Post-Harvest Handling of Wild Blueberry

Created by: Lily Calderwood, PhD, Extension Wild Blueberry Specialist and Brogan Tooley, Research and Extension Assistant. Revised February 2023.

Fresh pack, wild blueberries with red netting to hold them in and MOFGA certified stickers on a counter at a local storeDemand and price for all produce depends on quality. Characteristics that define high quality in wild blueberry include blue, clean, firm, dry, and whole. In general, customers buy fruit based on how it looks. Customers come back for more based on how the berries tasted and how long they lasted. Consistent high-quality products benefit the whole industry.

According to Charles Forney at the Atlantic Food and Horticulture Research Center in Kentville, Nova Scotia, the top four quality challenges for fresh wild blueberries are 1) the range of ripening stages due to field diversity, 2) raking technique, 3) bruising due to drop distances on the packing line, and 4) proper pre-cooling and storage temperature maintenance.

Produce remains alive after it is harvested. Being alive means that a berry continues to respire after being removed from the plant. Respiration is the continued intake of oxygen (O2) and release of carbon dioxide (CO2) and heat. Maine wild blueberries have thinner skins and are smaller than highbush blueberries. Therefore, there are more berries in a quart of wild blueberries than there are in a quart of highbush blueberries. Research needs to be conducted, but we suspect that wild blueberries generate more heat and more moisture per quart than highbush blueberries simply because there are more berries in each container with less air space between the berries. Having the ability to move cool air through wild blueberries before they reach the consumer is critical.

Steps to Improve Fresh Pack Quality – The Known Standard

  • Develop a harvest-to-consumer plan to share with your workers.
  • Improve harvest technique. Wild blueberries should not be harvested when plants are wet from morning dew or rain. Harvest early and late in the day when the air temperatures are cooler. To reduce splitting and bruising, gently tease the berries off the bushes as you pull the rake through the plants. Do not yank the rake forcefully through the plants in one swipe. Clean the rake frequently with cloth to keep it dry and free of debris.
  • Reduce compression. Do not overfill rakes, containers, or large bins. The weight of berries on themselves results in compression damage. As more berries are added to the rake or container, less air flow is possible between berries and between containers.
  • Reduce bruising. Bruising can occur at many points from the field to consumer’s hands. Mechanical harvesting has been shown to reduce marketable highbush and rabbiteye blueberry yields by 19-44% due to bruising. Traveling out of the field on bumpy terrain, winnowing, and drops on the packing line can all bruise berries. On fresh pack lines it is a good practice to reduce drop distances and cover sharp edges.
  • Reduce heat immediately. Cooling berries is the number one method of reducing respiration, the inevitable path to decay. Removing field heat starts in the field the second the berry comes off the plant. While there is color variation in the field, blue is a dark color and therefore absorbs heat. Fruit temperature can increase to above air temp in less than 1 hour. Berries in the shade will be 10° cooler than those in the sun. Some wild blueberry fields are very large. If you cannot get them out of the sun, you might consider bringing umbrellas or a mobile tent out into the field.
  • Pre-cool berries before packing. Berries should ONLY be precooled before packing, IF they are packed in a room that is the same temperature or colder than the berries. According to farmer experience and research to date, if you do not have the infrastructure to keep berries at a constant temperature all the way from the field to consumer, it is best not to pre-cool. Avoid temperature fluctuations at all costs. Processors also caution against cooling berries too quickly. Because of their thin skins, wild blueberries can burst or turn to mush if they are cooled too quickly but we do not yet have research-based information about how fast is too fast.

    Highlights

    Berries are alive in the field, after harvest, in transit, and on the table.

    Because they are alive, berries respire (let off CO2 and heat) which causes decline in quality.

    Respiration cannot be stopped but it can be slowed by cooling berries and keeping them cool.

    Even though many farms in Maine do not have the infrastructure to achieve a complete cold chain, cooling berries with moving air will extend shelf life and improve quality.

    Growers in Maine have developed their own methods of maintaining berry quality for up to one month. Current storage temperatures range from 40°F to 70°F.

Warnings aside, berries do need to be pre-cooled in order to extend shelf life. Once berries have been taken out of the field, continued removal of field heat is critical to reduce respiration and therefore decay. Pulling cold air across berries removes field heat more efficiently than air blown at stacks of berries from above. Berry containers should have slats or holes in them that allow for horizontal airflow. Research on highbush blueberries indicates that bringing the inside berry temperature down to 35°F in 2 hours results in longer shelf life than cooling down to 35°F in 48 hours. Again, this may be too fast for our smaller wild blueberries. Pre-cooling has also been shown to reduce fungal decay by 39% (Jackson and Sanford, 1989). Once berry temperature has been decreased, that temperature must be maintained all the way to the consumer.

