There are many variables to learning how to can food. Follow the guide below to determine how to treat the food, which equipment to use, and how to properly can the food to preserve vitamins and avoid developing botulism.
How Canning Works
Fresh foods contain a high percentage of water which makes them nutritious but also perishable. They can spoil or lose their quality due to growth of undesirable microorganisms, activation of food enzymes, reactions with oxygen, or moisture loss. Proper canning helps to prevent these things from happening. To properly can is to:
- Carefully select and wash fresh food
- Peel some of the fresh food (as the most effective method to clean)
- Hot pack many of the varieties of foods while canning
- Add appropriate acids (lemon juice and vinegar) to some of the foods
- Use jars and self-sealing lids to preserve food
- Process jars in a boiling-water or pressure canner for the correct period of time
- Store canned foods in appropriate locations
By following these procedures oxygen is removed, enzymes are destroyed, growth of undesirable microorganisms is prevented, and freshness is achieved with a high vacuum inside of the jars.
Selecting and Preparing Food
Selecting high quality food to can is the first step to successful canning. When choosing food to preserve, ensure that it is fresh and does not have signs of disease or mold. If you do encounter a portion that looks diseased or moldy, trim the section off and discard. To ensure best varieties for your purposes contact your county Extension Office for recommendations.
In order to maintain vitamin quality and freshness, vegetables are best to can 6 to 12 hours after harvest. Certain fruits such as apricots, nectarines, peaches, pears, and plums should be ripened for 1 or more days before processing. Fresh, home-slaughtered meats should be chilled and canned as soon as possible. Ice fish and seafood should have all organs removed immediately after harvest and canned within 2 days. If you are unable to can foods at their peak, make sure to chill them to slow quality depletion. *Note: do not can meat from sickly or diseased animals.*
After choosing the food you want to can, make sure to rinse and scrub vegetables and fruits to help curb the growth of bacteria. This form of preparation can help mildly but if you can peel the vegetable or fruit, this is the most effective form of cleaning. Blanching can also help. These methods plus proper processing can lead to safe canned food.
Canning is a fulfilling skill but can also be a safety hazard if not completed correctly. Clostridium botulinum is a bacteria that can grow in improperly processed canned goods that may cause botulism, a deadly form of food poisoning. This bacteria can survive in soil and water for many years without causing negative effects to people. Growth occurs when the bacteria are in an environment with the following:
- A moist, low-acid food
- A temperature between 40°F and 120°F
- Less than 2% oxygen
Follow this guide to ensure you’re using best practices to avoid possible growth of Clostridium botulinum.
Preserve Color and Flavor
To preserve the color and flavor of the food you are canning, you must remove oxygen from the food tissues and jars, quickly destroy the food enzymes, obtain a high vacuum in the jar and an airtight seal on the lid.
For best practice to maintain color and flavor in your canned food, complete the following:
- Select the best quality foods available that are in their peak freshness and without disease or mold.
- Use the hot-pack method to pack food into jars.
- Limit the amount of time prepared foods are exposed to oxygen- can them ASAP.
- Use a solution of 3 grams (3,000 mg) ascorbic acid to 1 gallon of cold water to keep the following foods in while preparing the canner load of jars: peeled and cut apples, apricots, cherries, grapes, mushrooms, nectarines, peaches, pears, and potatoes.
- Fill hot foods into jars and adjust the headspace based on the specifications in the recipe.
- Tighten screw bands securely before canning.
- Process and properly cool jars.
- Store jars between 50°F and 70°F in a relatively dark location.
- Make sure to can only the amount of food you estimate you will need for a year.
Ascorbic acid can be purchased in several forms. Pure powdered form can be found seasonally in stores that sell canning supplies. This form usually requires 1 tsp of powder per gallon of cold water. Vitamin C tablets can be found year-round in many stores. If you buy 500mg tablets you can crush and dissolve six of them per gallon of cold water to serve as your treatment solution. Commercially prepared mixes of ascorbic and citric acid can be found seasonally in stores that sell canning supplies. These are to be prepared by following the manufacturer’s instructions.
Canning Low-Acid Foods
Acidity levels are based on pH values of the food you are processing. Acids are items with values lower than 7.0 on the pH scale and bases are items with values higher than 7.0 on the pH scale. A food with a pH higher than 4.6 is considered a low-acid food and does not contain enough acid to stop the growth of the bacteria that causes the food poisoning botulism. These foods need extra heat to prevent this growth and should be processed with a pressure canner at 10-15 PSIG (PSIG means pounds per square inch of pressure as measured by gauge). Processing low-acid foods in a pressure canner can take anywhere from 20-100 minutes depending on the type of food as well as the arrangement of the jars in the canner. A boiling-water canner could be used for low-acid foods as well, but isn’t recommended as it takes much longer; anywhere from 7-11 hours.
