How to Prevent Yogurt from Turning Sour and What Are 5 Common Causes?

Sour Yogurt Starter
One of the main reasons for homemade yogurt turning sour is using a starter that is already sour. If the yogurt you are using as a starter is sour from the beginning, the yogurt you make will inevitably turn sour as well. So, if you want sweet yogurt, avoid using sour yogurt as a starter.

Old Yogurt Starter
If you want sweet yogurt, use fresh yogurt as a starter. Using old yogurt as a starter can not only ruin the taste of homemade yogurt but can also cause the yogurt to curdle, become watery, or separate.

Excessive Yogurt Starter
In addition to ensuring that the yogurt starter is not sour from the beginning, it's important not to use too much. If you add too much starter, the yogurt will turn sour. Generally, for every liter of milk, one tablespoon of yogurt starter is sufficient.

However, if you're using ingredients like starch, cheese, or powdered milk as a starter, the amounts will vary. Be mindful of the amount of starter you use to ensure your yogurt turns out sweet.

Warm Milk
For making yogurt, the milk should be at the right temperature before adding the starter. A temperature of 45°C is ideal for adding the starter. To check if the temperature is right, dip your finger into the warm milk for 10 seconds without burning your hand. If the milk is hotter than this, the yogurt you make will turn sour.

Storing Yogurt at High Temperatures Causes Sourness
Once the yogurt has set at room temperature and is ready, it should be placed in the refrigerator to firm up and fully set. If the yogurt is stored in a fridge that is too warm or if the cold air circulation is blocked due to an overstuffed fridge, the chances of the yogurt turning sour increase.

If the cooling cycle is interrupted or faulty, even yogurt that started off sweet can turn sour.