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
Expand All

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.


Click linked name(s) to expand


Materials and methods:

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

At two locations, Welch Farm and Blueberry Hill Farm (UMaine Experiment Station), we will evaluate wild blueberry quality under different storage conditions. We have chosen to focus on temperature as our study variable while maintaining proper relative humidity and airflow because these two factors are essential and known.

Welch Farm already has a homemade walk-in cooler that they keep at 46-50°F. At this site, we will build one additional 8x8ft coolbot room that will be kept at 40°F, 95% relative humidity (RH), with an industrial fan for air movement across the room. Temperature and RH will be monitored in each room using DigiSense meters. The new Coolbot will be in the same farm building as their current cold storage and serve as a demonstration and research site. Welch Farm will harvest their wild blueberries as they typically do, clean and pack them on their cleaning line, and fill both storage rooms with flats of wild blueberries. At Blueberry Hill Farm, we have more space to compare three temperatures: 34°F, 40°F, and 50°F, in a more controlled environment. In this location, one Coolbot built in 2020 already exists and we will build two more coolbots for this project. All three of these coolbots will be 8x8ft with the same LG Electronics air conditioner, R-value 25 insulation, and industrial fan. Berries will be harvested and packed to fill each coolbot equally. Temperature and RH will be monitored in each room using DigiSense meters.

At Welch Farm we will measure berry quality every 3 days for 15 days (total of 6 collection dates) each year because they typically sell fresh pints within 7-14 days of harvest. At Blueberry Hill Farm, we will measure berry quality every 3 days for 30 days (total of 11 collection dates) each year. Berry quality measures in both locations will include 100 berry weight, number berries shriveled per 100 berries, visual marketability ranking (on a scale of 1-5, where 1= not salable and 5 = excellent condition), in addition to pH and Brix using a hand held PAL-BX/ACID F5 Meter. Ten 100 berry samples will be taken from each storage room on each date for these measures. Ten additional pints will be taken out of each storage room and placed in ambient temperature for 24 hours when the same quality measures will be taken again. This amount of “transit” time may be adjusted to become more realistic. An additional ten pints will be left inside each storage room for the entire 15 and 30 day experimental duration where quality measures will be taken on day 1 and on the final collection date.  

Our experimental design is a complete block at Blueberry Hill Farm with two replications over the course of two years (see Experimental Design below). While the existing cold storage room at Welch Farm is not a true coolbot and the two cold storage rooms are fundamentally different, this site will serve as a comparison between a homemade room and a planned coolbot room. This location will also serve as a demonstration/educational site where farmers can discuss the use of each storage room first hand. This project will be conducted the same way for two seasons in a row (2021 and 2022) for two replications. We realize that three replications is the standard experimental minimum, however our budget did not allow for nine coolbots to be built or for a third year. We may be able to collect a third year of data in 2023 if funds become available. It is important that this experiment be conducted over several seasons because the outside weather conditions could impact the inside coolbot temperature, RH. There is a weather station at Blueberry Hill Farm which collects data that we can analyze with the berry quality data collected for this project. Data will be entered and analyzed by Dr. Calderwood and her team. One-way ANOVAs will be used to compare berry quality measures by temperature treatment at Blueberry Hill Farm using JMP Version 14.

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

Twenty fresh pack wild blueberry farms will be visited over the course this two-year project. Visits will occur during harvest and packing time in August. While the growers are especially busy at this time, we will coordinate with them to arrive on a packing day. We will bring a DigiSense temperature and humidity meter with us to measure the conditions of both the cleaning line room and the storage room if they have one. We will use these visits as an outreach opportunity to discuss post-harvest handling. Topics will include 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 will ask 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 addition to this discussion we will give them a paper copy of the UMaine Extension Post-harvest Handling for Wild Blueberries factsheet.

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.

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

Education & Outreach Activities and Participation Summary

Participation Summary:

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. Coolbot show and tell, discussion of results.

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 1.

The February UMaine Wild Blueberry Conference began in 2019 and is located in Bangor, ME. This project will be featured in both years. In 2020 there were 110 attendees.


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

2 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:

This project will really start in February 2021 but to date the first Coolbot was built at Blueberry Hill Farm, it runs, and we wrote up an initial report on costs. 

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.