Food Preservation by low temperatures
- Low temperatures are employed to stop chemical reactions and the action of enzymes in food and to stop or slow the development and activities of microorganisms found in food items.
- Lower the temperature the fewer chemical reactions will occur or enzyme action. This will also slow Microbial growth. A lower temperature can stop the development of microorganisms in any way.
- Every animal or plant food can be believed to include a range of yeasts, bacteria, and molds that need just conditions to grow in order to create undesirable changes to the food.
- Each microorganism in the world has an ideal temperature, or the best, to grow and a minimum temperature at which it is unable to grow. As temperatures drop from the optimal to the minimal temperature, its growth rate reduces and slows down at the lowest temperature.
- Cooler temperatures can stop growth, however, the slow metabolism could continue.
- So, cooling food from the normal temperature can have a different impact on the diverse organisms that reside there.
- A temperature drop of 10 degrees could hinder the growth of certain organisms, and slow the growth of others, but in a way that will depend on the type of organism.
- A further reduction of 10 degrees in temperature could slow the growth of additional organisms and slow the growth of the others.
- Storage at low temperatures can thus act as an important environmental factor that influences the nature of spoilage flora, which is predominant.
- The metabolism and growth of microorganisms are influenced by enzymes. The rate of an enzyme reaction is directly influenced by temperature.
- The primary aspect of this effect on temperature is the decrease in the growth rate of microorganisms when the temperature drops
Which Microorganisms grow at Low Temperatures
In general, freezing prevents the growth of most foodborne microorganisms and refrigeration temperatures slow growth rates. Commercial refrigeration temperatures, i.e., lower than 5 to 7.2 C, effectively retard the growth of many foodborne pathogens.
One notable exception is
- Clostridium botulinum type E with an ideal temperature that allows for the growth of approximately 3.3 C.
- Yersinia enterocolitis is able to survive and thrive at temperatures that are as low as 0 to 3 C. An earlier study recommended that the temperature should be between 1-7 C.
- However, there has been some concern regarding the limit of growth for Salmonella.
Other pathogens that are foodborne require a minimum temperature to develop below 7.2 C, and refrigeration is not a guarantee to stop an increase in size for a long time.
Some examples, as from various researchers of the low-temperature growth of microorganisms are interesting. The molds Cladosporium, and Sporotrichum have been discovered growing on food items in temperatures of -6.7 C and Penicillium and Monilia at 4 C. The growth of one yeast was observed at 34 C while two others developed at the temperature of -18 C (Michener and Elliott 1964). Bacteria have been found to grow at temperatures that are as low as -5 C on meats and 10 C on preserved meats as well as 11 C in fish. -12.2 C on vegetables (peas) and 10 C within ice cream. yeast at 5 C for meats, and -17.8 C on oysters; and molds at -7.8 C on meats and vegetables, and -6.7 C on berries.
Temperatures/Methods Employed In Low-Temperature Storage
Foods are stored at different temperatures to increase shelf-life. Four distinct methods of low temperature to preserve food items that include chilling, room-temperature storage refrigeration, freezing, and chilling
A. Common, or Cellar, Storage or Room-Temperature Storage
- The storage at room temperature is carried out at temperatures that are not too significantly lower than the outdoors temperature of the air (15-20 degrees Celsius).
- Cereals, potatoes and apples, and other similar foods can be kept for a brief time at room temperature.
- The degrading of foods like vegetables and fruits, caused by their microorganisms and enzymes can not be stopped in the room, however, it is slowed down when it is when the temperature is at room temperature.
- Foods stored in environments with low humidity at room temperature are prone to losing moisture. Foods that have high humidity are more susceptible to the development of microbial spoilage.
- In places where refrigeration is not readily available food storage at temperatures of room temperature is the norm.
B. Chilling, or “Cold Storage”
- Chilling storage occurs at temperatures that aren’t much above freezing. It usually involves cooling using ice or mechanical refrigeration.
