business development > your guide to food processing in Ontario > food processing and preservation
Food Processing and Preservation
Canadian consumers want safe, nutritious, inexpensive food throughout the entire year. Unfortunately, Canada's climate limits the production of food for up to six months of the year. Just store the food? Impossible. Once food is harvested, it begins to deteriorate immediately due to the following factors:
Because of the risk of spoilage,
much of our food is processed in some way to increase its availability. A food
is considered preserved once it is stabilized with respect to safety and quality.
Not all processing methods are applied to foods to achieve preservation. Some are also used to change or stabilize food texturally.
Methods of processing food can be divided into two main categories-chemical and physical.
Chemical Processing Methods
The following techniques use the chemical approach in processing food.
Intermediate Moisture Foods (IMF)
Binding the water that's present preserves intermediate moisture foods-for example, cookies, cake and bread. This reduces the availability of the water for deteriorative reactions.
Water is immobilized by adding permissible humectant additives such as glycerol, glycols, sorbitol, sugars and salts.
Generally, IMFs possess water activities that range from 0.6 to 0.85. This enables the food to be stable at room temperature, because the growth of most micro-organisms is inhibited at these levels.
Water Activity (aw)
Water is the most important factor in controlling the rate of deterioration of a food. However, knowledge of the moisture content of a food isn't sufficient to predict its stability. It is the availability of water for microbial, enzymatic, or chemical activity that determines the shelf life of foods. This water availability is measured as water activity (aw).
Water activity is measured on a scale of 0 to 1, where 0 indicates no water and 1 indicates all water. Food spoilage micro-organisms, in general, are inhibited in food where the water activity is below 0.6. However, if the pH of the food is less than 4.6, micro-organisms are inhibited when the water activity is below 0.85.
Addition of Chemicals
The addition of some chemicals inhibits microbial growth in foods. These chemicals include not only those classified as preservatives. Salt, sugars, wood smoke and some spices also inhibit the growth of micro-organisms. For more information on chemical preservatives, see Part 4: Food Ingredients in this guide.
Almost every food, with the exception of egg whites and soda crackers, has a pH value of less than 7. Foods can be broadly categorized on the basis of their pH as high acid, acid, medium acid or low acid. Examples of each category include:
Most micro-organisms grow best in the pH range of 6.5 to 7.5. Yeasts and moulds are capable of growing over a much broader pH range than bacteria. Few pathogens will grow below pH 4.0. This information is important, because it will help you in determining food stability with respect to microbial spoilage.
Physical Processing Methods
A number of physical methods are available to you for processing foods.
First, a science note: a pathogen is any microorganism that causes illness. Food pathogens cause food-borne illnesses such as food poisoning or food intoxication.
Sterilization destroys all pathogenic and spoilage micro-organisms in foods and inactivates enzymes by heating. All canned foods are sterilized in a retort (a large pressure cooker). This process enables food to have a shelf life of more than two years.
Foods that have a pH of more than 4.6, such as meat and most vegetables, must undergo severe heating conditions to destroy all pathogens. These foods are heated under pressure to 121°C for varying times.
Severe conditions are applied to ensure that Clostridium botulinum spores are destroyed during processing. These spores produce the deadly botulinum toxin under anaerobic conditions (that is, where there's no oxygen). The spores are destroyed by heat or are inhibited at pH values of less than 4.6. Therefore, a food with a pH of less than 4.6 that is packaged anaerobically, such as spaghetti sauce, doesn't need to undergo such a severe heat treatment.
Pasteurization is the process
of heating a food-usually a liquid-to or below its boiling point for a defined
period of time. The purpose is to destroy all pathogens, reduce the number of
bacteria, inactivate enzymes and extend the shelf life of a food product.
is a slight heat treatment, using hot water or steam, that is applied mostly to
vegetables before canning or freezing.
Microwave ovens are rarely used for processing large quantities of food. They
are mainly of interest if you cater to the convenience food market, with products
such as frozen entrées.
Frying differs from other methods of heat processing
in that the cooking medium is hot oil. Because of the big difference between the
temperature of the oil and the food, as well as the small size of the food pieces,
cooking is completed in a relatively short time-anywhere from 20 seconds to six
Refrigerators should be set to below 4°C to control the growth of micro-organisms
in foods. This lowered temperature also reduces the respiration rate of fruits
and vegetables, which retards reactions that promote spoilage.
Not all foods benefit from cold temperatures. For example, bananas turn black and bread goes stale when refrigerated.
While many home
freezers are held at -10°C, commercial freezers are under -18°C. At this
temperature, the growth of micro-organisms is almost stopped. Deteriorative microbial
reactions will still occur, but over a much longer time.
The shelf life of frozen foods is largely dependent
on storage conditions. Under ideal conditions, frozen foods can have a shelf life
of one year.
Irradiation is the controversial process of applying low doses of gamma radiation to food products. Forty years of research have shown that the process exhibits no safety hazard. As a result, irradiation is permitted in Canada to:
If irradiation becomes more widespread among various other food products, it is expected to replace fumigation, ensure hygienic quality and reduce the dependence on refrigeration.
Weigh Your Options: Batch vs. Continuous Processing
Food is processed in either discrete batches or a continuous system. Although there are advantages and disadvantages to each method, choice in the matter is restricted only to those replacing or setting up a new processing line. Generally, batch systems are used to produce small quantities of food, whereas larger volumes are required for continuous systems.
Evaporation is the partial removal of water from liquid foods by boiling. When
the operation is done under vacuum, boiling is avoided and the food's flavour
qualities are retained.
This process is carried out for three main reasons:
the nearly complete removal of water from solid foods. One of the oldest methods
of food preservation, it was traditionally carried out by the sun.
Spray drying and freeze drying are two drying methods used widely today. Spray drying is when a liquid food is atomized into a fine, dry powder. Examples include natural and artificial flavours and milk powders. Freeze drying involves first freezing the food and then driving off the ice, leaving a high quality, porous dried food such as instant coffee.
An emulsion is a system containing two liquid phases that don't mix, where one phase (dispersed phase) is distributed throughout the other phase (continuous phase) in the form of very small droplets. Generally there are two types of emulsions:
An example of an O/W emulsion is salad dressing, and an example of a W/O emulsion is butter.
is used to stabilize an emulsion. More specifically, it is the reduction in size
and the increase in number of droplets of the dispersed phase by the application
of intense shearing forces.
Extrusion is the process in which a food is compressed and worked to form a semi-solid
mass. This mass is then forced through a restricted opening, or die, to create
a desired texture or shape. The purpose of this application is simply to provide
a greater variety of textured foods to consumers.
|Creation Date:||30 August 2005|
|Last Reviewed:||11 July 2011|