The possibility of microwaving jam jars isn’t something that comes across many people’s minds until they need it—like if you want to slather some warm jam over a fresh-out-of-the-oven brownie. Or, maybe you’re hoping to use the microwave to sterilize your jam jar.
So, can you microwave jam jars?
Yes, you can microwave jam jars, provided they have a microwave-safe label. You should be able to find this label on the bottom of the jar if it exists.
However, before you heat your microwave-safe jam jar, you should follow some important guidelines. We’ll give you the low-down on them here.
Jump to:
The Science Behind Microwaving Glass
You can only microwave glass jam jars if they have a microwave-safe label. The reason is that non-microwave-safe glass often contains one or more of the following items that can cause them to crack or burst into pieces:
- Air bubbles
- Dye
- Metal
Why You Should Never Microwave Frozen Jam Jars
So, you know it’s safe to microwave jam jars, but there’s a catch—you should never microwave them when frozen.
That’s because glass is a terrible conductor of heat. So, when you apply heat to the outside of a jam jar by microwaving it, and the inside of the jar is still frozen from the food, a portion of the glass will expand more than the other.
The result is pressure building up on the jam jar, causing it to crack.
Therefore, even if your jam jar has a microwave-safe label, it’s never safe to heat it in the microwave if its contents are frozen.
How To Microwave Jam Jars To Warm Food
Microwaving food in a jam jar is fast and easy. We’ll walk you through the steps here.
Step 1: Remove the Lid
Jam jars almost always have metal lids, which are dangerous to place in a microwave. That’s because a microwave’s electromagnetic energy generates a strong electric field. The electric field is also in a concentrated environment, making things even worse.
The result is that the air becomes ionized, which will cause the metal to make a popping sound at best. But at worst, the metal will begin sparking in your microwave, creating a fire hazard.
Some jam jars have plastic instead of metal lids. More often than not, you’ll need to remove them too. However, you can check the label of the plastic top to see if it’s microwave-safe.
Step 2: Microwave for Short Periods
Even though many jam jars are microwave-safe, the general recommendation is not to microwave them for more than five minutes. But we don’t recommend setting your microwave for a prolonged heating period.
Instead, start with a modest time, such as one minute. Then check the jam jar, stirring its contents.
Continue heating the food in your jam jar in small increments until you’re satisfied with the food's warmth.
Step 3: Remove the Jam Jar from the Microwave
Even though glass doesn’t absorb heat well, it can still get extremely hot when you first remove it from the microwave.
For this reason, we encourage you to protect your hands by pulling out the jam jar with a towel or wearing oven mitts.
Then you’ll be ready to enjoy your jam or whatever food you have in the jar.
How To Microwave Jam Jars To Sterilize Them
So, now that you know how to microwave food in jam jars, it might get you wondering—can you microwave jam jars to sterilize them?
Yes, you can. Although many people think of microwaving jam jars to warm the food inside, you can also use a microwave to sterilize them.
Traditionally, people typically boil or bake jam jars for sterilization. However, tossing the jars in the microwave can be faster if you only have a couple that needs sterilizing. Below are the steps to do so.
Step 1: Wash the Jam Jar
It’s essential to remove all food from your jam jar and ensure you have it as clean as possible before you sterilize it. So, scrub it well with soap and water, then rinse it thoroughly.
Step 2: Wet the Jar
There’s no need to dry your jam jar after you wash it. Instead, you need to get it wet before putting it in the microwave. That’s because microwaves require water molecules to heat an object.
They do so by causing the water molecules to vibrate. The result is friction between water molecules, which releases heat into the food.
Therefore, your jam jar won't undergo sterilization if you don’t wet it before putting it in the microwave.
Step 3: Heat the Jar
Place your wet jam jar in the microwave and heat it on high for one minute. There’s no other trick here—your jar will come out squeaky clean and sterilized after that time.
However, we recommend setting your jam jar on a towel to air dry. That way, you won’t contaminate it by shoving your hand and a cloth inside to dry it.
Step 4: Tend to the Lid
We’ve already established that you can’t put metal lids in the microwave, and you can’t put plastic jam jar lids in it either.
So, you’ll need to pull out a pot, fill it with water, and boil your jam jar’s lid for ten minutes to sterilize it.
How Do I Know If My Jam Jar Is Microwave Safe?
The best way to know if your jam jar is microwave safe is to look for a “microwave safe” label at the bottom of the jar. All jam jars that can go in the microwave will have this label.
So, if you don’t see it, it’s best to assume that your jam jar can’t go in the microwave.
So, Can You Microwave Jam Jars?
Yes, you can microwave jam jars as long as they have a microwave-safe label. Doing so is an excellent choice for heating food and sterilizing empty jars.
That said, you should never microwave frozen jam jars, even if the glass is microwave-safe.
So, we’ll let you get warming up the delicious food in your jam jar or sterilizing your jars so that you can fill them again in the future.
