Factors Affecting The Growth Of Microorganisms In Foods

Interactions between microorganisms, animals, and plants are constant and natural. The ecological function of microorganisms and their role in the various biochemical cycles of nature are clearly documented. The human food supply is primarily composed of animals and plants or products that are derived from them, it’s reasonable to assume that the food we consume may contain microorganisms that interact with food.

Most microorganisms utilize our food to provide nutrients to support their growth. Naturally, this can lead to deterioration of food. In addition, by increasing their numbers using nutrients, producing enzyme changes and contributing off-flavors as a result of the breaking down a food item or the synthesis of new compounds , they could “spoil” the food. This is the normal result of the microorganisms’ actions as their primary function in nature is the conversion of reduced forms of nitrogen, carbon and sulfur found in dead animals and plants into the oxidized form needed by plants. These can then be used by mammals. Thus, by “doing their job” in nature, they cause food to become inedible to consumption. To stop this from happening, we reduce the interaction between microorganisms and our food items (prevent the spread of contamination) and eliminate the microorganisms that are present in our food or, at a minimum, alter the the conditions for storage in order to avoid their expansion (preservation).

If the microorganisms in question can be considered to be pathogenic in nature, their interaction with the food supply is crucial from a public health perspective from a health perspective. A lot of our food items encourage the development of pathogenic microorganisms, or serve as a conduit for these microorganisms. This is why we try to stop their entry and growth in our food or remove them through processing. Interactions between microorganisms with our foods can be beneficial as evidenced by the numerous foods that are cultured and consumed.

What are the main elements in these interactions? What is the reason this interaction is beneficial at certain times but not at other times? What are the reasons why certain foods encourage development of microorganisms much more quickly than others? What makes certain foods robust to microbial degradation? Food is the food source which is why the properties of food items are important to take into consideration. The kind of microorganisms that are present and the conditions in which they are found are crucial. However, the food source or substrate determines what is able or can’t grow. In analyzing the properties of the substrate or food one can draw predictions on the microbial fauna that might be created.

Understanding the elements that can influence or inhibit the growth of microorganisms is crucial to a better understanding of the fundamentals of the preservation and spoilage of food. The most important compositional elements of food items that affect the activity of microorganisms are the concentration of hydrogen-ions and moisture, oxidation reduction (O-R) potential nutrients, and the presence of inhibiting compounds or other barriers.

Thus, the ability of microorganisms in the food chain to grow or multiply in food is dependent on the environment in which they live. The extrinsic, intrinsic factors that are implicit in food and the various processes for food processing, all are all involved in the growth of microorganisms. These factors influence the microbial population in food as well as the specific metabolic pathways they employ to create the energy needed and other metabolic products.

A. Intrinsic factors Affecting The Growth Of Microorganisms In Foods

The parameters of animal and plant tissues that are intrinsic components of the tissues are known as intrinsic parameters. The parameters include:

• pH
• Moisture content
• Oxidation–reduction potential (Eh)
• Nutrient content
• Antimicrobial constituents
• Biological structures

Each of these substrate-limiting factors is discussed below, with emphasis placed on their effects on microorganisms in foods.

1. Effect of pH

• Each microorganism has a minimum or maximum pH, as well as an ideal pH for growth.
• Microbial cells are greatly dependent on the acidity of foods as they appear to have no way to regulate their pH internally.
• In general, molds and yeasts are more acid-tolerant than bacteria.
• The pH of food products is different, but most food items are acidic or neutral.
• Foods with pH values that are low typically aren’t easily spoilt by bacteria and are less vulnerable to spoilage by molds and yeasts.
• Foods with an intrinsically low pH will therefore tend to remain more stable in microbiological terms than neutral food.
• The high quality of the keeping properties of the following food items is due to their limiting pH levels: soft drinks, fruits sauerkraut, fermented milks and pickles.
• Certain foods are low in pH because of their inherent acidity. Other foods, e.g., the fermented foods have a low pH due to acidity that has developed due to the build-up of lactic acid in the process of fermentation.
• The growth of molds is possible in a wide spectrum of pH values than will most yeasts and bacteria. Additionally, the majority of molds are found in pH ranges which are too high for yeasts or bacteria.
• The majority of fermentative yeasts are supported by a pH of 4.0 to 4.5 such as in the juices of fruits and film yeasts. They also thrive on acidic food items like sauerkraut or pickles.
• However, the majority of yeasts fail to grow well in alkaline substrates , and have to be modified to alkaline mediums.
• The majority of bacteria are attracted by a pH close to neutral however, certain, like those known as acid-forming bacteria, are attracted by moderate acidity. others, e.g., the active proteolytic bacteria, are able to thrive in environments that have an elevated (alkaline) pH, such as that can be seen in the white of an egg that has been stored.
• Bacteria are generally more meticulous in their relationship to pH than yeasts and molds The pathogenic bacteria being most careful.

