Jam Making Comments
SAFETY FIRST
Beware: boiling fruit and jam will foam and increase in volume significantly - especially at the rolling boil stage - try to avoid more than half-filling your saucepan, jam kettle or stock pot to avoid boiling-over and a big sticky mess. It's better to do two batches than risk chaos and calamity that might put you off ever trying to make jam again. Stir gently - vigorous stirring can cause the mixture to suddenly erupt. WARNING: If you are using a gas hob NEVER leave jam or fruit boiling unattended - even if it looks like it is going to take a while. In the event the liquid does boil over it will almost certainly put the flame out and your house will slowly fill with gas. D A N G E R: Jam syrup is extremely hot, treat it like lava and keep kids/pets away when boiling and potting up.
SETTING POINT
I noticed when testing for the set that the plate from the freezer was getting condensation on it and this was affecting the jam I was trying to test - wipe the plate dry with a tea-towel before blobbing the jam on
As for the conundrum about boiling to a set, the principle used in determining when a set has happened is really a very simple one. A set is reliably achieved when the Jam mix has been boiled long enough to have reached a temperature of 105c To get to this temperature, a certain amount of water has to have been boiled out of the mixture you have extracted such that the ratio of cooked sugars to water is such that the mix left, is sufficiently of sugar and or fruit pulp, for its boiling point to have been raised by approximately 5 degrees over the boiling point of water. Obviously if you use less water in the mix, the less you will have to boil away and the quicker you will get to the setting point. To test for a set you can either use a specific jam thermometer, an electronic temperature probe for food use, with a temperature scale which goes at least as far as 105 degrees celcius or, you can use the 'spoon and plate method' All are equally valid. Obviously the results from using the 'spoon and plate method' are open to a wide margin of interpretation but one way of being more certain with this method is, if you push your finger into the blob of jam on the plate, sometime after it has cooled (about a minute) and you see wrinkles forming in the surface of the blob as you push it, you have a set and can stop the boil.
STERILISING JARS
Simply washing them in the dishwasher will do the trick, or washing in warm soapy water and putting in the oven at 100c for 20 mins.