Processing of Horticultural Crops

Introduction

Introduction
Jams, jellies and marmalades constitute an important class of preserved fruit products. Apple jam, pineapple jam, strawberry jam and mixed fruit jam prepared by using pulp of two or more fruits are quite common. Similarly, guava jelly and orange marmalade are also examples of such products.

A fruit preserve is made from properly matured fruit by cooking it whole or in the form of large pieces in heavy sugar syrup, till it becomes tender and transparent. A fruit impregnated with cane sugar and glucose, followed by subsequent draining and drying is called a candied fruit e.g. petha candy, ginger candy. Further, candied fruit covered or coated with a thin transparent coating of sugar, which imparts a glossy appearance is referred to as a glaced fruit. When candied fruit is coated with sugar crystals either by rolling it in powdered sugar or by allowing the sugar crystals from glucose syrup to deposit on it, it becomes a crystallized fruit. The use of high concentration of sugar in these products acts as the preservative. The products like Petha candy, aonla preserve, apple rings; candied citrus peels, ginger candy, ginger in syrup; bael, pineapple and carrot preserve are the commercial products in this category.

1. JAM: Jam is prepared by boiling the fruit pulp with a sufficient quantity of sugar to a thick consistency, firm enough to hold fruit tissues in position. The method for preparation of jam and jelly is the same except that pulp and pieces of fruit are used in jam while, for jelly making clear fruit extract is used. According to FPO specifications, minimum soluble solids in the final product shall not be less than 68 percent (w/w).

Procedure for jam preparation (Fig 9.1)

Fruit preparation: The fruit is washed thoroughly to remove any adhering dust and dirt. The fruit is then subjected to preliminary treatment which varies with the type of fruit.

• Strawberries are crushed between rollers; raspberries are steamed, crushed and passed through sieves to remove the hard cores.
• Plums, peach and apricots are heated with a small quantity of water until they become soft and are then passed through a wide mesh sieve to separate the stones.
• Fruit after softening by boiling with small quantity of water can be passed through the pulper to extract the pulp.
• Pears are peeled, cored and cut into small pieces.
• Mangoes are peeled, stones separated and then the slices are passed through a pulper.
• Pineapples are peeled, sliced and the cores punched. The slices are then cut into smaller pieces and passed through a screw type crusher to get a fairly coarse pulp suitable for making the jam.
• When two or more fruits or fruit pulps are mixed in appropriate proportion for preparation of jam, the jam is called as mixed fruit jam.

Addition of sugar: Generally 55 parts of cane sugar (sucrose) is used for every 45 parts of fruit for preparation of jam. The prepared jam should contain 30 to 50 percent invert sugar to avoid crystallization of sugar in the jam during storage. If the percentage of invert sugar (reducing sugar) is less than 30, then jam develops crystallization and if it is more than 50%, the jam will develop into a honey like mass due to the formation of small crystals of glucose. Sugar should not be added in excess as jam with higher total soluble solids becomes gummy and sticky.

Addition of acid, colour and flavour: Citric, tartaric or malic acid are used to supplement the acidity of the fruit for jam making. Addition of acids to fruits which are deficient in acid is required to get appropriate combination of pectin, sugar and acid for proper setting of jam. The pH of the mixture of fruit juice and pectin should be 3.1 before sugar is added. Only permitted edible food colours should be used and these should be added towards the end of the boiling process. Flavours are added at the end of cooking process and just before packing.

Boiling: The fruit is placed in the boiling pan along with a small quantity of water to facilitate pulping. It is then cooked sufficiently to liberate the pectin. After addition of sugar, the mixture is boiled to concentrate the soluble solids to about 68.5 percent and also to allow the necessary degree of inversion of the sugar. Boiling can be performed in steam jacketed kettle or stainless steel or aluminium boiling pan. Boiling in a vacuum pan using reduced pressure is used to prepare jam at lower temperature (65-75⁰C) to minimize undesirable changes and for retention of vitamin.

End Point: The end point can be determined by carrying out a jelmeter test. Generally, fruits which are fairly rich in pectin, the weight of the finished jam is one and a half times (11/2 times) the weight of sugar used. Jam containing 68.5 percent of soluble solids boils at 105⁰C at sea level. It should yield a definite quantity of the finished jam.

Storage: The jam is packed in sterilized glass jars. It should be noticed that unless the jars are stored in a fairly cool place, moisture will evaporate from the jam resulting in shrinkage of the jam. If jam is prepared from fresh, unsulphited fruit pulp then it is advisable to add about 40ppm of sulphur dioxide in the form of potassium meta-bi-sulphite to the jam, which is permitted by law. A layer of molten paraffin wax can be put on the top surface of the cooled jam in the glass jar, which on cooling sets. This acts as a safeguard against any possible moulding on the surface of the jam.