Mechanism

Numerous vital cell functions, including ATP synthesizing in bacteria, active transportation of nutrients, and cytoplasmic control are performed at the cell’s membrane. They are dependent on the potential energy stored within the membrane, in the form of the proton motive force. It is an electrochemical potential generated through the translocation of protons from the cell’s inside to the outside. Contrary to protons and other charge-producing molecules, lipophilic acid that is undissociated molecules are able to pass through the membrane and this, they are able to pass through an external environment with a low pH, where equilibrium favors the molecule that is not dissociated up to the pH that is high in the cell’s cytoplasm (around 7.5 in neutrophils). When the pH is higher the equilibrium shifts favor of the dissociated molecule as the acid ionizes creating protons that will eventually to in turn acidify the cytoplasm and reduce the proton’s pH motivation force. Cells attempt to keep the internal pH of its cells by neutralizing protons that are leaking in, however this can slow growth because it takes energy away from the growth-related processes. If the pH of the outside is sufficient low and the concentration of extracellular acid is high, the strain on the cell is too much, the cytoplasmic pH decreases to a point where it is no longer feasible to grow and the cell ends up dying.

2. Water activity (aw)

• Every organism has its own unique optimal aw, as well as its own specific range of development under a set of conditions.
• Most bacteria thrive in mediums with a water activity close to 1.00 (at 0.995 up to 0.998 for instance); i.e., they thrive in lower levels of sugar or salt.
• The lower limits that have been reported of aw that allow for the growth of certain food bacteria are 0.97 to Pseudomonas, 0.96 for Escherichia coli, 0.95 for Bacillus subtilis, 0.945 for Enterobacter aerogenes, 0.86 for Staphylococcus aureus as well as 0.93 in the case of Clostridium botulinum. Other bacteria can develop if the aw is less than 0.90.
• Molds differ greatly in the ideal Aw and the variety of aw needed for the development of Asexual spores.
• The minimum aw required for spores to germinate has been discovered at a value of 0.62 for certain molds, and up to 0.93 for other species (e.g., Mucor, Rhizopus along with Botrytis).
• Each mold has an optimal aw as well as a size of the aw range to grow. The most optimal aw is 0.98 to 0.98 for Aspergillus species., 0.99 to 0.98 for an Rhizopus sp. as well as o.99 for the Penicillium species.
• The aw should be lower than 0.62 to prevent all opportunities for mold growth. However, an aw that is below 0.70 prevents the growth of most molds responsible for food spoilage, and an Aw lower than 0.94 blocks molds like Rhizopus and lower than 0.85 hinders Aspergillus spp.
• Factors that influence the requirement for water of living things are (a) the nutritional characteristics of the substrate, (b) the level of pH in it (c) the amount of inhibiting chemicals, (d) accessibility of oxygen free as well as (e) the temperature. The range of aw that allows growth is limited when any of these environmental conditions are not in a good state and narrows even more in the event that two or more of these conditions are not in favor.
• A negative aw results not just in a decrease in growth rate but also in a lower maximum yield of cells.
• The more detrimental the substrate’s aw the more time (lag) in the initiation of growth or germination the spores. This is often as crucial in the preservation of food as decrease in the growth rate that the organism.
• It is generally accepted that bacteria need higher levels of moisture that yeasts and yeasts need more than molds. This demonstrates lower limits of aw in molds and bacteria. There are some notable variations to this generalization however, since certain molds have greater minimum aw requirement.
• Microorganisms that thrive in large amounts of solutes e.g. sugar or salt clearly have a minimal Aw. Halophilic bacteria require a small amount of sodium chloride dissolving for their growth. Osmophilic yeasts thrive when sugar is in high levels.

