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.
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).
Beyond Finished Foods: Production Environments
In contrast to the historical focus on testing finished products for pathogens just prior to release with little or no attention given to the processing operation and environment, new guidelines and regulations place more attention on environmental monitoring and entire process operation as means to prevent pathogen contamination.
The FSMA Preventive Controls rule, for example, focuses both on environmental monitoring and finished product testing for human food. In addition to recommending that raw materials, ingredients, and end products be tested, FSMA highly recommends environmental monitoring of pathogens in LMF and ready-to-eat (RTE) food processing environments. According to FSMA, “Foods such as peanut butter, soft cheeses, dried dairy products for use in RTE foods, and roasted nuts are among the products for which manufacturing operations would need to have an environmental monitoring program when such foods are exposed to the environment.”
In addition, when environmental monitoring results are gathered both prior to and following cleaning, manufacturers gain a good sense of the overall effectiveness of their hygiene controls and sanitation program. Armed with strong before and after data, they can make the necessary adjustments to improve cleaning strategies, practices, and training.
Carefully designed and implemented sampling programs also bring the benefit of detecting sites potentially harboring pathogens. To that end, LMF manufacturers are advised to perform environmental swabbing and analysis using a hygienic zoning system based on food safety risk. An example would be Zones 1 through 4, with Zone 1 being product contact surfaces, Zone 2 being surfaces immediately over or next to the product, then moving to Zones 3 and 4, with Zone 4 being furthest from the product.
Pathogen Control in LMFs
Every step of the LMF production chain—from sourcing of raw commodities and ingredients, preventing cross-contamination from harvest, to post-process, employing effective dry cleaning and sanitation processes, and implementing and monitoring validated lethal processes—is critical to ensure safer LMFs. Although today’s thermal (heat) processes coupled with continuous monitoring are probably adequate, there is significant room for improvement.
Thermal processes for nuts include oil roasting, dry roasting, and blanching as more traditional practices, but heat can also be applied through steam, infrared heat, and other means. Pasteurization has been successfully applied to raw almonds to reduce the presence of Salmonella. Some emerging technologies for LMFs include radio frequency and microwave heating, nonthermal plasma, pulsed light, UV light, irradiation, propylene oxide, ozone, and novel drying technologies such as microwave drying, vacuum drying, super-heated steam drying, infrared drying, and freeze drying. Although high-pressure processing has been successfully applied to high moisture foods, efficacy in LMFs is not well understood. Additional research is needed to understand these technologies’ application to LMFs.
Pathogen Detection Technologies and LMFs
Eliminating or preventing pathogens entering the production process through raw material screening and finished product testing are key to ensuring safe product is delivered. Unfortunately, processes that rely on inadequate or incorrectly used technologies can thwart a lot of well-meaning work.
High-performing pathogen testing technologies are able to identify intact pathogens, as well as pathogen cells that may have been damaged by freezing, drying, antimicrobial treatments, or other processing conditions. Pathogen detection methods typically require an enrichment step to allow bacteria to grow to detectable levels, and this nourishment and recovery step is especially critical for LMFs. Pathogens in these foods can be severely dehydrated due to the low water activity, and recovery and detection of desiccated bacteria from dry matrices and environments is critical.
Manufacturers of food—LMFs or otherwise—mostly utilize one of two test tools for detecting the bacteria in their low moisture products: culture-based tests and rapid methods. Traditional culture-based tests rely upon growth of pathogens in a selective media followed by counting of visible colonies based on certain traits, such as their ability to grow in the presence of a particular chemical (e.g., salts, bile) or their ability to utilize particular chemicals or nutrients. Rapid methods for foodborne pathogen detection have evolved over the last several years with fundamental advances in immunology and molecular biology and applications of these advances to testing methods. The accuracy of these rapid methods is generally validated against the same standard methods used in culture methods—FDA BAM, ISO, or USDA MLG, for example. However, compared to traditional culture tests, these rapid methods not only offer enhanced accuracy but drastically reduce the time-to-result of food testing (next day findings rather than three days to a week) and provide greater ease of use.
When it comes to rapid methods, antigen/antibody-based assays such as ELISA or lateral flow have been in use for many years, but a growing concern with these methods is the cross-reactivity with non-target organisms. DNA-based methods are generally considered to be more accurate, as they target a specific and unique DNA sequence of the bacteria.
Among the several kinds of DNA-based rapid methods, polymerase chain reaction (PCR) has been widely used for foodborne pathogen detection, and there are multiple vendors offering validated PCR methods to detect Salmonella, STEC (O157 and non-O157), Listeria spp., L. monocytogenes, Cronobacter, and other organisms. PCR uses Taq polymerase and repeated cycling (heating and cooling) to amplify DNA, relying on instrumentation capable of rapidly heating and cooling. In addition, PCR methods typically require multiple steps for processing enriched food samples and amplify target DNA for detection of pathogens.
Newer DNA-based methods such as the LAMP (loop mediated isothermal amplification) technology that 3M commercialized offer an alternative to PCR (see Table 2). These tests are also globally validated for their ability to detect the Salmonella, E. coli O157, Cronobacter, and other key pathogens implicated in LMFs, but with fewer steps and simpler instrumentation. LAMP uses Bst polymerase that has strand displacement activity, allowing amplification at a single temperature without the need for cycling through series of temperatures. In addition, Bst polymerase has been shown to be more resistant to inhibitors from media or food matrices that may compromise PCR results. The 3M Molecular Detection System based on LAMP integrates a proprietary bioluminescence solution for detection that offers a sample preparation process with only two transfer steps and no need for DNA extraction and purification steps.
The PCR and LAMP assays have been validated for various LMFs. Manufacturers need to select an appropriate method to fit their purpose and need based on comparative benefits such as cost, ease of use, and validations.
Lastly, whole genome sequencing (WGS) provides the complete DNA makeup of a test subject, allowing organisms to be differentiated with precision not possible with other technologies. WGS is an emerging technology for food safety applications, but it is mainly being used by regulatory agencies such as FDA and USDA to pinpoint sources of contamination during outbreaks. Its wide use for routine food safety testing is debatable given the cost and complexity of the method.
LMF, High Economic Pressure Businesses
The LMF industry is focusing on major changes to produce the highest food quality. New requirements are being enacted in supply chain controls, environmental monitoring programs, training, and recordkeeping. Greater enforcement of food safety laws and regulations is pushing LMF manufacturers to place safety at the forefront.
LMF processors are forced to balance countless procedural, competitive, and economic pressures alongside needs to cut testing time and release products to market faster. But as recent outbreaks and recalls attest, it’s imperative that they not take their eye off the ball when it comes to food safety. With goals of mitigating risk at every step and improving operational efficiencies and productivity, thoughtful workflows—from expertly designed, validated, and verified cleaning regimens to more automated pathogen testing practices to safe storage approaches—can help LMF products safely and sufficiently reach consumers.
Dr. Rajagopal is a senior global technical service specialist with 3M Food Safety. Reach him at firstname.lastname@example.org.
The “Control of Salmonella and Other Bacterial Pathogens in Low-Moisture Foods” book reviews the current state of the science on the prevalence and persistence of bacterial pathogens in low-moisture foods and describes proven techniques for preventing food contamination for manufacturers who produce those foods. Go to www.Wiley.com and type in book title to learn more.—FQ&S