How fast does milk go bad left out?

Quick Answer

Raw milk can go bad within 2 hours if left out at room temperature. Pasteurized milk typically goes bad within 4-6 hours if left out. The warmer the temperature, the faster milk will spoil. Milk should always be refrigerated at 40°F or below to extend its shelf life and prevent bacterial growth.

How Long Can Milk Be Left Out Before It Goes Bad?

Milk can go bad quickly if left out too long. Here’s a breakdown of how long milk can sit out before it spoils:

Raw Milk

Raw milk has not been pasteurized to kill harmful bacteria. It will spoil much faster than pasteurized milk.

– Room Temperature (68–72°F): 1–2 hours
– Warm Temperature (75–90°F): 30 minutes–1 hour

Raw milk should not be consumed if left out for more than 2 hours, as dangerous bacteria can multiply very rapidly at room or warm temperatures.

Pasteurized Milk

Pasteurized milk has been briefly heated to destroy bacteria. It lasts longer than raw milk but still should be refrigerated.

– Room Temperature (68–72°F): 4–6 hours
– Warm Temperature (75–90°F): 1–2 hours

Pasteurized milk should not be consumed if left out for longer than 6 hours. Even at cool room temperatures, harmful bacteria can still grow over time.

UHT Milk

UHT (ultra-high temperature) milk has been sterilized at very high heat to kill spores and microorganisms that can cause spoilage. It lasts the longest unrefrigerated.

– Room Temperature (68–72°F): 7-10 days
– Warm Temperature (75–90°F): 2-3 days

Once opened, UHT milk should be treated like pasteurized milk and consumed within 4-6 hours. The shelf-stable packaging prevents spoilage but not once air is introduced.

How to Tell if Milk Has Gone Bad

Here are the most common signs that indicate your milk has spoiled:

Sour Odor and Flavor

Fresh milk has a mild, sweet aroma and taste. As milk turns sour, an unpleasant sour odor becomes noticeable. Spoiled milk will have a distinct sour flavor.

Clumping or Curdling

Milk proteins will coagulate as the milk acidifies due to bacteria growth. Spoiled milk may have a lumpy, thickened texture or form chunks of solid curdles.

Change in Color

The color of bad milk can appear slightly yellow or greyed instead of a bright white. Fat separation may also be visible with a yellowish layer on top of whiter milk.

Gas Bubbles or Foam

Gasses are produced by bacteria as milk spoils, which can create visible bubbles or a foamy layer in the milk.

Mold Growth

If milk is left at room temperature for a very long time, dry, furry mold spots may start to appear on the surface, indicating advanced spoilage.

What Makes Milk Spoil Faster?

Several factors can accelerate milk spoilage, causing it to go bad faster than normal:

Higher Temperature

Heat dramatically accelerates bacterial growth. Milk spoils much faster at room or warm temperatures than properly refrigerated milk.

Repeated Temperature Changes

Fluctuating temperatures, such as warming milk up then cooling it down again, promotes spoilage. Try to limit temperature changes.

Age of Milk

Milk closer to the expiration date on the carton will go bad faster than very fresh milk.

Air Exposure

Once a milk carton is opened, oxygen helps accelerate food spoilage. Minimize transfers into other containers.


Dirty utensils or containers can introduce new bacteria. Always use clean, disinfected items when handling milk.

Light Exposure

Milk stored in transparent containers can degrade faster from light exposure. Store milk in opaque containers.

How Long Does Milk Last When Properly Stored?

To maximize freshness and shelf life, milk should always be refrigerated:

Milk Type Refrigerator (40°F or below)
Raw milk 4-7 days
Pasteurized milk 5-7 days
UHT milk (unopened) Until expiration date

The above times are for fresh, unspoiled milk. Once opened, milk should not be left refrigerated for longer than a week before consumption.

Properly stored milk stays fresh for:

– Raw milk: About 1 week
– Pasteurized: 5-7 days
– UHT: About 1-2 weeks after opening

Freezing milk can significantly extend its shelf life. Frozen milk stays good for:

– Raw milk: 4-6 months
– Pasteurized: 3-6 months
– UHT: 1 year

How to Store Milk to Prevent Spoilage

Follow these tips to keep milk fresh for as long as possible:

Refrigerate Immediately

Pour milk into a container and refrigerate within 1 hour of purchase. Do not leave milk out at room temperature.

Check Temperature

Use a refrigerator thermometer to verify the temperature stays at 40°F or below. The lower the temperature, the slower bacteria grows.

Watch Expiration Date

Store milk at the front of the fridge and check the expiration date regularly. Use oldest milk first.

Keep Lid Closed

Seal milk containers tightly after each use. Oxygen exposure speeds spoilage.

Avoid Temperature Changes

Prevent milk from warming up to room temperature then re-cooling (such as during meals) to limit bacteria growth.

Use Clean Utensils

Pour milk only into disinfected containers and use clean utensils to prevent contamination.

Don’t Return Milk to Fridge

Never put used milk back into the fridge after having some. Bacteria introduced can multiply quickly.

Does Spoiled Milk Go Bad Instantly or Over Time?