  • Organize efficient cool room space.
    • Install fans in packing rooms and coolers.
    • The width and height of stacks will impact how quickly field heat can be removed. This will change depending on the size, temperature, air flow, and amount of produce within your cool room space.
    • Putting wooden stakes between the berry totes as they are stacked will help the air flow keep them cool and dry.
  • Ensure the cool storage areas are airtight and leak-proof. This prevents the release of cool, dry air or the intake of warm, moist air which can delay fruit cooling and/or increase operational costs.
  • Consider installing your cool storage spaces (including CoolBot-regulated spaces) underneath or within other structures like garage bays or barns. When cool storage spaces are directly subject to external air temperatures and sunlight, the systems must work extra hard to maintain cool temperatures within the unit. However, if a garage bay or barn bears the brunt of the sunlight and provides shade for the  cool storage unit, the system will not be as strained. 
  • Improve worker environment. If the cooling infrastructure exists to keep the berries at a constant temperature while processing, berries can be packed after some of the field heat is removed. If not, it’s best to process as soon as possible and then begin the cooling process. Packing berries within 24 hours of harvest is ideal. Comfortable conditions for workers will improve berry quality. Providing GOOD LIGHTING, stools, hairnets, a bathroom, and a break room will only improve quality. Food safety requirements such as hand washing and jewelry removal go hand-in-hand with berry quality. Fans that blow air across the belts, not down onto the belts, can help keep the berries dry. Keeping the conveyor belts dry and clean will also maintain high-quality berries. The belts should be cleaned and wiped dry before every break and the end of the day. More information on Food Safety Best Management Practices for Wild Blueberry Producers can be found here: https://extension.umaine.edu/publications/4282e/

Timing Guidelines (based on UMaine research)

Removing field heat from harvested berries quickly is key to ensuring a long-lasting, high-quality product. Placing harvested berries in a cold room is not enough to cool the fruit quickly. Instead, fans can be used to move air across the berries to quickly and evenly cool them. This will reduce wetness on the surface of the fruit.

As soon as possible:

  • Harvested berries should be removed from the field or placed in shade (mobile tents or canopies are great for this!).
  • Harvested berries should be placed in cold storage areas ONLY IF cold storage can be maintained throughout the entire chain from harvest to consumer.

Within 5 days of harvest:

  • Harvested berries should be in the hands of processors or consumers.

By 14 days after harvest:

  • Harvested berries are generally unsaleable due to shrinkage, wetness, or rupturing.

Highbush Ideals

Highbush berries are stored at 32°F and 95% relative humidity (RH). Upon removal from 32°F, condensation will occur and berries will be wet. Retail chain warehouses will reject berries if they are too wet. Your goal is to keep the surface of berries dry. The 95% humidity within storage rooms allows the fruit to remain fresh without being wet. Remember that berry quality and extending shelf life begins with their treatment during harvest and in transport. Bruising is one of the top reasons for decrease in quality over the storage period. With proper precooling, packing, and storage at 32°F, highbush blueberries have a minimum shelf life of 14 days. To extend shelf life further the atmosphere in which the berries are stored and transported must be modified. Modified atmospheric packaging (MAP) or controlled atmosphere packaging (CAP) are different ways of trapping the right combination of O2 and CO2 inside the container in order to slow respiration. The best atmosphere for highbush blueberry was found to be 1.8% O2 and 12% CO2 where berries lasted 46 days and 97% of berries remained at high quality (Northwest Guide to Post-harvest Handling 2006).

