Improving Shelf Life of Fresh Pack Maine Wild Blueberries

Progress report for ONE20-359

Project Type: Partnership
Funds awarded in 2020: $28,270.00
Projected End Date: 02/28/2023
Grant Recipient: University of Maine
Region: Northeast
State: Maine
Project Leader:
Dr. Lily Calderwood
University of Maine
Marjorie Peronto
University of Maine Cooperative Extension
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Project Information


Family wild blueberry farms in Maine (20-200 acres) do not have the capital to invest in the development of complete cold chains that extends the shelf life of their berries from the field to consumers. There are also physical differences between highbush and wild blueberries that affect post-harvest handling such as small berry size, thin skins, and a range of ripeness in every pint. Therefore, Maine producers have developed their own innovative methods of maintaining berry quality, which has resulted in a wide range of fresh wild blueberry quality. Listening to growers, wild blueberry storage temperatures range from 40°F to 70°F and very few consider relative humidity or airflow as storage factors. The objectives of this project are to 1) identify the ideal storage temperature for fresh wild blueberries on small farms where only one cooling step is feasible, 2) survey the current temperature and relative humidity of fresh pack wild blueberry buildings in Maine, and 3) disseminate storage temperature findings and incorporate post-harvest handling education into the University of Maine Wild Blueberry Extension Program. Overall, this is a collaborative research and outreach project designed to reach both organic and conventional wild blueberry producers with the goal of improving the shelf life and quality of fresh packaged Maine wild blueberries. This will allow wild blueberry growers to reach worthwhile markets, thereby improving farmer livelihoods and supporting our regional food system.

Project Objectives:

The objectives of this project are to:

  1. Identify the ideal storage temperature for fresh wild blueberries on small farms where only one cooling step is feasible.
  2. Survey the current temperature and relative humidity of fresh pack wild blueberry buildings in Maine.
  3. Disseminate storage temperature findings and incorporate post-harvest handling education into the University of Maine Wild Blueberry Extension Program.

Questions we will answer include:

A. In the absence of a full cold chain (ability to keep berries cold from field harvest to market), what is the ideal temperature to which wild blueberries should be cooled to extend their shelf life as long as possible for the fresh pack market?

B. How does wild blueberry fruit quality change over the course of 30 days in wild blueberries that are stored at 34, 40 and 50 degrees for one week?


The University of Maine Wild Blueberry Extension Program is the go-to resource for wild blueberry farmers in the Northeastern US. Maine has 36,000 acres of commercial wild blueberry managed by 485 growers and processors. Over the past 100 years the UMaine Extension program has focused on crop and pest management in addition to frozen berry quality. Little research and education has been given to fresh wild blueberry quality and storage options for the small farmer.

The USDA NASS survey indicates that the percentage of berries sold fresh has increased from 0.4% of the entire Maine crop (380,000 lbs) in 2016, to 0.5% (350,000 lbs) in 2017, and 0.9% (450,000 lbs) in 2018 (NASS 2019). As Maine’s wild blueberry growers seek to diversify their operations from the frozen market for greater economic stability, many are interested in expanding direct fresh sales in Maine and across the Northeast region. At the recent 2020 UMaine Wild Blueberry Conference an event survey was distributed and we received 55 complete responses. Thirty one percent reported that they sell some portion of their berries fresh. These growers sell between 100 and 15,000 lbs of fresh wild blueberries per season and 18% of respondents indicated an interest in exploring fresh pack further.

Storing highbush blueberries at 32-34°F, 95% relative humidity with airflow from the field to consumer is standard wholesale practice (Boyette et al. 1993). Wild blueberry growers have a hard time following these recommendations for a few reasons. First, wild blueberries have different physical characteristics, which include smaller size, thinner skins, and a range of ripeness in every pint. Second, family wild blueberry farms have suffered very low frozen prices for six years, making it very difficult to invest in new infrastructure to diversify the blueberry products that they sell. Washington and Hancock counties of Maine are rural, underserved communities with Washington county having the third lowest household median income in the state and an 18% poverty rate (ME Dept of Labor 2016). In 2018, the Maine wild blueberry growers demanded that a Fresh Pack and Value-Added Committee form under the larger Wild Blueberry Commission of Maine. This group of 10 growers has indicated to UMaine Extension several times that fresh pack quality must improve and that identifying the proper storage conditions is the place to begin. Table 1 represents the current range of temperatures used to cool wild blueberries. 

