Keeping produce fresh may be more challenging than growing it. Without proper storage, even the most fruitful harvest is going to be a bust. From high-tech refrigeration units and monitors to basic root cellars, successful storage depends on keeping produce fresh and healthy, and is the key to maximizing profits.
“By the time you harvest something, most of the costs of production are sunk,” Chris Callahan, University of Vermont Extension Engineer said in a recent webinar. You can “capture higher prices at a time of the year when product is maybe not available from other folks.”
Factors affecting quality
Maintaining a proper temperature, which can range from anywhere between 32 degrees Fahrenheit for a carrot, to 50 degrees for winter squash, is the first critical aspect. Then, the relative humidity has to be adjusted. Losing moisture decreases the weight of the produce, and will decrease profits in items sold by the pound.
The production of ethylene, a naturally-occurring hormone, is also a concern during storage. Ethylene causes ripening. Some produce is much more sensitive to ethylene than others, and each type of fruit or vegetable emits differing amounts of ethylene. Apples, for example, are notorious for ethylene production. Carrots become bitter when exposed to the hormone, while cabbage will rot. So keeping high ethylene producers separate from ethylene-sensitive produce is a must.
Temperature will have a great impact on shelf-life, Cahallan said. Cold temperatures decrease the respiration rate of produce dramatically. Every chemical reaction — which respiration is — is dependent on temperature.
But cold isn’t all good. Chilling or freezing injuries can occur when things get too cold, and exactly what too cold means not only varies from product to product, but can vary across the body of a plant. Not all things freeze at 32 degrees Fahrenheit, either. Even though produce contains water, the water is not directly available, Callahan said, so some things can withstand lower than freezing temperatures. Some produce, like winter squash, suffers chilling damage well above freezing, too. So if the field temperature is less than 50 degrees Fahrenheit before harvesting occurs, the damage will be done, but won’t show up until the squash has been in storage.
Dessication is another concern. Refrigeration removes moisture from the air. Plant metabolism adds to the moisture loss. So adding back humidity to the air may be needed. To do so correctly, storage density needs to be taken into consideration. Density is measured in pounds per cubic feet. Higher densities will impact the storage climate, and temperature and relative humidity will need to be adjusted accordingly. Refrigeration units must also be sized properly to cool the room, even as the produce itself is producing heat.
In order to provide the best conditions for a variety of products, separating them into zones based on temperature and humidity needs, as well as ethylene sensitivity, is recommended. While having separate rooms may be ideal, there are other ways to achieve zoning. For example, putting carrots in a perforated bag will help them with their increased relative humidity needs over other products with similar temperature requirements.
A vapor barrier, made with moist burlap or greenhouse polyfilm, can also act to create a zone. One zone that might be overlooked is a pre-cooling zone, where field heat is removed initially, before putting any product into the storage unit.
When designing your storage unit, ask yourself “how long a term are you planning for?” Callahan said.
If storage is shorter term, you may not want to invest in top of the line material and equipment. Aside from a prefabricated walk-in cooler, other options include building a structure made of earthen materials, using insulated shipping containers, or building a room with materials such as galvanized roofing, fiber reinforced plastic, coated and sealed lauan or any material which is smooth, sealed and can be readily cleaned.
One concern is condensation. Wood and cellulosic materials will mold and mildew under moist conditions. Your storage unit should be structurally sound, durable, have internal and external moisture tolerance, possibly be reusable or portable, and sealed from rodents. Don’t forget to seal doors and latches, and use hardware cloth to reinforce vulnerable areas and keep rodents out.
Wood bins may be a problem in the future, with food safety standards, but can be used. Plastic bins are more portable, sometimes fold for storage, are readily cleaned and provide good air flow. Plastic clamshell totes can be used, but they are not ventilated, and will increase the humidity level.
Refrigeration systems and monitoring
A root cellar is a way of providing either cold or warm storage, Callahan explained. A root cellar can provide a relatively cool space in the fall, when the soil temperatures are lower than the air temperatures. But in the winter, when air temperatures drop, a root cellar protects against freeze damage, he said. You can also use fans and thermostats to exchange cool outside air with warm inside air to remove heat from a storage unit, and keep produce chilled.
More active storage requires a refrigeration unit with a compressor, to change the refrigerant from solid to liquid form, using pressure; a condenser unit, located outside of the refrigerated storage, which rejects the heat; and an evaporator, inside the cooler, to cool the air.
The evaporator can be a high humidity unit if needed. Or, if a higher humidity level is needed for some bins, burlap can be moistened and used to cover them. The burlap must be cleaned and sanitized routinely. A room humidifier can also be used, although maintaining the proper humidity levels can be challenging. An atomizing unit releases small droplets of water into the air, to be evaporated. Foggers and nozzles can also be used to add humidity, as can moist slabs of cement inside the unit. Humidity sensors are available, with the sling psychrometer being the most accurate.
Coolbots are room air conditioning units which can be used to cool storage areas. They are slow to lower temperatures initially, and slow to recover if the temperature of the room rises. They only cool to 35 degrees Fahrenheit. However, they are a lower cost option than commercial refrigeration units.
Thermostats, which control the temperature in the unit, need to monitored. Dial models are least accurate. The temperature can vary up to seven degrees from the reading, Callahan cautioned. Digital models are more accurate, and models with a remote sensor bulbs provide even more specificity, as their bulb can be placed exactly where you need it for best temperature regulation.
Coils on refrigeration systems need to be vacuumed and dusted off. Install coolbot systems correctly, or moisture will be a problem Scout daily for spoilage, using all five senses, and measure and monitor the temperature and relative humidity regularly. A digital monitor can send this information to your computer, or you can simply keep a log book. Don’t forget to calibrate. And keep the entire system clean.
“Measure. Know what the conditions are,” Callahan said. “Lasting quality depends on paying attention to storage.”