Changes in environmental conditions affect the life of microorganisms. The physical, chemical, biological factors of the environment can accelerate or inhibit the development of microbes, can change their properties or even cause death.
The environmental factors that have the most noticeable effect on microorganisms include humidity, temperature, acidity and chemical composition of the medium, the effect of light and other physical factors.
Microorganisms can live and develop only in an environment with a certain moisture content. Water is necessary for all metabolic processes of microorganisms, for normal osmotic pressure in the microbial cell, to maintain its viability. In different microorganisms, the need for water is not the same. Bacteria are mainly hygrophilous, with a moisture content of less than 20%, their growth stops. For molds, the lower limit of the moisture content of the medium is 15%, and with considerable air humidity and below. The precipitation of water vapor from the air to the surface of the product promotes the multiplication of microorganisms.
When the water content in the medium decreases, the growth of microorganisms slows down and may completely stop. Therefore, dry foods can be stored considerably longer than products with high humidity. Drying the products allows the products to be stored at room temperature without cooling.
Some microbes are very resistant to drying, some bacteria and yeast in the dried state can persist for up to a month or more. Spores of bacteria and mold fungi remain viable in the absence of moisture tens, and sometimes hundreds of years.
Temperature is the most important factor for the development of microorganisms. For each of the microorganisms, there is a minimum, optimum and maximum temperature regime for growth. By this property, microbes are divided into three groups:
- psychrophiles are microorganisms that grow well at low temperatures with a minimum at -10-0 ° C, an optimum at 10-15 ° C;
- mesophils are microorganisms for which the growth optimum is observed at 25-35 ° C, a minimum at 5-10 ° C, a maximum at 50-60 ° C;
- thermophiles are microorganisms that grow well at relatively high temperatures with an optimum growth at 50-65 ° C, a maximum at temperatures above 70 ° C.
Most microorganisms belong to mesophiles, for the development of which the temperature is 25-35 ° C. Therefore, the storage of food products at this temperature leads to a rapid multiplication in them of microorganisms and spoilage of products. Some microbes, with significant accumulation in products, can lead to human food poisoning. Pathogenic microorganisms, i.e. The causes of human infectious diseases also belong to mesophiles.
Low temperatures slow down the growth of microorganisms, but do not kill them. In chilled foods, the growth of microorganisms is slow, but continues. At temperatures below 0 ° C, most microbes stop multiplying; when the products are frozen, the growth of microbes stops, some of them gradually die off. It was found that at a temperature below 0 ° C most microorganisms fall into a state similar to anabiosis, retain their viability and with the rise in temperature continue their development. This property of microorganisms should be taken into account when storing and further cooking food. For example, salmonella can be stored in frozen meat for a long time, and after defrosting meat, they quickly accumulate in favorable conditions to a dangerous amount for humans.
When exposed to a high temperature, exceeding the maximum of endurance of microorganisms, their dying occurs. Bacteria that do not have the ability to form spores die by heating in a humid environment to 60-70 ° C in 15-30 minutes, up to 80-100 ° C – after a few seconds or minutes. The bacterial spores have a much higher temperature resistance. They are able to withstand 100 ° C for 1-6 hours, at a temperature of 120-130 ° C bacteria spores in a humid environment die in 20-30 minutes. Spores of molds are less heat resistant.
Thermal culinary processing of food in public catering, pasteurization and sterilization of food products in the food industry lead to partial or complete (sterilization) of the death of vegetative cells of microorganisms.
When pasteurized, the food product undergoes minimal temperature effects. Depending on the temperature regime, low and high pasteurization is distinguished.
Low pasteurization is carried out at a temperature not exceeding 65-80 ° C, at least 20 minutes for greater safety of the product.
High pasteurization is a short-term (no more than 1 min) effect on the pasteurized product of a temperature above 90 ° C, which leads to the death of the pathogenic non-spore-forming microflora and at the same time does not entail any significant changes in the natural properties of the pasteurized products. Pasteurized products can not be stored without cold.