Table 1. Responses to the question “If you sell fresh pack wild blueberries, do you currently cool them?” at the UMaine Wild Blueberry Conference in February 2020.



Number Responses




Yes, but not below 60°F.



Yes, 32-39°F



Yes, 40-49°F



Yes, 50-59°F



It is critical that we collaborate with wild blueberry growers to conduct an on-farm research study, and provide several outlets for education on wild blueberry shelf life extension. This will allow wild blueberry growers to reach worthwhile markets, thereby improving farmer livelihoods and supporting our regional food system.


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Materials and methods:

Objective 1.  Identify the ideal storage temperature for fresh wild blueberries on small farms where only one cooling step is feasible.

This two-year study is being carried out at Blueberry Hill Research Farm (BHF) in Jonesboro, Maine, at Welch Farm (RB) in Roque Bluffs, Maine. At RB there is an existing, homemade walk-in cooler (8ft x 24ft) made of a refrigerated truck trailer that is kept at 46-50°F by cooling with a 12,000 BTU air conditioning unit. At RB, an additional 8ft x 8ft cold storage unit was constructed using plywood, R-10 insulation panels doubled up, spray foam insulation, an industrial fan for airflow, a 12,000 BTU air conditioning unit, and a temperature and relative humidity (RH) sensor. This new cold storage unit was kept at 40°F with 95% RH and was installed in the same building as RB’s existing cold storage unit, enabling the farm to pack and process berries as usual while integrating the new unit into their production process. The existing cold storage unit at RB will provide a valuable comparison between a homemade cold storage unit and a planned unit. At BHF, there is an existing cold storage unit, constructed in 2020. An additional two cold storage units were constructed there so all three units are 8ft x 8ft, with the same R-value 10 insulation panels doubled up, spray foam insulation, industrial fans for airflow, 12,000 BTU air conditioning units, and temperature and relative humidity sensors. These three units are kept at 34°F, 40°F, and 50°F.

 Table 1. Summary of cold storage unit specifications.


Cold storage units

Target relative humidity (RH)

Fruit storage size

Harvest date




Roque Bluffs









12,000 BTU

12,000 BTU

R-10 (doubled)

R-10 (doubled)

Blueberry Hill Farm (BHF)







16,000 BTU

12,000 BTU

12,000 BTU

R-10 (doubled)

R-10 (doubled)

R-10 (doubled)


Berries at both RB and BHF were hand-raked. At BHF, berries were raked on July 29, 2021, then winnowed and transferred into industry-standard quart-size molded pulp produce baskets and immediately stored in the cold storage unit with baskets directly abutting one another. At RB, berries were hand-raked on August 9, 2021, then winnowed, put through a cleaning line, and transferred into industry-standard pint-size molded pulp produce baskets before immediate storage in the cold storage unit. At RB, berry quality was measured every 3 days for 26 days (total: 6 collection dates) because RB typically sells fresh pints within 7 – 14 days of harvest. At BHF, berry quality was measured every 3 days for 37 days (total: 11 collection dates) to mimic longer-term storage conditions for some growers. Berry quality measures taken at each site and during each sampling time included cold storage unit temperature and RH, internal temperatures of individual berries, internal pint temperature, berry firmness, and pint moisture level.

Continued berry sampling in the 2022 season (and hopefully beyond) is necessary to understand how outside weather conditions can impact the cold storage units’ internal temperatures and RH.

In addition to directly sampling berry quality, surveys of conditions and practices within fresh pack facilities were conducted and will continue in the 2022 season. These surveys occurred on a packing day during harvest. Investigators asked growers and processors about their current post-harvest handling and storage practices, facilities’ temperature and RH levels, timing for harvesting and processing, equipment providers and costs, customers, and markets. In addition, a portable temperature and RH sensor measured conditions in the processing and storage rooms. These surveys were conducted to foster discussion of current post-harvest handling and storage practices, and possibilities for improving these processes.

