How to Safely Pressure Can Onions: A Comprehensive Guide

Onions, a culinary cornerstone, add depth and flavor to countless dishes. Preserving them through pressure canning offers a convenient way to enjoy their pungent goodness year-round, especially when your garden overflows or you find a great deal. However, it’s crucial to understand that pressure canning is the only safe method for preserving onions due to their low acidity. This guide will walk you through the entire process, ensuring safe and delicious results.

Table of Contents

Understanding the Importance of Pressure Canning for Onions

Why can’t onions be safely preserved using a water bath canner, like jams and jellies? The answer lies in their pH level. Foods with a pH above 4.6 are considered low-acid and can harbor Clostridium botulinum spores, the bacteria responsible for botulism. These spores thrive in anaerobic (oxygen-free) environments, like the inside of a sealed jar.

A boiling water bath canner doesn’t reach a high enough temperature to kill these spores. Pressure canning, however, reaches temperatures of 240°F (116°C) or higher, which effectively destroys Clostridium botulinum spores, making it a safe method for preserving low-acid foods like onions.

Gathering Your Equipment and Ingredients

Before you begin, ensure you have all the necessary equipment and fresh, high-quality onions. This meticulous preparation is crucial for a safe and successful canning experience.

Pressure Canner: A pressure canner is specifically designed for low-acid foods. Make sure it’s in good working order, with a clean gasket and a properly functioning pressure gauge or weighted gauge.
Canning Jars: Use only jars specifically manufactured for canning. Check them for chips or cracks, especially around the rim, which can prevent a proper seal. Pint or quart jars are suitable for onions.
Lids and Bands: Use new, unused lids for each batch. The sealing compound on the lid is essential for creating an airtight seal. Bands can be reused if they are in good condition.
Jar Lifter: A jar lifter is an indispensable tool for safely lifting hot jars from the canner.
Bubble Remover/Headspace Tool: This tool helps remove air bubbles from the jars and accurately measure headspace.
Wide-Mouth Funnel: A funnel simplifies filling jars, minimizing spills and mess.
Large Pot: A large pot is needed for heating the onions before packing them into jars.
Cutting Board and Knife: For preparing the onions.
Clean Kitchen Towels: For wiping jar rims and general cleanup.
Onions: Choose firm, mature onions without blemishes. Select onions of similar size for even cooking.
Salt (Optional): Canning salt (also known as pickling salt) is recommended, as it doesn’t contain iodine or anti-caking agents that can darken the onions or cause discoloration.
Water: Use good-quality drinking water.

Preparing the Onions for Canning

Proper preparation is key to ensuring the safety and quality of your canned onions.

Wash and Peel: Thoroughly wash the onions under cold, running water. Remove the outer layers of skin, any blemishes, and the root end.
Slice or Dice: Slice or dice the onions to your desired size. Consistent sizes will ensure even heating during processing.
Hot Pack (Recommended): Place the sliced or diced onions in a large pot and cover them with water. Bring the water to a boil and simmer for 5 minutes. This “hot pack” method helps reduce shrinkage in the jars and improves the vacuum seal. A cold pack method is acceptable, but may increase the chance of siphoning.
Keep Onions Hot: Maintain the onions in the hot water until you are ready to fill the jars.

Packing the Jars

Proper jar packing is essential for achieving a good seal and preventing spoilage.

Sterilize Jars: While not strictly required if the processing time is 10 minutes or longer, sterilizing your jars ensures they are free from any harmful bacteria or mold. You can sterilize jars by boiling them in water for 10 minutes. Keep jars hot until ready to use.
Fill Jars: Using a slotted spoon, transfer the hot onions to the sterilized jars, leaving 1-inch headspace. Headspace refers to the space between the top of the food and the lid. Accurate headspace is crucial for proper sealing.
Add Salt (Optional): If desired, add 1/2 teaspoon of canning salt per pint jar or 1 teaspoon per quart jar.
Cover with Hot Cooking Liquid: Pour the hot cooking liquid (the water the onions were simmered in) over the onions, maintaining the 1-inch headspace.
Remove Air Bubbles: Use a bubble remover or other non-metallic utensil to gently release any trapped air bubbles by running it along the inside of the jar.
Adjust Headspace: If necessary, add more hot liquid to maintain the 1-inch headspace.
Wipe Jar Rims: Use a clean, damp cloth to thoroughly wipe the rims of the jars. Even a small amount of food residue can prevent a proper seal.
Apply Lids and Bands: Place a new lid on each jar, ensuring the sealing compound is facing down. Screw the band on fingertip tight. This means tightening the band until you feel resistance, then backing off slightly. Overtightening can prevent the jar from venting properly during processing.

Pressure Canning the Onions

This is the critical step for ensuring the safety of your canned onions. Always follow the manufacturer’s instructions for your specific pressure canner.

