Food microbiology of milk and dairy products. Microbiology of the most important food products. Pathogenic and opportunistic microorganisms cause foodborne diseases

Microbiology of milk. Microbes get into the milk already at the time of milking. The origin of milk microflora is very diverse. Some microbes live in the teat canals of the udder and are therefore always present in the milk produced. In addition, a lot of microbes get into milk from the surface of the udder, animal hair, from the hands of milkers, from manured bedding, inventory, etc., microbes can be introduced into milk by flies. Due to these sources, the number of microbes in 1 ml after milking increases from several thousand to tens and hundreds of thousands after processing - filtering, cooling and spilling. As a result, a very rich microflora is formed. Rapid cooling is a mandatory operation, otherwise the development of microflora in uncooled milk occurs quickly. This is facilitated by the favorable chemical composition of milk. In uncooled milk, the number of microflora increases by 2-3 times in 24 hours. When cooled to 3-8 ° C, the reverse picture is observed - a decrease in the number of microorganisms occurring under the influence of bactericidal substances contained in freshly milked milk. The period of delay in the development of microbes or their death in milk (bactericidal phase) is the longer, the lower the temperature of the stored milk, the less microbes in it. This phase usually lasts from 2 to 40 hours.

In the future, there is a rapid development of all microbes. However, lactic acid bacteria, if they were previously even in the minority, gradually become predominant. This is explained by the fact that they use milk sugar, which is inaccessible to most other microorganisms, and also by the fact that lactic acid and the substances secreted by some of them - antibiotics (nisin) inhibit the development of all other microbes. Gradually, under the influence of accumulated lactic acid, the reproduction of lactic acid bacteria also stops. In milk that has undergone fermentation, conditions are created for the development of mold fungi.

The most actively developing are oidium, penicillium and various yeasts. By consuming acids, desalination of products, mold fungi create the possibility of secondary colonization of the object with putrefactive bacteria. Ultimately, complete putrefactive spoilage of milk occurs.

In pasteurized milk, briefly heated to 63-90 ° C, the sequence of changes in microflora changes dramatically. Almost all lactic acid bacteria die, and the bactericidal substances of milk are completely destroyed. At the same time, heat-resistant and spore forms of microorganisms are preserved. Therefore, after some time, rapid reproduction of the preserved diverse microflora can begin in such milk. The absence of bactericidal substances, the small number or complete absence of lactic acid bacteria make milk “defenseless”. Under these conditions, souring of milk may not occur, but even a slight contamination with putrefactive or pathogenic bacteria leads to spoilage, making it dangerous for consumption. In this regard, it is clear why when trading pasteurized milk it is necessary to strictly comply with sanitary and hygienic requirements and observe temperature storage conditions.

BELNICKTIMP

Compiled by:

Safronenko L.V.- head. Department of Microbiology, UE "BELNIKTIMP", Ph.D. those. Sciences

Kuklyansky A.A.- ch. scientific co-worker, candidate of medical sciences

Minsk, 2001

MICROBIOLOGY OF MILK AND DAIRY PRODUCTS

(COMPENDIUM)

1. Basics of general microbiology

1.1. The main groups of microorganisms:

bacteria;

bacteriophages.

1.2. Microscopic methods

1.2.1. Basic rules for working with a microscope

1.2.2. Dark field microscopy

1.2.3. Phase contrast microscopy

1.2.4. Fluorescent microscopy

1.2.5. The main forms of bacteria

1.2.6. Bacteria staining methods

1.2.6.1. Preparation of working solutions of paints

1.2.6.2. Preparation of microscopic preparations

1.2.6.3. Simple coloring methods

1.2.6.4.- Gram stain

1.2.6.5. Capsule coloring

1.2.6.6. Coloring of spores and acid-fast bacteria

1.2.7. Determination of mobility crushed drop; hanging drop.

1.2.8. Morphology of actinomycetes and fungi

1.2.8.1. actinomycetes

1.2.8.2. Main groups of mushrooms

- ascomycetes;

- flour mushrooms;

- rhizopus;

- aspergillus;

- penicilli;

- cladosporium;

- milk mold;

- Alternaria;

- catenularia;

- yeast;

- yeast-like fungi.

