Thermal Preservation of Foods
Overview To understand the concepts that form the basis of thermal preservation of foods, you must become familiar with the associated terminology. In this lesson you will learn the meaning of terms such as blanching, pasteurization, commercial sterilization, D-value, Z-value, F-value, hermetically sealed containers, decimal reduction time and 12D concept of safety in the context of thermally processed foods. You will also learn about the containers that are used to package thermally processed foods.
ObjectivesThe overall goal of this lesson is that you gain an appreciation of how various food commodity groups are preserved through the application of thermal energy. More specifically, you will be able to:
- describe the scientific principles upon which the production of thermally processed foods are based;
- explain the terminology of importance for the thermal preservation of food systems
- differentiate between conduction and convection heating of foods during thermal processing of foods; and
- list the fundamental requirements of packaging materials used for thermally processed foods.
Link to Videos hosted on Connect for Students enrolled in FNH 200 102 You will need Flash to view the video in Connect. If you are already login, the link will take you to the video. If not, it will ask you to log into Connect first.
- Milk Pasteurization, about 10 minutes. The last few minutes is a summary.
- Butter Making, about 3 minutes. I must admit that it's not too relevant this lesson.
Videos on Youtube: How It's Made Tetra Pak Containers
Tetra Pak® A6 - Meet the filling machine for Tetra Evero® Aseptic
Amateur Videos Made by Judy: To review 'lectures' on thermal death curves, you may also watch these short video:
Logarithmic Graph: An Intro http://screencast.com/t/R7GNP2C4GK
Thermal Death Rate Curve - D value http://screencast.com/t/XeaptkN8z
Thermal Death Rate Curve - Different Microbs http://screencast.com/t/IeBP37JzH
Thermal Death Rate Curve - Different Temperatures http://screencast.com/t/sDY5fkQjSrT
Thermal Death Time Curve - Z-value http://screencast.com/t/rGa8T3Cv
Thermal Death Time Curve - Z-value Application (I made a mistake around 3:30. The Z-value is 40 oC, not 40 min.) http://screencast.com/t/I48gvPYa
Terms Used in Thermal Preservation The safety and storage life of many perishable foods can be enhanced by the use of high temperatures to inactivate undesirable disease and spoilage-causing microorganisms and to inactivate enzymes in food that can cause spoilage. Three categories of thermal preservation of foods are:
- commercial sterilization
In this section, we will discuss a number of terms and concepts commonly used when dealing with thermal preservation of foods on a commercial basis.
- 1 Blanching
- 2 Pasteurization
- 3 Commercial Sterilization (CS)
- 4 Determining the Appropriate Temperature and Time Needed
- 5 Heat Transfer Characteristics of Foods
- 6 Material Used for Packing Thermally Processed Foods
Blanching is a form of thermal processing applied mainly to vegetables and some fruit by exposing them to heated or boiling water or even culinary steam for a short period of time. Blanching is a food processing operation designed to:
- Inactivate enzymes in plant tissues so that enzymatic degradation does not occur in the interval between packaging and thermal processing or during frozen storage or in the early stages of food dehydration and after reconstitution of dehydrated plant foods.
- Wilt vegetable products to enable packing of the products into containers so that proper fill weights can be achieved.
- Drive off inter- and intracellular oxygen and other gases from plant tissues so that containers are not deformed by excessively high internal pressures due to expanding gases within the container and to permit formation of a vacuum in the container after thermal processing.
Pasteurization is a thermal process that involves using temperatures of at least 72°C for 15 seconds (high temperature short time or HTST process), prior to packaging.
Acid food products (pH < 4.6) are mainly pasteurized to inactivate spoilage-causing microorganisms. Pathogenic microorganisms cannot grow and do not survive very well in acid foods such as citrus juices or apple juice (with the exception of Escherichia coli 0157:H7, which will be discussed in Lesson 12).
