During my research into honey bee nutrition, I kept running into statements about the crude protein content of different pollen types. According to a University of Florida Extension Bulletin, the crude protein content of pollen ranges from 6% to 30% of the dry weight, depending on the floral source. Furthermore, the article claims that larval and newly emerged honey bees require a diet containing 20% to 25% crude protein.
Then the question came to me: what the heck is crude protein? How is it different from regular old protein?
According to various dictionaries, “crude” can mean unrefined, lacking finish or polish, lacking culture or refinement, rudimentary, natural, raw, or blunt.
But crude protein isn’t any of those. Instead, it is an estimated protein level. The estimate is derived from a chemical analysis of the amount of nitrogen in the pollen. The amount of nitrogen is then multiplied by a constant (usually 6.25-6.38) which yields an estimate of total protein.
But nitrogen is also found in other parts of the pollen grain, not just the protein. This other nitrogen throws off the estimate and creates the difference between true protein and crude protein. Fortunately, there usually isn’t too much of this other nitrogen, so the estimates are fairly good.
However, when it comes to nutrition, not all protein is usable by honey bees. Protein is made up of amino acids. There are many different protein-building amino acids, but only ten of them are needed by honey bees. The ten they need are: threonine, valine, methionine, leucine, iso-leucine, phenylalanine, lysine, histidine, arginine, and tryptophan. The others are known as non-essential.
The problem with the crude protein estimate is that it includes nitrogen from all sources including the non-essential amino acids. So, when evaluating pollen as a food source, you need to know it’s amino acid profile, not just the crude protein level.
Dandelion pollen, for example, can have a crude protein content of 22.69 (Jivan, A. et. al. 2011) but it is deficient in four of the essential amino acids: arginine, isoleucine, leucine, and valine.
The bottom line is that honey bees need a variety of pollen types in their diet to assure they get all the essential amino acids. Crude protein is an estimate of protein content, but it certainly does not tell the whole story.