Pectin is found particularly in the cores, pips and skins of fruits, together with some vegetables like lentils and split peas. The cell walls of unripe fruit contain pectose, an insoluble carbohydrate that changes to pectin when the fruits ripen. However, on ripening, the pectin changes to pectic acid and methanol, so it is best to make jelly with only just ripe fruit.
Pectin can be bought as a liquid or powder. Pectin is normally made from apple pulp or the white of citrus fruit skins, and is a classic by-product of the fruit juice industry.
How Pectin Jellies Are Formed
To make pectin jelly, sufficient pectin is needed and enough acid and sugar to generate the right structure. A pH of 3.2 - 3.46 is the best range for making homemade jelly - at pH 3.1 the liquid begins to seep out of the jelly, i.e. it weeps. Boiling for long periods with acid will destroy the pectin, as does a pressure cooker, both resulting in a weak jelly.
For sugar levels, fruit jellies need to be saturated to 69 - 72%. If the level of sugar is greater than 72%, a syrup forms and the jelly may not set at all, but the sugar can crystallise out on cooling. While jellies may set at 40% sugar, these jellies are stiff and may not keep, unless they've been really well sterilised. Finally, it helps to know that a high acidity means the exact sugar level is less important, so that a sourer fruit can take a larger quantity of sugar.
Some fruits have sufficient acidity and pectin that they will set with the simple addition of suitable amounts of sugar; these include:
- Sour apples
- Crab apples
- Currants - black and red
- Plums, especially damsons
- Borderline fruits: apricots, blackberries, loganberries, raspberries
Jam is usually made with roughly equal amounts of sugar and fruit, unless specifically stated in a recipe. If there is loads of pectin, then ¾ the weight of the juice is a reasonable quantity of sugar; however if the pectin is only just adequate, then the sugar should be lowed to 600 g (21 oz) of sugar per litre (1 litre, or quart).
Once the sugar has been added to the juice with enough pectin, boiling for no more than 10 minutes will make a jelly that will set on cooling. The temperature of boiling is the best test for levels of pectin: boiling should be at 104.5oC (220oF), but may get to 105.5oC (222oF), then the tests for settability are:
- Flake Test: dip a spoon into the hot jelly, take it out and twirl it until the jelly on it has cooled a little. Let some of the jelly drop off the edge of the spoon. If it forms a sheet and drops off cleanly as a flake, then the jelly will set when cold.
- Cold Plate Test: put a small blob of juice on a cold plate and leave to cool; push the blob sideways witha fingernail - if it wrinkles, it will set on cooling.
Pectin, sugar and acidity enable fruit juices to stiffen and jams, jellies and curds to be made.