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.
As an engineer, I love data. It turns out farmers do also. At least, Pete Johnson and Isaac Jacobs at Pete’s Greens in Craftsbury, VT do. “Is it working yet?” Isaac asks as I put the finishing touches on the remote data monitoring system we have been installing in the four zone drive-in cooler. “Just about… I think.” I say with trepidation. Isaac has been up and down in a scissor lift several times at placing and removing a sensor that was being difficult. And I’ve been wrestling with a data station to make it communicate over the wireless network so that we can actually see the data being collected by the new remote monitoring system.
Isaac Jacobs of Pete’s Greens (Craftsbury) installs a remote temperature and relative humidity sensor in the cabbage room of the farm’s cooler.
This is the first time we’ve installed this kind of system which consists of a “base station” and “remote sensors”. The remote sensors in this case measure temperature and relative humidity (RH), both parameters which can drastically influence the storage life, quality and food safety of produce. In fact, the motivation for multiple storage “zones” in the cooler is to provide each group of vegetables their desired set of conditions; e.g. potatoes at 38 °F and 90-95% RH, onions at 32 °F and 65-70% RH, cabbage at 32 °F and 99% RH and squash at 55 °F and 70% RH.
The items to the left of the laptop are the remote data logging system. Each of the five sensors can measure temperature and relative humidity over a wide range and report it back to the base station which uploads it to a website for review. Alarms can be set to alert users via email if conditions exceed high or low limits.
“What are those spikes in temperature?” Pete asks as we glance over the first set of data that pops onto the screen. There are spikes in temperature every 8 hours in the potato room that last about one hour. Not big spikes (2 °F above the nominal), but they stand out. And perhaps as important, relative humidity drops by 2% when the temperature goes up.
Later, when Isaac and I are looking at the electrical wiring to see where we can plug in additional sensors we note that evaporator defrost system in that room is on an 8 hour timer; that’s the source of the heat. “Well, they don’t even need to be on right now, that room isn’t even being cooled.” The circuit breakers for the evaporator heaters (intended to defrost the evaporator when it freezes up) are shut off, reducing the farm’s electric bill slightly over the next few months. And this is only two hours after we started getting data.
The power of data is three-fold. First it is inspired by inquisition. It then raises additional questions and with further review, it should answer questions and improve life. The principle behind this project is to allow for easier access to process data for Vermont’s growers and to demonstrate this type of system. As farms push the seasonal envelope in response to increased local demand, year-round production and long term storage of fruits and vegetables will be increasingly important. I plan to use this system at multiple locations in Vermont to collect data on refrigerated storage, greenhouse and high tunnel production, and whatever else comes along that is interesting and makes sense.
One question that seems to come up regularly is “how big a cooler do I need?“ I’d recommend grower’s review Table 7 in Heatcraft’s Engineering Manual which includes typical “product loading densities” in pounds per cubic foot. Based on your typical yields and acres in production you can use this to estimate a reasonably sized storage space.
“The three main objectives of applying postharvest technology to harvested fruits and vegetables are:
to maintain quality (appearance, texture, flavor and nutritive value)
to protect food safety, and
to reduce losses between harvest and consumption.
Effective management during the postharvest period, rather than the level of sophistication of any given technology, is the key in reaching the desired objectives. While large scale operations may benefit from investing in costly handling machinery and high-tech postharvest treatments, often these options are not practical for small-scale handlers. Instead, simple, low cost technologies often can be more appropriate for small volume, limited resource commercial operations, farmers involved in direct marketing, as well as for suppliers to exporters in developing countries.”
“VPR looks at the economic impact of food production in Vermont in the special series The Business of Food.
All this week during Morning Edition, we’re looking at the expansion of farmer’s markets from a summer calendar schedule to a year-round operation, how community planners in southeastern Vermont say it’s time to go beyond co-ops and plan for a “post-oil” world, how “farm to plate”initiatives have resulted in good social policy and how downtown planners are using food co-ops as a lure to enliven central business districts.
Listen this week at 7:50 a.m. during Morning Edition.”
Thanks to all of those who took the time to respond to the food storage survey several weeks back. The information you provided has been really helpful. A clear desire for additional programming in this area was expressed, and I’ll be working to develop that.
Here are the survey result highlights:
82% of respondents expressed interest in a formalized course on food storage topics with the overwhelming majority preferring classroom / workshop format and/or a webinar.
66% of respondents have plans to expand your food storage capacity in the next 24 months.
While most respondents expressed concern about the utility costs of their storage systems, relatively few (37%) currently know those costs.
60% are storing in multiple zones (Temp and RH), but the overall knowledge of optimal storage conditions is relatively low (see related post on USDA Handbook 66).
Winter markets included (% reporting sales to each)
Farmer’s Markets (41%)
Winter CSA (38%)
The most common systems in use are (% reporting use):
Walkin cooler (57%)
Chest freezers (55%)
Heated Winter Storage (39%)
Root Cellar / Other Cool Storage (not refrig’d) (31%)
Are you considering any equipment upgrades in the near future. Check Efficiency Vermont’s webpage to research available technologies and to see if any of their many incentives apply to you. They have a set of rebates specific to agriculture and some for commercial refrigeration which may apply to folks with refrigerated storage on their farm. Been thinking about an outside air economizer to take advantage of the chilly winter air in your walk-in? Check out the rebates. They even have some programs focused on heating, ventilation and air conditioning systems which may apply to greenhouse heating with a pellet furnace or boiler. Want to learn more about Efficiency Vermont, including where the money comes from? Read on…
The USDA regularly produces its Agricultural Handbook 66 – “The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks” to help guide long term storage of these products. The handbook is dense with info on optimal storage conditions for everything from Jerusalem Artichokes to Watercress. Each crop is given a brief overview which summarizes the expected loss when stored at certain conditions and also a summary of respiration rates to help with the sizing of any refrigeration that may be needed.
Many Vermont growers are probably familiar with the green book from 1986. But did you know that a newer version is available online? I was surprised to see how much the online version had that the printed version did not and (believe it or not) some recommendations have changed. As you are putting things in storage for winter markets and other outlets, consider reviewing the revised Handbook 66 online. You may be surprised by what you find.
I’m collecting information on Vermont’s commercial food storage practices. This is most commonly a walk-in cooler or freezer on the farm or at a food hub, but there is room in the survey to tell me all about the other ways you store food. The purpose of the survey is to provide a baseline against which to assess future growth and improvement, but also to determine where the need for more work and research is.
Please take 5 minutes to offer your perspectives by Wednesday, November 7, and spread the word.
Thanks to Eric Garza (UVM RSENR) for putting me onto this dense and fascinating report that summarizes the energy use in our food system. I was surprised to learn the per capita energy use in our food system increased significantly from 1997 to 2002.
From the report;
“Energy is an important input in growing, processing, packaging, distributing, storing, preparing, serving, and disposing of food. Analysis using the two most recent U.S. benchmark input-output accounts and a national energy data system shows that in the United States, use of energy along the food chain for food purchases by or for U.S. households increased between 1997 and 2002 at more than six times the rate of increase in total domestic energy use.
The use of more energy-intensive technologies throughout the U.S. food system accounted for half of this increase, with theremainder attributed to population growth and higher real (inflation-adjusted) per capita food expenditures.”