Where does vanilla flavor in maple syrup come from?
Maple syrup is 100% pure with nothing added. Vanillin, responsible for the smell of vanilla, has to come out of the tree, transform in the sap before boiling, or develop while the sap boils down into syrup. If we know where it comes from, are there choices we can make to increase vanillin in maple syrup?
Lignin, a complex polymer in wood and bark, is the raw material for vanillin. It flows in the sap out of the maple trees. Lignin degrades into vanillin though oxidation. So where does the oxygen come from?
My grandfather's maple syrup had wonderful vanilla notes. In his time, sap dripped into a bucket. Each drop increased oxygen in the sap. The buckets were dumped into a tank on a sled pulled by horses. The tank was emptied into a larger tank at the sugarhouse. Every time the sap was dumped into a new tank or it sloshed during transport, it increased dissolved oxygen in the sap. In the 1970s he installed tubing to transport sap, but he kept trees close to the sugarhouse on buckets for the grandchildren to carry.
Today, most sugaring operations use extensive tubing networks to transport sap from the trees to large stainless steel tanks. With the tubes, there are fewer opportunities to aerate the sap. That mean less vanillin and less vanilla aroma.
There is more to dissolved oxygen than vanillin. Microbes and yeasts are naturally present in the sap and drop out of the air. Those microbes transform sucrose the sap into glucose and fructose and transform many other compounds, too. In fact, maple syrup can contain over 200 chemical compounds, many of which we can smell and taste, and are created after the sap leaves the tree and before it is boiled. Those processes consume oxygen in the sap. When the oxygen is consumed, anaerobic bacteria start to flourish. Anaerobic bacteria produce unpleasant flavors.
Modern sap collection systems generally have fewer opportunities to aerate the sap than traditional sap collection in buckets, so this is a risk that larger sugaring operations have ignored.
In 2022, we added vacuum to increase the amount of sap we collected. To avoid the risk of anaerobic bacteria and to test whether we could increase vanillin, we installed an aeration system. There was some research on aeration to avoid off-flavors in late season syrup, but none on early and mid-season syrup. We had reasonable hypotheses, but we were stepping off the map when it comes to the science and practice of making maple syrup.
With aeration, we expected more vanilla aroma and darker syrup because of more fermentation. Both happened. We also got unexpected results. There were more floral and fruit aromas in the syrup - coconut, rose, peach, apple. They were strong enough that everyone could sense them, not just people with exceptional palates.
That part of the test was a success. There also were some downsides. We had much more organic material in the sap that had to be filtered out of the syrup. In retrospect, that is not a surprise. But it meant cleaning the filters many more times than normal.
Next season, we will have a testing protocol for measure dissolved oxygen and will cycle the aerator only when dissolved oxygen drops below a certain point. Hopefully we will reduce the need to change filters so often. We will also measure pH and temperature so we can keep making award winning syrup with classic flavors and turn up some of the unexpected flavors that make it so delightful.