Is it alright to drink sour milk? Understanding spoiled vs. fermented dairy

December 4, 2025 •

4 min read

According to the Food and Drug Administration, milk left at room temperature for more than two hours is no longer safe to drink. The crucial distinction lies in whether the milk is spoiled due to harmful bacteria or simply soured through controlled fermentation, which answers the question, is it alright to drink sour milk?.

Quick Summary

The answer to whether you can drink sour milk depends entirely on its source and state. Deliberately fermented milk, like buttermilk or kefir, is safe and beneficial. Pasteurized milk that has simply gone off due to spoilage bacteria is not safe to drink and can cause illness. Visual and olfactory cues are key to differentiating between the two, though slightly soured milk can sometimes be used for baking.

Key Points

Differentiate Fermented from Spoiled: Purposefully fermented dairy, like kefir, is safe, but accidentally spoiled pasteurized milk is not and can cause food poisoning.
Trust Your Senses: Use smell, sight, and consistency to determine spoilage. A rancid odor, yellow color, or thick lumps are clear signs to discard the milk.
Mildly Soured Milk has Uses: Slightly tangy, pasteurized milk that isn't curdled can be used safely in baking as a buttermilk substitute, as the oven's heat eliminates harmful bacteria.
Avoid Drinking Spoiled Milk: Ingesting truly spoiled milk can cause digestive issues like nausea and diarrhea due to an overgrowth of harmful bacteria.
Store Milk Properly: To extend shelf life, keep milk consistently refrigerated at or below 40°F (4°C) and avoid leaving it out for long periods.
When in Doubt, Throw it Out: If you are unsure about the state of your milk, the safest rule is to discard it to prevent potential illness.

The Critical Distinction: Spoiled vs. Fermented Milk

When milk turns sour, it's undergoing a chemical process. The crucial detail is what causes that process. There is a world of difference between milk that has gone bad by happenstance and milk that is fermented purposefully. The average commercially produced, pasteurized milk sold in most stores is a sterile product; once opened, it can be contaminated by various environmental bacteria. When these bacteria multiply, they cause spoilage, leading to the formation of lactic acid which gives the milk its sour taste and smell. This is not safe for drinking.

On the other hand, fermented milk products like kefir, yogurt, and cultured buttermilk are intentionally soured by introducing specific, beneficial bacteria (often lactic acid bacteria like Lactobacillus) in a controlled environment. This process creates healthy probiotic products that are safe to consume and offer numerous health benefits.

How to Tell if Your Milk Is Spoiled

It's easy to confuse slightly off-smelling milk with milk that's truly gone bad. Your senses are your best tool for this evaluation, as expiration dates relate to quality, not safety. Here's what to look for:

Odor: A foul, rancid, or excessively sour smell is a clear sign of spoilage. Fresh milk has little to no scent, while fermented milk has a distinct, but pleasant, tangy aroma.
Consistency: True spoiled milk will be thick, lumpy, or curdled, with solid pieces separating from the liquid. If you pour it into a clear glass, you'll see the separation.
Color: Spoiled milk can develop a yellowish or dingy color, distinct from its normal pearly white.

Can You Still Use Slightly Soured Milk for Baking?

Even if your pasteurized milk is slightly past its prime and a bit tangy, it might not be a lost cause. Many baking recipes, especially for biscuits, pancakes, or cakes, can use slightly soured milk as a substitute for buttermilk. The acid in the milk reacts with baking soda to create a light and fluffy texture. This process is generally safe, as the high temperatures in the oven will kill off most of the harmful bacteria. However, this only applies to milk that is just starting to turn. If the milk is lumpy, yellow, or has a strong, repulsive odor, it's best to discard it. Using overly spoiled milk can result in an unpleasant taste in your baked goods.

The Risks of Drinking Truly Spoiled Milk

Consuming milk that is genuinely spoiled carries risks of food poisoning. While a small accidental sip is unlikely to cause serious harm for a healthy adult, ingesting moderate or large quantities can lead to unpleasant digestive symptoms. This is especially true for children, the elderly, or those with compromised immune systems. The bacteria that grow in spoiled milk, like E. coli or Salmonella, produce toxins that can cause abdominal pain, nausea, vomiting, and diarrhea. In very rare and severe cases, a potent bacteria like Listeria could lead to more serious complications.

