I had the pleasure of joining the Vermont Vegetable and Berry Grower’s Association yesterday for their annual meeting. This event is packed with presentations ranging from the latest pests to farmer outreach in distant lands.
My talk was on crop storage, and it was a great chance to summarize my recent work in this area. I’ve been fortunate to have worked closely with several growers in this area including Isaac Jacobs and Pete Johnson of Pete’s Greens, Jon Satz of Wood’s Market Garden, Andrew Knafel of Clearbrook Farm, Howard Prussak of High Meadows Farm, and Jeremy Gildrien of Gildrien Farm. I’ve also benefited from conversations with Jim Thompson and the resources he helped create at the UC Davis Postharvest Technology Center.
The presentation slides are available at the VVBGA’s website. I’m still very grateful for the responses to the food storage survey, and we discussed these at the meeting. I also highlighted 5 things I think are critical considerations for VT growers storing vegetables and berries.
- Zoned Storage – While many are zoning (or grouping) their stored products based on optimal temperature and relative humidity (RH), it is also important to consider a zone for pre-cooling product as it comes into storage. The sudden addition of product with field heat and elevated respiration can contribute significantly to the cooling load in the room and could lead to slightly warming other crops already in storage. Additionally, we talked about the need to consider ethylene production of crops and also their sensitivity to it; sometimes requiring outside air exchange to remove the ethylene. Most are familiar with ethylene production from apples, but even common vegetable crops also produce some. Storage conditions for main crops as well as respiration rates and ethylene emission rates can all be found in USDA Handbook 66.
- Measurement and Monitoring – It is understandable that one should expect a cooler to be at the temperature you set on the thermostat. But I’m a believer in secondary, accurate measurement to confirm storage conditions. This means both temperature and RH. I urge growers to check it regularly (daily), and to keep track in some sort of log so that trends are captured. This can take the form of an advanced remote data monitoring system, but it can also take the form of a simple clipboard or notebook. The important thing is that the conditions are actually measured with an accurate device such as a certified and calibrated thermohygrometer or sling psychrometer and be recorded. Here’s a video showing how to use a sling psychrometer (equally useful in a greenhouse or cooler, although I recommend “slinging” for 1 minute or more, taking 3 readings to check for stability, and using a psychrometric calculator to determine RH as the slide calculator on the device is not terribly accurate.)
- Scouting – Despite all the best intentions; zoning your storage and confirming the conditions, sometimes you still run into problems. There are varietal differences in storage and many other factors that will influence how the crops keep in storage. So it is important to “scout” the storage as well. This can be daunting with bins and boxes piled high, but catching a problem early could help prevent a major loss. It is possible, as well, that you have to deviate from the published references for storage conditions for a certain crop. The verification of the storage conditions is the measurement step above, but the validation is the crop quality. The proof is always in the pudding.
- Cooler Audits – It is hard to make time to stop and smell the roses, and it is hard to take time to stop and audit your cooler. But there are things you can do on a routine basis that take little to no additional time.
- Check Door Seals – Walk inside the cooler, shut off the lights and look around the door for daylight. If you find spots with light shining through look more closely at the seal in that area, it may need repair or replacement. Look also for frost (on freezers) or condensation (on coolers) which can also be signs of air leakage.
- Door Closure Tightness – Even if your seals are in good condition, the door must shut snugly to have them work. Most commercial cooler doors have adjustable latches. Make sure there is no play in the latch when the door is closed, and adjust as needed so it closes tightly.
- Mold, Condensation – Keep an eye out for mold and/or water condensation, this may point to air circulation issues or dead spots of air flow that need to be addressed.
- Noise – Noise is energy, and if you get to know the typical “hum” of your compressor and fans, you’ll be able to tell when something is amiss. New noises or more frequent operation of the compressor can signal a significant change in the refrigeration system (a higher than normal load, or heavier work than normal.) Keep an ear out for new noises and do a complete walk around on a regular basis to catch maintenance issues early.
- Coil Cleaning – The air coils are the lungs of the system, and they need to be clear of debris. Regular coil cleaning should be added to any preventative maintenance or seasonal job list. If your system can’t reject heat (either inside the box or outside the box), you’re not cooling as effectively as you could. This definitely means reduced efficiency and increased energy use, but it could also mean reduced storage efficacy and premature spoilage.
- Mechanical Maintenance – A trained mechanical contractor should inspect your system on a regular basis (yearly prior to your main storage season). This will help minimize the chances of system failures and (worse) crop loss.
- Technical Resources – There are several excellent resources available on crop storage. The New England Vegetable Guide is an excellent overall crop guide that includes basic storage information. To dive a bit deeper, look at the USDA Handbook 66, note that the online edition has increased detail than the last print edition. I also recommend the UC Davis Postharvest Technology site which has a wide array of searchable resources, many of which are crop specific. If you get real deeply involved in environmental control (temperature and humidity), you might want to learn more about psychrometric charts and calculators. These allow you to very accurately understand the relationship of water vapor and air and are especially useful when used with a sling psychrometer.