Some handy information for Bottling tomatoes in particular but good generally...

Bottling tomatoes. . .

Tomatoes are best cooked before bottling. Chop them up into small pieces, and then cook gently in a saucepan (without adding any water or oil) until they soften and the juice runs from them. Add 1 teaspoon of salt and one of sugar per pound of raw tomatoes. If you like, you can also add some herbs. Then either pack the jars with the cooked chopped tomatoes and juice. Or, alternatively, puree the cooked tomatoes in a blender, and then fill the jars with the puree.

You can use special jars with rubber seals, such as Kilner jars. I find these expensive, and also usually too large to fit in my pressure cooker. We usually just use recycled jam or other jars for bottling. You need to check that the lids are in good condition and that they will seal properly. Most jars these days have a 'button' set in the lid so that you can see whether or not they are sealed - if the button is popped down, the jar is sealed, if it is up, it is not.   Whatever type of jars you choose, they should be washed thoroughly in hot soapy water and then rinsed with boiling water before you use them.

Once you have your full bottles of tomatoes, you need to sterilise them. Before you start, put the lids on the jars if you haven't already done so. If you are using Kilner jars with a metal screw top, screw it on all the way, then turn back a half turn. Jam jar lids should be screwed on fully, jars with clip on lids should be clipped down. Any bacteria present in the food will be killed by the heat treatment, and a hermetic seal on the jars stops any new bacteria getting in. Bottled tomatoes last a long time but not indefinitely, as chemical changes can still take place. In practice, so long as you eat one year's bottled food before the next harvest, this shouldn't ever be a problem.

There are three different ways of sterilising the jars; in a pressure cooker, in a hot water bath, or in the oven. Using a pressure cooker is in my opinion very much the best solution; it uses less gas or electricity, is much quicker, and is also safer, because the jars are heated to a higher temperature. In particular, non-acid vegetables - effectively all vegetables other than tomatoes - must only ever be bottled using a pressure cooker. The temperatures reached in a hot water bath or the oven are not high enough to kill the bacteria that cause botulism, which can be fatal.

To sterilise the bottles in a pressure cooker, you will need the cooker, set to its highest pressure setting, plus the cooker's trivet if it came with one. If you don't have a trivet, find some other way to keep the jars lifted off the bottom of the cooker, otherwise the direct heat will crack the glass. If you have a plate that will fit, this is ideal, or you can use several layers of folded cardboard.

Put in as many jars as will comfortably fit. Make sure that they don't touch, or they will crack. I use a piece of butter muslin wedged between the jars, just to make sure. Then add water to the cooker to come about 1 inch (2cm) up the side of the jars. Use at least 1.5 pints of water. Put the lid on the cooker, and bring slowly up to pressure.

Keep it at pressure for 7 minutes. Then turn off the heat, and leave the cooker on the stove to depressurise slowly. This is very important - if you let the pressure out quickly, the jars will crack. Once the cooker has depressurised and the bottles are cool, check the seal, label the bottles with the date, and then store in a cool dark place.

When you come to use the tomatoes, always check the seal again. For jamjars, if the button isn't down, and it doesn't 'pop' when you open it, don't use the contents. For Kilner jars, or jars with clip-down lids, the glass lid should stay in place when you either undo the clip, or unscrew the metal band. To remove the glass lid, pull on the tag on the rubber sealing ring until the seal breaks.