In low-acid food products such as milk, the basis for preservation by pasteurization is to inactivate pathogenic (disease causing) bacteria and viruses. Many spoilage-causing microorganisms can still survive typical pasteurization process conditions:
- For example, in milk, the proteolytic and lipolytic bacteria are more heat resistant and can survive the pasteurization process. This explains why the typical spoilage pattern of pasteurized milk reflects the proteolytic (protein degradation) and lipolytic (lipid degradation) action of the psychrotrophic, spoilage-causing bacteria.
- Because pasteurization does not kill all the psychrotrophic spoilage-causing bacteria in milk, pasteurized milk must be refrigerated to maintain shelf life quality.
The durable life date on milk containers reflects the storage life that can be expected when milk is held at 4 °C or lower.
Commercial Sterilization (CS)
The basis for preservation by Commercial Sterilization (CS) is to destroy both spoilage and disease causing microorganisms in low-acid and acid foods, thus rendering the food "commercially sterile".
Commercially sterile as described in the Food Regulations (Division 27) of the Food and Drugs Act of Canada "means the condition obtained in a food that has been processed by the application of heat, alone or in combination with other treatments, to render the food free from viable forms of microorganisms, including spores, capable of growing in the food at temperatures at which the food is designed normally to be held during distribution and storage". Therefore, commercially sterilization involves the destruction of spoilage-causing and disease-causing microorganisms. Commercially sterile foods may contain small numbers of thermophilic bacteria spores; however, the spores cannot germinate and produce actively growing cells at room temperature.
Canning involves heating the food with a minimum treatment of 121°C moist heat for 15 minutes.The process usually involves pre-sealing the food in containers prior to heating. Canning can be traced back to the early 1800's. It is called the"botulinum cook". Today, if a can of food is being sterilized, each food particle must receive the heat treatment (e.g. 121°C for 15 min).
When food is placed in a can, the heat treatment will change since heat transfer to the food takes place at a slower rate. Depending on the size of the can, the time to achieve sterility could be several hours. Most commercially sterile products have a shelf life of 2 years or more.
- Question: will commercially sterile products require a durable life date stamped on the label.
Ultra-High Temperature Processing (UHT) and Aseptic packaging
The combination of UHT and aspectic packaging is another form of commercial sterilization. It involves the application of "ultra high temperature" (heat) to food before packaging, then filling the food into pre-sterilized containers in a sterile atmosphere. This process will render the food shelf stable or commercially sterile without the need for refrigeration. UHT- Aseptic packaging is a relatively new development whereby food can be heated to 140-150°C very rapidly by direct injection of steam, held at that temperature for short period of time (e.g. 4-6 seconds) and then cooled, in a vacuum chamber to flash off the water added in the form of condensed steam. This is carried out as a continuous flow operation. The decrease in processing time due to the higher temperature, and the minimal come-up time and cool-down time leads to a higher quality product.
The UHT processed food is packaged aseptically into pre-sterilized containers. These are usually cartons made from laminated plastic, aluminum and paper, which are chemically sterilized with a combination of hydrogen peroxide and heat, and then filled in the same piece of equipment which is housed in a sterile environment. For more information about these cartons, visit the "Tetra Pak" aseptic technology (www.tetrapak.com).
There are other forms of packaging that can also be used in aseptic UHT processing: plastic cans, flexible pouches, thermoformed plastic containers, bag-in-box, and bulk totes. UHT-aseptically packaged products have a shelf life of 6 months or more, without refrigeration. It depends on the type of packaging being used. For example, Tetra Pak cartons can eventually be more prone to perforations in the packaging layers, whereas the newer plastic bottles are more resistant to pin hole formation allowing them to have a longer shelf life.
Some examples of food products processed with UHT are:
- liquid products: milk, juices, cream, yogurt, wine, salad dressings
- semi-liquid/solid products: baby foods; tomato products, fruits and vegetable juices, soups
Contrary to popular opinion, UHT processed milk and juices do not contain added agents to provide the long storage life at ambient temperature in the laminated cartons. The products are preserved solely through the application of heat. It is critical that the sterilized products are transferred to packaging equipment under aseptic conditions, to avoid contamination after thermal processing. Please note that many products that are UHT treated are not aseptically packaged. This gives them the "advantage" of a longer shelf life at refrigeration temperatures compared to conventional pasteurized products. However, this does not produce a shelf-stable product at ambient temperatures due to the possibility of post-processing recontamination.
