significantly more likely.

Second, salmonella can live for up to six months in dried foods, many of which have a long shelf life. Consequently, even if there are recalls on contaminated foods, many people will have the tainted product in their homes for weeks or months, extending the window in which salmonella could cause illness from the damaged foods if consumers are not made aware of the recalls.

Third, and perhaps most importantly, these outbreaks raise questions about whether sanitation standards in food processing plants are taking salmonella’s presence and increased temperature resistance in dry food products into account. According to Dr. Linda Harris, the Associate Director at the Western Institute for Food Safety and Security at the University of California, Davis, it only takes the ingestion of ten salmonella cells to make a person sick. So perhaps the standards for food products like chicken and eggs, which will be cooked by consumers thus eliminating any remaining salmonella, are not high enough for products such as pistachios, which will be eaten without any additional preparation.

This may be a moment for major food processing companies to reexamine their food safety practices. Enforcing more stringent inactivation measures for dry food products could be a step toward avoiding future outbreaks. However, perhaps a more immediate first step is to change household knowledge about salmonella. Greater public awareness of the potential dangers of salmonella in dry foods could be instrumental in preventing unpleasant, and potentially deadly, food poisoning for families across the nation.

 

Bibliography

Andino, A., & Hanning, I. (2015). Salmonella enterica: Survival, Colonization, and Virulence Differences among Serovars. The Scientific World Journal, 2015, 520179. http://doi.org/10.1155/2015/520179

Center for Disease Control. (2016). Retrieved from http://www.cdc.gov/salmonella/montevideo-03-16/index.html

Diez-Gonzalez, F. (Speaker). (2016, March 15). “Exploring the Secrets to Salmonella’s Success” MPR News. Retrieved from http://www.mprnews.org/story/2016/03/14/salmonella

Harris, L. (2009). Building a Better Understanding of Salmonella in Pistachios. Retrieved from http://www.foodhaccp.com/1news/050109o.html

 Mann, D. (2015). Survival of Salmonella in Cookie and Cracker Sandwiches Containing Inoculated, Low-Water Activity Fillings. International Association for Food Protection. Retrieved from https://iafp.confex.com/iafp/2015/webprogram/