Glycerin in fermentation airlocks

For people who truly want a no-babysitting approach to fermentation in an airlock jar, you need to mix glycerin with water in your fermentation airlocks.

You may have filled your airlocks with water to the fill line; inserted the airlock through the lid and clamped and left your vegetables to ferment; yet with time you noticed the water level in the airlock drop.

If the water drops below the inner piece of a 3-piece airlock, the water seal breaks. This allows air to enter the jar through the grommet hole. At this point, the Airlock Fermenter is no better than an open crock or mason jar with a threaded lid since it doesn’t completely restrict oxygen. This may lead to spoilage, and it certainly prevent ferments from reaching medicinal quality.

To solution to this is to check on the water level in the airlock around every four weeks. But this isn’t necessary when using glycerin in your airlock.

Since glycerin is stable and doesn’t evapourate, it provides a valuable solution to the airlock problem that all people fermenting with an airlock face, brewers and vegetable fermenter alike.

This article will explain what type of glycerin to use in your fermentation airlock, how to use it, and when glycerin in your airlock is not required.

What is glycerin?

Glycerin is a sugar alcohol derived from animals fats, plants oils, or petroleum.

Pure glycerin is non-toxic, odourless, sweet tasting, colourless, and has a syrup-like consistency. Vegetable glycerin is preferred for using in fermentation airlocks.

Depending on the grade, glycerin can be used in food, pharmaceuticals, and personal care products. It’s a common ingredient in foods to prevent drying out or to prevent the formation of ice crystals in frozen foods like ice cream.

In pharmaceuticals, it’s in some medications and used for suppositories or laxatives.

Since glycerin is hydrating to the skin, it’s used in moisturizers, hair gels, make-up, soap and other products. It’s specifically used in toothpaste to prevent it from drying out.

The grade of glycerin used in each context is determined by its purity, and its purity is determined by how it is made.

How is vegetable glycerin made?

It is made by heating vegetable oils – especially coconut, soy, or palm oil – with alkalis like lye or by using pressure. It can also be made synthetically from propene.

All vegetables oils have triglycerides, which are fat compounds made of a glycerin backbone with three fatty acid chains attached, hence, “tri”-glyceride. Pressure or heating alkali with vegetable oils cause glycerin to break apart from the fatty acid chains. 

Glycerin is then distilled to separate it from water. Using alkali may introduce impurities that are toxic to humans. Refining glycerin helps to remove some impurities, but it is hard to remove everything. The amount of impurities is reflected in the various grades of glycerin.

Food-grade glycerin

While there are several grades of glycerin – dynamite, commercial, chemically pure, or USP – the important thing when using glycerin in fermentation airlocks is to choose one that is fit for human consumption, that is, ultra pure.

Canada Food Inspection Agency and the U.S. Food and Drug Administration permit glycerin as a food additive, thought the grade of glycerin matters. 

Although you won’t be directly adding vegetable glycerin in your fermented food, it is possible for some suck back. This is a brewery term for liquids in the airlock pulled into the ferment when there are adjustments in atmospheric pressure or changes in temperature, such as transferring from room temperature to cold storage.

Choose glycerin that specifically says “food grade,” “appropriate for pharmaceutical and culinary uses,” or “Glycerin USP” like the one below. 

USP is a grade of glycerin that is high quality and suitable for pharmaceutical and food purposes. It stands for United States Pharmacopeia, which is a reference of uniform preparation standards for pharmaceuticals to ensure quality, purity, and potency. USP glycerin is available in drug stores and usually located on shelves near the pharmacy. Ask pharmacy staff if you have trouble finding it.

Choosing either food-grade or USP glycerin ensures the purest form of glycerin, free or with very low levels of impurities. 

Other varieties of glycerin are lower quality for industrial purposes, meaning not fit for human use, topically or internally.

Additionally, if you can find vegetable glycerin that is food-grade and non-GMO, even better. 

Non-GMO indicates that the vegetables from which oils were derived were not genetically modified. To my knowledge only the NOW brand of vegetable oil is both food-grade and non-GMO. Check your local health food store for this brand or other food-grade alternatives before ordering online.

How to use glycerin in an airlock?

Glycerin is thick. This with reduce the risk of back suck, but it also makes it hard to work with.

Instead of using straight glycerin in airlocks, mix some with water.

The 3-piece airlocks require just under 2 tablespoons of liquid to fill it to the fill line. The mini airlock takes 1 tablespoon. I use at least 1 part water and 2 parts glycerin. 

For example, in a 3-piece airlock, I use approximately 2 teaspoons water and 4 teaspoons glycerin for a total of two tablespoons (6 teaspoons). 

Using half glycerin and half water may allow too much evaporation to truly take the babysitting out of fermentation. 

Be sure to mix it with a wire or fork before pouring into an airlock. As long as you can see the glycerin texture in the water (thicker lines), continue to mix. All traces of the thick glycerin will disappear when it is mixed well. 

When glycerin isn’t required

For quick ferments that finish after 3-7 days at room temperature without needing a cold-storage phase to complete the fermentation cycle, glycerin isn’t essential. 

This applies to salsa, brine fermented beets, and fermented grated carrots, which are ready to eat in 3-7 days. Once the jar is opened to eat, you remove the airlock and replace it with a plug. No more needing to worry about the water level. 

It’s very unlikely that in 3-7 days water in the airlock would evapourate. If it’s hot enough to cause evaporation, it’s likely too hot for successful fermentation in the first place.

That said, if you plan to eat these quick ferments later, they should be stored in a fridge with an airlock and you may want to use glycerin in the airlock. Airlock jars (with the airlock inserted) stored in a fridge or root cellar will have a drop in the airlock water level. Expect it.

Such is the case with all fermented foods with a 12- to 13-week fermentation cycle, such as all cruciferous vegetables including brined Brussel sprouts or sauerkraut. This long storage time leaves much time for evaporation. 

Bottom line: If you are eating quick ferments right away, just use water in the airlock. If you are storing quick ferments in the fridge and delaying eating them, then you may want to use glycerin. 


Checking on your ferments in storage every four weeks is a hassle for some people and not possible for others who travel in the winter.

If the water levels falls below the inner piece in a 3-piece airlock, spoilage can occur due from entrance of oxygen.

Adding glycerin to your airlock with some reduces the rate of evaporation and the chance of spoilage. Glycerin is relatively inexpensive and commonly stocked in drug stores. Consider using glycerin in your fermentation airlocks from now on. 

And always remember to use the plastic cap on your airlock. It also reduces the rate of evaporation. 

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