- Questions: Will UHT products require a durable life date stamped on the label?
- What about products that have been UHT processed but not packaged under aseptic conditions?
Selection of Heat Treatments
The intensity of the heat treatment employed for a particular food preservation application depends upon a number of factors. The main considerations in selecting the required temperature-time conditions for thermal processing are:
- What is the objective or purpose? (blanching or pasteurization or commercial sterilization)
- Are there additional preservation steps? (is it combined with other preservation method?)
- What are the physical, chemical properties of the food? (Type of food)
- What is the heat resistance of microorganisms in the food?
Here are some examples to illustrate these points:
Foods that will be consumed within a short period of time after processing can have storage life extended by a combination of pasteurization and refrigerated storage (used for pasteurized milk and for pasteurized, vacuum packaged, cured meats). Longer storage times at ambient temperatures in evacuated sealed containers requires the use of commercial sterilization. The time-temperature combination required for pasteurization and commercial sterilization is determined by the most heat-resistant disease-causing and spoilage-causing microorganisms in the particular food commodity.
For a particular food commodity, the type of thermal processing operation and the rate of heat penetration into the slowest heating portion of the food within a particular container are governed by the food's physical properties (solid vs. liquid, or solid particles suspended in a liquid) and chemical properties (pH, fat content, presence or absence of heat-inducible thickening agents, food components that have protective or antagonistic effects on the thermal resistance of microorganisms). It is imperative that thermal preservation processes be designed so that the slowest heating portion of the food commodity receives the specified time-temperature thermal treatment to minimize risks of illness and/or post-processing spoilage.
The thermal processes applied to foods are governed by the heat resistance of the microorganisms in the food. In low acid foods which are to be thermally processed and vacuum sealed within gas-tight containers, the microorganism of most concern is Clostridium botulinum. The habitat of Clostridium botulinum can be soil (agricultural and forest), water (fresh, brackish and marine) and mud (fresh water and salt water). As a consequence, all foods of agricultural and fisheries origin must be considered as being potentially contaminated with Clostridium botulinum spores.
Thermal preservation treatments of low acid foods that are to be packaged and stored under anaerobic conditions must be designed to ensure the destruction of any Clostridium botulinum spores that may be present in the food with the provision of a wide margin of safety. Actively growing vegetative Clostridium botulinum cells produce a very potent neurotoxin.
Destruction of C. botulinum Spores
To determine the thermal resistance of heat-resistant spores in foods, "Inoculated pack studies" are carried out using a non-pathogenic spore-forming bacterium, Clostridium sporogenes PA3679 (a putrefactive anaerobe). Since PA3679 spores are more heat resistant than those of Clostridium botulinum spores, a process designed to kill PA3679 spores will definitely kill Clostridium botulinum spores with a wide margin of safety. The concept of "margin of safety" is later described in this lesson.
Determining the Appropriate Temperature and Time Needed
Thermal Death Curves
When microorganisms (bacteria, such as vegetative cells and spores, moulds, viruses and yeasts) are exposed to high temperatures capable of causing death of the organisms, one observes that the population is not killed instantaneously. We see that microbial death during thermal processing follows a logarithmic order. This means that bacteria are killed by heat at a rate that is nearly proportional to the number present in the system being heated.
The survivor curve or thermal death rate curve plotted in Figure 6.1 depicts the logarithmic order of death. You will note that the time taken to traverse one logarithmic cycle represents the time, at a constant temperature, required to kill 90% of a microbial population.
The exposure of microorganisms to a specific temperature for a period of time required to kill 90% of the population is defined as the decimal reduction time or D-value.
A sample calculation of the decimal reduction time follows:
If we were to start out with a population of 105 (=100,000) bacterial cells in a unit volume or mass of food at time 'A', only 104 (=10,000) cells would survive after one logarithmic cycle on the graph was traversed (time 'B'). The reduction in the number of survivors from 100,000 to 10,000 represents a 90% decrease in the number of survivors, as shown below:
- % Reduction in survivors
- = (Survivors at time 'A' - Survivors at time 'B')/Survivors at time 'A' x 100%
- = (100,000 survivors - 10,000 survivors)/100,000 Survivors x 100%
- = 90,000 Survivors / 100,000 Survivors x 100%
- = 90%
Assume that time 'A' is 5 minutes and that time 'B' is 10 minutes. During the time (10-5 = 5 minutes) that the survivor curve traversed one logarithmic cycle, 90% of the microorganisms were killed (10% survived) by the exposure to the constant temperature. The time taken to kill 90% of the microbial population was 5 minutes. The D-value at that particular temperature was 5 minutes.
Note from the preceding discussion that the D-value is a useful index of the heat resistance of a particular microorganism to the killing effects of heat at a particular temperature. However, also note that D-values apply to a specific microorganism under a specified set of conditions (temperature, type of food).
What effects do different conditions have on the D-value? If the temperature were increased, the D-value would decrease because the rate of microbial death would increase. The magnitude of the D-value depends on how the constituents of the food affect the sensitivity of the microorganism to the killing effects of heat. The protective effect of food constituents are discussed in more detail later in this lesson. Now think about the following question:
Thermal Death Time Curves
While the thermal death curve describes the rate of death of a particular microorganism under a specified set of conditions. A thermal death time curve (figure on right) can be constructed from a number of thermal death curves by exposing the microorganism to a variety of temperatures and determining the decimal reduction time at each temperature.
The thermal death time curve provides information about the time required to kill a particular microorganism in a particular food at a variety of temperatures. Take a close look at the figure on the right. For example, at a temperature of ~115°C, 10 minutes are required to kill the population of this specific microorganism.
All of the time-temperature combinations along the plotted thermal death time curve represent the same killing power, with lower temperatures requiring longer time of exposure. You should also note that any point above the line (e.g., 100 minutes at 110°C) will ensure that the microorganism is killed, while time-temperature combinations that fall below the plotted line (e.g., 100 minutes at 104°C) represent conditions that will not kill all of the microorganisms present.
You should also note in the thermal death time curve that vegetative bacteria cells have a much lower heat resistance than spores.
These two terms characterize a thermal death time curve. Potter and Hotchkiss (1995) define the z-value as "the number of degrees required for a specific thermal death time curve to pass through one log cycle". The z-value indicates the resistance of a microbial population to changing temperature. Different microorganisms in a given food will have different z-values. Similarly, a given microorganism will have different z-values in different foods.
The F-value, is a mathematically calculated number that describes the total lethal effects of the process at a specific temperature. The standard reference temperature is generally selected as 121.1°C (250 °F, temperature of steam under industrial pressure), and the relative time (in minutes) required to sterilize any selected organism at 121°C is known as the F-value of that organism.
In high acid food, we aim for a 100,000 times reduction of selected organism. Therefore F-value is 5 time the D-value at a specific temperature, usually at 121.1 °C. In low acid food, we aim to a 1,000,000,000,000 times reduction of selected organism. Therefore, the F-value is 12 times the D-value at a specific temperature, again usually at 121.1 °C.
From Potter and Hotchkiss (1995): the F-value "is the number of minutes at a specific temperature required to destroy a specified number of organisms having a specific z-value". The F-value is the equivalent of all heat considered with respect to its capacity to destroy spores or vegetative cells of a particular microorganism. In other words, it is a measure of "lethality" or the capacity of the heat treatment to sterilize.
Margin of Safety
Inherent in the thermal processing of foods is the concept of a margin of safety which refers to the probability that a container of food could still contain a viable spore of Clostridium botulinum after the completion of the thermal processing. Obviously, the goal is to ensure that the margin of safety is as large as possible (i.e., the probability that a spore of Clostridium botulinum survives the thermal process is as low as possible without causing undue heat damage to the quality factors and nutrient value of the food).