1.3. Features of metabolism in bacteria

1.3.1. energy exchange

1.4. Influence of environmental factors on the growth and vital activity of microorganisms

1.4.1. Temperature

- psychrotrophs;

- mesophiles;

– thermophiles

1.4.1.2. Pasteurization

1.4.1.3. Sterilization

- boiling;

– autoclaving;

- Sterilization with flowing steam;

– fractional sterilization;

- tyndalization;

- flaming;

– dry heat sterilization;

– filtering;

– radiation sterilization;

– chemical sterilization (disinfection);

– preparation of dishes for sterilization

1.4.2. Osmotic pressure

1.4.3. Humidity

1.4.5. Redox potential

1.4.6. growth inhibitors

1.4.7. Biological factors

- symbiosis;

- commensalism;

- antagonism

1.5. Cultivation of microorganisms

1.5.1. thermostats

1.5.2. anaerostats

1.5.3. Nutrient media

1.5.3.1. Nutrient Requirements

1.5.3.2. Quality control of culture media

1.5.3.3. Enterobacteria media

1.5.3.4. Media for anaerobes

1.5.3.5. Media for lactic acid bacteria

1.5.3.6. Media for staphylococci

1.5.3.7. Mushroom media

1.5.3.8. Environments for sanbakt. research

1.5.4. Seeding methods

1.5.5. Identification methods

– study of morphological and tinctorial properties;

– study of cultural properties;

– study of redox properties;

– study of the type of metabolism

1.6. Genetics of microorganisms

1.6.1. Mutations

1.6.2. Genetic recombinations

- transformation;

– transduction;

- conjugation;

- transgenic forms of bacteria with industrially valuable properties

1.7. Ecology of microorganisms

1.7.1. Soil microflora

1.7.2. Water microflora

1.7.3. Air microflora

1.7.4. Feed microflora

1.7.5. Microflora of the udder, intestines and skin of animals

1.7.6. Microflora of the gastrointestinal tract, respiratory tract and human skin

2. Special microbiology

2.1. Main groups of microorganisms found in milk and dairy products

2.1.1. Technically important microorganisms

2.1.1.1.1 Lactic acid bacteria (LAB)

lactococci;

leuconostoc;

thermophilic streptococcus;

thermophilic lactobacilli;

mesophilic lactobacilli

2.1.1.1. Other

2.1.2. Technically harmful microorganisms

micrococci;

putrefactive bacteria;

hay stick;

potato stick;

psychotrophs;

pseudomonas;

acetic acid bacteria;

butyric bacteria;

propionic acid bacteria;

bacteriophage

2.1.3. pathogenic microorganisms

causative agents of tuberculosis; causative agents of brucellosis; foot-and-mouth disease virus;

pathogenic Escherichia; salmonella; golden staphylococcus aureus; listeria

2.1.4. Sanitary-indicative microorganisms BGKP;

total number of bacteria

2.2. Microbiology of milk

2.2.1. The sources of the microflora of raw milk are the udders of cows;

staff hands;

equipment

2.2.2. The composition of the microflora of raw milk

2.2.3. The main phases of changes in the microflora of raw milk during storage

2.2.4. Influence of the conditions of primary processing, storage and transportation of milk on its microflora

2.2.5. Influence of milk processing modes at the enterprise on its microflora

2.2.6. Microbiological examination of milk

2.2.6.1. Sample selection

2.2.6.2. Determination of the total number of microorganisms

2.2.6.3. Determination of pasteurization efficiency

2.2.6.4. Quantitative accounting of ICD

2.2.6.5. Determining the amount of BGKP

2.2.6.6. Determination of the number of proteolytic bacteria

2.2.6.7. Determination of the number of butyric bacteria

2.2.6.8. Determination of the amount of yeasts and molds

2.2.6.9. Indirect methods for determining the total number of microorganisms in milk

2.2.6.10. Study of milk from mastitis cows

2.2.6.11. Determination of bacteriophage in milk

2.2.6.12. Determination of antibiotics in milk

2.3. Microbiology of starter cultures

2.3.1. Composition of microflora and forms of starter cultures used in the dairy industry

dry and liquid starter cultures;