 Data collection

Berries harvested at BHF were sampled two times per week over the course of 37 days from July 29 - September 3 while berries harvested at RB were also sampled two times per week over the course of 26 days from August 9 - September 3. The BHF units were constructed solely for this trial and therefore the doors of units were hardly opened unless berries were actively being sampled for this trial. The RB units were actively functioning on a farm and therefore integrated into the harvest, processing, and sales operations so they were opened as many as 30 times per day.

Temperature and RH data was gathered from a mounted sensor in the newly constructed cold storage units. A handheld DigiSense sensor or a separate, non-digital thermometer was used to verify the data on the mounted sensors. Berry and pint temperature were measured using a food-grade electronic thermometer (ThermPro Ultra-Fast Digital Food Thermometer TP-03B). Three separate berries were randomly selected and penetrated by the thermometer to obtain a temperature reading; these berries were then discarded from the quart or pint. Pint temperature was obtained by sticking the thermometer into the center of each pint three times, taking care not to place the probe right next to the edges or bottom of the pint container.

Berry firmness was measured by blindly selecting 3 separate berries and rating the firmness on a scale of 1 to 3, with 1 being a firm, salable berry and 3 being a mushy or nonsalable berry; these berries were then discarded from the pint or quart. Pint moisture was documented by visual inspection of the exposed top of the pints (as a customer would) and rating the moisture on a scale of 1 to 3, with 1 being dry and 3 being wet.

Marjorie Peronto taking berry measurements inside a cold storage unit in 2021.
Two pints, one from each cold storage unit at Roque Bluffs Farm. The pint on the left was kept slightly cooler than the one on the right. Berries on the right had better quality.

Data analysis Due to the nature of the data collected, especially the ranked data, much of the data failed the assumptions of normality and equal variance required to run parametric statistical tests. Transforming the data via a square root transformation visually improved the distribution, but the data continued to statistically fail the test of normality. All the data, including berry temperature, berry moisture and berry firmness, were transformed using a square root transformation prior to all statistical testing and statistical tests were carried out despite non-normality after establishing there were no serious problems with the data.

The effects of long-term storage on berry quality (firmness) were analyzed using a multivariate correlation to generate an R2 in Microsoft Excel (Excel® Version 2110) to observe the level of change over time. Overall treatment differences were tested using a full-factorial repeated-measures mixed model design in JMP (JMP®, Version 15.2) for berry temperature only. Here, the full-factorial model tested the effects of date, treatment and any interaction between date and treatment.

Objective 2. Survey the current temperature and relative humidity of fresh pack wild blueberry buildings in Maine.

Our goal is to reach twenty fresh pack wild blueberry farms. In 2021 we were able to interview seven farmers in-person. Visits occurred during harvest and packing time in July and August. While the growers are especially busy at this time, we coordinated with them to arrive on a packing day. We brought a DigiSense temperature and humidity meter with us to measure the conditions of both the cleaning line room and the storage room if they had one. We used these visits as an outreach opportunity to discuss post-harvest handling and an opportunity to learn from growers about why they process berries the way that they do. Topics included everything from the field to shipping such as harvest technique, bruising, handling from field to winnowing, cleaning, packing, pre-cooling, cold storage, humidity and airflow management. At the end of each farm visit, we asked three questions and document the answers: 1. Do you know the temperature and RH of your packing and storage rooms?, 2. How have you improved your fresh berry quality over time?, 3. Did you learn anything new from this visit?. 

In 2022 the temperature and relative humidity data points taken will be entered and used as educational material in presentations given at field days and conferences on this topic. In five to ten years, we plan to revisit these same facilities to see if post-harvest handling practices and conditions have changed.

On farm visit, fresh pack line in Beddington ME in 2021.
Fresh pack line at Roque Bluffs Farm in 2021.
An interesting way to store and sell berries to the public at a roadside stand in Cherryfield.

