Why you should test low-moisture foods

Does honey have an endless shelf life? In the right conditions, it can last indefinitely. When 3,000-year-old honey was found in a tomb in Egypt, scientists determined that it was still edible.

Honey’s unique chemical make-up – its acidity, low amounts of water and the presence of hydrogen peroxide – helps it stay pristine and unfriendly to bacteria and microorganisms. Bees create the food by collecting nectar, breaking it down in their stomachs and drying it out by flapping their wings, according to Smithsonian.com. It’s an ideal food source for their young with high calories and protective qualities.

But not all foods are so favored: honey is one of the few foods with an indefinite shelf life. Dried beans, rice, sugar, and salt also stay edible for years because of a common trait – low levels of water activity. Drying and curing food to reduce water and water activity is a common way to increase the shelf life of food.

Water activity in food

Water activity is a measurement of the availability of water for biological reactions. It determines the ability of microorganisms to grow. If water activity decreases, microorganisms with the ability to grow will also decrease.

Water activity is not the same as moisture content, but moist foods are more likely to have higher water activity than dry foods. Water activity and pH, or acidity levels, are key factors in predicting the growth of bacteria, yeast and molds in food.

Generally, bacteria need a water activity of .91 or higher to make themselves at home. However, certain molds can grow in foods with water activities from .80 to .50. See Table 1 and Table 2 for more detail.

Products with a water activity of .85 or lower are considered low-moisture foods (LMFs), but still have the potential to carry pathogens and cause outbreaks. Examples of LMFs are shown in Table 3.

Table 1. Water activity and growth of microorganisms in food
Water Activity Microorganism Growth Foods
1.0 to 0.95 Bacteria Meat, fish, sausage, milk
0.95 to 0.91 Bacteria Moist cheeses, cured meat (ham), fruit juice concentrate
0.91 to 0.87 Yeasts Fermented sausages (salami), dry cheeses, margarine
0.87 to 0.80 Molds Juice concentrates, syrups, flour, fruit cakes, jellies
0.50 to 0.70 Molds Honey and preserves
0.30 to 0.20 None Cookies, crackers, bread crusts
Table 2. Water activity required to support microorganism growth
Microorganism Water activity
Organisms producing slime on meat 0.98
Pseudomonas, Bacillus cereus spores 0.97
B. subtilis, C. botulinum spores 0.95
C. botulinum, Salmonella 0.93
Most bacteria 0.91
Most yeasts 0.88
Aspergillus niger 0.85
Most molds 0.80
Halophilic bacteria 0.75
 The rise of reported pathogens in low-moisture foods

The history of LMFs and their resistance to bacteria has led to a perception that these foods are invulnerable to safety issues, but that’s not always true. In the past 25 years, Salmonella outbreaks have impacted low-moisture products from black pepper to rice and toasted oats.

While bacteria don’t grow well in these LMFs, they can survive when they develop heat resistance and dramatically-slowed metabolism. According to the Journal of Food Science and Technology, reported outbreaks of Salmonella in this food category have risen in the past 10 years and have been primarily found in powders and nuts.

Table 3. Low-moisture foods and water activity

(adopted from Food and Agricultural Organization (FAO) and World Health Organization (WHO)

Food category Examples Water activity
Cereals and grains ·         Whole and milled grains (examples: wheat, barley maize, oats, rye) 0.6 to 0.7
·         Rice and rice products 0.22 to 0.6
·         Cereals and cereal products (example: breakfast cereals) 0.15 to 0.47
Confections and snacks ·         Cocoa and chocolate products 0.25 to 0.6
·         Other confections/confectionery (examples: marshmallows, candies) 0.4 to 0.68
·         Snacks (examples: chips, crackers) 0.09 to 0.46
·         Yeast
Powdered formula ·         Baby formulas 0.2 to 0.4
·         Milk powder 0.2 to 0.4
·         Drink mixes, dried/powdered nutrient supplements 0.2 to 0.4
Dried fruits and vegetables ·         Dried fruits (examples: raisins, dates, apricots, coconut) 0.5 to 0.7
·         Dried vegetables (examples: tomatoes, potatoes, carrots) 0.5 to 0.7
·         Dried/dehydrated mushrooms 0.5 to 0.7
·         Dried seaweed 0.5 to 0.7
Dried protein products ·         Dried dairy products (example: milk/whey powders) 0.2 to 0.4
·         Dried egg products (example: egg powders) 0.2 to 0.4
·         Dried meat other than sausages/salamis/jerky (examples: meat powders, gelatin, fish) 0.2 to 0.4
Honey and preserves ·         Honey, jams, syrups 0.5 to 0.7
Nuts and nut products ·         Tree nuts (examples: almonds, cashews, pecans, pine nuts) 0.4 to 0.5
·         Peanuts and peanut products (example: peanut butter) 0.4 to 0.5
·         Mixed and unspecified nuts 0.4 to 0.5
Spices and dried herbs ·         Fruit/seed-based (examples: paprika, black pepper, dill seed, fennel, cumin, allspice, nutmeg/mace, cardamom) 0.4 to 0.6
·         Root-based (examples: garlic, ginger, turmeric, onion) 0.4 to 0.6
·         Herb/leaf-based (examples: basil, bay leaf, mint, rosemary, parsley, sage, thyme, dill) 0.4 to 0.6
·         Bark/flower-based (examples: cinnamon, cloves, saffron) 0.4 to 0.6
·         Mixed/unspecified (examples: curry powder, tandoori, herb mixes) 0.4 to 0.6
·         Tea (examples: herbal, black teas) 0.4 to 0.6
Seeds for consumption ·         Tahini (sesame seed paste) 0.12 to 0.18
·         Halva/helva (confection made from sesame paste/tahini) 0.12 to 0.18
·         Sesame seeds 0.12 to 0.18
·         Other and unspecified seeds (examples: pumpkin seeds, sunflower seeds, poppy seeds, melon seeds, flax seeds) 0.3 to 0.4