sourdough for cottage cheese, sour cream and yogurt;

starter cultures for yogurt, Mechnikov yogurt,

ryazhenka, varenets;

starter cultures for acidophilic milk, acidophilic paste, acidophilus;

starter cultures for fermented milk drinks and low-fat sour cream;

kefir starter;

starters for cheeses with a low temperature of the second heating;

starter cultures for cheeses with a high temperature of the second heating;

leaven for sour cream butter;

bacterial concentrates and bacterial preparations

2.3.2. Basic rules for preparing starters

2.3.3. Starter cultures on pure cultures

2.3.4. Preparation of laboratory starter

2.3.5. Production starter preparation

2.3.6. Preparation of kefir sourdough

2.3.7. The vices of starter cultures

2.3.8. Rules for the use of starter cultures in production 2.4. Microbiology of fermented milk products

2.4.1. Sources of microflora of fermented milk products

2.4.2. Microbiology of kefir

2.4.3. Microbiology of cottage cheese

2.4.4. Microbiology of homemade cheese

2.4.5. Microbiology of sour cream

2.4.6. Microbiology of yogurt, Mechnikov yogurt, fermented baked milk, Varenets

Milk and sources of its contamination. Milk is the secret of the mammary gland of mammals. The composition of cow's milk in%: water 87%; milk sugar - 4.7%; milk fat - 3.9%; proteins - 3.3%; minerals - 0.7%; vitamins and enzymes.

“Milk,” wrote Academician I.P. Pavlov is an amazing food prepared by nature itself. It has been established that this product contains over a hundred of the most valuable components. It includes all the substances necessary for the life of the body, proteins, fats, carbohydrates, mineral salts, vitamins. Thus, in milk, nature "picked up" all the components in very good proportions.

Milk is a good medium for the reproduction and preservation of microorganisms. It is impossible to get sterile milk, because. in the teat canal (communicating with the external environment) there are representatives of the normal microflora of the udder: mammococci, micrococci, lactic acid streptococci and rods.

Origin of milk microflora. Sources of pollution. Milk in its composition is a favorable environment for the development and reproduction of various microorganisms, so you can always find one or another number of microbes in it.

Milk on its way from the udder to the consumer comes into close contact with a number of sources of contamination. These sources are far from equal in terms of both abundance and species composition of introduced bacteria.

Microflora obtained by milk from the udder. This source is put in first place because of its extreme constancy and absolute inevitability. The teat canal always contains bacteria: obligate - micrococci, mammococci (udder cocci are harmless) and facultative - lactic acid streptococci, there may also be pathogenic staphylococci. They form a "bacterial plug" of the teat canal, if it is not milked separately, this will lead to an increase in the number of bacteria in the total milk yield by three times.

The sanitary condition of animals also has a great influence on the bacterial contamination of milk during milking: animal skin, hands of a milkmaid, dust from bedding, dairy equipment and utensils.

Animal skin, as a source of pollution, is characterized by abundance and difficult to remove, due to contamination of the skin with dung particles. During milking, a real rain of E. coli, enterococci, aerobes and anaerobes, yeasts and molds, etc. must fall on the surface of the milk. ( the list of these microorganisms is very important, because. namely, they will constitute the normal microflora of milk). Therefore, the degree of bacterial contamination of milk depends on the method of processing the skin and udder before milking. In practice, one bucket is often used to wash the udder, one towel for the whole group, up to 214 million bacteria can be found per 1 cm 2 of such a towel.

