The Renewable Energy Group has released a free Feedstock and Biodiesel Characteristics Report, a fairly comprehensive analysis of – yep, you guessed it! – a whole bunch of animal fats and vegetable oils.
One area they analyzed was the fatty acid profiles of common fats and oils used as biodiesel feedstocks.
If you don’t know already, fatty acid chains are the long skinny carbon chains dangling from the alcohol on a fat or plant oil molecule. In naturally occurring oils the alcohol is glycerol, and holds onto three fatty acid chains. In biodiesel the glycerol has usually been replaced with a methanol, and connects to a single fatty acid chain.
(Note: most biodiesel brewers will have heard of free fatty acids, or FFAs. These are fatty acids that are no longer attached to their alcohol.)
As a reference point, the cetane molecule in diesel is 16 cabons long, and is fully saturated (coated, for lack of a better word) with hydrogen. (Of course the fossilized diesel fuel lacks the alcohol on the end.)
Okay, how to read this chart:
The first number shows how many carbons long the fatty acid is, and the second number tells how many hydrogens it’s missing (in these places the carbon double bonds to itself and the oil is considered unsaturated). If you look, most of these natural fats and oils are 16-18 carbons long, very similar to diesel!
You can guess cold weather flow properties by how unsaturated an oil is. Straight fully-hydrogenated chains pack together tightly, like uncooked spaghetti, and usually make a butter-like solid. Unsaturated chains (missing some hydrogens) have kinks and bends where the carbon double bonds to itself, making a tangly mess like cooked spaghetti. These unsaturated oils tend to flow better at cold temperatures (note: “hydrogenated” oils are unsaturated ones that have been treated to become saturated.)
Although unsaturated oils flow better, they also have slightly less power (less dense), and are less stable. Those double bonds are more susceptible to degradation by oxidation, breaking apart and/or reconnecting into a varnish.