Course:FNH200/Lessons/Lesson 06/Page 06.1

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6.1 Methods Used in Thermal Preservation

Terms to remember

  • Blanching
  • Pasteurization
  • Commercial sterilization
  • UHT processing
  • Aseptic packaging
  • Thermal Death Curves
  • Margin of safety
  • D-value, z-value, F-value
  • Inoculated pack studies
  • Tetra Pak
  • Retort
  • Cold point
  • Conduction
  • Convection

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:

  • blanching
  • pasteurization
  • 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.

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. Blanching is a food processing operation designed to:

  1. 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.
  2. wilt vegetable products to enable packing of the products into containers so that proper fill weights can be achieved.
  3. 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

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.

The basis for preservation by pasteurization is to inactivate pathogenic (disease causing) bacteria and viruses in low acid food products such as milk.

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 and acid foods, 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)

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).

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".

  • 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 extremely thermophilic bacteria spores; however, the spores cannot germinate and produce actively growing cells at room temperature, nor would they cause disease.
  • 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.

Ultra-high temperature processing (UHT) and Aseptic packaging:

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.

  • 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 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. 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 example 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 necessarily aseptically packaged. This gives them the "advantage" of a longer shelf life at refrigeration temperatures compared to conventional pasteurized (HTST) products. However, this does not produce a shelf-stable product at ambient temperatures due to the possibility of post-processing recontamination.

Video

You should now watch the video on milk and butter processing.

Note the following:

  • Three major processes used- clarification, homogenization, pasteurization
  • pasteurization of dairy products other than milk- the effect of other ingredients (e.g. in egg nog)
  • UHT process, aseptic packaging, Tetra Pak
  • salted versus unsalted butter- why does the latter have to be kept in the freezer?
Want to learn more?
  • Will canned food and aseptically packaged UHT food products require a durable life date stamped on the label?
  • What about products that have been UHT processed but not packaged under aspectic conditions?