How to Sterilise Your Jars

sterilising jars

The only thing for sterilising jars!

One of the highlights of my life at Wares of Knutsford is receiving the wonderful correspondence from you, the customer. I get loads of lovely emails saying how much you appreciate the products and service, but I also get quite a few technical queries. As would be expected most of the queries revolve around glassware and there is absolutely no doubt that the question most often asked is “how do I go about sterilising jars?” – for use in this, that and the other. So as our lovely new website includes a blog I have the opportunity to answer some of your questions and not surprisingly I’m going to start with the most popular one. You will also be pleased to hear that the answer to sterilising jars is not at all complicated. The key to understanding the process of sterilising jars is to understand exactly what we are trying to achieve and what the term “sterile” really means.

The Myths of Sterilising Jars

I’m afraid to say there are many myths surrounding sterilising jars and other glassware. I’m also sorry to have to tell you, for all practical purposes there is only one way to ensure that your jars and bottles are sterile. All sorts of methods from soaking in antiseptic, through heating in ovens at various temperatures, to standing immersed in boiling water are often banded around. The simple fact is none of these methods ensure sterility. Don’t get me wrong they do, to varying degrees, improve the pot life of your preserves, but they will not get your jars sterile.

sterilising jars

Perhaps it would be good at this point to define what is meant by sterile as I can see some of you scratching your heads saying “…..well I’ve done it that way for years and it’s been all right.” Sterile is an absolute term in other words something can’t be “nearly sterile,” “less sterile,” “more sterile,” and so on. Something is either sterile or it isn’t! And sterile is defined as “the absence of life.” In this instance of course the life we are concerned about is not wildebeest or great white sharks, but micro-organisms; bacteria and fungi.

Now all the ways of “sterilising jars” that have been handed down from our Grandmothers knee, if you pardon the mixed metaphors, all go some way to reducing the numbers of bacteria and fungi present on our glassware just prior to us filling it with our lovingly crafted preserves. Indeed, sometimes we will get lucky and the process we have used will have killed off all the microbes in some of the jars and the preserves will keep well. In other instances those few that remain have been severely weakened and take a long time to grow sufficiently to spoil our jam. But the point is we cannot guarantee the jars to be truly sterile and will always be living with “mixed results” when it comes to shelf life. In fact I need go no further than my earlier citation that sterility, or the lack of it, is the most common question I’m asked, to illustrate how hit and miss most “traditional” processes are.

So how do we go about sterilising jars

sterilising jars

Okay, I hear you cry, you’ve convinced us there is only one way to ensure sterility, will you please now tell us what that is!! Two words – Pressure Cooker. To ensure that all micro-organisms and their spores are killed off you must maintain a moist heat (most important) of 121oC for 15 minutes. Fortunately this is the temperature that most domestic pressure cookers operate at, so all you need to do is “cook” your glassware for 15 minutes in a pressure cooker and you are assured sterile jam jars every time. Now there is a bit more to the technique than that and I would suggest you read Jess’ blog post on his website by clicking here and also watch his video presentation. Having said that it is nothing any one of us can’t carry out with minimal effort and the purchase of a standard domestic pressure cooker. I hope this has gone a long way to clearing the myth of sterilisation and I assure you if you adopt this method mouldy jam will be a thing of the past.

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