Unit operations


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Unit operations in processing of fruits and vegetables:

Unit operations in processing of fruits and vegetables Presented by: Sakharam kale Scientist (AS&EM), HCP Division Central Institute of Postharvest Engineering and Technology, Abohar


Introduction In India, agriculture is still the mainstay of its economy due to huge production of food grains and horticultural crops India ranks second in the world in the production of fruits and vegetables Of the various types of activities that can be termed as agriculturally based, fruits and vegetables processing are among the most important Food processing prevents the physical, mechanical and/or microbial spoilage of food materials Food processing comprises of different processing operations called as unit operations The number and type of processing operations (unit operations) applied to fruit and vegetable products is immense and diverse. These operations can be grouped according to energy involved (e.g. heat), their mode of working (e.g. mechanical) etc. Example: Heat transfer operations is a group which is common to the manufacture of orange juice, strawberry-kiwi jam, canned peaches, French fried potatoes, cucumber pickles, sun-dried tomatoes etc. Some food processing methods utilize single unit operation whereas most of them include a combination of unit operations to complete the processing.


Group of operations Food processing operations Mechanical processing Peeling, cutting, slicing, size reduction, sorting, grading, mixing, emulsification Assembly operations Agglomeration, extrusion, forming Mechanical separations Screening, cleaning, washing, filtration, mechanical expression, centrifugation Mechanical transport Pumping of fluids, pneumatic conveying, hydraulic conveying, mechanical conveying Heat transfer operations Heating, blanching, cooking, frying, pasteurization, sterilization, evaporation, cooling, freezing, thawing Mass transfer operations Drying, extraction, distillation, absorption, adsorption, crystallization Membrane separations Ultrafiltration, reverse osmosis Fermentation Alcoholic fermentation, lactic fermentation, dairy fermentation Nonthermal preservation Irradiation, high pressure, pulsed electric field Packaging Filling, closing, aseptic packaging, active and intelligent packaging Groups of different unit operations


Materials handling Cleaning Separating Mixing Pumping Disintegrating Heat exchanging Evaporation and drying Forming Packaging Unit operations in horticultural crop processing

Materials Handling:

Materials Handling V ery common unit operation Encompasses those operations which move horticultural crops from the farm to packinghouse or processing plant and subsequently through the various unit operations in the plant Screw conveyor, bucket conveyor, belt conveyor, vibratory conveyor, bucket elevator, pneumatic conveyer etc C are is taken to maintain sanitary conditions, minimizing bruises and product loss, maintaining raw material quality like physical appearance, vitamin contents, minimizing microbial growth and minimizing other detrimental changes to the product quality during handling


Cleaning C ontaminating materials are removed from the food and separated to leave the surface of the food in a suitable condition for further processing Cleaning and sanitizing of fruits and vegetables is important to ensure that safe products are produced S ome of the fruits (like peach, apricot etc.) and vegetables that are peeled are seldom washed before peeling. Washing after peeling removes vitamins and minerals and hence should be discouraged Cleaning may range from simple removal of soil and other foreign matter on fruits by dumping in water (Fig. 2) to vigorous scrubbing of pineapples with high-pressure water jets Cleaning may be accomplished by using brushes, high velocity air, steam, water, vacuum, magnetic attraction of metal contaminants, mechanical separation etc. Use of clean water is essential for cleaning


Dry cleaning : Separation by air, magnetic attraction of metal contaminants or by physical methods depending upon the product and nature of the dirt/contaminants. Fruits and vegetables can also be brushed to remove the dirt Wet cleaning : Wet cleaning is more effective than dry methods for removing soil from root crops or dust and pesticide residues from soft fruits or vegetables. Soaking, spraying, flotation, washing etc. are the unit operations involved in wet cleaning. Washing is the main operation to clean the fruits and vegetables Dumping of apples in water Brushing and hand removal of damaged fruits


Screening separation of solids from the selected lot on the basis of size by means of screens of known apertures The raw material that stays on the screens is oversize and the material passes easily through these screens are of undersize Vibrating and oscillating screens are used for fine particles and sometimes the shakers are also used The basic purpose of any screen is to separate the feed consisting of a mixture of particles of different sizes into two fractions as: Oversize fraction: the material that stay on screen Undersize fraction: the materials that passing through the screens.


