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There are several ways to make sour yogurt and the method you choose will depend on your timeline, purpose for the yogurt and available equipment. The flavor of the sour yogurt will also vary a little with each method for a variety of reasons such as the number and type of active lactic acid bacteria, the remaining sugar in the yogurt and how sour the yogurt gets.
Extend the incubation time
By extending the incubation time you give the lactic acid bacteria in your starter more time to make lactic acid which makes your yogurt sour. Additionally the starter consumes more of the lactose in the milk making it less sweet.
To prevent contamination from the air you will need a container with a sealable lid. This will prevent any mold and other unwanted spoilage bacteria from entering the milk during incubation. When making normal yogurt the incubation time is short enough that any unwanted bacteria, yeast or mold don’t have enough time to get a foothold in the milk but with a longer incubation time you either need a strong starter like a heirloom starter or you need to take extra care to prevent contamination.
Equipment:
- 1 container with a sealable lid
- An incubation chamber (yogurt maker, oven with light on or other temperature controlled space)
- Double boiler
- Whisk
- Measuring spoon
- Thermometer
Instructions:
- Follow the instructions on how to make yogurt using your usual method (check this recipe if you don’t already make yogurt regularly)
- Ensure your container is sealed properly to prevent air flow into the container
- Incubate the yogurt 12-48 hours depending on how sour you want your yogurt (for a full explanation of what happens to yogurt during long incubation times check out this article here
- Once you have reached the appropriate sourness refrigerate it for at least 2-4 hours
Use extra starter
Sour yogurt gets sour when the lactic acid in the starter consumes most of the sugar in the yogurt and produces high amounts of lactic acid. To do this it needs a large population or cell count. The above method used time to allow the lactic acid bacteria to multiply and consume the lactose in the milk. This method jump starts that process by starting with extra starter from the get go.
To make yogurt you only need a teaspoon or so of starter so to make sour yogurt try using double the amount of starter. Incubate for the usual amount of time and test the result. If it is not sour enough try using even more starter.
To get predictable results using this method of making sour yogurt you will need to control the temperature and time you use to make your yogurt. A higher temperature will speed up some of the lactic acid bacteria species which can alter the flavor of the yogurt. The amount of time affects the population of the lactic acid bacteria in the milk which can quickly make the yogurt too sour for your purpose.
To prevent temperature and time from affecting the finished yogurt use an incubation chamber, dehydrator with temperature control or a yogurt maker. That way you reduce the number of variables in making your sour yogurt to only the amount of starter you are using.
Instructions:
- Follow the instructions on how to make yogurt using your usual method (check this recipe if you don’t already make yogurt regularly)
- Add double the amount of starter you usually use
- Whisk the starter into the milk thoroughly
- Incubate the yogurt 4-6 hours
- Refrigerate for at least 2-4 hours
Increase incubation temperature
Every living thing has an optimal temperature range which it thrives in, including lactic acid bacteria. For yogurt starters the range is between 108-120℉℃. Anything below this temperature and the lactic acid bacteria will multiply slowly causing the yogurt to become stringy or ropy. If the temperature is much above 120 for any period of time and the starter will die which will allow other bacteria to grow and spoil the milk rather than gelling it.
To produce sour yogurt by incubating the milk at a higher temperature the temperature needs to be as close to 120 as possible. This will allow the lactic acid bacteria to multiply quickly and consume more of the lactose in the milk. As the lactose falls the yogurt tastes increasingly sour.
To do this you can use an incubation chamber with a temperature control, a dehydrator set at 120. Using an oven with the pilot light on is only an option if your oven is well insulated and you can contain the heat from it in some way. Some yogurt makers have a temperature control which allows other fermentations to be incubated with them. To use them, set the temperature manually rather than using the preset buttons.
