Course:FNH200/Lessons/Lesson 05/Page 05.6

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5.6 Principles of Food Preservation

Food commodities are classified based on their shelf-life expectancy. Depending on the type of food and the type of preservation (processing) method used, their shelf-life can vary from a few days to several months or even years! Below is a summary of the three main classifications used: perishable, semi-perishable and shelf-stable foods:

Perishable Foods Semi-perishable Foods Shelf-stable Foods
  • Perishable foods are those foods that are not processed or are only minimally processed and have a shelf life of less than 60 days.
  • Spoilage of perishable foods is usually caused by microbial growth or senescence1
  • Examples of perishable foods are: meat, leafy vegetables, soft fruits, and milk.
  • Semi-perishable foods last between 2 to 6 months as a result of some form of preservation method.
  • Examples of semi-perishable foods are: ice cream, cheeses, and dry snack foods.
  • Shelf-stable foods have a shelf life greater than 6 months.
  • Examples of shelf-stable foods are: cereal grains, dehydrated pasta, some frozen foods, canned foods, and dehydrated vegetables.

1 Fruits and vegetables continue to respire after harvest. Respiration is fueled by carbohydrate metabolism that generates adenosine triphosphate (ATP) needed to promote various reactions in the tissues. When nutrients become exhausted, the tissues begin to deteriorate (soften, change colour, rot, produce off-odours). The deterioration is called senescence.

At the beginning of this lesson we learned about the many factors that contribute to food spoilage (deterioration). The following are the main goals for food preservation. This is just a brief introduction to the actual preservation methods that we will explore in the subsequent lessons (Lessons 6 to 10).

Preservation "goal" Preservation method(s) used
To control of Microorganisms

Controlling microorganisms by

  • keeping microorganisms out of food
  • removing microorganisms from foods
  • delaying the initiation of microbial growth
  • killing microorganisms or spores
1) HIGH TEMPERATURE- "HEAT"

Thermal processing involves the application of heat to inactivate enzymes and destory microorganisms.

  • Most bacteria are killed in the range o 82-93°C.
  • Spores are NOT destroyed even by boiling water at 100°C for 30 min.
  • To ensure sterility (total microbial destruction, including spores) a temperature of 121°C must be maintained for 15 minutes or longer.

There are various degrees of thermal processing:

  • blanching
  • pasteurization
  • commercial sterilization

Thermal processing will be discussed in more detail in Lesson 6.

2) LOW TEMPERATURE- "COLD"

Lowering temperature of a food decreases the rate of enzymatic, chemical and microbial reactions in food thus extending storage life.

Microbial growth rates decrease as temperatures decrease towards 0°C. Low temperatures, however, favour the proliferation of psychrotrophic microorganisms which ultimately cause spoilage of cold stored foods.

There are two main categories of low temperature storage of food:

  • refrigeration
  • freezing

Microorganisms are not easily killed by frozen storage of foods although death will occur slowly. Consequently, freezing cannot be relied upon to rid food of microbial contamination (Bacillus species and Clostridium species are virtually unaffected by low temperatures).

Some microorganisms can grow at temperatures as low as -9.5°C. Thus when food is held at improper frozen storage temperatures, microbial growth and spoilage can still occur, especially after thawing.

These preservation methods will be discussed in Lesson 7.

3) LOWERING WATER ACTIVITY (Aw)

Each specific organism has its own range of Aw in which it will grow. Bacteria normally need Aw of 0.90 and higher, yeast need >0.70, while moulds need 0.60-0.70 and higher. Of course, there are always some exceptions. For example, the pathogenic bacteria Staphylococcus aureus can grow at Aw as low as 0.83-0.84, while the yeast Saccharomyces. cerevisiae requires Aw of 0.90.

Drying

  • Microorganisms require free water in order to survive and multiply. Therefore, controlling water activity and water content of food can enable extension of storage life.
  • When free water is removed from the food and therefore from microbial cells, multiplication will stop since water will be unavailable for chemical, microbial and enzymatic reactions.

PLEASE REVIEW CONCEPT OF WATER ACTIVITY (AW) FROM LESSON 2.

Water activity in foods can be controlled (lowered) by:

  • Freezing water as crystals of pure water.
  • Physical removal of water from food (dehydration)
  • Removal of some of the water from food (concentration) by addition of substances that bind water in food making it unable to participate in chemical, microbial and enzymatic reactions (e.g. addition of salt or sugar, at high concentrations, to food).

The concept of food dehydration will be explored inLesson 8.

4) ACIDS

As discussed in Lesson 2, the acidity of a food can be described by its pH, which is the negative logarithm (base 10) of the hydrogen ion concentration. The pH of a food is an important factor that determines rates of chemical and enzymatic reactions as well as survival and growth of microorganisms in foods during processing, distribution and storage. The pH of solutions can vary between 0 (extremely acidic) to 14 (extremely alkaline). A pH of 7 defines a food that is neither acidic nor alkaline (i.e., it is neutral).

Only a few foods have a pH above 7; an example is egg albumen (white) which has a pH of 9. Most foods fall within the pH range of 2 to 7. The acidity of a food can be adjusted by the addition of food grade acids or alkalis or by acids produced through microbial fermentations. Beyond their influence on pH per se, some acids are also antimicrobial agents.

As we first learned in Lesson 2, pH 4.6 is a critical value in terms of microbial growth and stability, and we will explore in subsequent lessons, how the pH of a food is an important criterion in determining how the food should be processed or stored.

5) SUGAR and SALT

As discussed before, sugar and salt exert their preservative effects primarily through their effects on water activity of a food. Thus sugars and salt are employed in foods not only for their contributions to the flavour of foods but also because of their water binding properties. In addition, at very high concentrations, they may have a dehydrating effect on the microbial cells.

