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UVM Extension AgEngineering Blog

How Refrigerators Work: “Principles of Refrigeration”

Posted: May 10th, 2013 by Chris Callahan

1944 US Office of Education video on refrigeration principles.  Excellent.

Practical Produce Safety – April 11, 2013 – Vergennes/Monkton

Posted: April 12th, 2013 by Chris Callahan

Hans Estrin gets the class oriented for the day.

Hans Estrin gets the class oriented for the day.

Had a great time talking about all the ways we can get sick from produce related pathogens yesterday.  The good news is that the bulk of the workshop was focused on ways to mitigate the risks.

The Practical Produce Safety workshop series coordinated by Ginger Nickerson (UVM Center for Sustainable Agriculture) and Hans Estrin (UVM Extension Community Food Network) is an informative and pragmatic way to get up to speed on produce safety measures in field practices, harvest, rinsing, packing and storage. This session was co-sponsored by Annie Harlow with ACORN.

I was glad to contribute a little bit on the latter topic; storage.  Slides from my presentation are available here.

Thanks also to Rachel Shattman of Bella Farm for hosting a charrette walkthrough of her wash/pack and storage facilities.  That provided a very real component to the course (and the Apple Crisp was excellent as well!)

CoolBots(TM): Inexpensive Cold Storage

Posted: March 20th, 2013 by Chris Callahan

Demand for on-farm cold storage of produce and other Vermont agricultural products is increasing as local markets for these goods expand. I receive many inquiries regarding CoolBotsTM, an adaptation of a window air-conditioner to make a cooler out of an insulated space. This article is intended to collect related resources in one place and to also highlight some considerations adopters of CoolBots should be aware of.

In a nutshell:

A farmer-built cooler (photo from storeitcold.com).

These systems utilize a commercially available controller ($299) to allow the AC unit to run with a lower temperature than normal. Store-It-Cold, The manufacturer’s website has excellent resources and FAQ’s. They include a list of AC units that they have had positive experiences using. They are also very clear about who should consider NOT using a CoolBot. Applications for which the CoolBot is not well suited, according to the manufacture, include;

  • rapid “pull down” of temperature (e.g. high levels of field heat or frequent exchanges of product)
  • freezers – CoolBots perform best above 36 °F.
  • sites with many door openings per day (e.g. > 6 times per hour)
  • running through the winter – not a show stopper, but you need to be more careful about which AC unit you choose

Other things to be very aware of, according to the CoolBot controller manufacturer, include

A CoolBot installation (photo from storeitcold.com)

A report commissioned by NYSERDA summarizes the cost, energy efficiency, and greenhouse gas emission benefits of a CoolBot installation when compared to a conventional walk-in cooler system at certain conditions. The cost estimate of the CoolBot system (15,000 BTU/hr) is $750 installed compared to $4,400 for a conventional system (8′x10′ cooler box cost not included).

The authors conclude that a CoolBot system can result in approximately 230 kWhr/year of energy savings ($30/year at $0.13/kWhr VT average) when cooling 100 ft2 of cooler floor area to 35 °F (assumes Albany, NY conditions). It is important to note that this analysis highlights the main energy efficiency benefit of the CoolBot system comes from the reduced operating time of evaporator fans. High efficiency fans and improved controls exist for conventional walk-in systems and they are even supported by rebates from Efficiency Vermont. When the CoolBot system was compared with a conventional cooler that also had evaporator fan controls, the savings went the other way; i.e. the conventional walk-in system resulted in 74 kWhr/year savings.

Energy and Waste in the Food System

Posted: March 5th, 2013 by Chris Callahan

Although estimates vary, we invest about 14 calories of fossil fuel-based energy and 15-20 calories of energy in general into every 1 calorie of food produced.  And (here’s the kicker) 30-50% of the food produced never makes it to a digestive track.  So those energy input numbers are actually low by any true measure of efficiency and productivity.

Calorie for Calorie

A recent report from the USDA ERS sums it up this way, “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 the remainder attributed to population growth and higher real (inflation-adjusted) per capita food expenditures.”

Here is a similar idea, displayed in a slightly different way by the University of Michigan Center for Sustainable Systems (this uses units of millions of pounds, not energy).

