Canning: Types, Procedure, and Applications

A family cans green beans from their garden in a boiling water bath, the same way they can tomatoes every year. The jars seal, look perfect, and sit on the shelf for months. Weeks after a meal from one jar, a family member develops blurred vision and difficulty swallowing. The cause is botulism. The tomatoes were safe in a boiling water bath. The green beans were not. This one difference, acidic food versus low-acid food, is the most important idea in canning, and it is where most home-canning mistakes happen.

Canning is a preservation method in which food is sealed in an airtight container and heated enough to destroy the microorganisms that cause spoilage and disease. The seal then keeps new microbes out. Canned food usually stays safe to eat for one to five years, and sometimes far longer.

Two principles do all the work: heat kills the microbes already in the food, and the airtight seal prevents recontamination afterward. If either step fails, the food is no longer safe.

The amount of heat needed is not the same for every food. It depends on the acidity of the food, and this single fact is the key to understanding canning. It decides which of the two canning methods you must use, and getting it wrong is the main cause of dangerous canning failures.

Canning began in the early 19th century, when the French government offered a prize for a way to preserve food for Napoleon's armies. Nicolas Appert won it with the sealed-jar method. Glass jars later gave way to tin cans, and with mass production, canning spread worldwide.

Figure: Food preservation method: Canning

Types of Canning

There are two canning methods. Which one you must use is decided entirely by the acidity of the food, measured as pH. The dividing line is pH 4.6.

The reason is one dangerous organism: Clostridium botulinum. Its spores survive boiling. If they later germinate in a sealed, oxygen-free can, they produce botulinum toxin, one of the most poisonous substances known. Below pH 4.6, C. botulinum cannot grow or make toxin, so acidic food is safe after gentle heating. At or above pH 4.6, the spores can grow, so the food must be heated far hotter to destroy the spores themselves. This gives the two methods.

Boiling Water Bath Method (for high-acid foods)

This method is for high-acid foods, meaning a pH of 4.6 or below. Examples are most fruits, tomatoes with added acid, jams, jellies, and pickles.

The jars are filled with prepared food, sealed with lids, and immersed in boiling water for a set time. Boiling water reaches only about 100°C, but that is enough here. The acid already blocks C. botulinum, so the heat only needs to kill molds, yeasts, and ordinary bacteria, which die at these temperatures.

Some foods sit close to the line and need care. Tomatoes and figs are often near pH 4.6, so acid (such as lemon juice or citric acid) is added to bring them safely below it before boiling water canning.

Pressure Canning Method (for low-acid foods)

This method is for low-acid foods, meaning a pH above 4.6. Examples are vegetables, meats, poultry, seafood, and soups.

Because these foods do not have enough acid to block C. botulinum, boiling water is not safe for them. The spores would survive and could grow in the sealed can. Destroying the spores requires a temperature higher than boiling water can reach.

A pressure canner solves this. It is a heavy pot with a locking lid and a pressure gauge. By sealing in steam under pressure, it raises the internal temperature to about 116 to 121°C, well above the boiling point of water. This higher temperature destroys C. botulinum spores. This is why low-acid foods must always be pressure canned and never done in a boiling water bath.

Procedure of Canning

The exact steps vary by food, but the general sequence of canning is as follows. The thermal processing step near the end is where the acid rule from above decides the temperature and time.

Selection

• For canning, fruits, and vegetables should be fresh.
• The fruits should be ripe but firm and evenly matured.
• It should be free from blemishes, insect damage, soil, and dirt.

Sorting and Grading

• After the preliminary sorting, the fruits and vegetables are graded based on size and color.
• The grading is done by hand or with the grading machine (e.g., screen graders, roller graders, rope or cable graders).

Washing

• The graded fruits and vegetables are washed with water in different ways, such as; agitation, soaking, or spraying with water.
• Vegetables may be preferably soaked in a dilute solution of potassium permanganate to disinfect them.

Peeling, Coring, Pitting

• Peeling is done in various ways (such as; by hand or knife, machine, heat treatment, or boiling fruits or vegetables in a caustic soda or lye solution for 30 seconds to 2 minutes.
• Similarly, the coring and pitting of fruits and vegetables are done by hand or machine.

Blanching

• Treatment of fruits and vegetables with steam or boiling water for a specific period before cooling is termed blanching.
• The primary purpose of blanching is; to loosen the skin of fruits and vegetables, kill microorganisms, inactivate the enzymes, and improve flavor and color.

Filling of Cans

• Hot food is put into heated cans, which are also kept hot to allow the food to expand and remove any trapped air or gas.
• Before filling the fruits or vegetables, a small amount of syrup (for fruits) or brine (for vegetables) is poured into the can to provide medium to the contents.
• Filling cans is done either by hand or via machines. Additionally, cans are partially filled, leaving space for the food to expand during heating.
• After filling, brine or syrup is added.

Exhausting

• Exhausting is the method of removing air from the cans before they are closed.
• It reduces the pressure tension on can seams and internal corrosion and helps maintain the quality of products.

Sealing

• Cans are immediately sealed after removing air with the help of double seam can sealer.
• While sealing, make sure the temperature is not below 74℃.

