Why Is Yogurt Sour 

 June 5, 2022

By  Diane Miles

Yogurt is one of the most famous fermented foods in the world. It’s also a popular snack loved by kids and adults. However, many people wonder why yogurt is sour.

The sour taste of yogurt is caused by its bacterial strain content; when Lactobacillus, Bifidus, or other bacteria present convert the lactose sugars present in milk into lactic acid. The end result is a tangy, sour taste due to the high concentration of live bacteria present in the yogurt.

Learn more about why yogurt is sour by reading on below.

Why Does Yogurt Taste Sour?

There are many ways to make yogurt, but there are still fundamental principles that govern its creation. This requires the use of live microorganisms or bacteria together with pasteurized milk. When placed in the right conditions, the live bacteria convert the milk’s lactose sugars into lactic acid, and as a result, it has a sour taste.

The principles behind making Greek yogurt is similar, though it has much more protein and it requires straining – two factors which contribute to an even more sour taste compared to traditional yogurt.

Just because yogurt is sour doesn’t mean that it’s gone bad; it simply means that there are live, active cultures present in it. Store-bought yogurt is usually sweetened with fruits and other sweeteners to combat its sour taste, and when you make it at home, you can also sweeten it yourself.

What Factors Affect the Sourness of Yogurt

When making your own yogurt, you can control some of the factors that make it more or less sour. These are:

  • Temperature of the milk: Yogurt recipes typically require milk to be heated then cooled prior to culturing. Various temperature settings will result in more or less sour yogurt. If you use milk that is in temperatures below 77C, then it will be tarter while if the milk is cooked at 90C for 10 minutes, it will be significantly less tart.
  • Fermentation time: Yogurt is usually fermented for at least 24 hours which is adequate time for the lactic acid bacteria to convert the milk’s lactose sugars into lactic acid. However, during this time, yogurt also become too sour. If you want to reduce the sourness of yogurt, cut down the fermentation time to 12 hours.
  • Milk: Using higher fat milk will reduce the sour taste and instead result in a creamy yogurt. Skim milk contains more lactose resulting in a higher amount of lactic acid, contributing to the sour taste. You should also always use fresh, high-fat milk because old milk may have more bacteria in it, causing its acidity even before it’s been made into yogurt.

How Do You Know if Yogurt Is Sour or if It’s Gone Bad?

Whether your yogurt is homemade or store-bought, they can both be sour and go bad.

Here’s how you can tell if yogurt has gone bad using your senses:

  • Smell: Yogurt that has gone bad will smell of old, rotten milk instead of a clean, fresh smell.
  • Sight: Look for the presence of molds or yeast on the surface which can be green or any other color. In some cases, the mold can be white though they tend to have a crust around them. Yogurt will also develop an unusual amount of liquid on the surface or will curdle in the bottom if it is no longer safe to eat.

Yogurt that’s been around for 14 days after its sell-by date is also expected to go bad, especially if it hasn’t been stored in the refrigerator.


Many factors affect the sourness of yogurt, but this doesn’t necessarily mean it’s bad. You can always choose to sweeten it if you prefer to mask the sour taste, then enjoy all the nutrients and probiotics in it.

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