A Primer on Sucrose Solutions
The purpose of this three-part 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 hummingbird-pollinated 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.