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Writer and Founder of Microbiologynote.com. I am from India and my main purpose is to provide you a strong understanding of Microbiology.

Sourav Bio avatar
Sourav Bio
This article writter by Sourav Bio on July 31, 2022

Writer and Founder of Microbiologynote.com. I am from India and my main purpose is to provide you a strong understanding of Microbiology.

· 21 min read >

Food Microbiology

Food Preservation by Using High Temperatures

The utilization of high temperatures to preserve food items is based on their damaging effects on microorganisms as well as their spores.

Food Preservation by Using High Temperatures
Food Preservation by Using High Temperatures
  • Heating is one of the oldest methods used in food processing and preservation.
  • The utilization of high temperatures to preserve food items is based on their damaging effects on microorganisms as well as their spores.
  • The destruction of microorganisms through the heat process is thought to be caused by loss of protein lipids and, in particular, by the activation of the enzymes that are essential for metabolism.
  • The process of heating required to kill the organism or its spores is dependent on the type of organism, the state and the surrounding environment that is heated.

Factors Affecting Heat Resistance (Thermal Death Time)

Certain aspects are believed to affect the resistance to heat of spores, cells, or cells. These need to be taken into consideration when comparing microorganisms and when heat treatments to destruction of the organism are thought of as. The most well-known variables are:

1. The temperature-time relationship

  • The temperature and the time show the opposite relationship. The time required to kill cells or spores within certain conditions decreases when the temperature increases.

2. Initial concentration of spores (or cells)

  • The greater the number of spores or cells that are present, the more the need for heat treatment to kill them all. It has been proposed that the mechanism behind heat protection in huge microbial populations may be because of the creation of protective substances by cells.

3. Previous history of the vegetative cells or spores

The conditions in which cells have created and spores generated and their subsequent treatment will affect their resistance to the heat.

a. Culture medium

  • The medium where growth occurs is very crucial. The impact of the nutrients within the medium, the type, and amount differ depending on the species but generally speaking, the more favorable the environment for growth, the stronger the spores and cells.
  • The presence of a sufficient supply of growth factors and other accessories generally favors the development of spores or cells that are heat resistant. This could be the reason infusions of vegetables and liver extracts enhance heat resistance.

b. Temperature of incubation. 

  • The temperature of the growth of cells as well as the temperature of sporulation can affect their heat resistance.
  • It is generally true that the resistance rises as the temperature is increased to the optimal temperature for the organism. The resistance for many organisms, it increases when the temperature is near the limit of growth.
  • Escherichia Coli, for instance, is significantly more resistant to heat when it is grown at 38.5 C, which is close to its optimal temperature as opposed to 28, C. Spores from Bacillus subtilis, that were grown at various temperatures in peptone-water containing 1 percent were then heated.

c. Phase of growth or age

  • The resistance to heat of vegetative cells is influenced by the stage of development and also of spores with age.
  • Bacterial cells exhibit their highest resistance in the late lag phase, but they exhibit almost the same resistance in their stationary phase to the maximum which is followed by a decrease in resistance.
  • Cells are the least resistant in their initial phase of exponential growth. Young (immature) species are more resistant than mature ones. Certain spores show increased resistance in the initial weeks of storage. However, they later start to reduce their resistance.

d. Desiccation

  • Some bacteria’s spores that have been dried are more difficult to kill with the heat than those that are kept humid, but this is not the case for the spores of all bacteria.

4. Composition of the substrate in which cells or spores are heated.

The substance that the cells or spores are heated is of such importance that it needs to be described to determine if a death time in thermal form is to be considered meaningful.

a. Moisture

  • Moisture is a more effective killer than dry heat. as a result, dry substances require the most heat to sterilize than those that are moist.
  • In the laboratory for bacteriology, about 15-30 minutes at 121 C under the humid autoclave’s heat will cause sterilization of common materials. However, 3-4 hours at 160-180 C is needed in the event that the dry oven heat is utilized.
  • Spores caused by Bacillus subtilis can be destroyed within less than 10 minutes by steaming at 120 C However, in anhydrous glycerol, 170 C for 30 minutes is needed.

b. Hydrogen-ion concentration (pH). 