When milking cows by machine, many sources of contamination are eliminated, however, when milking machines are kept in an unsanitary condition, they become a significant source of microbial contamination (mainly psychrophilic bacteria). For example, if after disinfection with a 0.2% solution of chloramine, new milk hoses become almost sterile, then on old hoses with cracks on the inner surface, after the same treatment, up to 940 thousand bacteria were found per 1 cm2. Thus, the role of milk equipment is dual: on the one hand, milk equipment is the most perfect protection against contamination, and on the other hand, it can give milk its own microflora.

The source of contamination of milk can be dust from the distribution of feed and dry cleaning. The use of rotten straw as bedding increases the number of microorganisms, especially spore-forming and mold fungi in the air, along with dust, microbes also enter the milk.

It can be concluded that sources of pollution can be eliminated if zoohygienic rules for keeping cows and sanitary and hygienic conditions in the process of obtaining milk are observed. Having got acquainted with the sources of milk contamination, we got an idea about the composition of the microflora of fresh milk.

Changes in the microflora of milk during storage and transportation. Quantitative and qualitative changes in the microflora of milk depend on the temperature, duration of storage and its composition upon receipt. So, when milk is stored at 10 0 C, a successive change of phases occurs.

Bactericidal phase- the essence of this phase is that the number of microorganisms in freshly milked milk decreases during storage. These properties of milk are explained by the presence of various antimicrobial substances in milk: lactenins, bacteriolysins, lysozyme, etc. The duration of the bactericidal phase varies widely and depends on the following factors:

1. The number of bacteria that got into the milk during milking.

2. Storage temperatures (bactericidal properties of milk are preserved during the day if the temperature is below 10 0 C, and only 6 hours - at a temperature of 25 0 C).

3. From the individual properties of the animal's body and the lactation period.

Phase of mixed microflora. After the end of the bactericidal phase, when there are no substances in the milk that inhibit the development of microbes, and the storage temperature is above 10ºС, all microorganisms remaining by this moment begin to multiply in the milk. This phase is the period of the most rapid increase in the number of microorganisms. During this period, which lasts 12-18 hours, the microflora increases hundreds of thousands of times. The considered phase of mixed microflora is especially important from a practical point of view, since it is in this phase that milk reaches the consumer.

lactic phase. The moment when a noticeable increase in acidity is detected in the milk is taken as the beginning of this phase. From a certain point on, Str.lactis has a preponderance over all, as they multiply, the acidity of milk decreases to pH 4.0. Such acidity is unfavorable for streptococci; acid-resistant (pH up to 3.6) lactic acid bacilli begin to develop to replace them. Thus, here we can talk about two clearly distinguishable phases, replacing one another, in a certain sequence. An increase in acidity is detrimental to the putrefactive microflora, as well as to the bacteria of the Escherichia coli group.

The duration of the lactic acid phase is longer than any other phase, it can last for months without noticeable change in the microflora at the appropriate temperature. But it must be taken into account that the lactic acid phase as a whole covers the state of milk in which it qualifies as a fermented milk product.

Development phase of yeast and mold. This phase is of no practical interest and is unlikely to be observed under practical conditions (we give it for the sake of completeness). Normally, milk does not survive to this phase, once consumed, during the lactic acid phase. The external picture of the development of this phase is as follows: even during the lactic acid phase, separate colonies of Oidium lactis form on the surface of the clot, gradually closing into a continuous white fluffy film. At the same time, the appearance of membranous yeasts can be observed, later pigmented colonies of mold fungi Penicillium, Aspergillus appear, displacing Oidium. Rancidity appears in milk due to decaying fat, “moldy” and “yeasty” flavors. Then, under the mold film, the first signs of decomposition and peptonization of proteins begin to appear, in the form of a liquid from light yellow to dark brown. The layer of liquid increases due to the clot and, in the end, there is no trace of the clot: everything turns into a brown liquid, covered from above with a thick film of mold.

Normal microflora of milk. The entire microflora of milk is divided into normal and abnormal. The normal microflora includes those that are constantly present in milk, these are: lactic acid bacteria, micrococci, sarcins, enterococci, bacteria of the Escherichia coli group, butyric acid bacteria, putrefactive bacteria, molds and yeasts.