- The temperature of chilling is the difference between the typical temperature of the refrigerator and room temperatures, which is usually around 10 degC. They can vary between 0 and 15 degC. The temperature of chilling on the area close to the ice could be anywhere from 0 to 1 degree Celsius.
- It could use as the principal food preservation method or to preserve food items for a short period until a different preservative method is implemented.
- Most perishable food items, like eggs, dairy products seafood, meats, vegetables, and fruits, could be stored in chilled storage for a brief period without significant changes from their original state.
- Factors that influence the storage and chilling of food are the temperature of chilling, the relative humidity and air velocity, the structure of the air in the storage room, and other factors.
- Microbial and enzyme changes in the food can be prevented, but they can be slowed down.
Factor affecting in Chilling, or “Cold Storage”
The factors to consider in relation to chilling storage are chilling temperature and the relative humidity of the air velocity and the composition of the ambient air inside the storeroom, and the possibility of using ultraviolet rays and other radiations.
- The less temperature storage and the more expensive. So, while most food items are best stored at temperatures that are just above the freezing point, they’re not always stored at this temperature.
- Instead, the temperature of chilling is chosen in accordance with the food type and the period and conditions of storage.
- Certain foods have an ideal storage temperature or temperatures much higher than the freezing point and could be affected by lower temperatures.
- An example that is well-known of this is the banana. It is not recommended to be stored in the refrigerator It is best kept around 13.3 up to 16.7 C. Some varieties of apples go through “low-temperature breakdown” at temperatures that are near freezing. Sweet potatoes do best when they are kept between 10 – 12.8 C.
- The temperature of refrigerators is controlled by mechanical means, however it differs across different components, typically between 10 and 0 C.
(b) Relative Humidity
- The ideal level of relative humidity in the environment in chilling storage is dependent on the food that is stored as well as external factors like temperature, the composition of the atmosphere, and the effects of rays.
- Low relative humidity causes loss of moisture, and consequently weight loss, the wilting and softening of the vegetables, as well as the shrinkage of fruit.
- High relative humidity encourages the development of microorganisms that spoil food.
- The highest level of humidity, close to saturation, is necessary to support the majority of bacterial growth on the food’s surface and less moisture is required by yeasts, ranging from 90-92 percent, and even less by molds that can develop in an average humidity between 85 and 90 percent.
- Variations in humidity, and also in temperatures, during storage, can trigger “sweating,” or precipitation of water, which can affect the food. A wet surface can encourage spoilage by microbes, e.g. slime on the surface of the sausage.
- Controlling or ventilating the speed of airflow in the storage area is crucial in maintaining a consistent humidity in the room eliminating odors and preventing the growth of flavors and odors that are stale.
- The air circulation rate determines the speed of drying of food products. If sufficient ventilation isn’t available, foods in local regions with high humidity can be subject to microbial degradation.
(d) Composition of Storage Atmosphere
- The quantities and proportions of the gases that are stored in the environment affect preservation by cooling.
- In most cases, there is no effort to regulate the composition of the atmosphere, however, the food items stored in plants continue to breathe, using oxygen, and emitting carbon dioxide.
- In recent times, however, greater interest has been paid in recent years to “gas storage” of food products, in which the atmospheric composition is controlled through the introduction of ozone, carbon dioxide (experimentally) or any other gas, or the removal from carbon dioxide.
- Storage of gas (see pages 206 and 207) usually is used in conjunction with storage for chilling. It has been discovered that with optimal levels in carbon dioxide and ozone (1) the food item can be preserved for longer, (2) a higher relative humidity can be maintained without causing harm to the quality of preservation of specific food items or (3) an increased storage temperature is able to be utilized without affecting the storage time of the food . This is not achievable with normal chilling storage.
- It is particularly advantageous to have the ability to keep an extremely high relative humidity with no any risk of microbial contamination as many food items retain their original quality better when they are not able to lose much water.
- The best level of carbon dioxide present in the air is dependent on the food that is stored, ranging starting at 2.5 percent that is reported as the best for eggs, to 10 percent in chilled meat, up to 100 percent for bacon.
