Maple syrup farmers, or sugarmakers, use the term “sweetening the pan” at the start of the season. The ‘pan’ represents the maple evaporator which is used to boil the maple sap over the fire. The ‘sweetening’ happens during the year's first boiling of the raw maple sap.
Maple syrup evaporators may seem like mysterious machines made of stainless steel pans, dividers, flues, valves, levers, and floats. But they have one primary function: boil off water, and turn the watery maple sap (about 2% sugar) into thick maple syrup (66.9% sugar).
Evaporators are designed in such a way that raw sap enters one end, and maple syrup comes out the other end. The water in the maple sap is constantly evaporating, leaving behind the delicious sugars. As the water leaves, more raw sap is continually added so that the level of sap in the evaporator always stays the same. As the diluted sap enters from one end, the result is that a gradient of sugar concentration is developed across the pan. Each successive stage in the evaporator is slightly sweeter than the one before it.
To understand “sweetening the pan”, think of the first sap run in the early spring. After the sap is collected, the entire evaporator is filled with raw maple sap that is only about 2% sugar. It is nowhere near ready to become maple syrup, so we need to boil it for a long time to evaporate the water.
As the steam evaporates, more sap is continually added in order to keep the level of the sap constant. This keeps the sap at the start of the process at 2% sugar content. However, the sugar concentration at the end of the process keeps increasing until it eventually reaches 66.9%. At that point in the process, the gradient of the the pan ranges from raw sap at one end and finished syrup at the other end. The pan is then considered ‘sweetened’.
When the day’s sap supply is all boiled off, the evaporator is left just as it is: filled up to the same level with partially boiled sap. When the next sap run happens, the fire is started and the process begins right where it left off. At that point, the pan is already sweetened, there’s an established gradient of sugar content, and the amount of time that it takes to draw off more finished maple syrup is relatively short.
One downside to having a small operation is that our sugarhouses are typically unheated. That means that when we leave the evaporator full of sap, we need to watch the temperatures carefully. When they drop below freezing - or well below zero - then we need to start small fires to keep the sap from freezing and bursting seams in the evaporator. Minus ten degrees Fahrenheit can make for a cold morning to check on the equipment!