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Fermentation is not a fast process. There are no fermentation recipes found in microwave cookbooks so when I say fast I mean “faster than normal”. To make yogurt fast you need to understand what it is you are trying to do. Milk thickens to yogurt once the pH of the milk reaches 4.6 or lower.
To shorten the incubation time of yogurt, use more starter, increase the temperature, use low fat milk or pre-acidify the milk. This will shorten the time it takes for the milk to reach a pH of 4.6 where the proteins will begin to gel.
Yogurt is made when lactic acid bacteria found in the starter consume lactose in the milk producing lactic acid which acidifies the milk. When the milk reaches a pH of 4.6, proteins in the milk associate with each other causing the milk gel. Once the milk has gelled it is considered yogurt.
Milk needs to have a pH of less than 4.6 before it will start to gel, therefore to produce yogurt quickly you want to use the sugar (lactose) in the milk as fuel and acidify it as fast as possible. This is done by promoting the growth of lactic acid bacteria.
Incubate the milk at a higher temperature
The lactic acid bacteria which are present in the yogurt culture have a temperature range which they grow the fastest at. The usual range given for yogurt culturing is between 110-115℉. This produces a nice stable yogurt with good consistency but the goal here is not perfect yogurt but fast yogurt.
At the higher temperature range and even above up to 120 the lactic acid bacterial doubling rate of some of the lactic acid bacteria decreases. The doubling rate is the time it takes for the bacteria population to double, the shorter the doubling rate the faster the bacterial growth.
As the population of the lactic acid bacteria increases so does the amount of lactic acid which is produced. It is this acid which lowers the pH of the yogurt. The more active bacterial cells producing lactic acid the faster the milk will gel.
How to ferment yogurt at a higher temperature
To be able to make this work you will need an active yogurt culture which can be either from store bought yogurt, yogurt made from powdered yogurt starter or by using a heritage yogurt culture.
- Warm the milk to 180 and maintain for 10-20 minutes
To understand why heating the milk is an important step of making yogurt read this post about the benefits of heating milk prior to fermentation.
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- Cool the milk to between 115-120
The fastest way to accomplish this is to pour the milk into clean glass mason jars and place the jars in cool water. Be careful not to use cold water as the difference in temperature of the milk and the cold water may crack the jar or cool the milk too fast.
Slowly agitate the milk in the jars with a spoon and closely monitor the temperature. Remove the jars for the cool water bath once the temperature reaches 125 or so as the temperature of the glass will continue to lower the milk’s temperature rapidly.
- Stir in the yogurt starter
Once the temperature reaches 120 stir in the starter. Use a whisk to ensure the starter is fully mixed in. This will disperse the lactic acid culture evenly which helps all the bacteria cells to have access to enough lactose to begin to multiply.
- Place the lid and place the jars into your incubation chamber
An incubation chamber can be anything which has a controllable temperature and will maintain the temperature between 115 and 120 for at least 4 hours. For incubation chamber ideas you can read this post.
- Check the yogurt after three hours
Test the yogurt by tipping the jar slightly and watch to see if it has thickened at all. If it has not thickened at all, leave it for another hour. Once it has begun to thicken you will know the pH has dropped to 4.6 or below .
- Cool the yogurt
Once the pH has reached 4.6 the milk proteins will begin to gel. Cooling the yogurt will help to solidify the milk further making for a firmer creamier yogurt. Cool for at least 2 hours before using the yogurt.
It is important to remember that not all of the species of lactic acid bacteria which are present in your starter will react well to a warm incubation temperature, especially if you are using a heritage culture which has a wide variety of bacterial species in it. This may affect the texture, flavor and fragrance of the yogurt.
To ensure your culture remains viable, reserve some of it out of the high temperature batch to culture normally. This way you will not risk losing your heritage culture for the sake of one fast batch of yogurt.
Use more starter
The yogurt starter is the source of lactic acid bacterial species which ferments the milk sugar (lactose) and produces the lactic acid which lowers the pH of the milk causing it to gel. In most yogurt recipes small amounts of yogurt starter are used to ferment a batch of yogurt.
