Normally yogurt has a slightly tangy flavor which when added into a recipe, provides a nice tingly feeling in the mouth, adds to the mouthfeel and the taste of the recipe.  Most of the time cooks are interested in the creamy texture which yogurt adds to the recipe with a little addition of acidity from the lactic acid in the yogurt.  

Acidity translates to sourness in the mouth, the lower the pH of the yogurt the more sour it tastes.  There are several reasons why yogurt may become overly sour unintentionally (to learn how this can happen check out this article) but this article will focus on why you would and how you can increase the acidity (sourness) of homemade yogurt.

Why Make Sour Yogurt

You may want the lower lactose content, increased acidity, higher bacteria count, greater tendency to curdle or make sour yogurt for a specific recipe.  Sour yogurt has many uses and provides some unique benefits which normally incubated yogurt does not have.

Why sour yogurt has a lower lactose content

Lactose is the sugar found in milk which many people are sensitive to.  In fact most humans lose the ability to digest lactose once they are no longer babies.  Lactose is made up of two different mono-saccharides glucose and galactose.  The lactic acid bacteria have enzymes which divide these two sugars and use the mono-saccharides for respiration.  

As the lactic adic bacteria lower the amount of lactose in the milk it becomes more digestable for those who are lactose intolerant.

Benefit of the lower lactose content of sour yogurt

Since most of us cannot directly digest lactose (humans don’t have the enzymes needed) consuming yogurt with a lower amount of lactose improves its digestibility.  This is made apparent by those who are especially intolerant to lactose.  Many people who cannot drink milk can digest yogurt with little problem.  This is because there is less lactose in the yogurt and more bacteria which can continue breaking the difficult elements of the milk down.

Benefit of a higher probiotic count

If you have ever shopped for a probiotic pill you will know that the main advertisement for them is the live cell count with numbers ranging from millions to billions depending on the brand and delivery method.  It is assumed that the more living probiotic cells the better.  

When it comes to yogurt the bacterial culture takes time for the population of the bacteria to grow.  The main four bacterial species found in a yogurt culture continue to increase in population for 48-52 hours respectively.  Along with the long incubation time comes high amounts of lactic acid production leading to sour yogurt.

Make sour yogurt using a long incubation period

Yogurt is made with the addition of a yogurt culture, mostly made up of lactic acid bacteria, which consumes the sugar (lactose) in the milk and acidifies it by producing lactic acid.  Once the pH of the milk reaches somewhere around 4.6 the proteins in the milk begin to associate with one another forming a loose matrix which thickens the milk.

A long incubation period allows the lactic acid bacteria enough time to consume more of the lactose in the milk.  This lowers the sweetness of the yogurt as well as raises the acidity of the yogurt adding to its sour taste.  The longer the incubation period the lower the sugar content and lower pH of the yogurt.

Before making long incubated yogurt you have to understand some of the problems which can wreck the yogurt and why these problems happen.

Yeast overgrowth on surface of the yogurt

Yeast is a constant companion of any type of fermentation, including yogurt making.  Yeast species are present in every environment and will consume every type of simple sugars available.  They can grow rapidly and survive in acidic conditions.

To control the yeast population in a yogurt incubation it is important to limit their population count in the milk prior to incubation and boost the lactic acid bacteria’s ability to dominate the culture.  

Pasteurize the milk 

Don’t skip the pasteurization step prior to inoculation.  This will destroy any yeast found in the milk prior to inoculation.  Heat the milk to at least 180℉℃ and maintain it for 10-20 minutes.  There are several reasons why this is done.  To learn more check out this article.

Incubate in an anaerobic environment

It is true that yeast can grow in an aerobic and an anaerobic environment (with or without oxygen) but lactic acid bacteria prefer an anaerobic environment.  To encourage the lactic acid growth limit its access to oxygen.

Separated curds and whey

There are three components of milk which need to be balanced to make yogurt which is creamy and delicious.

