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these are the topics from food engineering unit operation
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Sorting & Grading Sorting and grading are terms which are frequently used interchangeably in the food processing industry, but strictly speaking they are distinct operations.
SORTING OF FOODS • Sorting may be regarded as a separation operation based on the differences in physical properties of the food raw materials or products such as colour, size, shape or weights of the food raw material. • Sorting is an important operation in controlling the effectiveness of many processes in food industry. • Sorted vegetables and fruits are better suited for mechanized operations of peeling, pitting and coring or blanching. • Food materials of uniform size or shape are better suited for efficient heat transfer during sterilization, pasteurization, dehydration or freezing.
SORTING METHODS • Weight Sorting • Size Sorting • Shape Sorting • Color Sorting
WEIGHT SORTING • Weight is usually the most precise method of sorting. • Meat cuts, fish fillets, fruits such as apples, pears and citrus fruits, vegetables such as potatoes, carrots and onions and eggs are sorted by weight using spring-loaded, strain gauge, or electronic weighing devices incorporated into conveying systems..
DISADVANTAGE OF WEIGHT SORTING weight sorting is the relatively long time required per unit and other methods are more appropriate with smaller items such as legumes or cereals, or if faster throughput is required.
SIZE SORTING • Size sorting is the separation of solids into two or more fractions on the basis of differences in size.
1) VARIABLE APERTURE SCREEN • variable aperture screens using cable, belt, roller or screen • Variable aperture screens with continuously variable apertures of roller, belt or screw type find use in size sorting of fruits and vegetables.
2) FIXED APERTURE SCREEN • fixed aperture screens using stationary, vibratory, rotary, gyratory or reciprocating screens. • Fixed aperture screens of flat-bed type are used in preliminary sorting of potatoes, carrots and turnips.
Multi-deck screens are used in size sorting of cereals, nuts and also partly processed and finished foods such as flour, sugar, salt, ground spices and herbs.
Drum screens are used for sorting peas, beans and other similar foods capable of withstanding tumbling action in a rotating drum screen.
SHAPE SORTING • Shape sorting is adopted when food raw materials contain undesirable material even after size or weight sorting and cleaning. • cleaned and size or weight sorted wheat may still contain weed seeds of similar size and weight compared to wheat. • Shape sorting on the basis of a combination of length and diameter is useful under such circumstances. • A disc sorter is used for shape sorting wheat, rice, oats and barley. • The principle is that disks or cylinders with accurately shaped indentations will pick up seeds of the correct shape when rotated through the stock, while other shapes will remain in the feed.
Accepted products and rejected products differ in their shape although their size are same
Photometric/ color sorting • Photometric sorting uses optical properties of foods to effect separation of desired material from contaminants. The goal is the separation of items that are discolored, toxic, not as ripe as required, or still with hull. • The color separator separates the fruits, vegetables or grains due to difference in color or brightness. • The color separators are generally used for larger crop seeds like peas and beans. These seeds differ in color because of varietal differences and also due to immaturity or disease. • Color sorters are also used for color sorting harvested foodstuffs, such as coffee, nuts, rice, and other cereals such as wheat or rye and pulses
WORKING • Two photocells are fixed at a particular angle, which direct both beams to one point of the parabolic trajectory of the grains. A needle is placed on the other side, which is connected to a high voltage source. When a beam falls on a dark object through photoelectric cells, current is generated on the needle. The needle end receives a charge and imparts it to the dark seeds. The grains are then passed between two electrodes with a high potential difference between them. The seed is compared with a selected background or color range, and is separated into two fractions according to difference in color. Since this machine views each produce individually, the capacity is low.
Reflectance properties are used to indicate: • Raw material maturity (e.g. color of fruit, vegetables and meat indicates ripeness and freshness characterize ;) • the presence of surface defects (e.g. worm holed cereals or nuts and bruised fruits) • The extent of heat processing (e.g. in manufacture of bread and potato chips or crisps).
OTHER SORTING METHODS • Sorting on the basis of surface roughness or stickiness may be used for separating seeds. • In Surface Texture/Roughness Separator the mixture to be separated is fed over the centre of an inclined draper belt moving in upward direction. • The round and smooth grains roll or slide down the draper at faster rate than the upward motion of the belt, and these are discharged in a hopper. The flat shape or rough surfaced particles are carried to the top of the inclined draper and dropped off into another hopper
GRADING OF FOODS • Grading is quality separation on the basis of an overall assessment of those properties, which affect the acceptance of the food raw material for processing, and finished food product for consumer acceptance and safety. The grading factors which determine the quality of the food include: • Process suitability • Consumer safety • Consumer acceptance.
PARAMETERS FOR GRADING OF FOOD • Size and shape as functional and acceptability factors, • Maturity to describe the freshness of eggs, ripeness of fruits and aging of meat, • Texture to grade the crumb structure in bread and cakes, crispness in apples and viscosity of creams • Flavour and aroma as indicators of ripeness of fruits as well as effectiveness of processing conditions, • Colour as indicator for consumer acceptability and effectiveness of process, • Blemishes such as cloudy yolk, blood spot and shell cracks in eggs, bruises in fruits and insect holes in coffee beans and cereals to indicate their defect and impurity.
Contaminants and undesired parts such as rodent hair and insect parts in flour, soil and spray residues on fruits and vegetables, microorganisms and their metabolites on meat, toxic metals in shell fish, hone fragments in meat products, pod residues in peas and beans and stalks and stones in fruits all these are the adverse qualities of the raw food materials.
GRADING METHODS Grading methods may be classified into two types: • Quality control procedures in which the quality of the food is determined by laboratory tests on samples drawn statistically from a batch of food. • Procedures in which the total quantity of food is subjected to physical separation in quality categories. This grading may be carried out manually or by specialized machines.
For proper grading, the food unit must be presented singly before the human grader or machine for assessment. These devices may be roller or vibratory tables or rotating wheels equipped peripherally with pneumatic devices which pick up food pieces, rotate them for viewing and then release them at a given signal.
Manual grading is done by trained operators who are able to assess a number of grading parameters simultaneously. For example, eggs are graded manually by candling.
Machine Grading • Machine grading is only feasible where quality of a food is linked to a single physical property, and hence a sorting operation leads to different grades of material. • But can be carried out by combining a group of sorting operations so as to separate the food units on quill it basis. • wheat of a particular variety may be graded by a combination of cleaning and sorting operations. Sometimes a single property may be helpful in grading the food. • peas of small size are recognized to be most tender and of highest quality so that size sorting of cleaned peas results in quality grading. Peas may also be graded on the basis of their density using flotation in brines of varying densities. • potatoes or high density, desirable for manufacturing French fries, potato crisps and dehydrated mashed potato, may be graded using Rotation in brines. • Mechanical grading is cost effective and efficient.