- Certain food items, e.g., apples the amount of oxygen and carbon dioxide is significant. A specific ratio of both gases is desired.
- Plant cells that are spiriform may release excessive carbon dioxide in the storage space for certain foods which is why a part of it needs to be removed.
- The combination of UV irradiation combined with chilling storage can help preserve certain foods, and can allow the use of a greater humidity or temperature for storage than could be achievable by chilling on its own.
- Ultraviolet lamps are installed in rooms to aid in the storage of cheese and meat.
Advantages of chilling
- There is not much difference in flavor color, texture, or color form.
- Fresh food can be stored in top condition for longer.
- The customer can choose from an array of convenience and fresh foods.
- Nutrients cannot be eliminated.
C. Freezing or “Frozen Storage”
- The preservation of food items in a frozen state is a crucial preservation technique for centuries, especially when outdoor temperatures for freezing were common.
- Thanks to the advancement of mechanical refrigeration as well as rapid-freezing process the market for frozen foods has seen rapid growth. Even at home food industry, freezing food items has grown to a large extent, now that deep freezers at home are easily accessible.
- When you store food in the typical conditions for freezing foods there is a complete stoppage of the growth of microbes completely and the activity of enzymes found in food is drastically reduced.
- The lower the temperature at which storage is kept the more sluggish will be any chemical or enzymatic reaction however, the majority of them will be slow to occur at any temperature that is new in storage.
- Thus, it is the norm to deactivate enzymes in vegetables by blanching or scalding before freezing if it is possible.
How to Perform Freezing or “Frozen Storage”?
There are a variety of steps to be taken in this process of Freezing also known as “Frozen Storage” such as;
(a) Selection and Preparation of Foods for Freezing
- The quality of food items that become frozen has paramount importance, as the frozen food will not be higher than the food before it was frozen.
- Fruits and vegetables are chosen according to their freezing potential as well as their maturation. They are cleaned, cut or cut, or prepared to be treated as needed.
- Most vegetables are scalded, or blanched, and fruit can be packed in syrup.
- The meats and seafood that are used are chosen to ensure quality and handled in a way that minimizes any microbial or enzymatic changes.
- Most food items are packaged prior freezing, however some food items are packaged in small chunks, e.g. the strawberries could be frozen prior to packaging.
- The blanching or scalding process of vegetables is typically performed using steam or hot water and the amount of treatment will vary based on the dish.
- This quick procedure of heating is designed to accomplish the following things:
- inhibition of most plants enzymes, which could otherwise result in toughness, changes to color, mustiness loss of flavor, softening, or loss of nutritional value
- diminution (as high at 100 percent) in the number of microorganisms in the food items,
- Enhancement in the color green of vegetables like broccoli, peas, and spinach.
- Wilting of leafy vegetables like spinach, which makes them pack better and
- movement of the air trapped within the tissues.
(b) Freezing of Foods
The speed of freezing foods is determined by a variety of variables, including the method used, the temperature, the circulation of refrigerant or air as well as the size and shape of packaging, and the kind of food.
- Sharp freezing: Sharp freezing generally is the term used to describe freezing in the air using the only circulation of air, or at best , with electric fans. The temperature typically is -23.3 C or lower but can range from -15 to -29 C as well as freezing can take anywhere from 3 to 72 hours. Sometimes, this is referred to as slow freezing, to differentiate it with quick freezing, where the food is frozen in a small amount of duration.
Quick freezing: Fast freezing is a broad term that generally refers to a freeze duration of 30 minutes or less, and typically it is the process of freezing small packs or food items. Quick freezing can be achieved by three different methods:
- (1) directly submerging the food or packed food in the refrigerant. This is for the freezing of brine-frozen fish or berries in specific sirups
- (2) in indirect contact refrigerant in which the food or the packaging is in direct contact with the channel through which the refrigerant -17.8 up to -45.6 C flows, or
- (3) Air-blast freezing in which frigid air in the range of -17.8 up to -34.4 C is blown across the frozen materials.
