A Primer on Sucrose Solutions
The
purpose of this threepart article on sucrose solutions is to provide information
on a topic that is a bit more complex than most people think. Much
of the published nectar data uses different units of measurement than those
used in books and on websites.
It is often a case of comparing apples and oranges. Hopefully after
reading this primer, you will have a better understanding of this subject.
Part I Basic Information and Calculations; Analysis of the 1 : 4 Solution
Concentration Units
When a solid is dissolved in a liquid to form a solution, the concentration is usually expressed in either of two ways:
(1) Weight of solid per volume of solution
The most common units are: grams (g) of solid/liter of solution
(2) Weight % = 100 x (weight of solid/weight of solution)
Since the weight of solution = weight solid + weight liquid,
Weight % = 100 x weight of solid/(weight of solid + weight of liquid)
The shorthand used to designate a weight % concentration is % (w/w).
In chemistry, the only time that a concentration is expressed as volume %
is when both components are liquids. The shorthand to designate volume
% is % (v/v).
If you mix Liquid A and Liquid B together, then
Liquid A
% (v/v) = 100 x (vol of liquid A/vol of solution)
If we wanted to determine volume % for a solid dissolved in a liquid, the equation would be:
Solid % (v/v) = 100 x (vol of solid/vol of solution)
The reason that chemists never use % (v/v) when working with solids in liquids
is that the characteristics of the solid (coarse powder, superfine powder,
etc.) directly affect how much is contained in a given volume. For
example, if you measure out 1 cup of superfine popcorn salt, it will weigh
about 30% more than 1 cup of kosher salt crystals. When you specify
one cup of a solid in a recipe, you have to be certain that everyone is using the same
material.
The simplest and best way around this issue is to always add solids by weight.
Weight is not affected by the characteristics of the solid and you will always
add the same amount. This is how solids are measured in the manufacture
of drugs.
The Standard Nectar Recipe: 1 Part Sucrose : 4 Parts Water
This is the standard recipe given for hummingbird nectar. The usual
instruction is to mix 1 cup of sucrose (table sugar) with 4 cups of water.
On virtually every website and in many papers, this recipe is incorrectly
called a 20% solution. The type of percentage is almost never designated
(v/v or w/w).
If we take 1 cup sucrose and add it to 4 cups water, what do we get?
We have to determine the volume of the solution in order to calculate
the % (v/v). I have access to a laboratory of calibrated containers
and scales, so I could perform these tests very accurately.
Start with a calibrated measuring container that will hold at least 5 cups
total. After we add 4 cups of water and 1 cup of sucrose, we can measure
the final volume of the solution. When we do this, we find that the
final volume is slightly more than 4.5 cups (4.54 cups to be exact).
Now we can calculate the % (v/v) of our 1 : 4 sucrose solution:
Sucrose % (v/v) = 100 x (1 cup sugar/4.54 cups solution) = 22.0% (v/v)
An important thing to remember is that 1 cup of sugar does not add 1 cup
to the final volume; instead 1 cup of sugar adds only about 0.5 cup.
Most authors incorrectly assume that 1 cup of sugar changes the volume by 1
cup. This
assumption results in significant errors, especially when the concentration is 1 : 3 or higher.
What is the 1 : 4 recipe in % (w/w)? To calculate this, we need to
measure the weight of 1 cup of sugar and the weight of 4 cups of water.
The CRC Handbook of Chemistry and Physics contains the information for water:
1 cup water = 236.17 g (at room temperature); 4 cups = 944.7 g
To measure the weight of sucrose, I took a 5 pound bag of C&H cane sugar
to the lab and made 10 separate measurements using a calibrated 8 oz measuring
cup. The average of the ten measurements:
1 cup sucrose = 206 g
Now we can calculate our 1 : 4 recipe in terms of weight:
Sucrose % (w/w) = 100 x weight sugar/(weight sugar+ weight water)
Sucrose % (w/w) = 100 x 206 g sugar/(206 + 944.7) = 17.9% (w/w)
The percentage of sucrose in our standard 1 : 4 nectar recipe is:
17.9% (w/w) or 22.0% (v/v)
In scientific literature, the concentration of sucrose solutions is always
given as % (w/w) or grams/liter. In order to be consistent, we should always refer
to sucrose solutions in % (w/w) rather than % (v/v). In any case, the
type of percentage, (w/w) or (v/v), should always be included with the concentration.
Part II Analysis of Other Nectar Recipes
There is an accepted range of sucrose concentrations used by most people.
On page 35 of Hummingbirds of North America, author Sheri Williamson writes:
"A solution of four parts water to one part table sugar is simple to prepare,
economical, and remarkably similar to the natural nectar of hummingbirdpollinated
flowers. The ratio need not be precise but should fall between three
and five parts water to one part sugar."
Let's look at the 1 : 5 and 1 : 3 solutions to see how they compare to the standard 1 : 4 recipe.
1 C sucrose : 5 C water
Using the same techniques as described in Part I, when we add 1 cup sucrose
to 5 cups water, the resulting solution has a volume of 5.537 cups.
Using this data and the weight of water and sucrose (from Part I), we can
calculate the % by weight and the % by vol.
% (w/w) = 100 x 206 g sucrose/(206 g + 1180.8) = 14.9% (w/w)
% (v/v) = 100 x (1 cup/5.537 cups) = 18.1% (v/v)
1 C sucrose : 3 C water
Adding 1 cup of sucrose to 3 cups of water results in a volume of 3.539 cups.
% (w/w) = 100 x 206 g sucrose/(206 g +708.5) = 22.5% (w/w)
% (v/v) = 100 x (1 cup/3.539 cups) = 28.3% (v/v)
Now we can create a summary table for our three recipes. I have also added a few other recipes for reference.
Summary Table
C Sucrose :
C Water