Research results and discussion:
Material list and costs of building a Coolbot cold storage room in Downeast Maine.

Maximum daily air temperature and relative humidity (RH) were collected at a field-based weather station 180 meters from the Jonesboro cold storage units and 9 miles from the RB cold storage units. Maximum daily outside air temperature ranged from 71°F to 88°F and showed the greatest correlation with the two colder storage treatments (34°F and 40°F) for the Jonesboro location. This suggests that the cold unit likely declined in efficiency when cooling to colder temperatures. This increased the temperature variability inside cold storage units. The warmer cold storage treatment (50°F) showed less temporal variability in air temperature fluctuations compared to the other units and outside temperatures. High variation in unit temperatures in the first week of storage may correspond to manual adjustments made by our team to obtain target temperatures. Outside RH ranged from 69% to 99% and correlated with all three cold storage units.

Cold storage air temperature of the Jonesboro units compared with outside maximum daily air temperature for BHF (Batch 1), on the dates that sampling occurred.
Cold storage unit relative humidity (bars) of the Jonesboro units compared with outside relative humidity (line) for BHF (Batch 1), on the dates that sampling occurred.

At BHF, when Batch 1 of berries first came out of the field following hand raking on July 29, 2021 (a full sun, 78°F day), internal individual berry temperatures averaged 75°F. The berries from this batch had cooled by the next sample date of August 3. Berry temperature fluctuations between 1 and 9°F were observed within the three temperature treatments during the 4 to 5 weeks after harvest. Units’ air temperature at time of sampling also fluctuated despite having preset target temperatures. The 50°F cold storage unit exhibited the most stable trend over time. The 34°F and 40°F degree units require some troubleshooting.

Berry temperature (°F; lines) and cold storage unit air temperature (°F; bars) by temperature treatment for Batch 1 at BHF (harvested and cooled 07/29/2021), monitored for 5 weeks following harvest.
Berry temperature (°F; lines) and cold storage unit air temperature (°F; bars) by temperature treatment for Batch 2 (RB harvested 8/9/21 and stored 8/10/21), monitored for 4 weeks following harvest.

Individual berry temperatures were not taken after harvest and before cooling for the second batch of berries (“Batch 2”), harvested on August 9, 2021, in RB due to logistics. Similar berry temperature fluctuations occurred often in response to fluctuations in cold storage unit air temperatures. The 40°F unit, maintained a relatively stable berry temperature trend over time. The higher temperature treatment, 50°F showed a steady decline in berry temperature over time. The shift in berry temperature mimics a shift in cold storage unit air temperature in the 50°F temperature treatment as air temperature dropped from 55°F on August 9 to 45°F on September 3.

Overall, average berry temperatures were significantly different across all temperature treatments for both Batches, with lower temperature treatments yielding significantly colder berries than warmer temperature treatments within the cold storage units. For Batch 1, the lowest temperature treatment (34°F) had an average internal berry temperature of 44.5°, while the middle treatment (40°F) and the warmer treatment (50°F) had internal berry temperatures of 45.5°F and 48.4°F, respectively. Batch 2 berries, stored in pints, had warmer internal temperatures compared to Batch 1 that were stored in quarts by an average of 7°F in the first 5 weeks. As a result, Batch 2 at RB internal berry temperatures were warmer than Batch 1 (although not statistically compared) with internal berry temperatures averaging 48°F and 51°F for the 40°F and 50°F treatments, respectively. The warmer internal berry temperatures in Batch 2 further exhibits the direct relationship between cold unit air temperature and internal berry temperatures.

Add graph

Ranked berry moisture data was variable and therefore decided to not be robust enough in this first year to present. Overall Batch 1 berries were very wet and not marketable due to the fact they were not cleaned after harvest. Berry moisture was clearly correlated to unit relative humidity. Cleaning removed most burst and damaged berries in Batch 2 leading to drier berries. Fluctuations in berry moisture greatly affect berry quality and berry firmness. As another indicator of berry quality, berry firmness declined drastically in Batch 1 over time with the greatest decline occurring under the warmest temperature treatments 40°F and 50°F. The coldest temperature treatment (34°F) saw declines in berry firmness over time, however, these reductions were not as dramatic as the warmer cooling treatments.