- A method of shipping of frozen, packaged food items involves the freezing of nitrogen in the cartoned food items in a specific aluminum container and food storage in the vessel.
- The initial low temperature, as well as the insulation, ensure that food items will stay in its frozen condition for the time you want it to be.
- Certain kinds of vegetables and fruits like shrimp, fish and even mushrooms are now being frozen using liquid nitrogen. Dehydrofreezing is a process whereby fruits and vegetables will have approximately 50% of their moisture removed prior to freezing.
Advantages for quick freezing
The benefits that are claimed to be gained from fast frozen over slow are
- smaller ice crystals are created and, consequently, there is less destruction of cells that remain intact the food.
- There is a shorter duration of solidification, which means shorter time for the diffusion of substances that are soluble and for the removal of ice
- There is a better preventative action against microbial growth.
- There is a faster the slowing down of enzyme action. Fast-frozen food items will, therefore, need to be able to melt to a state similar to the food that was originally frozen than slow-frozen ones. This may be the case for certain foods, e.g., vegetables however, it is not the case for all types of food. Fish research for instance has found no benefit when it comes to quick freezing versus slow freezing.
(c) Changes during Preparation for Freezing
- The speed and type of food deterioration prior to freezing will be based on the state of the food prior to harvesting or at slaughter and on the handling methods used thereafter.
- The temperature at which food is stored along with other environmental factors will determine the types of microorganisms growing and the types of changes that are created.
- The state of the food at moment that it was frozen determines the quality of the food that is frozen.
(d) Changes during Freezing
- The process of quick-freezing speeds up reactions of enzymatic and chemical nature in the food items and also stops the growth of microbial species.
- Similar results can be achieved through slow or sharp freezing, however with lesser speed.
- The physical effects of freezing are of major significance. There is an increase in size of frozen food and ice crystals develop and expand in size.
- They are usually larger when slow freezing is slower, and there is more accumulation of ice between cells of the tissue than rapid freezing. They can also smash cells.
- The water is drawn out of the cells to form frozen ice. This causes an increase in the quantity of solutes in the non-frozen liquor that will constantly drop in freezing point until a steady state is achieved.
- It is believed that crystals of ice break up the cells of tissues or microorganisms. However, some scientists minimize the significance of this kind of effect.
- The greater concentration of solutes within the cells accelerates the salting outprocess, as well as the dehydration, and the denaturation of proteins, and triggers irreparable changes in colloidal systems, for example the syneresis between colloids with hydrophilic properties.
- Additionally, it is thought to be the cause of killing microorganisms.
(e) Changes during Storage
- When food is stored in the freezer condition, the reaction of enzymatic and chemical substances occurs in a slow manner.
- Meat, poultry, and fish protein can be irreparably dehydrated. Red myoglobin in meat could be converted into brown metmyoglobin. Also, fish and meat fats can be hydrolyzed and oxidized.
- The unfrozen, concentrated mixture of salts, sugars, and so on. could ooze out of packages of fruit or concentrates after storage, forming a liquid that is viscous, referred to as metacryotic liquid.
- Variation in temperature of the food storage can result in an increase in size the crystals of ice and physical harm to food items.
- The food’s decapping will occur on its surface in the course of storage. If ice crystals are evaporated from an area near their surface, there is a deformity known as freezer burn develops on vegetables, fruits and meats or poultry, as well as fish. The spots usually appear dry or grainy and is brownish In this region, the chemical changes described above are triggered and tissues turn hard and dry.
- At temperatures that are freezing, the microorganisms’ vegetative cells that are not able to reproduce will eventually end up dying. There is a slow , but steady decrease in the numbers of viable microorganisms, as storage continues. There are species dying earlier than others, however representatives of the majority of species living for months or even for years.
(f) Changes during Thawing
- The majority of the changes which appear to occur during thawing are due to storage and freezing, but they are not apparent until.
- If the ice crystals break down the liquid is absorbed back into tissue cells or leaks away through the food.
- The slow, controlled thawing results in a greater water return to cells than fast melting and can result in food that is more similar to the original food item that was frozen.