Water is unavailable in different ways:

• Ions and solutes bind the water in solution. This means that any increase in amount of dissolved substances like salts and sugars can be seen as actually drying the substance. In addition, water is held together by solutes however, it is also a fact that water can leave the microbial cells via Osmosis when there is a higher amount of the solute outside than inside the cells.
• Colloids that are hydrophilic (gels) create water inaccessible. As little as 3 or 4 percent of agar in a solution could stop the growth of bacterial due to the fact that there is not enough water.
• The water that crystallizes or is hydrated is generally inaccessible to microorganisms. When water crystallizes in the form of ice, cannot is able to be utilized by microorganisms. The aw value of mixtures of water and ice (vapor pressure from ice multiplied by the vapor tension of water) diminishes as temperatures drop at temperatures below zero C. The Aw values for pure water are 1.00 at 1 C, 0.953 at -5 C, 0.907 at -10 C, 0.846 at -15 C, 0.823 at -20 C and further. In food the more ice gets created, the quantity of solutes present in the frozen water is increased, which lowers the Aw.

Factors that could affect the requirements of microorganisms comprise the following.

• The kind of substance used to lower the Aw. For many organisms, particularly molds, the most effective aw for growth is virtually independent of the type of substance that is used. Others are, however, more susceptible to less limiting aws with certain solutes than others. Potassium chloride for instance is usually less toxic than sodium chloride and, in turn, is less inhibitive than sodium Sulfate.
• The nutritional value of the cultivation medium. It is generally the more nutritious the environment for growth and development, the less the limitation Aw
• Temperature. The majority of organisms have the highest tolerance to temperatures that are low in temperatures that are near optimal.
• Oxygen supply. Aerobes grow at a lower rate when air is present than without air and the reverse is the case for anaerobes.
• pH. The majority of organisms are more intolerant of low pH levels near neutral than alkaline or acidic media.
• Inhibitors. Inhibitors narrow the options for development of microorganisms.

Methods to regulate the aw include (1) equilibration with controlling solutions, (2) determination of the water-sorption isotherm for the food (Iglesias and Chirife, 1976), and (3) addition of solutes.

Methods to measure or establish the aw value of food items include manometric techniques, and electronic devices. The freezing-point method is only feasible on liquid foods that have large aw values.

Mechanism

In the end, the impact of a decreased Aw on the nutrition of microorganisms is generally based on the sense that the cell needs that have to be handled by an aqueous environment are gradually closed. Alongside the impact on nutrients, a decreased Aw certainly has a negative impact on the function that the membrane of cells has to be maintained in a state of fluidity. The drying of the internal components of cells is likely to happen when cells are placed in an environment with a lower Aw to the point that the equilibrium of the water between the cells and the substrate takes place. While the mechanism isn’t fully understood, however, all microbial cells require the same powerful internal aw. The ones that are able to thrive in the extremes with a low aw have this ability due to their capacity to bring polyols, salts, and amino acids (and perhaps other compounds) to levels inside that are not just to keep cells from losing water, but also could allow the cells to remove water from its external water-stressed environment.

3. Oxidation–Reduction Potential

• It is known for a long time that microorganisms exhibit various degrees of sensitivity to the potential of oxidation and reduction (O/R Eh) of their medium for growth.
• The potential for O/R of a substrate can be described in general terms by the degree to which the substrate loses is able to gain electrons.
• If the element loses electrons the substrate gets oxidized the substrate that absorbs electrons is reduced.

