If you look at the top row, the numbers indicate how many carbon atoms are in the oil, a colon then the number of double bonds. The numbers in the middle of the chart are the percentage composition of the particular fatty acid found in the oils listed on the left.
Each type of fatty acid has a different freezing point, a different tendency to polymerize (dry into a film), and a different energy content.
Fatty acids containing double bonds are called unsaturated. The double bonding site is somewhat unstable and can break off or be chemically altered in the presence of heat and water. Unsaturated fats tend to spoil faster than fats without double bonds (saturated). If an oil contains too many double bonded sites the oil becomes a “drying oil”
Drying oils are not good for making biodiesel as they break down quickly. They usually contain three double bonds per fatty acid. Just to give you an idea, they are used in paints and varnish and dry quickly to form a tough film. Drying oils will age and turn acidic quickly, sometimes the biodiesel made from drying oils will degrade overnight. Examples of drying oils include Linseed oil, Walnut oil, and Poppy oil.
Cooking oils actually make the best biodiesel. These are unsaturated oils with a single double bond per fatty acid. Olive oil is probably the best oil for making biodiesel, since it ages slowly, but remains liquid to low temperatures, and contains a great deal of energy. Canola is another good cooking oil that contains mostly fatty acids with a single double bond. It has better cold weather properties than olive oil because most of the fatty acids have fewer carbons on average than Olive oil. Canola also has a little less energy than Olive oil, so fuel mileage would suffer, but you could continue to run B100 from canola at much lower temperatures than you could olive oil.
Hydrogenated oils are oils that have been chemically altered to remove the double bonding. This is done to lengthen the shelf life of the oil. While hydrogenated oils are used for cooking oils, they are unhealthy oils for our bodies. In addition it is difficult to make biodiesel from hydrogenated oils. Notice a trend here? The best cooking oils make the best biodiesel. There is a persistent myth on the internet that Chinese restaurants have the best oils for biodiesel, that’s true if they use unsaturated, non-hydrogenated oils. Look for restaurants that sell healthy foods and you’ll find the best oils. Not only do they use the better oils, but they are more likely to be lower in FFA than other restaurants due to their cooking practices.
Cooking practices play a vital role in WVO quality, primarily on FFA content. FFA or Free Fatty Acids are formed when the long carbon chain breaks away from the glycerin molecule. These fatty acids are acidic in nature and will turn to soap when using our normal base catalyst processing method. They can only be turned into biodiesel if we use Acid Esterification. The soap made from oils high in FFA can cause problems in processing. To make matters worse, FFA are hygroscopic, meaning they attract water. Oils high in FFA can contain as much as 50% water.
If the restaurant cooks at too high a temperature, the oil breaks down quickly and forms FFA. This is pretty simple, high heat makes FFA. They need to keep the oil temperature below 300F for healthy food and good biodiesel.
If the restaurant filters their oil every night before closing, the oil will last much longer. This is due to the fact that the breading and stuff that can be filtered out absorbs water from the atmosphere when the oil is cool, the water then reacts with the oil to make more FFA. Filtering has been shown to keep FFA levels below 1ml titrations for up to a week of heavy use. Without filtering the same oil can reach titration levels of 10 or 15 ml in just a few days.
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