In 2004, the United States consumed 140 billion gallons of gasoline and 40 billion gallons of diesel for on-road transportation (Annual Energy Outlook 2007). In an effort to reduce dependence on petroleum-based fuel products and reduce their negative impact on the environment, production of biofuels from renewable energy sources has experienced explosive growth. Like the rapid expansion of ethanol, biodiesel production has also grown quickly, albeit a smaller volume, to its 2007 level of 450 million gallons.
Biodiesel can be produced by a variety of esterification technologies using vegetable oils, animal fats, and recycled cooking oils as feedstocks. In general, oils and fats are filtered and preprocessed to remove water and contaminants, followed by mixing with an alcohol (usually methanol) and a catalyst (sodium or potassium methylate). Along with heat and mixing, the oil molecules (triglycerides) are broken apart into methyl esters and glycerin, which are then separated from each other and purified. Biodiesel is the name given to these methyl esters with crude glycerin the remaining co-product.
There are presently 171 companies with a combined annual capacity of 2.24 billion gallons that account for the 450 million gallons of biodiesel produced last year. In addition, 63 companies report plants under construction or expanding, which, if realized, would result in an additional 1.23 billion gallons of biodiesel production. With 0.07 gallons of crude glycerin generated for every gallon of biodiesel produced, one could expect approximately 158 million, 32 million, or 89 million gallons of crude glycerin generated from biodiesel capacity, 2007 volume, or production expansion estimates, respectively.
Glycerin has thousands of uses, with new uses continuing to emerge as new technologies are adapted. Refined glycerin is used to moisten, sweeten, and preserve foods; is widely used in drugs and pharmaceuticals; is used as a moisturizing agent or emollient for cosmetics and toiletries; keeps tobacco moist and soft to prevent breaking and crumbling during processing; is used to soften and reduce shrinkage during paper manufacturing; is used to size and soften yarn and fabric; and can be used to produce a renewable propylene glycol. With the expansion in biodiesel production, the U.S. crude glycerin market could be inundated with an ample supply that may become a new energy feedstuff for use in livestock diets.
Metabolism of Glycerol
Intestinal absorption of glycerol has been shown to range from 70 percent to more than 97 percent, likely due to its small molecular weight and it being passively absorbed rather than absorbed through a micelle as noted for medium and long chain fatty acids. Once absorbed, glycerol can be converted to glucose via gluconeogenesis or oxidized for energy production via glycolysis and the citric acid cycle, which under basal conditions can account for 60 percent of the metabolic fate of glycerol.
Caloric Value of Crude Glycerin in Poultry and Swine
Several scientists have reported that glycerin is an acceptable feed ingredient for poultry and swine, and when supplemented into the diet, it generally had no effect on animal performance, carcass composition, or meat quality. Although data assumed the metabolizable energy content of glycerin for monogastrics was 95 percent of its gross energy, data empirically determining the metabolizable energy content in crude glycerin is lacking. As a consequence, several studies were conducted by scientists in the U.S. Department of Agriculture/Agricultural Research Service and Iowa State University to ascertain the metabolizable energy content of crude glycerin in broilers, laying hens, and swine.
In three initial studies, a single source of crude glycerin (containing 86.95 percent glycerin, 0.028 percent methanol, 9.22 percent moisture, and 3.12 percent sodium chloride) was obtained from AGP, Inc., Sergeant Bluff, IA, from a biodiesel production facility using soy oil as its feedstock. Using standard animal experimental methods to determine the metabolizable energy value of a test ingredient, data obtained indicated a metabolizable energy value of this crude glycerin to be 3,434, 3,805, and 3,207 kilocalorie per kilogram (kg) for broilers, laying hens, and swine, respectively. Within each species, these values indicate that crude glycerin has similar energy content as corn and soybean meal, but only 40 to 45 percent the energy noted in animal fats or vegetable oils. This lower caloric value relative to animal fats or oils was expected given the fact that the fatty acid component of the triglyceride has been removed (i.e., methyl ester) during the biodiesel process.