Of these species, lactic acid bacteria are of particular interest. As their name suggests, their main waste product is lactic acid. Lactic acid bacteria are used in the manufacture of fermented milk products, cheese making, butter making. Therefore, we will give a detailed description of lactic acid bacteria. All lactic acid bacteria are united in the family:

L A C T O B A C T E R I A C E A E

Genus S t r e p t o c o c c u s Genus La c t o b a c t e r i u m

Str. lactis L.acidophilum

Str. cremoris L. bulgaricum

Str. thermophilus L.casei

Defects of milk of microbial origin. With long-term storage of raw and pasteurized milk, it begins to show signs of spoilage caused by reproduction of the above microflora. The nature of spoilage depends on the storage temperature and the type of prevailing microorganisms.

Ammonifiers(putrefactive microorganisms) can multiply at low milk storage temperatures, because belong to psychrophilic bacteria. In the process of decomposition of proteins, the consistency of milk changes, bitterness appears.

Butyric bacteria are widely distributed in nature. They are found in large quantities on care items, in feed and, if sanitary conditions are not observed, get into milk. During pasteurization, spores of butyric bacteria do not die; during long-term storage of milk, they break down lactose to butyric acid and gas, which give the milk a rancid taste and smell.

mold mushrooms form islands of colonies on the surface of curdled milk, give it a bitter taste and moldy smell. The presence of mold indicates long-term storage of the dairy product at low temperatures.

coli, found in milk in large quantities, gives it a stall smell, and at a favorable temperature ferments lactose with the formation of acid and gas. Milk containing E. coli cannot be used for the preparation of fermented milk products, cheeses, because. E. coli causes defects in them.

Causative agents of infectious diseases transmitted through milk. The causative agents of infectious diseases get into milk from sick animals, from the environment during transportation or processing. Microbes transmitted through milk fall into two groups. The first includes causative agents of zooanthroponoses that are transmitted from one animal species to another and from animal to human. These include: pathogens of tuberculosis, brucellosis, anthrax, foot and mouth disease, etc. The second group includes causative agents of anthroponosis- diseases that are transmitted from person to person (dysentery, diphtheria, typhoid fever, scarlet fever).

When pathogenic pathogens from sick people and animals get into milk, they multiply and accumulate toxins in milk, which leads to the occurrence of food toxic infections when such milk is consumed.

Disinfection on dairy farms should be considered as an important measure to complement the pasteurization of milk and aimed at preventing zoonoses and zoonotic diseases that are transmitted to humans through milk, including salmonellosis. Milking machines, buckets, cans and other containers should be disinfected; various chemicals are used for this, for example, soda ash and potassium hydroxide.

Preservation of milk by physical methods. Milk entering dairies is characterized by significant bacterial contamination (from hundreds of thousands to millions per 1 ml), especially during the hot season. Bacterial contamination of milk can be reduced if sanitary and hygienic conditions and timely cooling of milk are observed all the way from the udder to the consumer. Deep cooling immediately after milking is especially effective, as this lengthens and uses the bactericidal phase. Milk should be stored at a temperature not higher than 6-8°C, and preferably at 2-4°C.

Freezing milk allows you to stop bacterial processes in it for a long time. In this case, to prevent precipitation of casein, rapid freezing at minus 25°C should be applied. Cold does not cause the death of microorganisms, but transfers them to an anabiotic state, and when milk is thawed, their vital activity manifests itself again. Therefore, with the help of cold, only bacterially pure milk, in which there are few bacteria, can be preserved.

High temperature, unlike cold, causes the death of microbes, which increases the stability of the product, so milk processing by this method has become widespread.

boiling milk, although it provides a high sterilizing effect, it cannot be recommended for the dairy industry. When boiling, vitamins are largely destroyed, proteins are denatured, valuable calcium settles on the walls of dishes, the homogeneity of the fat emulsion is disturbed, therefore, instead of boiling, pasteurization of milk is used, after which the biological value of the product is preserved.

