Everything you need to know about making jam from scratch and an easy first recipe to get you started

Until recently there was a misguided belief that one should only use second-rate fruit for jam making. You can indeed make jam from less than perfect fruit, but the reality is that poor quality fruit makes poor quality jam. The secret to making really delicious jam is to make it in small quantities from gorgeous fresh fruit.

Before rural electrification, the soft fruit season was a pretty hectic time for those who wanted to have their shelves packed with jams and preserves for the winter. Now that fruit can be frozen at the peak of perfection, there is not such an urgency because you can make jam all year round. However, the best jam is still made from beautiful fresh fruit in season. If you don't want to spend your whole summer in the kitchen, the most practical approach is to freeze fruit in perfect condition in small, measured quantities so that you can make jam as you need it throughout the year. Jam made from frozen fruit will taste infinitely fresher and more delicious than a six- or seven-month-old jam even if it is made in peak season. Slightly under-ripe fruit makes better jam, because it has a higher acidity. The faster jam is made, the fresher it'll taste, so for that reason, we always warm the sugar.

In Ireland, the tradition of making and selling jam at farmers' markets and local fetes is alive and well. Homemade jam is much sought after. If you decide to sell your jam, cost it properly, taking jars, covers, labels and food costs into consideration. A formula used by many is food cost x 3. This would cover all the other items mentioned. If you are producing jam for sale you must contact your local health authority and comply with their regulations.

Pectin

Pectin is a jelling substance that occurs naturally in many fruits. It is most concentrated in pips, cores and skin. The cell walls of under-ripe fruit contain pectose, an insoluble substance that changes into soluble pectin as the fruit ripens. Slightly under-ripe fruits are best for jellies and jams.

Some fruits are high in pectin, while others have very little. One can compensate for those by mixing low and high pectin fruits such as blackberry and apple.

High-pectin fruits

Crab apples, blackcurrants, gooseberries, plums, redcurrants, cooking apples, cranberries, damsons, quince, oranges, lemons and many plums.

Fruits with quite high pectin

Raspberries, loganberries, boysenberries, tayberries and apricots.

Low-pectin fruits

Blackberries, blueberries, strawberries, rhubarb, elderberries, peaches, sweet cherries, dessert apples, pears, figs and marrow.

Do I need a preserving pan?

Great if you have one, but not essential. Alternatively, choose a low-sided, wide stainless-steel saucepan so that the jam cooks quickly. Avoid aluminium because the acid in the fruit will react with it and give a slightly tinny taste.

Should I use jam sugar?

Jam sugar contains added pectin. I don't use it because I don't like the solid texture of the jam that it makes. The only exception I might make is for strawberry jam, which is difficult to set. However, we usually use some redcurrant juice or lemon juice to bring up the acidity, but you could use a small proportion of jam sugar if you like. The end result may be more like bought jam than handmade jam, though. I use ordinary granulated sugar rather than caster sugar for jam making.

How to heat the sugar

Heat the sugar in a stainless-steel bowl in a moderate oven for about 15 minutes. It should feel hot to the touch. Be careful not to leave it in too long or the sugar will begin to melt around the edges of the bowl and will eventually caramelize.

Why heat the sugar?

The faster jam is made, the fresher and more delicious it tastes. If you add cold sugar to jam, it will take longer to return to the boil and will taste less fresh.

When do I add sugar?

Citrus fruit peel, blackcurrants, and gooseberries must be thoroughly softened before sugar is added, as sugar has a hardening on the fruit. If you add the sugar too early, no amount of boiling will soften the rind or skins. You can vary the amount of sugar you use to taste. For example, if the fruit is very sweet, use less sugar.

Should I skim the jam?

Don't skim the jam constantly while it's being cooked. Just skim it at the end, to reduce wastage. An old-fashioned tip that was passed onto me by my next-door neighbour Peggy Walsh is that if there's a bit of scum left after skimming, then drop a tiny lump of butter (about the size of a fingernail) into the jam. It will dissolve the remainder of the scum.

How do I know the jam has reached setting point?

If you want to take the guesswork out of it, buy a jam thermometer (quite an expensive piece of kit); when the thermometer hits 105ºC (220ºF), the jam is set. But we rarely use a sugar thermometer, because I want students to be able to judge when jam is set without any fancy equipment.