How to control contamination of low-moisture foods
Allergens_LMF_Final-almondsSeveral processes effectively kill bacteria in LMFs, especially pathogens such as Salmonella. Food safety experts have successfully employed thermal process and pasteurization to nuts such as almonds. Studies by the Food and Drug Administration (FDA) show that pasteurization of almonds – required in the United States, Canada and Mexico – does not affect the nutritional quality of the nuts. Animal feed and pet food manufacturers treat products with thermal processes (heat).

Several new approaches are also being evaluated. Scientists hope to validate that processes like radio frequency and microwave heating, nonthermal plasma, pulsed light, UV light, irradiation and others are effective and safe.

Tools for pathogen detection in LMFs

3M Molecular Detection System LAMP pathogen testingFood manufacturers should adopt a total solution from sample collection and preparation to identification, testing and monitoring. It is important to mitigate risk at every step to be efficient and productive. Processes can be streamlined through next-generation 3M™ Molecular Detection Assays. Accurate testing of pathogens is critical, but it can be complex and time-consuming. The 3M™ Molecular Detection System improves throughput in the lab process with automated efficiency and less chance for human error. A pet food manufacturer from Argentina shared their thoughts:

“After trying different methods for the detection of Salmonella in pet food with water activity less than 0.6, we ended up choosing the 3M™ Molecular Detection System, as the main method of internal analysis, due to its rapid application, its high precision, and its easy application, among others.”

The system is based on a combination of technologies – loop-mediated isothermal amplification (LAMP) of DNA and bioluminescence detection. It has assays for detecting the most likely pathogens to impact low-moisture foods including Salmonella, E. coli O157, and Cronobacter and can be used to test a wide range of dried foods, nuts, and powders.

The most valuable feature of MDS is its coherent results in time using different kinds of chocolate and confectionary with chocolate…We have stopped sending samples to third-party labs and we are able to deliver the final product in just 26 hours.

-Chocolate and confectionary manufacturer from Argentina

The 3M™ Molecular Detection System is the primary method of the USDA Food Safety and Inspection Service for the detection of Salmonella and Listeria

Contact a 3M Food Safety Expert

Need more information on the total 3M Food Safety testing solution? Simply reach out and we’ll be happy to help you with any questions you have.

Raj Rajagopal

Senior Global Technical Service, Food Safety Lab Raj Rajagopal has been working in 3M for the last 20 years in various groups and functions. Prior to 3M he worked in several academic institutions (University of Iowa, University of Georgia and University of Minnesota) in microbial physiology, molecular biology, anaerobic microbiology and genetics. He has a Masters and Ph.D. in Microbiology from India and MBA from University of Minnesota. In 3M, he started out in Biomaterials Technology Center and then in Biotechnologies Project. He was involved in antimicrobials development (FAME), microfluidic card for real time PCR and 3M cycler development. Later, he worked in 3M Pharmaceuticals doing genomics and proteomics research for couple years. He then moved to Safety and graphics group lab (formerly SS&PS lab) exploring food safety and hospital hygiene platform for Commercial Solutions Division and was instrumental in launching hospital hygiene platform. Recently, he worked on developing rapid bacterial testing (E. coli and coliform) for potable water quality. In addition, he also explored opportunity in water-energy nexus and identified several opportunities for 3M to pursue further. He joined Food Safety Department in June 2016 and has been supporting pathogen detection platform. Raj has 25 issued patents and 30 plus publications in peer reviewed journals.