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On the one hand, microorganisms are widely distributed in nature, and the types and quantities of microorganisms existing in different environments are different. On the other hand, food, from raw materials, production, processing, storage, transportation, sales to cooking, often comes into contact with the environment in various ways, leading to microbial pollution. Microbial sources of contaminated food can be divided into soil, air, water, operators, animals and plants, processing equipment, packaging materials and so on.

1. 1 microbial sources of contaminated food

1. 1. 1 soil

Soil contains a large number of carbon and nitrogen sources that can be used by microorganisms, as well as a large number of inorganic elements such as sulfur, phosphorus, potassium, calcium and magnesium, and trace elements such as boron, molybdenum, zinc and manganese. In addition, the soil has certain water retention, air permeability and suitable pH value.

(pH3.5~ 10.5), the soil temperature usually changes between 10 ~ 30℃, and the surface soil is covered to protect microorganisms from the harm of solar ultraviolet rays.

It can be seen that soil provides favorable nutritional and environmental conditions for the growth and reproduction of microorganisms. Therefore, soil is called "the natural culture medium of microorganisms" and "the base camp of microorganisms".

The number of microorganisms in the soil can reach 107 ~ 109 /g, and there are many kinds of microorganisms in the soil, among which bacteria account for the largest proportion, accounting for 70% ~ 80%, actinomycetes account for 5% ~ 30%, followed by fungi, algae and protozoa. There are great differences in the types and quantities of microorganisms in different soils. 3 ~ 25 cm underground is the most active place for microorganisms. There are more kinds and quantities of microorganisms in fertile soil and more yeasts in orchard soil. Microorganisms distributed in air, water, people, animals and plants will continue to enter the soil in addition to their own development. Many pathogenic microorganisms enter the soil with animal and plant residues and human and animal excreta. Therefore, there are both non-pathogenic microorganisms and pathogenic microorganisms in the soil. Generally speaking, non-spore bacteria live in soil for a short time, while spore bacteria live in soil for a long time. For example, Salmonella can only live for a few days to weeks, while Bacillus anthracis can live for several years or even longer. At the same time, there are soil-borne pathogens that can survive in the soil for a long time. Spores of molds and actinomycetes can also survive in soil for a long time.

1. 1.2 air

There are no nutrients and sufficient water conditions for the growth and reproduction of microorganisms in the air, and it is often irradiated by ultraviolet rays of the sun outdoors, so the air is not a place for the growth and reproduction of microorganisms. However, the air does contain a certain number of microorganisms, which are suspended in the atmosphere or attached to flying dust or droplets. These microorganisms can come from soil, water, human, animal and plant surface shedding and respiratory and digestive tract excretions.

Microorganisms in the air are mainly molds, spores of actinomycetes, spores of bacteria and yeasts. There are great differences in the number and types of microorganisms in the air in different environments. There are many microorganisms in the air in public places, streets, barns, slaughterhouses and poorly ventilated places. The more dust in the air, the more microorganisms it contains. The number of air microorganisms with serious indoor pollution can reach 106 /m3, while the number of microorganisms in places with fresh air such as oceans, mountains, villages and forests is relatively small. There may be some pathogenic microorganisms in the air, which come directly from human or animal respiratory tract, dry skin shedding and excrement, or indirectly from soil, such as mycobacterium tuberculosis, Staphylococcus aureus, Salmonella, Haemophilus influenzae and virus. The droplets ejected from the patient's mouth contain 1 ~ 20000 bacteria.

1. 1.3 water

In nature, rivers, lakes, oceans and other fresh water and salt water areas have corresponding microorganisms. Due to the differences in the types and contents of organic and inorganic substances, temperature, pH value, salt content, oxygen content and illumination in different waters, the types and quantities of microorganisms in various waters are obviously different. Usually, the number of microorganisms in water mainly depends on the content of organic matter in water. The more organic matter there is, the more microorganisms there are.

