Canning is a method of preserving, where food is placed in airtight, sealed containers and heat processed. This destroys micro-organisms and inactivates enzymes. Since the food in the container is commercially sterile1, it does not spoil under normal conditions. Canned foods are amongst the safest convenience foods available to the public. Canned foods are shelf-stable and can be stored up 5 years or even longer at room temperature.
History of Canning
In the late 18th century, Napoleon Bonaparte was concerned by the numbers of his soldiers who were dying from malnutrition. He offered a reward of 12 000 Francs for a new practical way to preserve food. Much work was done by various people on possible methods of food preservation, but Nicholas Appert, a chef and confectioner, was first to present Napoleon with a range of bottled fruit, vegetables and meat products that were heat preserved. In 1810, after he published his methods, he received his reward and used the money to establish a cannery. Initially Appert preserved his food by heating sealed bottles in boiling water, but within a few years changed to tinplate cannisters (cans) as his packaging of choice.
Commercial canning is done under tightly controlled conditions: strict plant and water sanitation; processing with the necessary time and temperature control; and careful sanitary handling after processing. As a result, the safety record of canned foods is excellent. There are, however, some factors that increase the risk of spoilage. One of the reasons why canned food may spoil is under-processing: high loads of micro-organisms in the raw ingredients can make it impossible for the normal heat process to render it commercially sterile. Another reason for spoilage is post processing contamination: either through defective double seams; physical damage; poor quality cooling water; or handling the cans while hot and / or wet.
Sterilization and Pasteurization
For canned foods, heat treatment generally falls into the following two categories:
- Sterilization: Heat treatment of low acid foods (pH>4.6) that leads to commercial sterile products. Heat processing is at high temperatures (±121°C), under pressure, with the objective of killing all viable micro-organisms, including the spores of pathogenic bacteria.
- Pasteurization: Heat treatment of acid foods (i.e. pH<4.6) that together with low pH, leads to commercial sterility. The process temperatures are slightly lower than sterilization (usually 70 – 100°C) and are designed to kill the vegetative forms of micro-organisms. These temperatures have little to no effect on the spores of bacteria, which are kept in check by the acid environment. The “pasteurized” products are shelf-stable as long as the pH remains low (<4.6).
Canning is well established in the area of food science and there are many textbooks and training courses available.
|1 “Commercial sterility” is the conditions achieved by application of heat which renders such food free from micro-organisms capable of growing in the food at temperatures at which the food is likely to be held during distribution and storage|
Advice to Consumers
Advice given to consumers regarding canned foods is as follows:
- Once opened the food will have the same expected shelf life as any other cooked food and any unused portions should be stored refrigerated in clean containers for three to four days maximum.
- Don’t purchase, or consume, bulging, rusted, leaking, or deeply dented cans.
- Discard heavily rusted cans. Cans that are heavily rusted can have tiny holes in them, allowing bacteria to enter. If you open the cans and there is any rust inside, do not eat the food.
- A sharp dent on either the ends or side seam can damage the seams and allow bacteria to enter the can. Discard any can with a deep dent on the seams. If a can containing food has a small dent, but is otherwise in good shape, the food should be safe to eat. Discard deeply dented cans. A deep dent is one that you can lay your finger into. Deep dents often have sharp points.
- Store canned foods and other shelf stable products in a cool, dry place: temperatures 15 – 25°C are recommended. Never store them above or beside the stove, under the sink, in a damp garage or basement, or any place exposed to high or low temperature extremes. Check your pantry or storage cupboard every few weeks and use the canned foods that you have within the recommended shelf-life.
- High temperatures (over 40°C) are harmful to canned foods. The risk of spoilage increases as storage temperatures rise. Canned goods designed for use in tropical areas are specially manufactured to take the high ambient temperature into account.
One of the incorrect, negative perceptions about canned food is that it is not as nutritious as other types of food. The original canned foods were made for military rations and it revolutionised the military at that time. At that time anything was better than nothing. The troops were much better nourished than ever before, but because of lack of understanding about food processing and lack of controls, the canned food was sometimes over-processed and as a result the nutritional value was compromised. But as the understanding of canning grew, the quality of the products became better.
Today canned food is minimally, but adequately processed. They are made from good quality raw ingredients, under strict hygiene conditions. Many studies have been done that show that the nutritional value of canned food is as good as that of its fresh or frozen counterparts. Thermally processed foods provide excellent nutrition over extended periods. Most crops, meat and fish are seasonal. Thermal processing allows seasonal products to be available all year round. Canned foods, in many cases, provide similar amounts of vitamins and minerals to fresh equivalents, and are a good source of protein and fibre.
Canned foods are controlled by strict legislation, and quality standards in South Africa are amongst the best in the world. The National Regulator of Compulsory Specifications (NRCS) has compiled standard and compulsory specifications for locally and imported canned meat and canned fish products. The Department of Agriculture has compulsory standards for certain fruits and vegetables, including those for export. Certain products are graded; the grade relates to appearance and not inherent quality.
Unsafe products are never offered for sale or donated for consumption.
Prepared for FACS by SFe (2019)