You will often see apples combined with low pectin fruits to make jellies or jams.
Pectin is technically a sugar, or a polysaccharide, even though it acts as a fiber in our digestive tract. This sugar is a long chain polymer composed of pectic acid and pectinic acid molecules. The bonds between the pectin molecules make chains that hold water and sugars in suspension within the fruit. The amount of pectin in apples is sufficient for making apple butter without the addition of commercial pectin.
Compared to Other Fruits
All fruits contain pectin in varying amounts. Apricots, rhubarb and strawberries contain low levels of pectin, though not usually enough to gel on their own. Apples and oranges are much higher in pectin content. Apples' pectin levels are the highest when they are just about ripe. As the softness of the apple texture decreases, the pectin levels also decrease.
Pectin Test
If you are not sure just how much pectin your apples contain, you can test them. By dropping a teaspoon of the juice from the apple into a dish containing 2 tbsp. of rubbing alcohol, you can expect to see a blob develop after a couple of minutes. This signifies that there is plenty of pectin present to form a gel.
Acid Level
Another component of making jelly with apples is the amount of acid present. Apples with high amounts of pectin tend to be characterized with a tart flavor. Many apple jelly recipes list lemon juice as an additive for apples lacking in tartness to bring the acidity level up to 3.5 on the pH scale. Levels above this mark will fail to cause the pectin to gel.
Action of Heat
Heat extracts the pectin from the apple tissues by dissolving the chains. As the bonds break apart, the pectin is distributed throughout the apple juice, the apple tissue and the added sugar. If you strain the apple mash, the resulting liquid jelly will have enough pectin to set the mixture into a gel as the mixture cools and the pectin chains bond together.
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