There are several modes milk pasteurization from healthy animals:

a) long-term - 63-65°C for 30 minutes;

b) short-term - 74-78°C for 20 seconds;

c) instant - 85-90°C without exposure.

When pasteurization is carried out correctly, about 99% of the bacteria contained in milk die, including non-spore pathogenic species (causative agents of tuberculosis, brucellosis, salmonellosis, pyogenic cocci), Escherichia coli and lactic acid bacteria.

After pasteurization, milk and cream must be cooled to + 4ºС to prevent the germination of spores and the reproduction of preserved thermophilic microflora.

Storing pasteurized milk at room temperature makes it possible for putrefactive and pathogenic bacteria to multiply freely, if they remain there, since bactericidal properties in pasteurized milk are inactivated. Pasteurized milk does not turn sour, but it can undergo putrefactive decomposition (peptonization) and acquire toxic properties during prolonged storage in the refrigerator. Thus, pasteurized milk cannot be stockpiled and stored for a long time.

Milk sterilization provides for the complete destruction of vegetative and spore forms of bacteria, which allows store such milk for a long time. Sterilized milk is prepared in three ways: a) milk is sterilized at a temperature of 140 0 C - 4 seconds, and then poured into paper bags with a polyethylene coating under aseptic conditions, such milk can be stored for 10 days at a temperature not exceeding 20 0 C; b) milk is bottled, sealed, and then sterilized at a temperature of 120 0 C - 15 minutes; c) milk is sterilized at 140 0 C - 2 seconds, bottled, corked and re-sterilized at 116 0 C - 15 minutes; such milk can be stored for up to 2 months.

Ultra high temperature treatment (UHT)- heating of milk to 140ºС within one second takes place in tubular apparatus by introducing chemically pure steam directly into milk, in a fully closed automated process. This eliminates oxidative processes that lead to the destruction of vitamin C, removes volatile substances of feed and stall origin. Such milk can be stored for a long time. As a result of such processing, spores also die, and all useful substances and trace elements in milk are preserved. In the manufacture of such milk, only high quality raw materials, because milk of the 1st and 2nd grade (according to GOST) will simply curl up. Especially for UHT milk, a new, aseptic version of the polyethylene-coated carton package was invented, such milk can be stored at room temperature.

canning the destruction of microbes or the creation of unfavorable conditions for the activity of microbes that cause spoilage of products is carried out. .
To prepare canned condensed milk in jars, it is sterilized at 115-118ºС for 15 minutes. At this temperature, vegetative microbes die, but some of the spore-forming microbes may remain. Surviving spores in favorable conditions can germinate, decompose the product with the formation of gases that cause the bombing of cans. To check the quality of sterilization, the jars are kept for 10 days at 37ºС. The absence of bombing indicates good sterilization of the jars, which allows them to be stored for a long time.

Condensed milk with sugar. Raw milk is first subjected to cleaning and the content of fat and solids is adjusted to a level that meets the requirements of GOST. Then the milk is heated to a boil and kept for about 20 minutes, while killing all microorganisms, with the exception of those resistant to high temperature. Pasteurized milk is concentrated to 1/3 of the original volume so that it contains no more than 26.5% moisture, and 43.5% sugar is added to it. With this ratio of water and sugar, a high osmotic pressure is created - conditions unfavorable for the development of Escherichia, lactic acid bacteria, yeast and many mold fungi. But in the presence of chocolate brown mold and colored micrococci with proteolytic properties, spoilage of the product occurs. Its safety in this case does not exceed 6-12 months. Compliance with technology and sanitary conditions in the production process allows you to save condensed milk with sugar for two years.

