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There’s some controversy about the safety of microwaving mason jars. Government authorities and mason jar manufacturers lay down specific rules about using the microwave with mason jars, and most of those rules involve very limited use.
There are two main issues with mason jars.
One, they’re made of glass. Modern mason jars will have a label on the bottom that reads “microwave safe.” Older Mason jars, or ones without that label, should not be heated in the microwave for fear of shattering.
Two, it’s dangerous to put the metal parts of mason jar lids in the microwave. There’s a very real danger of an explosion or a fire.
So how can you process foods in mason jars? You might want to sterilize one or more jars. You may want to thaw out food stored frozen in a jar. Or you’re thinking of heating or cooking an ingredient in the microwave to help you get ahead with preparing a dish.
Are any of these procedures safe? Let’s take them one by one.
Sterilizing a mason jar in the microwave
Those who use the microwave to sterilize mason jars give these directions:
Wash the jars to make sure they’re absolutely clean, with no specks of dirt or old food. Inspect them carefully for cracks. Do not use a jar that’s not perfectly whole and clean.
Fill the mason jar with water. DO NOT use the lid. You will need to sterilize the lid the old-fashioned way, on the stovetop.
The time required for sterilization:
1-2 minutes for jars less than 1-liter capacity.
3-4 minutes for jars with 1-4 liters capacity.
5 minutes for jars with more than 3 liters capacity.
The reason this method is not recommended is because of safety issues for you. The hot, wet jar must be removed from the microwave carefully, held by a towel or heatproof glove; otherwise, the hot glass will burn your hand.
The water inside may be at boiling temperature, so you must take great care to avoid spills. Very awkward.
You will have to set the jar down on another dry towel or wooden surface to avoid thermal shock, which would shatter it.
Mason jars with thin walls, or older models, are likely to shatter in the microwave. They aren’t meant to take extreme temperatures.
In other words: never put a mason jar in the microwave unless it has a label that specifically reads, “microwave safe.” This is especially important when handling colored mason jars.
And please, take all precautions when removing that hot jar from the microwave.
Is it safe to microwave frozen food that’s in a mason jar?
Authorities say no. As the cold jar heats, the glass can expand and shatter under the stress.
It’s far safer to thaw out the frozen contents by placing the jar in a basin of cool water. Change it every so often until you see that the food inside is thawing.
At that point, you can shake the contents of the mason jar out into a pot or skillet and reheat the food on your stovetop, or the oven.
In other words, never put a mason jar with frozen food directly in the microwave.
If you’re determined to use the microwave, you can place the mason jar, minus its lid, in a bowl with cold water and microwave the whole thing for 20 seconds. Take the bowl and jar out, change the water in the bowl, and repeat until the food in the mason jar is thawed.
If you don’t want to get the Mason jars, place a glass that is filled 90% with water next to the jars in your microwave. Place the glass away from your mason jar so that it continues boiling.
Prepping ingredients for cooking and canning in a mason jar/microwave
Attempting to can food in a mason jar/microwave situation is absolutely not recommended. Food doesn’t heat evenly in the microwave (that’s why there’s a revolving dish inside). You’d run the risk of producing contaminated food, and nothing is worth the danger of contamination.
In addition, you can’t have the metal lids on the mason jars for the danger of explosion and fire. Vacuum seals are equally unsafe.
The Ball Corporation, manufacturers of mason jars for generations, attaches a label to the jars that put the warning in capital letters: DO NOT ATTEMPT TO PROCESS IN MICROWAVE OVEN.
We take that seriously.
Still, we’re not against using the microwave when it’s safe to do so. While you can’t can foods in the microwave (mason jar or not), it’s useful in prepping ingredients before canning.
A booklet published by the Oregon State Extension points out that it is doable to microwave any food a person wishes to which includes apples used for applesauce, heating syrups that are used for canning fruits, and preparing pre-cooked fruits and vegetables during the hot pack method for canning.
Blanching vegetables in the microwave is, then, possible and safe. But you must stay alert to remove the product before it overcooks and becomes tough.
Under-cooking won’t destroy the enzymes which will spoil to be preserved.
It’s best to find a chart for blanching in the microwave. Good charts that show you how long to blanch each kind of vegetable, and more, are available online.
General rules for blanching in the microwave are:
- Make sure you’re putting the food in a microwave-safe dish. A good chart will indicate what size container to use per pound of produce.
- Wash, peel, and slice or cut the vegetables to desired sizes and shapes.
- Measure 1 quart or 1 pound of vegetables into recommended size casserole or dish.
- Add water. Do not add salt. Cover.
- Set the microwave on high for 1/2 the time, then stir. Cover the dish again and microwave for the second half. Stir again.
- Drain the vegetables from the hot water. Put them in ice water immediately.
- Drain the vegetables again. Dry them gently with paper towels. Pack them in freezer containers and store them in the freezer right away.