How to sterilise and prepare jars

When you make preserves, you need to ensure you exclude all bacteria from the jar or bottle you’re using, otherwise you run the risk that unwanted organisms can take hold, feed off your preserve, and spoil all your hard work. You can do this by sterilising your jars and lids before adding any food products.

Excluding air is also vital, as oxygen allows lots of different bacterial and fungal organisms to thrive. You can keep oxygen out of jars by ensuring they have sound/undamaged lids, and seal tightly. That means that if you’re hoping to reuse jars, you must only ever do so if the lids have no scratches, dents or otherwise damaged areas as these can all contribute to an imperfect seal.

Different methods

There are quite a few different ways to sterilise jars, so we’ve covered the most common ones here and then you can decide which works for your setup and needs (eg using a microwave will be no good if you want to process lots of jars, or if you’re living off-grid). Regardless of what method you use, you will need to begin with scrupulously clean jars. If you’re buying new jars from a wholesale supplier (in Australia, the two major suppliers are Plasdene and Cospak), they will be clean from the packaging and need only sterilising before filling. If you’re purchasing from discount stores or supermarkets, it’s advisable to wash new jars as if they were second-hand.

It’s also important to note that you should only ever use glass jars for preserving jams or chutneys that you want to be shelf-stable – plastic can’t safely withstand the heat involved. Likewise, it’s best to use metal lids with a “puff compound” (the raised rubber seal inside the lid) rather than plastic lids, as they seal more easily. If you’re using plastic lids, you’ll need to also use some kind of air excluder like wax discs, cellophane seals or food-safe liner that creates an air-tight seal.

A note on reusing jars

If you’re reusing old glass jars, as well as thoroughly checking lids for any damage, it’s a good idea to remove labels before putting preserves into them, as this allows you to inspect all surfaces for possibly chinks or cracks. Use any damaged jars for other things rather than preserving (or freezing), as extreme temperatures can lead to glass explosions and wastage of all your hard work.

  1. To clean glass jars, soak for a few hours in very hot water to loosen any labels and food residue.
  2. Peel off labels and scrape any remaining glue off with a flat knife and if necessary, a scouring pad.
  3. Then in clean, hot soapy water, use a scrubbing brush to thoroughly clean every surface – inside and outside the jar. Pay special attention to the ridges around the lip where the lid screws on.
  4. Rinse well in hot water and allow to air dry.

Now your jars will be clean, and ready to sterilise using one of the following methods.

1. Using an oven

This is our preferred method, especially when doing larger batches.

  • Place clean jars on a flat baking tray in the oven
  • Heat to 100-110C and keep there for at least 10 minutes
  • Remove just before filling, and fill when hot

2. In a Dishwasher

This method only works if you have a dishwasher that thoroughly cleans and doesn’t leave any residue whatsoever on glasses

  • Place clean jars in the top tray of the dishwasher
  • Run a short HOT cycle
  • (Shake off any remaining water droplets)
  • Remove just before filling, and fill when hot

3. In a microwave

Good for small, quick batches. DO NOT microwave lids. DO NOT microwave jars with metal clips or rings on them.

  • Place clean, wet jars in the microwave
  • Zap for 45-60 seconds on HIGH (longer for larger jars, shorter for smaller)
  • Remove just before filling, and fill when hot

4. Using a large pot

Good if you’re doing medium size batches, especially if you will be hot-water bath canning them after packing (eg for low acid pickles etc)

  • Use a large pot that is 10cm or more taller than your jars
  • If you have a rack that fits inside the base of this pot, place it in there. If you don’t fold a clean teatowel so it covers the base of the pot and place that in instead.
  • Position your jars in the pot  – opening facing up – so they are not touching each other or the sides of the pot
  • Cover with hot (not boiling) water. Make sure there’s at least 5cm of water above jar rims
  • Bring pot to the boil over high heat
  • Once boiling, start a timer and boil jars for a minimum of 10 minutes**.
  • Turn off heat, and take jars out with clean tongs or a jar lifter. Sit on a clean dry teatowel to dry, but don’t let cool down.
  • (You can leave the jars in the boiled water for up to an hour, so get this ready to time with when your preserve will be finished)
  • Fill while jars are still hot

5. In a bottle sterilizer

Alright for small batches

  • Use clean jars and follow manufacturer’s directions
  • Fill while hot

6. Using sterilizer tablets/solution

While often used in bottles for wine and beer making, this isn’t really a good way to sterilise jars for preserving, as it’s a cold process, which means you have to heat them up before filling. So it stands to reason that you may as well use one of the other methods and save yourself the money you might spend on sterilising tablets.

To sterilise the lids

If you’re reusing lids, make sure they are undamaged, with no dents, scratches or rusted areas, then wash & rinse them as for the jars and drip dry. For all clean/new lids, place them in a bowl and completely cover with boiling water shortly before you fill your jars. Let them sit in the boiling water for at least 10 minutes, then remove with tongs and shake off as much water as possible before applying to jars. NB – use rubber or silicone tipped tongs, as metal tongs can scratch the lining of the lids and thereby lead to rust & bacteria contamination.

So, there you go! Lots of different ways to sterilise jars for preserving. Have a play with a few different methods to find the one that’s right for you, and pretty soon it’ll become second nature to set up as part of your preserving process.

If you’re looking for a chance to practise your newly learned skills, why not have a go at one of these tasty recipes?

Nanna Anna

Permaculture Principle 2: Catch and store energy; 5: Use and value renewable resources and services; 6: Produce no waste; 9: Use small and slow solutions; 10: Use and value diversity

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