Separating Separating is done on the basis of a measurable physical property such as size, shape, colour and weight The purpose of this operation is to isolate desirable part of a raw material from another Separation generally termed as mechanical separation is divided into four groups - sedimentation, centrifugal separation, filtration and sieving sedimentation, two immiscible liquids or a liquid and a solid, differing in density, are separated by allowing them to come to equilibrium under the action of gravity Sedimentation may be a slow process and hence is often speeded up by applying centrifugal forces to increase the rate of sedimentation called as centrifugal separation Filtration is the separation of solids from liquids by causing the mixture to flow through fine pores Sieving, interposing a barrier through which the larger elements cannot pass, is often used for classification of solid particles


Common separating methods used in raw fruit and vegetable processing are sorting and grading Sorting : Sorting is the separation of foods into different categories on the basis of a measurable physical property Grading: After preliminary sorting, the fruit and vegetables are graded to obtain uniform quality with respect to size (Fig. 5), colour, texture, flavor etc. Sorting of fruits and vegetables Grading of onion bulbs on the basis of size


Peeling Peeling of fruits and vegetables is carried out to remove unwanted or inedible materials and to improve the appearance of the final product Hand/knife peeling : All fruits and vegetables Mechanical peeling : Used for peeling pears, apples, carrots, turnip, potatoes etc. Similarly, automatic peelers are used for peeling of peaches and cherries. Abrasive peeling : It is used for peeling potatoes, ginger, carrots etc . Flame peeling : Flame peeling is used in onions, garlic and brinjal (1000 o C). Peeling by heat or hot water : Peaches and potatoes are scalded in steam or boiling water to soften and loosen the skin, which is subsequently removed manually. Infra-red heat peeling can also be used for peeling of apples and tomatoes. Lye peeling : Fruit and vegetables like peaches, nectarines, apricot, sweet orange segments, carrots and sweet potatoes are peeled by dipping them in boiling caustic soda (1-2%) for 1-2 minutes.


Disintegrating Raw materials often occur in sizes that are too large to be used in processing Disintegration is an operation whereby large masses of foods are subdivided into smaller units or particles It can be accomplished by cutting, crushing, slicing, chopping, grating, pressing, homogenizing etc Fruit and vegetables are sliced to a desirable size either manually or by using semi or automatic slicing/chopping or dicing machines These unit operations increase the rate of drying, heating, cooling and improve the efficiency and rate of extraction of liquid components like fruit juices


Pulping Fruits like apple, pear, apricot, guava, plums, tomato etc., after preliminary treatment (crushing with or without heating), are passed through the pulper to extract the pulp Depending upon the type of fruit, various types of pulper like baby pulper , tomato pulper , mango pulper etc can be used

Juice extraction:

Juice extraction The fruits and vegetables like apple, pear, carrot, aonla, tomato etc. are grated in fruit grater to reduce their particle size. The grated mass is then pressed through basket press or hydraulic press to extract juice


Homogenization The fruit juices are generally homogenized to prevent sedimentation during storage. Homogenization imparts complete mixing of constituents in the juice. It has also an application in breaking the fat globules in milk


Pumping Pumping is an operation that moves foods from one location to another by mechanical action. The foods can be liquid, semisolid, paste or solid There are many types of pumps available in the market for general transfer processes. Food manufacturers often choose pumps that are able to move large quantities of foods, easy to maintain, a lower cost and able to preserve the physical properties of foods. Some pumps are employed for clean-in-place (CIP) purposes whereby the inner parts of pipes or instrument can be cleaned without dismantling


Mixing Many operations in food processing are dependent on effective agitation and mixing of the liquids Mixing implies random distribution of material throughout the mixed products. Different types of mixers depending upon the type of material to be mixed are used in processing of fruits and vegetables They may be used for mixing solids with solids, liquid with liquids, liquids with solids, gases with liquids, gases with solids etc


