Forced Air Cooling for Moisture Control on Fresh Wild Blueberries

Wild blueberries grow low to the ground and predominantly along the coastal regions of Maine making them very susceptible to wet conditions during harvest. Wet berries are not a great concern if they are going directly to a processor that will be immediately freezing them, but it is a major problem for farms that are selling freshly packed berries in quarts and pints directly to consumers. Damp berries are not attractive in their containers but even more importantly have a much shorter shelf life and are more likely to be a host for food safety concerns.

This past summer as Brodis Blueberries geared up with higher output packing and equipment and prepared to begin supplying Hannaford Grocery Stores with full pallets of pint boxes, the weather and thus berry conditions stopped us from shipping less than half of what we had planned. Even harvesting in our ideal window of late afternoon and evening was not possible on a number of days to do wet conditions, causing us to lose planned sales of $30,000. 

This problem was not just for Brodis Blueberries, but I have personally spoken to a half dozen growers who faced the same problems. Brodis Blueberries is one of a few farms in Maine that are participating in a University of Vermont (NECAFS) project through the FDA OARSA sampling for pathogens in fields and processing facilities. Preliminary results suggest that wetter conditions increased pathogen presence. As more growers begin ramping up their fresh pack sales, they cannot pick and choose short dry harvest windows as a very small farm can. Now we need to harvest in the rain to sell the quantity required.  

This is why a method of drying the berries is crucial for Maine’s wild blueberry growers. At Brodis, we have one of the more modern facilities with heat pumps cooling and dehumidifying our packing room but berries are still not drying to the extent needed.  According to Dr. Calderwood’s survey of fresh pack growers as part of a SARE Partnership Grant (# ONE20-359), growers indicated that fresh wild blueberry storage temperatures range from 40°F to 70°F, and airflow and humidity within the storage unit are not often considered. Our harvesting protocol is to harvest berries as dry and cool as possible and then store them in  our packing room overnight. This past year, we developed much greater markets and needed to harvest and store even more berries in our space which, compounded with the damp weather conditions, was not successful. We not only failed to pack and sell all we could have, the quality of those we did pack did not meet our typical expectations.

This proposal creates a unique space for the berries to be stored overnight, in better designed containers promoting air flow and fed by artificially dehumidified air to expedite the process. As designed, we expect to be able to not only improve the quality of the wild blueberries we typically sell but be afforded the opportunity we have never had to harvest in damper conditions. Especially in recent years we have experienced more diverse and extreme weather patterns. The addition of this drying system will benefit us in each extreme condition, because even in drought years, we have dew and damp days making harvest not possible.

Through the Cooperative Extension Wild Blueberry Specialist and the Wild Blueberry Commission of Maine, we have been sharing all that we have learned. Tours and workshops have shown other growers the value of the new “DH Harvesters” we have purchased and more and more growers are purchasing them. Two years ago we bought the first Optical Sorter to be used on wild blueberries here in Maine, which I shared at Dr. Calderwood’s Wild Blueberry Conference and Field Day in 2024. This summer 4 more growers tested similar optical sorters in their lines. We expect that this affordable drying system will greatly enable growers to expand both the window of time each day they can harvest and help them distribute a much higher quality product.

Adaptation of Forced Air Cooling

A common method used for precooling of packaged produce is forced air cooling (Callahan and Chamberlin 2018, Boyette & Rohrbach, 1993).  In this approach, airflow is preferentially forced in one direction through palletized cartons with the aim of ensuring intimate contact between the air and the product. This results in a higher coefficient of heat transfer when compared to “room cooling”, i.e. leaving a pallet of cartons sitting in a cooler with relatively still air. Forced air cooling reduces the pulp temperature of produce more quickly, thus reducing the respiration rate more quickly and initiating the cold chain. Project partner Callahan has developed small scale forced air cooling designs under a former NE-SARE funder project (#LNE16-347) and project partner Calderwood has adapted these designs to wild blueberry (Callahan and Chamberlin 2018 and Calderwood 2022). In this project the team will aim to adapt the forced air cooling methods to provide a combination of cooling and surface moisture removal from the berries.

Rolling carts or U-Boats will be the primary structure used to stack harvest bins of berries and to form the framing for the forced air cooler. The team plans to construct a “tunnel” using two rows of carts stacked with harvest bins of berries with approximately 16-24” of open space between them. On one end of this tunnel, a blower will be mounted to draw suction from the tunnel and blow air out into the room to be cooled and dehumidified by the existing heat pumps in the space. A light dimmer will be used to provide inexpensive speed (airflow) control of the blower. Coated tarpaulins will be used to cover half of the harvest bin / rolling cart width going outward from the center of the open tunnel and also the far end of the tunnel away from the suction blower. Additional blocking material will be used to ensure the airflow moves through the harvest bins to ensure intimate contact of air with berries.