Sterilization provides for the release of the product from all forms of microorganisms, including spores. Sterilization of canned canned food is carried out in special devices – autoclaves (under steam pressure) at a temperature of 110-125 ° C for 20-60 minutes. Sterilization provides the possibility of long-term storage of canned food. Milk is sterilized by ultra high temperature treatment (at temperatures above 130 ° C) for a few seconds, which allows you to preserve all the beneficial properties of milk.
The reaction of the medium
The life activity of microorganisms depends on the concentration of hydrogen (H + ) or hydroxyl (OH – ) ions in the substrate on which they develop. For most bacteria, neutral (pH about 7) or slightly alkaline medium is most favorable. Moldy mushrooms and yeast grow well with a weakly acid reaction of the medium. High acidity of the medium (pH below 4.0) prevents the development of bacteria, but molds can continue to grow in a more acidic environment. Suppressing the growth of putrefactive microorganisms during acidification of the environment has practical application. The addition of acetic acid is used in the marinating of products, which inhibits rotting processes and allows the preservation of products. The lactic acid formed during quenching also suppresses the growth of putrefactive bacteria.
Concentration of salt and sugar
Cookery salt and sugar have long been used to increase the resistance of products to microbial damage and better preservation of food.
An increase in the content of dissolved substances (salt or sugar) in the nutrient medium affects the amount of osmotic pressure inside the microorganisms, causes their dehydration. With an increase in the concentration of table salt in the substrate of more than 3-4% multiplication of many, including putrefactive, microorganisms slows down, at a concentration of more than 7-12% – ceases.
Some microorganisms need for their development in high concentrations of salt (20% and higher). They are called salt-loving, or halophiles. They can cause damage to salty foods.
High concentrations of sugar (above 55-65%) stop the reproduction of most microorganisms, this is used when preparing jam, jam or jam from fruit and berries. However, these products can also be damaged as a result of reproduction of osmophilic molds or yeast.
Some microorganisms need light for normal development, but for most of them it is disastrous. Ultraviolet rays of the sun have bactericidal action, ie, at certain radiation doses lead to the death of microorganisms. The bactericidal properties of the ultraviolet rays of mercury-quartz lamps are used to disinfect air, water, and certain food products. Infrared rays can also cause death of microbes due to thermal effects. The impact of these rays is used in the heat treatment of products. Negative effects on microorganisms can have electromagnetic fields, ionizing radiation and other physical factors of the environment.
Some chemicals can have a harmful effect on microorganisms. Chemicals that have a bactericidal effect are called antiseptics. These include disinfectants (bleach, hypochlorites, etc.) used in medicine, food industry and public catering.
Some antiseptics are used as food additives (sorbic and benzoic acids, etc.) in the production of juices, caviar, creams, salads and other products.
Different relationships can be established between different microorganisms: symbiosis is a mutually beneficial relationship; metabiosis – the vital activity of one at the expense of the other without causing harm; parasitism – the vital activity of one at the expense of another with causing harm to him; antagonism – one of the types of microorganisms depresses the development of another, which can lead to the death of microbes. For example, the development of lactic acid bacteria inhibits the growth of putrefactive, these antagonistic relationships are used in the souring of vegetables or to maintain normal microflora in the human intestine.
The antagonistic properties of some microorganisms are explained by their ability to release into the environment substances that have antimicrobial (bacteriostatic, bactericidal or fungicidal) action, antibiotics. Antibiotics are produced mainly by fungi, less often by bacteria, they exert their specific effect on certain types of bacteria or fungi (fungicidal action). Antibiotics are used in medicine (penicillin, levomycetin, streptomycin, etc.), in livestock as a feed additive, in the food industry for preserving food (nisin).
Phytoncides – substances found in many plants and food products (onion, garlic, radish, horseradish, spices, etc.) have antibiotic properties. Phytoncides include essential oils, anthocyanins and other substances. They are capable of causing the death of pathogenic microorganisms and putrefactive bacteria.
In egg white, fish eggs, tears, saliva contains lysozyme – an antibiotic substance of animal origin.