Course:FNH200/Lessons/Lesson 05/Page 05.6
- 1 5.6 Principles of Food Preservation
- 1.1 To control of Microorganisms
- 1.2 1) HIGH TEMPERATURE- "HEAT"
- 1.3 2) LOW TEMPERATURE- "COLD"
- 1.4 3) LOWERING WATER ACTIVITY (Aw)
- 1.5 4) ACIDS
- 1.6 5) SUGAR and SALT
- 1.7 6) OXYGEN
- 1.8 7) FERMENTATION
- 1.9 8) CHEMICALS
- 1.10 To Control of Enzymes and Oxygen
- 1.11 9) RADIATION (ENERGY)
- 1.12 Food Preservation Video Pt. 1 and 2
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|
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
1) HIGH TEMPERATURE- "HEAT"
Thermal processing involves the application of heat to inactivate enzymes and destory microorganisms.
There are various degrees of thermal processing:
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:
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.
PLEASE REVIEW CONCEPT OF WATER ACTIVITY (AW) FROM LESSON 2.
Water activity in foods can be controlled (lowered) by:
The concept of food dehydration will be explored inLesson 8.
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).
It is the oxygen in air or within a food that determines whether a food can support the growth of aerobic or anaerobic microorganisms.
Fermentation will be discussed in more detail in Lesson 9: Food Biotechnology.
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.
Similarly, antioxidants that are approved for specific uses may be added to delay the onset of oxidative rancidity.
To Control of Enzymes and Oxygen
Controlling enzymes and Oxygen by:
9) RADIATION (ENERGY)
Various forms of radiation (energy) can be used to preserve food.
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:
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?|