If pectin is lacking, the deficiency can be made up by adding a pectin-rich fruit or its juice, eg. apple is added to blackberries, gooseberries to strawberries. Alternatively, commercial or home-made pectin stock can be added. If in doubt about the setting quality of the fruit, test for pectin using the method below.
1. Prepare cooking apples, gooseberries or red-currants, and cook with enough water to cover until thoroughly pulped.
2. Crush with a wooden spoon and pour through a scalded jelly bag or double tea-towel.
3. The resulting stock can be added to fruit weak in pectin.
4. The usual allowance is 125 ml of stock to 2 kg of fruit, but more is required in vegetable jams, such as marrow where there is little natural pectin.
Choose firm, ripe fruit or a mixture of just ripe and slightly under-ripe fruits. Over-ripe fruit will not give a set and should be used for some other preserve such as syrup or pulp. To retain the natural green colour of gooseberries, really young, under-ripe fruit must be used. Ripe red gooseberries make a fragrant jam, but the skins, like those of blackcurrants, can be tough unless softened by long gentle cooking before the sugar is added.
Money is well spent on a good quality preserving pan which will give good service without constant fear of burning the jam. Aluminium, stainless steel, and unchipped enamel are good choices. Iron and zinc pans are not suitable as the fruit will react with the metal and spoil the colour and flavour of the preserve, as can also occur with exposed iron in a chipped enamel pan.
Copper and brass pans are also not recommended. They enhance the colour of jam, especially green gooseberries, but a considerable proportion of the vitamin C in the fruit is destroyed. If copper and brass pans are used for cooking, they must be cleaned with salt and lemon juice and not with metal polish.
It is important to have a large enough pan so that the jam can boil rapidly without it overflowing. A 6 litre capacity is recommended for yields of 2 - 3 kg. The inside of the pan can be rubbed with un-salted margarine or butter before use to prevent the jam from sticking, but this is usually unnecessary if a good quality, heavy preserving pan is used
1. Take 1 tsp of juice from the cooked down fruit and put it into a small glass or jar. Leave it to cool.
2. Add 1 tbsp methylated spirit. Do not stir but shake gently and leave for 1 minute.
3. Tip the glass and pour off the liquid carefully.
4. If there is plenty of pectin present there will be a single firm clot of jelly. If medium pectin there will be a soft clot or 2 or 3 smaller ones. If there is a feathery clot or no clot at all, pectin stock or some pectin-rich fruit must be added. Alternatively, commercial pectin may be added; this, however, is done after skimming the cooked fruit which is only boiled for a short time.
Either granulated or preserving sugar can be used for jam-making and it should generally be stirred into the fruit over low heat until thoroughly dissolved. Preserving sugar tends to cause less foaming. Warmed sugar dissolves more quickly than cold sugar. Once the sugar has dissolved, the heat can be increased to give a good rolling boil.
When a good quality pan is used, an occasional stir will be enough to prevent burning. Constant stirring tends to produce an excess of foam.
When the jam reaches setting point, remove the pan from the heat and remove the scum. This is done most economically with a warm perforated spoon or slice. The scum on jam made from fibrous fruits, eg. strawberries or apricots, may be difficult to remove, but for the appearance of the jam it is worth doing well. Where there is very little scum, eg. on blackcurrant jam, it can be gently stirred in with the addition of a little butter or margarine. The scum removed can be used to sweeten stewed fruit, or can be served over a steamed pudding. It comprises largely air bubbles, so its flavour, although fruity, is rather weak.
Using a small jar or cup pour the jam while really hot into warm, clean, dry jars, filling them right to the brim (since jam shrinks considerably on cooling). The modern jam jar with a twist-top is ideal for keeping jam in good condition especially in centrally heated homes. The twist-top must be put on the preserve as soon as it is potted as it acts as a seal.
Alternatively, with plain-necked jars, a transparent cellulose or waxed disc to fit the surface of the jam should be pressed on the hot preserve. The top cellulose cover can be put on at once while the jam is still very hot, but it is usually easier to wait until the preserve is quite cold. Never put it on warm jam as this encourages the growth of mould. Slightly dampen one side of the cover and place that side uppermost over the jam before securing with a rubber band or string.
Label the jars with the type of preserve, the date, and any other details about the fruit or the recipe which will be of interest later. Store preserves in a cool, dry place, preferably dark. The store for jams that are not sealed with twist-tops should have some ventilation. Because of variations in utensils and ingredients, even a well-tried jam recipe may differ from household to household. The following recipes have been thoroughly tested but yields may vary slightly according to the equipment used.