  • In general, spores or cells are the most resistant to heat when placed on a substrate close to or neutral.
  • A rise in alkalinity or acidity speeds up the killing process by heating, however, an alteration to that acidic aspect is far more powerful than a rise in alkalinity.
  • Spores from B. subtilis that were heated to 100 C in 1:15 m phosphate solution, and adjusted to different pH levels resulted in the results.
  • Other examples will be discussed during the discussion of heating processing of canned food items.

c. Other constituents of the substrate. 

  • The only salt that is present in large amounts in many food items is sodium chloride that in small amounts is a protector for certain spores.
  • Sugar may protect certain organisms or spores, but not other organisms.
  • The ideal concentration for protection depends on the type of organism. it’s high for some organisms that are osmophilic and low for other ones, the highest for spores and low for non-osmophilic cells.
  • The sugar’s protective effects could be due to a reduction in the amount of aw. A lower aw d can cause an increase in the observed resistance to heat.
  • Solvents are different in their effects on bacteria. For instance, glucose is able to protect Escherichia bacteria and Pseudomonas fluorescens better than sodium chloride , at temperatures that are close to the limit for growth.
  • In contrast glucose provides virtually no protection and is dangerous to Staphylococcus aureus. In contrast, sodium chloride has a great protection.
  • Since the amount of solutes could influence the process of heating required for sterilization, canners often further categorize foods as high-soluble solids foods, including sirups and concentrates as well as food items with lower soluble solids such as fruits vegetables, meats, and fruits.
  • Colloidal substances, particularly fats and proteins, act as shields against heat.
  • Antiseptic and germicidal compounds in the substrate help in the destruction and destruction of organisms. So, heat and hydrogen peroxide can be used to decrease the sugar’s bacterial content and is the foundation of the milk production process.

Heat Resistance Of Microorganisms And Their Spores

  • Microorganisms’ heat resistance generally is measured by their thermal death time which is the amount of time required at a particular temperature to kill an arbitrary amount of organisms (or the spores) in a specific environment. Sometimes, this is known as the absolute thermal death rate to differentiate it from the general thermal death time, which is to kill the majority of cells or spores that are present, and the rate of death, which is the percentage of killing.
  • The thermal death zone, currently rarely used is the temperature required to kill all organisms within 10 minutes.
  • The findings of various researchers on the relative resistance to heat of molds, yeasts, and bacteria, as well as their spores don’t completely match due to differences in the types of cultures and conditions of heating. So, only generalizations can be made, based on the results of a few researchers using as an example.

1. Heat Resistance of Yeasts and Yeast Spores

  • Their resistance to humid heat is dependent on the species and strain, and, obviously depending on the substrate where they’re being heated. The ascospores of yeasts require just 5-10 C more heat to cause death than the vegetative cell from which they originate. 
  • The majority of ascospores die at 60 C for between 10 and 15 minutes. A few are more resistant, however, they are not able to withstand even a short heating time of 100 C. Vegetative yeasts generally are destroyed by 50-58 C for between 10 and 15 minutes. 
  • Both yeasts, as well as their spores, are killed by pasteurization procedures which are given to by milk (62.8 C for 30 min or 71.7 C for 15 sec) as well as yeasts are easily killed during the baking of bread when the temperature inside is around 97 C.

2. Heat Resistance of Molds and Mold Spores

  • The majority of species of molds, as well as their seeds, get destroyed by moist heat of 60 C within 5 to 10 minutes, but certain species are significantly more resistant to heat.
  • The asexual spores are much more resistant than normal mycelium and require temperatures of 5-10 C greater to kill within a specific period.
  • There are many species of Aspergillus, and certain species other species of Penicillium, as well as Mucor, have a higher resistance to heating than other molds. One resistive mold to heat on fruits is called Byssochlamys fullva (Paecillomyces) which has resistant ascospores.
  • The pasteurization treatments that are given to milk generally kill all molds and their spores. However, some aspergilli spores that aren’t typically present in milk, might be able to survive such heat treatment.
  • Sclerotia are particularly difficult to kill with high temperatures. Certain species can withstand a heat treatment of between 90 and 100 C for a short time and are known to cause spoilage in canned fruit. It was discovered that 1,000 minutes in 82.2 C or 300 min at 85 C was needed to kill the sclerotia that is a type of Penicillium.
  • The mold spores have a high resistance to dry temperatures. Research from various researchers suggests that dry heat of 120 C for up to 30 minutes will not eliminate some of the resistant mold spores.

