Yes, you can often eat butter that smells slightly cheesy, as it's usually a sign of natural fermentation (butyric acid) in raw or cultured butter, but if it smells strongly rancid (like vomit or sweaty feet), looks moldy, or tastes bitter/sour, then toss it; a mild cheesy scent from aging or improper storage is generally fine, especially for cooking, but strong off-smells mean spoilage.
Ya, your butter is rancid. It won't kill you, but it's not a popular flavour.
Spoiled butter can smell (or taste) sour or tangy, and can change in color and texture. Mold or colored spots are a clear indication that butter has gone bad, but even a subtle shift from pale yellow to dark is a sign that it's time to throw it out.
The first clue that your butter has gone bad will be the smell. Any sour or off-putting smell means it's a goner. Same goes for taste: If it tastes sour or off, toss it out.
Raw or cultured butters can smell cheesy as they're made using fermentation or with cream that hasn't been pasteurized, which can produce cheese-like aromas.
If you're unsure if your butter is OK to eat, simply give it a smell. If it's rancid it will most definitely smell off, and if you're willing to give it a taste, it should taste unpleasantly sour. At that point it's time to reach for fresh butter. Another factor to consider is whether or not its salted.
Yes, butter left out can develop a cheesy smell and flavour, due to a bit of fermentation and aging, causing the development of butyric acid. It's still fine to use, though, as long as the slight change in taste isn't too off-putting.
Eating expired butter is generally harmless but can cause temporary digestive discomfort, like vomiting or diarrhea, due to its nasty taste. Butter is less likely to harbor harmful bacteria due to its high fat content. The salt in salted butter also helps keep out microbes.
TRUST YOUR SENSES: LOOK, SMELL, AND TASTE BEFORE YOU WASTE
Look at it: if you notice any mold or discoloration, it's best to throw it out. Next, smell it. Fresh butter has a mild, creamy scent. If it smells sour or rancid, it's time to discard it.
In the refrigerator, well-wrapped salted butter can last up to five months without spoiling. Salt is a preservative that prevents microbial growth and extends shelf life. Unsalted butter can last up to three months in the fridge. (If the butter is unwrapped, shorten those projections to about two months in the fridge.)
If you've gone to use butter and found it to be past its expiration date, don't fret. Odds are, this butter is probably safe to consume, provided you keep an eye out for signs of spoilage. While many dairy products have a short shelf life, butter is an exception to this rule.
Although the butter is not a highly perishable food, it does undergo spoilage by bacteria and molds. The main source of microorganisms of butter is cream, whether sweet or sour, raw or pasteurized1. Yeast and molds are important spoilage microorganisms of butter and can result in surface discoloration and off-flavor.
Many of the oxidation pathways are not entirely understood. Salted butter was developed to prevent spoilage, and to mask the taste of rancid butter. A sour-bitter taste is identifiable with rancidity (i.e. soapy, baby-vomit, blue cheese). Rancid butter becomes yellow to brown and the flavor becomes harsh.
Fresh butter smells clean and slightly sweet. If it starts to smell sour, metallic, or like the inside of your refrigerator, there's a good chance it's spoiled." In general, we recommend that if your butter doesn't smell right, it's best to avoid it.
No problem. Instead, a bigger concern should be if your feet smell like cheese.
As with many things in life, there's a general rule of thumb to keep in mind: Butter will stay fresh in the refrigerator for 1 to 3 months, whether it's salted or unsalted and whether you've opened it or not.
The symptoms usually start within 1 to 2 days after eating contaminated food. Symptoms can start between a few hours and a few weeks later. In most cases, these symptoms will pass in a few days and you will make a full recovery.
But if you can't tell by looking at it, smell it. Rancid butter will start to smell like sweaty feet, rotten cheese, or even vomit. If all else seems fine, the final test is to taste a small portion. Butter that has gone bad will have a sour-bitter taste.
Smell: If butter smells sour, cheesy, or otherwise off, discard. Taste: If you notice a sour, bitter, or off flavor, discard. If the butter simply absorbed flavors from nearby garlic or onions in the refrigerator, it's still safe to use in cooking (although you might want to avoid using it for baking applications).
Brevibacterium linens, a bacterium that loves cheese rinds, is closely related to a bacterium that loves the sweaty crevasses of our feet: Brevibacterium epidermidis.
A mild cheese-like smell in these butters is completely normal, but they should still taste like butter, not cheese. However, if your butter smells sour or strongly rancid and has a very tangy or off flavor, it may have gone bad.
This is common in older refrigerators. There is mild seepage of the coolant gas into the storage area and some foods particularly absorb this odor. For butter, you can take a knife and shave off all sides of the sticks where the odor accumulates.
The USDA's FoodKeeper app offers this guidance for storing butter: Butter may be left at room temperature for one to two days. One to two months when stored in the refrigerator. Six to nine months if stored frozen.
Eating rancid butter is unlikely to lead to serious illness, but it might cause an upset stomach or mild digestive issues, such as nausea. If you suspect butter is spoiled, it's best to err on the side of caution and toss it.
Salmonella can grow in butter at 25°C and is not eliminated by refrigeration or freezing (Sims et al., 1970). E. coli O157:H7 has been responsible for outbreaks of infections associated with dairy products (Meng et al., 2001).