Since Clostridium botulinum is ubiquitous in the environment where food materials are grown and harvested, the assumption is made that all foods to be preserved are potentially contaminated with Clostridium botulinum spores and thus must be processed accordingly.
As the spore population in a food system is increased, the time required at a particular temperature to kill all the spores increases. This phenomenon is inherent in the logarithmic order of death of microbial cells and spores. During preparation of foods for processing, efforts are made to minimize the microbial population in the foods to be processed. Microbial and spore populations can be minimized by proper handling techniques, the use of extensive washing, and the use of peeling and trimming procedures to remove as many as possible of the spores that may be present on the food.
Typically for low acid foods (pH greater than 4.6) a thermal process of 12D is applied. A low acid food (e.g. milk, meat, poultry, fish, vegetables) is subjected to a thermal process so that the slowest heating portion of the food is exposed to an amount of thermal energy such that the microbial spores present experience the equivalent of 12 successive decimal reduction times. This has the capability of killing 1012 (one trillion) spores of Clostridium botulinum per container. Since the natural levels of contamination of foods with C. botulinum are much lower than that, a large margin of safety is introduced for thermally processed foods. The D-value is temperature dependent. Thus, the higher the temperature the lower the D-value and the less time taken to achieve the 12D "botulinum cook" for low acid canned foods.
The 12D heat treatment employed for low acid foods would be excessive and unnecessary for acid foods (pH of 4.6 or less). With acid foods, temperatures at or below 100°C for a few minutes should be an adequate heat treatment. Typically, a 5D thermal process is usually used for acid foods. Another important factor to be considered is that C. botulinum is very sensitive to acid and it will not grow in foods at pH 4.6 or below.
Heat Transfer Characteristics of Foods
The rate and mechanism by which heat is transferred through a food material during thermal processing is very important in determining how long it will take the slowest heating part of the food (the cold point) to reach the desired time-temperature combination required to ensure killing of Clostridium botulinum spores with an adequate margin of safety. Factors that affect the heat transfer characteristics of a food are
- the consistency of the food (liquid -viscous or non viscous- or solid)
- the chemical composition of the food.
- other factors that are important are the container size, shape and composition.
Foods that are thermally processed after being packed in containers (such as metal cans, glass bottles, plastic pouches) are exposed to an environment of pressurized steam within a vessel called a retort.
A retort operates very much on the same principles as the pressure cooker or pressure canner with which you may be more familiar. Packages of foods are placed in the retort after which the retort is sealed and the air within is vented by purging the retort with steam. Once the retort is properly vented, the steam pressure inside is increased to achieve the desired processing temperature.
A processing temperature of 250°F (121°C) is achieved by establishing a steam pressure of 15 pounds per square inch within the retort. In comparison, the normal household pressure cooker and pressure canner operate at 10 pounds per square inch steam pressure (116°C or 214°F). The containers of food in the retort are bathed in an atmosphere of hot, high pressure steam. Heat is transferred by the hot steam condensing on the containers of food with the heat transferred through the walls of the containers (glass, metal or plastic) to the food inside the container. If the food is a solid (salmon, for example), heat energy is transferred by conduction (Figure on right).
Heating food by conduction is a slow process. The cold point (illustrated in Figure 6.4) is in the centre of the container if the container is cylindrical (e.g., canned salmon). Foods that are non-viscous liquids (canned evaporated milk, for example) heat by convection.
Foods that are a combination of solids and liquid components heat up by a combination of convection and conduction heating. Note the location of the cold points in relation to conduction heating (in the geometric centre of a cylindrical container) and convection heating (one third of the way up the centre axis measured from the bottom of the cylindrical container). The cold point of a container of food must receive the required amount of thermal energy to ensure killing of Clostridium botulinum spores that may be present and to ensure a sufficient margin of safety.