3. Heat Resistance of Bacteria and Bacterial Spores

  • The resistance to heat of the vegetative cells of bacteria differs significantly with the species, ranging from some pathogens that can be easily eliminated to thermophiles that require a few minutes at temperatures of between 80 and 90 C.
  • Some general assertions are possible to make about the resistance to heat of the vegetative cells of bacteria:
    1. cocci tend to be more resistant than rods, but there are some interesting exceptions.
    2. the more extreme the optimal and maximum temps for development, the more it is likely that the heat resistance likely to be.
    3. bacteria that clump up in capsules or clumps of them are harder to eliminate than those that do not.
    4. cells with high levels of lipids are more difficult to eradicate than other cells.
  • Keep to keep in mind the fact that temperatures death rates (and the ones that will be announced in the future for the spores) are for various amounts of cells (or spores) that are heated in various substrates. They could be lower or higher in other conditions.
  • The resistance to heat of bacteria spores is greatly dependent on the bacterium species and the conditions of the process of sporulation. The resistance at temperatures of 100 C can be as low as 1 minute up to 20 hours.
  • It is generally true that the spores of bacteria that have the highest optimal and maximum temperatures that allow growth tend to be more resistant than those of bacteria which grow most efficiently in lower temperature. A sporeformer that is growing in an additional sporogene that has a higher degree of heat resistance could have an increased resistance, e.g., Clostridium perfringens that is growing with C sporogenes.

4. Heat Resistance of Enzymes

  • Although most food and microbial enzymes are destroyed at 79.4 C, some may withstand higher temperatures, especially if high-temperature-short-time heating is employed.
  • One of the purposes of a process that is thermal (Witter 1983) is to inhibit enzymes that could cause product degradation when stored.
  • Generally speaking, processes that use heat to kill microorganisms can activate enzymes that are of interest. However, there are some notable instances of exceptions.
  • For example, some hydrolases (proteinases and lipases) will retain a substantial level of activity after an ultrahigh-temperature process. The remaining enzymes’ activity could affect the quality of the product that was processed during long-term storage.
  • Another enzyme, bovinephosphatase is utilized as an “monitor” during the pasteurization process of milk.
  • The presence of bovine enzyme found in milk processed typically means that milk wasn’t pasteurized. False positives can occur in the event that there are high levels of microbial-based an enzyme called phosphatase have been detected.

Thermal Destruction of Microorganisms

Microorganisms’ heat resistance is typically expressed as follows:

  • Thermal Death Time (TDT): The term The term Thermal Death Time (TDT) is the amount of time required to kill a specified amount of microbial cells or spores at a certain temperature in a specific environment. It is sometimes called the absolute temperature death time. In this way , the temperature remains constant , and the amount of time needed to end all cells is determined.
  • Thermal Death Point (TDP): The Thermal Death Point (TDP) is the temperature needed to kill a certain number of microorganisms over an fixed time period, usually 10 minutes.
  • Decimal Reduction Time (D Value): The D value represents the time in minutes needed to eliminate 90% or one milligram of bacteria. Ex: The 12D idea is utilized in the heat processing of high-pH food items (pH > 4.6 and low acid foods like beans, corn and meat) to eliminate the most resistant spores that are heat-resistant from the pathogenic bacteria Clostridium botulinum. This means that the food items undergo heat treatment to decrease the number of C. botulinum spores to twelve log cycle. Its 12D value, at D121.1oC is approximately. 2.8 or roughly. 3.0 min.