Microorganisms in fresh water can be divided into two categories: one is freshwater aquatic microorganisms, which are used to living in clean lakes and reservoirs, mainly autotrophic microorganisms, and can be regarded as indigenous microorganisms in the water environment, such as sulfur bacteria, iron bacteria, Chlamydomonas and cyanobacteria containing photosynthetic pigments, green sulfur bacteria and purple bacteria. Some saprophytic bacteria, such as chromobacter, achromobacter and micrococcus, can grow in clean water with low nutrient content. There are also some aquatic species in mold, such as some species of Hydromycetes and Sargassum, which can grow on decaying organic residues. In addition, unicellular and filamentous algae and some protozoa often grow in water, and their numbers are usually small. The other is corrupt aquatic microorganisms, which multiply with the corrupt organic matter entering the water area and gaining nutrition, and are the important reasons for water pollution and spreading diseases. Among them, the number of G- bacteria is the largest, such as Proteus, Escherichia coli, Enterobacter aerogenes and Alcaligenes, as well as some Bacillus, Vibrio and Spirillum. When the water is polluted by soil and human and animal excreta, the number of intestinal bacteria will increase, such as Escherichia coli, Streptococcus faecalis and Clostridium welchii, Salmonella, Clostridium perfringens, Bacillus anthracis and tetanus, and there will be ciliates, flagellates and protozoa in the sewage. The pathogens of animals and plants entering the water body can not survive for a long time because the environmental conditions of the water body can not fully meet the requirements of their growth and reproduction, but a few pathogens can survive for several months.

Seawater also contains a large number of aquatic microorganisms, mainly bacteria, which are halophilic. The common bacteria in offshore waters are Pseudomonas, Achromobacter, Flavobacterium, Micrococcus, Bacillus and Phagocytosis, which can cause corruption of marine animals and plants, and some of them are pathogenic bacteria of marine fish. There are also pathogenic bacteria in seawater that can cause human food poisoning, such as Vibrio parahaemolyticus.

Mineral water and deep well water usually contain a small amount of microorganisms.

1. 1.4 human and animal bodies

The skin, hair, mouth, digestive tract and respiratory tract of human body and various animals such as dogs, cats and rats contain a large number of microorganisms, such as unwashed animal fur and skin microorganisms, which can reach 105~ 106 /cm2. When people or animals are infected with pathogenic microorganisms, different numbers of pathogenic microorganisms will appear in the body, some of which are pathogenic microorganisms of people and animals, such as Salmonella, Mycobacterium tuberculosis, Bacillus berghei and so on. These microorganisms can pollute food through direct contact or excretion through respiratory tract and digestive tract.

Mosquitoes, flies, cockroaches and other insects also carry a large number of microorganisms, among which there may be many pathogenic microorganisms, and their contact with food will also cause microbial pollution.

1. 1.5 processing machinery and equipment

All kinds of processing machinery and equipment do not have the nutrients needed by microorganisms, but in the process of food processing, because the juice or particles of food adhere to the inner surface, at the end of food production, the machinery and equipment are not completely sterilized, so that a small number of microorganisms grow and multiply on them and become microbial pollution sources. This kind of mechanical equipment will cause microbial contamination of food through contact with food in the later use.

1. 1.6 packaging material

All kinds of packaging materials will also carry microorganisms if they are not handled properly. Disposable packaging materials usually contain fewer microorganisms than recycled materials. Plastic packaging materials will absorb dust and microorganisms because of their charge.

1. 1.7 raw materials and auxiliary materials

1. 1.7. 1 animal raw materials

Healthy livestock and poultry before slaughter have a sound and complete immune system, which can effectively prevent and prevent the invasion and spread of microorganisms in muscle tissue. Therefore, normal tissues (including muscle, fat, heart, liver, kidney, etc. ) are generally sterile, and microorganisms always exist in the body surface, coat, digestive tract, upper respiratory tract and other organs of livestock and poultry. For example, the number of microorganisms in unwashed animal fur and skin can reach 105~ 106 /cm2. If feces are polluted by hair and skin, the number of microorganisms will be more. The number of microorganisms in freshly discharged livestock manure can reach 107 /g, and the number of microorganisms in rumen components can reach 109/g.

There may be microorganisms in organs and tissues of sick livestock and poultry, such as Mycobacterium tuberculosis and foot-and-mouth disease virus of sick cattle. These microorganisms can break through the body's defense system and spread to other parts of the body, most of which are pathogenic bacteria. When animal skin is stabbed, bitten or suppurated, bacteria will exist in lymph nodes. Some bacteria will be swallowed up or destroyed by the body's defense system, while others may remain and cause diseases. Some livestock infected with pathogenic bacteria have clinical symptoms, but a considerable number of them are asymptomatic carriers. In the process of transportation and captivity, due to the stimulation of crowding, fatigue, hunger and panic, the immunity of these livestock and poultry decreased and showed clinical symptoms, and pathogenic bacteria spread to the outside world, resulting in mutual infection of livestock and poultry.

