To answer such questions, we have developed several methods; including both the physical and chemical methods used for the control of microorganisms. With the use of such methods we are able to significantly decrease the number of microbes present at out area of interest. And for that purpose; the different physical methods used by us include: Heat, autoclaving, filtration, radiations, low temperature and high pressure etc. while, the different chemical methods include disinfectants etc.

PHYSICAL METHDOS TO REMOVE AND CONTROL THE GROWTH OF MICROBES
Maryam Sana
Maryam Sana

PHYSICAL METHODS TO REMOVE AND CONTROL THE GROWTH OF MICROBES

Keywords: Filtration, High Pressure, Radiation, Low Temperature, Sterilization, Pathogenicity, Pasteurization, Flaming, Boiling, What is Microbial Control, Different Physical Methods Use For The Control Of Microorganisms.

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What Is Microbial Control?

Microorganisms are classified as the microscopic organisms of versatile nature that are present everywhere around this planet. As they are found everywhere, many of these microbes are seen to be involved in various activities like breaking down of waste material, in the digestion of food, etc.

While few are pathogenic as well, meaning that they have a potential of causing serious diseases like tuberculosis; which is caused by mycobacterium tuberculosis etc. Though they are some helpful and some pathogenic, disease causing microbes but their control is necessary for the life to exist because by control we mean of destroying the only unwanted microbes not every microbial life, as they also play an important role for the life to exist on this planet. And with the control of microbes, we have made improvements in many fields of life like; water treatment plants, preservation and sterilization of food is possible also in medical fields (sterilized medical equipment’s).

But the main question arises there is; as microbes are so small in size that we can’t even see them without an unaided eye, how can we control them? Or destroy the unwanted microbes?

To answer such questions, we have developed several methods; including both the physical and chemical methods used for the control of microorganisms. With the use of such methods we are able to significantly decrease the number of microbes present at out area of interest. And for that purpose; the different physical methods used by us include: Heat, autoclaving, filtration, radiations, low temperature and high pressure etc. while, the different chemical methods include disinfectants etc.

With the use of such methods we are able to significantly decrease the number of microbes present at out area of interest

Different Physical Methods Use for the Control of Microorganisms:

Heat

Pasteurization and Sterilization

Filtration

Temperature and High Pressure

Desiccation

Osmotic Pressure

Radiation

Sterilization

Sterilization is a process that kills or destroys all living microorganisms including viable pores. Physical method of sterilization include heat, filtration and radiation.

Different Physical Methods Use for the Control of Microorganisms

Dry heat sterilization

Dry heat sterilization usually carried out in hot air oven. In this method microbes are killed by charring, oxidative damage and denature of cellular proteins. The temperature range used is 160 to 180 degree Celsius for 2 hours. The dry heat sterilization provides the benefit of good penetrability and non-corrosiveness so it is used for sterilization of glassware’s surgical instruments and some chemical like liquid paraffin, fats, glycerol etc.

Flaming is a direct method of dry heat sterilization. Hot air sterilization is a type of sterilization microbiological ovens are exposed to 171 degrees Celsius for 1 hour160 degree Celsius for 2 hours or longer depending upon the amount of volume. They are used for sterilizing glassware’s, metal instruments and inert material like powder, oils that are not damaged by excessive temperatures.

Moist heat sterilization

In this method microbes are killed by denature and coagulation of proteins.

Moist heat at a temperature below 100 degrees Celsius 

Pasteurization is the process of pasteurization is used for sterilization of perishable foods like fruits and vegetable juices and dairy products specially milk. Water bath is used for sterilization of vaccines, body fluids and serum samples at 60 degrees Celsius for 60 minutes. In Fractional distillation, samples are heated for 3 days for about 80 to 85 degree Celsius for 30 minutes. This method is used for sterilization of egg based and serum based media.

Moist heat at temperature 100 degree Celsius

Boiling of samples in water for 15 minutes kills the microorganisms except some fungal spores. In the steaming procedure the samples or culture media is exposed to a temperature of 100 degrees Celsius for 90 minutes at this time and temperature all the microorganisms except some fungal spores get killed. This method is used for sterilization of some culture media which are sensitive for high temperature. This type of media gets spoiled at high temperature of autoclaving. In this process of sterilization, the media or samples such as eggs and gelatin are steamed at 100 degrees Celsius for 20 minutes and this procedure is carried out for 3 days in order to kill all microorganisms and resistant fungal spores.  

Moist heat at temperature above 100 degrees Celsius

In this method of sterilization an autoclave is used which is exposed to a temperature of 121 degrees Celsius for 15 minutes at 15lbs pressure.it is used to sterilize surgical instruments, glassware’s as well as all the water containing samples which cannot be sterilized by dry heat. Filtration is a method of sterilization in which microbial population is removed and is passed through a filter of appropriate pore size. The microbial load remains on the filter and get a sterile sample.

Pasteurization

Louis Pasteur was the first scientist who develop a process of pasteurization in 1860s. Pasteurization is a method in which many substances such as milk and other food products are treated with controlled heating at temperature well below boiling point without changing the taste of product. It also helps to prevent food from spoilage of beer and wine. Pasteurization does not sterilize a beverage, but it does kill many pathogens present and drastically slows spoilage by reducing the level of nonpathogenic microorganisms. There are two types of pasteurization 1) Flash pasteurization which is also known as high-temperature short-term (HTST) pasteurization. 2) Hold pasteurization which is also known as Vat or batch pasteurization.

 Milk pasteurization was introduced into the United States in 1889.Milk can be pasteurized in two ways. In the older times Batch pasteurization method is used in which the milk is held at 63 degrees Celsius for 30 minutes in order to kill pathogenic microbes. In this method, product is held in a specific temperature range for long time. It also helps to balance temperature and length of time of treatment. In broad perspective, milk pasteurization effectively reduces total bacteria count by 97% to 99%, due to the fact that the most common milk borne pathogens such as Tubercle bacillus, Salmonella, streptococcus and Brucella organisms do not form spores and are quite sensitive to heat. Large quantities of milk are now usually subjected to flash pasteurization or high-temperature short-term (HTST) pasteurization, which consist of quick heating to about 72 degrees Celsius for 15 seconds using a plate heater exchange, then rapid cooling. HTST pasteurization not only kills pathogens but also lowers the number of total bacteria counts so that quality of milk keeps well under refrigeration. Many heat resistant (thermoduric) bacteria survive through pasteurization process but they are unable to cause spoilage in refrigerated milk.

Ultra-high temperature (UHT) pasteurization is an alternative of high temperature short-term (HTST) pasteurization in which milk is heated to a temperature of 138 degrees Celsius for 2 or more than 2 seconds. It is a method in which in which pasteurized milk is stored without refrigeration in big sealed containers for a long period of time.  At very high temperature the proteins in the milk changes and cause slight change in the smell and taste of milk. This method of pasteurization is useful in those areas where there is no or limited facility of refrigeration. Products other than milk such as yogurt, cheese, ice cream have different pasteurization time and temperature. In order to test that whether a product is pasteurized or not temperature test is taken. For example, in milk an enzyme is present called as phosphatase. Phosphatase test is taken in order to determine that milk is pasteurized or not because during pasteurization phosphatase will have been in activated. 

There are two types of pasteurization

Heat

We have seen that in different laboratories heat is used to sterile different laboratories equipment’s like media, glass-wares etc. also heat preserved food canes are used for preservation of food. So how heat provide us sterility? As we all know that every living organism possess different enzymes which are biological catalyst, speed up the biochemical reaction and convert only the specific substrate into the product. These enzymes work best at its optimum conditions like opt. temp and opt. pH etc. so, by heating the instrument which we want to sterile (free from microbial life) causes the temperature of instrument to increase with which the enzymes present in microbes starts to denature as heat causes the temperature to increase with which the destruction of enzymatic and proteins 3-dimenstional structure is lost, resulting the death of microbes.

Though there are several heat resistant microbes as well, and this resistant varies differently among different microbes. We explain them through the concept of:

Thermal Death Point (TDP): (the lowest temp. at which all the microbes present in a liquid suspension are killed within 10 minutes)

Thermal Death Time (TDT): (the min. time required to kill all the bacteria present in a particular liquid at a given temperature.)

Decimal Reduction Time (DRT) / D value: (specifically related to bacterial heat resistance. It is basically the time, in minutes, which causes the death of about 90% of the bacterial population at a given temperature)

Dry Heat

Many times we want to sterilize the object but without burning it to ashes. So in this case what we basically do is to sterile the object or instrument using dry heat. The benefit of applying the dry heat is that it causes the oxidation of molecules present inside of the microorganisms, which ultimately causes the proteins and other biological molecules to lose electron thus, resulting denaturation. As their structure is lost, they are unable to perform their function correctly thus, stops working and result into the death.  You can see the importance of this method, as it not only causes the bacterial destruction but also used when dealing with viruses and vegetative spores, who have the ability of resisting themselves when other methods are applied.

The benefit of applying the dry heat is that it causes the oxidation of molecules present inside of the microorganisms, which ultimately causes the proteins and other biological

Though it is quite useful method but it has different disadvantages as well; like conduction of heat through dry air requires much more time then moist air, it requires extremely high temperature of about 338°F for at least 120 minutes, thus very high temperature for the longer period of time is required for this process.

Moist Heat Sterilization

Moist heat; as name suggest include water vapors as moisture. It causes the killing of microbes primarily through coagulation of proteins (denaturation) via breaking the hydrogen bonds which gathers the 3-D structure of proteins.

One type of moist heat sterilization is “boiling”, which causes the death of bacterial pathogens, viruses, fungi and some spores as well within a time of about just ten minutes. However, some endospores require more time to be destroyed with this method, e.g. some endospores of bacteria have the ability to resist boiling for more than twenty hours. Thus, boiling is not always used as the most reliable process for sterilization, though boiling at high altitudes can cause the death of most pathogens. Thus, to make the sterilization reliable, we require to increase the temperature above that of boiling temperature, which will be achieved through autoclave which work with the principle of having steam under high pressure in a closed container.

Autoclaving is considered as; one of the preferred method of sterilization, unless it causes any kind of damage to the material which requires to be sterilized. The increase in pressure in autoclave causes the temperature to increase. For instance; at sea level when the pressure is about 15 psi, the temperature will increase up to 121°C. but when we increase the pressure to about 20 psi, the temperature will rise to almost 126°C.

Though it is quite useful method but it has different disadvantages as well; like conduction of heat through dry air requires much more time then moist air, it requires extremely high temperature of about 338°F for at least 120 minutes, thus very high temperature for the longer period of time is required for this process.
Moist Heat Sterilization
Moist heat; as name suggest include water vapors as moisture. It causes the killing of microbes primarily through coagulation of proteins (denaturation) via breaking the hydrogen bonds which gathers the 3-D structure of proteins. 
One type of moist heat sterilization is “boiling”, which causes the death of bacterial pathogens, viruses, fungi and some spores as well within a time of about just ten minutes. However, some endospores require more time to be destroyed with this method, e.g. some endospores of bacteria have the ability to resist boiling for more than twenty hours. Thus, boiling is not always used as the most reliable process for sterilization, though boiling at high altitudes can cause the death of most pathogens. Thus, to make the sterilization reliable, we require to increase the temperature above that of boiling temperature, which will be achieved through autoclave which work with the principle of having steam under high pressure in a closed container.
Autoclaving is considered as; one of the preferred method of sterilization, unless it causes any kind of damage to the material which requires to be sterilized. The increase in pressure in autoclave causes the temperature to increase. For instance; at sea level when the pressure is about 15 psi, the temperature will increase up to 121°C. but when we increase the pressure to about 20 psi, the temperature will rise to almost 126°C.

Sterilization will be more effective when the microorganisms are either in contact with steam directly or are contained in small liquid volume. At this time, the moisture present in the form of steam is at a pressure of about 15 psi with temperature of about 121°C, under these conditions, almost all type of life present on the material will be killed and also even the endospore within just 15 minutes of autoclaving.

Sterilizing of the solid surface requires the steam to be in contact with it. Thus, special care must be under taken to sterilize bandages, glass-wares etc. for instance; aluminum foil is resistant to moist heat, thus paper should be used to wrap the material.

Such products, like petroleum jelly or mineral oil, which don’t allow penetration by moisture are not stabilized by the methods used for sterilizing the aqueous solutions. Retorts are the large industrial autoclaves, but in household pressure cooker used commonly, the same principle is applied.

Autoclaving procedure is used for sterilizing culture media, solutions, intravenous equipment, dressings, applicators, instruments, and various other which are able to withstand high pressures and temperatures. Extra time is required by heat in reaching the center of solids such as canned meat, this is because such solid materials do not develop the efficient heat-distributing convection currents which occur in liquids. Several commercially available methods can indicate whether heat treatment has achieved sterilization. Some of these are chemical reactions in which an indicator changes color when the proper times and temperatures have been reached

Sterilizing of the solid surface requires the steam to be in contact with it

Tyndalization: It is also a type of moist heating, which involves the boiling of material for about one and a half hour but heat the material on three consecutive days. In first day, boiling causes the death of vegetative cells, but not the spores. On second day, those spores which are not killed on the first day of boiling will germinate and result into vegetative spores, the second day boiling causes the death of these spores. On the third day, any spore present on the material which survives the previous 2 days of boiling will be killed on third day. Now the material is completely free of microbes, thus expected to be completely sterilized.

Tyndalization: It is also a type of moist heating, which involves the boiling of material for about one and a half hour but heat the material on three consecutive days. In first day, boiling causes the death of vegetative cells, but not the spores. On second day, those spores which are not killed on the first day of boiling will germinate and result into vegetative spores, the second day boiling causes the death of these spores. On the third day, any spore present on the material which survives the previous 2 days of boiling will be killed on third day. Now the material is completely free of microbes, thus expected to be completely sterilized.

Filtration

Filtration is the physical process. Filtration is the passage of liquid or gas through a screen like material with pores small enough to retain microorganism. A vacuum is created in receiving in flask air pressure then forces the liquid through the filter. The filters divided in two types: Depth filters and Membrane filters.

Filtration is the physical process. Filtration is the passage of liquid or gas through a screen like material with pores small enough to retain microorganism

Depth filters:

Consist of fibrous and granular material that have been bonded thick layer filled with twisting channels of small diameter. The solution containing microorganism is sucked in through this layer under vacuum and microbial cells are removed by physical screening and also by absorption of surface material. Depth filters are Candle filter, Asbestos filter, Sintered glass filters.

Membrane filters:

Microbiological membrane filters provide a useful way of sterilizing materials such as: Vaccines, Antibiotic solutions, Animal sera, Enzyme solutions, Vitamin solutions.  Solutions that are may be damaged or denatured by high temperatures or chemical agents. The filters contain pores small enough to prevent the passage of microbes but large enough to allow the organism free fluid to pass through. The liquid is then collected in a sterile flask.

Depth filters:
Consist of fibrous and granular material that have been bonded thick layer filled with twisting channels of small diameter. The solution containing microorganism is sucked in through this layer under vacuum and microbial cells are removed by physical screening and also by absorption of surface material. Depth filters are Candle filter, Asbestos filter, Sintered glass filters.
Membrane filters
Microbiological membrane filters provide a useful way of sterilizing materials such as: Vaccines, Antibiotic solutions, Animal sera, Enzyme solutions, Vitamin solutions.  Solutions that are may be damaged or denatured by high temperatures or chemical agents. The filters contain pores small enough to prevent the passage of microbes but large enough to allow the organism free fluid to pass through. The liquid is then collected in a sterile flask.

Membrane filtration use for many purposes

These circular filters are porous membrane. These filters are only 0.1 mm of viruses but filtration is very slow. In different varieties of pore sizes are available, the membranes with pores about 0.2 m in diameter are used to remove most vegetative cell. These filters are used to sterilize some solutions such as Ophthalmic solutions, Culture media, Oils, Antibiotics, Pharmaceuticals and Heat sensitive solutions. Membrane filters remove microorganism. Filtration also sterilize the air. High efficiency particulate air HEPA filters are one of the most important air filtration systems which remove 99.97% of 0.3m particles.

Laminar flow biological safety cabinet force air through HEPA filters, then project a vertical curtain of sterile air cross the cabinet opening, this protect a worker from microorganism being handled within cabinet prevent contamination in room. The person uses this cabinet while working with dangerous agents such as: Mycobacterium tuberculosis, Tumor viruses, Recombinant DNA.

HEPA Filters

HEPA filters composed of randomly arranged fibers composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting its functions are Fiber diameter, Filter thickness, and Face velocity. The air space between HEPA filter fibers is typically much greater than 0.3 μm. Unlike sieves or membrane filters, where particles smaller than openings or pores can pass through, HEPA filters are designed to target a range of particle sizes. These particles are trapped through a combination of the following three mechanisms:

Diffusion    An enhancing mechanism that is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 μm in diameter, which are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either interception or impaction; this mechanism becomes dominant at lower air flow.

Interception   Particles following a line of flow in the air stream come within one radius of a fiber and adhere to it.

Impaction   Larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fiber separation and higher air flow velocity.

HEPA filters composed of randomly arranged fibers composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers

Gas filtration

HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, cigarette, pet or flatulence odors call for the use of an activated carbon (charcoal) or other type of filter instead of or in addition to a HEPA filter.

Low Temperature

Low temperatures affect the number of the microorganism either directly or indirectly. Direct effects include decreased growth rate, enzymatic activity, change of cell composition and differential nutrient requirements whereas indirect effect include solubility of solute molecules, nutrient diffusion, osmotic effects on cell membrane and density. Due to low temperature, growth is prolonged in lag phase which results in less growth rate and final cell number.

Refrigeration

It is a method to preserve food or sometimes laboratory cultures at temperature 0-7℃ that reduces the metabolic rate of microbes so that they cannot reproduce or release toxins. Psychrotrophs grow slowly at refrigeration temperature causing the spoilage of food forming slimy layer on surface of food, changing the taste or color. Example of food spoilage bacteria are: Lactobacillus, Leuconostoc, Enterobacter and Streptococcus. Pathogenic bacteria do not thrive at refrigeration temperature except Listeria and Yersina enterocolitica.

Psychrotrophs grow slowly at refrigeration temperature causing the spoilage of food forming slimy layer on surface of food, changing the taste or color

Freezing

Temperature below 0℃, first used in 1842 freezing food in ice and salt brine. Freezing inactivates the microbial growth but not completely destroy them. It also slows the chemical changes and is a process in which temperature is reduced to which water changes into ice crystal lowering the water activity in food. Many types of food can be freeze without change in texture, color and taste of food which can be preserved up to months. Bacterial spores and vegetative cells of micrococci, staphylococci and streptococci are resistant to freezing. Bacteria or medical cultures for long term use or transport are frozen at -70℃ or lower.

Types of freezing:

Air freezing

Food frozen by still or blow forced air and is cheapest method

Indirect contact

Food placed in metal surfaces but faster than air freezing and expensive

Direct contact

Products placed in refrigerant having ultra-low temperature or in nitrogen tanks -196℃. It is also fast and expensive.

Products placed in refrigerant having ultra-low temperature or in nitrogen tanks -196℃. It is also fast and expensive.

High Pressure

It is a non-thermal process for preservation of food which inactivates vegetative microorganism cause food borne disease. High pressure ranges from 100-1000MPa with less effect on food quality. The main target of high pressure is to inactivate microorganism due to which food is safer and used for long time.

High Pressure
It is a non-thermal process for preservation of food which inactivates vegetative microorganism cause food borne disease. High pressure ranges from 100-1000MPa with less effect on food quality. The main target of high pressure is to inactivate microorganism due to which food is safer and used for long time.

Effect on bacteria

Gram-positive bacteria are more susceptible to pressure than gram-negative bacteria, often bacterial spores. High pressure treatment is efficient in removing Listeria monocytogenes from processed meat and cheese and also harmful microorganism causing food spoilage like E. coli, Salmonella, Vibrio, yeast and molds. High pressure with temperature is effective to reduce bacteria spores e.g. spore count of Clostridium sporogenes is reduced when treated with combined pressure-temperature at 690 MPa and 80℃ for 20 minutes and Bacillus stearothermophilus treated with combined pressure moderate temperature 70℃.

Effect on yeast and molds

Yeast and molds are less resistant to pressure than bacteria. They are inactivated by pressure ranges from 200-400 MPa but often by pressure at 400 MPa. Saccharomyces cerevisiae more resistant than gram negative bacteria. It has been proved that S. cerevisiae is not to be inactivated by pressure equal to 400MPa in orange juice. The resistant of this microorganism increased when the concentration of sugar in the environment also increased. This can cause a problem during the treatment of fruit based preparation containing a strong concentration in sugar content. These fruit juice which are preserved are sold in Japan and United States.

Desiccation

Basically, desiccation is the state of extreme dryness, or also it is the process of extreme drying that can be used to control the growth of microorganisms. So without the water microorganisms cannot replicate or develop, however some might be suitable for a long time. But when the water becomes available they start growing again. So the process of desiccation does not provide the complete sterilization.

Drying up has the static impact on the microorganisms and the absence of the water will repress the activity of the microbial enzymes. So the freeze dried foods and dehydrated foods i.e. these do not require the refrigeration because the absence of water will inhibit the growth of microorganisms. The pharmaceutical companies as a desiccation tool often use the freeze drying in order to increase the shelf life of the products. Like for example the vaccines and some other injectable. They done this by removing the water from the material and by sealing the material in a container so the material can be easily stored shipped and also reconstituted to in to its original form. The preservation will only be possible when the decreased water substance will stop the activity of the microorganisms and also stop the activity of the enzymes that cause spoilage and sometimes cause degradation of the substance. In order to produce the tablets and wafers, this will be the best example from the pharmaceutical industry.

So, drying is also called as the method which may be used in order to preserve the food and it will work by the removal of the water from food so that will stop the growth of the microorganisms. In the ancient time, for the preservation of food, open air drying by using the sunlight and also wind has been practiced. The drying process will be greatly speed up with the electric food dehydrator or solar and these will give more accurate results. The water will be easily removed by the evaporation like sun-drying, air-drying, wind-drying or smoking. But in the process of freeze-drying the first step is the frozen of food, then after this step the water will be removed by the process known as sublimation. Yeast, molds and bacteria needs water in the food in order to grow. But the drying will effectively stop their growth in the food, means it will prevent them so they cannot survive in the food. And the process of freeze drying is performing by using the unique equipment’s. There are two components that are very common in all types of the freeze dryer’s i.e. Vacuum pump and the condenser. The vacuum pump is used in order to reduce the ambient gas pressure in a vessel that is containing the substance to be dried, while condenser is used to remove the moisture by the condensation on the surface that is cooled to -40°C to -80°C.

