How Does Milk Spoil
Milk spoils when bacteria multiply rapidly in warm temperatures. Bacteria convert lactose into glucose and galactose producing lactic acid. This lactic acid then forms a curd that can quickly curdle the milk within a day. When the milk starts curdling, it is considered spoiled and should be discarded immediately.
Milk is compromised due to over-population of bacteria, reducing milks quality, taste, and texture. Milk spoilage is caused by an overpopulation of bacteria, which reduces the texture, flavor, and overall quality of milk. Like sour milk, spoiled milk will also eventually separate, but in this case, it has done so due to the bacteria from the spoilage, not good bacteria. Pasteurization lowers the number of bacteria in the milk, which, in turn, increases the time it has to go bad.
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Pasteurization does not, however, work on every single bacteria, and it is those hardy survivors who ultimately make the milk spoil. After opening your home-made milk, new bacteria may get inside and grow, ultimately leading to the spoilage of your milk. If temperatures are above 40F, the bacteria can begin to grow in the milk, causing the rot and the smell.
| Temperature | Storage |
| At room temperature | 2 hours |
| In refrigerator | 12-14 days |
If you are using the rotting milk to bake, the oven heat will kill off most of the harmful bacteria, so you have a lower chance of getting ill from it. As certain non-harmful microbes grow, they make enzymes that help them break down milk, causing it to curdle and producing that “off” smell that we associate with spoiled milk. Milk is kept cold (in a refrigerator) to slow growth of these remaining microbes.
Some non-harmful microbes will still grow, and it is the growth of these non-harmful bacteria that causes milk spoilage. Psychrotrophic, or cold-resistant, bacteria are what causes refrigerator spoilage, and the rate of psychrotrophic bacteria is similar for both types of milk. Often, it is the psychrotrophic bacteria that grow under cold conditions that are the culprits for spoiling your milk. Milk production involves multiple steps aimed at killing off the psychrotrophic bacteria in order to prolong the shelf-life.
Heating destroys the bacteria which may be present in raw milk and which can lead to human infections (often called pathogens). Most milk is pasteurized today, a method which uses a temperature–typically 15 seconds at 72 degC (162 degF)–to kill pathogenic bacteria (those causing illness). Milk sold today is pasteurized, meaning the milk is heated for a short time to kill disease-causing bacteria; however, the process does not kill all bacteria. It is worth noting that pasteurization is used on the vast majority of commercially produced milk, and most dangerous types of bacteria known to cause foodborne illnesses, like E. coli, listeria, and salmonella, are killed in the process.
Pasteurized milk has much less bacteria, which is why the lactic acid building process is slower. There are many different types of lactic acid bacteria, each one giving the milk product a different flavor. Most cheeses are made by indirect acidification, which is when you add the bacteria to milk, which creates the acid indirectly by turning the lactose into lactic acid.
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Lactic acid bacteria are added to fresh, liquid, pasteurized skimmed milk or partial skimmed milk, producing a thick, tart, buttermilk. As the lactic acid builds up, the pH of the milk rises, until Streptococci cannot grow anymore. Within 12-24 hours, raw milk will start coagulating, or beginning to thicken and separate, because of a rapid buildup of lactic acid caused by all of those happy bacteria chomping on lactose up there. The sugar (lactose) in milk allows the bacteria to grow and multiply, leading to spoilage.
If stored at temperatures higher than 40degF, the milk will start showing signs of spoilage including an off-sour smell, an off-flavor, and curdled texture. If you notice a sour or slightly bitter flavor after taking a small drink of milk, that is a clue that your milk has soured. The short answer is that sour milk is milk that has developed a tart flavor, and that milk has developed this flavor either because it is starting to spoil, or because it has been deliberately made to sour by fermentation or acidification. There is a distinct line between sour milk, which is still safe for consumption, and spoilage milk, which is well past its shelf life or has not been refrigerated correctly.
It is important to note that the expiration dates differ depending on the type of milk you buy, the way that the milk is stored, and the way it is packaged. Using the expiration dates, or having a basic idea of how long milk takes to spoil, can give a rough indication of time. The best-by date is a manufacturers best guess at how long milk should be left in your refrigerator before you notice any spoilage, whether it is through odor, taste, or both. Every batch of pasteurized milk is different, and there is going to be more or less of certain non-harmful microbes left in it, so really, the best-before date is an estimate of when the milk that has more microbes left will spoil.
In this article, we will discuss how long milk may stay safe beyond its shelf life, and we will explain what different dates on food and beverage labels mean. For the purposes of your average dairy consumer, there is not really any hard-and-fast rule for which type of milk ages more quickly. The rate of spoilage depends on so many variables–manufacturer, manufacturing methods, milk formula, facility hygiene, storage temperatures, pH, and moisture levels, to name just a few–that one slight variation in only one of those variables–the producer–could make one specific carton of milk have slightly longer shelf life than the other.
There is also a high probability that there is fecal contamination, and thus, raw milk can contain harmful bacteria, such as Salmonella and E. coli O157, that can cause serious illness. Milk naturally contains bacteria that can spoil and cause diseases, such as Escherichia coli and Salmonella. Milk goes through a heat treatment – pasteurization – to eliminate the germs that cause spoilage and food-borne illnesses, but some strains of bacteria can survive the heat treatment – pasteurization – as spores, causing the milk to become sour when stored.
This curdling process can be done with pasteurized milk or with raw milk (not pasteurized), but the number of cultures or bacteria needed may vary depending on what you are using. Sour milk made through fermentation — like those found in cultured dairy products like buttermilk and kefir — is made by adding lactic acid bacteria, like lactobacillus, to pasteurized milk and letting it sit for several hours at between 104-111 degrees F. According to a 2018 study published by the Journal of Dairy Science, soured milk is caused because bacteria continues to grow even after milk has been pasteurized (which involves heating milk to destroy pathogens) and bottled.
Why does milk spoil even when refrigerated?
The bacteria that result in the milk decaying and smell can begin to thrive if the temperature rises over that. (However, for what it’s worth, milk can typically be left out of the fridge for up to two hours without experiencing any major problems.)
How can you tell if milk is spoiled?
The sour smell of spoiled milk is a result of the bacteria’s production of lactic acid. Additionally, a somewhat yellow tint and lumpy texture are indications of rotting. A sour flavor and odor, color change, and lumpy texture are all indications that your milk has gone bad and might not be safe to consume.
What happens if you drink spoiled milk?
Beyond an unpleasant taste, a small sip of spoilt milk is unlikely to produce any symptoms. Larger quantities of spoilt milk might upset your stomach and result in nausea, vomiting, and diarrhea like a food-borne illness. The majority of the time, ingesting spoilt milk-related symptoms go away within 12 to 24 hours.