Colorado State University Extension
SafeFood Rapid Response Network
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New Home Drying Recommendations
Edited by: Mary Schroeder, M.S., R.D. & Pat Kendall, Ph.D., R.D., Colorado State - Summer 2004
The practice of home food drying is gaining in popularity as an alternative to home canning of surplus fruits and vegetables. Home dried foods are very convenient to take along when camping and hiking, or just when eating on the run. Until recently not much thought was given to the safety of home drying methods. It was assumed that their low moisture content would prohibit microbial growth. However, recent research findings have shown that microorganisms, such as Salmonella and E. Coli O157:H7, are hearty enough to survive basic drying methods, and that new procedures are needed to ensure the safety of home dried foods.
Vegetable Drying Studies
Two studies conducted at Colorado State University evaluated the influence of pre-drying treatments and aerobic storage on inactivation of a five-strain mixture of Salmonella (7.8 log CFU/g) during dehydration of carrot slices. Treatments evaluated in the first study included three pre-treatments (3 min steam blanching, 3 min water blanching, and immersion in a salt solution (3.23% NaCl), one post drying treatment (dehydrating, then oven heating for 15 minutes at 175ºF) and an untreated control. All carrot slices were dehydrated at 140ºF (60ºC) for 6 hours.
None of the treatments in the first study effectively reduced bacterial populations. Therefore, a second study was performed to evaluate longer blanching times and new pre-drying treatments. Treatments evaluated in the second study included the following: 1) untreated control, 2) 10 min steam blanching, 3) 4 min water blanching, 4) 4 min blanching in 0.105% citric acid (1/4 teaspoon citric acid per 1 quart water), and 5) 4 min blanching in 0.210% citric acid (1/2 teaspoon citric acid per 1 quart water). Blanching carrot slices for 4 min in water or either of the citric acid solutions effectively reduced Salmonella populations. Interestingly, control and steam blanched samples had bacterial populations >1.9 log CFU/g after 6 hours of drying and 30 days storage, thereby illustrating the resiliency of some microorganisms and their potential food safety risk.
Fruit Drying Studies
Various pre-drying treatments for fruits have been explored as well. Two studies evaluated whether pre-treating either inoculated apple or peach slices with sodium metabisulfite or acidic solutions enhanced inactivation of Salmonella during dehydration and storage. After 6 hours of dehydration at 140ºF (60ºC), microbial counts on sodium metabisulfite, ascorbic acid and citric acid treated slices displayed much greater reductions (3.8 to 5.6 log CFU/g on apples and 4.3-6.2 log CFU/g on peaches) than sterile water (3.2-3.4 log CFU/g) respectively. Bacteria were still detectable on the stored slices by direct plating after 28 days, except on ascorbic acid treated slices.
New Drying Recommendations
Based on the above research findings, the take-home message for consumers is that pre-treating fruits and vegetables with an acidic solution prior to drying enhances the destruction of potentially harmful microorganism during home drying. For vegetables, water blanching is recommended, and in particular, blanching with a solution that contains 1/2 teaspoon of citric acid per quart of water before dehydration is encouraged. For drying fruit or making fruit leather, immersion in acidic solutions prior to dehydration is recommended to promote safety of the dried product.
The CSU Extension Fact Sheet Drying Vegetables (#9.308) has just been revised 7/04 with the new guidelines. This along with the revised fact sheets on Drying Fruits (#9.309) and Leather and Jerkies (#9.311) are currently available.
The 15-page comprehensive booklet Drying Foods on dehydrating fruits, vegetables, leathers and jerkies will be available for purchase this autumn from the Extension Resource Center. Please call 970-491-6198 for more information, or visit the publications section of the Extension website stated above.
Sources:DiPersio, P.A., P.A. Kendall, M. Calicioglu and J.N. Sofos. (2003). Inactivation of Salmonella during drying and storage of apple slices treated with acidic or sodium metabisulfite solutions. Journal of Food Protection. 66:2245-2251.
DiPersio, P.A., P.A. Kendall and J.N. Sofos. (2004). Inactivation of Listeria monocytogenes during drying and storage of peach slices treated with acidic or sodium metabisulfite solutions. Food Microbiology (in press).
DiPersio, P.A., P.A. Kendall, Y. Yoon and J.N. Sofos. (2004). Inactivation of Salmonella during drying of Nantes carrot slices treated with blanching or immersion in 3.23% NaCl before drying, or oven heating after drying. IFT Annual Meeting Poster.
DiPersio, P.A., et.al. (2004). Inactivation of Salmonella During Drying and Storage of Nantes Carrot Slices Treated With Steam, Water or Acid Blanching Before Dehydration. Poster session: International Association of Food Protection.
