To Pulse or not to Pulse? Does Pulsed Vacuum Improve Maple Sap Yields?
R. Burl Hanson, Research Scientist, Northwestern Wisconsin Maple Products Laboratory
Funding: Nevada Maple Products Association Inc.
Maple syrup producers have gained greatly in efficiency, yield, and tree health over the past few hundred years by constantly improving the process. We have moved from the wide bark slashes made by Native American to drilled holes of an inch diameter gradually down to the current 5/16 inch in an effort to do less damage to the tree.
We have placed pipelines through the woods connecting trees together with gravity flow into centralized tanks gaining labor efficiency. We have applied a constant vacuum to the pipelines to gain production of 20-30 percent over gravity feed. We have learned to replace the tip of the spile each year to deter bacteria from entering the tree, and recently moved to check valve replaceable tips to prevent backflow—each step improving the yield of sap per tree hole. We have experimented with different vacuum levels, pipeline sizes, and various and sundry other changes to improve our sap/cost ratio.
We have adopted reverse osmosis sap filtering, ultraviolet sap sterilization, air infiltration evaporator boiling, pressure syrup filtering with diatomaceous earth and dozens of other innovations in the past 50 years as research guides our future.
Many years ago, as a young man helping his father on our Wisconsin dairy farm, I too tried using a vacuum milker pump to increase sap yield. We hooked directly to the barn vacuum pump and applied a continuous vacuum of about 15 lbs. It was successful and we got higher yields on vacuum assisted taps and adopted vacuum pipeline decades ago.
On the farm, vacuum pumps ran milking machines to automate milking cows. The milking machines copied the pulsing suction of a calf drinking at the spigot. The machine pulsed the vacuum once per second to match the calf and a hand milker’s rate. I wondered then if a pulsed vacuum would change the sap yield over a continuous vacuum. Dad had no interest in letting me take his Surge milker to the woods much less hook it to a tree, so the thought remained idle until last year.
Fifty years later, having retired from a career in scientific research, I was preparing for maple syrup season getting equipment stored in the old farm milkhouse, still as it was when Dad quit milking cows 30 years ago. I noticed the Surge milking machines still on the rack near the bulk tank. I remembered my curiosity about pulsed vacuum.
Milking machines of the 20th century had a mechanical pulsator on them, two small pistons that slid back and forth actuated by the vacuum, pulsing the vacuum sent to the the actual cow milking part, four metal teat cups with flexible rubber “inflations” that did the squeezing with each pulse. With equipment available and a background in scientific research to guide me, and funding available, I could finally try to answer the question “To pulse or not to pulse?”
I cleaned, oiled the pulsator, and tested the Surge milker—it worked fine. I set it to 60 pulses per minute, the cow rate. I brought it to a tree adjacent to the sugarshack where I could observe it while cooking syrup. I removed the 4 teat cups and blocked off two of them (only wanted two for my test) and hooked the other two to plastic hoses going to freshly drilled 5/16 inch taps, 2 inches deep. Of four taps, two would be pulsed and two would have continuous vacuum. For two weeks I measured the output and sugar content of each pair of taps—one coming directly into the milker bucket and the other into a special tank. After two weeks, I switched the pulsed and continuous vacuum taps so I could rule out tap hole variation.
Results: Statistical analysis shows with significance of P less than 0.001 that pulsing vacuum gives a 20.1-26.2 percent greater sap flow than continuous vaccum. Whichever set of taps had pulsed vacuum gave at least 20% higher sap yield than continuous vacuum. Sugar content was the same for all taps all periods.
Discussion: The mechanism underlying greater sap flow with pulsed vacuum is not intuitive. Pulsing may provide the tree with a brief respite that continuous vacuum does not allow. Trees are thought to have their own intrinsic pulse as they push sap from the roots to the leaves at least according to some biotreeologists. It is possible that we are tapping into this synchronically with the tree’s own biorhythmic nature.
As pulsing is clearly a yield enhancer, we anticipate rapid and extensive adoption in 2012. The only contraindication is that checkvalve wear was accelerated due to the 60 times a minute the ball rolled to open and close. Additional testing showed a new checkvalve would last for 46 days average (46 +-5 Standard error 2.5), an average season.
Future: With additional research funding we hope to continue with efforts to understand what is happening. We plan to do experiments varying the pulse rate from once per minute to 100 times per minute in steps of 10. We have developed and patented a simplified pulsator (nicknamed Pulsing Mathilda) that is connected at the vacuum pump and pulses the whole pipeline at once. We have patented this method and are licensing it at 18 cents per tap hole with discounts for larger producers. Call for direct sales or dealership opportunities. Research contributions of signed blank checks are welcome.
Reported April 1, 2011.