• Oxidation can also be accomplished by using oxygen as demonstrated in the following example.  2Cu + O2 → 2CuO
• Thus, a substance that is able to release electrons is a great reducer, while one which is quick to absorb electrons is a great agent for oxidizing.
• If electrons transfer from one compound to the other the possibility of a differentiating effect is produced in the compounds.
• More highly oxygenated the substance is, the greater positive potential its electrical power and the more severely reduced a substance is, the less positive the electrical power potential. If the amount of both oxidant and reducer equal, a zero electrical potential is present.
• The O-R power potential of a system is typically written as Eh and is measured and expressed in millivolts (mV).
• It can be measured through (1) the specific O-R capacity of the original ingredient, (2) the poising capacity, i.e., the resistance to changes in the potential of the food, (3) the oxygen tension in the air around the food as well as (4) the access the environment is granted for the foods.
• Aerobic microorganisms must have positively Eh value (oxidized) to grow while anaerobes require the opposite of Eh levels (reduced).
• The main ingredients in food which help maintain reducing conditions include -SH groupings in meats, ascorbic acid and reducing sugars found in fruit and vegetables. Regarding the highest positive and negative mV levels and the fact that they are not essential for the development of anaerobes and aerobes however, these extreme values could be deadly for the particular category.
• Certain aerobic bacteria can actually thrive more efficiently under conditions that are slightly less favorable These organisms are called microaerophiles. Microaerophilic bacteria can be identified as campylobacters and lactobacilli.
• Certain bacteria are able to flourish under either anaerobic or aerobic conditions. They are also known as facultative anaerobes. The majority of yeasts and molds found in or on food items are aerobic, however certain types are facultative anaerobes.
• Concerning the Eh of food items Plant foods, and especially juices from plants, typically contain Eh values ranging 300-400 mV. It’s not a surprise to discover that aerobic molds and bacteria are the main cause to spoilage of foods similar to this.
• Solid meats contain Eh values around -200 mg; in minced meats minced meats, the Eh is usually about 200 millivolts. Different kinds of cheeses are reported to contain Eh values that are on their negative sides, ranging from 20 to around 200 mV.

Mechanism

Microorganisms influence their environment by affecting the Eh of their surroundings when they grow, just like they alter pH. This is especially true of aerobes that can reduce the pH of their surroundings however anaerobes can’t. When aerobes multiply, oxygen in the medium decreases and this causes a reduction of Eh. The growth rate isn’t slowed however, but not as much as one would expect because of the capability of cells to utilize hydrogen-accepting and O2-donating substances in the medium. As a result, the medium is less efficient at the process of oxidation and more abundant in reducing substances. The Eh of a medium may be reduced by microorganisms due to the production of certain metabolic byproducts, such as H2S that is able to reduce Eh down to 300 mgV. Since H2S is a strong redox agonist with O2 It will build up only in anaerobic conditions.

Eh is a function of the acidity of substrate and the relationship between these two variables is defined by following method:

where R = 8.315 joules, F = 96,500 coulombs, and T is the absolute temperature.

In relation to the impact of Eh on the production of lipids by Saccharomyces cerevisiae, it has been established that anaerobically cultivated cells produce lower level of total lipids of lipids, the glyceride percentage is highly variable and a lower proportion of phospholipids and sterol components in comparison to aerobically-grown cells. The lipids produced by cells that are anaerobically grown is characterized by a large amount (up to 50 percent of the total acid) of 8:0-14:0 acids, and a lower level of unsaturated fatty acids within the phospholipid component. In cells that were grown aerobically it was found that 80-90% of the fat acid component was linked with glyceride. The phospholipids were found to have an 18:1 or 16:1 acid. Contrary to the aerobically-grown cells, anaerobically developed S. cerevisiae cells were discovered to have a lipid requirement and sterol requirement.

4. Nutrient Content

In order to grow and function normally, the microorganisms of importance in foods require the

following:

• water
• source of energy
• source of nitrogen
• vitamins and related growth factors
• Minerals

(a) Water

• Water plays a crucial function in the development of foodborne microorganisms.
• In comparison to the four other kinds of substances, the molds are the least needed and are followed by Gram-negative bacteria yeasts, Gram-positive bacteria.

(b) Sources of energy

• In order to provide energy microorganisms that live in food can utilize alcohols, sugars, along with amino acids.
• Some microorganisms are able to make use of complex carbohydrates like starches and cellulose to provide sources of energy, by first converting these compounds into simple sugars.
• Fats are also utilized by microorganisms to provide energy, however, these substances are attacked by only a tiny amount of microbes found in food items.

(c) Nitrogen sources

• The most important nitrogen sources utilized by heterotrophic microorganisms are amino acid.
• Many other nitrogenous compounds can be used to serve this purpose for different kinds of organisms.
• Certain microbes, like can utilize nucleotides as well as free amino acids while others are able to make use of proteins and peptides.
• In general, simple compounds such as amino acids will be utilized by almost all organisms before any attack is made on the more complex compounds such as high-molecular-weight proteins. Similar is the case with polysaccharides and fats.

