Preparatory Operations In Food Processing (part1)

Preparatory Operations In Food Technology (part1) 

In the food industry several conscious operations are performed to meet the consumer requirements. These are aimed at ensuring that

the raw material is of the desired quality and free from unwanted constituents. These operations are collectively callod preparatory

operations and include:

1. Handling and transportation of raw materials

2. Cleaning

3. Sorting and grading

4. Peeling, shelling, skinning

5. Removal of inedible constituents

6. Size reduction

7. Mixing

8. Filtration

9. Prevention of enzymatic browning

1. Handling And Transportation Of Raw materials :

Under no circumstance can inferior raw material yield a good quality end product (Proper handling and transportation of raw materials

is, therefore, essential to produce good quality end products.Moreover, if the raw material is contaminated before being processed or preserved, greater efforts will be required in later stages to salvage its positive quality potentials. For that reason, as soon as (the raw material is

harvested, animal slaughtered, or fish is caught, 'it has to be given adequate treatment. Proper containerization is essential for these raw materials. Suitable containers must be free from contaminants such as ferilizer residues, chemicals, soil, etc. Quite often second-hand sacks are used for packaging or transporting grains. These may have been used for animal feed, fertilizer, cement or other similar products. The raw material gets contaminated in such bags.

         Any aspect of the container construction that predisposes food raw material to injury must be avoided, since this will pave way for

contamination and growth of microorganisms resulting in accelerated spoilage. The size and type of container is also important. Some raw

materials such as tomatoes cannot be packed in deep containers, while onions or potatoes, that are texturally hard, may be transported in bags. In the Pakistani markets, fruits and vegetables to the tune of 40% are lost as a result of improper containerization and handling.

                  Meat carcasses are usually hung in spegial transporting vehicles that are equipped with refrigeration system. In Pakistan and other

d eveloping countries, meat is transported from the slaughterhouses to the market in all sorts of available vehicles including bicycle, animal

d riven carts (donkey cart, tonga, etc.), rickshaw and others. By the time it reaches the sale point, it gets heavily contaminated.

                                                   Fish has to be brought to the market soon after catching as it deteriorates rapidly after death. In most paris of Pakistan it is transported in baskets packed with ice. Eggs have special trays in which these can

be placed and the trays are stacked one over another for safe transportation.

2. Cleaning :

Food raw materials carry severai contaminants on their surface. Microorganisms are invariably present in all foods. Carrots, potatoes and other crops that grow beneath the soil surface possess larger and diversified microflora than oranges, bananas or tomatoes. The raw material also gets contaminated with soil, sand and stones from the field. The grains carry foliage, twigs, stalks and other plant materials extraneous to them. Pieces of metals, lubricating oil, grease, and pieces of ropes, strings, etc., may also be present in one food material or another. .In addition, pesticide residues and residues from fertilizers

applied to crops also form part of the contaminants.

                         The cleaning operation separates contaminants from the food material and disposes off the same. Once the food surface has been reasonably cleaned, effort is made to limit recontamination and to leave the cleaned surface in the desired condition it has been observed that vegetable retailers in Pakistan wash their produce in canal or stagnant water. Such a practice adds fine soil particles to the vegetables and leaves the surface contaminated, thus providing a reverse effect.

                          There are primarily two methods of cleaning, depending upon the raw materials and contaminants: dry and wet cleaning.

1. Dry Cleaning :

This is especially suitable for grains and other crops that are dry and would introduce technical problems if soaked in water. Particles of different sizes are separated by screening-a technique applied in the home for separating coarse particles from flour. If the contaminant and food raw material are of dđifferent densities, then aspiration is a suitable method.

                          Winnowing, finds application in the separation of wheat from straw, rice from husks, groundnut skins from roasted shelled groundnuts and bran from endosperm in flour milling. As practiced at home, food to be cleaned is placed in a shallow wide-mouthed tray or basket and the contents propelled some 10-12 cm upwards into the air. The lighter

particles are blown off by wind or air current. This action repeated several times brings about complete separation. Industrially, the process

of winnowing is mechanized. Fig. 3.1 illustrates a combined pneumatic and screen separator for grain cleaning in the industry.

