I'm vegetarian and I make a lot of jam.
The gelling agent in jams is pectin - it's naturally found in fruit. Certain fruits have more pectin in than others, so you can get a better 'set' from fruits such as apples, citrus fruits and currants. Soft fruits like strawberries and apricots are low in pectin, so extra pectin will need to be added so the jam won't be too runny.
You can make low-pectin fruits set better by combining them in a jam with a high-pectin fruit, which is combinations like apple and blackberry jam are common. Or you can add a squeeze of lemon juice to soft fruit, to help it set.
You can also buy pectin powder to get a good set - I imagine this is used commercially too. It's a dried powder made from pectin extracted from apples and is completely safe for vegetarians/vegans to eat.
I have never heard of jam being made with gelatin or isinglass, but you could check with the company if it makes you feel better :-)
The only way to be completely sure what goes into something is to make it yourself! A simple recipe for strawberry jam is:
Serves: makes 4 x 500g jars
Prep: 20 min
Cook: 1 hr 10 min
Ingredients
1.5kg Strawberries
1 lemon, juice only
1.25kg Sugar
1. To prepare your strawberries, remove all tops and greenery, and chop large fruits into chunks.
2. Put the strawberries and lemon juice into a large, heavy-based saucepan and simmer them very gently for an hour.
3. Add the sugar and turn up the heat until the temperature rises to setting point (104C).
4. Check that setting point has been reached with a sugar thermometer or using The Wrinkle Test (see below).
5. Once the jam has reached setting point, skim any scum off the top of the jam. Set aside until a skin starts to form.
6. Pour into sterilized jars.
7. Seal them and leave to cool. Label them and store in a cool place.
Cook's note: If you don't have a sugar thermometer, you can do the 'The Wrinkle Test'. Chill a plate in the fridge, spread a small teaspoon of jam on it. Allow to cool and push your finger through it. The skin on the jam should have just about set, and should wrinkle up. The two halves of the jam either side of your finger mark should remain separate.
(this recipe uses the natural pectin in lemon as the gelling agent).
Best of luck!
- Rachel S.