(d) Vitamins

• Microorganisms can need B vitamins in small quantities and nearly all natural foods contain a large amount for the organisms that cannot synthesize their vital requirements.
• In general Gram-positive bacteria are most synthetic, and therefore must be fed with at least one of these substances prior to them growing.
• The Gram-negative bacteria and the molds can synthesize the majority or all of their needs. This means that these two types of organisms can be seen on food items that are low in B vitamins.
• Fruits are generally lower in B vitamins than meats. This fact, coupled with the typical low pH and high Eh from fruits help to explain the typical degradation of these products caused by molds and not by bacteria.

5. Antimicrobial Constituents

• The sturdiness of certain foods against microorganisms’ attack result from the existence of specific naturally occurring compounds that exhibit antimicrobial properties.
• Certain plant species are thought to contain essential oils which contain antimicrobial properties.
• Some of them are eugenol found present in the cloves and allicin found in garlic, cinnamic aldehyde and eugenol found in cinnamon as well as allyl isothiocyanate found in mustard, eugenol as well as thymol in sage, as well as carvacrol (isothymol) and the chemical thymol found in oregano.
• The cow’s milk is a rich source of antimicrobial compounds, including lactoferrin (see below) conglutinin, conglutinin and the lactoperoxidase process (see further).
• Raw milk is reported to have an inhibitor of rotavirus that could block up to the level of 106 PFU (plaqueforming units)/ml. Pasteurization destroys it.
• Casein from milk as well as certain free fat acids have been proven to be antimicrobial in certain conditions.
• Eggs are a source of lysozyme as do milk. This enzyme, in conjunction conalbumin, provide fresh eggs with an effective antimicrobial system.
• The hydroxycinnamic acid derivatives (p-coumaric caffeic, ferulic, p-coumaric, and chlorogenic acid) are found in fruit vegetables, tea, Molasses and other plant sources show antibacterial and antifungal properties.
• Lactoferrin is a glycoprotein that binds iron that inhibits various food-borne bacteria, and is used as a microbial blocker on carcasses of beef.
• Ovotransferrin is believed as the inhibiting compound in egg whites that blocks Salmonella enteritidis.
• Cell vacuoles in cruciferous plants (cabbage, Brussels sprouts, turnips, broccoli etc.) contain glucosinolates that, when damaged or broken result in isothiocyanates.

6. Biological Structures

• The natural cover of certain food items provides excellent protection from the introduction and damage caused by spoilage bacteria.
• In this category are things such as the seeds’ testa as well as the outer covering that covers fruits, the outer shell of nuts, and the hide of animals and egg shells.
• For nuts like pecans and walnuts the shell, or covering can be sufficient to block the entry of all bacteria. After cracking, naturally nuts are subject to spoilage due to mold.
• The membranes and outer shell of eggs, when intact will block the entry of almost every microorganism that can be found in eggs when they are kept under the appropriate conditions of temperature and humidity.
• The fruits and vegetables that have damaged cover get spoiled significantly faster than those that aren’t damaged.
• The skin that covers fish and other meats like pork and beef prevents food from spoilage and contamination. foods, mainly due to the fact that it tends to dry quicker than freshly cut surfaces.
• In total the six variables are nature’s method of protecting animal and plant tissue from microorganisms. When determining the amount of each one’s presence in a specific food, one can determine the kinds of microorganisms expected to multiply and, in turn, the stability overall of this specific food.
• Their analysis could also assist in determining the age of the food, and perhaps the handling history of a specific food item.

What is Lactoperoxidase System?

