I boil mine till it's the right consistency, then pour into the equivalent of kliner jars, put the lid on, and that's it. The lid expands upwards under the force of the steam, the air expands in other words...then when the passata cools down, the air shrinks [technical I know...] and the lid pops because it's created a vacuum seal. Lost one jar in 30 over the last few years....still using it from last year, in fact, I opened one to make pizza the other night, lovely
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