Farming Magazine - October, 2012


Sugaring in the Heat

Improve your chances of maple success
By Tim Wilmot

The record heat wave of March 2012 presented maple producers with a difficult situation: how to collect sap and produce a salable product in weather that is unsuitable for normal production. While sap may flow even in hot weather, filtering and boiling a sugar solution that has been warmed for hours in a leafless forest, at temperatures that cause microorganisms to grow exponentially, is quite challenging. Equally challenging is collecting enough sap to make a decent crop of syrup before the maple buds start to swell and ruin the flavor. A previous article discussed possible alterations to the maple forest as the climate changes and grows hotter; this article will discuss adaptations that producers may employ for sugaring in a warming climate.

Photos by Bob M. Montgomery Images,

One of the most important adaptations to climate change is for producers to recognize that the timing of the season has shifted, and to be ready to start sap collection earlier than some fixed, traditional date. There is ample evidence that significant changes have already occurred. A few years ago we obtained records from a sugar maker in Washington County, in central Vermont, that were kept from 1870 to 1940. In the 1870s, the farmer's normal tapping date was around April 1, and the season ended around May 7. By the late 1930s, the tapping date was around March 13 and the season ended around April 15. This shift of both ends of the season at this location, as well as the shrinkage of the season from an average of 39 days to 33 days in length, corresponds well with more recent data collected and analyzed by Dr. Tim Perkins at the University of Vermont Proctor Maple Research Center (PMRC). Perkins' data showed that between 1963 and 2003 producers who had sugared continually in New England during that period were starting 8.2 days earlier and finishing 11.4 days earlier.

In recent years, increasing numbers of Vermont producers with tens of thousands of trees have started tapping as early as January just to be finished by late February or early March. Many more producers tap sooner than they used to, and even when doing so the most common regret described in my surveys of sugar makers is, "I should have tapped earlier." Data from these producers, as well as research from the PMRC, indicates that this practice has not led to reduced yields, even when the season does not start for several weeks after tapping. When trees are cold, the microorganism growth in the taphole that causes them to "dry up" or become nonfunctional is minimal. There may be a slight reduction in flow late in the season compared to a taphole drilled several weeks later, but early tappers usually make up for this deficit by catching the first sap runs of the season, even when they come in midwinter.

Vacuum is an obvious and widely employed adaptation to aid sap collection in warmer weather. In the spring, when maples freeze and then thaw, pressure is generated in the trunk that causes sap to drip from a taphole. When there are long intervals between freeze-thaw events, sap collection into buckets is intermittent, and total sap yield is often poor. Vacuum in tubing provides a constant force - lower pressure in the tubing compared to the tree - which can maintain sap flow long after the last freeze-thaw event. Thus, in sap seasons with few freeze-thaw events, sap yield via vacuum collection often greatly exceeds yields without vacuum. Even without connection to a vacuum pump, a properly designed tubing system on a slope can generate enough vacuum to produce substantial sap yields in unfavorable weather.

To keep sap flowing from the taphole as long as possible, microorganism growth must be kept to a minimum. With temperatures in the 30s and 40s, microorganism growth in the taphole is usually slow; when temperatures are in the 60s and above, growth is explosive. The most effective way to reduce this growth is to use a new spout or spout tip every year, because used plastic spouts, no matter how well they're cleaned, are likely to contain bacterial biofilms in the porous plastic substrate. These biofilms generate new growth, which reacts with the tissues of the wood to plug sap vessels. Research into the use of food-grade alcohol as a taphole sanitizer is incomplete at this date, but in the future this may be a way to retard microorganism growth. Having new or very clean tubing downstream from the taphole can also help reduce taphole infection. When tubing is older, dirty and/or contains contaminated sap, backward flow of sap into the taphole, which can occur during a freeze or when vacuum is interrupted in the line, can infect tapholes with microorganisms. Research has shown that check-valve spouts reduce this infection by preventing backflow.

Once sap leaves the tree, the objective is to keep it as cool as possible until it is boiled. Do spout color, tubing color and tubing size have some influence on sap temperature? Spout color (the common materials with tubing are black nylon, clear polycarbonate or stainless steel) may influence both taphole and sap temperature, although research has not yet demonstrated differences, and spout temperature is strongly influenced by the temperature of the tree trunk. Drop line and lateral line tubing can be obtained in shades from white to dark blue; it would seem logical that lighter colors would heat more slowly. Mainline is also available in shades from gray to black. While many producers prefer dark mainline, especially on cold slopes in cool weather where ice can prevent flow to the sugarhouse for much of the day, in hot weather black is likely to be a disadvantage. Painting the top of the mainline white can serve to reflect the sun; however, in addition to this being a very inconvenient task if wire ties are facing up, it has been reported that some oil-based paints are incompatible with some mainline materials and may cause the plastic to deteriorate quickly. The paint may also make it impossible to get a good vacuum seal when attaching a lateral line saddle.

Tubing size can make a big difference in sap temperature. Bob White, of Underhill, Vt., who has a warm, southwest facing sugar bush, used an infrared thermometer to measure temperatures in tubing of various colors and sizes. He reported that during March 2012, sap was overheating less because of tubing color and more because of too little sap in certain mainlines. His original intention was to carry mainline to the most distant corners of his sugar bush; however, he found situations where a small trickle of sap from a few taps was being greatly heated in a large tube, and now strives to reduce mainline length and keep a greater volume of sap in his mainlines. One of the advantages of a dual conductor, or wet line/dry line, system is that the wet line is designed to be nearly full of sap, since it doesn't carry vacuum, and thus the sap can arrive cooler at the tank.

As warm syrup arrives at the sap tank and sits waiting to be boiled, it deteriorates as microorganism activity splits some of the sucrose into glucose and fructose. In addition, some sugar is consumed by the activity of these organisms. Two technologies may help keep sap from further spoiling. Some producers claim that a UV light system designed for maple can make a significant difference in reducing bacteria. However, these units are expensive, and not every UV light has been shown to be worth the trouble. Another technology is the heat exchanger, a method (and possibly also the equipment) borrowed from the dairy industry. The idea is to pass the sap through a heat exchanger, which is connected to a cool water source such as a spring or deep pond, prior to it entering the tank. In either case, it is still important to process the sap into syrup as soon as possible.

When the sap runs for a long time between freezes, the sugar content usually declines noticeably. Using reverse osmosis (RO) to remove most of the water in the sap is essential for reducing boiling time, particularly with sap that is low in sweetness, and thus RO has become an essential adaptation for warm-weather sugaring. When sap is extremely warm, as it was during mid to late March 2012, it becomes so laden with bacteria, yeasts and molds that it is capable of damaging RO components. Nick Atherton, RO expert at CDL USA in St. Albans, Vt., reported that a number of producers plugged their RO membranes irreversibly by trying to push severely damaged sap through them. To recover the flow before the damage is irreversible, a time-consuming process of membrane cleaning needs to be followed and may include several cleaners. Producers in a hurry may regret skipping these steps. RO sap filters are not capable of removing heavy microorganism loads prior to the sap reaching the expensive membranes. Atherton recommends dumping questionable sap in very warm weather rather than hoping it can somehow be converted into salable syrup.

Boiling sap in warm weather often leads to excessive foaming. This can be difficult to control, especially in organically certified operations where organic vegetable oils must be used as defoamers. Joel Boutin, extension specialist from Quebec, recommends turning down the burner (on an oil-fired evaporator) to reduce foaming in these situations, however, this increases the amount of time needed to make syrup.

The end result of sugaring in very warm weather may be syrup that is not table grade. Syrup that is made from sap with a large proportion of invert sugars (glucose and fructose) is often too dark for a table grade, although it may have a perfectly acceptable and desirable taste. Proposed changes in the international grading system for syrup may soon allow this syrup to be called grade A, with a strong taste, recommended for cooking. A fair percentage of the syrup made in and after the hot weather of 2012 was, however, off-flavored and cannot be sold as table grade. "Buddy" was the most common off-flavor in 2012, as maple buds in many areas started to swell in late March, causing any syrup made after this point to retain an unpleasant flavor. It is critical that sugar makers recognize this off-flavor and not try to put damaged syrup in retail containers. This off-flavor cannot be diluted by blending it with a large amount of good-tasting syrup; this would result in more off-flavored syrup. Instead it is sold at a discount as "commercial." According to Bruce Bascom, of Bascom Maple Farms in New Hampshire, there is usually a strong demand for this lower-cost syrup among the manufacturers of foods like bacon and cereal, who want to add a small amount of maple syrup to their product, but add so little that the taste is largely determined by other ingredients. Bruce continues to buy large quantities of this syrup, but recommends that sugar makers not make ropy, unfilterable syrup from heat-spoiled sap in the hopes of getting some return on their labor - he just doesn't want this syrup at any price.

Because sugaring takes place in what is traditionally late winter weather, sugar makers are usually familiar with methods and materials to aid in processing cold or frozen sap: thawing lines, clearing ice from releasers, using a heat exchanger to heat sap, etc. Sap is well preserved in cold weather, and many of the issues presented above do not arise in a typical sugaring season, except in the final few days. We can only hope that coming seasons will be on the cool side and that the extreme heat of 2012 was an anomaly, but more than ever, sugar makers need to be prepared for both sides of the equation as the climate continues to change.

The author is maple specialist with University of Vermont Extension and Proctor Maple Research Center in Underhill Center, Vt.