Types of Pre-cooling

Types of Pre-cooling

    Types of pre-cooling methods
    A. Cold air
    i. Room cooling
    ii. Forced air cooling (presser cooling)
    B. Cold water / Hydro cooling
    C. Top icing – direct contact with ice
    D. Evaporation of water from produce – i. Evaporative cooling
    ii. Vacuum cooling
    E. Hydrovac cooling – combination of hydro and vacuum cooling
    Commodity –wise cooling methods

    Cooling methods

    Commodities

    Room cooling

    All fruits and vegetables

    Forced air cooling

    Fruits and fruits type vegetables, tubers and cauliflowers

    Hydro cooling

    Stems, green leafy vegetables, fruits and fruit type vegetables

    Package icing

    Roots, stems, cauliflowers, green onion, brussel sprouts

    Vacuum cooling

    Stems, Leafy and flowers type vegetables

    Transits cooling
    -Mechanical
    -Top iceing & channel icing

    All fruits and vegetables
    Roots, stems green leaf vegetables and cantaloupes

    A. Cold Air cooling
    i. Room cooling
    • In room cooling, heat is transferred slowly from the mass of the produce (by convection) to the cold air being circulated around the stacked containers.
    • This is most common and widely used method. Here cold air is passed from the fan and cool by convection process.
    • Its commonest use is for products with relatively long storage life and marketed soon after harvest.
    • Advantages of this room air-cooling are that produce can be cooled and stored in the same room without the need of transfer and hence it is economical.
    • Under this system, cold air from evaporator enters the room, moves horizontally and then passed through the produce containers and return to the evaporator.
    Disadvantage
    It takes more time to cool the products- the removal of heat slowly makes this system unsuitable for highly perishable commodities. This is because the product needs at least 24 hours to reach the required storage temperature.
    Almost all crops are suitable for this type of cooling but it is mainly used in citrus fruits, potato, onions, garlic, citrus etc.
    ii. Forced air cooling or pressure cooling
    • In this system ‘cold air is passed by force from one side to other side using big fan’.
    • Cold air movement is through the containers rather than around the containers.
    • Cooling is 4 to 10 times more rapid than room cooling and its rate depends on airflow and the individual volume of produce
    • Air is blown at a high velocity leading to desiccation of the crop. To minimize this effect, air is blown through cold water sprays.
    • It is slow compared to hydro cooling but is a good alternative for crops requiring rapid heat removal which cannot tolerate wetting or chlorine of cooling water.
    • Adequate airflow is necessary. This is because fruits in the center of packages tend to lose heat at a slower rate, compared to those on the exterior.
    • This system is also called as high humidifier. High RH of 90 - 95% is to be maintained in the precooler to avoid dehydration during cooling.
    • This system can be applied to all crops particularly berries, ripe tomatoes, bell peppers and many other fruits, cabbage, green peas, cucumber, brinjal, muskmelon, watermelon and mushroom.
    • Baby corn : 5 – 6 hr cooling at 2 - 40C,
    • Leafy vegetables : 1 – 2 hr cooling at 6 - 80C (too low temperature causes chilling injury)

    Difference between Forced air cooling and Vacuum cooling

    Forced air cooling

    Vacuum cooling

    The air passes over the surface of the crop, cooling the outside while the inside is cooled by heat transfer from inside to the outside for the crop.

    In cooling chamber, pressure (reduced) is exactly the same around the produce and in the centre of the produce. This means the cooling is very even and quick throughout the crop.


    Crops usually pre-cooled by forced air

    Anona

    Citrus

    Litchi

    Plantain

    Avocado

    Coconut

    Mango

    Pomegranate

    Banana

    Cucumber

    Mangosteen

    Prickly pear

    Barbados cherry

    Eggplant

    Melons

    Pumpkin

    Berries

    Fig

    Mushrooms

    Rhubarb

    Breadfruit

    Ginger

    Okra

    Sapota

    Brussels sprouts

    Grape

    Orange

    Snap beans

    Cactus leaves

    Grapefruit

    Papaya

    Strawberry

    Capsicum

    Guava

    Passion fruit

    Summer squash

    Carambola

    Kiwifruit

    Persimmon

    Tomato

    Cassava

    Kumquat

    Peas

    Tree tomato

    Cherimoya

    Lima bean

    Pineapple

    Yam

    B. Hydro cooling/Cold water cooling
    Principle - ‘the transmission of heat from a solid to a liquid is faster than the transmission of heat from a solid to a gas’. i.e. water is better heat conductor than air.
    In this method cold water is used for quick cooling of a wide range of fruit and vegetables. Hydro cooling avoids water loss and may even add water to the fruit. Under this method, water is usually cooled by mechanical refrigeration, but ice may be used to make process faster. Chlorine (150-200ppm)/Iodine/Nutrients/Growth regulators/ Fungicides can be added in water to sanitize/ improve nutrient status and prevent post harvest diseases of the produc.
    For quick cooling of the produce, cold water must constantly be passed over the crop. This can be done by submersing the crop in cold water which is constantly being circulated through a heat exchanges.
    Cooling time
    • 2 min for asparagus(long & narrow) & Leafy vegetables (more surface to volume ratio)
    • 10 minutes for small produce like capsicum (large and globular)/ cherries/tomato
    • up to 1 hour for large products such as melons.