- The reddish or pinkish liquid that is produced by meat that has been frozen is known as bleeding or drip and the liquid that oozes out of the vegetables or fruits on the freezing is known as leakage.
- The flabbiness or wilting of vegetables as well as the squishiness of fruits after their thawing is mostly caused by physical damages when frozen.
- When the food is thawed, the efficiency of the enzymes in the food will rise however, the time to the action to take place will be relatively small if the food is used quickly.
- If the process of thawing is relatively quick and the food is consumed quickly, there will be no issues with the growing microorganisms as temperatures are too low to allow any significant quantity of growth.
- The thawing process is extremely slow or the food that has been thawed is allowed to remain at room temperature does it provide the possibility of a significant amount of activity and growth of microorganisms.
- The kind of organisms that are growing are influenced by the temperatures at which they are during the thawing process and how long when the foods were allowed to sit the following thawing.
(g) Disposal of Thawed Foods
- Sometimes, power outages cause either complete or partial thawing of frozen foods. Fruits that have been frozen can be frozen and refrozen.
- Foods and vegetables that are cooked can be frozen and refrozen in the event that the packaging has frozen.
- Refrozen food items will have large crystals of ice, and they may exhibit a loss in the form of a fluid (syneresis) and a tinge of mushiness.
- If frozen, flesh-based foods are suitable if their temperature falls below 3.3 C. In the case of uncertainty, food must be removed.
(h) Precooked Frozen
- Foods that are frozen and precooked contain a range of different kinds of food and food items that they are able to be easily discussed.
- The majority of these foods are made up of fish, meat or poultry products, e.g., soups or stews, creamed items stews, pies chicken or fish that is fried such as chow mein barbequed meats, meat loaves and chicken as the king.
- Certain bakery products such as fruits and even vegetables can be cooked before being frozen, but.
- The precooking process is typically sufficient to kill pathogens that are present in the raw material. It also helps to reduce the amount of microorganisms in the material.
- The cooking process does not kill any staphylococcus toxin found in the food.
- Enterococci endure freezing and remain longer in storage than bacteria with coliforms, and are suggested to be used as “indicator bacteria” to detect contamination of the feces.
- It is particularly important to ensure that there is no contamination of food items during the cooking process, since any spoilage or pathogenic organism that is introduced will find the competition of other organisms significantly diminished and the food could be a more suitable cultivation medium than the raw material, if the opportunity for growth was provided. This is why it is crucial to ensure that cooling and freezing are completed quickly after cooking so that there is an opportunity for growth.
- If these frozen food items that are precooked remain at warm temperatures for too long after they have thawed it could result in an increase in growth and production of toxin by food poisoning organisms. However, this has never been previously reported.
- The cooking process of “warming the up” of these foods at the restaurant or in the home will not be enough of a treatment to significantly reduce the amount of organisms present , or ensure that any pathogens that are present are killed or removed.
Types of freezing
- Slow freezing process: It is also referred to in sharp freezing. The food is frozen at temperatures that range from -4oC to 29oC. It can take three to seventy-two hours in the conditions. Freezing at home is accomplished using this method.
- Quick freezing process: The temperature utilized during the process of quick freezing vary from -32oC up to 40oC. Food is frozen so fast that fine crys-tals begin to form. The time needed for rapid freezing is considerably less than slow freezing. When quick freezing is used, large quantities of food items can be frozen within a brief time. The use of a very low temperatures for freezing and storing frozen goods can increase the cost, but it is it is desirable for many items with regard to retention of nutritive value and palatability.
- Dehydro freezing: Dehydro-freezing of vegetables and fruits is drying food to approximately 50% of its original weight and volume , after that freezing the food to keep it in good condition. The quality of dehydro-frozen fruits and vegetables is similar to the quality of fruits and vegetables that are dried prior to freezing. The price is slightly lower due to weight and volume reductions in packaging, freezing, storage and shipping.
