The persistence of pathogens and viruses in ingredients and ready-to-eat foods has wide-ranging impacts on our food supply, and ultimately our health. Because low-moisture foods (LMF) such as nuts, dried fruits, cereal products, and chocolate are often used as ingredients in the manufacture of other foods, they carry significant potential for the amplification of outbreaks and recalls over a wide variety of products.
There has been worldwide recognition of the need to more seriously manage the microbiological hazards associated with these products. In particular, the underlying factors that mediate pathogen survival in LMF under standard processing and storage conditions—as well as mechanisms for isolating and inactivating them—have yet to be fully understood.
To address this need, IAFNS supported a body of research on this topic. Part of these studies were performed as part of a multi-center research collaboration between the University of Guelph, Health Canada, and North Carolina State University. This collaboration between investigators in the United States and Canada enabled the investigators’ diverse perspectives and expertise to strengthen this line of research.
For example, foodborne bacterial and viral pathogens such as Listeria monocytogenes, Salmonella, and norovirus can survive in LMF and in dry food processing environments for months, or even years. Whether—and for how long—these pathogens remain capable of causing human illness is not completely understood. According to Dr. Jeff Farber of the University of Guelph, “the increased awareness of the importance of low-moisture foods as a possible vehicle for foodborne illness has already led to better approaches towards prevention and control and will continue to do so in the future.”
Furthermore, the lower moisture content of dry foods and manufacturing environments can substantially increase the heat resistance of foodborne pathogens and can increase their tolerance to further treatments, posing a vexing food safety challenge. As such, IAFNS supported further work by Dr. Meijun Zhu at Washington State University to comprehensively evaluate L. monocytogenes survival in LMF during heat processing, and to examine factors impacting their resistance. According to Dr. Zhu, “L. monocytogenes can survive in LMF for an extended period. The desiccation stability of L. monocytogenes in LMF is impacted by water activity, food matrices, and storage temperature. The thermal resistance of L. monocytogenes in LMF is inversely related to water activity and depends on the food composition. In general, L. monocytogenes is more stable at lower water activity and fat-rich food matrix.”
The key findings from this series of studies and their impact on public health are summarized below.
Identifying Novel Genes That Facilitate Survival of Salmonella in LMF
Ten genes important to Salmonella survival on LMF were impaired and their survival studied on pistachios. Pistachios were treated with pathogens and then measured after treatment, after drying, and after 120 days. The findings support a mutagenesis and sequencing strategy to identify genes key to Salmonella survival on LMF.
Modeling L. monocytogenes Survival on Model LMF
Three model LMF were inoculated with a 4-strain cocktail of L. monocytogenes to evaluate their survival under long-term (8-12+ months) storage at 23°C and 4°C. Decreases in L. monocytogenes during storage on the LMF were the result of both cellular inactivation and transition to a viable-but-not-nonculturable state. The surviving cells — specifically after long-term storage at 4°C on the chocolate liquor and pistachios — remained infectious and capable of replication. These findings will help predict future microbial health risk incidents. The paper also calls for adding LMF to food safety questionnaires used during listeriosis outbreaks because of this concern.
Two decontamination methods were studied for inactivating a cocktail of Salmonella or Listeria monocytogenes inoculated on dried strawberry, dried apple, raisins, chocolate crumb, cornflakes and pistachios. One method was based on an acid-ethanol sanitizer and the other combined UV radiation, ozone and peroxide. Both methods show promise in reducing risks in LMF depending on the type of pathogen and product.
Foodborne viruses such as norovirus and hepatitis A virus (HAV) are highly transmissible, persist in the environment, and resist inactivation. Foods can become contaminated with these viruses during harvest, handling or processing. This study compared a bead-based magnetic assay with an existing International Organization for Standardization (ISO) method for virus recovery and tested it on chocolate, pistachios and cornflakes. Thus, depending on the food matrix and the virus, the bead-based assay efficiently and rapidly extracts viruses from LMF.
L. monocytogenes Survival and Virulence on Apples, Strawberries and Raisins
The survival of Listeria monocytogenes was measured during long-term storage on three fruits. After dry inoculation and storage at two different temperatures and humidity levels, the results show that L. monocytogenes is rapidly inactivated during storage on raisins and dried strawberries at 23°C, but capable of long-term survival at 4°C.
This study examined the survival of foodborne viruses in LMF during 4-week storage at room temperature. It also evaluates a treatment geared toward inactivating viruses. Pistachios, chocolate and cereal were inoculated with hepatitis A virus and two related viruses. Then viral survival was measured over a four-week incubation at room temperature. The study found that while foodborne viruses can persist for a long time in LMF, combining UV radiation, ozone and peroxide as a treatment may represent an effective inactivation method.
Viable but Nonculturable
In this study, dried apples, strawberries and raisins were mixed with a five-strain cocktail of Salmonella and then dried. However, Salmonella could not be recovered, even after being enriched. The use of microscopy methods revealed that 56-85% of Salmonella cells were still viable despite their nonculturable state. These data suggest that the unique combination of stressors on dried fruit can keep pathogens viable but undetectable by culture, posing hidden risks for food safety.
The safety of low moisture foods in the food supply is capturing more scientific attention. Examining the survival and inactivation of pathogens and viruses on specific foods under common production conditions provides insights into the extent of contamination and methods to prevent it. This new information fills critical knowledge gaps and identifies cutting-edge decontamination tools that empower food safety professionals to reduce risks in low-moisture foods and prevent foodborne illness outbreaks.
The Institute for the Advancement of Food and Nutrition Sciences (IAFNS) Food Microbiology Committee continues to proactively improve the understanding and control of microbial food safety hazards to enable scientifically informed decision making. IAFNS committed to leading positive change across the food and beverage ecosystem. IAFNS is a 501(c)(3) science-focused nonprofit uniquely positioned to mobilize government, industry and academia to drive, fund and lead actionable research. iafns.org.