    Hydro-cooling has the advantage over the pre-cooling method where it helps in cleaning the produce, provides fast, uniform cooling for commodities. It is faster than forced air cooling. Hydrocooling can be achieved by immersion or through means of a chilled water shower. Not all crops can be hydrocooled, because they need to be able to tolerate wetting, chlorine, and water infiltration.
    Disadvantage -Tank water can be contaminated with micro organisms which can result in increased levels of spoilage during subsequent storage or marketing so chlorine should be added to avoid the problems.

    Two types of hydro coolers are generally used.
    i. Shower/batch type - The water showers over the commodity, which may be in bins or boxes, or loosen a conveyer belt. A common design is to transport the crop on a perforated conveyer belt (the speed of the conveyer can be adjusted to the time required to cool the crop) and cold water is pumped from the tank and allowed to fall on the produce in sprinkled type and then falls through to the tank below then filtered, recycled and re cooled (Fig.3).
    Efficient cooling depends upon adequate water flow over the product surface. For product in bins or boxes, water flows of 75-100 lt. /min./ft.(400-600 l/min/m2) of surface area are generally used.
    Shower batch type hydro cooler
    Fig.: Shower/Batch type hydro cooler
    ii. Immersion type – It is simplest type of a hydro-cooler in which produce is dipped in cold water. Here product are normally in bulk, is in direct contact with the cold water as it moves through a long tank of cold. This method is best suited for products that do no float, because, slow cooling would result if the product simply moved out of the water. Immersion hydro coolers convey product against the direction of water and often have a system for agitating the water. Depth of the water tank should be >30 cm and water tends to penetrate inside fruits, particularly those that are hollow such as peppers. Water temperature also contributes to infiltration. It is recommended that fruit temperature is at least 50C lower than liquid.
    Eg.: Radish, Asparagus, Artichoke, Green onion, capsicum and leafy vegetables.

    Crops normally hydro cooled

    Artichoke

    Carrot

    Kiwifruit

    Potato(early)

    Asparagus

    Cassava

    Kohlrabi

    Pomegranate

    Beet

    Celery

    Leek

    Radish

    Belgian endive

    Chinese cabbage

    Lima bean

    Rhubarb

    Broccoli

    Cucumber

    Orange

    Snap beans

    Brussels sprouts

    Eggplant

    Parsley

    Spinach

    Cantaloupe

    Green onions

    Parsnip

    Summer squash

    Cauliflower

    J. artichoke

    Peas

    Sweet corn

    C. Top icing
    • This is one of the oldest ways to reduce field temperature.
    • It is commonly applied to boxes of produce by placing a layer of crushed ice directly on top of the crop.
    • It can also be applied as an ‘ice slurry’made from 60% finely crushed ice, 40% water and 0.1% sodium chloride to lower the melting point of the ice.
    • Ice slurry give greater contact between produce and ice compared only top icing, and therefore result in quicker cooling.
    • The main use for top icing is for road transport and it can be applied shortly after harvest. Top-ice on loads should be applied in rows rather than a solid mass. It is important not to block air circulation inside the transport vehicle. Ratios of water to ice may vary from1:1 to 1: 4.
    • Direct contact between the produce and the ice provides fast, initial conduction cooling. However, as the ice melts, an air space is created between the ice and the produce and the conduction cooling stops. Subsequent cooling is by radiation and convection, both of which are slower processes than conduction.
    Package ice can be used only with
    • During transport to maintain a high relative humidity for certain products
    • Water tolerant, non-chilling sensitive products
    • With water tolerant packages such as waxed fiberboard, plastic or wood.
    • It also increases costs because of the heavier weight for transportation and the need for oversized packages. In addition to this, as water melts, storage areas, containers, and shelves become wet.


    Crops suitable for ice cooling
    Belgian endive

    Chinese cabbage

    Leek

    Sweet corn

    Broccoli

    Carrot

    Parsley


    Brussels sprouts

    Green onions

    Pea


    Cantaloupe

    Kohlrabi

    Spinach


    D. Vacuum cooling
    • Vacuum cooling takes place by water evaporation from the product at very low air pressure (At a normal pressure of 760 mmHg, water evaporates at 100ºC, but it does at 1ºC if pressure is reduced to 5 mmHg.).
    • Most rapid and uniform methods of cooling. Products that easily/rapidly release water may cool down rapidly.
    Eg.: Most suitable - Leafy vegetables, cabbage
    Not suitable - Tomato with low ratios between mass and surface area and effective
    water barrier like wax on surface is not suitable.
    Produce is placed in a strong, airtight, steel chamber. Moisture loss is achieved by pumping air out of the chamber containing the product and reducing the pressure of the atmosphere around the product. It causing the water in the produce to vapourize. Cooling occurs because the heat energy for vapourization comes from the produce.
    Vacuum cooling causes about 1% product weight loss for each 50 of cooling.
    This method is also used to cool the products like beans, carrots, capsicum, celery, corn, lettuce, mushrooms, spinach, sweet etc.
    High cost and sophistication operation needed.


Last modified: Tuesday, 26 June 2012, 4:53 PM