• It is an inhibiting system that is found naturally in bovine milk and it is composed of three elements which are thiocyanate, lactoperoxidase, and H2O2.
• Three components are necessary for antimicrobial activity, and Gram-negative psychrotrophic like the pseudomonads can be quite sensitive.
• The lactoperoxidase amount required is 0.5-1.0 per milliliter, while bovine milk usually contains about 30 ppm.
• While both thiocyanates as well as H2O2 are present in milk, the amounts differ. For H2O2, approximately 100 U/ml is needed in the inhibitory system. However, less than 1-2 U/ml is typical in milk.
• A thiocyanate level that is effective is about 0.25 millimeters, while in milk, the amount varies in the range of 0.02 between 0.02 and 0.25 mM.
• The lactoperoxidase mechanism that is present in milk raw was initiated by adding thiocyanate at 0.25 mM , along with an equivalent volume of H2O2, shelf life was extended to five days as opposed to the 48-hour shelf life for the controls.
• The system performed better when it was at 30*C, compared to 4*C.
• The antibacterial effects increase when acidity is high and the cytoplasmic membrane is thought to be the target of the cell.
• In addition to the immediate addition of H2O2 an exogenous source could be created by adding glucose or glucose oxidase.
• To prevent the addition of glucose directly to oxidase, the enzyme is immobilized on glass beads to ensure that glucose can be produced only in the quantities required through the use of b-galactosidase immobilized.
• The system proved effective on goat’s milk to fight P. fluorescent as well as E. Coli, in which E. coli’s growth was stopped for 3 days, and the latter was controlled for two days at temperatures of 8*C.
• The lactoperoxidase mechanism can be utilized to preserve the raw milk of countries in which refrigeration isn’t common.
• The addition of 12 ppm of SCN and 8 ppm H2O2 is likely to be completely safe for consumers.

B. Extrinsic Parameters

The extrinsic properties of food are not influenced by substrates. They are the characteristics associated with the conditions of storage which impact both the food and its microorganisms.  Those of greatest importance to the welfare of foodborne organisms are as follows:

1. Temperature of storage
2. relative humidity of environment
3. presence and concentration of gases
4. presence and activities of other microorganisms

1. Temperature of Storage

• Microorganisms, both as individuals and collectively, thrive in a broad temperature range. Thus, it is important to take a look at the range of temperature that is suitable for important organisms in food items to assist in choosing the right temperature for the storage of different kinds of foods.
• The lowest temperature at which microorganisms have been observed to grow is -34*C. The highest is around 100*C.
• It is standard to classify microorganisms into three categories in accordance with their requirements for temperature to grow.
  • Psychrotrophs: Organisms that thrive at or less than 7°C, and achieve their ideal temperature between 20*C to 30*C are called psychrotrophs.
  • Mesophiles: Those who grow in a range of 20 to 45 degrees Celsius with optimal temperatures between 30*C and 40*C are often referred to as mesophiles.
  • Thermophiles: the ones that grow at or above 45*C and have the optimal temperature between 55*C and 65*C are called thermophiles.
• Regarding the bacteria that are psychrotrophic, species, as well as strains, can be found in the genera listed below: Alcaligenes, Shewanella, Brochothrix, Corynebacterium, Flavobacterium Lactobacillus Micrococcus, Pectobacterium, Pseudomonas, Psychrobacter, Enterococcus, and many more. The psychrotrophs most frequently found in food items are those belonging to families Pseudomonas as well as Enterococcus . These organisms thrive at temperatures in the refrigerator and cause spoilage of 5-7°C in meats as well as poultry, fish eggs, and other food items that are normally stored in this range of temperatures.
• Mesophilic strains and species are recognized across all genera and can be observed on food items kept in refrigerators at temperatures of up to. They do not appear to develop at this temperature, but they do develop at temperatures that are within the mesophilic range, if other conditions are favorable. It is worth noting that certain organisms can thrive in a temperature range of 0°C to 40*C. One of these organisms is Enterococcus Faecalis.
• The most important thermophilic bacteria in food are part of Bacillus Paenibacillus Clostridium Geobacillus, Alicyclobacillus, and Thermoanaerobacter. While not all species from the genera in these are thermophilic they are of particular significance to food microbiologists and food technologist working in the industry of canning.
• Like molds, which can grow in larger ranges of pH levels as well as osmotic pressure and nutrition content and osmotic pressure, they also have the ability to flourish in wide ranges of temperature just as bacteria. A variety of molds can develop at temperatures that are cold including some varieties that belong to Aspergillus, Cladosporium, and Thamnidium that can be found on eggshells, on the sides of beef, or even on fruits.
• The yeasts are found in mesophilic and psychrotrophic temperature zones, but rarely within that thermophilic zone.
• The quality of the food product should also be considered when the selection of a suitable temperature for storage. While it is tempting to keep all food items at temperatures that are below freezing or in the refrigerator however this isn’t necessarily the most efficient way to maintain high quality in certain food items. For instance, bananas will last better when kept at 13-17°C rather than 7 to 7°C. Many vegetables thrive at temperatures around 10°C including celery, potatoes as well as cabbage, and others. In all cases, the effectiveness of temperature storage is dependent in large part on the humidity (RH) that is present in the area and its presence, or lack of gasses like CO2 and O2.

