Wednesday, March 28, 2012

Dehydrating temperatures

There seems to be a bit of controversy regarding the temperature for drying fruits and veggies. Even some books and dehydrator manuals recommend drying at 135 to 140 degrees.

However, there is something I have found that they rarely seem to mention, and that is "case hardening"

 Case hardening is caused when the temperature is too high. It seals the surface, not allowing all moisture on the inside to escape.  That puts your food at risk for spoilage later down the line.

"Fruits are best dehydrated between 125°F and 135°F – any hotter than that may cause the skins of certain fruits to get crusty i.e. 'hard' - this is known as 'case hardening' which prevents the inside of the fruit from drying properly. Don't be tempted to turn the food dehydrator on high to speed up the process!"

"Case hardening is a result of partially dehydrated food that. Case hardened food has the outermost portion of the food dried while the interior remains moist. This situation typically results when too high of a drying temperature is used. Case hardened foods will spoil due to microbial growth. Moisture from the interior of the food will migrate to the exterior."

LINDA'S NOTE: I am convinced that the higher temperatures are often based on those used for canning.

 "Dehydrating doesn't subject foods to high temperatures associated with traditional canning methods. For safety reasons, low-acid foods are heated to temperatures of 240F degrees in a pressure canner. High-acid produce reaches a temperature of 212F in a water-bath canner.

When a raw food is heated to an internal temperature of 120F or higher, much of it's nutritional value is lost, especially enzymes. Canning also leaches out water soluble vitamins and minerals, which further depletes the healthful qualities of raw-living foods"

LINDA'S NOTE: The above temperatures are for canning... Canned food is stored in some form of liquid, which is, in essence a petri dish ready to grow all sorts of bacteria. That is why it is steralized at a high temperature. Since dehydrating removes the moisture, the risk of spoilage is profoundly reduced, if not totally elemenated.

Yes, you can find articles and recommendations all over the internet recommending the higher temperatures. But I have found my foods dry better at lower temperatures and I have never had an issue with molding foods.  It takes a little longer, but I am convinced the finished product is safer and has a higher nutrition value.

 In this situation, you must decide for yourself whether to dehydrate at a higher temperature and risk case hardening which can lead to spoilage


  1. for awhile I was trying to follow a raw diet and was dehydrating foods at a low temp with my excalibur (in the live foods range, around 104* but I also had case hardening with some things, like carrots. I have only been able to successfully dry the previously frozen bagged carrots at such a low temp.
    I am no enjoying the raw foods because of health reasons. (Several fruits and veggies seem to make the inside of my mouth break out in cancr sores unless they have been cooked (Canned) first.
    Anyway, the case hardening seemed to happen at lower heat levels. I live in the PNW.

    1. That's very interesting. YOu do understand that case hardening is a term used for food that is dry on the outside and still a bit moist on the inside?

      That is different from foods that are uniformly dry. Carrots will tend to be hard when they dry. YOu will need to find a way to rehydrate them or grind them before eating them.

      I'm not sure what your dietary restrictions include, but you might want to look at the Backpacker's Coleslaw recipe here.

      It calls for shredded cabbage and carrots that are marinated before drying. They are absoolutely delicious and easy to eat.

      Sounds like you may have some sensitivity to certain foods. I'm thinking that in the dehydrate form they are more concentrated. That might be a problem for you.