By August 30, approximately 4-5 weeks after entering the cold storage unit, both batches of berries had clearly lost their volume and form at the 50°F cooling treatment and were classified as “unsaleable”. Mold growth was first observed on Batch 1 berries on August 26, approximately four weeks after entering the cold storage unit, across all three cooling treatments. Mold growth was first observed on Batch 2 berries on August 30 at the 50°F cooling treatment and on September 3 at the 34°F cooling treatment. Mold growth was not as prominent in Batch 2 at the 40°F cooling treatment. Mold growth was also observed on the walls of some cold storage units toward the end of sampling due to condensation.


Growers currently store their fresh pack berries for 24 – 48 hours on average. The length of this trial (4 – 5 weeks) greatly exceeds the current storage time because our aim is to extend fresh pack shelf life. Our short-term goal is for growers to be able to deliver higher quality fresh pack wild blueberries to consumers in Maine with a long-term goal for growers to be able to ship high quality fresh pack berries to markets outside of Maine.

This trial’s year one results clearly showed the importance of relative humidity and outdoor weather conditions in wild blueberry storage. Fluctuations within the units’ air temperature and humidity combined with the natural respiration and ripening processes contributed to the decline in berry firmness and losses of berry shape and volume and the mold that was observed. These changes were particularly evident in the 50°F temperature condition. Under the 34°F temperature condition, these changes and losses were less pronounced. Relative humidity and temperature variation within the units may be related to berry respiration, whereby berries continue to convert glucose into carbon dioxide, water vapor, and heat, even after being harvested. Like most produce, blueberries produce ethylene gas as they ripen, which accelerates their ripening as the rate of respiration increases. Storing berries at colder temperatures reduces the rate of respiration and can thus slow the ripening process. For every 10°F reduction in temperature, the respiration rate decreases by 50% (Callahan, 2018). Slowing the rate of respiration would also reduce the amount of heat and water vapor being produced by the berries. The berries in this study did not leave the cold storage unit and therefore only represent the first part of the fresh wild blueberry journey from the field to consumer. If the 34°F berries were removed and transported or stored at a temperature warmer than 34°F, condensation would occur, reducing berry quality dramatically.

Overall, RB’s Batch 2 berries were less moist and of better quality than berries harvested at BHF due to better raking techniques and cleaning. Berries in pints (Batch 2) showed a trend towards having a warmer internal temperature than berries in quarts (Batch 1), yet this must be confirmed in 2022 by removing confounding variables. Possible reasoning for this is that the smaller pints are more vulnerable to temperature fluctuation within cold storage temperatures whereas quarts were able to hold their cooler temperature until the unit’s temp dropped back down. Berries from both batches stored at 50°F had significantly warmer internal temperatures than those stored at 34°F and 40°F. Additionally, when relative humidity was low, our measure of berry moisture was also low indicating that our subjective method of monitoring berry moisture is a good indicator of relative humidity. Berry moisture presented significant treatment differences in Batch 2, such that the 40°F cooling treatment had significantly higher moisture than the 50°F treatment. This indicates that more condensation may develop on the berries at this temperature or the rate of cooling was a factor in moisture buildup within the system. The coldest temperature treatment (34°F) saw declines in berry firmness over time, however, these data were very variable and need to be repeated in 2022.

Some Maine wild blueberry growers have cold storage rooms, yet their berries remain warm and wet. Observationally, the harvest and cleaning techniques used greatly impacted berry quality, even more so than the temperature at which they are stored.