Points to be Considered Before Freezing Food
- Vegetables: The process of blanching (dipping the vegetables in boiling water for 2 to 3 minutes) Vegetables that are blanched prior to freezing decrease the number of microorganisms present and removes air from tissues, making them less bulky and improves their color. Its main purpose is to inhibit enzymes, which could cause a decline in palatability color and ascorbic acid content in storage.
- Fruits: Enzymes in fruits can be activated by boiling, however this isn’t done since it imparts the fruit with an uncooked taste and a soft texture. Instead, the fruits are cut in sugar syrup, or even sugar to avoid the oxidation. Sugar is not only a boost to sweetness, but also helps preserve the flavor and scent.
- Meat and poultry: Meat and poultry need the wrapping of only for freezing. After the animal is slaughtered pork, meat and poultry must be chilled immediately to prevent spoilage. The tendency for the fat from the poultry and pork to go rancid in storage in freezers is made worse by the storage prior to freezing.
- The temperatures of the refrigerator are between 0 and 7 degrees Celsius and are ideal to store an abundance of perishable and semi-perishable items.
- Temperatures less than 5 degC can stop the growth of many foods poisoning bacteria.
- C. Botulinum Type E, non-proteolytic and F strains and V. parahaemolyticus Y. enterocolitica L. monocytogenes, as well as certain A. Hydrophila species that are associated with gastroenteritis may develop at temperatures of 4 degrees Celsius.
- The growth of the majority of food spoilage microorganisms can be effectively inhibited below 5 degrees Celsius.
- For food items that are perishable A temperature of 4°C is considered to be a good temperature for refrigeration.
- Commercial food processors can make use of as little as 1 degC to cool of perishable food items.
- The relative humidity as well as the proper distance of the item is monitored in commercial refrigeration.
- Raw and processed food items from animal and plant sources and ready-to-eat food items are preserved through refrigerating.
- To ensure a long shelf life, other preservation techniques are incorporated into temperatures that are refrigerated.
Advantages of Freezing
- A variety of foods are able to be frozen.
- Naturally colored color and flavor and nutritional value that is retained.
- Texture is generally superior to other ways of preserving food.
- Food items can be frozen faster than they could be canned or dried.
- Simple procedures.Adds ease of cooking food.
- The proportions of the kitchen can be adjusted to the requirements of your home, unlike other preservation techniques. The kitchen remains cool and cozy
Food items that are frozen can be kept for a long amount of time. It is usually around 3 months. Deep freezing is the diminution of food’s temperature to the point that the microbial activity ceases
Disadvantages of Freezing
- The texture of certain foods is not ideal due to the freezing process.
- The initial investment and the cost of maintaining freezers are expensive.
- Storage space is limited by the capacity of the freezer
Effect Of Subfreezing And Freezing Temperatures On Microorganisms
- It’s hard to speak about the impact of freezing on microorganisms due to of the multitude of factors and the observed effects.
- One of the major problems with these findings is that it’s impossible to research the impacts of freezing of cells and without also observing the effects that result from cooling (decrease to zero C) as well as the effects of freezing. Marth (1973) was the first to have explained the phenomenon of freezing microorganisms as being a process that involves
- The cells are cooled to the temperature of 0 C,
- further cooling via the possibility of intra- or extracellular the formation of ice crystals,
- the concentration of intracellular and extracellular solutes,
- the storage of cells in a frozen state and
- freezing of cells and the substrate.
- The result of freezing is usually significant reductions in the quantity of viable organisms present in the food.
- This decrease in numbers that are recoverable could be due to sublethal or lethal causes.
- A large number of cells die from freezing, however, this isn’t an act of sterilization. The most commonly utilized methods to preserve culture are through freezing and storage that is frozen generally with liquid nitrogen.
- The lethal effects are believed to be due to flocculation or denature of the essential cells’ proteins or enzymes, possibly because of the higher concentration of solutes in unfrozen water or possibly partly due to physical damage caused by crystals of ice.
- A rapid decrease in cell temperature from a temperature that is optimal to zero C can cause death. This is known as cold shock. It is believed to be due to the alteration of lipids in the membrane, which can cause an increase in the permeability of the cell, or to the release of inhibitors of repair enzymes, e.g., an inhibiting ribonuclease.