Heat Penetration

The rate at which heat is introduced heat into food items is essential to know to determine the process of thermal transformation required to ensure its preservation. Since each part of food contained in a bottle or container needs to undergo a proper heating treatment to prevent spoilage, the portion that gets the most heat is the most crucial and the rates of changes in temperature of this part-mostly near the center of containers for food that heats by conduction and further down when convection is the method of heating-are assessed.

The transfer of heat through an outside source to the central part of the container could occur through conduction, which is the process where heat travels from molecule to molecule through convection, in which heat is transferred via the moving gases or liquids or gases; or, as is typically the case, via the combination of conduction with convection. Conduction is slower in food but it is rapid in metallic materials. The rate at which heat is transferred through convection is contingent on the potential for flow of liquid currents and the speed of flow of these currents.

Conduction and convection play a role in the heating of food both can work simultaneously or sequentially. If food particles are suspended in liquid, the food particles are heated through conduction while the liquid is heated by convection. Certain foods alter their consistency when heated and a broken curve develops. This happens with the sugar-rich syrups and brine-packed whole grain corn, as well as certain thick tomato juices and soups. The elements that determine the length of time it takes to bring the central part of the food container up to the temperature for sterilization are the following:

  1. The material of which the container is made: Glass has a slower rate of heat absorption than a metallic can.
  2. The size and shape of the container: The bigger can it is and the larger it is, the longer it takes to reach a certain temperature in the center, because the distance from the middle of the bigger can is greater, and it has less surface area per weight or volume. Thus, cans with larger sizes are heated more frequently, however they do not reach the same extent in the middle. Naturally, the shape of the can will determine the radius. A long slim cylindrical form is more efficient at heating than the same size with a smaller cylindrical shape.
  3. Initial temperature of the food: Actually the temperature of food inside the can before it goes to the retort (steam sterilizer) is not a significant factor in the amount of time needed to allow the inside in the can attain the temperature for the retort. For the food with a low temperature initially heats up quicker than food that is heated to a higher temperature. But, the food that has the higher temperature at the beginning is in the danger zone for microorganisms over longer and the average temperature of heating is much higher than that of the food inside the can that has an initial temperature lower. The high temperature of the initial stage is essential in the processing of food items that are heated slow, like the cream corn variety, pumpkin and even meat.
  4. Retort temperature: Replica cans of food that are placed in retorts with different temperatures are able to reach their respective temperatures nearly at simultaneously. However the fastest heating will take place in the most hot Retort, and food items would be heated to lethal temperatures the fastest.
  5. Consistency of can contents and size and shape of pieces: These are all significant in their effects on the penetration of heat. The size and shape of food pieces and the process they undergo when cooking, warrant their classification into three categories.
    1. Pieces that retain their identity i.e., do not cook apart: Examples are beets, peas, plums as well as asparagus and whole-grain corn. In the case of small pieces placed in brine, like peas, the heating process is similar to like the water. In the event that the parts are larger the heating process is delayed because the heat has to get into the center of the piece before the liquor can be heated to the temperature required for retorting. Large beets and large stalks of asparagus cook more slowly than smaller ones.
    2. Pieces that cook apart and become mushy or viscous: They heat slowly because the majority of heat is absorbed through conduction, rather than convection. This is the case with cream-style corn, squash sweet potatoes, and pumpkin.
    3. Pieces that layer: Hence convection currents flow generally between the two. Spinach is horizontally layered, resulting in the “baffle-board effect” that disrupts convection currents. The layering process is greatly affected by the amount of filling in the can.
  6. Rotation and agitation: Rotation and agitation in the food’s container during the process of heating will increase heating when the food is at the level of fluid. However, it could also trigger undesirable physical changes in certain food items. There is a minimal distinction in the time of processing of foods that are able to allow free convection currents , and also have extremely small particles, like peas. However it is extremely beneficial to agitate for foods that are layered with other ingredients, such as tomatoes, spinach as well as peach half halves. In the case of older equipment, it’s not feasible to move cans at speeds greater than 10-12 rpm, but modern methods of end-overend rotation allow for greater speeds. Rotation is a great option for evaporated milk in cans as well as shaking for food items in the form of purees or pastes. The brined process used for whole-kernel corn involves heating in a continuous cooker with high boiling liquid and the contents of the can mixed , by being placed on the edge of a spinning reel or through rotation on rollers.