Comparison Table: Fermented vs. Spoiled Milk


Feature	Fermented Milk (e.g., Kefir)	Spoiled Pasteurized Milk	Intentionally Soured Milk (Homemade Buttermilk)
Source	Produced by adding specific, beneficial bacteria.	Result of uncontrolled microbial growth after pasteurization.	Fresh milk with an acid (vinegar or lemon juice) added.
Safety	Generally safe and offers probiotic benefits.	Can cause food poisoning and is not safe to drink.	Safe for cooking and baking.
Odor	Pleasantly tangy and clean.	Unpleasant, pungent, or rancid.	Tangy and acidic, without a rancid smell.
Consistency	Smooth, uniform, and slightly thicker than fresh milk.	Thick, lumpy, and curdled.	Curdled texture due to protein separation.
Best Use	Drink straight or use in smoothies.	Discard or use for very specific, low-level baking applications (when only mildly off).	Substitute for buttermilk in recipes.

Conclusion

Understanding the nuanced difference between intentionally fermented dairy products and expired, spoiled milk is essential for safe consumption. While products like kefir are health-beneficial, drinking milk that has gone bad due to spoilage bacteria is risky and can lead to illness. Trust your senses: if your pasteurized milk smells rancid, is lumpy, or discolored, the safest course of action is to throw it out. For milk that is only slightly soured, consider using it in baked goods where the heat will neutralize any potential pathogens. When in doubt, it is always best to discard it and choose safety over waste.

Using Soured Milk in the Kitchen

Even slightly soured milk can be a useful ingredient. Here are a few ways to use it up before it's too late:

In Baked Goods: The acidity is perfect for recipes that call for buttermilk, like pancakes, scones, or cakes. The extra tang can add a delightful depth of flavor.
As a Meat Tenderizer: The acid can help break down proteins in meat. Use it as a marinade for chicken or fish before cooking.
For Making Cheese: With a bit of practice, slightly soured milk can be used to make simple homemade cheeses, such as cottage cheese.
As a Soup Thickener: A small splash can add richness and a creamy texture to soups and stews.

Remember, this is only for milk that is just slightly off. The moment it shows signs of excessive spoilage, such as excessive lumps or a truly foul odor, it should be thrown away.

Frequently Asked Questions

No, it is not safe to drink pasteurized milk that has gone sour. Pasteurization kills most bacteria, so when it sours, it's due to opportunistic, harmful microbes. This milk is considered spoiled and should be avoided to prevent food poisoning.

The key difference is the bacteria involved. Sour milk can refer to a cultured product like buttermilk, made with beneficial bacteria. Spoiled milk, however, refers to pasteurized milk that has gone bad from an uncontrolled, potentially harmful bacterial overgrowth.

Yes, if the milk is only slightly soured and not excessively spoiled (no thick curdles, no mold). The mild tanginess can work as a substitute for buttermilk in recipes for pancakes, biscuits, and some cakes. The heat from baking will kill any remaining bacteria.

For a healthy individual, a small sip of spoiled milk is unlikely to cause serious harm, though the unpleasant taste will likely deter you from drinking more. Mild digestive discomfort might occur, but it typically passes quickly.

Beyond a sour taste, signs of badly spoiled milk include a pungent, rancid odor, a thick and lumpy or curdled texture, and a yellow or off-color appearance. If you observe these signs, the milk should be discarded immediately.

Fermented dairy products like yogurt and kefir contain probiotics, which are beneficial for gut health and aid digestion. They can also improve lactose tolerance for some individuals, boost the immune system, and have been linked to other health benefits.

No, leaving pasteurized milk out on the counter can lead to rapid and dangerous spoilage from uncontrolled bacterial growth. While some people intentionally sour raw milk for baking, it is not recommended with pasteurized milk due to food safety risks.