Low moisture foods (LMFs)—foods that are naturally low in moisture or made through processes such as drying or dehydration from higher moisture foods—include but are not limited to cereals and grains, flours, milk powder, powdered infant formula, spices, chocolate, dried fruits and vegetables, nuts and nut products, dried protein items, coffees and teas, pet food, and animal feed. LMFs have low water activity, a measure of free water that is an important factor in food safety because it determines the amount of water available to help microorganisms grow.
For many years, it was thought that LMFs were safe from microbial contamination. After all, LMFs are defined as having water activity levels less than 0.85 and most bacteria (including pathogens like Salmonella and E. coli O157) need water activities of 0.91 or higher to grow.
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However, just because these bacteria have growth challenges doesn’t mean they can’t survive. Numerous outbreaks of foodborne illnesses have been linked to LMFs contaminated with Salmonella spp. (peanut butter, chocolate, milk powder, crackers, almonds, infant cereals, spices), Bacillus cereus (rice, nuts, herbs, spices), Cronobacter sakazakii (powdered infant formula), Clostridium spp. (herbs, spices, dried tofu), Shiga toxin-producing E. coli (STEC) strains (flour, walnuts, almonds, rice, seeds), and Staphylococcus aureus (rice, seeds, nuts, almonds). It is generally agreed that pathogenic bacteria can remain viable in these foods for long periods of time and, given the opportunity and right conditions, can grow and cause illness. Several studies have documented long-term survival of pathogens in LMFs, and Salmonella spp., STEC, and Cronobacter survive from days to years in low moisture conditions. In addition, the pathogens show increased resistance to heat treatment in LMFs and exposure to low water activity confers cross-tolerance to other stresses, including low pH, bile salt tolerance, resistance to disinfectants, UV irradiation, and heat. The pathogens in LMFs have also been shown to have a low infectious dose (10 to 100 CFU) to cause illness. This is well documented from several studies of Salmonella outbreaks from LMFs (chocolate, peanut, paprika powder, and others), where very low numbers of cells were present in the contaminated product (about 13 CFU/g) in contrast with the high infectious dose (>105 CFU) for other contaminated foods.
Consequently, there is a global recognition that these foods need to be monitored and managed for microbiological hazards, and many regulatory agencies including FDA, USDA, Health Canada, European Food Safety Authority, and Codex have developed guidelines for managing these foods. FDA has developed the Preventive Controls rule for human food and animal food that can come in contact with humans. Similarly, the Codex Alimentarius Commission has developed a Codex Code of Hygienic Practice for Low Moisture Foods. Increased surveillance of LMFs has been implemented under the Food Safety Modernization Act (FSMA), Canadian Food Inspection Agency’s Food Safety Action Plan, and Codex guidelines. In addition, several industry guidelines describe methods to limit or reduce Salmonella and other pathogens in nuts, spices, and other foods (see Table 1).
Pathogens are most often introduced in LMFs via contaminated ingredients or cross-contamination during processing. Regulatory agencies such as FDA therefore recommends conducting hazard analyses for preventive controls for human food, and manufacturers need to consider the potential for biological, chemical, and physical hazards relating to their raw materials and other ingredients (ingredient-related hazards), processes (process-related hazards), and the food-production environment (facility-related hazards). Regulatory guidelines also recommend good hygienic practices, hygienic design of equipment, proactive maintenance programs, control of incoming materials, and effective ingredient control in the LMF establishment to prevent contamination. The Codex advises that special attention be paid to those products exposed to the processing environment following a pathogen reduction step (such as almonds and pistachios), products that are not subjected to a pathogen reduction step (such as flour and dry mixes), and products for which ingredients are added after a pathogen reduction step (such as herbs and spices).