Milk goes bad if not stored in a cool place because it contains bacteria that multiply quickly at room temperature, causing it to spoil and become sour.
Fresh milk naturally contains microorganisms, primarily lactic bacteria. These tiny creatures, invisible to the naked eye, feed on lactose, the natural sugar found in milk. By digesting this lactose, they produce lactic acid, which is responsible for the increasing acidity of the milk. It is this acidity that causes the curdling of the milk, giving it a sour taste and a thick texture, forming those famous not-so-appetizing chunks. The more bacteria there are, the faster your milk spoils: warm temperatures and poor hygiene all promote their rapid multiplication.
Milk curdles due to a key reaction called lactic fermentation. Here's what happens: the lactic bacteria naturally present in the milk begin to consume the lactose (the sugar in milk). By digesting it, they mainly transform it into lactic acid. The more active these bacteria are, the more lactic acid is produced. As a result, the pH of the milk decreases, it becomes more acidic, and the milk proteins (caseins) clump together. This clumping phenomenon leads to the formation of curds, changing our milk from a liquid and homogeneous state to that curdled texture that is only welcome when making cheese (and much less pleasant in morning cereal!).
The higher the temperature rises, the faster the microorganisms in the milk develop, much like us when summer arrives: things get moving! At room temperature (around 20°C to 25°C), bacteria multiply quickly, which significantly accelerates the production of lactic acid. The result: milk spoils rapidly and becomes undrinkable. In contrast, a quick chill to around 4°C considerably slows down bacterial activity. These little critters are not very fond of the cold; they remain somewhat dormant and will therefore work at a slower pace. Hence the importance of always storing your milk in the fridge as soon as possible!
When milk is exposed to light, particularly sunlight or fluorescent light, certain vitamins (like vitamin B2) are destroyed. This phenomenon gives milk a strange taste known as "lightstruck flavor," which is not exactly pleasant. Furthermore, when oxygen comes into contact with milk for too long, it leads to the oxidation of fats, gradually changing the taste (and you can forget about your morning latte). The result is an unpleasant flavor, slightly metallic, and an odor that is far from appealing. That's why your milk is better protected in opaque, sealed packaging to avoid these nasty surprises.
Always keep the milk in the cold zone of the fridge, ideally at a temperature around 4°C. Avoid placing it in the door, even though it's convenient: every time you open it, it gets a blast of warm air! Also, make sure to tightly close the bottle after each use to limit the presence of air inside. Never pour milk that has already been served from your bowl or glass back into the bottle, as it brings in a whole bunch of microbes that spoil the milk faster. A little tip: prefer opaque bottles or keep your milk away from direct light, as this reduces the chemical reactions responsible for bad taste and strange odors. Finally, nothing beats a regular check of the expiration date to avoid unpleasant surprises at breakfast!
Did you know that, apart from animal milk (cow, sheep, or goat), plant-based milks (soy, almond, oat, etc.) can also spoil? However, their deterioration is mainly due to the growth of molds and bacteria different from those found in traditional dairy products.
Yogurt, derived from the fermentation of milk, is produced by deliberately adding certain lactic acid bacteria such as Streptococcus thermophilus and Lactobacillus bulgaricus. These microorganisms convert lactose into lactic acid, giving yogurt its tangy flavor and enhancing its preservation.
Once upon a time, before the invention of the refrigerator, milk was often stored in crushed ice or quickly turned into cheese or butter to extend its shelf life. One more reason to appreciate our modern refrigerators!
Contrary to a common belief, the sour smell and taste of spoiled milk primarily come from the production of lactic acid through bacterial fermentation, rather than from direct chemical contamination.
Here is the translation: "Good hygiene of containers, storing in the fridge in the coldest zone (typically at the back of the bottom shelf), as well as avoiding temperature fluctuations, significantly prolongs the shelf life of milk. Some also try to transfer it to an airtight container, protected from air and light, to limit its deterioration."
Even when stored in the fridge, milk naturally contains a small amount of bacteria. Over time, these microorganisms slowly multiply and produce acids that give milk its characteristic sour taste. This is why, even when refrigerated, milk eventually deteriorates, although more slowly than at room temperature.
UHT milk (Ultra High Temperature) is thermally treated at high temperatures, neutralizing the microorganisms present. However, once opened, it is exposed to ambient air and can become re-contaminated by bacteria and mold. Therefore, it should also be stored in the refrigerator after opening and consumed quickly.
Several signs can indicate that milk has gone bad: a sour or unpleasant smell, a change in texture with the appearance of lumps or clots, a slightly yellowish color, or an unusual tangy taste. In these cases, it is better not to consume the product.
It is strongly advised not to consume spoiled milk. Even if it is only slightly sour, milk can contain harmful bacteria that may cause digestive issues or more serious health problems. It is better to discard any milk that has an unusual color, smell, or texture.
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