Tesi Johnson, Gleaner Writer
Generous helpings of guava jelly on everyone's favourite Excelsior Water Crackers are a quick and delicious snack. - PHOTO BY TESI JOHNSON
This week, our series, 'The Science Behind Cooking', looks at what happens when jams and jellies are made as a result of pectin present in the fruit used to make them. The first in Tesi Johnson's examination of the chemical changes that take place when food goes from one state to another, focused on how whisking egg whites can turn them into a delicious meringue.
PECTIN IS a water-soluble carbohydrate that is found in the skin and core of many fruit. Under acidic conditions, pectin forms a gel and can be used as a thickening agent in cooking. So, when fruits are cooked in a sugar solution with the correct proportion of pectin and acid is optimum, the mixture reacts to form a jam or jelly.
The formation of a jelly occurs by way of the water-holding network within the juice or crushed fruit that pectin produces. Before gelling starts, individual molecules of pectin are surrounded by water molecules, which isolate them from each other. Under acidic conditions, the pectin will lose some of its attraction for these water molecules. So if the acid content of the fruit is low, lemon juice can be added to make the fruit mixture more acidic. Now, once the pectin has loosened its hold on the water molecules, something more attractive must pull the water away from the pectin, and that is sugar.
This is why artificial sweeteners cannot make jelly using ordinary pectin, because they do not attract water as sugar does, even though they have a sweet taste. Further, the sugar acts as a preservative to prevent the growth of micro-organisms. Now, with the water away, pectin is free to bond with other pectin molecules to form a three dimensional network. And this network gives the stiffness to the finished product.
Some fruits have enough natural pectin to make high quality jellies, whereas others require added pectin. All fruits have more pectin when they are under-ripe, and as the fruit ripens the level of pectin falls. In overripe fruits pectin becomes pectic acid which does not form jelly with sugar solutions. Where commercial pectin is needed, they are available on the market in two forms: liquid and powdered. These are usually extracted from apples or citrus fruits.
With commercial pectin, it is possible to make a jelly at room temperature, but the heat of cooking speeds up setting, and sterilises mixture. The addition of too much pectin will give the jam or jelly a dull or uninteresting flavour; and not adding enough pectin leaves you with a syrup-like mixture, so using the correct portions is always
important.
Some fruits that contain low pectin levels and may need added pectin to make jelly are mangoes, passion fruit, melon, pineapple, bananas and guava. However, citrus fruits, plums, apples and cranberries, among others, have high levels of pectin, and may not need any added pectin to form a jelly. At times it may be advantageous to add pectin even where the fruit may have a high level of natural pectin, because you can then use a riper fruit to get more flavour.
Guava Jelly (using added pectin)
3 1/2 cups prepared juice
(about 2 1/2 lb. fully ripe
guavas)
4 cups water
1/2 cup fresh lemon juice
Few drops red food
colouring (optional)
6 cups sugar, measured into
separate bowl (See tip below.)
1/2 tsp. butter or margarine
3 oz. liquid fruit pectin
METHOD
1. Slice guavas, place in a saucepan, add water to cover and bring to boil.
2. Reduce heat to low, cover and simmer for five minutes.
3. Thoroughly crush cooked guavas and place three layers of damp cheesecloth or jelly bag in large bowl.
4. Pour prepared fruit and liquid into cheesecloth. Tie cheesecloth closed; hang and
let drip into bowl until dripping stops. Press gently.
5. Measure exactly 3 1/4 cups juice into 6- or 8-quart saucepan.
6. Add lemon juice and food colouring.
7. Stir sugar into juice in saucepan. Add butter to reduce foaming, if desired.
8. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly.
9. Stir in pectin quickly.
10. Return to full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with metal spoon.
11. Ladle quickly into prepared jars, filling to within 1/8 inch of tops.
Source: http://divinerecipes.com
Guava Jelly (without added pectin)
2 1/4 cups guava juice
2 cups sugar
1/2 cup lemon juice
METHOD
1. Choose firm, slightly under ripe guavas. Wash, cut off ends, and slice.
2. Place slices in a large pot with enough water to just cover them. Boil until fruit is very soft (15 to 20 minutes).
3. Pour fruit into a bag made of cheese cloth and squeeze the juice through the cloth into a container. For clear juice, do not squeeze the bag.
4. Pour 2 1/4 cups guava juice into a pot, then bring to the boil.
5. Add lemon juice and sugar, stir until well mixed, then cook at a full rolling boil.
6. After a few minutes of boiling, test to see if jelly is ready by putting a teaspoonful in a saucer, letting it cool, and running a clean finger through it. If the line made by the finger does not close up, the jelly is ready.
7. Pour jelly into hot sterilised jars and seal.
Source: http://www.fao.org
Testing for pectin
Mix 1 teaspoon of cooked, cooled crushed fruit with 1 tablespoon of rubbing alcohol.
In a closed container, shake the mixture gently.
Juices from fruit that is high in pectin will form a solid gelatinous lump. If the fruit is low in pectin, it will form only small rubbery particles.
Source: www.gi.alaska.edu/ ScienceForum/fun.html
