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CAROL DAVIDSON COOKING PANCAKES at The National Maple
Syrup Festival.
Osmosis is the natural diffusion, without pressure,
of a solvent through a semipermeable membrane from a
solution with low solute concentration to a solution with
high solute concentration. On the other hand, reverse
osmosis uses high pressure to force a solvent from an area
of high solute concentration through a membrane to an
area of low solute concentration. It was achieved in 1959
when a UCLA professor, Samuel Yuster, and two of his
students, Sidney Loeb and Srinivasa Sourirajan, trying to
find a way to extract pure water from salt water, produced
a synthetic membrane from cellulose acetate polymer. The
membrane rejected the salt, but let the water pass. The
operating pressures of the membrane ranged from 800 to
1,000 lb/in! for sea water and 200 to 400 lb/in! for brackish
water. Today reverse osmosis is the most efficient way of
ridding both salt and fresh water of impurities. And, as
maple syrup producers have found out, an efficient way of
capturing them.
Straight from the tree, sap has a sugar content
of 2 percent; to increase the content to 67 percent (or, to
turn sap “into thick, sweet syrup that piques the palate”),
producers must reduce the water. “Reverse osmosis,”
Burton says, “is a process we do with the sap before it even
gets to the evaporator. What we do is discard the water and
capture the impurities – we want the impurities, the sugar.
What happens when you filter the sap is you go from
2 percent sugar content to 4 percent; then if you filter it
again, you go from 4 percent to 8 percent. Some producers
are even going from 8 percent to 16 percent before it goes
to the evaporator.”
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DRAWING OFF A BUCKET OF 100% PURE MAPLE SYRUP.
According to Dr. Timothy Perkins, director of
the Proctor Maple Research Center at the University of
Vermont, reverse osmosis uses pump-generated pressure
to push water through a membrane in a high-pressure
chamber. “The sugar in sap can’t pass through the pores
in the membrane, so it’s retained and concentrated;
however, the water does pass through the membrane and
is discarded or used for washing or rinsing cycles.” Perkins
says reverse osmosis pressure vessels generally work under
a maximum pressure of 200 to 300 psi, though that can vary
greatly depending on the vessel and membrane.
Reverse osmosis as a viable method for the
concentration of sugar in sap was first tested in 1966 at
the USDA Maple Research Laboratory. The first reverse
osmosis system was installed in 1971; by the early eighties,
a total of about 100 machines were operating in Canada
and the US.
Something producers need to be wary of
when using reverse osmosis is the contamination of the
membrane by microorganisms in the sap. Because sap
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