- In a normal microbiological analysis of frozen food items, a decrease in the number of cells may not indicate the death of the population. It is possible that some cells could be in an injured or damaged condition which hinders their recovery to allow for the enumeration.
- Cells that are in this condition are referred to as frost-injured or freeze-injured and metabolically damaged.
- The freezing of microorganisms within a food could result in a cryoinjury. Because these cells are repaired if the time for repair is allowed or other nutritional factors are included in the media for enumeration They aren’t really dead.
- The focus on microorganisms that are injured by freezing food items is a result of concerns about the importance of bacteriological guidelines on these items, the quality of enumeration techniques, and the pathogenicity of damaged pathogens, as well as the analysis of the viability of numbers of microorganisms found in frozen food.
Response of Microorganisms to Freezing
The following factors or variables can be observed during freezing and could explain why some microorganisms die, others are injured, and others are not.
The type of microorganism as well as its status: Resistance to freezing is contingent on the type of microorganism and its growth phase and whether it’s an infected cell or a spore. Christophersen (1973) classified microorganisms in terms of their sensitivity to freezing.
- Susceptible: Vegetative cells of mold, yeast, and several gram-negative bacteria could be within the first category.
- Moderately resistant: Grampositive organisms, including staphylococci and enterococci, are likely to be part of the second category.
- Insensitive organisms: The 3rd group is mainly spore formers, as the spores from bacilli as well as Clostridia have a high resistance to freezing. Bacteria living in the logarithmic phase of growth are more likely to be killed than those in other stages.
- The freezing rate: The freezing rate is at a threshold temperature that can cause lethal consequences; therefore, speedier freezing rates are likely towards being less harmful as the critical temperature range will be traversed more quickly.
- The freezing temperature: High freezing temperatures can be deadly. More organisms are activated between -4 and -10 C than at temperatures ranging from -15 to 30 C.
- The time of frozen storage: The killing rate at first in freezing is fast but is then later followed by a gradual decline of microorganisms, called the death of storage. The quantity of living organisms decreases as you longer storage time. The storage of frozen food items in the critical temperature range will have a greater speed of decrease than when stored at higher or lower temperatures.
- The kind of food: The food’s composition determines the death rate of living organisms in freezing and storage. Proteins, salt, sugar colloids, fats, and many other ingredients can protect, while high humidity and low pH could accelerate deaths.
- Influence of defrosting: The response of microorganisms to the rate of defrosting varies. Rapid warming has been found to be harmful to some bacteria.
- Alternate freezing and thawing: Alternate freezing and thawing are said to accelerate the death of microorganisms, but it appears to not always work.
- Possible events during freezing of the cell: When the temperature drops as the cell are cooled, more and more water is frozen. The unfrozen or frozen water at every temperature is more and more enriched with solutes (salts and proteins nucleic acids, salts, etc.). This may alter the pH of the cell and enhance electrolytes, alter the colloidal state as well as denature proteins and increase the viscosity. Ice crystals may be formed outside of cells (“extracellular”ice”) and pull water from the cell, causing a dehydration or concentration effect. Crystals inside cells can form and develop or crystallize into the cell, resulting in changes in permeability or “holes” within the cell’s membrane and wall. The theory is that intracellular ice can be more damaging to cells than crystals of ice that are extracellular.
Factors Affecting Storage of Foods at Low Temperatures
1. Low Temperatures
- Different kinds of yeasts, bacteria, and molds can thrive in food items when temperatures are higher than those of water (above 2 degrees Celsius).
- The exponential phase and lag get larger as the temperature is decreased.
- The time for the generation of microbial cells increases by 10 degrees Celsius for each increase in temperature for storage and also doubles the enzyme activity.
- At lower temperatures at low temperatures, the growth of the majority of microorganisms ceases, with the exception of yeasts, psychrophilic microorganisms, and molds.
- A large number of cells are injured in a sublethal and lethally way at temperatures below freezing.
