Processing canned food in a high-pressure canner or a boiling water-bath canner uses high temperature to prevent the growth of bacteria and destroy enzymes. Done properly, canning with intense heat prevents the growth of bacteria, destroys enzymes, removes oxygen and creates a high vacuum seal. A good seal on canning jars keeps liquids in, and air and microorganisms out. If food is not canned properly then bacteria, yeasts and moulds inside the jar can spoil food and make you sick. However you must be vigilant about botulism prevention.
Botulism is a type of food poisoning caused by bacterium that can produce a deadly toxin inside a sealed jar of food. If you eat even a tiny amount of contaminated food, botulism can cause serious illness or death. Spores of the botulism bacteria are present everywhere. Botulism bacteria are heat resistant and can survive high temperatures. It grows in moist, low acid foods with little to no oxygen. Improperly canning foods provides ideal conditions for botulism if not done properly.
There is no way to tell if canned food is contaminated with botulism because the food may not look, taste or smell spoiled. The only safe way to avoid botulism is by properly canning low acid food in a high-pressure canner. Low acid foods like meat, fish and poultry and vegetables (except tomatoes with added lemon juice or vinegar) must be processed or cooked in a high-pressure canner.
- A high-pressure canner cooks under pressure. The pressure inside the canner increases the temperature of boiling water from 100°C (212°F) up to 116°C (240°F). This is the minimum temperature necessary to destroy botulism spores and the only way to guarantee safe canning.
Food acitidy and processing methods
Successful home food preservation involves time, reliable recipes and equipment; quality product and common sense. Reliable canning practices remove oxygen, destroy enzymes, prevent growth of bacteria, moulds and yeasts; and form a tight seal. They include pressure canning and water bath canning.
The acidity of the food being processed is key to deciding which canning method to use. Acidity is measured using the term “pH”. The lower the pH number of the food, the more acidic it is.
- Pressure canning must be used for all low acid foods, meat, fish, poultry and all vegetables except tomatoes.
- Water bath canning is used for high acid foods (fruits) and foods whose acidity has been raised with the addition of vinegar (pickles).
Low acid foods have a pH higher than 4.6. These foods must be processed in a high-pressure canner. A high-pressure canner is a large, cast-aluminum pot with a locking lid and a pressure gauge. Cooking under pressure increases the temperature of boiling water from 100°C (212°F) up to 116°C (240°F). This is the minimum temperature necessary to destroy botulism spores and the only way to guarantee safe canning. Low acid foods include:
- vegetables including most tomatoes
- meat and poultry
- fish and shellfish
- soup and milk
- mixtures of low and high acid foods, such as spaghetti sauce with meat, vegetables and tomatoes
High acid foods have a pH lower than 4.6. These foods can be safely canned using a boiling water canner which heats food to 100° C (212°F) at sea level. The acidity of these foods prevents botulism bacteria from growing. The heat will kill most yeasts, moulds and bacteria that could be present. High acid foods include:
- jams, jellies and marmalades
- fruit butters
- tomatoes with added lemon juice or vinegar
How to be sure your food is safely canned
- Always use a high-pressure canner to process low acid foods.
- Say no to “oven canning” and “open kettle canning” as these methods are proven to be unsafe.
- Also say no to multi-cookers for canning. Multi-cookers are appliances that have several functions: browning, slow cooking and pressure cooking. Even though some of these are advertised for home canning, the National Centre for Food Preservation (US) does not recommend them for canning. For more information see: https://nchfp.uga.edu/publications/nchfp/factsheets/pressurecookers.html
- Use only safe canning recipes with tested proportions of ingredients. Never adjust the amounts of water, vinegar or other ingredients in the recipe as this can change acidity levels and make canning unsafe. Safe recipes have been tested for pH and tell you which canning method to use. Here are links to safe canning recipes:
- Do not puree food before canning because proper processing times for pureed foods (such as baby food) have not been determined for home use.
- Before you open canned food, make sure the jar lid is firmly sealed and concave (curved downward) and does not move when pressed.
- If in doubt, throw it out.
Hot packing is when you heat food to boiling and simmer for 2 – 5 minutes before filling jars and processing them. Hot-packing helps to remove air around the food which improves shelf life. Hot-packing also shrinks the food which allows more food to be added to each jar.
Raw-packing is when you tightly fill jars with fresh but unheated food and then pour hot liquid over top to cover the food. The disadvantage of raw packing is that air can become trapped around the food, which can cause discoloration in storage. Use a spatula around the sides of the jar to remove air around the food.
Safe canning recipes give processing times at sea level. If you live at altitudes above 305m (1,000’), you must increase the processing time because water boils at lower temperatures as the altitude increases. There is a risk that not all of the bacteria in food will be killed if the cooking time is too short. Much of the prairies and some of the far north lie above 300m. The elevation in Saskatoon is 488m above sea level.
To find out your elevation put your location into the finder in this link: https://whatismyelevation.com
Follow the instructions for increasing processing time in your safe canning recipe.
To adjust processing times, consult this link: https://www.healthycanning.com/altitude-adjustments-for-pressure-canning/
Basic canning process
|Boiling water-bath canner:
Large pot that is at least 7 cm (3”) taller than your jars
|Can be used for sterilizing jars and for processing high acid foods. Never use for processing low acid foods.