US Food System - flows in million pounds

And it isn’t all about energy efficiency and renewable energy and boring engineering BTU, calorie and bean counting (although I do like counting beans).  The food wasted post-harvest is a real loss that we can do something about. (NOTE: The report linked above where I take the 30-50% waste figure from was done by a UK engineering trade organization, IMECHE).

Some of the loss occurs in storage, and I think we all can agree that we can do better with our storage practices. Regardless of whether you are root cellaring, using a CoolBot(TM) or a commercial walk-in cooler, the principles remain the same. Some loss occurs in transport and distribution which speaks to the benefit of the broader food system considerations espoused by UVM’s Food Systems Spire and the Vermont Farm to Plate Initiative. Some, of course, occurs in the kitchen or in consumer storage and suggests we have some work to do with consumers as well.

As one grower recently said to me, “By the time we put food in our farm cooler, 99% of our cost is sunk into that product. We gotta pay attention to what goes on in there and make sure we get paid for it.

FIDO – A Do-it-yourself Temperature Monitor with Text Message Alerts

Posted: March 2nd, 2013 by Chris Callahan

As I’ve mentioned in several other posts, I think the continual monitoring of conditions in greenhouses and food storage spaces is incredibly important for quality and safety and insightful for any operation. There is a really clever design for a do-it-yourself temperature monitoring system called Fido, on the FarmHack site.  It uses an Arduino control and electronics platform, a cheap cell phone, and a few other pretty inexpensive pieces to do the job.

“A farmer-built electronic tool that can monitor greenhouse temperature, record greenhouse data, and alert the farmer to problems in the greenhouse via cell phone text message. This tool will be much more affordable and useful than commercially available greenhouse alarms (which rely on landline connections or internet connections, which usually aren’t available in the greenhouse).

I’ll be trying to add RH monitoring to this soon, and will update the post when that is complete.

Wash and Pack Stations

Posted: February 16th, 2013 by Chris Callahan

This is a great video overview from the Cornell Cooperative Extension Small Farms Project with helpful guidance on wash and pack station layout. More on the Cornell Small Farm Project can be found here.  This video was part of a NE-SARE funded project.

Some highlights:

  • Cover – prepare for all conditions; rain, wind, cold, heat
  • Lighting – ensure plenty either naturally or artificially
  • One Directional Flow – dirty on one end and clean on the other, work zones, specialty zones
  • Avoid extra and repeated steps
  • Bench/Counter Height – 30″ is comfortable for an average person
  • Some benches lower than the working surface for packing bins (keep the top of the bin level with the counter)
  • Containment – back splash and fencing to prevent loss of produce to the ground
  • Water sources – consider overhead routing and multiple taps
  • Water drains – where will the water go?
  • Float valve for wash tanks
  • Dedicated area for record keeping, dedication to record keeping
  • Marked containers, color coding bins (dirty vs. clean, crop specific)
  • Creature comforts such as floor mats, etc.
  • Logistics – pre vs. post market

Bioenergy NOW!

Posted: January 29th, 2013 by Chris Callahan

The Vermont Bioenergy Initiative, a project of the Vermont Sustainable Jobs Fund recently released a series of videos summarizing their bioenergy related projects through-out Vermont. Visit the YouTube channel for all the videos.

2013 VVBGA Annual Meeting

Posted: January 29th, 2013 by Chris Callahan

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.

 

Remote Data Monitoring – 1st Install at Pete’s Greens

Posted: December 22nd, 2012 by Chris Callahan

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.

Installing Remote Sensor in Cabbage Room

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.

Setting up the Data Logger

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.

UC Davis Guide: Small-Scale Postharvest Practices

Posted: December 18th, 2012 by Chris Callahan

This guide from the UC Davis Postharvest Center is jam packed with information relevant to VT’s small-scale growers seeking improved post-harvest handling and processing. I think it is a nice companion to the New England Vegetable Management Guide and USDA’s Handbook 66. The UC Davis Postharvest Center has a wonderful on-line library with many other titles which may be of interest including an overview of small-scale cold room options and one on root cellars and other passive storage options.

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:

  1. to maintain quality (appearance, texture, flavor and nutritive value)
  2. to protect food safety, and
  3. 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.”

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