Dear Dr. Mitcham, My name is Tristan L. and I am in the 7th grade and am doing my science fair project on storage methods for strawberries. I have read a couple of your articles about strawberry post harvest technology and was wondering if you could clear up a few questions for me?
I stored 4 containers (holding 5 strawberries in each) in the fridge and examined them every day for 10 days and recorded how they decayed. I found that the strawberries stored in the clamshell containers (that they were bought in) went bad the fastest. My other methods of storage were Debbie Meyer Green bag (designed to absorb ethylene gas), Ziploc Fresh Produce bags (designed with Moisture Vents) and a plate covered in saran wrap.
All the containers formed mold on day 10 (except the clamshell). The clamshell containers were not edible on day 6 – but did not have mold, they shriveled and were very mushy. Is this because they were exposed to more carbon dioxide than the others? How does carbon dioxide affect strawberries in the refrigerator? What other factors could have made the clamshell containers become non-edible the fastest?
I am concluding that the Debbie Meyer bags worked the best because they absorbed the ethylene. Would you agree that this is reasonable? (T.L.)
Thanks for sharing the results of your science fair project with me. One of the things your experiment showed is that in addition to decay, water loss which leads to shrivel and loss of firmness is an important factor in the deterioration of strawberry fruit after harvest. In a future experiment, you could record details of shrivel and firmness of the berries as another parameter of product deterioration.
All of your berries were at the same temperature if they were in the same compartment of the refrigerator, but the relative humidity surrounding the fruit were changed by the storage conditions. The relative humidity around the fruit will be kept high when the fruit are in a plastic bag or covered with Saran wrap, while in the clamshell the relative humidity will be lower. The clamshell does not result in a high CO2 atmosphere around the fruit because there is free air exchange through the large vents. In the Debbie Myer bags, and the Saran wrap covered tray, CO2 would tend to accumulate within the bag. I am not familiar with the Ziplock Produce Bags, but I imagine they may allow some CO2 to accumulate as well – your results would confirm this. Because CO2 is a large molecule, it can be retained more easily than water vapor and this is why it accumulates while moisture is retained. The berries produce CO2 and give up water vapor naturally and this is the source of these compounds within the package.
My experience with the Debbie Myer bags is that they do not work much better than a plain plastic bag. They do not seem to absorb much ethylene, and ethylene gas only has a minor effect on the decay of strawberries. The presence of CO2 would be a much stronger effect. Your data seems to support that as well.
The most important factors for maintaining the quality of strawberry fruit after harvest are temperature (as close to 32 as possible), high relative humidity to reduce water loss (90 to 95% is best – 100% humidity can lead to water droplets that promote decay), and then elevated CO2 concentrations (10 to 15% is best). Good luck on your project.