Common low-acid foods that people can:
- Red meats
- Fresh vegetables (except for some tomatoes)
Canning High-Acid Foods
Acidity levels are based on pH values of the food you are processing. Acids are items with values lower than 7.0 on the pH scale and bases are items with values higher than 7.0 on the pH scale. A food with a pH lower than 4.6 is considered a high-acid food and contains enough acid to stop the growth of the bacteria that causes the food poisoning botulism. These foods can be processed in a boiling-water canner, taking anywhere from 25 to 60 minutes. They could also be processed in a pressure canner but it is not recommended as it would take anywhere from 55 to 100 minutes.
Common low-acid foods that people can:
- Jams, jellies, marmalades, fruit butters
- Some tomatoes
*Note: tomatoes are listed in both sections because different types of tomatoes have been found to have different pH values. See this guide for estimated pH levels per species.*
How Hot-Packing Works and When to Use it
Hot-packing is the process of heating freshly prepared food to be packed into jars before processing. This is helpful when working with foods such as high-acid foods that will be processed in a boiling-water canner. The alternative to hot-packing is called raw-packing and it is the practice of tightly filling jars with freshly prepared, unheated food. This can be beneficial for food that will be processed in a pressure canner such as low-acid foods.
Hot-packing is beneficial because it removes excess air from foods that could cause discoloration and reduce the shelf-life of the canned goods. Many fresh foods contain 10-30% of their mass in air and hot-packing reduces that amount significantly. This process also shrinks the food which allows for more food to be packed into each jar.
To hot-pack prepared food, put freshly prepared food in boiling water, simmer it for 2 to 5 minutes and then promptly fill jars loosely with the heated food. *Note: whether you choose to hot-pack or raw-pack your food, the juice, syrup, or water that you add to the food should be heated to boiling before putting in the jar for processing.*
After processing hot-packed food it can be hard to immediately see the benefit of the process, but after a few months it is easy to see the superior color and flavor retention as compared to raw-packed food of the same variety.
Most canning takes place in glass jars with self-sealing lids. When used properly these jars and lids create excellent vacuums and proper seals to ensure safety and freshness. Before each canning, all empty jars should be washed in a dishwasher or by hand. If washing by hand make sure to rinse thoroughly as remaining detergent residue can cause unnatural flavors and colors. If your jar has a white film, aka mineral deposits, create a soaking solution from 1 cup vinegar (5 percent acidity) per gallon of water and soak the jars for several hours, then wash in your dishwasher or by hand.
Some food products, such as jams, jellies, and pickled foods processed less than 10 minutes, must be packed into sterilized jars. To sterilize empty jars submerge them right-side-up in a boiling-water canner with the rack in the bottom. Fill the canner with enough warm water so the water level is 1 inch above the tops of the jars. Bring the water to a boil and boil for 10 minutes. Reduce the heat in the canner but leave the jars in the hot water until it is time to fill them. When ready to pack jars, remove and drain sterilized jars one at a time with a jar lifter, saving the hot water in the canner for processing the jars once they are packed.
If you did not sterilize your jars before packing, submerge them in enough water to cover the jars and then simmer the water until it is time to pack them with food. A dishwasher may also be used for cleaning and preheating the jars if they are washed and dried on a complete regular cycle- make sure to leave the jars in the closed dishwasher until you are ready to pack them with food.
Headspace and Packing Jars
The headspace refers to the unfilled space between the lid and the liquid in the jar. When food and liquid are heated they expand and the headspace allows room for this expansion to happen without affecting the integrity of the jar. The amount of expansion is determined by the amount of air content in the food as well as by the processing temperature that you are using with your canner. Air will expand dramatically when heated to high temperatures; the higher the temperature, the greater the expansion.
Appropriate headspace is indicated in recipes and should be followed to ensure jar integrity as well as a proper vacuum in cooled jars. In general, for items being processed in a boiling-water canner, the headspace should be:
- ¼-inch for jams and jellies
- ½-inch for fruits and tomatoes
In general, for items being processed in a pressure canner, the headspace should be:
- 1 to 1-¼-inches in low-acid foods
- Always double-check and use the headspace indicated on your verified recipe. *Note: Only use recipes that have been tested and verified to ensure best practices have been followed.*
After food has been packed into the jars, take a flat plastic spatula and release the air bubbles between the food and the jar. To release the bubbles, slowly turn the jar and move the spatula up and down to allow air bubbles to escape. This step can be skipped for jams, jellies, or all liquid products such as juices.
Prepare the jar for placing lids by measuring the headspace and cleaning the jar rim with a dampened paper towel. Cleaning the rim allows for a better seal with the lid’s gasket. Uncleaned rims may cause seal failures and ultimately spoiled foods.