Milk does not immediately go from fresh to rotten, but spoils slowly over time. Bacteria produces acids and gas that chemically changes the milk’s odor, flavors, and textures.

The signs of spoilage get progressively worse if milk is left out:

– 0-2 hours: Minimal odor or taste change. Safe to drink if chilled quickly.

– 4-6 hours: Noticeable sour aroma and slight curdling. Milk should be discarded and not consumed.

– 8-12 hours: Distinct sour smell, coagulation, discoloration, gas bubbles visible. Milk is completely spoiled.

So time is critical. Milk does not instantly become unsafe, but passes through stages of spoilage if unrefrigerated. Always err on the side of caution and discard milk if signs of spoilage appear.

Can You Safely Drink Spoiled Milk?

No, you should never knowingly drink or eat spoiled milk. Spoiled milk contains harmful bacteria that can cause food poisoning.

Consuming bad milk can lead to:

– Upset stomach
– Abdominal cramps
– Nausea and vomiting
– Diarrhea
– Headaches and fever

Some bacteria in spoiled milk like Salmonella, E. coli, and Listeria are very dangerous. Symptoms may take 1-3 days to appear but can be severe or even life-threatening.

Even a small sip should be avoided, especially for those with compromised immune systems like older adults, infants, and pregnant women. The risk is simply not worth the consequences.

Is It Safe to Taste Test Sour Milk?

It’s never completely safe to taste expired or sour milk. Even a small amount could make you sick. The bacteria risk rises the longer milk sits out.

However, some people may take a quick sip to confirm it’s spoiled. Make sure to immediately spit it out without swallowing if the taste is rancid or sour.

Do not continue drinking and never let children or those with immunity issues taste test milk. Only healthy adults should attempt to minimize the swallowed amount.

The safest option is always to smell and visually inspect milk rather than tasting. Your nose and eyes can easily identify spoiled milk. When in doubt, just throw it out.

Can Spoiled Milk Be Saved or Salvaged?

No, there is no way to safely salvage milk once it has spoiled. The bacteria cannot be reversed and any signs of curdling, odor changes, or gas mean milk must be discarded.

Do not:

– Boil or reheat spoiled milk – This will not kill the bacteria or toxins
– Add salt or lemon – Acidity and salt do not stop bacterial growth
– Skim the curds – Removing visible curds does not make the underlying milk safe
– Strain the milk – This cannot remove the microbes and waste

Any milk showing even early signs of spoilage should be thrown out. Never consume spoiled milk or try to salvage it. Prevention by refrigeration is better than illness.

Will Sour Milk Curdle When Cooked?

Yes, spoiled milk will form very noticeable curds and clumps if heated or used in cooking. The proteins and fats coagulate from the acidity and heat.

You may see:

– Thick clumping or curdling as milk is heated. The liquid becomes very thick and chunky.

– Large clumps of solids floating in the remaining liquid.

– A clearer, yellowish whey seperating out from white curds.

– Viscous lumps sticking to the pan that are hard to wash off.

This curdling is caused by the bacteria and acidity. Never cook with spoiled milk or consume any cooked milk that has thickened or curdled.

Is Curdled Milk Safe to Drink?

No, do not drink milk if it has curdled. Curdling or clumping is a sign of spoilage. The proteins coagulate and floating chunks form from acidic bacteria breaking down components.

Curdled milk may contain harmful pathogens like Staphylococcus species, Salmonella, E. coli, and Listeria. Consuming could lead to food poisoning.

Discard any amount of milk that has started to coagulate or clump, even if you can strain out the curds. The underlying milk is also unsafe to drink and the bacteria cannot be fully removed.

Can Curdled Milk Be Used in Cooking?

It is risky and not recommended to use curdled milk in any recipes or cooking. The milk is spoiled and acidic, and heating curdled milk will cause it to break down further.

You may see:

– Increased clumping and curdling
– Whey separation
– More acidity oozing out from coagulated proteins
– Curd chunks resistant to mixing or dissolving

Not only will the flavor be very sour, but the spoiled milk is unsafe. Curdling means bacteria has multiplied, and heating does not kill the toxins. Never use curdled milk in baked goods, soups, sauces, or other dishes.

Does Refrigerating Curdled Milk Make It Safe to Drink?

No, refrigerating curdled milk does not make it safe to drink. The proteins have already coagulated from acidic bacterial growth. Simply cooling back down will not reverse the spoilage.

Dangerous bacteria like Listeria can continue growing even at refrigerator temperatures. And the toxins released by the microbes still persist.

Never consume or use curdled milk in any way, even if you attempt to chill it after seeing curds. Always err on the side of caution and discard any amount of curdled milk.


Milk can spoil quickly when left unrefrigerated. Raw milk goes bad within 2 hours and pasteurized within 4-6 hours if left out. Always refrigerate milk right away and check for signs of spoilage like odor changes, curdling, gas bubbles, or mold growth before drinking. Never consume milk that smells or tastes sour. Refrigeration at 40°F or below is critical to prevent bacterial growth and keep milk safe and fresh for the longest possible shelf life.

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