Plus, how is it different than soured milk?

What Is Sour Milk?

The short answer is that sour milk is milk with a tart taste, which the milk gets either because it is beginning to spoil or because it was soured intentionally through fermentation or acidification.

Sour Milk vs. Soured Milk

The terms "sour milk" and "soured milk" are often used interchangeably, which is confusing. Technically speaking, there is a difference between sour milk and soured milk — milk that is soured on purpose is "soured milk" whereas milk that turns sour on its own is "sour milk."

Sour Milk vs. Spoiled Milk

First, let's clarify something: Just because milk is past the sell-by date on the milk carton, it does not mean that it needs to be poured down the drain. If the milk has been properly refrigerated, it may be drinkable for up to a week after the date on the label.

Yes, sour milk is safe to use

But there is a clear line between sour milk that is still safe for consumption and spoiled milk that is way past its expiration date or was not properly refrigerated. The best way to assess in which category that bit of leftover milk in your fridge falls is to follow your nose and taste buds. If the milk does not have any off-putting smell or taste, you can still use it. If milk has gone a little sour, it is still safe for consumption. However, you need to ask yourself whether you find it appetizing enough to use it. Sour milk is not to be used for pouring over breakfast cereal but, instead, for baking.

But spoiled milk is a different story...

All milk sold today, unless it's raw milk, is pasteurized. This prevents serious diseases caused by bacterial infections such as tuberculosis and brucellosis. Milk that has turned excessively sour, yellowish or lumpy, however, for example milk that was left for an extended period of time on the kitchen counter on a hot day, falls in the category of spoiled, not soured.

Besides the unpleasant taste and smell, spoiled milk can cause nausea, stomach cramps, or diarrhea. If you use spoiled milk for baking, the oven heat destroys most of the harmful bacteria so there is less of a chance that you will get sick from it. But the baked goods might have an off taste so it's probably not worth using a whole bunch of good ingredients to salvage a bit of milk only to end up with a cake that tastes off. If in doubt whether your milk is just a bit sour or full-blown spoiled, it is best to throw it out.

Pouring homemade buttermilk into a glass
Credit: Fascinadora / Getty Images

Types of Soured Milk

The two types of soured milk are soured milk made by fermentation and soured milk made by acidification.

Soured milk made by fermentation – as in cultured dairy products such as buttermilk and kefir – is produced by adding lactic acid bacteria such as lactobacillus to pasteurized milk and incubated at 104 to 111 degrees F for several hours. This process does several things: It increases the shelf life of the milk product because it slows down microbial growth and it enhances the taste and the digestibility, as the lactic acid bacteria produce acid from the lactose in the milk. Fermented sour milk has a higher acidity (lower pH), a tangy taste and a thicker consistency than regular milk, and it contains less lactose. There are many different types of lactic acid bacteria, and each gives a milk product its own distinct taste.

Likewise, there are many different variations of soured milk products around the world. In the western hemisphere, cultured buttermilk is the most widely known and available. Other fermented dairy products include kefir, kumis, yogurt, and probiotic drinks like Yakult.

For soured milk made by acidification, an acid is added to pasteurized milk to thicken it and give it a tangy taste. There are no lactic acid bacteria, no fermentation, and no incubation involved — so it is something that you can easily do at home.

a jug of milk about to be poured down the drain of a stainless steel sink
Credit: Nadia Hassani

How to Use Sour Milk

Don't toss that milk that has gone just a bit sour — it can be used in a variety of ways:

Scones and Biscuits
Sour milk is great for scones and biscuits. In combination with baking soda, it acts as a leavening agent and gives them a slight tang. Note that for sour milk to have this rising effect, the recipe must contain baking soda, not baking powder, which already contains acids.

The same applies to pancakes: adding sour milk makes them fluffier but again, baking soda is required so the acid in the sour milk can have its full effect.

Cakes and Muffins
If you have a cake or muffin recipe that calls for buttermilk, you can use sour milk instead. Again, be sure it contains baking soda.

Sour Milk Substitutes

Recipes with buttermilk are much more common than recipes with sour milk. However, with the increasing awareness of food waste and limited availability of ingredients, this is changing.

If you realize in the middle of baking or making pancakes that you have run out of buttermilk, then whipping up a cup of homemade sour milk is a quick, economical, and easy substitute. The substitution goes in both directions so when a recipe calls for sour milk, you can simply use buttermilk instead.

slice of waste not cake topped with powdered sugar
Credit: Diana Moutsopoulos

Recipes with Sour Milk

A lot of Depression-era recipes actually call for sour milk, given that resources were scarce and people strove to avoid food waste. Try these recipes if you find yourself with some sour milk in the fridge: