Fruits suitable for jelly are guava, sour apple, plum, Karonda, Wood apple, loquat, papaya, etc. Some fruits have low pectin content, so they are also used for jelly after adding pectin powder. Apricot, Pineapple, Strawberry, Raspberry, etc.

Fruits can be divided into four categories according to their pectin and acid-

1. Rich in Pectin and Acid- Crab Apple, Sour Guava, Grapefruit, Lemon, Orange, Sour Plum, Jamun.
2. Rich in pectin but low in acid- apple, unripe banana, unripe fig, sour cherry, guava, orange rind.
3. Low in pectin and rich in acid- sour apricots, sweet cherries, pineapples, strawberries, and sour peaches.
4. Low in pectin and acid- Apricots, peaches, pomegranates, strawberries, raspberries, and other ripe fruits.

Ingredients

Flowsheet for processing of jelly

Procedure for jelly preparation

(A) Fruit selection: Fruits should be sufficiently ripe, but not overripe, and have a good flavour. Slightly underripe fruits produce more pectin than overripe fruits; As the pectin present during ripening decomposes into pectic acid, which does not form jelly with acid and sugar.

(B) Pectin requirement: Usually 0.5 to 1 percent pectin in the extract is enough to make a good jelly. If the pectin content is high, a firm and hard jelly is formed and if it is low, the jelly may fail to solidify. Pectin, sugar, acid, and water are the four essential components of jelly and should be present in approximately the following proportions: pectin 1 percent, sugar 60 to 65 percent, fruit acid 1 percent, and water 33 to 38 percent. However, the exact proportion of sugar depends on the pectin grade.

Pectin grades: Grades of pectin means the weight of sugar required to set one gram of pectin under suitable conditions to form a satisfactory jelly. For e.g. 100-grade pectin means 100g of sugar is required for the setting of 1 g pectin.

Determination of pectin content

1) Alcohol test- This method involves the precipitation of pectin with alcohol, the outline of which is given below-

One teaspoon of filtered fruit extract is taken into the beaker and cooled and 3 teaspoons of methylated spirit are gently poured from the side of the beaker which is swiveled to mix and left to rest for a few minutes.

a) If the extract is rich in pectin, a single, transparent lump or clot will form. To make jelly in this type of extract, sugar has to be added in equal quantity to the juice.

b) If the extract contains moderate amounts of pectin, the clot will be less firm and fragmented. Three-fourths quantity of sugar has to be added.

c) If the extract is low in pectin, many small granular clots will appear. Only half the quantity of sugar is added.

2) Jelmeter Test – Jelmeter is held in the left hand with the thumb and forefinger. The lower part of the Jelmeter tube is closed with a little finger. The filtered extract is spooned with the right hand into the jelmeter until it is full to the edge. While holding the jelmeter, the little finger is removed from the lower end and the extract is allowed to flow or drip off for exactly one minute, at the end of which closed again with the finger.  The level readings in the jelmeter are noted. Sugar is added to the extract as per the readings.

(C) Acid Adding

The jelly formation of the extract depends on the amount of acid and pectin present in the fruit. Of the three acids found in fruits, citric, malic, and tartaric, tartaric acid produces the best results. The total acid in the final jelly should be at least 0.5% but not more than 1% because the acid in excess can cause syneresis.

Jelly pH: The strength of the jelly increases with an increase in pH until it reaches the optimum. The addition of more acid beyond this level reduces the strength of the jelly. The pH of the jelly can be controlled either by adjusting the pH of the pectin extract or by adding an appropriate buffer. The fruits also contain salts like sodium citrate, sodium or potassium nitrate, etc. If the acidity of the extract is high, the finished jelly will stiffer. The optimum pH of the pectin solution is between 3.1 and 3.3 for the best jelly setting.

(D) Sugar adding

It is the essential component of jelly that provides sweetness as well to the body. If the concentration of sugar is high, the jelly holds less water resulting in the jelly becoming hard, probably due to dehydration. Boiled with an acid to invert sugar (sucrose), it is hydrolyzed into dextrose and fructose, the amount of inverted sugar depends on pH and duration of boiling. Due to the partial inversion of sucrose, jelly contains a mixture of sucrose, glucose, and fructose. This mixture is more soluble in water than sucrose alone and therefore the jelly can hold more sugar in the solution without crystallizing.

Sugar in jelly is added according to the pectin test, if pectin is high, then an equal amount of sugar is added, and if there is less pectin, sugar is added only in half the amount.

(E) Determination of end-points: Determination of end-points in jelly can be done using the following methods:

1) Drop Test: A drop of concentrated mass is dropped into a glass full of water. The movement of the drop to the bottom without disintegration (dispersed) indicates the endpoint.

2) Cold plate test: A drop of boiling liquid in the pan is taken and placed on a plate and allowed to cool quickly. If the jelly is ready, the mixture on the plate will shrink when pushed with a finger. The main drawback of this method is that while the drop is cooling on the plate, the jelly mixture continues to boil in the pan and risks overcooking the product or losing the correct setting point.

3) Sheet or flake test: This test is more reliable than the plate test. While boiling the jelly, take out a little jelly in a spoon or wooden spatula and cool it down a bit. After that, it is dropped. If the product falls like a sheet or flakes, it means that the endpoint is reached and the product is considered ready, if it falls as a continuous stream or syrup, it will need to be further cooking.

4) Temperature test: A solution containing 65% TSS boils at 105 °C. Heating the jelly to this temperature will automatically increase the concentration of the solids to 65%. This is the easiest way to figure out the endpoint

(F) Problems with jelly

1) Failure of jellies to set: Sometimes jelly does not set due to the following reasons:

i) Acid or Pectin Deficiency: The jelly may fail to set due to a lack of acid or pectin in the fruit from which it is made. It may also fail to set due to insufficient cooking of the fruit to obtain pectin resulting in insufficient extraction of pectin and acid.

ii) Adding too much sugar: If sugar is added in excess of the required quantity, it results in the formation of syrup or extremely soft jelly. This can be corrected by adding fresh clear juices rich in pectin.

iii) Cooking below the endpoint: If cooking is stopped before the sugar concentration reaches 65 percent, the jelly may fail to set and remain syrupy or overly soft.

iv) Over-cooking: If cooking is continued beyond the endpoint, the jelly hardens due to high concentration. This occurs when both acid and pectin are abundant in the juice and not enough sugar is added. If the acid is high, the pectin breaks down and forms a syrup-like jelly.

2) Syneresis or weeping of jelly: The phenomenon of spontaneous release of fluid from the gel is called syneresis or weeping of jelly. This is due to the following factors:

i) Excess acid: As a result of excess acid, the structure of jelly breaks down due to hydrolysis or decomposition of pectin.

ii) Very low concentration of sugar or soluble solids: This allows the pectin network to hold more fluidity than would be possible under normal conditions.

iii) Insufficient Pectin: This results in the formation of a network of pectin that is not sufficiently dense and hard to hold sugar syrup.

iv) Premature Gelation: Gelation occurs due to the breakdown of pectin during the pouring of jelly into containers. The jelly weakens and breaks.

v) Fermented jellies: Fermentation usually occurs in jellies that have syneresis.

3) Cloudy or foggy jelly: This may be due to the use of the non-clear extract, use of immature fruits (immature fruits contain starch which is insoluble in juice), over-cooking and cooling, non-removal of scum, defective pouring (when the jelly is poured from a great height, the air is trapped in the form of bubbles and the jelly becomes opaque) and premature gelation also causes a cloudy or foggy jelly.

4) Formation of crystals: The addition of extra sugar to jelly can lead to the formation of crystals.