Store the Bounty: Post-Harvest Produce Handling & Storage
By Christopher W. Callahan, UVM Extension
Intro to Vegetable Storage
Remember when the first seed you ever planted actually germinated and the phosphorescent green of life reached for the sky? And then the plant grew and you produced something. Food. Living food. It's a remarkable thing and what's even more remarkable is that harvested crops continue to live. This continuation of life, post-harvest, is an important factor in our ability to store food for future consumption far beyond the traditional growing season.
Produce is alive, it continues “respire.” This respiratory metabolism converts sugars such as glucose to water and carbon dioxide by using up oxygen in the air. That conversion is great if you’re a piece of fruit hoping to attract an animal to help propagate the genes you carry in your seeds. But if you’re the human that cultivated the crop, you generally want to preserve the quality of the produce as long as possible, which usually means slowing down respiration and preventing water loss. How can we do that? We store harvested produce at relatively low temperatures and often at high relative humidity. Table grapes, for example, deteriorate more in 1 hour at 90 °F then in one day at 39 °F or one week at 32 °F (Thomson et al 2008). The effect of temperature on respiration rate, and therefore shelf life, varies significantly by crop.
Fresh produce is alive, with respiratory metabolism continuing. This is part of what makes fresh produce so healthy and nutritious, but it also means we need to take special care in post-harvest handling and storage. Graphic by Virginia Jaquish, UVM Extension.
Ideal Crop Storage Conditions
Each crop, however, is somewhat unique in its “ideal” conditions for long-term storage. Some crops can be stored at or near freezing temperatures (e.g. carrots, beets, cabbage) without any damage, while others are very sensitive to chilling injury (tomatoes, squash) and need higher temperatures. To avoid water loss from fresh produce, high humidity is often an ideal condition. However, some crops are “cured” to provide a dry, papery outer layer (e.g. onions, garlic, winter squash) for protection and they prefer dry conditions to avoid mold growth on that outside skin. Additionally, some varieties within a crop have been found to “keep” better than others. If you want long-term storage, look for “good keeper” or “stores well” in the seed description. Some of these varieties will actually improve over time in storage, with increasing sweetness and improving texture.
Luckily, the ideal conditions for various crops have been studied and are well-documented. One of my favorite references is the USDA Handbook 66: The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. This is a free PDF, available online with a chapter on each major crop that outlines post-harvest handling and storage conditions for maintaining quality.
Over the years, several storage guides have consolidated storage conditions into common zones of storage. These include cold/humid, cold/dry, cool/humid, warm/dry, and warm/humid. The chart below shows some of the typical crops stored in these different zones for reference.
There are five common storage zones (combinations of temperature and humidity conditions) that most crops can be stored in for optimal storage life and quality. Photo courtesy of Chris Callahan, UVM Extension.
You can also use a crop storage planner that allows you to enter the specific crops you want to store. The planner groups these crops into specific storage zones and also estimates the amount of space you need based on the amount you want to store. The planner is based on the information from USDA Handbook 66.
Examples of Storage Systems
Root cellars are one of the earliest known forms of produce storage used in temperate climates. The oldest known examples in North America are in Newfoundland and some still stand that date back to 1839. Root cellars have traditionally been a mix of earthen and stone masonry construction usually (bot not always) constructed below grade to provide cool temperatures in summer and to prevent freezing in winter. The conditions are generally very humid. Modern root cellars have been constructed including some built into basements and several books exist providing guidance (see Bubel & Bubel, 1991 and Maxwell, MacKenzie and Churchill, 2010.)
Although Root Cellars are a great, low-energy, passive way of holding storage crops, the rapid reduction of temperature in produce directly following harvest greatly reduces the respiration rate and prolongs quality. This is especially important for commercial growers, and it is often hard to achieve this with a root cellar alone. Having a refrigeration system enables very high quality storage conditions. Refrigeration systems range from small contained systems like a household refrigerator to large rooms with evaporators inside and condensers outside. With complexity and size comes cost, of course. Walk-in coolers can be constructed from scratch using dimensional lumber, rigid insulation and smooth and cleanable finish surfaces. Prefabricated and modular cooler walls are also available. Some care should be taken during construction to prevent rodent access. It is also a good idea to plan on routine cleaning and maintenance regardless of the type of cooler you have.
Ron Khosla, a farmer from New York, invented a really clever way of getting some refrigerated storage using a window air conditioner. Since an air conditioner is just another form of a refrigeration system, it can be “tricked” into making a space even cooler than normal. The CoolBot, available from StoreItCold is a small controller that converts an air conditioner into such a refrigeration system.
It is amazing to see how well some storage crops actually keep in unintended storage solutions. Some people find just the right conditions for carrots in an outside stairwell that provides basement access. Others store cured onions or garlic in a barn corner that has just enough frost protection from an odd heat source. Last year, I had a butternut squash that sat on top of my kitchen refrigerator for 9 months and was absolutely perfect and delicious when we finally cooked it up in June.
Further Reading and Other Resources
UVM Extension Ag Engineering Crop Storage Resources Page. http://blog.uvm.edu/cwcallah/crop-storage-resources/
Gross, K. (2014). USDA Handbook 66: The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. USDA ARS. Retrieved from https://www.ars.usda.gov/ARSUserFiles/oc/np/CommercialStorage/CommercialStorage.pdf
Callahan, C. W. (2015). Crop Storage Planning Tool. UVM Extension. Retrieved from http://blog.uvm.edu/cwcallah/crop-storage-planning-tool/
Bubel, M. and Bubel, N. (1991) Root Cellaring: Natural Cold Storage of Fruits & Vegetables
Storey Publishing. https://www.storey.com/books/root-cellaring/
Maxwell, S., MacKenzie, J., and Churchill L. (2010) The Complete Root Cellar Book: Building Plans, Uses and 100 Recipes. https://www.motherearthnews.com/store/product/the-complete-root-cellar-book
Chris Callahan is the Assistant Extension Professor of Agricultural Engineering at the University of Vermont. His work focuses on the application of the engineering practice to food systems. He is a frequent contributor to UVM Extension's blog, where his expertise and wealth of knowledge is a resource for farmers in Vermont and beyond: http://go.uvm.edu/ageng.