Why does milk turn sour? Why does milk sour?

Milk is rich in nutrients, especially protein and fat. Milk is easily contaminated by microorganisms in the process of placement, processing, transportation, etc., which will contain a certain number of microorganisms, if the temperature is appropriate, these microorganisms will grow and multiply, produce the same enzymes, and the nutrients in the milk are decomposed into substances with a very small amount of molecules, and finally decomposed into peptides, organic acids, etc., and the milk will be spoiled. At this time, the spoiled milk will emit an ammonia odor and sour taste, and it will also have a sour taste when smelling or tasting, and the protein in the milk will clump due to the acidity.

If the milk becomes sour, do not eat it, although yogurt is also produced by microbial fermentation, but because the milk is contaminated with microorganisms, there may be more than lactic acid bacteria in the milk.

There may also be other pathogenic bacteria.

and toxin-producing bacteria, and can make some bacteria with weak pathogenicity grow and multiply in large quantities, which will cause foodborne diseases after human consumption.

And in the process of spoilage, toxins are produced, and if you eat sour milk, it is very likely to cause food poisoning.

The reason why milk ferments into yogurt is due to lactic acid bacteria. Lactic acid bacteria are a class of anaerobic bacteria that are widely distributed in nature and can use glucose or other sugars to produce large amounts of lactic acid through fermentation. There are many kinds of them, and the common ones are lactobacilli, lactis streptococcus, bifidobacteria, etc.

Under anaerobic conditions, the original natural lactic acid bacteria or artificially added lactic acid bacteria in milk can decompose the lactose in milk to produce a large amount of lactic acid, resulting in a decrease in the pH value of milk and a sour milk turn. The acidic conditions can agglutinate and precipitate the casein in the milk, so that the milk begins to thicken, becoming a sour and thick yogurt.

If the milk is left for a few more days, it will become sour. It doesn't take two days for the milk to sour and turn into curd, two seconds is enough.

We need to put a little vinegar in the milk and the curd separates out at once.

Curd is a casein, milk protein. It dissolves in milk just as sugar dissolves in water. However, in order for the casein in milk to be separated from the fat, acid needs to be added.

But no one adds acid to milk. So, why does it still turn sour?

It's all the fault of tiny, yeast-like bacteria. Bacteria are always in the air. It starts working as soon as it falls into the milk, that is, it turns lactose into lactic acid. The milk curdles because of the acidity.

In order not to make the milk sour, it must be boiled. In this way, the fungus inside is killed.

Of course, curdling of milk can also occur when it is cooked. This is because before that, bacteria were already making acid.

The main difference between physical change and chemical change is whether new substances are formed, and the main factor that milk will become sour after a long time is the chemical change caused by lactic acid bacteria.

The main factor that causes milk to become sour over time is the chemical changes caused by lactic acid bacteria. Milk has lactose in it, so it's a little sweet. Lactic acid bacteria, on the other hand, secrete enzymes that turn sugars into lactic acid.

The reason why milk becomes sour is because lactose turns into lactic acid, and in addition to lactic acid, small amounts of alcohol, succinic acid, formic acid, and carbon dioxide are usually produced.

1.Wipe the leather shoes - first use a brush to brush off the dirt on the upper, and then use gauze to evenly apply the expired sour milk on the upper, and wipe it with a dry cloth after drying, the upper can be as bright as new, and can also prevent the leather surface from drying out.

2.Skin care - many people use expired milk to skin care, this practice does have a certain effect, because expired milk will produce lactic acid, which is a kind of fruit acid, which can soften keratin and have a moisturizing effect.

3.Remove ink from clothes - Wash your clothes with clean water first, and then pour in expired sour milk and gently scrub them to remove ink from your clothes.

4.Wipe the floor - dilute the expired sour milk with water, then sprinkle it on the floor and wipe it with a mop, so that the floor can be as shiny as new.

5.Wipe down leather furniture – If the expired milk has a small amount of precipitation, but the smell hasn't changed, it can be used to wipe down leather furniture to restore its shine and also repair fine cracks.

Milk turns sour because there are bacteria in it. Milk is very rich in protein, but the protein is easy to spoil, and if left at room temperature for too long, it can easily become sour and smelly. Some anaerobic bacteria or facultative anaerobic bacteria will produce lactic acid after anaerobic respiration in a hypoxic or anaerobic environment, so it causes milk to turn sour, and such sour milk is inedible, because many of the bacteria in it are toxins themselves, which is equivalent to moldy food.

How to store milk:

1. The milk should be placed in a cool place, and if the cap has been opened, it is best to put it in the refrigerator. Do not expose milk to sunlight or light, which can destroy several vitamins in milk and also cause it to lose its aroma.

2. When the milk is placed in the refrigerator, the bottle cap must be closed to prevent other smells from being skewered into the milk. Pour the milk into a cup, teapot and other containers, if you do not finish drinking, you should close the lid and put it back in the refrigerator, and do not pour it back into the original bottle.

3. Overcooling also has a negative effect on milk, and its quality will be damaged when the milk is frozen into ice. Therefore, milk should not be frozen, just put it in the refrigerator and refrigerate.