2. Relative Humidity of Environment

• It is important to consider the RH that exists in the storage space is crucial from both the perspective of aw within food products and also the growth of microorganisms on the surfaces.
• If the aw of the food is set to 0.60 it is essential to store the food in conditions that do not permit the food to absorb water from the air, and thus enhance its own Aw on the surface as well as subsurface to an extent that there is a possibility of microbial growth.
• When food items with low values of aw are placed in conditions with high RH The food items soak up moisture until an equilibrium is achieved.
• Also, food items that have a high aw loss of moisture when they are in a room with low RH.
• There is a correlation between RH and temperature which must be taken into consideration when choosing the right storage environment for food items. In general the greater the temperature is, the lower the RH and the reverse is true.
• Foods that suffer from damage to the surface caused by yeasts, molds, and some bacteria must be stored in conditions with low RH.
• Incorrectly wrapped meats like whole chickens and cuts of beef tend to experience a lot of surface spoilage when stored in the refrigerator prior to when the meat is spoiled deeply due to the high RH in the refrigerator in addition to the fact that meat-spoilage bacteria are essentially aerobic in nature.
• While there is a way to decrease the risk of surface spoilage in certain food items by storing them in low temperatures of RH however, it must be kept in mind that food items will be dehydrated to the air in these conditions, and consequently end up being unappealing.
• When choosing the most suitable RH’s environmental conditions it is important to pay attention to both the potential for surface growth as well as the quality that is maintained in the food items that are being considered.
• By altering the gaseous environment It is possible to limit surface spoilage while lowering RH.

3. Presence and Concentration of Gases in the Environment

• Carbon dioxide (CO2) is the only most significant atmospheric gas that is used to manage food-borne microorganisms.
• Together CO2 and O2 are the second gases that are most significant that are present in the modified atmospheric packaged (MAP) food items.
• Ozone (O3) is another atmospheric gas that is known to have antimicrobial properties. It has been tested over several decades as a method to prolong the shelf life of specific food items. It has been proven that it is effective in battling a range of microorganisms. However because it is an extremely antioxidant and a strong oxidizing agent, it shouldn’t be applied to foods with high lipid content as it can create increased rancidity.
• The protozoan was three-times more sensitive O3 when it was 25*C than 5*C.
• It is permitted in foods that are sold in Australia, France, and Japan and, in 1997, it was granted GRAS (generally considered to be uninvolved) classification within the United States for food use.
• In general, O3 levels of 0.15 to 5.00 per milligram in air have been proven to stop the growth of certain spoilage-related bacteria and yeasts. Ozone can be used as a food-sanitizing agent.

4. Presence and Activities of Other Microorganisms

• Certain foodborne bacteria produce substances that are either inhibiting or fatal to others. they are antibiotics, bacteriocins hydrogen peroxide, as well as organic acids.

C.  Implicit Factors

• The third set of variables that play a role in determining the character of microbial relationships that are found in food products can be described as Implicit factors that determine the properties of the organisms themselves, and how they react to their environment, and how they interact with each other.
• Microorganisms may inhibit or boost the growth of each other.
• Different reactions and actions that could be beneficial or harmful to microorganisms include predation, parasitism and amensalism. asymbiosis and allotropy. neutrality.
• The organisms could produce substances that are harmful or injurious to other organisms like antibiotics, bacteriocins and antibiotics, as well as hydrogen peroxide, as well as organic acids.

D. Food Processing factors

• When food products are processed microorganisms are exposed to a variety of chemical or physical tensions.
• These processing elements include heating, freezing drying, osmotic effect Irradiation, as well as a variety of chemicals.
• Heating can reduce the level of microbial activity in food products by causing damage to the cytoplasmic membrane. It also alters enzyme and metabolic processes.
• Freezing slows bacteria’s growth by having the inhibitory effect of decreased pH and an increase of Aw.
• Drying can reduce the growth of microbes since it triggers metabolic damage that hinder the proliferation of cells.

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