A metal rack that fits the bottom of the pot keeps jars from breaking and allows boiling water to circulate around jars.
Large cast-aluminum or porcelain-covered steel pot with a locking lid and pressure gauge, specially made for canning food.
|A safe way to process low acid foods.
Use according to manufacturer’s instructions.
Tip: Pressure canners are expensive but worth the cost. To keep costs down, several families can pool resources and share this equipment.
|Canning jars, lids and rims||
Glass jars and two-piece canning lids (snap lids and screw bands) designed for home canning are a good choice for home canning. Jars come in many different sizes: 250 ml, 500 ml, 750 ml, 1 L and 1.9 L.
Glass jars and metal screw bands (unless rusted or warped) can be used many times.
|Magnetic wand||Useful for picking up lids without using your hands.
Prevents cross contamination.
|Jar lifter or tongs||Use for lifting hot jars.|
|Bubble remover or spatula, and head space tool||Plastic tool used to remove air bubbles and to measure the space between the top of the jars and the food inside it.|
|Funnel and ladle||Wide mouth funnel should fit in jars.
Makes filling jars neater and prevents spills.
|Paper towels and/or clean dishtowels||For wiping rims clean before sealing.|
|Other||You will need other tools like pots, spatulas, bowls, measuring cups, and knife to make your recipe.|
Before you start, get your tools, equipment, and ingredients ready.
- Wash your hands with soap and warm water for at least 20 seconds. Sanitize counter tops, cutting boards and utensils with hot, soapy water. Even better, use a weak bleach solution (5 ml (1 tsp)of household bleach to 750 ml (3 cups) of water to clean surfaces. Rinse with water.
- Clean and sterilize jars, funnel, ladle and magnetic wand. First, wash in hot, soapy water. Rinse well. Place jars and utensils in the large pot with enough water to cover them and bring to a boil. Jars must be boiled for at least 10 minutes to sterilize. Turn off the heat and leave the jars in the canner until you are ready to fill them. Jars may also be washed in the dishwasher and kept hot after they cycle until ready to use.
- Prepare the recipe. Use only a safe recipe and do not adjust the ingredients.
- Soak lids in hot (not boiling) water for at least 5 minutes before filling your jars. This will soften the seals so you get a good fit.
- Fill the jars using the funnel and ladle. See Hot-packing vs Raw-packing for tips.
- Remove air bubbles and adjust the head space. Check the recipe for the correct amount of headspace. Low acid foods need 2.5 cm (1”) head space
- High acid foods need .5 cm (1/4”) head space for jams and jellies; and 1 cm (1/2”) for fruit, pickles chutney or relish.
- Wipe the rim with a clean paper towel to remove any stickiness or food residue.
- Centre lids on the jars and secure with a screw band until finger-tip tight.
- Process jars in the appropriate canner using the jar lifter to place the jars into the high-pressure canner or boiling water bath.
- High-pressure canner for low acid foods: Adjust water level as directed by canner manufacturer. Lock the canner lid and place over high heat. Vent the canner to allow steam to escape according to manufacturer. When pressure reaches the right level, begin timing according to the recipe. Regulate heat to maintain pressure level for your altitude. When processing time is done, turn off heat. Wait for pressure to drop to zero. Wait 2 more minutes before carefully removing the canner lid by turning the lid away from you.
- Boiling water bath for high acid foods: Fill pot so that jars are covered by at least 2.5 cm (1”) of water. Cover pot with lid. Bring to a rolling boil. Process (boil) jars for time stated in recipe. Add more boiling time for altitudes higher than 305 m (1,000’). Boil 1 additional minute for each additional 305 m (1,000’) of elevation.
- Let jars sit in the canner for 5 minutes before removing.
- Using a jar lifter, remove the jars from the canner onto a towel or cloth. Leave some space between jars. Allow to cool. You will hear a popping sound as lids seal. Fully sealed lids are concave – curved downward. Leave jars to cool completely for 12 – 24 hours.
- Label your jars with contents and date. Store in a dark place that is below 35ºC (95ºF). The ideal temperature range is 10ºC – 21ºC (50ºF - 70ºF).
- After you open a jar, store it in the refrigerator.
Fill canner with 5 to 7 cm (2-3 inches) of hot water. Load jars on the canner rack and lower rack into canner. Fasten canner lid securely.
Leave weight off vent port or open petcock. Heat until steam flows. Exhaust 10 minutes. Place weight on vent port or close petcock (canner will pressurize during the next 3 to 5 minutes.)
Begin timing process when dial gauge indicates recommended pressure has been reached or weighted gauge begins to jiggle. Regulate heat to maintain steady pressure.
At the end of process time, turn off heat, remove canner from heat, remove weight from vent port or open petcock. Wait 2 minutes. Remove lid carefully, away from you.
Fill canner halfway with water and preheat water. Load jars on rack. Lower rack into canner. Add boiling water so water level is at least 2.5 cm (1 inch) above jar tops. Bring water to boiling, cover canner.
Set time for minutes required for processing. Maintain gentle boil through processing time. Shut off heat, remove jars and place spaced apart out of drafts to cool.
Home food preservation means retaining food quality and providing safe food. Do it right and enjoy the fruits of your labour year-round.
Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010) Putting Food By: Fifth Edition. New York, New York: Penguin Publishing Group.