Thermal processing

• It consists of heat treatment sufficient to eliminate spoilage microorganisms’ growth.
• High-acid foods are processed at about 100°C in a boiling water bath, while low-acid foods such as vegetables and meats are processed at about 116 to 121°C in a pressure canner.
• The total time required to sterilize the can depends on various factors such as; size, processing temperature, rate of heat penetration at the center of the can, pH of food, and the type and number of microorganisms present.

Cooling

• After thermal processing, cans are immediately cooled at 39℃ to stop the cooking process to prevent overheating/ burning.
• Usually, cans are cooled by dipping them in cold water.

Labeling and Storage

• After cooling, the cans are labeled, packaged, and stored in a clean and dry place.
• Storage temperature should be maintained based on preserved food types.

Applications of Canning

The applications of canning food preservation methods are as follows;

• It is used to preserve varieties of foods such as; fruits and vegetables.
• It provides convenience and accessibility by allowing ready-to-eat or easy-to-prepare meals throughout the year.
• Canned foods are crucial for emergency preparedness, providing a reliable food supply during emergencies.
• It supports local agriculture by preserving locally grown produce.
• Canning reduces food waste by preserving surplus food.

Advantages of Canning

Some of the advantages of the canning food preservation method are as follows;

1. It provides a long shelf life of preserved food (i.e., 1 to 5 years; in some cases, it can increase to more than 20 years).
2. It provides higher stability of preserved food than traditional packaging methods.
3. Materials used (i.e., tin or glass jars) are reusable.
4. The pull-off lid makes food packed by the canning method easy to open.
5. Foods preserved by the canning method are of high quality as few or fewer preservatives are added.
6. Foods preserved by the canning method are easy to prepare and store.
7. Canned food does not require a refrigerator for storage.
8. It does not require cold chaining during transportation.

Disadvantages of Canning

Despite having benefits, canning also has some disadvantages, which are as follows;

1. The glass jar might break.
2. Breaking of seals can cause spoilage.
3. If low-acid food is not processed correctly, Clostridium botulinum spores can survive and grow in the sealed can, producing a dangerous toxin. This is why low-acid foods must be pressure canned and processing instructions must be followed exactly.
4. The can food does not taste the same as fresh food.
5. Can food may have fewer vitamins and minerals than fresh food. Canned fruits and vegetables have about 65% fewer vitamins and minerals than freshly picked ripe fruits and vegetables.
6. Canning requires much time.

Precautions

Some of the precautions that are needed to consider while canning are as follows;

1. Before storing, the sealing of canned food should be checked appropriately.
2. Always store canned foods in a clean, dry, and dark place.

How to Remember

The one number that runs all of canning: pH 4.6.
Above 4.6, the botulinum spore can grow, so you need a pressure canner (hotter than boiling). Below 4.6, acid blocks the spore, so a boiling water bath is enough. If you remember one fact about canning, remember this line.

Which method for which food — "High acid, low heat. Low acid, high heat."
High-acid foods (fruits, tomatoes, jams) take the gentler boiling water bath. Low-acid foods (vegetables, meat, fish) take the hotter pressure canner. The acid does the safety work the extra heat would otherwise have to do.

Why pressure, not just longer boiling?
Self-check: could you make low-acid food safe by boiling it longer? No. Boiling water cannot go above 100°C no matter how long you wait, and botulinum spores survive 100°C. You need higher temperature, which only pressure provides. This is the point students most often miss.

Key exam facts in one table

Where Students Get Confused

Why can't you just boil low-acid food for longer?
Because boiling water never gets hotter than 100°C, and C. botulinum spores survive that temperature indefinitely. More time at 100°C does not help. Only a higher temperature destroys the spores, and only a pressure canner reaches it. This is the single most important idea in canning.

A perfect seal means the food is safe.
Not by itself. The seal only keeps new microbes out. If the food was under-processed and live C. botulinum spores remain inside, the airtight, oxygen-free can is exactly the environment they need to grow. Safe canning needs both correct heat and a good seal.

Botulism spoils the food so you can smell it.
Often not. C. botulinum may leave the food looking and smelling normal. This is why following correct processing is essential; you cannot rely on your senses to detect it. (This connects to the spoiled-versus-unsafe distinction covered in the food spoilage article.)

Tomatoes are acidic, so they are always safe in a boiling water bath.
Many tomatoes sit close to pH 4.6, and some varieties rise above it. That is why acid is added before boiling water canning, to bring them safely below the line. "Acidic enough" is not something to assume.

References

1. Frazier WC, Westhoff DC, Vanitha NM. Food Microbiology. 5th ed. New Delhi: McGraw Hill Education; 2014.
2. Jay JM, Loessner MJ, Golden DA. Modern Food Microbiology. 7th ed. New York: Springer; 2005.
3. U.S. Department of Agriculture. Complete Guide to Home Canning. Agriculture Information Bulletin No. 539. Washington, DC: USDA; 2015. Available from: https://www.nifa.usda.gov/about-nifa/how-we-work/extension/national-institute-food-agriculture-usda-complete-guide-home-canning
4. U.S. Food and Drug Administration. Acidified and low-acid canned foods; regulation of Clostridium botulinum and pH 4.6. Available from: https://www.fda.gov/food