You will appreciate that if a food formulation is changed such that the mechanism of heat transfer is altered from convection heating to conduction heating, and if the processing times are not altered to accommodate the change in mechanism of heat transfer, the food could be under processed and could pose a potential health hazard with respect to botulism. In the past, outbreaks of botulism have occurred due to such changes in heat transfer resulting from changes in product formulation or in the changes in product piece size which can affect the rate of heat transfer.
Protective Effects of Food Constituents
Sugars, oils, fats and salt can have the effect of protecting spores and vegetative cells from the killing effects of heat, thus requiring use of longer exposure times or higher temperatures for processing those foods exhibiting these protective effects. Certain spices, increased acidity of foods, and some antimicrobial agents have the effects of decreasing the resistance of microorganisms and spores to the killing effects of high temperatures (D and z values are decreased). Food processing companies must be very careful in re-evaluating the lethality of the thermal processes they used after they reformulate foods that are preserved by thermal processing (commercial sterilization, pasteurization).
When canning foods at home, be sure to process all low-acid products in a pressure canner following the manufacturer's instructions closely. Any deviation from those instructions could substantially decrease the margin of safety of the process being used.
According to the Canadian Food Inspection Agency, "Improperly prepared home-canned, low-acid foods (e.g. corn, green beans, mushrooms, spaghetti sauce, salmon) are most likely to represent a risk for botulism. Outbreaks of botulism have also occurred in Canada's Inuit populations when people have eaten improperly prepared raw or parboiled meats from marine mammals."
Material Used for Packing Thermally Processed Foods
Steel Body Cans
Steel body cans with a thin layer of tin ('tin cans") are the most widely used containers. They withstand high temperatures and pressure differentials and are not readily breakable. The can lids provide a good indication of the presence of a vacuum and thus a hermetic seal (a seal that is impervious to the transmission of gases, water and microorganisms). The steel, and very often the tin plating, must be protected with lacquers to minimize reaction of the metals with food constituents.
Cans can be made with easy open features. Some have hinged covers or slip-on covers for easy access. Paint cans often have a removable plug on the top for access and for reclosing.
Glass jars are more resistant to corrosion and reaction with food constituents and allow the consumer to see the contents in the container. However, glass is heavy and bulky and must be packaged with extra protection to prevent physical breakage of the glass during transportation. Glass filled containers must also be processed in the retort with extra care to prevent breakage due to thermal shock.
Sterile Cartons (eg. Tetra Pak)
Sterile cartons are made from laminated plastic, aluminum and paper. They are commonly known as Tetra Pak. Carton laminates are first sterilized by treatment with hydrogen peroxide followed by forming and filling of the cartons under sterile conditions. A chemical sterilizing agent must be used for this type of packaging material since exposure of the laminated material to high temperatures required for heat sterilization would destroy the packaging material.
If you have an UHT juice or milk container in your home you may wish to look at the laminate film after you have used the contents. Can you locate most of the layers of the laminate?
The retortable pouch is a relatively new form of packaging. The pouch is made of a laminate of plastic films and aluminum.
heat penetrates these pouches faster due to their thinner profile, allowing the 12D thermal process at the cold point to take place in a much shorter period of time than the conventional metal can or glass bottle.
Nutrient retention is superior in foods thermally processed in the retortable pouch. The pouch itself must be packaged in an outer protective carton to minimize physical damage to the food due to handling by the consumer and to avoid inadvertent puncture of the container.
New types of plastic packaging materials that can be produced in the shape of a can or bottle. The plastic materials can be hermetically sealed and thermally processed in a steam retort to achieve the 12D process for low acid foods. Newer plastic bottles can be used with UHT-Aseptic packaging technologies. Some of these containers are used for foods that are ready to eat. These containers, unlike metal cans and glass bottles can be placed in the microwave oven.
FNH 200 Course content on this wiki page and associated lesson pages was originally authored by Drs. Brent Skura, Andrea Liceaga, and Eunice Li-Chan. Ongoing edits and updates are contributed by past and current instructors including Drs. Andrea Liceaga, Azita Madadi-Noei, Nooshin Alizadeh-Pasdar, and Judy Chan.