to measure the sugar
concentration of sap and syrup
you ever looked at sunlight passing through a glass of clear water?
What does it look like? Does it look different if you fill the same
glass with iced tea? With orange juice? With chocolate milk? As you
have probably noticed, sunlight passes through different solutions
(liquids like water and milk) in different ways.
Now imagine two
glasses of water. A pinch of sugar is placed in one glass, and a whole
tablespoon of sugar is placed in the other. Both glasses are mixed
until the sugar is completely dissolved, and then they are placed
in a sunny windowsill.
While our eyes
do not notice it, light passes through each glass in a slightly different
manner - like what you noticed with the iced tea and milk. This is
because the sugar concentration of the water causes light to "refract,"
or scatter when it passes through the water solution. The glass with
more sugar would cause the sunlight to refract more than the glass
with only a pinch of sugar.
can determine sugar concentrations by measuring how light is refracted
in a sugar solution. Maple sap and syrup sugar concentrations can
be measured with these types of refractometers.
producers want to measure the sugar content of sap (1-6% sugar), syrup
(66-67%), and concentrated sap (between 6 and 66%), refractometers
are available with several different scales. Some refractometers can
measure the low concentrations of sugar present in fresh sap, while
others measure the higher concentrations found in syrup.
scales are 1) 0-36%, 2) 28-55%, 3) 54-70%, and 4) 68-92%, but the
scales may vary. A refractometer with the low scale would be useful
for measuring sap, while a higher scale would be necessary for syrup
Once you have
obtained a refractometer, you may be interested in measuring the sugar
content of sap from maple trees in your yard or schoolyard, or you
may want to measure the sugar content of maple syrup you have purchased
or made yourself. You may even be interested in participating in the
Cornell Sugar Maple
Tree Improvement Program.
can be obtained from most maple syrup supply companies, and vary from
$100 to over $300. Some companies to consider include Waterloo/Small
or Dominion and Grimm.
about the scale you might have at home in your bathroom. Before getting
on it, do you check to see if it is pointing to zero? If it is not
at zero (for example, if it is pointing at 4 pounds), you would probably
turn the knob to adjust it. When you adjust a bathroom scale to read
zero when nothing is on it, you are calibrating it. A refractometer
needs to be calibrated in the same way.
To calibrate a
refractometer, you will use a drop of water containing no sugar. As
with the scale, you will adjust the refractometer to read 0 when it
is measuring the water sample. The refractometer should read 0 for
a water sample because there is no sugar in the sample - much like
the scale should read 0 when nobody is standing on it.
To calibrate your
refractometer, place a drop of water (preferably distilled) on the
dark circular or rectangular area and close the cover. A shadow or
dark area is visible on the scale inside the eyepiece. Turn the calibration
screw until the shadow falls on the zero mark. Open the refractometer
cover and dry the cover and glass prism using soft tissue paper or
a cotton cloth.
optical components of a refractometer change slightly at different temperatures,
so it must be calibrated as temperatures change. You should check the
calibration of your refractometer (see if it reads 0 for water) after
every sap or syrup sample. Once it is stable (you have not needed to
adjust it for several samples), you will only need to check the calibration
after every fifteenth reading, or after each cleaning. If you are waiting
a long time before making measurements (more than a couple of minutes),
you should calibrate before and after each sample.
If in doubt, check
the calibration. Your values will be meaningless if your refractometer
is not properly calibrated.
are now ready to measure the sugar concentration of a sap or syrup
sample. Only one drop is needed for a sample, though it is important
that the sample is very fresh. Old samples (for example, a droplet
that has been hanging on a spile for several minutes) will have lost
water to evaporation, so their refractometer readings will be inaccurate.
Place a drop of
sap on the refractometer, close the cover, and quickly read the scale
(the line at the top of the darker area). Readings should be taken
to the nearest 0.1 percent. The refractometer should be dried with
tissue paper and rinsed with water after each reading. If you are
working rapidly, dry the refractometer after each measurement, and
rinse and dry it after 10 to 15 measurements.
While many refractometers
auto adjust for the sap or syrup temperature, some may come with a
chart that requires mathematical adjustments. Check the documents
that come with your refractometer to see whether you need to adjust