As ethanol production continually increases in North America, the volume of the by-product distillers grains also continues to increase.
Of the distillers grains produced in North America, 90 percent is going into animal feeds with 80 percent of that going into ruminant diets. With such a great deal of distillers going into animal diets, public concern has arisen over the safety of the product.
A recent survey done by the FDA further fueled concern when they announced that 53 percent of domestic distillers grains sampled had antibiotic residues.
A major concern with the use of distillers grains in food animal diets is that, if active, these residues could potentially lead to the development of bacterial resistance in both humans and animals. In addition, active antibiotic residues could be detrimental to the animals consuming them and the humans consuming tissue from those animals.
Distillers grains are primarily produced through dry or wet milling. However, ethanol producers are continually working to create new distillers grain varieties in order to meet the needs of the feed animal producer.
When producing distillers grains through dry milling, the whole grain is milled and used for ethanol production. Wet milling, on the other hand, separates out all of the grain components and only uses the starch for ethanol production. The remaining portion of the grain is used to create a variety of other marketable products.
With each method, the grain or the starch is added to water to create a mash. The mash is heated and enzymes are added to the mixture which allows the starch to breakdown and form glucose.
Next, the mash is fermented. During fermentation, yeast is added to the mixture. The yeast converts the glucose to alcohol. However, during this step bacteria can compete with the yeast, utilizing the glucose to grow. This in turn reduces the amount of ethanol produced from the fermentation process.
In order to combat these competing bacteria, antibiotics are added to the fermentation process. Antibiotics can kill the unwanted bacteria during the fermentation process allowing the yeast to utilize all of the glucose.
Once fermentation is complete and all of the glucose has been converted to alcohol, the alcohol is separated from the water and solids through a distillation column. In the distillation column, heat and other separation methods remove all water from the alcohol producing 100 percent pure ethanol.
After distillation all that remains are the solid (wet cake) and liquid (thin silage) components of silage. The silage is made up of protein, fiber, oil, and non-fermented starch at a concentration three-fold greater than that in the original corn. Additionally, antibiotic residues from the fermentation will be present.
In order to separate the liquid portion of the silage from the solid portion, centrifugation is used. The wet portion, or thin silage, is evaporated further to produce a thick syrup called condensed solubles. The condensed solubles can then be added to the wet cake to produce a variety of products.
The condensed solubles can be mixed back with partially dried wet cake to create 65 percent moisture distillers wet grains with solubles (DWGS), or they can be dried to create distillers dried solubles (DDS). If fed without condensed solubles, the partially dried wet cake is known as distillers wet grains (DWG).
Distillers wet grains can be dried further to produce dried distillers grains (DDG). The most common practice is to dry distillers wet grains with solubles to 10 to 12 percent moisture creating distillers dried grains with solubles (DDGS). Additionally, modified distillers grains with solubles (MDGS) can be produced by creating 50 percent moisture wet cake with condensed solubles.
Antibiotics have been used in fuel ethanol production for decades in order to improve production efficiency by reducing levels of contaminating bacteria present during the fermentation of ethanol, allowing for a more productive fermentation.
There are two major concerns with the use of antibiotics in ethanol. The first concern is the potential for bacteria to develop resistance, rendering antibiotics useless against them. The second concern is the potential for antibiotic residues to end up in animal feeds and potentially animal tissues used for human consumption.
Antibiotic resistance develops in bacteria during ethanol production due to the misuse of antibiotics. This includes antibiotic overdosing when no effect is observed and underdosing when efficient control is observed.
Overdosing can lead to an increased chance that antibiotics will not be inactivated during the distillation process. This would result in their presence in distillers grains.
Consumption of distillers grains containing antibiotic residue by animals could result in humans consuming those animals and in turn acquire resistance to the antibiotics used in ethanol production.
In addition, overdosing affects the rate of fermentation. Underdosing, on the other hand, leads to a lack of effectiveness of antibiotics. It also carries a greater risk of causing resistance, as bacteria can become accustomed to the antibiotic in low doses and thus develop resistance mechanisms.
It is the responsibility of the FDA Center for Veterinary Medicine (FDA/CVM) to approve the use of drugs in animal feeds, as well as monitor and establish limits for feed contaminants. The use of antibiotics in ethanol production could result in antibiotic residues in the distillers grains by-product. Thus, antibiotic residues in distillers grains used as feed or feed ingredients are considered feed additives and regulated by the FDA.
Currently the FDA has several concerns with the use of antibiotics in the fuel ethanol industry. Their primary concern is that antibiotic residues may be present in distillers grains and that those residues could be transferred to animal tissue upon ingestion.
However, it is unlikely that antibiotic residues in distillers grains would transfer into animal tissue due to the low levels present in distillers grains, as well the pharmacology of the antibiotics used in ethanol production.
The second concern expressed by the FDA is the potential harm to humans who eat tissues containing antibiotic residues. The FDA is specifically concerned with the probability that consumption of contaminated animal tissues could result in antibiotic resistance in humans.
Many countries currently place maximum residue limits in foodstuffs of animal origin to combat this problem. Finally, the FDA expressed concern for the health of animals fed distillers grains containing antibiotic residues.
The only antibiotic currently approved for use in ethanol production is virginiamycin. The FDA/CVM issued a letter of no objection for the use of virginiamycin in the fermentation phase of alcohol production at 2 to 6 ppm. In addition, the CVM did not object to potential residues of 0.2 to 0.5 ppm in distillers by-products. This statement was based on an animal diet containing no more than 20 percent distillers grains. Moreover, it was stated that the FDA/CVM is unlikely to take regulatory action against dried distillers grains-containing feed with residual levels of virginiamycin below 0.5 ppm. Levels below 0.5 ppm pose no concern to broilers, turkeys, swine, or cattle consuming the feed, nor to the humans consuming food derived from those animals.
Although antibiotic residues have been confirmed in distillers grains by the FDA, the activity level of those residues has never been tested. It is possible that the residues present have been inactivated by the ethanol production process, as the antibiotics are exposed to high and low temperatures and a wide pH range.
Further studies will help to confirm the potential implications of feeding distillers grains with antibiotic residues.
For more information regarding beef feedlot nutrition or any other beef-related topic, please visit the U of M Beef Team website at: http://www.extension.umn.edu/beef.