Blanching It involves the dipping of fruits and vegetables in boiling water or steam for short periods followed by immediate cooling The basic objectives of blanching are: to inactivate the enzymes to cleanse the product initially to decrease the microbial load to preheat the product before processing to soften the tissue for facilitating compact packing in the cans to expel intercellular gases in the raw fruit to prevent excessive pressure built up in the container to allow improved heat transfer during heat processing to ensure development of vacuum in the can to reduce internal can corrosion Different types of blanchers are used in blanching of fruits and vegetables


Pasteurization Pasteurization is a relatively mild heat treatment in which the food is heated below 100 o C to destroy selected vegetative microbial pathogens or to inactivate enzymes The pasteurization of liquid foods (fruit juices, milk, milk products, liquid egg etc.) is carried out in continuous heat exchanger The product temperature is quickly raised to the pasteurization levels in the first heat exchanger, held for the required length of time in the holding tubes and quickly cooled in a second heat exchanger.


Sterilization Sterilization is a more severe heat treatment given to a food to destroy both spoilage and pathogenic micro-organisms, after packaging the food in a hermetically sealed container . The thermal processing criterion for acid and medium acid foods (pH<4.5) is the destruction of heat resistant vegetative micro-organisms or enzymes. The low acid foods such as mushrooms, potatoes, peas and other vegetables are processed at elevated temperatures (115-121 o C). Whereas, the acid foods like peaches, pears, pineapple and other fruits are processed at 100 o C or lower for adequate inactivation of enzymes.


Cooling Cooling is the removal of heat which is added during processing. Cooling may be done to the degree where food is chilled to refrigerated temperature. Freezing Freezing is the reduction in temperature generally by super cooling followed by crystallization of water, nucleation and finely crystal growth. Most of the fruits and vegetables are frozen to increase their shelf life considerably.


Evaporation Evaporation in the food industry is used principally to concentrate foods upto 2 to 3 folds by removal of water All liquids boil at lower temperatures under reduced pressure and are the key to modern evaporation In many cases the evaporation is carried out under vacuum so that the water boils off at lower temperature, which avoids thermal damage to the final product Vacuum evaporators and multi-stage evaporators can easily remove water.


Drying It involves removal of water with minimum damage to the food Sometimes driers take the foods very close to total dryness that is to 97-98% solids Driers are used to prepare well known products like milk powder and instant coffee The Sun and solar drying, atmospheric dehydration including batch processes (kiln, tower, and cabinet driers) and continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers) are used in drying of various products of fruits and vegetables. Subdivision of a liquid is the basic principle behind the widely used spray driers Vacuum shelf, vacuum belt, vacuum drum and freeze driers are the type of driers that can be used for drying and dehydration of high valued products


Forming Many categories of the food industry require forming operation It is a size enlargement operation to obtain desirable shapes and sizes It is an important unit operation in the breakfast cereals and snack food industries The characteristic shapes of the popular breakfast cereals are the result of pressure extrusion through dies, together with adherence operating conditions like pressure, temperature, dough consistency and other variables


Packaging Packaging of food is required to protect the food from microbial contamination, dirt, dust, light, moisture and the losses. Functions of packaging Protection and preservation Containment Waste reduction Marketing Information Traceability Convenience Tamper indication The foods are packaged in metal cans, glass, plastic bottles, paper and metallic films, pouches etc. Now a days the packaging of food products has emerged as an important industry and automatic packaging units are in great demand The containers for packaging are automatically formed, filled and sealed by passing through machines.


Conclusion India ranks second in production of fruits and vegetables Huge production of fruits and vegetables in India is still accompanied with large (up to 18%) amount of postharvest losses in fruits and vegetables Such losses will be eliminated only through the processing of these commodities Hence it is important to understand and study the different unit operations involved in the processing of fruits and vegetables which will create awareness and interest about processing and thereby will help in reducing the postharvest losses.

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