Preparing and Applying Lids
Some recipes suggest that you preheat the lids before processing. To do this, simmer the lids at 180°F without boiling them. This is supposed to help soften the sealing compound but is not necessary with Ball or Kerr lids.
Once the jar is packed and the rim is cleaned, place one of the lids, gasket-side down, onto the cleaned jar-sealing surface. Fit the metal screw band over the flat lid and tighten the lid to the manufacturer’s specifications. If the lids are applied correctly the contents in the jar will contract during the cooling stage which will pull the self-sealing lid firmly against the jar to form a high vacuum, preserving the food.
If a lid’s ring is too loose some of the liquid could escape from the jar during processing and would not provide a secure seal. If a lid’s ring is too tight air cannot vent during processing and food will discolor during storage. Over-tightened lids can also lead to buckled lids and broken jars, especially if the jars are processed in a pressure-canner.
Selecting the Correct Processing Time
It is essential to choose the correct processing time in order to keep the food at a high enough temperature for long enough to maintain food integrity and safety. This can be achieved by following these suggestions depending on which canner you are using.
For boiling-water canners to destroy microorganisms in acid foods you must process the jars for the correct number of minutes in boiling water and cool the jars at room temperature. The food could spoil if you process for fewer minutes than your recipe specified or cool the jars in cold water.
For pressure canners to destroy microorganisms in low-acid foods you must process the jars for the correct number of minutes at the correct pressure and allow the canner to cool at room temperature until it is completely depressurized. The food could spoil if you process for fewer minutes than your recipe specified, process at a lower pressure than specified, fail to exhaust the canner properly or cool the canner with water.
Download these tables to be able to decide what pressure and process time your food needs and/or to verify that your recipe has the correct information. These tables work by choosing the style of pack, hot or raw, finding the correct jar size (½ pints and pints require the same and 1-½ pints and quarts require the same), then choosing the correct altitude.
Properly Cooling the Jars
Jars will be hot when you remove them from the canner. Use a jar lifter to pull the jar from the canner and place on a towel or a rack to cool without causing damage to your countertops. The jars should remain cooling at room temperature for 12 to 24 hours. When you remove the hot jars from the canner do not retighten their lids. If you attempt to retighten the lids the gasket could end up damaged, causing seal failures and spoiled food. If you raw-packed the food you will likely see a lower liquid level after the jar cools; during processing the jars will vent air and the food will shrink. Resist the urge to open the jar and add more liquid; this breaks the seal and will end in spoiled food if placed in storage.
Testing Jar Seals
Once the jars have cooled for 12 to 24 hours, remove the screw bands and test the seals with one of the following options:
- Press the middle of the lid with your finger. If the lid springs up when you release your finger the lid is not properly sealed.
- Tap the lid with the bottom of a teaspoon. If there is a ringing, high-pitched sound the jar is sealed properly. If it makes a dull sound, the lid may not be properly sealed. A dull sound may also occur if food is touching the bottom of the lid; if you can see food touching the bottom of the lid try one of the other two methods to verify a quality seal.
- Hold the jar at eye level and look across the lid. If the lid is properly sealed it should be concave (slightly curved down in the center). If the center of the lid is flat or bulging, it may not be properly sealed.
Screw bands should not be stored on jars. After they have been removed to test the seal wash, dry and store them in a dry area. If left on stored jars they become difficult to remove, often rust, and may not work properly again.
Storing Canned Foods
After testing the jar seals and finding that they are properly sealed, make sure that the screw bands are removed, wash the lid and jar to remove food residue, rinse and dry jars. Make sure to label and date the jars and store them in a clean, cool, dark, and dry place. *Note: do not store jars above 95°F or near hot pipes, a range, a furnace, under a sink, in an uninsulated attic, or in direct sunlight. If you store your jars in these conditions the food will lose quality in a few weeks or months and it may spoil. Dampness could corrode metal lids, break seals, and allow recontamination and spoilage.*
Accidental freezing of canned foods will not cause spoilage unless the jars become unsealed. Freezing and thawing could soften food and compromise the quality of the food. If the jars must be stored where they may freeze, wrap them in newspapers, place them in heavy cartons, and cover with more newspapers and blankets.
Reprocessing Unsealed Jars
If a lid fails to seal on a jar, remove the lid and check the rim of the jar for small nicks or other inconsistencies. If the jar has nicks or other problems, trade it out for a new one and reprocess within 24 hours using the same processing time and procedure. If you don’t have the time or do not want to reprocess the jars the food could be frozen instead. To do this, leave a headspace of 1-½ inches. If reprocessing or freezing will not work, an unsealed jar could be stored in the refrigerator and consumed within several days.