Milk with microbial contamination
作者：admin 点击次数：134 发布时间：2025-06-04
Fresh cow's milk, horse milk, goat's milk and their products, such as milk powder, condensed milk, etc., are used by humans to make up for the lack of breast milk, used to feed infants and young children, and are also commonly used supplementary foods for adults. Among them, fresh milk and its products are the mainstay. Because milk is rich in proteins, carbohydrates, lipids, inorganic salts and various vitamins, it is also the object of microbial light, and their growth and reproduction will lead to various deterioration phenomena of milk and dairy products. Abnormal milk production and deterioration of fresh milk in dairy cows are the result of microbial contamination and growth metabolism.
(1) Milk abnormalities and deterioration caused by microorganisms
1. Pathological abnormal milk
Chronic and acute mastitis is a common epidemic in the cattle industry. Cows with mastitis not only produce less milk, but also have a change in the composition of their milk, making them abnormal milk.
Some microorganisms that cause mastitis, such as coliform and staphylococcus, can produce strong hormones, which cannot be destroyed and are not suitable as raw materials for dairy products, and can also produce severe food poisoning and acute enteritis or infants after eating
Digestive disorders in children, etc. 2. The deterioration phenomenon of normal milk due to bacteria produces acid, gas and coagulation: bacteria use the carbohydrates in milk to produce lactic acid, butyric acid and carbonic acid can increase the acidity of milk, which is manifested in the reduction of pH and the coagulation of casein; The production of volatile acids and other gases can cause cattle to foam.
Alkali production: the sugar is broken down into carbonates, which makes the milk alkaline and accompanied by the metamorphic phenomenon of increased viscosityPeptone: the emulsion coagulates under non-acidic conditions under the action of extracellular enzymes secreted by microorganisms.
Changes in odor: microorganisms use the nutrients in milk to produce some substances, so that milk has sour odor, bud odor, unclean odor, pentyl alcohol odor, vinegar odor, fruit odor, benzene odor, fish odor and other odors.
(2) Dominant microorganisms in milk
The dominant types of microorganisms in fresh milk are bacteria, yeasts and a few molds, namely lactic acid bacteria, chemical bacteria, lipolytic bacteria, butyric acid bacteria, gas-producing bacteria, alkali-producing bacteria, molds and yeasts.
1. Streptococcus lactis
This bacterium is suitable for growth at 30-35°C and produces nistreptodin, which is mainly caused by the natural rancidity of fresh milk.
2. Streptococcus lactis
This bacterium is suitable for growth at 30°C and has a strong ability to break down proteins. 3. Streptococcus faecalis
This is the intestinal bacteria of humans and animals, which can be found when the sanitary conditions are poor, and can grow in the range of 10~45 °C
Long.
4. Liquefied streptococcus
This bacterium strongly breaks down proteins, and casein can produce a bitter taste when broken down
5. Streptococcus thermophilus
This bacterium is suitable for growth at 40~45 °C, and does not grow below 20 °C. 6. Lactobacillus acidophilus
This bacterium is suitable for growth at 37~40 °C, and does not grow at below 15 °C.
7. Aging bacteria
The phenomenon of transforming insoluble proteins into a soluble state under the action of proteases is called protein ization. The main bacterial species that cause the peptone of proteins: Bacillus subtilis, Bacillus licheniformis, and Bacillus cereus in the genus Bacillus, the suitable temperature range for growth is 20-40 °C; Pseudomonas fluorescens and Pseudomonas putrefinosa in the genus Pseudomonas have a growth temperature range of 25~30 °C.
Other bacteria: lipolytic bacteria, mainly gram-negative bacillus agenes, such as Chromobacter, Pseudomonas, etc.; Gas-producing bacteria that break down sugars to produce acids, such as coliforms; Alkali-producing bacteria, bacteria that can decompose organic acids in milk, the result of decomposition causes the pH of milk to rise, mainly Gram-negative aerobic bacteria, such as alkaloidens faecium, Actagenes viscolactis, etc.
(3) Microbial changes when fresh milk deteriorates
1. The fresh milk just extruded during the inhibition period can inhibit and kill the microorganisms in the milk because it contains antibodies from the animal body and other antibacterial substances, so the fresh milk is placed at a suitable temperature for a certain time without deterioration, which can generally last for 12 hours. Of course, the retention time is related to the number of bacteria in the fresh milk, and this period is the antibacterial period.
2. Streptococcal lactic acid stage
The amount of antimicrobial substances contained in milk is limited, and when the antibacterial substances are reduced or disappear, microorganisms such as Streptococcus lactis, lactobacillus, Escherichia coli and some proteolytic bacteria in milk begin to grow and multiply. Among them, the growth and reproduction of lactic acid cocci is absolutely dominant, and the decomposition of lactose and other sugars produces lactic acid, which makes the acidity of the emulsion continue to increase, and the emulsion appears to clot. When the acidity rises to a certain limit (pH=4.5), Streptococcus lactis itself is inhibited and no longer reproduces, and the number begins to decrease.
3. Lactobacillus stage
During the growth of Streptococcus lactis, it is also accompanied by the growth of lactobacillus. When the pH drops to about 6, lactobacilli begin to grow, and when the pH drops to about 4.5, Streptococcus lactis is inhibited, but lactobacillus continues to reproduce and produce acid due to its strong acid resistance, and a large number of clots appear in the milk, accompanied by whey precipitation.
4. Fungal phase
With the rise of acidity, when the pH reaches 3.0~3.5, the growth of most bacteria is inhibited, or even dies, at this time, only yeast and mold can still adapt to the strong acid environment, and use the lactic acid or other organic acids in it to survive. As the acid is utilized, the acidity of the emulsion decreases, the pH rises, and gradually approaches neutrality, at which point the milk loses its value as a food.
5. Aging bacterial period
After the above stages, the lactose content in milk has been basically consumed, while the protein and fat content are relatively high. Therefore, at this time, the bacteria that can decompose proteins and fats become active, the curd is gradually digested, the pH of the milk continues to rise, and it is converted to alkaline, accompanied by the growth and reproduction of spoilage bacteria, such as sporium, pseudomonas, proteus, etc., so that the milk has a putrid smell. At this point, the phenomenon of alternating microflora in the milk ends.