Thanks to “Betty Botulism” who appropriately pointed out the lack of guidance in these pages regarding the potential for serious illness resulting from foodborne pathogens in improperly preserved and stored foods (though there’s no need to be rude Betty!)
Based on review of several university and ag extension station documents about home dehydration, and consultation with a specialist at Washington State University, I can report that Betty’s grave concerns are not supported by the facts when proper food prep, home dehydration and storage practices are followed. Food drying is a centuries-old method, and as long as the food is fully cooked in advance, taken quickly down to a very low water content, and kept dry and uncontaminated during handling or storage, the risk of pathogen growth and resulting illness is very low. See the following guidance from a food specialist at WSU, and additional links at the bottom.
Adapted from information provided by Margaret A. Viebrock, Director, WSU Extension Douglas County , Waterville, WA 98858
Research on home dehydration is focused on fruits, vegetables and some types of meats, not whole meals. However, the considerations here are not that much different, except it is a low-acid pre-cooked mixture and it needs to get dryer than fruit products. Issues that need to be addressed include:
- Get the water content low enough within the first 2-4 hours to prevent harmful bacterial growth. The only preservation factor in drying these mixtures is the control over water content (though thoroughly cooked foods will be less contaminated than raw foods).
- There is no data to verify how quickly in the drying process the water content is reduced to less that 10-15% of the total moisture for each exact mixture. For this reason, it’s important to spread out the puree and little pieces evenly in a very shallow layer on the dehydrator tray to aid rapid drying.
- Make sure all the cooking, pureeing and spreading out are done in very sanitary conditions – thoroughly wash and disinfect the food processor, dehydrator trays, any utensils, and your hands, before handling the cooked food.
- If the product is crispy dry when taken out of the dehydrator, and kept from re-absorbing moisture in the packaging, it should be okay in ambient storage. That means the end point needs to be crispy, brittle dry. Test for brittleness (or, for fruit, lack of stickiness) after fully cooling the food. Package dehydrated food only after cooling it down fully.
- Thorough cooking to minimum safe temperatures before pureeing should kill pathogenic vegetative (growing) bacterial cells, it will not kill spores.
Plus, even after cooking, a product could be re-contaminated with Staph or other harmful bacteria as it is pureed and laid out on the dryer trays (surface as well as hand contaminants). However, spores that might have survived in the mixtures after cooking are not a big concern due to the open air nature of the drying (anaerobic conditions can foster growth of some nasty pathogens) and the very dry nature of the final product, as long as the product remains very dry through storage.
- Treat rehydrated food as perishable. Once the food is rehydrated, it will require refrigeration to control pathogen growth. Any leftovers should be discarded.
- Keep the final product from re-absorbing moisture during storage, which could allow bacteria growth. If it is dried to the brittle stage it is not necessary to freezer-store the food; plus, depending on the packaging, the product could re-absorb moisture in the freezer leading to bacteria growth.
Final note from Magaret: If these recommendations aren’t followed we cannot guarantee the safety of the end product. As I consistently tell people when it comes to food safety, it’s not like making cookies where you can substitute or change ingredients or even the directions. Food safety is based on scientific principles, and when the actions or methods are changed it changes the final product which in turn will no longer follow the research or principles.
For more useful information on dehydration methods and food safety, see the following links: