Frequently Asked Questions

The following information is intended to help answer basic questions frequently asked about producing maple syrup and about the Minnesota Maple Syrup Producers Association. It is not intended to provide everything needed to make quality maple syrup. There are many printed and on-line resources that can provide additional information and more detailed answers to your questions. Better yet, become a member of the MMSPA and get your answers first hand.

Minnesota Maple Syrup Producers' Association - Reliable information on Maple - Tapping Maple Trees - Gathering/Storing Sap - Evaporating Sap / Finishing the Syrup - Filtering Finished Syrup - Bottling - Licensing / Selling to Public - End of Season / General

Minnesota Maple Syrup Producers' Association

Q. How do I become a member of MMSPA and what are the benefits?

1. Join us! Membership is affordable, high benefit, and fun. Membership application forms (online and printable) are available on this site mmspa-membership. MMSPA welcomes hobbyist and back yard producers, and commercial/licensed producers alike; and maple equipment suppliers and dealers, value-added processors, buyers, historians and supporters of high quality Minnesota pure maple syrup. The price of a single membership is inexpensive ($25 annually), but the information, sugar house tours, friends and contacts are “priceless”. MMSPA members pride themselves in learning from each other and sharing information among all members, beginning or experienced. Members also receive the quarterly MN Maple News newsletter, a subscription to the North American Maple Digest, member alerts and email tidbits. Meetings are held twice a year, in May after the season and in early October. Meetings are rotated around the state and include tours of member maple operations and other local attractions.

Reliable Information on Maple

Q. What’s a good source of maple information?

1. The North American Maple Syrup Producers Manual, 3rd Edition is a fantastic resource.  It can be downloaded for free.  Many MMSPA members also order the spiral bound hard copy version for sugarhouse  reference.  Several universities in the maple producing area (Great Lakes, St. Lawrence River, and Appalachian regions) have maple extension and research programs with excellent websites.  Vermont, New Hampshire, New York (Cornell), Ohio State, and Wisconsin are a good start.  State maple producer associations also post helpful insights and information.  And many maple supply and equipment dealers host open houses and educational workshops.

Tapping Maple Trees

Q. I tapped a few trees in my yard. The sap started to flow, but then stopped. Did I do something wrong?

1. Even during the “sugaring season”, it is not unusual for sap to flow for a few days, then stop for a few days, in response to weather patterns.  You likely did nothing wrong, just wait for temperatures to get into that freeze-thaw cycle again.

Q. How do I know when to start tapping?

1. .  If you use just a few taps to produce pure maple syrup, you can watch the weather forecast to determine when a stretch of warm (45-50 degree F) weather will arrive.  Sap runs when day time temperatures reach above freezing and then go back below freezing at night.  If above freezing temperatures aren’t reached til later in the day, little sap will flow on that day.  Ideal flow seems to occur when overnight lows are 28-29 degrees F and warm quickly after dawn.  Sunny days seem to also help pressurize trees for greater sap flow, and barometric pressure can be a factor too.  In southern Minnesota the beginning of the season is around the 1st of March.  In northern Minnesota the season often begins after the 15th of March.  The season typically lasts about six weeks, ranging from four to eight weeks.  Some producers place pilot taps in a select tree or two, and when those start running they put in the rest of the taps.  Large producers with lots of taps generally start tapping before the season, often in February, so that all of their taps can be in place when the season starts.  Trees start to heal the tap holes as soon as they are drilled, so there’s a trade off between getting all the taps out early in order to catch every drop of the season, versus having the taps at their freshest just as the larger runs of the season start up (the first couple runs may be small).  But small runs are sometimes the most delicate gold of the season.  The first warm spell usually doesn’t start the season.  The start of the season usually occurs when water starts working its way into the root system.  During the first warm spell(s) the snow is oftentimes just recrystallizing rather than melting into the ground.