When constructing a cold storage unit, it is important to have a well-insulated and tightly sealed cold storage unit. Our units exhibited condensation, which caused mold buildup, which we plan to troubleshoot to eliminate this mold in 2022. Tilting the air conditioner back slightly so that condensation water drips out and away from the berries stored inside is critical. Managing relative humidity and air movement inside cold storage rooms is necessary to achieve high-quality berries. Constructing a unit akin to the cold storage units studied in this trial can cost a few thousand dollars per unit (including all materials and labor, building one unit in 2020 at BHF cost $2,600), and operational costs for the system are similar to that of a refrigerator or walk in cooler (Callahan, 2013). Cooling to lower temperatures (such as 34°F and 40°F) will cost more than cooling to higher temperatures (such as 50°F) but the improved length of shelf life and resulting high-quality berries will likely justify the additional expense.

Recommendations to Growers

  • Field conditions at the time of harvest and the method of harvest impact berry quality and cannot be fixed with cooling measures.
  • Consistently cool temperature is best.
  • Temperature fluctuations cause wet berries.
  • The earlier harvested berries can enter a cold storage unit, the better.
  • For more information on post-harvest storage of wild blueberries please visit the Quality and Food Safety page of our website:
Participation Summary

Education & Outreach Activities and Participation Summary

5 Consultations
1 Curricula, factsheets or educational tools
2 On-farm demonstrations
1 Online trainings
1 Tours
1 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

135 Farmers participated
15 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

A collaborative research and outreach program will reach both organic and conventional wild blueberry producers. Growers and stakeholders will be invited to participate in educational events and encouraged to use the resources created via online and in-person methods. Calderwood will distribute event and webinar announcements through both paper and online means (newsletters, listservs, social media, and UMaine Wild Blueberry calendar). Our database of wild blueberry growers and stakeholders currently has 750 recipients in ME. We will measure our outreach success with surveys distributed to participants at field days and newsletter, in addition to sticker board surveys at conferences. Objective 2 (farm visits) of this project is an outreach objective with data collection included. This shows our commitment to including farmers throughout our research and educational work.  


Summer UMaine Blueberry Hill Farm Day is an annual field day typically attracting 100-200 attendees to the Experiment Station in Jonesboro, ME. A cold storage unit show and tell and discussion of the project was presented at the 2021 event on July 8. 100 attendees were present. 

Calderwood presents the three cold storage units built at Blueberry Hill Farm for this project at the Blueberry Hill Farm Field Day in 2021.
Winnowing berries before putting them in quarts for cold storage experiment.

Post-harvest Handling Field Workshop will be held at Welch Farm in Roque Bluffs in year 2 of this project. We expect approximately 30 people to attend and discuss storage and shelf life of this crop. Research and education knowledge to date will be discussed.

Calderwood organized the first UMaine Downeast Agriculture Conference in 2020 specifically for farmers in rural Hancock and Washington Counties of Maine. This event brings the agricultural community together through research-based education and farmer discussion. In January 2020, there were 60 attendees. A talk on this project will be included in year 2.

The February UMaine Wild Blueberry Conference began in 2019 and is located in Bangor, ME. This project will be featured in both years. In 2021 a session was held titled "Fresh Pack Line Improvements" which 35 growers attended and can be found here.  


Factsheet: The existing “Post-harvest Handling of Wild Blueberry” factsheet will be updated with information gained from this project.

Webinar: A webinar will be held live and recorded for posting on the Cooperative Extension Maine Wild Blueberry website: The contents of this webinar will include how to make and run a coolbot, report on the current conditions in existing cool rooms, and suggest ways to improve their efficacy.

Research Reports: Calderwood completes an annual Research Reports document, which contains a farmer friendly report on all research projects conducted on wild blueberry. This project will have it’s own annual report available to growers in print at the Wild Blueberry Conference and online.

Wild Blueberry Monthly Newsletter will be used to advertise events and report on timely project findings.  Past newsletters and reports can be viewed here:

Learning Outcomes

24 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

After the 2021 UMaine Wild Blueberry Hill Farm Field Day which has 100 attendees, 24 indicated learning something new about cold storage and/or coolbot units. Of those 24, 8 indicated that they learned something new that they planned on implementing in their operation. 

After the 2021 UMaine Wild Blueberry Virtual Conference session on fresh pack processing which had 35 attendees, 11 indicated learning something new from the session and 4 indicated that they would make changes to their operation from knowledge gained during the session. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.