1. The protein
2. The milk fat
3. The water content

Each of these three components add to the texture and mouthfeel of the yogurt.  When making long fermented yogurt these three can get out of balance and result in excessive amounts of the water from separating from the milk solids forming curds and whey.

Although this is part of the process of making cheese it needs to be avoided when making yogurt and when making sour yogurt the chances of it happening are higher.

As the milk acidifies the proteins in the milk are attracted to one another with a stronger bond.  As this bond strengthens the water in the milk is forced out of the matrix which makes up the texture of the milk.  

To prevent this from happening there needs to be enough fat in the milk to act as a buffer to this effect and the milk needs to remain undisturbed as much as possible.  Use higher fat milk when making sour yogurt to produce a creamy sour yogurt and incubate it without testing or moving it for the duration of the incubation period.

Moldy off flavors

Another constant companion of fermentation is mold.  It is not always bad in the case of cheese or kogi making. Koji is used to make a wide variety of traditional asian foods but when it comes to yogurt making it produces an unpleasant off flavor which can linger in the mouth long after tasting it.

To prevent mold growth follow the recommendations for preventing yeast growth in yogurt but add two other things.

1. Clean all equipment with hot running water prior to use

Mold can consume any type of carbohydrate including long chain starches which yeast and bacteria cannot use for energy and in the process of its growth it produces toxins which inhibit the growth of other microorganisms.  It is these toxins which produce the off flavors in spoiled food.  By washing all equipment prior to use you are not only destroying mold spores and removing mold growth but you are washing away much of these toxins from your equipment.

2. Only use fresh milk 

Old milk will have a higher amount of mold species and population than fresh milk.  Due to the health standards in most countries milk produced in industrial dairies is safe to consume up to and a little beyond its best before date but when using old milk for sour yogurt the toxins produced by the ever present mold are concentrated and highlighted.  

The mold itself will be destroyed by the pasteurization but the toxins will remain in the yogurt and due to the lower sugar content they will be more noticeable.

How to make sour yogurt using an extended incubation period

Using an extended incubation time will give the lactic acid bacteria a longer active period to consume the lactose in the milk and produce lactic acid.  To extend the incubation period you will need an incubation chamber which can maintain a steady temperature for 12-24 hours or more (depending on your purpose). 

Equipment:

• Incubator which can maintain a temperature range between 108-115℉ (42-46℃)
• Double boiler
• Wooden spoon
• Whisk
• Container to hold the yogurt
• Teaspoon 

If you don’t have an incubator checkout this article which gives you several options. If you decide you want to purchase a yogurt maker you can use this Amazon affiliate link to a collection of yogurt makers currently available or if you want to go higher end and purchase an instant pot like this one on Amazon

Ingredients:

• Fresh milk
• Yogurt starter (affiliate link)

Instructions:

• Heat the milk in the double boiler to 180℉ (82℃) 
• Maintain temperature for 10-20 minutes
• Pour milk into container and cool to 115℉ (46℃)
• Add 1 tsp yogurt starter to cooled milk
• Whisk milk and starter together
• Place in incubator and leave undisturbed for 12-24 hours
• Move yogurt to the fridge and cool for 2-4 hours before use

Note:

The longer the yogurt incubates the less lactose will remain in the milk.  As this happens it will become increasingly sour.

Make sour yogurt using extra starter

This method of making sour yogurt is usually discovered accidentally.  It happens when an extra yogurt starter is used at the beginning of the incubation period.  When an extra starter is used the starting population of the lactic acid bacteria is higher and therefore multiplies faster than otherwise.

The growth of bacteria in a substrate is an exponential growth not a linear one.  It is like multiplying by two as opposed to adding two to a total.  

2 X 2 = 4 X 2 = 8 X 2 = 16  as compared to 2 + 2 = 4 + 2 = 6 + 2 =8 

When extra starter is added it is like you are starting at 4 X 2 or 8 X 2 which leads to a much faster population growth and much more lactic acid production which makes the yogurt sour.  

Before making sour yogurt by adding extra starter there are some things which are important to understand.  Yogurt made this way can be unpredictable and can have an unfamiliar texture which some find unpleasant.

