Processing of Horticultural Crops

Types of Unit Operations

1. RAW MATERIAL HANDLING
Material handling include varied operations as hand or mechanical harvesting on the farm, transportation in trucks or refrigerated vehicles of perishable produce to the market or to the processing plant or to store/godowns. For conveying, wide variety of mechanical conveyors is used depending upon the type of material. Common conveyors used in the processing plant include screw conveyor, bucket conveyor, belt conveyor and vibratory conveyor. Throughout these operations care 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 etc. It also includes other unit operations like receiving, cleaning/washing, sorting, grading, peeling, halving, slicing, blanching etc for preparation of fruit and vegetables for processing.

Raw material selection/receiving

• Fruits and vegetables should be ripe but firm, evenly matured, free from blemishes, insect damage and malformation.
• Harvesting at proper maturity is an important step in selection of raw material.
• Most of the fruits are harvested at soft ripe stage.
• Vegetables except peas, beans etc are harvested at mature stage to enable them to withstand cooking during sterilization.
• Some vegetables like green beans, greens peas, ladies finger should be tender and free from soil, dirt etc.

2. WASHING/CLEANING
Cleaning is the unit operation in which contaminating materials are removed from the food and separated to leave the surface of the food in a suitable condition for further processing. Cleaning can be performed by using:

• Wet procedures: Soaking, spraying, floatation, washing and ultrasonic cleaning.
• Dry procedures: Separation by air, magnetic attraction of metal contaminants or by physical methods depending upon the product and nature of the dirt.
• Fruit and vegetables are generally washed with water to remove dust, dirt and adhering surface micro-flora.
• Fruits like peach, apricot etc that are lye peeled are not washed before peeling.
• Washing after peeling removes vitamins and minerals and should be discouraged.
• Different methods of washing include soaking or agitating in water, washing with cold or hot water sprays etc.
• Mechanical washers involve agitating or tumbling the commodity on moving belts or revolving screens while they are immersed in water or subjected to water sprays.
• Washing by using high pressure sprays is most satisfactory.
• Detergents are frequently used in the wash or rinse water.
• Vegetables may be soaked in dilute solution of potassium permanganate or chlorine (25-50 ppm) for disinfection.

3. SEPARATING
It involve separating a solid from a solid like peeling of potatoes, separating a solid from a liquid as in filtration or a liquid from solid as in pressing of juice from a fruit. It might involve the separation of a liquid from a liquid as in centrifugation of oil from water. It might also involve removing gas from a solid or liquid as in vacuum removal or air from canned food during canning. Common separating methods used in fruit and vegetable processing are discussed as under:

• Sorting: Sorting is the separation of foods into different categories on the basis of a measurable physical property. Sorting and grading ensures the removal of inferior or damaged commodity. For sorting, inspection belt can be used, in addition to trained personnel who detect poor quality produce unsuitable for canning. Automatic colour sorters can be used for sorting to reduce labour cost.
• Grading: After preliminary sorting, the fruit and vegetables are graded to obtain uniform quality with respect to size, colour etc. Grading can be done either manually or with the help of mechanical graders. Different types of mechanical graders include screen grader, roller grader, rope or cable grader, conveyor grader etc.
• Peeling, coring and pitting: These are the primary unit operations for preparing fruit and vegetables for processing.
• Peeling of fruit and vegetables is carried out to remove unwanted or inedible material and to improve the appearance of the final product.
• The main consideration for peeling is to minimize cost by removing as little of the underlying food as possible and reducing energy, labour and material cost to a minimum.
• The peeled surface should be clean and undamaged.
• Depending upon the commodity, peeling and coring methods can be selected such as 1) hand or knife peeling 2) machine/abrasive peeling 3) flash steam peeling 4) lye (caustic) peeling 5) flame peeling.
• Cores and pits in fruits like apple, peach, apricot etc are removed by hand or by machine (de-corer).

4. DISINTEGRATING
It covers wide range of operations that are used to sub-divide large masses of foods into smaller units or particles. It may involve cutting, slicing, chopping, grating, pressing to extract juice, pulping, homogenizing etc.

