Precooling 2

Factor affecting cooling

    Heat in the crop due to surrounding or due to metabolic heat.

    Packaging material used.

If cooling medium is air, the boxes having produces should have holes for ventilation. For hydro-cooling and icing, the box used must withstand direct contact with water. Plastic boxes or fibre-board boxes should have wax coating. Heat is removed by evaporative cooling, so crops should not be sealed in polyethylene bags.

Pre-cooling the crops after harvest may have a large benefit on their subsequent maximum storage life. Several method of pre-cooling are commercially and which method is most suitable depend on the crops, storage life required and economics.

Different methods of cooling are:

v    Icing

v    Room Cooling

v    Freezed Air Cooling

v    Hydrocooling

v    Vaccum-Cooling

Icing: This has been used for many decades and is also called contact or top-icing. In this process crushed ice is directly placed on the top of crop which are in the boxes. The ice melts and the cold water runs down through the crops, cooling it. Ice can be placed in the form of slurry which is hosed onto the top of the crop from a tank.

Ice-slurry- 60% of crushed ice + 40% of water and 0.1% of NaCl

Ice slurry gives better contact between produce and ice than top-icing and results in quicker cooling.

Use:

Top-icing is for road transport and it can be applied shortly after harvest.eg field-packed lettuce

Room Cooling: Crop is simply placed in cold store. The type of room used may vary , but generally consist of a refrigeration unit across which cold air is passed from a fan. The air circulation is such that air is blown across the top of the room and falls through the convection. if it is blown in at the bottom of the store, than plenum chamber is used.

Advantage: No special equipment or facilities is used so cost is low.

Disadvantage: It takes time to reduce the temp. of produce.

Forced-Air Cooling: The principle of this type of cooling is to place the crop into the cold room but to arrange the airflow pattern so that it is directed trough the crop. The heat given out from the surface of the crop is than carried away in the single stream of cold air, thus settling a temp. gradient  and cooling the crop quickly. The common type of pre-cooler which uses this method is the letterbox system .It is important to achieve an even distribution of air through the crop to ensure uniform cooling.

The efficient forced-air cooling system can pass air of a very high velocity and thus lead to desiccation of crop. To reduce this effect various system like ice bank cooler have been devised.Thier cooling have been immersed in water so that the water freezes building up ice around them. Water is than pumped over the ice and sprayed in a fine mist in a countercurrent of air entering the plenum chamber. The air passes through the filter to take out liquid water particles from it and then passed through the crop. The effect is that air is both cooled and humified. Humidity is close  to 100% RH in a well constructed system and will results in minimum desiccation of the crop during cooling. Portable ice bank coolers have been constructed. These are small  insulated trailer which can be towed into the field and driven from the power take-off of a tractor. These are used for strawberries, where it is important to begin cooling directly after harvest. Other system have been developed to provide high-velocity high-humidity air for rapid pre-cooling. These are marketed under trade names like ‘Humifresh, ‘Humicold’  and ‘Airspray’.

HYDROCOOLING: The transmission of heat from solid to a liquid is faster than the transmission of heat from a solid to gas. Therefore cooling crops with cooled water can be very quick and result in no loss of weight. If the crop is simply immersed in ice-water, the water in direct contact with the crop will heat up and rate of cooling will be slow. To achieve the maximum effect the cooled water must be constantly passed over the crop. This can be done by submersing the crop in cold water which is constantly being circulated through a heat exchanger. Where crop are being transported around  a packhouse in water, the transport can be hydrocooler. Common design is to transport the crop on a perforated conveyer belt over a tank of water. Water is being pumped from the tank over the cooling coils or block of ice, and allowed to fall on a crop and then through the tank below. Speed of the conveyer can be adjusted to the time required to cool the crop. Hydro-cooling helps in cleaning the produce but water can be contaminated with the microorganism, which can result in increased level of spoilage. Cl may be added to the water. It is unsuitable for many crops. Eg. Capsicums which were hydro-cooled had a higher incidence of rot during storage than those which were not. This was due entrapment of water between fruit and calyx.

Cooling time depend on the size of produce .Asparagus spears take 2 min. while capsicum takes 10 min. Tomato, Melon, Leafy vegetable can be hydro-cooled.

VACCUMCOOLING: In forced air cooling air passes over surface of the crop ,cooling the outside of the crop, while the inside is cooled by heat transfer from inside to outside of the crop, whole in vaccum cooling the reduced pressure is exactly the same around the leaves in the centre as it is around the leaves on the outside. This means that cooling is even and quick throughout the crop. The speed and effectiveness of cooling  of crop depend upon ratio of mass and surface area of the crop, so it is suitable for leafy crop like lettuce. Where there is low ratio of mass to the surface ratio or an effective barrier to water loss from the crop surface , Vaccum cooling can be slow. eg. Tomato with thick wax cuticle. Vaccum coolers are constructed to withstand the vaccum therefore tend to be expensive to construct. They are made from steel and are usually cylindrical in shape to withstand low pressure. The vaccum is achieved by a vaccum pump attached to the cylinder and the moisture from the cooling coils. Inspite of the high cost they are used for lettuce in USA and Europe because the method is quick. Loss of water not only affects the quality of crops. In order to reduce this ,crop may be sprayed with water before loading into vaccum