Question
As our technical services office goes out to inspect our customers' banana ripening rooms, as part of our ripening review, we test the pressurized rooms for proper airflow through the banana boxes. We are currently using a Dwyer magnahelic with a range of .00 - .50 inches of water. Our understanding at this time is that the industry standard for recommended air pressure differential is between .08 and .35. We have also heard that for banana processing it is recommended that we have at least .30. Do you know why these two different standards exist? Our initial guess was that the industry standard is a general standard set forth by the refrigeration, or some other-related industry. As for the recommended banana processing air pressure differential, I can see why you would want .30 in a banana room - you would achieve greater pulp temperature consistency, and the room would respond faster to temperature changes made by the ripener.
Anyway - I'd like to get your input and hopefully incorporate your suggestions into our program. Thanks for your help and assistance, and maybe we'll be seeing you at the next postharvest class sometime soon (we've got a few new techs that need to be schooled!). (N.C.)
Answer
Good question. I am not sure where these pressure difference recommendations came from, but let me give you a little background. The key physical constraint to good operation of a pressurized ripening room is airflow through the boxes. I recommend 0.3 cfm per pound of bananas. An air pressure difference across the pallets is required to produce this airflow. Low pressure differences will produce adequate airflow if boxes have 3% to 5% vent area, vent holes aligned well between boxes, and interior packing materials (like liners) do not block the vents. So actually a pressure drop recommendation can only be made for a particular type of box and packing method.
It id difficult to measure airflow through boxes in a ripening room. Instead, I recommend measuring banana temperatures towards the end of the ripening process when the bananas are rapidly respiring. If the pulp temperature difference between the warmest and coolest fruit is less than 1oF then there is adequate airflow. Measure the pressure difference across the pallets and this will be a level that produces adequate airflow for the given box and packing method. Repeat this procedure a number of times to determine the relationship between pressure drop and temperature difference across the pallet. I am sure you will find that there is a range of pressures that are required for proper ripening in a particular box. I hope this helps.
-Jim Thompson