Q: We received a load of mixed vegetables from one shipper and only a portion of the load was showing freezing damage. The rest of the product pulped adequate temperatures and had no indication of freezing damage. How is it possible that the pulp temperatures can vary so much within the trailer?
A: To understand how this could have happened, we begin by reminding you that trailers are responsible for maintaining temperatures during transit. They are not meant to cool down or warm up the product. Three main reasons come to our mind that could cause freezing damage to only a portion of a load: a) Product loaded warmer than the reefer unit set temperature; b) bad loading pattern blocking the airflow or blocking the air chute; or, c) poor trailer insulation or extreme weather conditions.
a) Product loaded warmer than the set temperature
The BOL indicates temperatures are to be maintained at 33oF. The product at loading is pulping 38oF; the freezing point is 30.5oF. The reefer is set at 34oF degrees on continues mode. When this occurs, the return air sensor is going to start picking up the temperature of the product (38oF) and the message to the reefer unit would be that it needs to lower the discharged temperature immediately to lower the temperature in the trailer. Therefore, the product exposed directly to the discharge air from the chute may exhibit freeze damage because the discharged air temperature could be lower than the freezing point of the product.
b) Bad loading pattern blocking the airflow or blocking the air chute
A loading pattern that does not allow the air to circulate properly may result in the return air sensor reading temperatures above the reefer unit set point, therefore sending the message that cooler air needs to be discharged. Normally a blocked air chute will result in warmer pulp temperatures at the front of the truck and cooler pulped temperatures at the back of the truck. This is because as the front of the truck begins to warm up, the reefer unit believes it needs to expel colder air, and thus the product nearest to the end of the chute gets chilled.
c) Poor trailer insulation or Extreme Weather conditions
Product loaded too close to the walls on a trailer with poor insulation may create freezing damage to pallets nearest to the sides of the trailer. This could be the result of harsh temperatures during the winter or hot outside temperatures during the summer. Similar to the previous example, to compensate the reefer unit will usually discharge cooler temperatures or warmer temperatures than the set point in order to correct the problem. For instance, if a truck travels through the northern US or Canada during the winter and it is not properly insulated, there is a strong likelihood that the pallets closest to the exterior walls could experience some freezing damage. Conversely, despite proper trailer insulation, sometimes extreme weather conditions such as severe cold or intense heat may influence the temperatures inside the trailer.