The desiccation is basically the state of drying while freeze drying is the dehydration process that is used to preserve the perishable material, and then make this material more convenient for the transport purposes. It is also called as lyophilization or cryodesiccation.

So the susceptibility to the desiccation varies widely i.e. Neisseria gonorrhea can only survive one hour. Mycobacterium tuberculosis may survive for many months. But some of the viruses like Clostridium spp. and Bacillus spp. may survive for the longer time. And these viruses are impartially show resistant towards desiccation.

Basically, desiccation is the state of extreme dryness, or also it is the process of extreme drying that can be used to control the growth of microorganisms

Osmotic Pressure

The osmotic pressure is basically the pressure that must be applied to a solution in order to prevent the inward flow of the water, across the semipermeable membrane. In other words, it is also defined as: the minimum pressure which is needed to nullify the process of osmosis. The osmotic pressure phenomena arise from the tendency of the pure solvent in order to move over a semipermeable membrane and also in to the solution that has the solute to which the membrane is impermeable. 

The process of osmosis causes the water to flow from the area of low solute concentration to the area of the high solute concentration. Until these two areas have the same or equal ratio of the solute to water. Normally the solute will diffuse towards the equilibrium also, so however all the cells are surrounded by the lipid bilayer cell membrane that will permit the flow of water inside and outside of the cell. But sometimes will restrict the flow of solute in different circumstances.

In the natural environment microorganisms are constantly facing changes in the osmotic pressure. The water may tend to flow through the semi permeable membranes, i.e. cytoplasmic membrane of the microorganisms is towards the side where high concentration of the dissolved material i.e. solute is present. In other words, water moves from lower solute concentration to the higher solute concentration.

Hypotonic:  So when the concentration of the dissolved solute is higher inside the cell than the outside, the cell is said to be in the hypotonic environment and the water will flow in to the cell. So the rigid cell wall of the fungi and bacteria will prevent the busting or the plasmolysis. (When the water leaves the cell, the plasma membrane will shrink away from the cell, cells may not die but they usually stop growing).

Isotonic: So when the inside and outside concentration of the solute is same, the cell is said to be in the isotonic environment. The water will flow equally in both outside and inside. These both environments hypotonic and isotonic are not harmful to the microorganisms.

Hypertonic:  when the concentration of the dissolved solute is higher in the outside environment than the inside, the cell is said to be in the hypertonic environment. So in this condition water will flows out of the cell and may resulting in the shrinkage of the cytoplasmic membrane, and the cells becomes dehydrated and the growth of the cells become inhibited. Means it causes the cell to shrink and lose its turgidity.

 

When the concentration of the dissolved solute is higher in the outside environment than the inside

In order to create the hypertonic environment for the microorganisms the very most common substances that are used are salt and sugar that will also prevent the microorganism from growing. These substances are widely applied in the food preservation processes.

Table salt i.e. sodium chloride is the primary ingredient that is used in the meat curing process. Addition of salt and the removal of water from meat create the solute-rich environment where the osmotic pressure pulls water out of the microorganisms by retarding their growth. So the 20% salt concentration requires for this purpose.

Sugar it is also used to preserve the foods either I the syrup with the fruits like pears, peaches, apple, apricots etc. in the crystalline form where the preserve material is cooked in the sugar in to the point of crystallization and resultant product will be stored dry. So the basic purpose of the sugaring is in order to create the environment that must be hostile to the microbial life and prevent the food spoilage. It can also be used for the purpose of non- food preservation.

Canning of jams is preserves with the high concentration that will stop the growth of the bacteria through the hyper tonicity. So the same effect may be obtained by placing the food in the salt brine or also by the salt curing meats. Molds basically are more tolerant to the hyper tonicity.

