Revisiting.... Refrigeration!!!

Trailer refrigeration is one of, if not, the most important factor in ensuring the delivery success of fresh fruits and vegetables. The object of trailer refrigeration is to prolong storage and shelf life, and to maintain quality by lowering the commodity temperature so that respiration, metabolic deterioration, and decay are slowed down. One of the most important things to remember is that the function of a trailer’s refrigeration unit is to maintain the temperature of the commodity, not to lower the temperature; therefore, pre-cooling of fruits and vegetables prior to shipping is essential.

So now, let’s look at some of the key factors in trailer refrigeration.

Methods of refrigeration

1. Mechanical refrigeration: this operates by absorbing heat at one point and dispensing it at another. This is accomplished by circulating a refrigerant, most commonly Freon, between two points.

2. Ice: in addition to mechanical refrigeration, ice is sometimes used to help maintain humidity inside the trailer. The main drawbacks for ice are its weight which cuts down the allowable product weight, the arranging of replenishing ice while in transit, and the requirement of water-resistant shipping containers. Recommended procedure for top icing is to apply the crushed ice on top of the load in 3 length-wise windrows. The thermostat should be set to 35F to allow the ice to melt steadily during the trip; a colder setting may crust or freeze the ice and block air circulation, which then affects trailer air circulation and causes heat to build up in the interior of the load.

Air Circulation

Air circulation is critical to ensure uniform temperatures throughout the load. There are two methods of circulating air in refrigerated trailers:

1. Top Air Delivery: This is the most common design in trailers. Ceiling ducts are used to direct the air from the blower to the rear of the trailer. Load patterns that provide unblocked air channels throughout the load to allow the air to return to the blower intake are required. Load locks must also be used to prevent the load from shifting or toppling backwards and blocking air circulation between the load and the rear doors. Recessed groove floors and inverted-rib sidewalls are also designed to increase the amount of air that can be circulated through the load. Finally, bulkheads are designed so they do not block the air returning to the blower.

2. Bottom Forced-Air Delivery: In this method, air is forced under and up through the load. The air returns to the lower over the top of the load and through the top of the bulkhead.

Humidity

Most fruits and vegetables require high relative humidity to prevent dehydration and to keep them looking fresh and crisp. Some moisture loss during transit has to be accepted, but it must always be minimized, by the following procedures:

1. Using top or packing ice where possible.

2. Thorough pre-cooling to reduce the temperature difference between the product and the surrounding air.

3. Keeping the refrigeration unit’s coil only a few degrees colder than the desired transit temperature.

4. Waxing or using semi-permeable wraps to reduce evaporation.

Do you have expertise in trailer refrigeration? Please share! Also have a look at some of our numerous posts that discuss the popular topics of temperature, loading/load compatibility, and how a refrigeration unit works!!!!

References:
Transportation. RBCS Handbooks. http://rbcs.com/HandbookArticles/transpor.htm. Accessed on Nov 3, 2011.