This issue has been addressed on the blog before, but it bears repeating with so many people new to prepping and hoping to do so easily with items they already have on hand.
The premise of dry canning is that the oven heat will kill bugs and drive out excess moisture. Doing so in a canning jar means that when the jar of food is removed from the oven, the lid and band can be applied quickly so that a vacuum seal is created as the jar and its contents cool.
However, there are numerous problems with this approach to food preservation. Here are the bad:
- The oxygen is still in the jar. People use oxygen absorbers to remove oxygen from package containers so that their food lasts longer.
- It's not cheaper than other forms of food preservation. Even if the jars are free, the cost of the lids alone makes using Mylar bags and oxygen absorbers more economical.
- Canning jars are made from annealed glass, not tempered glass. Annealed glass does not tolerate temperature changes, such as may occur when the heating element in the oven cycles on and off or when the jars are removed from the oven to the counter to cool. There is the risk of the glass shattering and causing substantial harm, especially to the eyes. (Yes, temperatures get high in boiling water baths and pressure canners, up to about 260 degrees. But it's a gradual temperature change, not the rapid changes caused by direct contact as happens when jars are set on pre-heated oven racks or removed from ovens to cool on counters.)
- Exposing food to unnecessary heat shortens the shelf-life. It hastens deterioration, spoilage, and nutrient loss. It's one of the primary enemies of long-term food storage. All food is supposed to be protected from light, oxygen, humidity, and heat. Dry canning drives out moisture, but any foods you purchase for long term storage should already be dry. Dry canning also kills insects, but so will freezing. And freezing won't deteriorate the quality of dry foods.
But you don't just have to take my word for it. Take a look at what a nutrition professor and cooperative extension food safety specialist has to say:
Issues with dry canning
Here are some issues to think about with this concept:
- This process is not “canning” just because it uses canning jars.
- It is unknown if this process can sterilize the food, although it might cause vacuum sealing of jars. This would not be a time and temperature combination known to kill many bacterial spores or mold spores.
- This method does not remove all of the oxygen from the jar before sealing and may trap moisture from the food if condensation occurs. While the presence of spores would not be an issue in very dry foods, any moisture pockets say from condensation or incompletely dried foods could be a problem. Moisture in the jar with some retained oxygen could support the growth of airborne molds or even bacteria not killed by the low heating in some seemingly dry foods. Some foods may seem dry to the consumer but still have enough moisture in them to come out upon heating and closing up in a container. This could be especially true of home dried foods.
- The dry oven process used at home has never been shown to sterilize these various dry foods or produce the claimed extended shelf life with quality. In fact, there is no known researched shelf life for foods packaged just this way at home. (If someone can lead me to the research I have not been able to find, please let me know.) Extended shelf life expectations are available for dry foods prepared and packaged by other methods (see the Utah State University Extension references below).
- This heating could even make the quality of some foods worse. This could be either by moisture condensation upon cooling, or if the food is lipid-containing nuts and grains, increased enzymatic reaction causing rancidity.
- A major manufacturer of canning jars and lids in the U.S. does not support the use of their jars and lids/sealing compound in this manner.
- Because this type of process is not recommended, doing it can be a waste of resources, time and energy.[1]
Remember, the idea behind everything we do to preserve food for our family is to preserve their lives. Engaging in unsafe practices, especially when it comes to food, may be counterproductive.
References:
[1] Elizabeth L. Andress, "'Dry Canning' Isn't Canning to Me," National Center for Home Food Preservation, 16 April 2020, https://preservingfoodathome.com/category/canning/ (accessed 23 July 2020).
https://aces.nmsu.edu/covid19/canning-dried-food.html#:~:text=%22Dry%20Canning%22%20is%20method%20of,heated%20to%20200F%20or%20hotter