The yeasts and molds are more resistant to the high osmotic pressure, while Staphylococci spp. is fairly resistant to the high osmotic pressure.

The yeasts and molds are more resistant to the high osmotic pressure

Radiation

Radiation has varying effects on the cell growth, depending on the intensity, wavelength and duration. Radiation that has ability to kill microorganisms are of two types: ionizing and non-ionizing.

Ionizing radiation:

Radiation that has a shorter wavelength than that of nonionizing radiation usually less than about 1nm, gamma rays, X-rays and high energy electron beams are the example of ionizing radiation. That why they carry much more energy. Gamma rays are emanated by certain radioactive elements such as cobalt. By accelerating electrons to high energies in accelerator electron beams produced and X rays, which are produced by machines in a manner similar to the production of electron beams. Gamma rays have good ability to penetrate deeply but it requires hours to decontaminate large masses. High energy electron beam has low penetration power as compared to gamma rays, it requires only few seconds to sterilize. Ionizing radiation are cause of water ionization, which creates highly reactive hydroxyl radical. These radicals are capable to microorganism by reacting with cellular components particularly DNA and damaging the cell.

Target theory of damage by radiation is that it uses packet of energy, ionizing particles, pass through or close to cell; these constitutes “hits’ one or few hits may only cause no harmful mutations, these are used for sterilization purposes. But higher exposure rate cause serious mutations.

In food industry radiation are largely use for the food preservation. Low level (ionizing) radiation use in many countries for decades, also, approved by United State for the preservation of meat, vegetables and spices. High energy electron beams used especially for the sterilization of pharmaceutical and surgical devices such as, syringes, studs, surgical gloves and catheter. Ionizing radiation also use against bioterrorism in postal services, it uses electron beams to sterilize mail.

Nonionizing radiation:                      

These radiations have higher wavelength as compare to ionizing radiation, Wavelength normally more than 1nm. Best example of nonionizing radiation Ultraviolet (UV) light. UV light damage the DNA of exposed cell by the formation of thiamine dimer (bond between adjacent thiamine). Thymine dimer cause mutation and halt the replication of DNA during replication. Most microorganisms killed by UV light at a wavelength of 260 nm, this particular wavelength absorbed by cellular DNA. The use of UV light for the control of microorganisms in air is also common. UV lamp also called germicidal lamp commonly used at different places such as, hospitals rooms, nurseries, operating rooms, cafeterias and hostel rooms. It is also used to disinfect vaccine and other pharmaceutical and medical products.

One major problem related to use of UV light against microbes is that, as the UV light has low penetrating power so the microbe to be killed must be fully exposed to UV light. Organisms which are protected by some solid covering remain unaffected.

Another disadvantage of UV light is that it can be harmful for human eyes, and longer exposure to UV light can cause skin cancer.

Another disadvantage of UV light is that it can be harmful for human eyes, and longer exposure to UV light can cause skin cancer

Sunlight also contains some fraction of UV light, but the wavelength of these light is shorter. Sunlight UV radiation are most effective against bacteria, which are screened out from the Ozone layer. Antimicrobial effect of sunlight is due to the formation of singlet oxygen in the cytoplasm. Pigment produced by bacteria protect them against radiation.

Based on different experiments, it has been reported that visible blue light (470 nm) kill methicillin-resistant Staphylococcus aureus (MRSA) in laboratory cultures and mice. It can be used for the treatment of skin infections.

Microwave:

Microwave do not have serious effect on microorganisms, bacteria can be easily isolated from the interior of recently operated microwave. Microwave heat the moisture that is present in the food, heat will kill most of the vegetative pathogenic. As, the moisture content is unevenly distributed in the solid food so, solid food will heat unevenly. Due to this reason, pork cooked in microwave oven has been responsible for the outbreaks of Trichinellosis. 

As, the moisture content is unevenly distributed in the solid food so, solid food will heat unevenly
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