I have been making jams for many years and have never used this method although I know my mother used to boil the jars and lids. What I do is before using them I wash them in the washing machine in the high temperature program (65C). I usually wash them in the evening and they stay in the washing machine all night.
Would you believe that in the morning they are still warm? So I think they are well sterilized.
Wash the strawberries with plenty of cold water and remove the stems. Place them into a colander so that they dry and then into a large pan. As I like some chunks of fruit in the jam, I leave them whole but know that this will take longer to set as by crushing them it releases the natural pectin so it can thicken.
Add sugar and brandy, then cover with cling film and place into the fridge overnight.
The next day bring to a boil. When they begin boiling lower heat to the minimum and you will need to stir it once in a while. You must remember that the mixture will rise as it boils.
As it boils you will see some foam emerging on the top. That should not worry you because that’s just a lot of air trapped in the bubbles from the boiling but it’s best to remove it as the jam looks better without it. Otherwise it is harmless. You may avoid foaming by adding 1 teaspoon of butter or margarine to the mix but this will help early spoilage of the jam, so it is better to skim it.
After hours of simmering you will notice that your jam is still too runny. What you have to do is put some of the boiling jam in a spoon and let it run back into the pan. If all the jam falls away from the spoon it is not ready yet. You must keep checking this during the last half an hour at least every 10 minutes and when a last drop does not drop down but rests on the spoon this means that it is set.
Add the lemon juice and stir and remove from the heat and let it cool down. Don’t worry if the jam is still runny, as it cools down it will thicken. Place jam in the jars, cover with a lid, label and store in a cool, dark cupboard for at least two to three months otherwise refrigerate.