- Injury and death are extremely intense during the initial freezing phase and, in turn, decrease. Cold temperatures that are low are more deadly.
- More microorganisms are activated at temperatures ranging from -4 to -10 degrees Celsius rather than at -15 or -30 degC.
- The temperature fluctuations in food storage that is low-temperature could have an impact on growth, an injury that is sublethal, and the death of microorganisms.
- The temperature rise of 4.0 to 12 degC could cause the rapid growth of psychrophilic as well as mesophilic bacteria as well as the formation of spores.
- The temperature fluctuations of frozen food items can lead to the chance of injury or death to microbial species due to the repeated damage caused by chemical reactions and mechanical damage due to huge crystals of ice.
- Dead microbial cells emit intracellular enzymes (e.g. proteinases, proteinases and Lipases) These enzymes could be a source of food-related toxins and decrease the quality of food.
- The cooling rate is vital in limiting the growth rate of microorganisms. The slow cooling of food can lead to allow microbial growth. This is often the case in an enormous amount of warm or hot food items in a large (deep) containers.
- Foods that are refrigerated (at around 4 degrees Celsius) are likely to have a short shelf time due to the growth of microbial. In frozen food microorganisms are not likely to develop, but there will remain after long storage.
- A rapid thaw is essential to limit the growth of microbial. If food is slowly thawed the temperature on the surface of the food will rise which will allow an increase in microbial activity while the inside remains frozen.
2. Food Environment
- In the case of storage at lower temperatures, intrinsic as well as extrinsic elements of food products can impact sublethal and lethal injuries, and the viability of microorganisms.
- A food that has a high solid content (sugar and lipids, proteins salt, as well as other ingredients) could help to protect and encourage the growth and survival of microorganisms even at low temperatures.
- The shelf life of food can be increased through any of the factors like low pH and low aw or Microbial inhibitor(s) as well as the use of modified or vacuum packaging.
- When frozen food is thawed the ice melts, and water is absorbed into the food. This causes an increase in the amount of aw that is present in the area of melting and makes it vulnerable to the growth of microbial.
3. Characteristics of Microorganisms
- Certain psychrophilic microorganisms can grow at temperatures of as low as -10°C and numerous mesophilic, as well as thermophilic cells, are sublethally injured and die.
- In general, rod-shaped Gram-negative bacteria are more prone to the damaging effects of freezing than Gram-positive spherical-shaped spherical-shaped bacteria.
- Microorganisms vary greatly in their sensitivity or resistance to damage caused by freezing. The degree of resistance to freezing can vary based on the type of microorganism as well as its growth stage and also the vegetative cell, or the spore.
- Microorganisms can be classified in three categories based on their degree of sensitivity to freezing: sensitive (vegetative yeast cells, molds, as well as many Gram-negative bacteria) moderately resistant and non-sensitive (spores from Clostridium as well as Bacillus) Microorganisms.
- Bacteria living in the logarithmic stage are more susceptible to freezing than other phases.
- Spores are not likely to lose their viability in frozen food items.
- When food is stored in the freezer condition reactions of enzymatic and chemical nature take place in a slow manner.
- Certain heat-stable enzymes (released by the lysed or bacterial cells) can catalyze reaction when temperatures exceed -20 degrees Celsius in a very slow manner and decrease the quality of food.
4. Time of Frozen Storage
- The rate at which microorganisms die is extremely high in the first few minutes of the process of freezing.
- Then comes the gradual decrease of microorganisms in storage, which is referred to as storage death.
- The amount of microorganisms that are viable affects the duration of storage.
- Food Microbiology by William C Frazier and Dennis C Westhoff, 5th Edition.
- Fennema, O., & Powrie, W. D. (1964). Fundamentals of Low-Temperature Food Preservation. Advances in Food Research, 219–347. doi:10.1016/s0065-2628(08)60102-0
- Food Preservation by Low Temperatures. (2016). Food Microbiology: Principles into Practice, 34–43. doi:10.1002/9781119237860.ch29