The maple season is now underway and this is a good time to talk about handling your sap during and after collection. How you handle your sap prior to boiling will strongly affect the quality of the syrup you make. When quality syrup is the goal, timing is everything, and the clock starts as soon as the sap leaves the tree and doesn’t stop until it hits the evaporator.
When sap comes from the tree, it is sterile. That all changes once the sap starts to drain from the taphole. The air and surfaces surrounding the tap contain an abundance of microbes. The sap supplies the food source and a media for the microbes to grow and multiply. Research at Center Acer in Quebec found 21 different strains of microbes present in sap. At first you would think that could be problematic, but the reality is, you need certain strains of bacteria to produce the color and flavor that is unique to maple syrup. For microbial growth you also need the right temperature. Once the environment warms the sap, microbes multiply rapidly. Producers can monitor the potential for microbial growth by checking the temperature of sap. If the temperature is close to freezing, growth is suppressed. Below 40 degrees Fahrenheit, the growth of bacteria is slow, but once the temperature rises above 50 Fahrenheit microbial growth is rapid. The chances for 50 degrees and above temperatures are greatest at the end of the season.
When sap leaves the tree, the sugar is 100% sucrose. Once the sap is exposed to bacterial action, a small fraction of the sucrose is converted into glucose and fructose, often referred to as “invert sugars.” When maple sap containing sucrose, glucose, and fructose is heated, you create an amber color and a unique maple flavor. The problem is when undesirable bacteria begins to outnumber the good bacteria. This changes the chemistry of the sap. As the invert sugar level increases, syrup begins to take on a darker color and a stronger maple flavor. This produces the different grades of syrup. Syrup early in the season has a light color and very mild flavor. The maple syrup produced at the end of the season is often darker and stronger flavor. Syrup containing higher levels of bacteria can develop a very strong almost bitter off-taste known as sour syrup. The syrup consistency takes on a thick almost rubber like appearance and is often referred to as ropey syrup. Sour sap is often confused with buddy syrup because it happens most often at the end of the season. Buddy syrup is caused by sap coming from trees where the buds are getting ready to bloom. The chemistry is completely different from sour sap. Sour sap can happen any time during the season when a warm spell causes extreme flushes of bacteria growth. Sour sap can be prevented with good sanitation practices. Buddy syrup is a natural occurrence every year at the end of the season.
The quality of syrup produced from buckets and bags is best early in the season. Once the hole is drilled and the spout is exposed to the air, microbial development and taphole healing begins. Your season has begun, and you are now on the clock. A normal season for a bucket, bag or gravity tubing producer is 4 to 6 weeks. During the cold periods early in the season, the sap stays fresh just like it would if you put it in your refrigerator. Keep your sap below 40 degrees Fahrenheit and you are fine, but let it heat up to over 50 degrees and you asking for trouble. That happens readily at the end of the season. What many producers forget is that the bucket is an incubator for bacteria if it is not cleaned out regularly throughout the season. Leaving sap sit in a dirty bucket for any length of time is a problem. Remember bacteria does not grow in a clean dry bucket. If you are in a warm spell wash out your buckets and place them upside down next to the tree. If you are in a extended cold period, you should collect your buckets and let them hang until the next run. And never let stale sap sit a bucket, hot or cold.
As for tubing, we have discussed tubing sanitation multiple times over the years and those articles are in the Ohio Maple Blog Archive. Keep your lines as clean as possible throughout the season. This is difficult unless you are on continuous high vacuum. I know it sounds expensive to run the pumps 24/7, but it works to your advantage by keeping the lines cool and dry when the sap is not running. Another essential is to follow the tubing sanitation guidelines, installing new spouts every year, and new tees and drops every three years. You will improve the quality of your syrup.
Once you get the sap to sugarhouse, there are additional things you can do to improve quality. Sap that is going to be stored for longer periods of time needs to be stored in a stainless steel tank. Avoid poly tanks for sap storage. Plastic tanks are incubators for bacteria. Older galvanized tanks, like galvanized buckets, need to be discarded because of the risk of lead contamination. For the backyard producer, make sure your tank is in the shade. Pack around it with snow if possible. You can even freeze some sap and put it in the tank during warm spells. What ever it takes to keep your sap cold, take those necessary precautions. Anytime your sap reaches 50 degrees Fahrenheit and you can’t immediately cool it back down, boil immediately.
What about the evaporator? Boil your sap as quickly as possible. If you are using a reverse osmosis machine, make sure you do not let your concentrate sit. Boil it as soon as it comes through the RO. You double, triple, and in some cases, quadruple the sugar concentration in your sap, and bacteria builds fast in concentrated sap. If you are using a small evaporator, it is a good idea to drain and flush your rig. Leaving partially boiled sap on an evaporator in a warm sugarhouse can result in ropey syrup. Once the syrup is filtered get it into a barrel or a container as fast you can. Do not let it sit around. Pack your drums hot and do not open them until you are ready to use them. Do not store syrup drums in a warm building. Move them into the basement where it is cool or package the syrup at 185 degrees Fahrenheit shortly after the season. From the tree to final container, paying attention to details pays big dividends.
Author: Les Ober, Geauga County OSU Extension 
Does spoiled sap just affect syrup quality? I would think all the boiling would
Remove any danger of consuming it.
Hmmm…that’s a legitimate question. First things first – bad sap makes bad syrup, there is no getting around that. But from a health risk perspective, I honestly don’t know. You can eat anything once, right? Best policy if you’re sure it is spoiled, dump it. Even if using in cooking applications, off-flavor of bad syrup will carry into whatever end product you make.