Q. How big does a maple tree need to be before it can be tapped?

1. The North American Maple Syrup Producers Manual recommends tapping trees no less than 10 inches in diameter at chest height.  Use one tap per tree until they are at least 18 inches in diameter, then two taps per tree can be ok.  Tapping more than two taps per tree is no longer recommended regardless of size.  Research has shown that the added sap from a third tap is not that much in comparison to potential affect to tree health; so it is better to maintain tree health with just two taps than to get just that little more sap.

Q. Can you use the same tap hole every year? How about tapping the same tree year after year?

1. Once a tree is large enough to tap, and if the tree remains healthy, the same tree can be used for many, many years.  Tap holes will scar over shortly after the season ends; the scars surround the tap hole and will not conduct sap.  So, next years tap holes need to be offset and not reoccupy the scarred wood.  Best practice is to not “girdle” the tree by tapping in successive years at the exact same height.  Many producers will pick a direction (clockwise or counterclockwise) for these successive tap hole offsets, so that they can keep better track of where the previous years tap holes are located.  By the time they work their way around the tree, the tree has grown enough additional diameter that fresh wood has grown over the old scars.
   1. Only tap maple trees with healthy crowns, and avoid tapping trees that have been recently damaged or show signs of poor health.  Some producers will give the trees a rest after major seasonal events like drought or if there is significant damage from ice storms or deep frost.
   2. Previous tap holes will develop a scar roughly twice as wide as the tap and will be quite long, reaching 6-12 inches above and below the tap hole.  This scar area becomes closed off to the tree when it heals and is called nonproductive wood.  The next seasons’ tap holes should be drilled several inches to the side, and/or above or below the scar length to ensure clean sap wood.
   3. This is a main reason why trees should be 10 inches or more in diameter before tapping.  It takes this much circumference on the tree trunk to support annual tapping and still maintain tree health and growth into the future.
   4. Tap hole wood shavings should be whitish-cream colored.  Brown or dark colored shavings indicate rotted or unproductive wood, and those tap holes should not be used.

Q. How deep do you drill for tapping?

1. Approximately 1 1/2 to 2 inches deep, maybe slightly more if the bark is thick.  The most sap flow comes from the wood closest to the bark.  Past about 2 inches deep doesn’t gain much sap flow and can become detrimental to the tree.  Tap holes should be drilled slightly at an up angle into the tree, so that the tap slants slightly downward to drip.  This is so that sap doesn’t get trapped and spoil, or freeze in the tap hole and potentially crack the bark.  Use a sharp bit when drilling tap holes.  A sharp bit cuts wood cells, leaving the flow channels open.  A dull bit will rub the wood as it cuts, smearing and partially closing off the cellular channels.

Q. How hard do you pound the tap in?

1. You want the tap to be snug and firm, but not pounded so hard that the bark cracks or splits.  The purpose of the tap is to get the sap away from the bark so that it can drip into a container or flow into a tube.  Taps are usually tapered so that a snug fit can be achieved with moderate tapping of a hammer or rubber mallet to seat the tap.  Snug enough to hold up a bag or pail, and not enough to crack the bark, that’s about right.

Q. I’ve heard that in the old days some producers put peroxide or even formaldehyde in tap holes to “sterilize”.  Is that recommended?

1. No!  It is illegal to use these kind of solutions in tap holes.  Research has shown that it is actually harmful to trees and makes much larger scars in trees, resulting in much more nonproductive wood, and introduces dangerous materials into the sap.  Tap holes will be clean enough when drilled.  Do NOT introduce anything but the tap into the tap hole.  Don’t blow in the hole to clean it out, as that just introduces bacteria from your mouth into the tap hole.  As long as shavings don’t interfere with the seal, they’re not a problem.  If need be, a problem shaving at the lip of the tap can be removed with a clean stick or wire.  And don’t “clean” the tap holes at the end of the season, just pull the taps and be done.  The trees are much better at healing the hole when left alone.

Gathering/Storing Sap

Q. Can I use milk jugs to collect sap?  How about detergent containers or used buckets?

1. Many small and back yard producers use various food grade containers to collect sap.  Plastic milk containers can work for a hobbyist, but on big run days it is not uncommon to get 2-4 gallons of sap from a tap and a lot of sap can be lost to overflow.
   1. Only use food grade containers, and only use containers that do not have an odor, as this will transfer to the sap and be concentrated in the syrup.  Containers that held pickles, sour kraut, cheese or other strong flavors should be avoided.
   2. Do not use any soldered pails and perhaps avoid metal containers altogether, as some solders contain lead that can be transferred to the sap and concentrated in the syrup.
   3. Sap should be kept cold and boiled as soon after collection as possible.  Keep collected sap in the shade, and cooled by snow or other means.  The fresher the sap, the more tasty the syrup.  When it comes to maple sap, if it ain’t boiling it’s spoiling!
   4. Never use containers that held chemicals or petroleum products.
   5. Only store sap in food grade containers.

Q. What is Reverse Osmosis and is it right for me?

1. Many people are familiar with under-sink reverse osmosis systems that remove contaminants from drinking water.  In maple syrup making, the clean water is set aside for later washing and rinsing of equipment, and the other liquid stream, the concentrated sap, is retained for syrup making.  Reverse Osmosis can separate out 1/2 to 2/3 of the water from sap at the hobby level, and some large scale producers with much more industrial Reverse Osmosis systems can achieve 3/4 to 7/8 water removal from sap.  Using Reverse Osmosis can yield significant savings in fuel and labor, since all that separated water does not have to be boiled off during evaporation.  Interestingly, if you concentrated sap all the way to syrup by Reverse Osmosis, without any boiling, you would end up with a clear, sugary syrup with little taste other than sweet.  Boiling, and the chemical reactions that occur during boiling are an important part of maple syrup flavor development.  There is a lower point at which Reverse Osmosis becomes impractical for the back yard hobbyist.  With even the smallest hobby Reverse Osmosis units, some 20-25 gallons of super-clean Reverse Osmosis water or distilled water will be needed for cleaning and rinsing of the unit.  In practice, deploying the Reverse Osmosis unit for sap batches smaller than 40-50 gallons becomes counter-productive, due to time investment for set up and cleaning, and the need to purchase distilled water to complete the cleaning and rinsing.  There is a lot of information online and from dealers regarding Reverse Osmosis systems, options and performance.

Evaporating Sap / Finishing the Syrup

Q. What are the best fuels to use for evaporating maple sap?

1. Most small producers initially boil their maple sap over a wood-fueled fire for several hours in a pan especially fashioned for boiling sap. Concentrated sap is typically removed from the wood fire before it is “done,” and then “finished” over a gas stove where the heat can be more effectively controlled (i.e. shut off) when the syrup is “done.” Large commercial producers with thousands of taps may use LP gas or fuel oil to fire their large capacity evaporators.  Equipment manufacturers are constantly introducing new technology to make modern evaporators more efficient.
   To make and maintain a rapid boil in the evaporating pan, most producers burn wood, fuel oil, propane, or natural gas.  Some industry research and development is working toward steam and electrical heat sources as a means to increase efficiency and reduce greenhouse gas emissions.  Many producers will use one fuel type for the main boiling work, and then transfer near-syrup to a finishing pan on a gas burner or stove where finer control of the heat source is possible.  For larger operations, heat recovery from the steam released from boiling, through heat exchangers, is a significant source of energy for pre-heating sap.