Sanitary and microbiological characteristics of milk. In order to prevent the spread of infectious diseases through milk, strict veterinary and sanitary supervision is carried out over animals and dairy industry enterprises (control of raw materials and production processes). The milk supplied to the dairy plant from the manufacturer, depending on the sanitary-microbiological and physico-chemical parameters, is divided into two grades. Milk of the 1st grade must have an acidity of 16-18ºT (according to Turner), microbial contamination according to the reductase test is not lower than 1st class and a degree of purity of 1st group according to the standard. The acidity of milk of the 2nd grade can be in the range of 16-20ºT, the microbial contamination according to the reductase test is not lower than class 2 and the degree of purity according to the standard is not lower than group 2. At the same time, the assessment of milk upon acceptance is carried out according to the worst indicator.

The indicators by which the grade of delivered raw milk is determined are shown in Table 1.

Table 1

Milk grade indicators

An increased content of somatic cells in milk indicates the presence of acute inflammation of the udder (mastitis). The use of milk with a high content of somatic cells in it for food purposes is not allowed. In addition to the loss of technological properties, such milk contains toxins.

The acidity of milk is an indicator that indirectly confirms its microbial well-being. With an increase in the number of bacteria in milk, its acidity also increases. Reduced acidity indicates that chemicals have been added to the milk in order to falsify its quality. And this is dangerous, since all substances used for falsification are toxic to humans.

Pasteurized milk produced by dairy industry plants is divided into two groups according to the total number of microbes and coli-titer: A and B. (Table 2).

Milk and dairy products are essential commodities. They are especially needed by children and the elderly. There is an opinion that it is useless to drink milk after treatment. Is it so? What if you drink raw milk? What do we know and what have we never thought about when buying most people's favorite product?

bacteria

Raw milk always contains a certain amount of bacteria. They are found even when sanitary and hygienic rules are observed. If you add imperfect milking conditions, the milk literally teems with microorganisms. There are enough microbes on the surface of the udder and hands of the milkers. Milking equipment, dishes and even the air are saturated with different types of microbes. Can raw milk be eaten? What happens to dairy products after processing?

Fresh milk has a very diverse microflora. It can detect lactic acid bacteria, E. coli, enterococci, yeast and other types of microorganisms. Among them are those that determine the various undesirable properties of milk. It can be bitter, bluish, reddish, viscous, have an unpleasant odor. When the microbiology of milk and dairy products is carried out, specialists often find pathogens of various kinds of infectious diseases and food poisoning. These are dysentery, brucellosis, typhoid fever, tuberculosis, Staphylococcus aureus, salmonella and other types of dangerous microorganisms.

What happens after milking?

When storing milk, the ratio between microorganisms undergoes some quantitative and qualitative changes. The nature of these processes depends on the temperature, the duration of storage of the product and the initial microflora. At the same time, 60-80 degrees is the pasteurization temperature of milk, when some bacteria die, while others reduce their activity. This is how the shelf life of the product is extended.

Fresh milk contains bactericidal lactenins, due to which the development of bacteria is delayed in the first hours after milking. This period is called the bactericidal phase. The higher the temperature and the more bacteria, the sooner the product will start to spoil.

What the microbiology of milk and dairy products tells consumers

Freshly milked milk will have a temperature of about 35 degrees Celsius. When the temperature drops to thirty degrees, the bactericidal phase will last about 3 hours. Of course, if the product is not contaminated with bacteria in excess. At twenty degrees, the shelf life of milk will be about 6 hours, at ten - about a day, and at five - one and a half days. If the temperature is zero, the bactericidal phase will last 48 hours. But you should always take into account the number of bacteria in milk: the more of them, the sooner it will spoil.

The faster the milk is cooled, the longer the bactericidal phase will last. The higher the temperature, the faster the bacteria will multiply. The phase in which different bacteria begin to develop during the first hours of storage is called the phase of mixed microflora.

What processes are taking place?

As the microbiology of milk and dairy products shows, towards the end of this phase, lactic acid bacteria actively develop, causing an increase in acidity. As the acid accumulates, other bacteria develop. Others, such as putrefactive ones, begin to die off. The lactic acid bacteria phase begins and the milk ferments. If you do not comply with the storage conditions in the first hours, then the temperature of milk pasteurization of 60-80 degrees will provoke its coagulation and the product will become unusable.