After livestock and poultry are slaughtered, they lose their innate defense function, and microorganisms invade tissues and multiply rapidly. Improper sanitary management in the slaughter process will lead to extensive microbial pollution. At first, when the unsterilized knife was used for bloodletting, contaminated microorganisms were brought into the blood and spread to all parts of the carcass with the short weak circulation of the blood. Microbial contamination may occur in every link of slaughter, segmentation, processing, storage and meat distribution.

The state of livestock and poultry before slaughter is also very important. Give them enough rest and good feeding before slaughter, and keep them in a quiet and comfortable environment. In this state, glycogen in their muscles will be converted into lactic acid. The pH value of the carcass decreased to 5.6 ~ 5.7 within 6 ~ 7 hours after slaughter, and decreased to 5.3 ~ 5.7 within 24 hours due to the increase of lactic acid. Under this pH condition, contaminated bacteria are not easy to reproduce. If livestock are in a state of stress and excitement before slaughter, the stored glycogen will be utilized, and the pH value of animal tissues will be close to 7 after slaughter. Under such conditions, the infection of spoilage bacteria will be more rapid.

The inside of fresh eggs laid by healthy birds should be sterile, but microorganisms and even fresh eggs can often be found in fresh eggs. Source of microbial contamination: 1) ovary. Pathogens enter the ovary through blood circulation and enter the egg when the yolk is formed. Common infectious bacteria are Salmonella pullorum and Salmonella gallinarum 2) Excretion cavity (reproductive tract). There are a certain number of microorganisms in the excretory cavity of birds. When the egg is discharged from the excretory cavity, microorganisms attached to the eggshell can enter the egg through the eggshell due to cold contraction. 3) environment. The barrier effect of fresh eggshells is limited. There are many pores with the size of 4 ~ 6 microns on the eggshell, and all kinds of external microorganisms may enter, especially eggs that have been stored for a long time or washed. Under the condition of high temperature and high humidity, microorganisms in the environment are more likely to invade eggs through water infiltration.

Fresh milk always contains a certain amount of microorganisms. This is because even in the breasts of healthy dairy animals, there may be some bacteria, especially papillary ducts and their branches, which often live with specific breast flora. There are mainly micrococcus, streptococcus and lactobacillus. When dairy animals suffer from mastitis, their breasts also contain pathogenic bacteria that cause mastitis, such as Streptococcus agalactiae, Corynebacterium pyogenes, Streptococcus lactis and Staphylococcus aureus. When suffering from tuberculosis or brucellosis, there may be corresponding pathogens in milk.

Fish live in water, and there are many kinds of microorganisms in water, so there are a certain number of microorganisms in fish's body surface, gills and digestive tract. The number of bacteria attached to the surface of live fish per square centimeter is 102~ 107, and the number of bacteria in the intestinal juice of fish per milliliter is 105~ 108. Therefore, the bacteria carried by freshly caught fish are mainly bacteria in aquatic environment. There are mainly Pseudomonas, Flavobacterium and Achromobacter. There are also Alcaligenes, Aeromonas and Brevibacterium in fresh water.

Fish in offshore and inland waters may be polluted by human or animal excreta and carry pathogenic bacteria such as Vibrio parahaemolyticus. They exist in a small amount in fish and will not directly endanger human health, but if they are not properly preserved, pathogenic bacteria will cause food poisoning after they multiply in large quantities. Pathogens found in fish may also include Salmonella, Shigella and Vibrio cholerae, Erythrina erysipelas and Clostridium perfringens, which are also polluted by the environment. The caught fish may be further polluted by various microorganisms on land during transportation, storage, processing and sales. These microorganisms are mainly micrococcus and Bacillus, followed by Proteus, Escherichia coli, Serratia, Streptococcus and Clostridium.