The amount of starter ranges from a teaspoon for a quart to ¼ cup for the same quart container of milk. How much you use is up to you, both will work with varying results depending on incubation temperature, type of milk and how you like your yogurt.
When you want to speed up the fermentation, increase the amount of starter. This will increase the amount of active lactic acid bacteria in the milk, which in turn reduces the time it takes for the pH to drop to 4.6.
How to use extra starter to reduce fermentation time of yogurt
The process of making yogurt with extra starter uses the same method as most yogurt making recipes with the exception of doubling or tripling the starter used. Ensure you are using an active culture with a short lag time (not a powdered starter) as the activity of the starter is vital to fast fermentation.
- Prepare the milk as above
Follow the procedure for making yogurt outlined in the preceding section until you add the starter.
- Add double or triple the amount of starter you normally would use
The extra starter puts a larger amount of active lactic acid bacteria into the milk. This shortens the amount of time it takes for their population to be high enough to begin lowering the pH of the milk.
- Incubate the milk normally
Place the milk in your incubation chamber. Incubate the milk between 110-115℉. The higher population will grow rapidly within this range and quickly acidify the milk.
Use high lactose milk
High lactose milk will provide easily fermentable sugars to the lactic acid bacteria. This will help to lower the doubling time of the lactic acid bacteria which in turn reduces the amount of time it takes to lower the pH to 4.6.
Milk with a lower fat content has a higher lactose content in it. Products like skim milk, 1% or other low fat milk products will ferment faster. One of the important factors in fermentation of any type is the availability of food sources to the culture, when it is easy for the bacteria to access less energy is used to get the needed food and more energy is spent on growth.
How to use high lactose milk to speed fermentation time
This method is as simple as using low fat milk during the yogurt making process. The lactic acid bacteria will not have to work as hard to multiply and the pH will fall faster than higher milk fat content milk.
The yogurt will not be as thick as whole milk yogurt because of the lack of fat in the milk but it will gel at the usual pH of 4.6. Once it has reached this level, place it in the fridge for an hour or two to cool before using it.
Pre-acidify the milk
Cow’s milk has a natural pH close to 7.0 which makes it neither acidic or basic. This pH needs to be lowered to at least 4.6 before the milk will gel to produce yogurt. This happens normally as the lactic acid bacteria use lactose in the milk and produce lactic acid as a byproduct but if you pre-acidify the milk with something the amount of time it will take to reach a pH of 4.6 will be greatly reduced.
The benefit of this is the fermentation time can be controlled by the amount of acid added. The lower the starting pH the milk has the shorter the fermentation time.
The drawback is the yogurt will have a different flavor and texture than normally made yogurt. The addition of an acid will quickly lower the pH but has no effect on the lactose content. The result will be either an overly sweet yogurt or a yogurt which will continue to ferment making it extremely tart.
You have to be careful to not add too much acid to the milk as if it lowers the pH too quickly the milk will curdle making it unusable for yogurt.
- Acetic acid powder
Used for making certain types of cheese, acetic acid powder can lower the pH of milk quickly and curdle it without any lactic acid bacterial action. One of the most known cheeses made with acetic acid is mozzarella. When making yogurt only use a small amount of acetic acid to lower the pH a small amount leaving the lactic acid bacteria to finish the job
- Liquid acetic acid
Otherwise known as vinegar liquid acetic acid has been used to acidify food to help preserve it forever. To use it to acidify milk prior to fermentation only use small amounts as it adds volume to the milk and will affect the texture of the yogurt.
The vinegar will add flavor to the yogurt depending on the type of vinegar you use. For the most unnoticeable flavor use distilled white vinegar. Only use a small amount of vinegar otherwise the yogurt may become overly tart.
If you are using the yogurt for making dips and other creamy recipes then any type of vinegar will do but if you want it for putting on cereal or in smoothies consider using white vinegar which has the least amount of additional flavors.
- Powdered lactic acid
Powdered lactic acid can be found online and is used in the process of making Sake when the fast method is used. It will not add different flavors to your yogurt as it is the acid which is produced by the lactic acid bacteria during fermentation.