Unrefrigerated yogurt

Is it dangerous to eat yogurt that has been left out overnight?

Will unrefrigerated yogurt spoil? I left a tub of strawberry yogurt out overnight and don’t know if I should pitch it or if it will be safe to eat. It looks and smells fine but I really don’t want to poison myself!

Answer from Green Energy Efficient Homes

Your yogurt is probably safe to eat. Yogurt will not spoil if left out for a short time – 24 hours or less. Don’t forget that yogurt, along with cheese, sour cream, kefir, and buttermilk are created by bacterial action on milk, and were developed by early farming peoples as ways of preserving milk, centuries or millenia before modern refrigeration was invented. Cheese is a great way to store milk protein and fats for long periods – my nephew led a 75 day canoe trip through Canada’s arctic last summer, and they carried something like 40 lbs of unrefrigerated cheese (cheddar), some of it for over two months. Unrefrigerated yogurt won’t last that long but certainly it will last at least a day.

Yogurt is a live culture of lactose-eating bacteria. The bacteria convert the lactose (a complex sugar in milk) into lactic acid; the acid causes the milk to curdle and provides the thick yogurt texture. Acidic foods are much less prone to spoilage than neutral or alkaline foods, and acid is especially important in inhibiting the growth of botulin-producing bacteria (botulin is the toxin that produces botulism, one of the worst forms of food poisoning). That’s one reason why unrefrigerated yogurt can last quite a long time without spoiling, and yogurt kept in the fridge lasts even longer – often several months.

The most common form of spoilage in yogurt that has been left out or has gone past its best before date, is mold growing on the surface. It may be safe in such cases to skim off the mold and eat the remaining yogurt, but I don’t recommend it. Mold growth in an acidic food can actually reduce the acidity of the food, which means that while the mold itself might not make you sick, the reduced acidity can make the yogurt susceptible to other forms of spoiling, including the botulism mentioned above. So if you are thinking about eating yogurt that has been left out, or yogurt that’s past its best before date, be sure to check for signs of mold before trying it.

Making yogurt yourself

I love doing just about everything myself, and I have made my share of yogurt over the years. It’s not a lot of work and as long as you use a good starter, and follow directions carefully, the results are typically as good as or better than store-bought yogurt. If you do make yogurt yourself you will understand why unrefrigerated yogurt is not a major food safety problem.

Along with the natural yogurt to use as a starter, you’ll also need something to keep the growing yogurt culture at the proper temperature for a few hours. Yogurt cultures grow best at 100F or 38C. I made a fair bit of yogurt a decade ago, with a yogurt maker similar to the one pictured here. I would bring the milk to a simmer (160F on my candy thermometer) to kill any bacteria in it that might compete with the yogurt, then let it cool, covered, to body temperature, then pour it into yogurt-making cups that already have a tablespoon of starter culture in them. I would give the yogurt a very gentle stir, then cover it for 7 or 8 hours inside the yogurt maker.

Longer incubation times make the yogurt thicker but also more acidic, and if you wait too long the yogurt can get granular. Once it has set, you can let it drop to room temperature and keep it unrefrigerated for at least a day. You’ll save a lot of money making your own yogurt. In my neighborhood a 24 oz container of yogurt costs about $3.19, while I can buy that much milk for about $1.35 and turn it into yogurt for about 1 cent worth of electricity. You can make your own yogurt with any milk from skim to 3.25%. If you really want to economize you can even make it partly with powdered milk (but you still need to scald the reconstituted powdered milk).

You can also make yogurt in an Instant Pot. We bought one of these around 3 years ago and use it for everything – making yogurt (both the pasturization phase and the incubation phase, separate cycles), making soups, cooking beans, sauteeing vegetables. It’s probably my favorite kitchen appliance; I’m pretty sure I’ve cooked over 100 kg of beans in it, since I eat a ton of homemade hummus!

I’ve noticed that North Americans in particular are paranoid about food poisoning and tend to throw everything in the refrigerator, and then throw it out if it accidentally gets left out for more than an hour or so. I once talked a health food store owner into giving me an entire case of fruit-bottom yogurt that was already a month past its best before date, and a month later I finished the last tub of it; not one of them had spoiled. I lived in Costa Rica for a year, and noticed that even supermarkets there leave eggs at the end of a regular grocery aisle; no one there seems to put eggs in the fridge. We ate plenty of unrefrigerated eggs during the year we lived there and never had a problem with spoilage.

As I said above, and elaborate on in my main article Energy saving refrigerators, refrigeration is a very recent invention, and especially for foods that were created as ways of extending the shelf life of perishable agricultural products – foods such as yogurt, cheese, pickles, sauerkraut, jams and marmelades, smoked meats, etc. – the food is not automatically dangerous to eat just because it hasn’t spent its entire life confined to your refrigerator. Of course, you should apply common sense here: if you see any signs your yogurt has turned, throw it out. One common sign of spoilage is bulging of the container, which suggests fermentation is happening inside; note that this bulging can also be caused by expansion of the air in the tub as the yogurt warms when left out, and the less yogurt there is in the tub, the more likely it is to expand. Other spoilage signs are pretty obvious – mold spots, a moldy or yeasty taste, or a sense of effervescence or bubbling in your mouth. In any of these situations you should stop eating the yogurt and throw it in the garbage or compost or down the sink.

Because fruit yogurts are sweeter, there is an increased chance of fermentation in these than in plain yogurt. I would personally be more willing to risk eating plain unrefrigerated yogurt that had been left out for a full 24 hours, than fruit-bottom or blended-fruit yogurt left out the same amount of time.