Why extra starter can affect the texture of the yogurt

Every living thing has different environmental conditions in which it excels in.  whether it is the ambient temperature, pH, availability of oxygen or a myriad of other things.  When you use extra starter to make sour yogurt the conditions for growth of some of the cultures in the milk may be more conducive for growth than others and since there is a higher number of them they can quickly overtake the milk.

With one or two types of lactic acid bacteria dominating the culture the yogurt will have a different texture than when many species of lactic acid bacteria compete for resources.  

Some lactic acid bacteria produce higher amounts of biofilm, some produce carbon dioxide and less lactic acid while others produce organic compounds which can be unpleasant.  The challenge is you don’t know which one will dominate the culture and why.

Extra starter increases chance of curdling

Yogurt when made with the recommended amount of starter will acidify over a longer period of time.  This allows time for the proteins to develop the loose matrix with each other which incorporates the fat and water borne proteins in the milk.

When extra starter is used the pH of the milk is lowered much faster and as a result the proteins form strong bonds faster, forming curds and driving out the water (the whey) from suspension.  This can be demonstrated by adding vinegar to milk and stirring.  The lowering of the pH of the milk curdles the milk quickly.

How to make sour yogurt using extra starter

To remediate the risk of making a yogurt with an unpleasant texture or curdling either use a fresh yogurt starter which is active or use a heritage starter.  The fresh active starter will have a culture which is competing well amongst themselves, keeping one species from dominating the incubation.

Heritage starters have many bacteria and yeast species in them making up a very stable culture which self-regulates its population.  With the higher number of species one or two find it hard to dominate, this produces a more predictable product.

Equipment:

• Incubator which can maintain a temperature range between 108-115℉ (42-46℃) 
• Double boiler
• Wooden spoon
• Whisk
• Container to hold the yogurt
• Tablespoon 

Ingredients:

• Fresh milk
• Yogurt starter

Instructions:

• Heat the milk in the double boiler to 180℉ (82℃) 
• Maintain temperature for 10-20 minutes
• Pour milk into container and cool to 115℉ (46℃)
• Add 1-2 tbsp yogurt starter to cooled milk
• Whisk milk and starter together
• Place in incubator and leave undisturbed for 4-8 hours
• Move yogurt to the fridge and cool for 2-4 hours before use

Note:

Time is the ultimate factor when it comes to sour yogurt.  It is important to leave the yogurt undisturbed for at least 4 hour but the longer you leave it the more sour it will get.

How to make sour yogurt quick and easy

Sour yogurt has many uses in cooking.  It can be used in quick bread baking to give the rising agent a boost, add tang to a sauce, add acidity to a dip and be used in marinades to help penetrate the meat’s surface and draw in the flavors.

When you don’t have any sour yogurt on hand it can be made quickly and easily with the addition of some type of acid and left to sit for 4-24 hours.  Some recipes for sour yogurt are actually recipes for curd which is an unfermented thickened milk made by adding acid to milk to lower the pH of the milk which causes the proteins to associate with one another (just like yogurt).  

This type of sour yogurt still has the normal amount of lactose in it as the lactic acid bacteria has not had the time or population to consume it so it cannot be used as a replacement for those who are lactose intolerant but it can be used as a substitute for sour yogurt in recipes.

Two ways to quickly make sour yogurt

1. Add some acid to the milk during incubation

When making yogurt you can add some acid like acetic or citric acid by adding some vinegar or lemon/lime juice to the cooled milk once the starter has been added.  The additional acid will quickly lower the pH of the milk and produce sour milk in 4-6 hours.

2. Add some powdered acid to finished yogurt

If you have some yogurt which you need to sour quickly you can add some powdered citric, acetic or lactic acid.  These can be found at beer and wine making stores or online.  

Simply add a small amount of powdered acid to the yogurt and gently mix it in.  The acid will quickly lower the pH of the yogurt making it tangy.  Be careful to not mix too much as it may cause it to curdle.