Sucrose

% (w/w)

% (v/v)

1.0 : 5.0

14.9

18.1

1.0 : 4.5

16.2

19.9

1.0 : 4.0

17.9

22.0

1.0 : 3.5 
19.9

24.8

1.0 : 3.0

22.5

28.3

1.0 : 2.0

30.4

39.4

1.0 : 1.0

46.6 
64.8

This table can be used to compare flower data with our standard recipes.
Part III A Look at Two Published Papers
The two articles that I am going to examine are:
(1) "Rufous Hummingbird Sucrose Preference: Precision of Selection Varies with Concentration"
CHARLES R. BLEM, LEANN B. BLEM, JOEL FELIX and JENNIFER VAN GELDER
A PDF of this article is available here.
(2) "Nectar Sugar Composition in Relation to Pollination Syndromes in Sinningieae"
MATHIEU PERRET, ALAIN CHAUTEMS, RODOLPHE SPICHIGER, MAURO PEIXOTO and VINCENT SAVOLAINEN
A PDF of this article is available here.
The article on Rufous Hummingbird sucrose preferences has been widely quoted
and discussed. What sucrose concentrations did they really test?
On page 2 of the article in the "Methods" section it states: "All solutions
were made up as mass/volume percentages, where a 40% solution is 40 g of
sucrose in 100 ml of water." We can use this information to calculate the
sucrose concentrations. Here is a summary table:
Rufous Preference Summary Table
Published
% Sucrose

Actual
% (w/w)

"10%"

9.1%

"20%"

16.7%

"30%"

23.1%

"40%"

28.6%

"50%"

33.4%

"60%"

37.5%

"70%"

41.2%

What the authors of this study call "50%" is actually only 33.4% (w/w).
If we wanted to compare what they tested with published nectar data or our
own nectar recipes, we would have to convert the concentration units so that
we were comparing "apples to apples."
I picked the second paper because it is an excellent example of how to properly
report nectar data. It also contains some interesting information on
the different kinds of sugar that are found in flower nectar. The sugar concentration
of the nectar is reported as % (w/w), which in this case is g sugar/g nectar.
The percentages (not concentrations) of sucrose, glucose and fructose are
calculated based on the total weight of sugar. For example,
% Sucrose = 100 x (weight of sucrose/total weight of sugar)
In the discussion, they mention that their result for hummingbird flowers
of 23.9% (w/w) is consistent with several other published studies.
If we wanted to duplicate the sugar content of flowers in our feeders, we
would use a recipe of 1 C sucrose : 3 C water, which results in a sucrose
concentration of 22.5% (w/w). Note that 23.9% (w/w) is very close to
the “30%” solution that was tested in the first paper.