There are hundreds of compounds that comprise hog odors — but only a few appear to be the “bad guys” responsible for most of the stink.

Jacek Koziel figures he is halfway through developing a catalog of all the complex compounds that are contained in hog manure. He has amassed data on 200 compounds, and figures that number will double before he's through.

Even so, he's already confident he's identified the worst offenders that come from a select few chemicals embedded in manure compounds.

The engineer's confidence stems from the unique $130,000 instrument that resides at Iowa State University's Atmospheric Air Quality Laboratory in Ames. The multi-dimensional-gas-chromatography mass-spectrometry-olfactometry (MDGC-MS-O) instrument breaks down the gases and chemicals in livestock and poultry odors. ISU owns the only instrument of its kind in the country devoted to identifying animal-feeding odors. The instrument is normally used to capture the pleasant smells of cosmetics or the distinctive aromas given off by coffee, beer or tobacco products.

“With this multi-dimensional capability, we can isolate and identify chemicals that are causing the greatest amount of odor, using separation and spectrometry technology. On the one hand, the instrument separates and identifies gases that are in the air, and on the other hand, we are coupling it with olfactometry analysis,” says Koziel, who works with ISU's Department of Agricultural and Biosystems Engineering, focusing on air quality and livestock odor research.

For olfactometry, three graduate students and Koziel take turns at stationing themselves at the instrument's sniffing port to try and label, in layman's terms, what a hog odor smells like. They then match it to a compound the instrument has identified.

Samples are also sent to ISU agricultural engineer Steve Hoff's olfactometry lab, where trained odor-sniffing panels provide their perceptions.

To obtain air samples, a novel air sampling technology known as solid-phase microextraction (SPME) is employed.

“This technology allows researchers to sample very low concentrations of odorous gases,” observes Koziel. SPME is being used in the laboratory and in the field. ISU researchers use SPME to evaluate the effectiveness of odor control technologies, air sampling inside swine barns and downwind from hog operations. Odor samples collected on SPME are analyzed using the MDGC-MS-O system.

The Bad Guys of Odor

Since the instrument arrived at ISU last December, it has pinpointed a few “needles in the chemical haystack” of hog odors that stand out as truly offensive to most people, says Koziel.

At the top of the odor list are: para-cresol (smells like a barnyard); 4-ethyl phenol (smells like roadkill); and hydrogen sulfide (smells like rotten eggs).

A second tier of the worst offenders includes gases that emit smells similar to wet feed, old socks or body odor, mothballs and old leather shoes.

Dust Carries Odor, Too

“What we have found out is that the smallest dust particles have the capacity to carry the most odor, because they are made up of ‘fine dust’ that contains mainly dried feces,” explains Koziel.

Larger, “coarse dust” particles carry dried feces, but also transport dander or hog skin, feed, insects, etc.

Hog odor compounds are “sticky” chemicals, facilitating transport by dust particles. The majority can travel short distances, but only the smallest, most odorous compounds remain intact for long distances, he says.

The ability of these compounds to also adhere readily to people and building materials is a chemical phenomenon of adsorption. When odor particles cling to people's skin and hair, one shower may not be enough to totally dislodge them.

Koziel says odorous chemicals can become permanently adsorbed into a variety of equipment and building materials in a hog barn.

Developing Solutions

Because there are really only a few compounds that appear to be the cause of most hog odors, Koziel is confident that a multi-disciplinary approach will produce viable solutions.

As it stands, biofilters have been very successful at changing the intensity of offensive odors. Koziel learned from fellow agricultural engineer Steve Hoff about the mobility of hog odors via dust. “If you can control dust emissions, you can go a long way toward controlling odors in animal-feeding operations,” he notes.

Using diet, combined with environmental and engineering modifications, Koziel believes the problem of hog odors can be minimized.

Besides his current work, he also plans to revisit testing of manure additives. Previous work only analyzed their impact on manure. He hopes to expand that research by using the new, sensitive instrument to analyze gas production, and then sniff those gases to determine if odor control is improved with the use of the additives.