To meet the growing demand for local food, more and more growers are producing storage vegetables as a way to extend the marketing season long after the production season is over. To be successful, this requires slowing down the natural process of aging and decay. Vegetables are alive, even after harvest, but the reality is they are doomed to die - the only question is when. You can think of storage as a life support system designed to reduce a crop's respiration rate, which uses up energy, and minimize its transpiration rate, which causes dehydration. In addition, a good storage system will avoid damage to crop quality from chilling injury, exposure to ethylene or off-odors.
Storage life depends on the condition of the crop, so careful handling is critical, and only mature, undamaged produce should be harvested for storage. Crops last longest when held under optimal storage conditions, which for winter squash is 55 degrees Fahrenheit and 50 to 70 percent relative humidity.
Photos by Vern Grubinger.
First things first: planning ahead for storage
The steps growers can take to optimize the storage life of vegetables happen in three stages: from field production, to harvest and handling, to storage.
The first stage is still in the field, looking ahead to the quality of the crop at harvest. Since quality only goes down in storage, it makes sense to start with the best quality possible. That involves the usual array of best management practices, from a good soil fertility program to timely and adequate irrigation to effective pest management. It also includes selecting varieties that are well suited to storage and timing the harvest to make sure the crop is mature but not past its prime and starting to senesce.
Harvest and postharvest handling is the second stage. The primary concern here is to avoid any and all injury to the crop that will reduce its ability to store well. Cuts, bruises, exposure to direct sunlight and high temperatures should all be avoided. Prompt removal of field heat is essential to slow down the aging process right away. Some crops will require trimming or washing to meet market demands, others may need curing to get them into the best condition for storage. Washed products may be sanitized to kill off the microbes that promote decay.
Crops will be packed in some way for storage, and this must be done with care to avoid injury. Storage containers must allow the right amount of air exchange for the crop, and should not pile the crop so deep that bruising occurs.
Curing is an important part of the second stage for some crops. It's used to promote the drying down and toughening up of external tissues, which reduces moisture loss in storage as well as entry points for decay organisms.
Just like storage conditions, crops vary in their optimal curing conditions. For garlic and onions, cure at 68 to 86 degrees Fahrenheit, 70 percent relative humidity (RH) with good airflow for two weeks or until necks and skins are completely dry and tight. For table stock potatoes, cure at 55 to 65 degrees Fahrenheit, 95 percent RH for two to three weeks. For sweet potatoes, cure at 80 to 86 degrees Fahrenheit and 85 to 95 percent RH for four to seven days. Cure winter squash at 80 to 85 degrees Fahrenheit for 10 to 20 days to heal wounds; curing may not benefit mature, undamaged fruit.
Curing is important to promote long storage life for some crops like onions. Curing can be done in the field when the weather is warm and dry, or in a greenhouse. Temperatures should be held below 85 degrees Fahrenheit, with good air movement. Curing is complete when necks are completely dry and tight.
The final stage is storage. Vegetable shelf life is manipulated in storage by managing temperature, humidity and the combinations of crops that are stored together (to avoid ethylene injury and odor transfer). The better a storage system can maintain the optimal temperature and relative humidity for different crops, the longer the crops can be held in a marketable condition. Of course, it isn't always possible to meet the needs of every crop you want to store, so do the best you can with the facilities you have, recognizing that suboptimal conditions will shorten the storage time you can expect.
A quick Internet search will bring up sites that show long lists of vegetable and their optimal storage conditions. The most comprehensive explanation of individual crop storage requirements and handling guidelines is USDA Agriculture Handbook Number 66, The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks, which is online at: www.ba.ars.usda.gov/hb66/contents.html. A one-page summary of vegetable crop post-harvest handing information can be found in the 2012-13 New England Vegetable Management Guide on page 26, which is online at: www.nevegetable.org/index.php/cultural/foodsafety?start=2.
For practical reasons, growers need to group storage crops into common categories so facilities can be created to provide several different conditions. There are basically five sets of ideal conditions for winter storage vegetables. These are: cold and moist, cold and dry, cool and moist, warm and moist, warm and dry. Below are the specifics of these conditions and the crops suited to them that are commonly stored into the winter on northeastern vegetable farms.
Cold and moist = 32 degrees Fahrenheit and 90 to 95 percent RH. Beets, cabbage, carrots, cauliflower, leeks, rutabagas and turnips.
Cold and dry = 32 degrees Fahrenheit and 65 to 70 percent RH. Garlic and dry onions. (Store seed garlic at 50 degrees Fahrenheit.)
Cool and moist = 45 degrees Fahrenheit and 90 percent RH. Potatoes for table stock.
Warm and moist = 57 degrees Fahrenheit and 85 to 90 percent RH. Sweet potatoes.
Warm and dry = 55 degrees Fahrenheit and 50 to 70 percent RH. Winter squashes, including pumpkins.
You may not need all these "ideal" conditions for the crops you have, or you may not be able to afford to create them, especially if you are only storing small amounts. In that case, slightly less than ideal conditions can work, too - you'll just have to sell crops sooner rather than later in order to avoid storage losses. For example, a single storage can work for potatoes and roots crops, held 38 degrees Fahrenheit and 90 percent RH. A single storage also works for winter squash and sweet potato held at 55 degrees Fahrenheit and 60 to 70 percent RH.
Problems that can crop up in storage
Chilling injury. Some crops are more sensitive to low temperatures than others. If your storage system does not have accurate temperature control, then it is best to err on the side of caution with sensitive crops and aim for a few degrees above the optimal temperature. Potatoes and sweet potatoes are very sensitive to chilling injury. Onions, winter squash and carrots are moderately sensitive, meaning they can tolerate brief exposure to suboptimal temperatures. Cruciferous storage crops are the most tolerant of cold temperature and least susceptible to chilling injury.
Odors. Crops can pick up off-odors from other crops, so some storage combinations that should be avoided include: apples or pears with celery, cabbage, carrots, potatoes or onions; celery with onions or carrots.
Ethylene. This gas is naturally produced by some crops in storage and can cause other crops to age faster, develop off colors or blemishes. Storage crops affected by ethylene include cabbage and carrots. Stored crops that produce a lot of ethylene include apples, pears and tomatoes. These two groups should be stored separately. If they are stored in the same area, separate them by impervious plastic barriers.
For more information, view a table summarizing the harvest and postharvest needs of fall/winter storage crops at: http://extension.unh.edu/agric/Docs/HandoutOnePostharvestStorageChart.pdf and the slide show "Maintaining Quality of Winter Vegetables in Storage" by Ruth Hazzard of UMass Extension, at: http://extension.unh.edu/agric/Docs/5_Hazzard_postharvest.pdf.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.