2. JELLY: Jelly is prepared by boiling the fruit with or without addition of water, straining the extract and mixing the clear extract with sugar and boiling the mixture to a stage at which it will set to a clear gel. The jelly should be transparent, well set, but not too stiff and having original flavour of the fruit. It should be of attractive colour and should keep its shape with a clean cut surface. In the preparation of jellies, pectin is the most essential constituent. Pectin is present in the cell wall of fruits. In order to get a good quality jelly fruits rich in pectin, but deficient in acid should be preferred.

Procedure for jelly preparation (Fig 9.1)

Selection of fruits: The fruits should be sufficiently ripe, but not over ripe and they should have good flavour. Slightly under-ripe fruit yields more pectin than over-ripe fruit; as during ripening the pectin present is decomposed into pectic acid, which does not form a jelly with acid and sugar. The amount of pectin extracted from a fruit depends on the degree of disintegration of protection during the heating process.

Pectin requirement: Usually 0.5 to 1 percent of pectin in the extract is sufficient to produce a good jelly. If the pectin content is in excess, a firm and tough jelly is formed and if it is less, the jelly may fail to set. Pectin, sugar, acid and water are the four essential constituents of a jelly and must be present approximately in the following proportions:
Pectin 1 percent
Sugar 60 to 65 percent
Fruit acid 1 percent
Water 33 to 38 percent
However, the exact proportion of the 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. e.g. 100 grade pectin means 100g of sugar is required for setting of 1 g pectin.

Figure 9.1: Flow sheet for preparation of jam and jelly

Theories of jelly formation: Jelly formation is due to the precipitation of pectin rather than its swelling. Only when the pectin, acid, sugar and water are in definite equilibrium, the precipitation of pectin takes place. The rate of precipitation is influenced by the following factors:

1. Concentration of pectin in the solution
2. Constitution of pectin
3. Hydrogen ion concentration (pH) of the pectin solution
4. Concentration of sugar in solution
5. Temperature of the mixture

There are several theories to explain the formation of jellies.

1. Fibril theory: When sugar is added to the pectin solution, it destabilizes the pectin-water equilibrium and the pectin conglomerates forming a network of fibrils through the jelly. This network of the fibrils holds the sugar solution in the inter-fibril spaces. The strength of the jelly depends on the structure of the fibrils, their continuity and rigidity.

2. Spencer’s theory: Pectin particles are negatively charged. A pectin solution is most stable in neutral pH range. Increase in acidity or alkalinity decreases its stability. In jelly formation, sugar acts as a precipitating agent and the presence of acid helps it.

3. Olsen’s theory: If pectin is taken to be a negatively charged hydrophilic colloid, the following may be assumed:

• Sugar acts as a dehydrating agent, which disturbs the equilibrium existing between water and pectin.
• Sugar does not dehydrate the pectin micelles instantaneously, but requires time to bring about equilibrium.
• If the negative charge on the pectin is reduced with the help of hydrogen ion concentration, pectin precipitates and coalesces in the form of a fine network of insoluble fibers, provided that the sugar is present in sufficient concentration.
• As the system reaches equilibrium, the jelly strength becomes the maximum.
• Salts and other components which cause a change in the ultimate jelly strength of the system, may function either by changing the rate of gelation or by affecting the ultimate structure of the jelly or by a combination of both.

4. Hinton’s theory: This theory is based on the assumption that pectin’s are complex mixtures of variable composition. According to it, gelation of pectin is a type of coagulation in which the coagulated particles form a continuous network. Pectin like organic acid, dissociates in solution. It is only the non-ionized and not the ionized pectin, which enters into jelly formation.
pH of jelly: The pH of the jelly can be controlled either by adjusting the pH of the pectin extract or by adding suitable buffers. In case the acidity of the extract is more, the prepared jelly shall be stiffer. The optimum pH of the pectin solution for best jelly setting range between 3.1 & 3.3.

Determination of end-point: The end-point in jelly can be judged by using following methods:

1) Cold plate test: A drop of the boiling liquid from the pan is taken and placed on a plate and allowed to cool quickly. If the jelly is about to set, the mixture on the plate will crinkle when pushed with a finger. The main drawback in this method is that while the drop on the plate is cooling, the jelly mixture continues to boil in the pan and there is a risk of over-cooking the product or of missing the correct setting point.

2) Sheet or flake test: This test is more reliable than the plate test. A small portion of jelly is taken with a large spoon or wooden ladle, cooled slightly and then allowed to drop off. If the jelly drops like syrup, it requires further concentration. Falling of the drop in the form of flakes or sheet indicates the end point.