Fresh Pack COOLING Cost Analysis (as of 2019)
Products & Resources Source  Product Details Initial Cost Additional Cost to Run
Walk-in Cooler (Insulated box with cooling system, floor not included) Hoodmart 12′ x 12′ x 9′ (844 cu. Ft.) $10,000 $160/month
Hoodmart 8′ x 8′ x 9′ (353 cu. Ft.) $7,000 $130/month
CoolBot Walk-in Cooler CoolBot Cooling system Controller with Wifi (for a pre-insulated room) $400 $60/month
LG Air Conditioning Unit (price depends on BTU*) $400-$880
CoolBot Cooling System with Wifi + insulated box $4,000+ $60/month
Refrigerated Truck (Rental) RYDER;

Hermon, ME

Refrigerated Box Truck   Length: 16′, Load capacity 7,000 lb. $194/day (+) Fuel
RYDER;

Hermon, ME

Refrigerated Truck   Length: 22′-26′, Load capacity 7,000-13,000 lb. $215.95/day (+) Fuel

*The larger the space, the greater number of BTUs needed using an LG Air Conditioner with CoolBot Technology

About CoolBot

CoolBot is a cheap, refrigeration technology used on small farms across the US for on-farm produce cooling. The CoolBot device overrides a common air conditioning unit to hold a set temperature instead of turning off once a set temperature is reached. Small growers typically build a simple insulated room inside an existing building and install the AC unit with the $400 CoolBot. It is designed to work in a 8ft x 8ft insulated room at a temperature no lower than 32°F and in situations where the door will not open more than 6 times per hour. Visit the University of Vermont Ag Engineering Blog for specific instructions on how to build a CoolBot:

https://blog.uvm.edu/cwcallah/2013/03/20/coolbotstm-inexpensive-cold-storage/.

A stack of red blueberry crates is loosely wrapped in painter's plastic as it sits in front of a wooden wall with blower and motor installed in the wooden wall.

The image above shows a forced air cooling system, built in an afternoon for a few hundred dollars by following the University of Vermont’s specifications on their website (https://blog.uvm.edu/cwcallah/2019/06/17/forced-air-cooling-field-trial-results/). The system is not operational at the time of the photo.

Resources

If you are looking to buy or sell wild blueberries, please visit the Marketplace on the UMaine Wild Blueberry website: https://extension.umaine.edu/blueberries/marketplace/ or Contact lily.calderwood@maine.edu.

USDA Commercial Storage Guidelines in Agricultural Handbook 66: https://www.ars.usda.gov/ARSUserFiles/oc/np/CommercialStorage/CommercialStorage.pdf

Interested in selling to Hannaford? GAP Certification online course is now available: https://extension.umaine.edu/blueberries/wp-content/uploads/sites/41/2010/06/GAP-GHP.pdf

https://gaps.cornell.edu/gaps-online-course/

FDA’s Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables: https://www.fda.gov/media/117408/download

Pacific Northwest Guide to Post-harvest Handling of Highbush Blueberries:

http://berrygrape.org/post-harvest-handling-of-blueberries/

North Carolina Extension Guide to Post-harvest Cooling and Handling of Highbush Blueberries: https://content.ces.ncsu.edu/postharvest-cooling-and-handling-of-blueberries

References

Callahan and A. Chamberlin. 2017. New England Vegetable and Fruit Conference. Presentation. http://blog.uvm.edu/cwcallah/files/2019/09/CALLAHAN-Precooling-and-Curing.pdf

Boyette, M., E. Estes, and B. Cline. 1993. Postharvest cooling and handling of blueberries. North Carolina State University Extension. AG-413-07. https://content.ces.ncsu.edu/postharvest-cooling-and-handling-of-blueberries.

Callahan, C. and A. Chamberlin. (2019, June 17). Forced Air Cooling: Field Trial Results. UVM Extension AG Engineering. Retrieved February 6, 2023, from https://blog.uvm.edu/cwcallah/2019/06/17/forced-air-cooling-field-trial-results/ 

Donahue, D. W., Benoit, P. W., Lagasse, B. J., & Buss, W. R. (2000). Consumer and instrumental evaluation of Maine wild blueberries for the fresh pack market. Postharvest Biology and Technology, 19(3), 221–228.

Donahue, D. W., & Work, T. M. (1998). Sensory and textural evaluation of Maine wild blueberries for the fresh pack market. Journal of Texture Studies, 29(3), 305–312.

Gross, K., C. Yi Wang, and M. Saltveit. 2016. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. USDA Agricultural Research Service. Agriculture Handbook Number 66.

Jackson, E., K. Sanford. Quality maintenance in fresh lowbush blueberries; Current knowledge and selected literature. 1989. Food Processing Research

Jackson, E. D., Sanford, K. A., Lawrence, R. A., McRae, K. B., & Stark, R. (1999). Lowbush blueberry quality changes in response to prepacking delays and holding temperatures. Postharvest Biology and Technology, 15(2), 117–126.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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