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Once you have activated your kefir grains and they are making kefir regularly you may wonder if you are fermenting the milk the correct amount of time. This is very subjective and depends on how sweet or tart you like your kefir.
Kefir grains use lactose found in milk to acidify it. When it reaches a PH of 4.6 it will begin to gel and can then be considered kefir. This process can take 12-24 hours but can vary depending on the grain to milk ratio, temperature, type of milk, activity of the grains and personal taste.
The ambient temperature of the room
The various cultures which make the kefir grain their home have a temperature range of 60 – 90 degrees but they are most active between 65-85 degrees. Anywhere below 65 degrees their metabolisms will slow down and they will take longer to acidify the milk. Anywhere above 85 degrees some of the strains in the kefir will die out and above 90 several will not survive if this temperature is maintained over a longer period of time.
The kefir grain to milk ratio
The higher the ratio of grains to milk the shorter the time it takes to ferment. This is because the cultures in the grains need food and if there are a lot of grains per cup of milk the milk will become acidified sooner.
Over time the grains will continue to grow and the ratio of grains to milk will change unless you remove some from the fermentation jar.
Experiment a little to see what ratio you like and try to stay as close to it as possible. If you find that you have extra grains there are lots of things you can do with them. For some ideas read this post on what to do with too many kefir grains.
The type of milk used
Milk of different types have different chemical makeups which vary in the available lactose, fat and protein content. The grains use all of these for energy but the lactic acid bacteria which produce the lactic acid need simple sugars like lactose to grow. The more lactose available in the milk the faster the milk will turn to kefir.
How tangy you like your kefir
Kefir can be made to suit your needs. It can be made sweet and fresh by fermenting it until it just starts to gel or it can be fermented longer to become a tangy kefir with lots of lactic acid in it.
The tanginess of the kefir depends on two things:
The amount of remaining sugars in the kefir.
The cultures in the kefir grains will grow surprisingly fast when there is available milk sugars. The lactic acid bacteria can double in population every 2 hours if there is enough available sugars for it to grow. As the sugars get consumed their growth will slow down but will still continue to consume the sugar in the milk.
The overall PH of the kefir
As the lactic acid bacteria grow it produces lactic acid which lowers the PH of the kefir until it reaches a PH of about 4.2. Once it gets this low the action of the lactic acid slows due to lack of sugar but if left long enough it will continue to reduce the PH. The range of acidity which some species of lactic acid bacteria can produce is down to a PH of 3.5.
The difference is the overall PH of the kefir and the remaining amount of sugars, mostly lactose, which is found in the milk. The cultures in kefir grains grow surprisingly fast when there is a high amount of food energy available but as the food energy is consumed the growth slows and consequently the rate it acidifies.
Kefir is great for those who are lactose intolerant because the lactose is severely reduced as the kefir ferments. If you are lactose intolerant the amount of time it takes to make a kefir which is low enough in lactose to be comfortably consumed is longer than those who are not lactose intolerant. You may have to ferment your kefir 48 hours before you can use it. It all depends on your situation.
The current state of the kefir grains
Kefir grains are full of living cultures of bacteria and yeast which need food energy to survive. These cultures are hardy and can survive a wide variety of environmental conditions like dehydration, cold temperatures or highly acidic PH. The challenge is that the cultures in the grain grow at different rates depending on the environment.
This is what people mean when they talk about the health of their kefir grains or letting your kefir grains acclimatize to any new conditions. Every time the environment changes the ratio of cultures will change in the kefir. Since there are up to 50-60 different cultures in the grains at any environmental changes can affect the rate the kefir ferments.
Kefir grains which have been refrigerated for a long period of time may need longer to ferment until the population of the cultures equalizes. It may take a few batches of milk before the grains produce a pleasant kefir you like.
How do you know when kefir is ready?
Once the milk has reached a PH of 4.6 it will begin to gel and at that point would be considered kefir but when it is ready is up to you. It may take some experimentation to find just how long you ferment it with how many grains and at what temperature.
Kefir can be made with a short fermentation time producing a kefir with lots of milk sugars and whole proteins and tastes sweet and slightly fizzy or it can be fermented longer to lower the lactose and increase its acidity making it tangy and less fizzy.
How you like your kefir is up to you so do some experimentation with varying temperatures, fermentation times and types of milk.
What if I let it ferment longer?
The longer you let your kefir ferment the tarter it becomes. This is because as the cultures ferment the milk it consumes the sugars in the kefir. As the lactic acid bacteria consume the sugars they produce lactic acid which lowers the PH of the kefir. The acidity of the kefir will reach a threshold at about a PH of 4.2 but as the sugars are consumed the kefir will get sourer.
The milk fats will continue to solidify and form curds on the top of the container. If left long enough it will begin to smell like cheese.
Kefir made with kefir grains is a very stable environment and will remain usable for a long time but it may no longer be to your liking to drink it or pour it on your cereal. It is okay there are lots of things to do with over fermented kefir just check out this post of 23 ways over fermented kefir.
How do you know if kefir is bad?
If you detect mold growing on the surface of the jar or it begins to smell off, like sulfur or rotten milk then your kefir should not be consumed. Although this is a very rare occurrence as kefir made with kefir grains as it is a very stable product.
Even after the grains have been removed the active cultures in the grains will continue to protect it from spoilage by maintaining an environment which the spoilage bacteria cannot grow in. At the same time it will compete for food within the culture and as it ages the population of each culture will rise and fall depending on the available food sources. This makes the kefir a very stable micro-environment which ages well.
Since the cultures in the milk continue to ferment the milk even in the fridge it is best to consume your kefir within a week of making it before it gets too sour for you to drink it.