Question: What is pectin?
Production of fruit jams was known for hundreds of years and today the commercial and "home-made" jams use century-old techniques. The important parts of the jam are sugar and pectin, and with gradual boiling it can preserve the foods.
Pectin is a large and complex branching carbohydrate molecule. In under-ripe fruit, pectin is present in relatively small quantities with protopectin being predominant. As fruit ripens, the protopectin gradually becomes converted into pectin. Over-ripe fruit has higher levels.
However, pectin concentration varies based on the type of fruit and the ripening stage of the fruit itself. Also, certain parts of the fruit (such as peels in citrus and core in apples) can have substantially more pectin than the fruit "flesh".
The amount of sugar in jam making is recommended between 60-65 per cent. Sugar is added to bind to the water molecules and free-up the pectin to form networks, producing the required jam textures.
One of the common errors is attempting to reduce the sugar without considering potential negative effects. Insufficient sugar can leave too much water in the fruit and pectin cannot form a gel by linking to another pectin molecule. Often, prolonged boiling is seen as a way of reducing the water content of fruit but this can increase the sugar and pectin as well as cause an undesired impact on the final colour.
On the other hand, if too much sugar is added, the jams will probably crystallise on storage.
In addition to sugar and pectin, for jams to form properly, they require a specific acidity, preferably around the pH of 3.3. Some fruits are naturally acidic and do not require addition of acids. The acids that are commonly added (such as citric acid) assist in the promotion of gel formation by reducing the negative electrical charge found on the pectin branches, therefore allowing the pectin molecules to form stronger networks.
During the boiling of jam, a considerable amount of foam may start to appear on the surface. This is trapped air caused by the hot jam circulating through the mixture all the way up to the surface. There are two ways to deal with this; adding small a amount of fat (such as butter) before boiling which will reduce the surface tension; or physically removing it once it's cooled.
Response by: Dr Nenad Naumovski, University of Canberra
Brought to you by the Fuzzy Logic science show, 11am Sundays on 2XX 98.3FM. Send your questions to firstname.lastname@example.org