Heat Treatments Employed In Processing Foods

  • The amount of time and temperature for heat-processing food is contingent on the impact heat can have on the food as well as what other methods for preserving food are used.
  • Certain food items, like peas and milk are able to be heated only to an extent, without causing changes in appearance or in palatability. However, other items such as pumpkin or corn are able to undergo more intense heat treatment with no any noticeable change.
  • The higher the degree of heat treatment the higher the temperature, the more organisms will be killed, until the temperature that causes sterilization of the food. If none of the organisms have been killed, or the heating process must kill any spoilage organisms that could be present or the food should be treated followingward in order to stop or delay the growth of the remaining spoilage organisms.
  • When canning, an effort is to eliminate any organism that may spoil the food when it is later handled. pasteurization, the majority spoilage organisms are killed , but some survive and need to be prevented by low temperatures or other method of preservation if the risk of the food is to avoid spoilage.
  • The various degrees of heat that are used to cook food could be classified as
    1. Pasteurization
    2. heating at about 100 C, and
    3. heating above 100 C.

1. Pasteurization

  • Pasteurization is a treatment that kills some but not all microorganisms that are present. It usually requires the use of temperatures lower than 100 C. The heat treatment can be accomplished by steam or dry heat, hot water or electrical currents and the product is then immediately cooled after the treatment.
  • Preservatives used to aid in pasteurization are (1) refrigeration, e.g. or of milk (2) eliminating microorganisms usually through the use of a sealed container (3) maintaining anaerobic conditions as in sealed, evacuated container, (4) adding large amounts of sugar, such as sweetened condensed milk and (5) presence or the addition of chemical preservatives e.g. organic acids in pickles.
  • The duration and temperature used in the process of pasteurizing are based on the process employed as well as the type of product being treated.
  • The high-temperature-short-time (HTST) method employs a comparatively high temperature for a short time, whereas the low-temperature-long-time, or holding (LTH), method uses a lower temperature for a longer time.
  • A few examples are provided of pasteurizing treatment given to various kinds of food. The minimum heat treatment for market milk is 62.8 C for 30 min using the holding method. It is in the holding method, at 71.7 C for at least 15 seconds in the HTST method, and in the HTST method, at 137.8 C for at least 2 seconds in an ultrapasteurized process.

Use of Pasteurization

  • where more intensive heating treatments could harm any quality or flavor of product like with milk from markets.
  • When the goal was to eliminate pathogens such as in the case of market milk
  • in cases where the primary spoilage organisms aren’t resistant to heat, like the yeasts found in juices of fruit,
  • Any spoilage organisms that survive are removed through additional methods of preservative to be used, such as when chilling market milk and
  • When competing organisms need to be eliminated, which allows the desired fermentation to occur, typically through the addition of starter organisms such as cheese production.

Methods of pasteurization

The duration and the temperature of the process of pasteurization depend on the method used and the material that is used.

  • High temperature and short time (HTST )method – This method uses relatively high temperatures for a brief time . For milk, the temperature is 72degC for 15 seconds.
  • Low temperature and higher time (or) Holding method (LTH) method – Itutilizes a lower temperature to allow to last for a longer duration . For milk , it’s 62.8degC in 30 mins.
  • They are comparable and can eliminate all molds, yeasts Gram negative bacteria and numerous gram positives and are the most resistant to heat of the nonspore-forming bacteria that cause disease Mycobacterium tuberculosis as well as Coxiella burnetii.
  • The treatment for pasteurization of juices from fruit depends on their acidity as well as whether they are bulk , in bottles or in the can.

Limitations

However this method kills bacteria or pathogens that are present in the milk, which ensures that it is safe to drink without risk to health, but there are some limitations that are that are discussed in the following sections:

  • Due to the loss of specific enzymes from food during pasteurization, some think raw dairy is superior over pasteurized milk.
  • There is a belief that pasteurizing milk using the HTST method could reduce by 1/3 the amount of thiamine that is present in milk and half of vitamin B12.
  • The survival of heat resistant pathogens is increasing the likelihood of bacteria being present even after pasteurizing food.

2. Heating at about 100 C

  • In the past, home canners were able to process all food items for various durations at temperatures of the temperature of 100 C at or lower. This was enough to kill all bacteria, but not the spores of bacteria in the food, and was often sufficient to preserve lowand medium acid foods.
  • Today, however, many home canners are using pressure cookers to process foods that are less acidic. A lot of acid food items are able to be processed at temperatures of 100 C or lower, like sauerkraut, highly acid fruits and so on.
  • A temperature of around 100 C is achieved through boiling food that is liquid by submerging the food’s container in boiling water or exposure to steam flowing.
  • Certain very acidic foods, e.g., sauerkraut could be preheated to a temperature that is less than 100 C in order to be packaged hot and not further heated-processed.
  • Fresh vegetables are blanched prior to drying or freezing requires the heating of the vegetables for a few minutes at around 100 C.
  • When baking, the temperature of cakes, breads or other bakery goods can be reached but does not reach 100 C when there is moisture present, even though the oven’s temperature is higher. The temperature of canned foods that are cooked in the oven should not surpass the boiling point of the liquid in.
  • As will be demonstrated later in the next section, spores from bacterial organisms that can remain in the bread baking process (maximal temperature around 97 C) can cause the ropiness.
  • Simmering begins with gentle boiling, at approximately 100 C.
  • When roasting meat, the temperature of the meat is 60 C in the case of rare beef. It can go up to even up to C in well-cooked beef, and up to 85 C for a roast of pork. Frying can make the outside of the food extremely hot, but the inside typically does not exceed temperatures of more than 100 C. It is an undefined term that has no significance.
  • Within the industry of food the term “cook” refers to a particular time and temperature that is required for an industrial process. Heating up food could refer to anything from a slight rise in temperature, to heating up to 100 C.

Methods include

  1. Baking
  2. Simmering (gentle boiling with the temperature about 100o C.
  3. Roasting
  4. Frying
  5. Cooking
  6. Warming up
  7. Blanching

3. Heating above 100 C or Sterilization

  • Temperatures of more than 100 C typically are achieved by steam pressure inside steampressure sterilizers, or Retorts.
  • The temperature of the retorts rises with increasing steam pressures. In other words, with no pressure, the sea temperature is 100 C and with 5 lbs of pressure at the temperature is 109 C when 10 lbs, 115.5 C; and with 15 lbs, 121.5 C.
  • If liquid food items are to be sterilized prior to their introduction into sterile containers using high steam pressures, they are employed to maintain an extremely high temperature for couple of minutes.
  • The milk can be heated at temperatures as high as 150 C using steam injection , or infusion. This is followed by “flash the evaporation” of condensed steam as well as rapid cooling.
  • Processes such as this for milk have been referred to as ultrahigh-temperature, or UHT, processes. With enough holding time in the range of several minutes, this process could “sterilize” milk.
  • The use of heat treatments for processing canned food will be addressed further in the next section.

4. Canning

  • It could be described as ” the process of preserving food items in sealed containers that generally, heat treatment is the main element in the preservation of food and the destruction of every kind of microorganism that exists, like Clostridium Botuliniumspores”.
  • Canning (also called the sealing of hermetically-sealed containers) is made in glass containers, aluminum and plastic pouches.
  • The process of canning was invented through Nicolas Appert (called father of canning).
  • When it comes to canning the process involves careful preparation of food that is packed into an sealed glass, tin or plastic containers that are placed under a defined temperature (above 100oC) for a specific duration of time before being cooling.
  • While heating, there is elimination of oxygen, and a the sealing process is hermetic to prevent post-process contamination. Also, it is essential to boil the food inside the container in order to kill microbes before sealing the container (either in the beginning or when the food is still in the boiling process) to limit and stop any new microorganisms being introduced into.
  • Following the thermal process of the container, it must be immediately cooled to a temperature of around 38oC in order to avoid any adverse effects of heat on flavor, texture or texture of the food items.
  • So, it sterilises the food and allows it to remain for a longer time without the risk of spoilage caused by unwelcome microorganisms.