• Slicing, chopping, cutting and dicing: 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 are collectively called as size reduction. These unit operations increase the rate of drying, heating, cooling and improve the efficiency and rate of extraction of liquid components like fruit juices.
• Juice extraction: For juice extraction, the fruits and vegetables like apple, pear, carrot, aonla etc are grated in fruit grater to reduce their particle size. The grated mass is then pressed through basket press/hydraulic press to extract juice.
• Homogenization: Homogenization of milk causes disintegration of fat globules in milk or cream from large to minute globules. The smaller fat globules then remain evenly distributed throughout the milk or cream with less tendency to coalesce and separate from the water phase of the milk. Disintegration of fat globules is done by forcing the milk or cream under high pressure through a valve with very small openings. Similarly, fruit juices are homogenized to prevent sedimentation during storage.
• Pulping: For extraction of pulp, the fruits like apple, pear, apricot, guava, plums, tomato etc after preliminary treatment (crushing with or without heating), are passed through the pulper. With the action of blades/flights in the pulper, the fine pulp is forced through the openings of the screen/sieve which is collected at one end, while, seeds, skin and core is forced through other end of the pulper. Depending upon the type of fruit, various types of pulper like baby pulper, tomato pulper, mango pulper etc can be used.

5. PUMPING
This unit operation is used for moving liquids from one processing step to another. Single screw type and gear type pump are used for this purpose.

6. MIXING
There are different types of mixers depending upon the type of material to be mixed. They may be used for mixing solids with solids, liquid with liquids, liquids with solids, gases with liquids etc.

• For simple mixing of dry ingredients, a conical blender may be used.
• A ribbon blender with rotating mixing elements is used to mix sugar with other dry components to produce fluffy dry mix.
• Propeller type agitator mounted within stainless steel vat is used for mixing solids into liquids to dissolve them as in case of salt and sugar solution.

7. HEATING
Heating of foods is carried out to destroy the micro-organisms, to preserve the food as in case of pasteurized milk and canned peas and to make them more tender and palatable as in cooking operations. Foods are heated by conduction, convection, radiation or their combination. Most of the foods are sensitive to heat and prolonged heat may cause burnt flavour, dark colour or nutrition loss. It is therefore desirable to heat such foods rapidly and cool immediately. Foods may be heated or cooked by using hot water toasters, direct steam, direct contact to flame or microwave cookers.

a. Blanching: Treatment of fruit and vegetables by dipping in boiling water or steam for short periods followed by immediate cooling is called blanching. The basic objectives of blanching are:

• Inactivation of 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 and
• To reduce internal can corrosion.

b. Pasteurization: Pasteurization is a relatively mild heat treatment in which the food is heated below 1000C 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 cooked in a second heat exchanger.
• For viscous fluids, a swept surface heat exchanges (SSHE) is used to promote faster heat transfer and to prevent surface fouling problems.
• In package pasteurization is similar to conventional thermal processing of foods, but is caused at lower temperatures.
• The extent of heat treatment required to stabilize a food is determined by the D value of the most heat resistant micro-organism or enzyme present in the food.
• Milk pasteurization is based on D60 and a12 logarithmic cycle reduction in the numbers of Coxiella burnetti while liquid whole egg is treated to produce a 9 D reduction in numbers of Salmonella seftenberg. Since colour, flavour and vitamins are also characterized by D values, therefore HTST (High temperature short time) process is used for retention of nutritional and sensory quality.
• In milk processing, the low temperature long time (LTLT) process operating at 630C for 30 minutes (holder process) cause greater changes to flavour and loss to vitamins than HTST processing at 71.80C for 15 seconds.
• Flash pasteurization uses higher temperature and shorter times (HTST) for example 880 C for 1 s, 94 0 C for 0.1 sec or 100 0 C for 0.01 sec. for milk and is known as Higher heat shorter time processing.
• For milk pasteurization, inactivation of alkaline phosphatase is used as indicator of pasteurization.
• Liquid egg pasteurization is based of measurement of a- amylase activity.

c. Processing: Heat processing consists of heating cans to a predetermined time and temperature combination of heating to eliminate all possibilities of microbial spoilage.

• Over cooking should be avoided as it spoils the texture, flavour and appearance of the product.
• In continuous non-agitating cookers, the cans travel in boiling water in crates carried by over-head conveyors on a continuous moving belt.
• In continuous agitating cookers, the sealed cans moving on the belt are rotated by a special mechanical device to agitate the contents of the cans. This helps in reducing the processing time.
• Generally all fruits and acid vegetables can be processed satisfactorily in boiling water (100oC) and non acidic vegetables (except tomato and rhubarb) are processed at higher temperatures of about 115-121oC under pressure.

d. 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 (115-1210C).
• Acid foods like peaches, pears, pineapple and other fruits are processed at 1000C or lower for adequate inactivation of enzymes.