Fermenting your milk kefir

Basic fermentation

Excellent, so with all of our supplies on hand, we’re ready to get to work. Growing kefir often involves two separate fermentation cycles, the primary fermentation, before filtering your grains from the cultivated kefir, and an optional secondary fermentation, which continues after filtering. We are dealing here with primary fermentation.

Make the right choices

Depending on how you ferment and store your kefir, the taste and composition will vary widely. The taste can range from smooth and creamy (like an improved version of store-bought kefir), to an explosion of creamier, slightly sparkling, and slightly cheesy flavors. The texture can be smooth and quite thick, until grainy and almost watery.

While you can produce some interesting flavor and texture differences by varying every aspect of how you perform primary fermentation, we recommend, at least initially, that you follow the guidelines we offer below and play with it. developing more flavors and textures through any secondary fermentation you might do.

To get the right primary fermentation, we have to consider a trio of variables. These are the length of the fermentation period, the ambient temperature and the milk / grain ratio.

1. The length of the fermentation period

It’s still flexible, but we find, as it’s widely accepted in the kefir community, that 24 hours (plus or minus a few hours) is ideal. This will keep your grains happy and, provided you use the right milk to grain ratio, will produce a nice thick kefir, without over-separating the curd and whey.

2. Ambient temperature

Unless you have cooler, warmer spaces in your home, the room temperature may be something that you can’t over-control.

A warmer room temperature will certainly speed up the fermentation rate considerably. Therefore, to keep the fermentation time of kefir constant, you may need to adjust the milk to grain ratio seasonally, as the room temperature fluctuates.

The sweet spot temperature is between 18 ℃ and 20 ℃, but anything from 16 ℃ to 26 ℃ is fine.

3. The milk / grain ratio

The milk-to-grain ratio is the main variable you will have to play around with. At room temperature between 18 ℃ and 21 ℃, and aiming to complete primary fermentation within 24 hours, your optimal ratio of milk to kefir grains will vary between 10 parts milk to one part grain, (10: 1) and 20 parts milk to one part grain (20: 1).

When the ambient temperature rises above 21 ℃, the optimum ratio further increases significantly. At 26 ℃ and above, you could end up working with as little as 100: 1 (just 10 grams of grains per liter of milk).

This ratio is by weight, so at 10: 1 you will add 10 gr. of grains to 100 ml of milk. 

Keep in mind that these ratios apply to grains fed daily that are well adapted to their environment and to the milk you feed them. For grains that has just been shipped to you, we recommend that you start with 10: 1, regardless of the temperature. Then once you find your primary fermentation is ready to harvest well before 24 hours, add more milk next time.

Start of fermentation

So let’s say you start at 10: 1 and use 15g of kefir grains. Place a clamp lock pot on a set of digital scales and reset the reading to zero. Add the grains, reset the scale and measure in 150 ml of milk. Close the lid and swirl the pot to mix. You are now ready to set the jar aside in a suitable location and let it do its business. It doesn’t matter whether it’s somewhere exposed to light or not.

Checking and shaking

Every 4 to 8 hours, it’s time to check the progress of the ferment, then to shake the pot. Check the progress before shaking, as you cannot effectively measure the progress of your ferment until about an hour after shaking.

While the probiotic kefir microorganisms are active throughout the mixture, the grains themselves will land on top, expanding downward as the curds melt around them.

The purpose of shaking the pot is to recombine and disperse the forming curd and whey, exposing the grains to the curd more consistently. If you don’t, the grain will actually become deprived of its nourishment, causing the microorganisms to turn on themselves. In this case, the yeast elements tend to start to take over, which means the mixture can start to ferment too quickly, and you end up with unbalanced grains and a grainy, over-split kefir.

This shake should actually be quite vigorous, turning the pot upside down and inverting as you do it. You don’t have to worry about what harms your beans, they aren’t bothered by even the most brutal shaking!

If your grains end up being too leavened, don’t worry either, just lower the grains to milk ratio, shake the pot more often, and on a few batches they’ll sort themselves out.


During the fermentation period, you will periodically hear gas hissing around the sealing ring of the lid. This is perfectly normal and we personally prefer not to open our jars to stir or vent gas during fermentation. However, if you’re concerned about the risk of your pot breaking, you can release the clamp just enough to manually release any built-up pressure without letting too much air (and oxygen) in from the outside.

So when is he ready?

The first signs of action

At first your beans will float upwards and nothing will happen for a while.

After a few hours, you will notice that the composition of the mixture changes noticeably at the top, with curd and clear liquid whey starting to form a suspension around the grains.

As you can see in the picture, the rest of the mixture in the lower part still looks like milk. There will be a separate separation between these upper and lower areas.

Some people will say that kefir can be strained at this point, but while it is perfectly acceptable and safe, our preference is to stay on board at least a little longer.

The first step in preparation

In this image, you can see that pockets of whey are now forming in the top half of the mix, while the separation between the top and bottom may not be (but not always) as sharply linear as before.

You may also be able to detect by inspecting closely that the lower part where the whey has not yet visibly formed no longer looks as much like milk as before, having taken on a slightly curdled consistency.

This is the first point of preparation, when you can go ahead and put in some effort if you want to. Otherwise, you’ve entered the sweet spot, where you can choose your preferred time to harvest.

It’s time to harvest

Here we can see that the curds and whey pockets have constantly spread all over the mixture and the clear liquid whey is starting to build up at the bottom.

Now might be the perfect time to strain! This moment needs to be watched carefully, as another hour or so (or less at higher room temperatures), and this mixing would have progressed a lot.

Go further

As fermentation continues, the clear liquid whey at the bottom will continue to increase until it reaches about a third of the entire mixture.

The kefir harvested at this point is still perfectly safe and flavorful, with some brewers preferring to take their primary fermentation this far.

However, it may have become more difficult to filter, as the curd has become more coagulated. It can also be difficult to recombine the curd and whey after the kefir has been filtered, without using a blender. For this reason, you may find the resulting texture thinner and a bit grainy – perhaps less appealing than when harvested a bit earlier, but it all depends on your personal preferences.

And remember, if you like funky flavored kefir, or are concerned about lactose, you can always do secondary fermentation!