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When you first start making milk kefir you will notice that your kefir grains will hang out in different locations in the milk. Most of the time they will hang out at the top of the jar.
Although kefir grains are denser than milk they are encased in a biofilm which is less dense than milk. CO2 produced by yeast and lactic acid bacteria can get caught in the grains which helps to raise the grains to the top.
Cow’s milk is mainly made up of fat, protein, and lactose. Each of these basic ingredients have a different density and each of these can vary depending on the health of the cow, stage of lactation and the breed. Other factors include the collection and processing method used, transportation and handling of the milk. Therefore the density of milk can be anywhere between 1.026 g/cm3 – 1.034 g/cm3 at 20°C.
The density of kefir grains is about 1.03 g/cm3 and is fairly steady in its composition. It may vary a little due to the composition of the cultures in the grains but since there have not been many studies concerned with its density it is not known what range it has.
Since the densities of the two is so close if bubbles form and get caught in or on the kefir grains they will act as little floatation devices and float to the top of the milk. In most cases this does not pose a problem but it produces a kefir which may have an uneven fermentation from the top to the bottom.
This can be seen by inspecting the kefir jar after 24 hours of adding the grains. On top you will notice the grains and thickened milk mixed in together. The further down the jar the higher the PH and the thinner the consistency.
What to do about it
For Small batches
If you make kefir in small batches like a quart sized Mason jar it will not be a problem.
Put the grains in first then pour in the milk. This will help mix the kefir which comes along with the grains into the milk spreading the kefir culture into the milk and as a result the bacteria and yeasts will be evenly mixed into the milk. Give the milk a stir before covering the jar to ensure the kefir grains have come in contact with as much of the milk as possible.
If you are available during the day you can give your kefir a stir every once and a while to redistribute the grains and to mix the growing culture around the jar.
For Larger batches
It is of more concern when larger batches are made. When making larger batches it is difficult to ensure that the milk has been properly inoculated with the kefir culture. If the grains float to the top of the jar the milk on the bottom may not have enough contact with the grains or the kefir which came along with the grains.
When the grains get concentrated at the top of the container the milk may get little contact with the cultures.
There are two problems with this.
The kefir may not thicken appropriately
The solids in milk begin to gel as the kefir acidifies causing the kefir to thicken. If the grains are concentrated at the top of the milk this acidification will be uneven, causing the milk at the top of the container to thicken more rapidly than the milk of the bottom.
It may appear to have thickened until the grains are strained out and you may find milk which has not thickened on the bottom.
When this happens give the container holding the kefir a good stir and ferment it for another 12-24 hours until it has properly acidified.
Some of the kefir may have a higher amount of lactose left
The sugar in the milk (lactose) is consumed by the cultures in the grains. This makes it easier to digest for those who are lactose intolerant. If the milk is not evenly fermented through there may be a higher amount of lactose left in the milk making it less digestible.
When this happens give the container holding the kefir a good stir and ferment it for another 12-24 hours until it has properly acidified.
When larger batches are being made it is important to mix the grains around in the milk occasionally to redistribute the grains within the milk. This will ensure that the culture in the grains will be distributed evenly in the milk.
Grains which sink to the bottom
There are a few reasons why kefir grains may sink to the bottom.
The grains are small with few crevasses for the CO2 to get caught in
As mentioned above kefir grains are denser than milk so they should sink. It is only the bubbles of CO2 gas which act as little flotation devices which get caught in the grains. If there are few places for the gas to get caught then the grains remain on the bottom of the jar.
The yeasts in the grains are not very active
The CO2 is made by the yeasts as a byproduct of their respiration. If the yeasts are not very active then they produce less gas and the grains do not float. To help the yeasts to be more active try placing the container in a warmer location.
Fermenting kefir under pressure
Some people like to ferment their kefir in an anaerobic environment. Which means without oxygen. This can be accomplished in a few ways such as using a Mason jar with a sealed lid or some other sealed jar.
Kefir grains in a pressurized jar can sink to the bottom because the CO2 which the yeast produces will go into solution rather than form bubbles on the grains surface.
This can be dangerous as the sealed jar tends to hold pressure in and can burst the jar sometimes with great violence which can cause serious injury from flying glass.
If you are going to ferment your kefir in an anaerobic environment either use a vapor lock or burp the jar regularly to prevent pressure buildup.