Because biodiesel can be produced from a variety of feedstocks (soy, canola, corn, and waste cooking oils, and animal fats), the composition of crude glycerin can vary, but usually ranges from 78 to 85 percent glycerin, eight to 15 percent water, two to 10 percent sodium chloride, 0.5 percent free fatty acids (although non-acidulated products may be up to 25 percent free fatty acids), and approximately 0.5 percent methanol. It would be expected that a compositional difference in crude glycerin will affect its metabolizable energy value, but it is unknown whether the feedstock source will also affect the metabolizable energy value. Consequently, another series of experiments have been conducted determining the metabolizable energy value of crude glycerin obtained from biodiesel production facilities using soybean oil (six samples), tallow (one sample), yellow grease (one sample acidulated and one sample non-acidulated), and poultry fat (one sample) as their feedstock. In broilers these products are being compared to a refined U.S. Pharmacopoeia (USP) grade glycerin, while in swine these products are additionally being compared to soybean oil and lard. At this time the poultry and swine trials are complete and show a wide variation due to the sample source. Although not statistically analyzed, on average, samples from soybean oil have a similar energy as previously determined and noted above. As the level of residual fat (or fatty acids) increased in some of the crude glycerin samples tested, the metabolizable energy increased, as expected. Lastly, in the swine trial, all of these samples had a lower metabolizable energy level than that determined for soybean oil or lard. In order to delineate whether these metabolizable energy value differences are due to the feedstock source or the composition of the crude glycerin, the crude glycerin samples are being analyzed for glycerin, moisture, sodium chloride, and residual fatty acids.
Crude Glycerin in Swine Diets
An additional sample of crude glycerin was obtained from AGP (84.51 percent glycerin, 0.32 percent methanol, 11.95 percent moisture, and 2.91 percent sodium chloride) for a performance study in swine from eight to 133 kg. Overall, pig performance, carcass composition, or meat quality was not affected by crude glycerin supplementation of five or 10 percent. Because the level of methanol in the sample was above the recommended level, the frequency of lesions associated with methanol toxicity was also evaluated. In this study there were no notable differences in lesion scores due to the level of crude glycerin supplementation, suggesting that levels of methanol in crude glycerin, slightly above that suggested to be safe, did not have a detrimental effect on pig productivity or health.
Methanol levels in crude glycerin warrant special consideration because it is a potentially toxic compound and is not completely recovered from the crude glycerol co-product. Metabolism of methanol results in accumulation of formate, which is the metabolite principally responsible for the toxic effects of methanol. Consequences of methanol poisoning are central nervous system depression, vomiting, severe metabolic acidosis, blindness, and Parkinsonian-like motor disease.
In the United States, no specification for crude glycerin use in animal feed has been published. The Food and Drug Administration (FDA) addresses pure glycerin under Code of Federal Regulation (CFR) 582.1320 as a substance that is generally recognized as safe (GRAS) for general purpose use in animal feed when used in accordance with good manufacturing or feeding practices. With no GRAS regulation or American Association of Feed Control Officials definition listing specifications for crude glycerin use in animal feeds, specifications for pure glycerin defined under USP and Food and Chemical Codex (FCC) specifications are used for guidance.
Methanol levels, however, are not specifically listed in the USP or FCC specifications, such that the FDA has decided to address free methanol levels under CFR 573.640, requiring that levels of methanol in methyl esters of higher fatty acids should not exceed 150 parts per million (0.015 percent) or a level shown to be safe for use in animal diets. Specific toxicological studies on methanol in swine are lacking, but the previous study cited above suggests that levels of methanol slightly above that cited by the FDA have no impact on pig productivity or lesion scores in key methanol metabolizing tissues.
With a metabolizable energy value of crude glycerin being similar to corn and soybean meal, crude glycerin can be used as an excellent source of calories in monogastrics. Although crude glycerin may “look” somewhat similar to vegetable oils, it appears to have only 40 to 45 percent the caloric content as vegetable oils or animal fats. In addition, crude glycerin is a viscous liquid that may present handling, diet mixing, and feed flowability characteristics that are different than other liquid feed ingredients. Additions of crude glycerin up to 10 percent appear to have little impact on animal performance, carcass composition, or meat quality. Lastly, levels of other compounds in crude glycerin (i.e., methanol, sodium chloride, and free fatty acids) should be monitored for potential impacts on feed manufacturing considerations, metabolizable energy value to the animal, and animal performance responses.
References for this review or additional information on this subject is available from the authors: Brian Kerr, (515) 294-0224, firstname.lastname@example.org, or William Dozier, (662) 320-7505, email@example.com.
August 2008 RENDER | back