1. 1.7.2 Plant raw materials

Healthy plants are in extensive contact with nature during the growing period, and there are a large number of microorganisms on the body surface, so the harvested grains generally contain microorganisms in their original living environment. According to the determination, each gram of grain contains thousands of kinds of bacteria. Most of these bacteria belong to Pseudomonas, Micrococcus, Lactobacillus and Bacillus. In addition, the grain also contains a considerable number of mold spores, mainly Aspergillus, Penicillium, Alternaria, Fusarium and so on, as well as yeast. Plant pathogens and intestinal microorganisms, as well as pathogens from human and animal feces, can also adhere to the surface of plants. Healthy plant tissues should be sterile or have few bacteria, such as fruits or vegetables, which sometimes look normal, and there may be some microorganisms in their internal tissues. Some people isolated yeasts from apple, cherry and other tissues, and isolated yeasts and Pseudomonas from tomato tissues. These microorganisms invade and exist in fruits and vegetables during flowering.

There will be a large number of pathogenic microorganisms in infected plant tissues, which invade the tissues through different ways such as roots, stems, leaves, flowers and fruits during plant growth.

Fruit and vegetable juice is made from fresh fruit through processing. Because the raw materials of fruits and vegetables contain microorganisms themselves, and they will be re-infected during processing, there must be a large number of microorganisms in fruit and vegetable juice. The pH value of fruit juice is generally between 2.4 and 4.2, and the sugar content is high, which can reach 60 ~ 70 Brix. Therefore, the microorganisms living in fruit juice are mainly yeast, followed by mold and a few bacteria.

In the process of grain processing, some microorganisms on the surface of grain can be removed by washing and cleaning, but some processes will pollute it again by microorganisms carried by the environment, machines and operators. Most commercially available flour contains thousands of bacteria per gram, and also contains about 50 ~ 100 mold spores.

1.2 ways of microbial contamination of food

Food may be contaminated by microorganisms in the process of production, processing, transportation, storage, sales and eating, and the ways of pollution can be divided into two categories.

Endogenous pollution 1.2. 1

All animals and plants used as food raw materials are called endogenous pollution, also called first pollution, because of their own microorganisms. For example, in the life of livestock and poultry, there are always certain groups and quantities of microorganisms in the digestive tract, upper respiratory tract and body surface. When infected by pathogenic microorganisms such as Salmonella, Brucella and Bacillus anthracis, pathogenic microorganisms will exist in some organs and tissues of livestock and poultry. When poultry are infected with infectious diseases such as pullorum and typhoid fever, pathogenic microorganisms can invade the ovaries through blood circulation and be contaminated by pathogenic bacteria when egg yolk is formed, so that the eggs laid also contain corresponding pathogenic bacteria.

1.2.2 Exogenous pollution

In the process of food production, processing, transportation, storage, sale and consumption, the pollution of food by microorganisms through water, air, people, animals and mechanical equipment is called exogenous pollution, also known as secondary pollution.

1.2.2. 1 Through water pollution

In the process of food production and processing, water is not only the raw material or ingredient of many foods, but also an indispensable substance for cleaning, cooling and freezing. Cleaning equipment, floors and utensils also requires a lot of water. Various natural water sources, including surface water and groundwater, are not only the source of microbial contamination, but also the main way of microbial contamination of food. Tap water is purified and disinfected from natural water for drinking. Under normal circumstances, the bacteria content is less. However, if the tap water pipe leaks and the pressure in the pipe is insufficient, it will temporarily become negative pressure, which will cause microorganisms around the pipe to leak into the pipe and increase the number of microorganisms in tap water. In production, even if the water source that meets the hygienic standard is used, the scope of microbial pollution will be expanded due to improper methods. For example, in the process of slaughter, depilation and evisceration in slaughterhouses, microorganisms in fur or intestines can lead to mutual infection between carcasses through the diffusion of water. If the production water is polluted by domestic sewage, hospital sewage or toilet feces, the number of microorganisms in the water will increase sharply. Water not only contains bacteria, viruses, fungi, leptospira, but also may contain parasites. Using this water for food production will cause serious microbial pollution, and may also cause food pollution by other toxic substances, so the hygienic quality of water is closely related to the hygienic quality of food. The water used for food production must meet the drinking water standard, and tap water or deep well water should be used. Circulating cooling water should be prevented from being polluted by livestock manure and leftovers.

