Biofuel co-products as livestock feed - Opportunities and challenges

155Chapter 8Utilization of crude glycerin in beef cattleJ.S. DrouillardKansas State University, Department of Animal Sciences and Industry, Manhattan, Kansas 66506-1600, United States of America.E-mail: jdrouill@ksu.eduABSTRACTIncreased world demand for renewable fuels has prompted rapid expansion of the biodiesel industry, in whichanimal fats and plant oils are converted to combustible fuels and significant quantities of an edible byproduct,glycerin. Crude glycerin, which is approximately 75 to 85 percent glycerol, also contains water, minerals, and smallamounts of residual methanol. Crude glycerin has a variety of applications in livestock feeding. Given its humectantproperties, it is effective in agglomerating small feed particles, thereby reducing dust and maintaining homogeneityof mixed feeds. As a pelleting aid, it decreases energy expenditure associated with pelleting and improvesdurability and hygienic quality of pelleted feeds. In ruminants, the impact of glycerin on ruminal fermentation iswell-documented. Acetate:propionate ratio generally decreases with addition of glycerin, presumably improvingenergetic efficiency. Inhibitory effects of glycerin on the fermentative activities of fibre digesting bacteria and fungiare evident, providing a plausible explanation for the decreases in fibre digestion often observed in vitro and in vivo.The deleterious effects of glycerin on fibre digestion are most evident when animals are fed diets containing higherlevels of starch. It is apparent that the gut ecosystem adapts to the presence of glycerin, though it is not clear ifthis is a consequence of long-term population shifts or changes in capacity for substrate utilization by individualspecies of micro-organisms. Prior exposure to glycerin does enhance its utilization in cattle fed grain-based diets.Moreover, populations of pathogenic E. coli O157:H7 have been partially suppressed in the presence of glycerinwhen fed at low levels in the diet. Crude glycerin is a viable energy source for cattle, particularly when fed at lessthan 10 percent of the diet dry matter.INTRODUCTIONGlycerin (glycerol) can be derived through production ofalkyl esters (biodiesel) from plant oils or animal fats. Of thethree processes available for alkyl ester production – oilconversion to fatty acids followed by acid-catalysed esterification;direct acid-catalysed esterification with methanol;and base-catalysed trans esterification with methanol – thebase-catalysed esterification is most economical, and thereforethe most frequently employed process for biodieselproduction (Van Gerpen, 2005). In base-catalysed esterification,fats and oils are reacted with methanol in the presenceof potassium hydroxide, yielding glycerin (Figure 1) and alkylesters. Residual methanol is reclaimed via distillation, andglycerin is recovered through evaporation following removalof methyl esters. Each 100 kg of oil or fat yields approximately10 kg of glycerin (National Biodiesel Board, 2008).Historically, glycerin has had a broad range of applicationsin human foods and pharma ceuticals, and has beenused industrially for production of synthetic polymers, cosmeticsand personal care products. It can be modified toyield mono- and diglycerides, which are important classesof emulsifying agents. Glycerin is a sweet (~60 percent thesweetness of sucrose), viscous liquid that has been usedin beverages as a thickening agent, and exploited in foodsystems as a result of its humectant properties (SDA, 1990).This latter attribute makes it attractive as an addition to animalfeeds for texturing properties and dust control. Photo 1illustrates the effect of glycerin when added at 12 percentof the diet dry matter in a typical feedlot ration. Levels of4 percent or more are relatively effective in aggregatingsmall feed particles, thus reducing dust and fines. In its pureform, glycerin is colourless; however, the colour of crudeglycerin ranges from light amber to deep brown, and differencesare largely attributable to varying concentrations ofFIGURE 1Chemical structure of glycerin (glycerol)H 2CHCH 2COHOHOH