Post harvest Management of Horticultural Crops (2+1)

Pre-cooling

Pre-cooling refers to removal of field heat (quick cooling) after harvest; if not, its deterioration is faster at higher temperature of 1 hour at 320C = 1day at 100C or 1 week 00C. The entire products must be pre-cooled as early as possible to the recommended storage temperature and relative humidity. Pre-cooling is done just above chilling and freezing temperature.

Advantages of pre-cooling:

• It removes the field heat
• Reduces the rate of respiration and ripening
• Reduces the loss of moisture
• Reduce bruise damage during transits
• Reduces the production of ethylene
• Reduces /inhibits the growth of spoilage organisms
• Eases the load on the cooling system (refrigeration) of transport or storage chamber
• Above factor helps in extends the product shelf life


Pre cooling depends on the following factors

• Air temperature during harvesting (during summer pre-cooling time is more)
• Time between harvest and precooling
• Nature of the crop (High perishable crop require immediate pre-cooling)
• Difference in temperature between the crop and cooling medium
• Nature/Velocity of the cooling medium
• Rate of transfer of heat from the crop to the cooling medium.
• Type of package material used – Use of water proof ventilated boxes for good air circulation in the room is helpful. Plastic boxes/ fiber board cartons which have been treated with wax will render them water proof.


Choice of pre-cooling method depends:

• On the nature of the produce
• Economics of the process

Mechanism of pre-cooling - Conduction and convection are the two main heat-transfer mechanisms used for cooling of produce. With conduction, the heat is transferred within a produce to its coldest surface. This is direct movement of heat from one object to another by direct methods (from fresh produce to water or warmer to cooler).With convection, the heat is transferred away from the surface of the produce via a cooling medium such as moving water or air.

Potatoes/ apples/cauliflower/orange and other fruits (bigger mass and lesser surface area) and vegetables require more time to pre-cool than produce which is having smaller mass and large surface area like lettuce/green onion/ carrot tops/peas/corn/brussel sprouts. This is because of the heat from the inside of the crop has to move to the surface before it is transferred in bigger produce.

The rate of cooling depends on individual volume and the exposed surface of product. The difference in temperature between product and the refrigerating medium also needs to be taken into account.
For example: large exposed surfaces, leafy vegetables cool almost 5 times faster than large fruit such as melons (more volume, less surface).

Heat with in the crop comes in two ways:

• Through the convection from the surrounding air mainly from the sun in the form of radiation
Eg.: Crop harvested in the early morning will be cooler, since the sun has not been able warm the surrounding air or crop and lower metabolic heat.
• From the metabolic heat from chemical reactions within the crop(respiration)
When the heat is removed by way of evaporative cooling then the fresh produce must not be sealed in moisture proof film like polyethylene bags.

Pre-sorting - Pre-sorting of produce is usually done to eliminate injured, decayed and other unwanted produce before cooling and handling. Pre-sorting will save energy in that culls will not be handled.