Q. How will I know when my syrup is “done”?

1. Syrup is done when it reaches the proper concentration of sugar and other solids.  Fresh raw sap typically runs about 2-3% sugar and finished syrup is in the range 66.0% to 68.9% Brix.  Brix is essentially percent by weight.  Maple syrup by weight is 1/3 water and 2/3 sugar/solids.  In cooking and baking recipes, maple syrup substitutes by volume; one cup of maple syrup represents about 3/4 cup sugar plus 1/4 cup liquid.  But back to the original question, “When is my syrup ‘done’?”  There are several tools and techniques that can help a syrup maker determine when syrup reaches the desired concentration.
   1. Before modern density instruments were devised, visual techniques like “sheeting” and “aproning” were used to check concentration progress.  Early in the boiling process the liquid will behave just like water, and will drip off a spoon or spatula like water.  As the liquid becomes thicker it will start to come off the spoon less as a drip and more as a thin sheet.  With experience, a syrup maker can tell when the liquid should be transferred to a finishing pot or pan, and maybe even can tell when syrup is finished.  The bubbles in the boiling liquid will also become smaller and more foamy-looking as the liquid gets thicker.  Be mindful whenever coming toward the end of a boiling stint that the amount of water in the concentrate is becoming less and less at a faster and faster rate, so don’t become so distracted with your density measurements that you fail to watch what’s happening in the pan.  Reducing the heat as the liquid approaches syrup concentration is prudent, so that liquid always remains in contact with the bottom of the pan for heat transfer.  Too little liquid contact with the bottom of the pan will allow the pan to overheat and burn/scorch the sugars, resulting in an off flavor in the syrup or even a wrecked boiling pan.
   2. Some producers watch the temperature of the boiling liquid to determine when syrup is “done”.  Maple syrup at correct density boils at 7-8 degrees F above the boiling point of water, or 219-220 degrees F, if you are at sea level and at standard barometric pressure of 29.2 mm Hg.  Reliance on only a thermometer for determining when syrup is “done” is a difficult task.  An elevation change to 500 feet above sea level will lower the boiling point of water by about 1/2 degree F, compared to the boiling point at sea level.  Weather will have an hourly influence on water boiling point.  Reliance on this method for final finishing requires first measuring the temperature of rapidly boiling water with a precise thermometer calibrated with 1/4 degree F marks or better, and then measuring the temperature of the near-syrup.  Digital kitchen thermometers can be very helpful too.
   3. A very reliable and not overly expensive tool used by many producers is a syrup hydrometer, which is specifically calibrated to measure syrup density by floating the hydrometer in syrup, either hot (211 degrees F) or cold (60 degrees F).  By repeatedly testing the liquid as it concentrates, the density of the liquid can be monitored until the desired density is reached.
   4. Refractometers, which measure the refraction of light between syrup and air, are also used by many producers and competition judges for exact density determinations.  Refractometers as optical instruments can be either manual, or digital.

Filtering Finished Syrup

Q. I washed my cloth/Orlon filters after last season and now I’m having trouble filtering, and my syrup has a bad taste.  What might be the problem?

1.  When washing filters, it is important NOT to wring them out.  The wringing tends to break the fibers that form the filter, resulting in some particle leakage through the filter(s).  That’s hard to believe, and most producers learn the hard way that it is actually true, and need to replace filters after having some fine sediment or a film of sugar sand form on the bottom of  containers after bottling.  After use, filters should be washed by rinsing thoroughly in hot water, or better yet immersing in boiling water to remove any sugar and sugar sand particles, and then rinsing in hot water.  Filters should be allowed to thoroughly dry before storing to avoid any chance for mold or mildew formation that would lead to a musty or moldy off flavor.  Maple syrup is very good at picking up the flavor(s) of things it comes in contact with.

