Thermal Preservation of Foods Flashcards
What are the three categories of thermal preservation of foods?
blanching
pasteurization
commercial sterilization
What is blanching?
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.
What is blanching as a food processing operation designed to do?
- 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
What is pasteurization?
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.
What is the basis for preservation by pasteurization?
To inactivate pathogenic (disease causing) bacteria and viruses in low acid food products such as milk.
What food products are pasteurized?
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.
Can microorganisms survive pasteurization?
In low-acid and acid foods, many spoilage-causing microorganisms can still survive typical pasteurization process conditions:
- 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.
What bacteria in milk can survive the pasteurization process?
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.
What bacteria (classification) survives pasteurization?
Psychrotrophic spoilage-causing bacteria
What is commercial sterilization?
This thermal process 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 (also known as “canning”). Other forms of CS involve heating the food before it is aseptically packaged (UHT-Aseptic packaging).
What is the basis for commercial sterilization?
The basis for preservation by CS is to destroy both spoilage and disease causing microorganisms in low-acid and acid foods, thus rendering the food “commercially sterile”.
What does commercially sterile as described in the Food Regulations (Division 27) of the Food and Drugs Act of Canada mean?
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 extremely thermophilic bacteria spores.
True or false?
True
Commercially sterile foods may contain small numbers of extremely thermophilic bacteria spores; however, the spores cannot germinate and produce actively growing cells at room temperature, nor would they cause disease.
If a can of food is being sterilized, what heat treatment must each food particle receive?
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.
What is the shelf-life of a commercially sterile product?
Most commercially sterile products have a shelf life of 2 years or more.
What is the basis for ultra-high temperature processing?
The basis of UHT and aseptic packaging is 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.
What is ultra-high temperature aseptic packaging?
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.
How are UHT processed foods packaged?
The UHT processed food is aseptically packaged 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.
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.
What is the shelf-life of UHT-aseptically packaged foods?
UHT-aseptically packaged products have a shelf life of 6 months or more, without refrigeration. It depends on the type of packaging being used.
Give examples of food products processed with UHT.
- liquid products: milk, juices, cream, yogurt, wine, salad dressings
- semi-liquid/solid products: baby foods; tomato products, fruits and vegetable juices, soups.
UHT processed milk and juices contain added agents to provide the long storage life at ambient temperature in the laminated cartons.
True or false?
False.
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.
Describe the graph of microbial death during thermal processing.
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.
What does one logarithmic cycle represent?
The survivor curve or thermal death rate curve 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 time required to kill 90% of the microbial population exposed to a specific temperature 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’).
How do you calculate the D value?
What is the D value useful for?
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.
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 is a thermal death rate curve?
It describes the rate of death of a particular microorganism under a specified set of conditions.
A thermal death time curve can be constructed from a number of thermal death rate curves by exposing the microorganism to a variety of temperatures and determining the decimal reduction time at each temperature.
Note: vegetative bacteria cells have a much lower heat resistance than spores.
What is a retort and what is its function?
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.
How does a retort heat a solid food?
If the food is a solid (salmon, for example), heat energy is transferred by conduction
How does a retort heat non-viscous liquids?
Foods that are non-viscous liquids (canned evaporated milk, for example) heat by convection.
How does a retort heat foods that are a combination of solid and liquid components?
Foods that are a combination of solids and liquid components heat up by a combination of convection and conduction heating.