Instructions:
- Follow the instructions on how to make yogurt using your usual method (check this recipe if you don’t already make yogurt regularly
- Set your chosen incubation appliance to 120
- Incubate the yogurt 4-6 hours
- Refrigerate for at least 2-4 hours
Use high sugar milk
One of the limiting factors for the growth of lactic acid bacteria is the availability of fermentable sugars. Since bacteria need simple sugars for their respiration and they cannot break down starches or protein to use as fuel like other organisms, having an abundance of sugar allows the culture to increase its population rapidly.
As the culture grows it produces lactic acid which acidifies the milk causing it to gel into yogurt. By using milk with a high sugar content the starter will be able to produce more lactic acid than other types of milk. To take advantage of this use skim or 2% milk as they have a higher sugar content than milk with a higher fat content. With more lactose available the starter will be able to grow bigger.
You can also add additional sugar to the milk prior to heating it to give the starter an additional boost if you are after a really sour yogurt. To do that add no more than a teaspoon per quart of milk. Any more and the lactic acid bacteria can grow too quickly and curdle the milk, making for curds and whey rather than creamy yogurt.
Instructions:
- Follow the instructions on how to make yogurt using your usual method using low fat milk
- If you want additional sourness add a teaspoon of white sugar prior to heating the milk
- Incubate the yogurt 4-6 hours
- Refrigerate for at least 2-4 hours
Add additional acid
Sour yogurt is sour due to a higher concentration of acid in the yogurt. The preceding methods of how to achieve this have been focused on the lactic acid bacteria which produce lactic acid but that is not the only method of making your yogurt sour. You can also add additional acid directly to the yogurt.
To do this you can add the acid either prior to or after incubation. You can get powdered acetic or lactic acid from home winemaking stores. Acetic acid is used often to adjust the acid level in wines whereas lactic acid is used in the making of Saki. Either acid will work although lactic acid will produce a sour yogurt which is closer to the flavor of naturally fermented sour yogurt.
When adding acid to your milk prior to incubation be sure to only use a small amount. Add it after the milk has been heated and gently stir it in before incubation. If you add too much acid at the beginning your milk will curdle as it heats up. This is because the proteins in the milk will begin to associate with each other when the pH of the milk falls below 4.6.
When adding additional acid after incubation, sprinkle the acid on the surface of the yogurt and gently fold the acid into the yogurt prior to refrigeration. This will allow the acid to permeate throughout the yogurt and allow the yogurt to set in the fridge. If your yogurt’s pH falls too low it will cause the whey in the yogurt to separate from the curds which will destroy the texture of the yogurt.
Problems with making sour yogurt
Sour yogurt has a lower pH than normal yogurt, which is why it is sour, but this can cause problems as the lower pH affects the behavior of the proteins in the milk. As milk sours it changes the proteins in the milk to associate with one another which is why the milk thickens into yogurt. If the association is strong enough it will curdle by clumping together in tight bonds and expelling water and fat.
There are three things to know to avoid this from happening.
- Keep the acidification process slow
When the acidification is too fast the consistency of the milk does not change fast enough to prevent the protein molecules from clumping together causing curdling.
- Use whole milk
Whole milk is thicker with more fat than skim milk. The fat resists being expelled from the association of the protein, maintaining the yogurt’s consistency.
- Do not disturb the milk while incubating it
When milk is disturbed it encourages the water in the milk to separate from the yogurt. As milk acidifies it is the proteins which associate with one another not the water molecules. Once disturbed the water will be forced out of the way and clumps of proteins will form and the milk will curdle.
Sour yogurt is not easily eaten
Sour yogurt is high in lactic acid making it very tart. Not the typical yogurt flavor which makes it difficult to eat plain or with a little fruit. Since you may be making sour yogurt as part of a specific diet, consuming it like you would normal yogurt is not pleasant. Instead you can use it as a base for savory meals, use it to make dips and sauces or as a cooling topping for spicy meals.
Sour yogurt is best used as an ingredient rather than the main item of a meal. To get the most out of your sour yogurt do not heat it too much. Instead add it at the end of the cooking period to preserve any good for you lactic acid bacteria and vitamins which can be affected by heat.