How to Sterilize Jars For Preserving

Correctly sterilizing your jars, lids and bottles is a vital part of the preserve making process as it will stop the contents spoiling. With a little know how you can confidently, easily and safely sterilize jars time after time.   

Everything You Need to know to Correctly Sterilize Jars

These guidelines are only for preserves made with sugar and vinegar as the preservative –  Jams, jellies, chutneys and the like.  

Greater precautions are needed for canning fruits and vegetables where the jar contents are not preserved and shelf stable with sugar or acidity.  

Why Sterilize Jars for Jams, Jellies and Preserves

I love to preserve and probably make a small batch of jam or jelly every few weeks – its a relaxing and fun hobby, and it is very satisfying to bring some product home from the market, or friend’s or family member’s garden and make something either for us or to be given away 

Sterilizing the jars for jams, jellies, chutneys, pickles and relishes will remove bacteria and mould spores and stop the contents spoiling.   

After all the effort and expense to make jam, it would be a shame to take a jar from the cupboard and find it has gone mouldy.  At best it is a waste, at worse there could be a bug in your preserve that could make you quite unwell.  

Jars and bottles are easy to sterilise and have ready and warm when you need them.   

Wash & Check Jars Before Sterilizing  

Before sterilising your jars must be scrupulously clean, with no stuck on food or dust. 

 I put all my empty jars through the dishwasher and check they are clean before they are put away, even so some will slip through with some food stuck on and I nearly always find dust in them, even though they have been stored in a cupboard with their lids on.  

It is vital to check jars for chips and cracks. A chip on the rim will stop the lid sealing and a cracked jar can shatter when you fill it.   I’ve once found a piece of broken glass at the bottom of a brand new jam jar!  Check and then double check! 

Three Ways To Sterilize Jars for Jam Making 

1 – Sterilizing Jam Jars in Boiling Water 

This is the only method recommended for home preservers to fully sterilize jam jars. 

1. Place a rack at the bottom of a large maslin pan or saucepan. Place the jars on the rack the right way up. 
2. Completely submerge the jars. Make sure the water is at least 5cm/2″ above the top of the jars. 
3. Bring the water to the boil, 
4. Start timing: boil the jars for ten minutes. 
5. Remove the jars before filling, taking care not to burn yourself. Place them upside down on a clean tea towel to drain.

How to Sterilize Jam Jar Lids In Boiling Water

Place lids or rubber seals in a small pan and fill with 4 inches of water, heat and simmer for 10 minutes, turn the heat off and cover the pan until you are ready to seal the jars.

2- Sterilizing Jars in the Oven

Sterilizing jars in the oven is not thought to be as effective as using a water bath as above. However, we’ve used this method successfully for several years.

Heat the oven to 160°C 320°F Gas Mark 3. Wash and dry the jars thoroughly. Place in a roasting tray, and then into the oven for 15 minutes.

Remove from the oven and fill immediately, using a jam funnel.

3 – Sterilizing Jars in a Microwave Oven

Again, this is not as effective as using a water bath. Wash and rinse the jars, and do not dry them. Place them, still damp, in the microwave. Microwave on high for 1 minute.

How to Avoid Thermal Shock By Warming Sterlized Jars

It’s very important that glass jam jars are hot before you fill them with boiling jam or chutney. Adding hot jam to a cold jar will result in a thermal shock one or more of your jars will probably break or shatter. There are three easy ways this can be done. 

1 – Warming Jars in the Oven 

Using the oven to warm jars before filling. It’s not recommended as a way of fully sterilizing jars, because unlike submerging jars in water, air is poor at transferring heat.

However, the oven is a handy way to heat jars before filling to avoid thermal shock. 

1. Place jars in the oven on a baking tray
2. Set oven to 160C/Gas mark 3. 
3. Let the oven come up to temperature. By allowing the jars to warm slowly as the oven, the chances of jars breaking from thermal shock are minimized. 

2 – Warming Jars in a Dishwasher

1. Place the jars in the dishwasher, open side down. 
2. Run the dishwasher on a full cycle on it’s hottest setting. This is normally about 70°C/ 160°F
3. Once the dishwasher has run, fill the hot jars immediately with the freshly made jam. 

This method does require some careful timing: you don’t want the jars to cool before filling them, but a full dishwashing cycle can take longer than it takes to make jam. You might find it useful to time a full cycle of your dishwasher, so that you can start making jam timed to coincide with the machine’s cycle. 

3 – Warming Jars in a Microwave 

If you find that you haven’t prepared enough jam jars for all your preserve, then you can warm a jar in the microwave.

1. Fill it about one quarter full of water, and place in microwave with a plastic lid on top.
2. Microwave at full power for 2 minutes (timing may vary according to the power of your machine).
3. Remove from the microwave, and pour out all the water. Stand the jar upside down on a clean tea towel for a couple of minutes to drain. 
4. Fill with jam or preserve while the jar is still hot. 

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