Summer heat in the Philippines soars to as high as 40 degree Celsius during mid-day. The crops grown inside the greenhouse and in the open field absorb much of the field heat.  It is therefore very important for the farm to remove the field heat.  

The process of removing the field heat is what we call pre-cooling.  It is the first operation in the cold chain.

Some of the more common pre-cooling method includes :

• Room Cooling;

• Forced Air Cooling;

• Hydro-cooling;

• Icing; and

• Vacuum Cooling.

Vacuum cool system

Room Cooling. Product is simply placed in a cooler, and over time, the product temperature decreases nearly to the cooler temperature. This process takes a long time, but requires no other equipment than a cooler.

Forced Air Cooling.   It is very similar to room cooling, except that the cold air is pulled by an electric fan through the stacked boxes of vegetables. Because of the increased air flow, the cooling rate is much faster than with room cooling, so that heat is taken from the vegetables in a very short time. Cooling with forced-air is also more uniform, so that vegetables in different boxes cool at much the same rate.

Hydro-cooling.  Cool water is sprayed over the product, or the product is immersed in cool water. 

Icing.  This method is applicable only  is used for product that can withstand direct contact with ice, such as beets and broccoli.  The ice can be finely crushed, flaked, or in a slurry with water. Packaging used with icing must also be able to withstand wetting.

Vacuum cooling .  The product is placed in a vacuum, so that the atmospheric pressure around the product is reduced. This reduces the water vapor pressure around the product, and when it is lowered below that inside the product, water evaporates from the product.  This in turn lowers the surface temperature of the product.

After pre-cooling activity, the produce are then transferred to the intermediate storage area where storage temperatures should be low as possible to slow product metabolic processes and discourage pathogen growth, but high enough to avoid freezing or injuring the product.

So, can salad greens subject to vacuum cooling improve the quality ?

 CONCLUSION

Fact #1.  There are several methods of doing pre-cooling activity. Vacuum cooling is just one of 5 methods discussed. Pre-cooling activity is just one of the several farm activities that will determine the final quality of the fresh produce.  

Fact #2. Fresh produce that are subject to vacuum cooling does not assure you of longer shelf life if any of the following conditions are present:

• succeeding cold chain is broken;
• the temperature of the intermediate storage area is outside the optimum level;
• fresh produce with physical damage are not removed; or
• oxidizing agent is present in the final packaging.

Fact #3.  The reason why vegetables are also called fresh produce is because they are meant to be consumed when they are relatively fresh and not two weeks after harvest.