With an increase in acid concentration, even lactic acid bacteria are suppressed. The first to die off In the future, the growth of mold and yeast begins. These microorganisms contribute to the formation of alkaline protein breakdown products. Now the acidity is reduced, and putrefactive bacteria can again develop.

Studying the microbiology of milk and dairy products, you will learn a lot of interesting things about these products. For example, at temperatures below 10-8 degrees Celsius, lactic acid bacteria practically do not multiply. But cold-resistant bacteria are slowly gaining strength. For example, decomposing proteins and fats Pseudomonas. These bacteria contribute to the bitter taste of milk.

In order to preserve the freshness and vitamins necessary for a person in milk, on dairy farms it is cooled to 6-3 degrees. Only chilled it is delivered for processing. There, the milk is cleaned from mechanical impurities, pasteurized or sterilized. Chilled, it is poured into containers and sent for sale.

As for fermented milk products, they play a big role in nutrition.

These are dietary products, and even medicinal ones. Compared to whole milk, the fermented milk product has an increased storage stability. In addition, it is more easily absorbed by our body. But for all its value, it is a source of pathogenic bacteria if the preparation technology and storage conditions are violated. It is very important to ensure a normal course during the fermentation process.

When storing such a product, the process of development of yeast and mold can begin in it. Dangerous microorganisms get into the product from the equipment, clothes and hands of workers, and, of course, from the air. A spoiled product has an unpleasant odor, a taste defect and other signs of spoilage.

Milk and most dairy products are a breeding ground for various microorganisms, both pathogenic and spoilage microorganisms.

Milk obtained from sick animals is dangerous to health, it can cause infectious diseases, staphylococcal toxicosis and other food poisoning.

Fresh milk obtained from healthy animals has bactericidal properties. The bactericidal phase lasts from several minutes to 45 minutes if the milk is at a temperature not higher than 0 ° C. Then the number of microorganisms begins to increase, and the faster, the higher the storage temperature of milk.

Raw milk can contain micrococci, streptococci, as well as Klebsiella, Yersinia, Proteus and Escherichia coli (coliforms), etc. If the storage and sale conditions are violated, microbes in milk and dairy products multiply rapidly, which leads to an unpleasant taste, changes in the properties of milk and his damage.

When lactic acid bacteria begin to predominate and acidity increases, the milk turns sour, the development of many other bacteria is suppressed. Then the lactic acid microflora gradually dies off, creating conditions for the growth of yeast, mold fungi, and then rotting microorganisms.

Pasteurization of milk is carried out in order to destroy pathogenic microorganisms and reduce the total contamination of milk. Milk is pasteurized at 76 ° C with a holding time of 15-20 s. After pasteurization of milk, a certain amount of thermophilic and heat-resistant bacteria (including enterococci) and spores remain. Such milk should be stored at a temperature of +4 °C for no more than 36 hours. Sterilized milk contains practically no microorganisms and can be stored for a long time.

Dairy products(sour cream, cottage cheese, kefir, curdled milk, etc.) have greater storage stability than milk. They are an unfavorable environment for the development of many pathogenic bacteria. This is due to the increased acidity of the products and the antibiotic properties of some starter cultures.

In the manufacture of fermented milk products, starters are used that contain pure cultures of lactic acid streptococci, Bulgarian and acidophilus bacilli, or mixtures thereof. For the production of kefir, the so-called kefir fungus is used - a symbiosis of yeast and other microorganisms.

Cheeses are obtained by fermenting milk with lactic acid bacteria, and then introducing rennet, which activates the coagulation of milk. Next, the process of maturation of the cheese takes place - under the action of the microbes of the starter, lactic acid and propionic acid fermentation occurs. As a result, milk sugar is fermented, proteins are partially broken down, and a specific taste and aroma appear. The carbon dioxide released during these processes forms cheese eyes.

In the production of some soft cheeses, cultures of mold fungi from the genus Penicillium are used.

Cheese spoilage is most often due to mold, the development of butyric acid bacteria leads to bloating, and some lactic streptococci to bitterness.