Summer 1998
Only the nose knows? Not for long
by Tammy Freund, extension communications intern
It
used to be that the only way to verify how something smelled-be it good,
bad, or maybe even ugly-was to ask someone else to take a whiff.
That's generally a safe technique, unless you're dealing with legal issues such as livestock odor and separation distances in Iowa.
Enter olfactometers and electronic noses, bits of technology still under development. Electronic noses use sensors to detect specific gases. Their potential uses extend beyond measuring livestock odors.
The electronic nose has the potential to be a "quality control device," said Earl Hammond, ISU professor in food science and human nutrition. "The electronic nose will give an objective, consistent result that can be used to predict how people will react." An electronic nose, for example, can be used as a research tool, assessing aroma to develop a uniform food product.
The trick to making an electronic nose represent a human nose is finding the correct combination of gases and relative concentrations.
"The electronic nose has sensing probes that when exposed to volatile compounds in the atmosphere are absorbed on the sensors, changing electrical properties to give a signal," Hammond said. "Using a variety of probes, one can correlate a response of sensors with various aromas."
Inside a livestock confinement building, the odors of concern are primarily ammonia and hydrogen sulfide; however, outside the building volatile organic acids are the big issue.
"There are a couple hundred different volatile organic acids in swine buildings. These are the ones that both carry the scent, and stick to your clothes," said Jeff Lorimor, ISU Extension agricultural and biosystems engineer.
Currently, olfactometry is the method used to measure odor concentration. "Odorous air captured from buildings, pits, or fields is collected in odorless tedlar or PVC bags and brought to the labs for analysis," Lorimor said.
In the lab, the air is processed through the olfactometer and "sniff panel" (composed of four people) to determine its detection threshold. The four panelists determine only whether or not they can detect the odor, rather than judging if it is weak or strong, or good or bad. They smell mixtures of the odorous air from the cylinders, diluted by specific amounts of fresh air. The dilution rates are adjusted until the panel members detect the odor—that dilution ratio is the odor threshold. The higher the threshold, the more odorous the compound is.
"The concentration of the mixture is increased a step at a time as less and less fresh air is added. At each concentration level, panelists sniff three ports: two have fresh air; one has odorous mixture," Lorimor said. The panelists must indicate which port they think the odor is in without communicating with each other during the tests. If they do not indicate the correct port, the odor concentration is increased, and they smell it again. When enough of the panelists indicate the correct port, the test terminates and a statistical program is used to calculate the odor threshold. The greater the threshold number, the greater the raw odor. For instance, a threshold of 1,000:1 is 10 times as strong as 100:1 threshold.
"A lot of work is being done to put a number on odors in order to get an objective result," Lorimor said. "Current research has created a recipe of chemicals that smell like hog manure to the human nose." If this recipe can be programmed into an electronic nose the variation from the subjectivity in the human sniff panel might be removed. As of now, however, olfactometry, and the use of human noses, is the best method available to determine odor concentrations.