                            Electronic metal detectors are employed to separate metallic contaminants such as nuts, bolts, pieces of broken equipment parts, etc. These remove both ferrous and non-ferrous particles. Strong magnets are employed to eliminate nails, screws, pieces of wires and other ferrous contaminants from the grains.

                      Other techniques. used in dry cleaning of food commodities employ the principle of abrasion and brushing.These techniques are especially useful for heavily contaminated foods such as potatoes, carrots, radish, turnips, and other roots and tubers that grow beneath the soil surface. Special brushes loosen and sometimes remove the adhering soil on these commodities in conjunction with other methods of cleaning.

2. Wet Cleaning :

In wet cleaning, water, which is an efficient and cheap liquid/solvent, is exclusively used for the purpose. The water for cleaning must itself be of appropriate standards, i.e., it should be clean and free from pathogenic microorganisms and toxic substances. It should not contain dissolved solids and should be soft. Proper arrangements for disposal of wastewater are a pre-requisite in the food industry.

                           Heavily contaminated raw materials such as potatoes and carrots are soaked for some time to loosen the adhering soil before applying any other technique for complete removal of the contaminants.3 The soaking process is made more efficient if the product is moved in water or the water is agitated. Use of ultrasonic waves above 16 kHz (20 to 100 kHz) for cleaning some raw materials has gained popularity in the food industry. When these waves are passed through the cleaning fluid, violent agitation of particles occurs. This results in loosening of contaminants from the surface of raw material. Spraying may be

employed separately or in conjunction with soaking for washing fruits and vegetables. The efficiency of the spraying process depends upon the water pressure and its volume.

                      Flotation and sedimentation are used when density or buoyancy of the contaminant is different from that of the raw material. The principle is similar to aspiration but in this case water is the medium rather air. In cleaning of ice and pulses (daats) destined for cooking, the grains are soaked in water, all light particles fioat and are removed from surface. The heavier contaminants such as stones settle at the botton and are removed. Raw peas contain stones along with pieces of leaves pods and stalks. In a stream of water, stones sink to the bottom and pieces of leaves and stems float to the surface, leavinga layer of "clean peas in the middle. Contaminants from fresh milk, fruit juices and syrups are normally removed by filtration.

                     A combination of different cleaning techniques is often applied in the food industry to achieve the goal. For example, potatoes are soaked in water, brushed and then sprayed to effectively remove.the adhering contaminants. Similarly, in cleaning grains, especially wheat for the manufacture of flour, several techniques are employed. These include screening, magnetic separation, aspiration and washing before milling into flour.

                            When wet cleaning is employed, dewatering is essential, since excess water usually has an adverse etfect on the subsequent

processing steps. Dewatering may be affected at home by using a towel but in the industry where large quantities of raw material are handled, mechaniçal dewatering equipment is essential. Vibrating screens, dewatering reels, centrifuges or even drying equipment may be

employed for this purpose.

3. Sorting And Grading :

Sorting is the separation of raw materials into categories of different physical characteristics such as weight, size, shape and colour, while grading is separation into categories by quality. The two operations together provide a means of segregàting raw material into identical lots, which ensure that finished products are also of uniform quality.

                                        in the food industry, where-huge quantities of raw materials are handled by machines, sorting and grading form an essential part of the processing. Sorting is one way of ensuring uniformity in quality finished products. Hence, while the raw material is being sorted, It is partly graded for quality as well. Normally, sorting and grading are carried out as integral operations. However, the finished product is often graded again into one of the several grade categories depending upon its quality. Raw materials are sorted on the basis of weight, size, shape colour.

1. Weight Sorting :

Weight sorters are machines that weigh the individual item and separate into different categories. Eggs are normally sorted by weight

and packed in trays. Some food materials that are sold by weight, e.g., meat cuts, pass through a computer controlled machine which weighs

the item, records its weight, computes the price and prints-out a weight label that is affixed to the pack.