One of the preservatives in cured processed meats is salt, while sugars in jams and jellies prevent growth of bacteria and yeasts (except those that are tolerant to low water activities and moulds which can grow under conditions of low water activity).

6) OXYGEN

It is the oxygen in air or within a food that determines whether a food can support the growth of aerobic or anaerobic microorganisms.

  • Thus moulds can be inhibited from growing on foods by excluding oxygen. This is the function of waxes applied to rounds of cheese during aging and also is the function of paraffin wax placed on top of jams and jellies.
  • However, the removal of oxygen from low acid foods with a high water activity can pose a potential health hazard because conditions can be created whereby anaerobic disease-causing microorganisms, such as Clostridium botulinum, may be able to proliferate and produce toxins that could cause disease when the toxin-containing food is consumed. Vegetables and fish can be safely stored in an oxygen-free environment only if the Clostridium botulinum spores are killed by the application of heat. This will be discussed in more detail in Lesson 6.
7) FERMENTATION
  • Specific microorganisms (starter cultures) are cultured in certain foods to facilitate chemical changes in the foods such that the foods have a longer storage life.
  • Inhibitors such as acids, alcohol and bacteriocins (antimicrobial agents) are produced by the starter cultures. The inhibitors delay or prevent growth of undesirable microorganisms.

Fermentation will be discussed in more detail in Lesson 9: Food Biotechnology.

8) CHEMICALS

Microorganisms that can cause deteriorative changes in foods can be controlled by the use of chemical agents that have antimicrobial properties. Only a few such agents are permitted for use in Canada and their use as preservatives is defined within The Food and Drug Regulations of Canada.

  • Examples of antimicrobial agents added to some foods are:
    • Sodium propionate - may be added to bread formulations as a mould inhibitor.
    • Sodium benzoate - may be added to some acidic foods to delay growth of acid tolerant spoilage bacteria.

Similarly, antioxidants that are approved for specific uses may be added to delay the onset of oxidative rancidity.

  • For example, VITAMINS C and E (lesson 2). Another antioxidant, Butylated hydroxyanisole (BHA), is added to the packaging material for some breakfast cereals to react with oxygen before it can enter the package and react with sensitive constituents in the breakfast cereal to cause oxidative rancidity.
To Control of Enzymes and Oxygen

Controlling enzymes and Oxygen by:

  • inactivating endogenous enzymes; and
  • preventing or delaying chemical reactions in the food
9) RADIATION (ENERGY)

Various forms of radiation (energy) can be used to preserve food.

  • Ionizing radiation or "food irradiation" can be used to inactivate microorganisms in food, and to destroy storage pests (insects, mites, flies), thereby extending the storage life of the food.
  • Microwave treatment of food can be used to inactivate enzymes and microorganisms through generation of high temperatures as a result of the interaction of the microwave energy with water in the food.
  • Infrared radiation is used to toast foods, to keep foods hot and to cook foods.
  • Ultraviolet energy is used to sterilize air and water used in food processing, particularly in the beverage industry.

This topic will be explored in more detail in Lesson 10.

Enzymes in foods are controlled by many of the same techniques described above to control the activity of microorganisms in foods.

As an example of controlling enzymatic activity, the enzyme system that causes browning of fruit and vegetable tissues will be used as a model for discussion.

Refer again to the example of the browning reaction caused by polyphenol oxidase given earlier in this lesson. We have all had the experience of observing apple tissue turning brown after it has been cut and exposed to oxygen in the air. The enzymatic browning reaction can be inhibited in the following ways:

  • The apple slices can be dipped in hot water to cause heat denaturation of the polyphenol oxidase. This will adversely affect the physical properties of the apple but this is not of much concern during the manufacture of apple sauce or pie filling.
  • Oxygen can be excluded from the surface of the apple tissue by immersing the apple slices in water. Oxygen will not be completely excluded from the tissue surface but the rate of diffusion of oxygen to the tissue is impaired by the water. The availability of oxygen becomes rate limiting and thus slows down the reaction rate. The slices will eventually turn brown but the time required for browning to occur will be much longer than the time required during exposure of the apple tissue to oxygen in the air. This practice is widely used to delay browning of fruit and of peeled and sliced potatoes in the food processing and food service industries and in the home.
  • Acid conditions can be created on the apple slices by applying acids such as citric acid. Polyphenol oxidase activity is inhibited by the more acidic conditions created by adding citric acid or lemon juice (which contains citric and ascorbic acids), thus slowing down the rate of the enzyme catalyzed browning reaction.
  • Chemical reducing agents can be applied to the surfaces of apple slices to remove oxygen from the surface. Ascorbic acid (vitamin C) is often employed as an antioxidant, acting as an oxygen scavenger to delay the onset of enzymatic browning by reacting with oxygen before the oxygen can take part in the browning reaction.

If you have ever preserved fruit such as apple slices or peaches in your home, you probably have sprinkled the sliced fruit with a commercial preparation of ascorbic acid and citric acid to delay the onset of browning before the fruit is frozen. Sulfur dioxide or metabisulfites have also been employed to inhibit the browning reaction, although the use of these substances is now severely restricted because of the sensitivity of certain segments of the population to sulfur dioxide and sulfites in foods.

Want to learn more?
  • It is fairly simple to observe firsthand the relative effects on the rate of browning that result from the treatments described above.
  • Slice an apple and compare the rates of browning in apple slices left exposed to the air to those that have been dipped in boiling water for several minutes, or stored under water, or treated with a commercial preparation of citric acid and ascorbic acid, or lemon juice.

Video

Food Preservation Video Pt. 1 and 2