Q. What is wrong with my syrup?  It looks very cloudy?

1. Cloudiness usually occurs in unfiltered or partially filtered syrup.  The cloudiness comes from suspended sugar sand.  Gravity filtering using pre-filters and Orlon bags is a simple method for filtering.  Filter presses are a more advanced method of filtering using pressure and filtering materials to produce crystal clear syrup.  Occasionally, slight cloudiness after filter pressing may occur if the filtering materials slip through the filter press.  Some producers let sugar sand sediment settle after placing the syrup in a stock pot or pan.  After a couple of days, the clear syrup can be decanted off.  This last method tends to waste some good syrup that is still slurried with the sediment, and longer exposure to the sugar sand can impart a slight bite or metallic taste to the syrup.
   1. In some seasons, sugar sand/niter is much more pronounced, resulting in filtering challenges, particularly later in the season when lower sugar content requires more sap and longer boiling times to make a given volume of syrup.  Sugar sand is mostly made up of calcium malate, the calcium salt of malic acid.  Malic acid is the main organic acid in maple sap and is the least soluble component of the sap.  Malic acid is important for flavor, but the excess precipitates as the calcium salt.  Malic acid is the same organic acid that gives tartness to apples, and some chefs like to use malic acid in cooking instead of citric acid from citrus fruits, because the malic acid adds a more complex flavor to foods.
   2. Sugar sand/niter is not harmful, but most consumers do not like the chalky or gritty mouth feel that comes with the sediment, or the appearance of cloudiness or bottom sediment in the container.

Bottling

Q. I heat up my syrup before I bottle it but I’m getting a lot of spoilage. What can I do?

1. To become maple syrup, sap needs to be concentrated to within the range 66.0 to 68.9% sugar/solids (Brix) content.  Below this level the liquid is considered concentrated sap, and subject to mold or ferment due to excess water content.  If you aren’t concentrating to at least 66% Brix then you aren’t producing maple syrup, and it will not be shelf stable.  Syrup in excess of 68.9% Brix is sugar oversaturated and that excess sugar will likely crystallize to the sides of the container during storage.  See the North American Maple Syrup Producers Manual for additional insights.

   1. There are many recommendations out there about proper temperature for bottling syrup.  Generally, recommendations are to bottle at 180-185 degrees Fahrenheit, high enough for hot packing, but just below the point where additional sugar sand/niter would develop (requiring additional filtering).  In a finishing pan, the temperature next to the heat source can be higher than in the rest of the pan, and excess temperature there may induce additional sugar sand/niter to precipitate.  Some producers fight this kind of situation by using some method of stirring the syrup during heating, in order to avoid having too-hot syrup develop near the heat source, and to gain consistent temperature throughout the finishing pan.
   2. Once caps are on the bottles/containers and tightly sealed, lay the containers on their sides for 30 seconds to a minute so that the hot syrup comes in contact with the inside of the cap and also sterilizes the neck of the bottle/container.
   3. After opening, containers of maple syrup should be stored in the refrigerator.

Licensing / Selling to Public

Q. I love making maple syrup. I add taps every year, and I now have more than my family and friends can use. I think I’d like to sell to the public. Do I need a state license to do that? How do I obtain a license?

1. Generally speaking, a license is required to sell maple syrup to the public.  However, there are exceptions if syrup is produced exclusively from trees on land you own or control, and if no “off farm” ingredients, such as flavorings, or sap from other producers are added.  Another exemption can be found in Minnesota’s Cottage Food Laws, which allow small producers to sell directly to consumers, under certain conditions.  Pure maple syrup is a premium product that is expensive to buy, and expensive to make.  Minnesota Pure Maple Syrup is a high quality product, and potential sellers of maple syrup should carefully consider whether they can routinely produce high quality syrup.  Maple syrup is also a food, and needs to be carefully guarded as such, to uphold Minnesota’s reputation for quality maple syrup.  To look into Minnesota’s food licensing requirements, cottage food laws, or product-of-farm exemptions, start by checking here: Cottage Food Laws, and Commercial License, and for product-of-farm exemptions scroll down to item #30 in the section Food Types Allowed.  Carefully read through Minnesota Department of Agriculture and Department of Health guidance to determine which route is best for your situation.  To find your local Minnesota Department of Agriculture food inspector call 651-201-6062.  MMSPA cannot provide specific guidance, as every individual situation is unique.