2. Size Sorting :

Size sorting is important for an effective use of processing machinery. Foods of similar size are convenient for mechanized operations like peeling (potatoes), blanching, extraction (oranges juice), canning, dehydration and freezing. A common sizę sorter used in homes

is the fixed aperture screen (sieve) employed for removing tealeaves from prepared tea beverage, or for sifting wheat flour. Similar sieves, called screens, of different dimensions and apertures are used in the food industry. These are operated by electric motors and may have

different types of motions such as vibratory, rotary, gyratory, etc. Roller sorters, adjusted to different apertures, are employed, especially for

large items such as citrus fruits including orange. These may have a fixed aperture to sort the material into two categories only or variable aperture for separating into several sizes.

3. Shape Sorting :

Shape sorting is not very common except for removal of contaminants from the raw material. Thus, when grains, such as wheat, have been cleaned of all possible contaminants, some weed seeds may stilI remain because of their being similar in size and weight to the wheat

grains. In this case wheat is passed through shape sorters, which removes all contaminants having the same size or weight but different shape.

4. Colour Sorting :

Colour sorting is important where uniformity of colour will fetch a higher price for the product. This is carried out with the help of manual labour on inspection belts-a method that is expensive and has several limitations. Electronic colour sorters are in operation that scan the colour of each individual granule with the aid of photocells. The equipment allows materials of the matching colour to pass through while rejecting granules of different colours by means of an air stream.

4. Peeling, Shelling, Skinning :

Nature protects food crops and animals from the attack of external forces by providing a protective covering. This covering known as peel, husk, shell or skin, depending on the raw material, is often inedible and has to be removed. Peeling is commonly practiced in fruits, vegetables and tubers. Several peeling methods are employed. Hand peeling using ordinary or peeling knife requires minimal investment and is only cheap where labour is inexpensive. It has been replaced with more modern methods, though some fruits like mangoes are still hand peeled.

                      Peeling by heat is advantageous as losses are minimum and the process can easily be automated: In this method boiling water or steam loosen the peel that can be easily slipped by hand or a scrubber or water spray. Flame peelers are employed for peeling onions, garlic, tomatoes and peppers by exposing them to direct flame or to hot causes steam to develop under the skins and puff them so that they can

be washed away with water.

                         Mechanical peeler is more suited to apples, carrots and potatoes. It consists of an upright cylinder, provided in the bottom with a

rapidly revolving disc that undergoes undulatory movement (Fig 3.2). The inner walls of the cylinder and disc surface are coated with abrasive carborundum. Continuous abrasive peelers have also been developed. In lye peeling (Fig. 3.2), hot sodium hydroxide solution (1 to 2%) is used for about 30 seconds to 2 minutes or longer, depending upon the raw material. The lye dissolves the peels. This method is suited to all shapes, sizes and varieties. Peeling losses are less as compared to hand peeling and the method is amenable to large-scale production.

                       Besides these conventionally used methods, a number of new techniques have been developed. These are freeze peeling, vacuum peeling, dry caustic peeling, acid peeling, calcium chloride peeing and ammonium salt peeling. Major emphasis in all the efforts has been to (a) minimize the peel loss, (b) improve the quality of peeled material, (c) reduce pollutión of wastewater disposal problems and (d) reduce the cost of peeling. Table 3.1 gives comparative efficiencies of

conventional peeling methods.

                       Some raw materials e.g., eggs and groundnuts have tick and hard protective covering called shell. Removal of the shell is kown as shelling. Some shells, such as are found on coconut, are so hard that considerable mechanical force may be necessary to crack then open peas, pistachio nuts, coconuts, almonds, or walnuts must be shelled to obtain the edible portion from them.

5. Removal Of Inedible Constituents :

Fruits like pineapple have a central core, which although edible in some cases, must be removed during processing. Removal of the core is termed as coring. Dates and plums have pits or seeds located centrally in them. The removal of these seeds is done in an operation

referred to as pitting. Some raw materials may be plucked with stems. Examples are tomaloes, strawberries and some vegetables. Removal of

the stem is known as stemming and is usually done by hand or vibrating screens. A mild heat treatment (blanching) helps easy removal of stems.

            Trimming involves discarding of discoloured, decayed or damaged portions of the raw material and is usually done after other

preliminary operations have been completed. Green or under ripe portions; damage/decaying parts, over-ripe and bruised portions of raw materials or the eyes in potatoes are removed during trimming.