Everything you need to safely can homemade fruit sauces at home - about the jars, the method, processing times and how to know for certain that the seal has worked.
This started with a reader's question: how to bottle our strawberry coulis so it keeps.
Our strawberry coulis can be stored safely in the fridge for a few days, but canning will ensure it keeps for months. The process is simple but you must follow certain rules to avoid it spoiling.
Lets get straight into it.
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Acidity is what makes canning safe.
Our first rule of canning:
The bacterium behind botulism (Clostridium botulinum) cannot grow in a high acid environment. Instead it thrives in low acid, oxygen free conditions which is precisely what you might have with low acidity foods kept a sealed jar (such as garlic in oil).
This is an important rule to follow as botulism is serious and improperly home-canned low-acid food is the most common cause of it and you cannot identify that it is present by sight, smell or taste. Therefore these rules must be adhered to.
This is why the acidity of what you're canning determines the method:
High-acid foods (pH 4.6 or below) - these are most fruits, jams, pickles, fruit sauces. These can be safely processed in a boiling water bath at 100°C/212°F.
Low-acid foods (pH above 4.6) - these are vegetables, meats, most tomato sauces, anything with added onion, garlic or peppers. These cannot be safely water-bath canned and they need a pressure canner reaching 116-120°C/240-250°F, because botulism spores can survive the temperature of boiling water.
Examples of high acid (below 4.6) sauces you can comfortably can using a water bath are:
- Strawberries - around pH 3.5
- Cherries - pH 3.2 to 4.5
- Apple sauce - pH 3.3 to 3.6
These high acid foods will only need around 5 minutes of processing.
For low acidity foods such tomato sauce, a chutney with onions in it, or anything with vegetables you will need to follow the process of pressure canning.
Water bath canning
What you will need
Jars - The thick canning glass jars, so they withstand the heat, with a rim heavy enough to form a proper seal. For ease of use they should be round rather than square so they are not a nuisance to pour and sauce does not become lost in the corners.
In the US these are Mason jars and in the UK they are Kilner jars. What matters is the two-part lid: a flat lid with a sealing compound, plus a screw band.
Why does water bath canning require a two part lid?
The two parts do to separate jobs.
The flat lid has a ring of sealing compound on the inside. This is the part that forms the seal during processing. As the compound softens against the jar rim and the jar cools, a vacuum forms pulling down the flat lid and setting it against the glass, thus creating an airtight barrier.
The screw band only holds the flat lid in place during processing.
They are separate so that air can escape during processing. As the jar heats, the contents expand and force air out through gaps in the lid. A single lid screwed on tight will trap the air and a vacuum will not form on cooling and it will not make a seal. The two part lid lets the flat lid lift very slightly to vent, then settle back down.
Once cooled, the lid will be held in place by the vacuum created. The band should be removed or it will rust shut and be difficult to remove a few months later. Removing the band also shows if the jar is improperly sealed as there won't be a vacuum and the lid will come away easily.
Note: Flat lids are single-use because once the compound compresses, it won't reliably seal again. Bands are reusable indefinitely. So buy replacement flat lids in bulk and keep the bands forever.
Check the jars carefully before you start:
- ensure there are no cracks or chips around the rims
- the lids lie flat and are not dented or rusty
- the sealing compound is intact on the flat lid
If in doubt about a jar, test it: half-fill it with water then close it, turn it upside down and shake. If it leaks then the seal is faulty and use another lids.
A canner - You will need a water bath canner for high-acid foods (a large pot with a lid and a rack to place inside the pot to rest the jars on so they do not touch the hot base), or a pressure canner for anything low-acid. Read the manufacturer's instructions before you start.
Sterilising
Anything that might touche the sauce needs to be sterilized:
- The jars
- The flat lids
- A spoon or metal funnel for filling
Screw bands won't touch the sauce, so they do not need to be sterilized.
Wash everything on the hottest dishwasher setting or by hand in hot soapy water, then place everything in a large bowl and rinse through with boiling water.
Use the jars while they're still hot. Pouring hot sauce into a cold jar can crack the glass whereas hot sauce into a hot jar will stop bacteria getting into the jar.
Filling the jars
- Fill freshly sterilized jars while they are still hot, with the sauce (also hot) to prevent development of bacteria without a need for using preservatives.
- Use a sterilized spoon or metal funnel to fill the jar or bottle three quarters full.
- Then using an oven mitt or tea towel, gently tap the jar on the counter so the sauce settles and releases trapped air bubbles. Continue filling till the sauce is almost to the top.
- Leave a quarter inch headspace (below the rim) so that when the sauce is processed it has room to expand, expelling air and creating a vacuum. This will prolong freshness of the sauce as air accelerates decay.
- Wipe the rim with a clean damp paper towel - a sticky rim is the most common cause of a failed seal.
- Close with the flat lid and screw band firmly, but not forced. Close it as tightly it as you would a jam jar that you intend to open again yourself without sweat and tears.
Heat Processing
This is the most important part of the canning process and one that will ensure your products can be stored safely.
Heat processing will vacuum pack the sauce thereby destroying any microorganisms and prolonging the freshness of the product.
