Why is Yogurt Sour? | Bacteria and Lactic Acid

Yogurt is sour because it is made by bacteria which ferment milk breaking down the lactose into glucose and galactose. When the glucose is broken down lactic acid is released which gives the yogurt its sour taste. The older the yogurt is, the more of the lactose and glucose will have broken down and the sourer it will taste. Pot set, Greek and natural yogurts will taste more sour than flavored yogurt.

This article will explore why yogurt has a sour taste, what you can add to your yogurt to make it less sour and how to know when it has gone bad.

Lactic acid and sour yogurt

The bacteria in yogurt will ferment the lactose in the yogurt making lactic acid.

It is this lactic acid that gives the yogurt its sour taste.

There are millions of bacteria in each tub of yogurt digesting the lactose and creating the delicious yogurt we all love. These friendly bacteria are not harmful to humans and they can continue to live in the yogurt even when it is packaged.

The acidic environment of the yogurt made by the fermentation process will curdle the milk separating the curds which is the thick milk proteins from the whey, the liquid yellow material that can be strained of yogurt before it is packaged.

If you notice yellow whey liquid sitting on the top of your opened yogurt, this is because it has separated out again or is from the continued fermentation by the bacteria. You can strain it off or mix it into the yogurt.

Why does the sour taste in yogurt increase over time?

As the yogurt sits for longer in your fridge the bacteria that are in the yogurt will continue to break down the lactose into glucose and galactose. When the glucose is broken down (glycolysis) through the citric acid cycle anaerobically (without oxygen) by the bacteria the by-product of the process is lactic acid.

As this process continues for longer and longer, more lactic acid will build up in the yogurt giving it a sourer taste over time.  Pouring off any excess whey or liquid on top of the yogurt can help to reduce the sour taste.

You could also take the opportunity to mix through some stewed or fresh fruit. The sweetness of the fruit will help to mask the sour taste of the yogurt.

Should yogurt taste sour or has it gone bad?

Most natural yogurts will have a sour taste. This includes unflavored, pot set and Greek yogurt. Yogurts without the sour taste often have additives including fruit or sugar which can mask the sour taste.

If yogurt tastes too sour, smells like vinegar or has blue or black mold on top then it is not good to eat. If the container has expanded and the lid appears to be bloated then it might also be off. If you taste the yogurt and it doesn’t taste right or is past it’s expiration date then get rid of it.

It is not worth risking your health by eating yogurt that has gone bad.

How to get rid of the sour taste in yogurt

If you are not a fan of sour yogurt then it is time to take some steps to make it more delicious. Here are some easy ways to get rid of the sour taste in yogurt.

1. Pour off the excess whey

Whey will separate out from the curds as it sits for longer. Pouring off this whey can decrease the sour taste of the yogurt and leave the thick curds behind. Strained yogurt is delicious and straining it overnight in the fridge is a great way to make labneh.

2. Mix through fresh, frozen or poached fruit

Add fruit is a great way to cover the sour taste of natural yogurt. Mix through poached, fresh or frozen fruit through the yogurt and it will taste delicious. Turn it into a balanced meal by adding oats and nuts to add extra protein.

3. Eat as soon as you buy it

The less time the yogurt sits around, generally the less sour it will be. A newly opened, newly produced pot set natural yogurt will be less sour than one closer to its use by date or one that has previously been opened. There will have been less bacterial activity and therefore less lactic acid produced.

4. Keep yogurt at fridge temperature

Keeping yogurt cold can help to reduce the bacterial activity and sourness. The more time the yogurt stays closer to room temperature the sourer it will taste. Avoid leaving yogurt out of the fridge for longer than 2 hours if you want to eat it at a later date.

Is it OK to eat sour yogurt?

It is fine to eat sour yogurt as long as it has been kept at or below fridge temperature, is within its use by date and has been sealed properly. If the yoghurt smells bad, has mold growing in it or looks a strange color then do not eat it.

Most natural yogurts will have a naturally sour taste thanks to the bacterial activity. Yogurt that does not have added sugar will not have this sourness masked by the sweetness of the sugar.

The sour taste of yoghurt is an acquired taste but once you get used to it, it is hard to go back to sweetened yoghurt.

Why is Yogurt Sour? | Summary

Yoghurt is sour because of the lactic acid produced by the bacteria in the yoghurt. These good bacteria are great for our bodies and help to make yoghurt what it is. A delicious, sour treat that tastes good with and without anything added to it.