Add Water to Canner: Pour the amount of water specified in your pressure canner’s instruction manual into the canner. Typically, this is about 2-3 inches of water.
Load Jars into Canner: Use a jar lifter to carefully load the filled jars into the canner, ensuring they do not touch each other or the sides of the canner.
Secure the Lid: Secure the canner lid according to the manufacturer’s instructions. Make sure the vent pipe is open or that the lid is properly locked, depending on your model.
Vent the Canner: Turn the heat to high. Allow the canner to vent steam for 10 minutes. This crucial step removes air from the canner, ensuring the correct temperature is reached.
Pressurize the Canner: After venting, close the vent (if applicable) and allow the pressure to build to the recommended level for your altitude. Refer to the processing chart below for the correct pressure and processing time.
Maintain Pressure: Once the correct pressure is reached, reduce the heat to maintain a steady pressure. Monitor the pressure gauge closely, adjusting the heat as needed. Start timing the processing time once the correct pressure is reached. Maintaining consistent pressure is critical for safe canning.
Processing Times and Pressures:

Jar Size	Processing Time (Minutes)	Pressure (Dial Gauge)	Pressure (Weighted Gauge)
Pints	40	11 psi (0-2,000 feet) 12 psi (2,001-4,000 feet) 13 psi (4,001-6,000 feet) 14 psi (6,001-8,000 feet)	10 psi (0-1,000 feet) 15 psi (Above 1,000 feet)
Quarts	40	11 psi (0-2,000 feet) 12 psi (2,001-4,000 feet) 13 psi (4,001-6,000 feet) 14 psi (6,001-8,000 feet)	10 psi (0-1,000 feet) 15 psi (Above 1,000 feet)

Altitude Adjustment: Adjust the pressure according to your altitude. Higher altitudes require higher pressures due to the lower boiling point of water. The chart above provides guidelines. Always consult your pressure canner’s manual for specific instructions.
Depressurize the Canner: Once the processing time is complete, turn off the heat and allow the canner to depressurize naturally. Do not force cool the canner by running it under cold water, as this can cause the jars to break or seal improperly.
Wait 10 Minutes: After the pressure has returned to zero, wait 10 minutes before carefully removing the lid. This allows the pressure inside the jars to equalize, preventing siphoning.

Cooling and Storing Your Canned Onions

Proper cooling and storage are the final steps in ensuring the longevity and safety of your canned onions.

Remove Jars from Canner: Using a jar lifter, carefully remove the jars from the canner and place them on a towel-lined surface, leaving space between the jars for air circulation.
Do Not Disturb Jars: Avoid touching or tilting the jars while they are cooling.
Listen for the Seal: As the jars cool, you should hear a popping sound as the lids seal. This is a good indication that a vacuum has formed.
Check the Seals: After 12-24 hours, check the seals by pressing down on the center of each lid. If the lid doesn’t flex or pop, it is properly sealed. Alternatively, you can remove the band and lift the jar by the lid. If the lid holds, it is sealed.
Unsealed Jars: If a jar hasn’t sealed, you have two options: reprocess it with a new lid within 24 hours, following the same processing time and pressure, or refrigerate the onions and use them within a few days.
Remove Bands: Once you’ve confirmed that the jars are sealed, remove the bands. This prevents moisture from accumulating under the bands, which can lead to rust.
Wash Jars: Wash the outside of the jars with warm, soapy water to remove any residue.
Label and Date: Label each jar with the contents and the date.
Store in a Cool, Dark, Dry Place: Store your canned onions in a cool, dark, dry place, such as a pantry or cupboard. Properly canned onions can last for 12-18 months.

Safety Precautions

Always follow tested recipes and processing times: Do not experiment with your own recipes or shorten processing times.
Inspect jars for damage: Do not use jars with chips, cracks, or other damage.
Maintain proper headspace: Accurate headspace is crucial for a proper seal.
Adjust for altitude: Adjust processing pressures according to your altitude.
Never force cool the canner: Allow the canner to depressurize naturally.
Check seals before storing: Ensure all jars are properly sealed before storing.
If in doubt, throw it out: If you have any concerns about the safety of your canned onions, discard them.

Using Your Canned Onions

Canned onions are a versatile ingredient that can be used in a variety of dishes. They are a convenient way to add flavor to soups, stews, sauces, casseroles, and more. Before using canned onions, inspect the contents of the jar. If the liquid is cloudy, the lid is bulging, or the onions have an unusual odor, discard the jar immediately. When using canned onions, you may want to rinse them to remove some of the salt. They may be softer than fresh onions due to the canning process, so adjust cooking times accordingly.