The USDA recommends correct canning methods are followed and health and safety guidelines carefully adhered to in order to prevent any risk of illness.
Once the jars are closed, place in a canner filled with water. The best for home use are the boiling water canner or the pressure canner. For safety reasons, read through and carefully follow the instructions.
The temperature for heat processing will vary depending on the acidity value of the foods being canned for example, alkaline foods require higher temperatures (around 116℃-120℃/240℉-250℉ compared to acidic foods (100℃/212℉).
For higher altitudes adjust according to the temperature at which water boils for you.
Our strawberry coulis and cherry sauce require just 5 minutes in a boiling water canner - add 1 minute for every 1000 feet above sea level.
During the heating process the pressure increases and air and contents of the jar expand forcing excess air to escape through the screw cap.
On cooling, the pressure decreases and as the air has escaped, creates a vacuum which seals the lid pulling it downwards.
A lid which appears sucked downwards indicates that a vacuum has been created and the canning has been successful.
Set the jars aside aside to cool for 12 to 24 hours during which time the contents will settle.
Do not retighten their lids, but once they have cooled replace the two-part screw caps and lids with a regular single screw cap (if left on the two-part cap may become difficult to remove), then store the jars safely.
Check the seals
After 12-24 hours, check each jar:
- The lid is sucked slightly downwards - it should be concave, not flat or domed
- Press the centre - it shouldn't flex or pop, if it springs back, it hasn't sealed
- Tap it with a spoon - a sealed jar rings clear and high, an unsealed one thuds
Once you're satisfied, remove the screw band and replace it with a plain single cap for storage. Bands left on can rust in place and become impossible to remove and can also cover a faulty seal.
Storing
Label the jars with the contents and date them. Then store them somewhere cool, dark and dry. Properly sealed high acid sauces will keep for 6 to 8 months and retain their good quality.
Once opened: refrigerate and use within a week.
Troubleshooting
The jar didn't seal - you can still fix it within 24 hours. Replace the flat lid with a fresh one, check the rim is clean and undamaged and reprocess. Or skip the reprocessing and simply refrigerate it, unprocessed sauce will keep for 4 days in the fridge, or a month in the freezer.
Mould, or the colour has changed - discard it, don't taste it, don't scrape the top off! This usually means either the fruit was past its best before dates, or the equipment wasn't properly sterilised.
The lid is domed or bulging - discard it and don't open it near food or surfaces you'll use again. A bulging lid means gas is being produced inside, which means something is growing in there and its spores might be transferred to clean surfaces..
When in doubt, throw it out - a jar of sauce is not worth the risk.
Top tips for canning
Canning safely prolongs the freshness and quality of sauces as long as the following are adhered to:
- select fresh ingredients
- lemon juice (and sometimes sugar) act as preservatives
- use sterlized jars suitable for canning with self-sealing lids
- hot filling the jars (see below)
- process filled jars in boiling water or a pressure canner for the correct amount of time.
- check seals are intact
- cool, label and store in a cool dry place.
Frequently asked questions
Flat lids are single-use (the compound compresses once and won't reliably seal again), bands are reusable indefinitely. Buy replacement flat lids in bulk and keep your bands forever.
This is only possible with added acid. Tomatoes are on the pH 4.6 acidity borderline with modern varieties being often less acidic than older ones. To can them in a water bath safely you need to add either bottled lemon juice or citric acid. Standard guidance is for 2 tablespoons of bottled lemon juice per litre. Use bottled juice and not fresh, because its acidity is consistent. If your sauce has onions, garlic or peppers, it is no longer high acid but alkaline and you will need a pressure canner.
Not the flat lids as they are single-use. The sealing compound compresses when it seals and cannot be expected to do so a second time. The screw bands can be reused multiple times as long as they are not rusted or bent.
The flat lid should be slightly concave, it should not flex when you press the centre and should ring clear rather than thud when tapped. Check after 12-24 hours of cooling and not before.
Reprocess within 24 hours with a fresh flat lid, or refrigerate the sauce and use it within four days. You can also freeze it for up to a month. This way nothing is wasted, it will just need to be used sooner.
For naturally high acid fruits like strawberries and cherries, they are acidic enough on their own. Lemon juice can act as a preservative and helps colour retention, but it is not actually required for safety. For anything borderline acidic such as tomatoes, figs, certain pears, added acid is a safety requirement, not an option.
They will last six to eight months to be good quality, stored somewhere cool, dark and dry. Once opened, refrigerate and use within a week.
Anything low-acid such as vegetables, meat, fish, beans, most savoury sauces and anything containing onion, garlic or peppers. These require a pressure canner, which reaches temperatures a boiling water bath simply cannot. Water bath canning low acid food is the single most common cause of home-canning botulism.
For processing times under 10 minutes such as high acid fruit sauces, you will need to sterilize them first. A 5 minutes process is not long enough to sterilize the jars, whereas jars processed for 10 minutes or longer are effectively sterilized during processing.
Sources
Canning safety guidance in this article follows the USDA and National Center for Home Food Preservation recommendations, including their guidance on processing temperatures for food preservation.
If you're canning anything beyond the high-acid fruit sauces covered here, check their guidelines directly before you start.
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