Q. What is the required information on labels for maple syrup produced and sold in Minnesota to the public?

1. See Minnesota Department of Agriculture (MDA) food-labeling-requirements.  This MDA web page also contains a section specific to maple syrup that describes specific requirements for label and text sizes, and describes required content for the principal and information display panels.   In general, labels must include 1) the identity of the product (i.e. pure maple syrup), 2) the net quantity of contents (i.e. 8 ounce, 12 ounce, etc.), 3) the ingredient list - including major food allergens, and 4) the business name and address.  If health or nutritional claims are made on a food label, the product is subject to special nutritional labeling.  Many maple syrup suppliers sell plastic bottles having preprinted nutrition facts, or have rolls of labels that list maple syrup nutritional facts.  Use of the respective USDA maple syrup grades and classes is optional (not required) for maple syrup produced and sold in Minnesota.  However, if grades and/or classes are displayed on the label, the contained syrups must meet the USDA minimum standards established for each grade and class displayed.

End of Season / General

Q. I made maple syrup for the first time this year. I tapped, gathered, boiled and now I am ready to wrap things up. What do I do about the holes in the trees? Do I need to plug them with something? How do I get the sap to stop coming out?

1. The sap will stop flowing on its own as the temperatures get warmer and the trees make leaves.  As soon as you drill the tap hole, the tree begins to heal over (a reason to not tap too early before the season starts).  The tree and bacteria will do that job, you don’t need to plug or do anything to the holes.  Let them heal by themselves!  Be sure to remove all taps and clean, sterilize, dry, and store all equipment, and then enjoy dreaming of next spring!

Q. I have used up my canning jars and I’d like to get real maple syrup bottles for next year. I also want to get some better equipment. Where do I get these things?

1. Several MMSPA members are equipment dealers.  See the member directory.  Many maple equipment dealers also sell supplies like bottles, decorative materials, and signs.  Many other sources can be found by searching online.  With the large increase in back yard syruping in the past few years, some hardware and supply stores, even regional chains, have begun stocking simple supplies for back yard producers.  Join MMSPA and talk to other producers and ask where they obtain their supplies.

Q. What is the recommended way to clean my taps, buckets, etc. at the end of the season? How much rinsing is recommended?

1. End of season cleaning is VERY important!  It’s a lot easier to clean things in the warmer weather of April and May at the end of the season than it is in February and March at the beginning of the next season!  And there’s usually a lot less elbow grease needed to do cleaning sooner than later.
   1. Most equipment surfaces that touch maple sap or syrup can be washed with hot water, or boiled in hot water.  Mild dish soap (unscented) can be used on hard surfaces and equipment (but not on filter materials!!) and these should be rinsed thoroughly and dried thoroughly before storage.
   2. Remember that maple syrup is very good at picking up any flavor it comes in contact with.
   3. Cleaning recommendations for tubing vary and are evolving.  Consult the North American Maple Syrup Producers Manual, University extension websites, and equipment dealer websites for the most current advice and recommendations. 

Q. How about cleaning my evaporator pan?

1. In the case of polished stainless steel pans, special care should be taken not to scratch the inside surface of the pan with an abrasive.  Some producers soak the pan in hot water and vinegar or use special commercial pan cleaner solutions available from equipment suppliers.  Dispose of cleaning solutions according to manufacturers recommendations, and be sure to make a complete and thorough rinse of any equipment exposed to these solutions.  The underside of evaporator pans also should be cleaned to remove soot which otherwise impedes the transfer of heat when boiling.

Q. I just want to build a basic "back yard" maple sap cooking set-up. Where can I learn how to build a small "cooker"? How do I locate needed equipment?

1. The North American Maple Syrup Producers Manual describes maple evaporator set ups ranging from basic to advanced.  Maple syrup equipment dealers handle a range of needs from small hobbyists to large commercial producers, and are good contacts.  Key aspects are safety, weight, and energy efficiency.  Several state universities in the maple-producing area (Great Lakes, St. Lawrence River, and Appalachian regions of US and Canada) have extensive maple programs and websites.  There are also many good books on back yard maple syrup making.