Pressure canning onions is a rewarding way to preserve this essential ingredient. By following these guidelines carefully, you can enjoy the taste of homegrown or locally sourced onions year-round while ensuring the safety of your preserved food. Remember that safety is paramount in home canning. Always prioritize tested recipes and processing times, and never hesitate to discard any jars that show signs of spoilage. With a little practice and attention to detail, you’ll be enjoying delicious, home-canned onions in no time.

Why is pressure canning necessary for onions, instead of water bath canning?

Onions are a low-acid food, meaning they have a pH level above 4.6. This is significant because Clostridium botulinum, the bacteria that produces botulism toxin, thrives in low-acid environments. Water bath canning, which reaches a maximum temperature of 212°F, is insufficient to kill these bacteria and their spores. Therefore, using a water bath for low-acid foods like onions can lead to botulism poisoning, a potentially fatal illness.

Pressure canning, on the other hand, reaches temperatures of 240°F or higher. This high heat, achieved through the increased pressure, is necessary to effectively destroy Clostridium botulinum spores and ensure the safety of the canned onions. Properly following pressure canning guidelines is crucial to eliminate the risk of botulism and create shelf-stable, safe food.

What kind of pressure canner should I use for canning onions?

There are two main types of pressure canners: dial-gauge canners and weighted-gauge canners. Both are effective for canning onions if used correctly, but they operate differently. Dial-gauge canners feature a gauge that indicates the pressure inside the canner, requiring users to monitor and adjust the heat to maintain the proper pressure. These gauges need to be tested for accuracy annually.

Weighted-gauge canners use a weight that jiggles or rocks to indicate the pressure is correct. These are considered more foolproof because they don’t require gauge testing and maintain pressure automatically once the weight is jiggling or rocking at the appropriate frequency. Ensure whichever type you choose is specifically designed for pressure canning and has a capacity large enough for your canning jars.

What is the proper jar size and headspace for pressure canning onions?

For safety and consistent results, it’s essential to use the correct jar size and headspace when pressure canning onions. The National Center for Home Food Preservation recommends using either pint or quart jars for canned onions. Using jars larger than quart size isn’t recommended because the heat penetration may not be adequate to destroy all botulism spores.

Headspace refers to the empty space left at the top of the jar between the top of the food and the lid. Leave 1 inch of headspace when packing your onion pieces into the jars. This allows for expansion of the food during processing and helps create a proper vacuum seal. Too little headspace can lead to jar breakage or seal failure, while too much can lead to spoilage.

Can I add butter, oil, or flour to my onions before pressure canning them?

No, it is strongly discouraged to add butter, oil, or flour to your onions before pressure canning. These ingredients can interfere with the heat penetration within the jars, potentially preventing the destruction of botulism spores. Fats like butter and oil can also become rancid during storage, affecting the quality and safety of the canned product.

Adding thickeners like flour can also alter the density of the food, further hindering heat penetration and increasing the risk of spoilage. For safe pressure canning, stick to tested and approved recipes that do not include these ingredients. After opening a jar of properly canned onions, you can add butter, oil, or flour as desired during cooking.

How long should I process onions in a pressure canner?

Processing times vary depending on the jar size and altitude. For pint jars, process for 30 minutes at 11 pounds of pressure for dial-gauge canners (at altitudes of 0-2,000 feet) and 10 pounds of pressure for weighted-gauge canners (at altitudes of 0-1,000 feet). For quart jars, process for 40 minutes at the same pressure levels. Always adjust pressure according to your altitude.

It is critical to follow the recommended processing times exactly. Underprocessing can result in spoilage and potential botulism poisoning. Overprocessing can soften the onions excessively, but it is safer than underprocessing. Consult a reliable source like the National Center for Home Food Preservation for specific processing times and altitude adjustments.

What happens if the pressure drops during the canning process?

If the pressure drops below the recommended level during the canning process, it’s crucial to take immediate action to ensure the safety of your canned onions. Do not open the canner or remove the jars. Doing so could cause the jars to break due to the sudden change in pressure and temperature.

Instead, bring the canner back up to the required pressure as quickly as possible. Once the correct pressure is reached, begin the processing time again from the beginning. This will ensure that the onions are processed for the full duration required to destroy any harmful bacteria. Make a note of the incident and consider discarding the batch if you are uncertain about its safety.

How should I store my pressure-canned onions?

After processing and cooling, remove the jars from the pressure canner and check the seals. The lids should be concave and not flex when pressed in the center. If a jar hasn’t sealed properly, refrigerate the onions immediately and use them within a few days, or reprocess them with a new lid within 24 hours.

Store properly sealed jars in a cool, dark, and dry place, away from direct sunlight. Ideal storage temperatures are between 50°F and 70°F. Properly canned onions can be safely stored for up to one year. Before using, check for any signs of spoilage, such as bulging lids, unusual odors, or mold. Discard any jars that show these signs.