The Canning Procedure

a. The Heat Process

  • The goal of the canner is complete sterilization of all food items, however, it isn’t always able to achieve it.
  • Instead of eliminating all microorganisms that are present in the foods, the process could eliminate all the bacteria that can cause food to spoil under normal storage conditions and could leave some incapable of growing, which makes the food can “commercially sterilized,” “practically sterilized,” or “bacterially inactive.”
  • The heating processes required to preserve canned food are based on factors that determine the resistance to heat of the organisms that are most resistant to spoilage and the factors that affect the rate of heat penetration.
  • If the retort temperature is higher, the timeframes would be reduced and the process will differ based on the types of canned food items and the sauces utilized in the can, the dimension and form, beginning temp of the meal and many other variables.
  • The cooking time of food made up of fragments as well as a finely split element in brine or water is reduced with the help by the “Strata-Cook” method.
  • The ingredients, e.g., creamed whole-grain corn, whole-grain and brine are separated from each other inside the container and kept separate when the process of heating.
  • This method makes use from the reality that tiny variations in the water content of food that is heated by conduction don’t have much impact on the speed of heat absorption and the thin layer of food is more likely to heat up than one that is thicker.
  • The fact that it does not dissolve also assists in the penetration of heat.
  • Acidic foods require less heat than foods with neutrality. Certain very acidic foods like sauerkraut can have a hot packaging and need no heating. Certain food items, e.g., globe artichokes, which can be damaged due to high temperatures, are acidified , and transformed at lower temperature.

Methods of Heating

  • The heating is usually carried out in retorts, either using steam or not, depending on the food needs. The HTST heat process, which is currently employed for certain fluid food items require special equipment to sterilize the food in bulk, sterilizing lids and containers, and sealing and filling the containers that are sterile under safe conditions.
  • The Dole method is an illustration of the HCF or heat-cool-fill method.
  • In the SC which is also known as sterilizing and sealing process, sterilization of food is done prior to when sealing the container.
  • In the PFC, or pressure-fillercooker, method, the food is sterilized by high-pressure steam and filled into the can; then the can is sealed and the heat processing is continued as long as necessary before cooling. When dehydrocanning is used, e.g., of apple slices foods are dried down to approximately half of its original weight prior to the canning process.
  • Another method of heating cans is through directly gas-fired flame steam injection, or by heating in the form of a fluidized mattress of fine solids as well as by using the hydrostatic sterilizer. This comprises an upright tank that has conveyors that move cans through a water pipe, upwards into live steam then out and up through another water leg.
  • When using the “Flash 18” method the canning process is carried out in the very high-pressure (18-psi) chamber.
  • The product receives an HTST treatment in order to bring it up to temperatures for processing. The cans are then filled and sealed and partially cooled within the chamber.
  • The use of heat is also being incorporated with other preservative agents, e.g., antibiotics radiation, chemicals, or, e.g., hydrogen peroxide. So far, the majority of this research is experimental.

