How to kosher a microwave

The microwave is probably the appliance that raises the most questions in a kosher kitchen, and for good reason: it does not cook like any other device. While an oven uses dry heat and a stovetop transmits heat by direct contact, the microwave heats the water already inside the food, and that generates steam which circulates throughout the interior.

That technical difference is what defines the koshering method, and why you cannot simply apply the same approach as with a conventional oven.

Why steam changes everything

A central principle governs these cases in halacha: k'bolo kach polto, "as it absorbed, so it expels". A utensil is purified by the same medium through which it absorbed flavour. If it absorbed through boiling liquid, it is koshered with boiling liquid; if it absorbed through direct fire, it needs direct fire.

The microwave absorbs through steam, so it is koshered through steam. That is why the procedure consists of deliberately generating a large amount of steam inside the cavity, so it reaches every surface that came into contact with the splatters and vapours of the previous food.

If you are interested in the technical detail of why the microwave oven heats by agitating the water molecules in food, the full physical explanation is there. For kashrut, what matters is the consequence: steam is generated, and steam carries flavour.

The procedure, step by step

1. Deep cleaning. No visible food particle may remain. Pay attention to splatters on the roof of the cavity, which is where they build up most and where people look least. Remove the turntable and also clean the ring and the base.
2. 24 hours without use. This step is not optional. Halacha considers that after 24 hours the absorbed flavour degrades and loses its status, which makes the koshering valid. If you used the microwave two hours ago, you have to wait.
3. Generate the steam. Place a container of water inside — preferably one you will not use afterwards for food, or a disposable one — and bring it to a boil until the interior is completely fogged up. This usually takes between 5 and 10 minutes depending on the appliance's power. If the water evaporates completely, refill and repeat.
4. Let it act. Once the steam has been generated, leave the container inside for a few more minutes with the door closed, so the steam also reaches the inner face of the door and the corners.

The turntable: the detail people overlook

The rotating glass plate receives food spills directly, so it needs its own treatment. Since it is glass, opinions vary: some hold that a thorough wash suffices, others require immersion in boiling water, and the most practical solution many families adopt is simply to buy a spare plate and keep one per category of use.

The strategy that avoids all of this

Most observant families do not kosher the microwave every time they switch use, for a simple reason: it is tedious. Instead they apply two customs that solve the problem at the root:

• Always cover. Heat food with a microwave-safe lid or wrap. If nothing evaporates freely, there is no flavour transfer and the microwave stays neutral. This is why in many kosher kitchens the plastic lids sit next to the microwave and not in a drawer.
• Assign it a single category. Dedicate the appliance exclusively to pareve, or exclusively to dairy, and handle the rest on the stovetop or in the oven. It is the simplest option for anyone just setting up their kosher kitchen.

Special cases

Microwaves with grill or convection. Those functions generate dry heat, not steam, so the steam method does not cover them. For that part a libun-type procedure is needed, similar to a conventional oven. If you have a combination appliance and used the grill, ask about it specifically.

Stainless steel vs. plastic or enamelled interior. Materials absorb differently, and some authorities are stricter with certain coatings. It is one of the points where it is most worth asking about your particular model.

Koshering for Passover. Passover rules are stricter than those of the rest of the year. The base procedure is the same, but many communities add requirements. Before the holiday it is worth reviewing your certifier's specific guide and our article on Passover dishes.

Frequently asked questions

Can I use vinegar or lemon in the water?
It is not necessary for the koshering itself — what purifies is the steam, not the additive — but it helps loosen dirt and neutralise odours during the prior cleaning step.

Is putting in just a glass of water enough?
A small glass probably will not generate enough steam to saturate the whole cavity. A wide container with plenty of water is better, so that the steam is abundant and sustained.

I bought a used microwave, is this enough?
The procedure is the same, but since you do not know what was cooked in it, this is a case where it is especially worth consulting the community rabbi before using it.

Further reading

The other koshering procedures are covered in how to kosher an oven, how to kosher a dishwasher and hagalah for metal utensils.

As an external source, Star-K's Kashrus Kurrents publication has detailed technical material on koshering appliances, and the Orthodox Union Passover section covers the additional requirements of the holiday.