Q. I hear about grades of syrup, but I don’t know what that means. How do I know what grade my syrup is, and does it matter?

1. Grade A Maple syrup is graded by Color, Clarity, Density and Flavor.  Presuming that clarity and density are adequate, color and flavor become the determining factors for maple syrup grade.  Maple flavor intensity is closely correlated with color, so color (defined as percent light transmittance, %Tc) is typically used to determine grade.  It is possible for syrup color and syrup flavor to not match, particularly in blended maple syrups.  A darker flavored, lighter colored syrup would best be labeled as the darker grade.
   1. The International Maple Syrup Institute (IMSI) developed the current standard classifications for grades of maple syrup, which are endorsed by the North American Maple Syrup Council (NAMSC).  The US Department of Agriculture (USDA) and state regulators have adopted these standards.  Grade A maple syrup can be sold at retail and has four classes (GoldenDelicate, Amber/Rich, Dark/Robust, and Very Dark/Strong).  Syrup that has clarity, off flavor or other damage that keeps it from being sold as Grade A is now referred to as “Processing Grade”.  If that syrup still has fairly good maple taste and can be used in food manufacturing, it is Processing Grade.  This grade of syrup must be sold in containers no smaller than five gallons in size.  Syrup failing to meet processing grade is called substandard.  There is no longer a “Grade B” maple syrup in the marketplace, having been replaced by the newer Grade A “Very Dark/Strong” class.
   2. Generally speaking, grading of syrup is required in Minnesota only if your customers want it graded, or if you are selling syrup outside of Minnesota, or if you are entering contests and competitions.  However, if you claim on a label that your syrup is a certain grade and class, then it must meet the USDA requirements for that grade and class.
   3. Color grading can be accomplished by use of a grading kit or digital photometer.  Most grading kits consist of pre-filled colored sample bottles, or colored reference slides.  Digital photometers are typically small handheld instruments that accept a small sample vial.  In both cases, reference samples are compared to your sample.
   4. See the North American Maple Syrup Producers Manual, or the maple syrup grading article at the University of New Hampshire website - maple grading article.

Q. My syrup has a bad smell and/or a bitter taste. What gives?

1. There can be many reasons for bad smells and bad tastes or “off flavors” in maple syrup.  Some are “natural”.  Others can be avoided by better practices.  Most often, off flavors occur when:
   1. You’ve waited too long to boil down your sap.  Sap can spoil and become sour if it is not cooked when “fresh” and kept cold in storage.  Given time and warmth, bacteria will eat the sugars in the sap, leaving a sour byproduct and reducing the sugar content.  The resulting syrup will also be darker in color.  Potential for bacterial spoilage increases dramatically as the season progresses.
   2. You may be using unclean sap collecting and storage equipment, which makes for another source of introducing sugar-eating bacteria.
   3. You may have cleaned equipment without adequate rinsing, leaving a chemical or detergent taste on the equipment, that gets transferred to the maple sap/syrup that comes in contact.  You can never “over-rinse” your equipment after cleaning.
   4. You may have used a storage container that was not food grade or clean enough.  Pickle buckets and similar containers tend to yield pickle-flavored syrup regardless of cleaning efforts!
   5. You may have tapped an unhealthy tree or off-colored wood in a tree, which can introduce earthy-musty-mushroomy flavors.  A good tap hole will yield cream-colored shavings, not brown, when drilled.
   6. You may be using an unclean boiling pan.  Sugar sand/niter can build up as a scale on the bottom of a boiling pan, allowing for a bitter, scorched or metallic flavor to develop.
   7. See the University of Vermont flavor and off flavor summary at: Map of Maple, or the North American Maple Syrup Producers Manual.
   8. Toward the end of the season, the trees will start to add other organic compounds to the sap in preparation for leaf-making.  This happens about when the leaf buds begin to swell.  The sap will start to become discolored, and/or take on a bark-like taste or odor.  Maple syrup makers describe the bitter, sometimes chocolaty flavor that results as “Buddy”.