b. Pressurized Packaged Foods

  • The pressurized liquids or pastes also known as aerosols, are packed under the pressure of the gas that propels them, which is usually nitrogen, carbon dioxide, or nitrogenus oxide, to disperse the food as spray, foam or liquid. A variety of foods are being packed in this manner, e.g., whipped cream and other toppings drinks concentrates, salad dressings and condiments, oils jellies, flavoring ingredients.
  • The foods that are pressurized are susceptible to microbial spoilage , unless appropriate preservation techniques are employed.
  • Acid food items can be cooked or canned and then gassed. However, the process can cause food contamination.
  • Canning with aseptic technology is an option for lowand medium acid food items. The requirements for processing pasteurized food, e.g., whipped cream, and other toppings are comparable to those with or without gas pressure.
  • The gas utilized for propellants could affect the types of organisms that are that are likely to develop. Nitrogen, for instance, does not block aerobes when the presence of oxygen was minimal while carbon dioxide could inhibit aerobes in the same situation.
  • Carbon dioxide under pressure kills numerous microorganisms, such as molds and aerobic bacteria however, it does not hinder the lactic acid bacteria Bacillus coulans Streptococcus Faecalis or even yeasts. Nitrous oxide suppresses a variety of fungi.

c. The Cooling Process

  • After an application of the heat the food containers are chilled as fast as is practical.
  • The cans can be chilled in the retort, or in tanks through the immersion in cold water or spraying water.
  • Large cans and glass containers are chilled more slowly to reduce strain and even breaking.
  • This process requires the use of hot (or spray) water (or spray) and the temperature of which decreases as cooling proceeds.
  • The final cooling of containers typically occurs through air flow.

Two approved methods of canning are

  1. Water-bath canning: Water-bath canning, often referred to as boiling-water method of canning , or called hot water canning. It is the simplest and most efficient method to preserve high-acid foods. The jars are submerged into the water, and then warmed up until an appropriate temp. (100oC) during a specified amount of time. This procedure is enough to kill yeasts, molds enzymes, and certain bacteria, making it safe to consume in the future . For instance, acid-based foods like fruit butters and spreads, fillings for fruit pie sauerkraut, pickles, jams, pickled vegetables, and jellies are safe to be made through the boiling process of water bath canned. This method can’t be used to processing foods with low acidity since it does not reach a the temperature of super-high temperatures to kill heat-resistant bacteria or the heat-stable toxins.
  2. Pressure canning: Pressure canning is the process of pressure canning the kettle is large and steam is created in a compartment that is locked. The jars that are filled with steam in the kettle are heated to the temperature of 116oC when under the pressure of a certain amount, which is determined by a gauge. It’s useful for processing vegetables and other foods with low acidity (i.e. meat, poultry, seafoods etc.). In this process, C. botulinum (the bacteria that causes botulism poisoning in food) is eliminated in foods with low acid when they are prepared according to the proper time and temperatures inside pressure canners.

References

  • Food Preservation by High Temperatures. (2016). Food Microbiology: Principles into Practice, 12–33. doi:10.1002/9781119237860.ch28
  • Jay, J. M. (1998). High-Temperature Food Preservation and Characteristics of Thermophilic Microorganisms. Food Science Texts Series, 347–369. doi:10.1007/978-1-4615-7476-7_16 
  • https://courseware.cutm.ac.in/wp-content/uploads/2020/06/C7-2.pdf
  • http://www.jiwaji.edu/pdf/ecourse/microbiology/Food%20Preservation%20by%20use%20of%20high%20temp.pdf
  • https://www.open.edu/openlearncreate/mod/oucontent/view.php?id=196&section=8.4.3
  • https://www.slideshare.net/eishashahid1/food-preservation-or-food-preservation-by-high-temperature
  • Jay, James. (2000). High-Temperature Food Preservation and Characteristics of Thermophilic Microorganisms. 10.1007/978-1-4615-4427-2_17. 
  • https://www.scribd.com/presentation/143172055/Preservation-by-Use-of-High-Temperature
  • https://nofima.com/worth-knowing/food-preservation/#:~:text=Sterilization%20uses%20the%20most%20extreme,pathogenic%20bacteria%20in%20the%20food.
  • http://ecoursesonline.iasri.res.in/mod/page/view.php?id=111436
  • https://www.brainkart.com/article/Preservation-by-High-Temperature_33479/
  • https://biologyease.com/preservation-by-high-temperature/
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MN Editors
This article writter by MN Editors on August 02, 2022

Microbiology Notes is an educational niche blog related to microbiology (bacteriology, virology, parasitology, mycology, immunology, molecular biology, biochemistry, etc.) and different branches of biology.

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