1.2.2.2 Through air pollution

Microorganisms in the air may come from soil, water, human beings, shedding of animals and plants, and excreta of respiratory tract and digestive tract, and they may pollute food with the flying or settling of dust and water droplets. Microorganisms contained in human sputum, nose and saliva drops include pathogenic microorganisms, which can be eaten directly or exist in secondary pollutants when people talk, cough or sneeze. When people talk or sneeze, the distance from human body 1.5m is a direct pollution area, and large water droplets can be suspended in the air for 30 minutes. Water droplets can be suspended in the air for 4 ~ 6 hours, so food exposed to the air will inevitably be contaminated by microorganisms.

1.2.2.3 Pollution caused by contact between humans and animals

People engaged in food production, if their bodies, clothes and hats are not often cleaned and kept clean, there will be a large number of microorganisms attached to them, which will cause pollution through contact with food through skin, hair, clothes and hats. In the process of food processing, transportation, storage and sales, if you come into direct or indirect contact with rats, flies, cockroaches, etc. , but also cause microbial contamination of food. Experiments show that each fly carries millions of bacteria, 80% of flies carry dysentery bacilli in their intestines, and the feces of mice carry pathogenic microorganisms such as salmonella and leptospira.

1.2.2.4 pollution caused by processing equipment and packaging materials

All kinds of machinery, equipment and packaging materials used in food production, processing, transportation and storage always carry different amounts of microorganisms before disinfection or sterilization, which becomes a way for microorganisms to pollute food. In the process of food production, the more equipment without disinfection and sterilization, the more opportunities for microbial contamination. Food that has been disinfected and sterilized will be contaminated again if the packaging materials used are not sterilized.

1.3 Growth and decline of microorganisms in food

After food is contaminated by microorganisms, the species and quantity of microorganisms will constantly change with the changes of food environment and food properties. The main feature of this change is the increase or decrease of the number of microorganisms in food, which is called the growth and decline of food microorganisms. The growth and decline of microorganisms in food usually have the following laws and characteristics.

1.3. 1 before treatment

Before food processing, both animal raw materials and plant raw materials have been contaminated by microorganisms to varying degrees. In addition, the chances of food being contaminated by microorganisms during transportation and storage are further increased, thus increasing the number of microorganisms in food raw materials. Although some kinds of microorganisms will pollute food and die because of environmental discomfort, the total number of surviving microorganisms generally increases instead of decreasing. This characteristic of microbial growth and decline is obvious in raw materials of fresh fish and fruits and vegetables. Even though strict hygiene measures have been taken in the transportation and storage of food raw materials before processing, microorganisms that have been polluted and existed as early as the origin of raw materials will still exist if they are not sterilized.

1.3.2 in the process of processing

In the whole process of food processing, some processing techniques such as cleaning, heating disinfection or sterilization are not conducive to the survival of microorganisms. These treatment measures can significantly reduce the number of microorganisms in food, and even almost completely eliminate microorganisms. However, if the microbial pollution in raw materials is serious, it will slow down the decline rate of microorganisms during processing. Microbial secondary pollution may also occur in many links of food processing. Under the condition of good production conditions and reasonable production technology, there is less pollution, so the total number of microorganisms contained in food will not increase obviously; If the microorganisms remaining in food have a chance to reproduce during processing, the number of microorganisms in food will suddenly increase.

1.3.3 after processing

If the storage conditions are suitable, microorganisms will grow and multiply, which will deteriorate the food, because there are still microorganisms left in the processed food or contaminated by microorganisms again. In this process, the number of microorganisms will rise rapidly, and when the number rises to a certain extent, it will not continue to rise. On the contrary, the number of viable bacteria will gradually decrease. This is due to the large consumption of nutrients required by microorganisms, which makes the deteriorated food not conducive to the continued growth of microorganisms and gradually dies. At this time, the food is inedible. If there are other kinds of microorganisms in spoiled food, and they can adapt to the substrate conditions of spoiled food and get opportunities for growth and reproduction, then the number of microorganisms will increase again. If the processed food is no longer polluted, and the remaining microorganisms are in unsuitable conditions for growth and reproduction, the number of microorganisms will decrease with the extension of storage date.

Due to the different types of food, processing methods and storage conditions, the growth and decline of microorganisms in different foods can not be exactly the same. It is of great significance to fully grasp the growth and decline characteristics of microorganisms in various foods to guide food production.