Another way to tell when your jam will set is to put a plate in the fridge to chill. When the jam looks as though it's almost set, take a teaspoonful and put it onto the cold plate. Push the outer edge of the jam puddle into the centre with your index finger. If the jam wrinkles even a little, it will set.

Covering jam jars

When the jam has reached setting point, pour it into sterilised jars and cover immediately. One can still buy packets of jam covers in most shops and supermarkets. These are made up of three elements, a silicone disc of paper, a large round of cellophane and a rubber band. Cover each jar with a silicone disc (slippy side down onto the surface of the jam). Wet one side of the cellophane round, then stretch it over the jar, and secure with a rubber band. If the cellophane disc is not moistened, it will not become taut when the jam gets cold.

Alternatively, use screw-top lids, which should be sterilised in boiling water and dried before use. Later the jars can be covered with doilies, rounds of material or coloured paper. These covers can be added secured with plain or coloured rubber bands, narrow florists ribbon tied into bows or ordinary ribbon with perhaps little dried flowers or herbs.

What can go wrong?

1. Mould on the top

If mould starts to grow on top of jam, I just spoon it off, give it to the hens and continue to eat the rest. Keep the jam in the fridge from then on and use it as soon as possible. When we were little, Mummy would just tell us to stir in the mould and eat it because it was penicillin and good for us – I'm not sure about that but I am alive to tell the tale! Having said that, eating mouldy shop-bought jam is a different matter and certainly not advisable. If you remove jam to a separate dish to serve it in, do not add it back to the main pot afterwards or it will go boozy. Mould grows on top of jam when:

(a) jars are not properly sterilised

(b) the fruit was picked while wet

2. Crystallisation

Sugar crystals appear on top and sometimes through the jam. The jam is safe to eat but will taste very sweet and gritty. Crystallisation is caused when:

(a) too much sugar is added

(b) the sugar is not properly dissolved

(c) the jam is over- or under-boiled

3. Fermentation

When fermentation occurs, the jam will start to bubble and can smell gassy when the lid is removed. Jam that has fermented should not be eaten. Fermentation can occur when:

(a) the jam is undercooked

(b) the fruit was wet when harvested

(c) the jars were not properly cleaned and sterilised

Storing jam

Normally, there is no need to store jams in a fridge. They should be stored in a cool, dry, airy place.

Recipes

Redcurrant juice

This juice can be frozen for use another time if necessary. Use it to bring up the acidity and pectin level when making jam.

To obtain 450ml (¾ pint) of juice, put 450g (1lb) of redcurrants (they can be fresh or frozen) into a stainless-steel saucepan with 175ml (6fl oz) of water. Bring to the boil and simmer for about 20 minutes. Strain through a fine sieve.

Raspberry, boysenberry, tayberry or loganberry jam

If you've never made jam before, this is a good place to start. Raspberry jam is the easiest and quickest of all jams to make, and one of the most delicious. Loganberries, boysenberries or tayberries may be used in this recipe, too. Because it uses equal amounts of sugar and fruit, you don't necessarily need as much as the recipe calls for. Sometimes when I'm trying to take the mystery out of jam-making for students, I put some scones into the oven, then make jam, and by the time the scones are out of the oven, the jam is made. It's that easy!

Makes 3 450g (1lb) pots.

900g (2lb) fresh or frozen berries

900g (2lb) white sugar, warmed

(use 110g/4oz less if the fruit is very sweet)

Wash, dry and sterilise the jars in the oven for 15 minutes. Put the berries into a wide, stainless-steel saucepan. Mash them a little and cook for 3–4 minutes over a medium heat until the juice begins to run, then add the warmed sugar and stir over a gentle heat until the sugar is fully dissolved.

Increase the heat, bring to the boil and cook steadily for about 5 minutes, stirring frequently (frozen berries will take 6 minutes).

Test for a set by putting about a teaspoon of jam on a cold plate and leaving it for a few minutes in a cool place. Press the jam with your index finger. If it wrinkles even slightly, it is set. Remove from the heat immediately. Skim and pour into sterilised jam jars. Cover immediately.

Keep the jam in a cool place or put on a shelf in your kitchen so you can feel great every time you look at it! Anyway, it will be so delicious it won't last long!

• This extract is taken from Forgotten Skills of Cooking by Darina Allen (Kyle Cathie, £30) with photography by Peter Cassidy