Preparatory Operations In Food Processing (part2)

Preparatory Operations In Food Processing (part2)

6. Size Reduction :

Size reduction or disintegration is an important operation at home as well as in the food industry and has several functions, eg: -

1.A definite size range may be a consumer requirement, e.g., production of carrot slices or dices.

2.It may help in the extraction of desirable constituents from the raw material, e.g., milling wheat grains for the production of flour or crushing fruits for juice.

3.Size reduction facilitates mixing of various ingredients, as smaller sized particles are easier to mix, e.g., production of cake mixes and baby foods.

4.Some other operations in food processing and preservation are facilitated by smaller sized particles. For example, when potatoes are to be fried or boiled, smaller pieces expose more surface area, hence are easier to process.

                       In food processing several operations demand prior size reduction of the raw material. Examples are blanching, canning,

dehydration, freezing, etc. The type of size reduction treatment given depends upon the kind of raw material, consumer preference and

process demand. Thus, in canning, dehydration or freezing of fruits and vegetables, some raw material may be sliced (e.g. carrots, mangoes),

diced. (pineapples, potatoes) or shredded (onions, cabbage). When fruits are processed for the production of beverages, the raw material is crushed and squeezed to release juice from the tissues. For some fruits, e.g., mango or guava, a special process known as pulping assembles the fleshy portions.

                         Grain milling also involves size reduction operations. Different -fractions of the milled product form special market categories: whole-wheat flour (brown flour), white flour, semolina, etc. form special proprietary products of the wheat grain.

                           Meat for canning and for use in meat dishes is cut into smaller pieces, while for sausages, burgers and other similar products it is macerated or minced. The minced meat provides a larger surface area, which facilitates mixing of other ingredients and subsequent formation of stable meat emulsion.

7. Mixing :

Mixing is an opération in which uniform combination of two or more substances is affected. The two components in a mixture may both be solids, or liquids, or ene solid and one liquid, and even a gas being mixed with a gas or a liquid, Powdered solids are mixed together for particular reasons.

                             Powdered solids are mixed together for particular reasons. Powdered skim milk, sugar, cocoa, chocolate, malt extract, processed cereals and flavourings are mixed in the production of baby foods, beverages and other products, In self-raising flours and cake mixes, solid ingredients including white flour, baking powder and other components: are mixed together. The most homogeneous mixture is obtained when all ingredients are of similar size, shape and density.

                           Mixing of two miscible liquids like water and milk does not pose a problem to the food processor. However, immiscible liquids like oil and water are difficult to mix. When these are mixed intimately, the product is called an emulsion. Emulsions can be easily formed between two immiscible liquids when a mutually'miscible substance called emulsifier

is added to the mixture. Examples of emulsions are homogenized milk, butter, margarine and mayonnaise.

                   In the manufacture of syrups and sugar-based confectionery. water is mixed with soluble and insoluble solids, respectively. Sugar and common salt constitute soluble solids and will form a homogeneous solution when mixed with water. Starch and wheat flour (called plastic

solids) are insoluble and will not mix well with water. For such materials kneading is done to blend thoroughly with water. Production of dough in bread making is an example of mixing insoluble solids with liquid, where the solids absorb water under the influence of mechanical action.

               Sometimes gases need to be mixed with liquids. In carbonated beverages, carbon dioxide is mixed under pressure with a homogeneous solution of sugar and other ingredients in water, In the production of omelet, cakes and some types of biscuits, air is incorporated in the eggs by whipping. Similarly, in the manufacture of hydrogenated vegetable

oils, hydrogen is mixed with oil in a metal catalyzed chemical process. The hydrogen is accepted at the double bonds of the unsaturated fatty acids of the oil in the presence of the catalyst, nickel. Mixing of gas with gas poses no problems in the industry, as the gases are readily miscible. In cold storage chambers sometimes an atmosphere of ozone or carbon

dioxide is maintained to prolong the shelf life of fresh fruits and vegetables.

8. Filtration :

Filtration is the separation of insoluble compenents of solid-liquid mixture by passing the same through a material that allows the liquid to pass and retains the insoluble solids. In food industry, filtration is a widely used operation. Water for use as a food ingredient, especially for carbonated beverages, is treated with chemicals to sediment dissolved

inorganic materials and filtered before being used in the production processes. In canning, sugar and water are mixed to prepare sugar

syrup, while brine is prepared by dissolving common salt in water. In both cases impurities are removed by filtration. In the dairy industry fresh milk is filtered prior to any treatment. Fruit juices, likewise, are filtered to remove coarse particles, skins, seeds and other fibrous tissues.                       Depending upon the application, different filter media are employed. At home, one may use cheese-cloth (muslin) for fruit juices. In the industry, filters constructed from porous carbon, porcelain, perforated metal plates, rigid wire meshes or fused alumina find applications. In addition, filters made from silk, wool, cotton or jute are also used. A good filter is one, which will offer minimum resistance to the flow of product and retain maximum residues. It has also to be non-toxic and relatively inexpensive.

                                       Sometimes when large quantities of a product are required to be filtered, the filter surface may get blocked, especially when the solids are very finely divided or are of a slimy nature. In such cases substances known as filter aids are employed. They help to retain the porousness of the filter thereby facilitating filtration. Examples of such substances are kieselguhr, diatomaceous earth and activated charcoal.

9. Prevention Of Enzymatic Browning :

In certain plant foods enzymes promote discolouration, especially after these have been peeled or injured. These enzymes catalyze the oxidative polymerization of polyphenolic substrates contained in the food. The polymerized products of this complex reaction

give rise to browning as observed on the surface of cut potatoes. bananas and apples after exposure to air. At home, such reactions are avoided by promptly dropping the material in water soon after cutting. Changes in colour (browning) are prevented by mild heat treatment

called blanching or by the use of certain chemical substances.

1. Blanching :

Blanching (also known as scalding) is the heating of some plant food materials in not water or live steam for a very short time (arranging  from few seconds to a few minutes) mainly to destroy active food enzymes. However, blanching also serves to:

1.Loosen the skin, e.g. tomatoes.

2.Clean the product by helping to contaminants such as soil, insects, microorganisms, etc.

3.Remove tissue gas from leafy vegetables (e.g. spinach), thereby reducing their volume and facilitating close filling.

4.Fix the green colour in vegetables (peas, spinach).

5.Remove slime-forming substances in vegetables (okra).

                                 At home, blanching is done by dipping freshly prepared (peeled, sliced, diced) raw material in boiling water for 2 to 5 minutes. In the industry, prepared raw material is exposed to steam in a blancher for similar period.

Tray type steam blancher

                   The effectiveness of blanching is determined by evaluating he activity of peroxidase or catalase, as these are the most heat-resistant and widely distributed in plant tissues. The heating time required t destroy these enzymes depends upon several factors such as the type of raw material, size of particles and heating method. In general, at 100

peas require from 1 to 1%, spinach 1, green beans from 1 to 4, okra 1- 2, mushrooms 4 - 5 and turnips from 3 - 4 minutes.

                     Quite often certain chemicals are added to blanching water to improve the effectiveness of the process. Calcium chloride is added to firm fruits. Ascorbic acid, sodium chloride, magnesium oxide, sodium

metabisulphite, potassium metabisulphite, sodium bicarbonate and others are useful in the preservation of colour and retention of vitamin C.

2. Use Of Chemicals :

Control of polyphenoloxidase in some fruits usually cannot be achieved by heating, since heat damages some desirable sensory properties. Chemical substances are, therefore, utilized to retard enzyme catalyzed oxidative browning. These serve to inhibit enzymes, alter substrates, or limit entrance of oxygen. Among these are sulphur dioxide

(sulphites and sulphurous acid), some organic acids (e.g., citric and malic), ascorbic acid and sugar.

                   Sulphiting agents, such as sulphur dioxide and salts releasing this gas (Na- or K-metabisulphite), are highly effective in controlling both enzymatic as well as non-enzymatic browning. They also check the!

growth of microorganisms, act as bleaching agents, antioxidants or reducing agents and carry out various other functions. Sulphites interact with the enzyme or substrate. The raw material, e.g. grapes for drying, is' exposed to SO2 fumes generated from burning sulphur in a closed chamber. Because of the inherent side effects introduced when SO, gas is used, dipping in solutions of Na- or K-metabisulphite is preferred. In this case SO, is released much more slowly, the bleaching action is minimized and the apparent toxicity and bad taste often associated with bigh levels of the gas are avoided. Sulphur dioxide or matabisulphite

treatment has certain limitations. Besides being toxic to the workers limplicated as initiator of asthmatic reactions), it bleaches natural colour

E come fruits and may cause corrosion of cans. It is discouraged for fruits and vegetables that-serve as major-sources of thiamine, since this

vitamin is destroyed by these chemicals.

                   Ascorbic acid is the best-known alternative to sulphites. It is substances in reduced and colourless state. Concentration of about 0.3% ascorbic acid in sugar syrup is usually required for complete effectiveness. However, lower concentration (about 0.1%) is frequently used. Sugar syrup lessens enzymatic browning by acting as a barrier to the entrance of oxygen. The organic acids lower pH to a value les suitable for enzyme activity.