Olfactory.the Science of Smell Issue 2 PDF

7/30/2019 Olfactory.the Science of Smell Issue 2 PDF

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The Science of Smell Part 2:

Odor Chemistry

PM 1963b May 2004

Odor chemistry is complex and still poorly

understood. More than 75 odorous com-pounds, in varying proportions, have beenidentified in livestock manures. Knowing thechemical basis of odors derived from animalmanure is helpful to understand how odordevelops and what can be done to design andmanage manure systems and avoid nuisancecomplaints.

Biochemistry of manure odorGroups of primary odorous compounds

include volatile organic acids, aldehydes,ketones, amines, sulfides, thiols, indoles, andphenols. All of these groups can result fromthe partial decomposition of manure. Ma-nure breakdown is accomplished by a mixedpopulation of anaerobic bacteria, which iscommonly grouped into acid-forming ormethane-producing classes. Acid formers areresponsible for the initial breakdown of com-plex molecules into short-chain compounds,including organic acids. Methane bacteriafurther reduce organic acids to methane andcarbon dioxide.

Figure 1 provides a simple overview of the

breakdown process. The breakdown of pro-tein proceeds to ever-simpler proteoses,peptones, peptides, amino acids and finally,to ammonia and volatile organic acids such asformic, acetic, propionic, and butyric acids.Due to the presence of sulfur in certain aminoacids (sulfur averages about 1 percent of mostproteins), various sulfides and mercaptans canbe expected as a result of protein catabolism.

Carbohydrates in animal waste include sug-

ars, starch, and cellulose. Starch and celluloseare broken into glucose (sugar) units as thefirst step of decomposition. Under anaerobicconditions, sugars are broken into alcohols,aldehydes, ketones, and organic acids. Theseintermediate compounds are odorous and canbe further metabolized and transformed intomethane, carbon dioxide, and water (non-odorous end-products) if conditions allow themethane-producing microorganisms to func-tion.

ProteinCarbohydrate

Complexsubstrate(manure)

Lipid

Peptones, peptides, amino

acids, organic acids, sulfides,

mercaptans, phenols

Fatty acids, alcohols,

acetate, organic acids

Alcohols, aldehydes,

ketones, organic acids

Figure 1. Manure breakdown chain.


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Fats are esters of the tri-hydroxy alcohol calledglycerol. Bacteria use fats as an energy source,hydrolyzing them first to the correspondinglong-chain fatty acids and alcohols. These ac-ids, along with those produced in the deami-nation of amino acids, undergo further break-down in which acetic acid is cleaved from theoriginal acid. Acetic acid is then potentiallyutilized as an energy source, yielding methaneand carbon dioxide as end-products.

Examination of the metabolic pathways for thebreakdown of manure components indicatethat the following components are expected toresult in: organic acids, alcohols, aldehydes,sulfides, simple hydrocarbons, carbon diox-ide, ammonia, and methane. The presence of

this mixture of organic materials and ammoniain an aqueous solution leads to the formationof several other groups, as reaction products.For example, ammonia in water-- a H+ recep-tor -- may be expected to react with acids andalcohols to yield amides and amines. Also,hydrogen sulfide in water may combine withalcohols, aldehydes, and acids to form mer-captans, thiols, and thioacids.

An accumulation of these intermediate metabo-

lites results in an offensive smelling product,whereas containment of intermediate com-pounds for sufficient time allows methaneproducers to act and metabolize most of theodorous compounds into non-odorous meth-ane. Background levels of sulfur in water mayalso be a source of odor.

Physical chemistryAny compound occurring in the atmosphere

must have escaped the liquid phase. Thus,vapor pressure is an important factor which,within specific types of compounds, generallydecreases with increasing molecular weight.

The solubility of a compound in water is an-other important factor in evaluating its signifi-cance as an atmospheric constituent. Insolublegases, such as methane, escape immediately

after being produced, whereas more solublecompounds, such as ammonia, are retainedin solution and can engage in biological andchemical reactions. Solubility of many com-pounds, and hence odor, is markedly influ-enced by the solution pH. Hydrogen sulfide isa particularly good example of the pH effect.Under conditions of high pH, almost no odoris detected whereas under acid conditions, theH+ and HS- ions combine, escape, and pro-duce the typical sulfide odor (H

2S). Ammonia

(NH3) is another good example of pH effect.

The NH3in an acid medium accepts H+ to

produce ammonium (NH4

+) which stays in so-lution and does not volatilize. Even with a pHup to 8, ammonia remains relatively soluble inliquids and little odor is detected. Above a pH

of 9, however, ammonia is rapidly volatilized.

No single compound has been identified asa good predictor of odor sensation acrosssituations in the field. Because of this, humanpanelists conduct odor measurements andquantify odor intensity and unpleasantness.

Odor characterizationBased on psychological tests, seven primaryclasses of olfactory stimulants have been

found to preferentially excite separate olfac-tory cells. These classes are: 1) ethereal, 2)camphoraceous, 3) musky, 4) floral, 5) minty,6) pungent, and 7) putrid. The nervous sys-tem integrates the responses from a number ofcells to determine the identity of the primaryodor stimulus being received. The intensity ofthe perceived odor class is related to the num-ber of receptors bound and the degree of exci-tation of the olfactory cells. Table 1 shows the

variation in concentration needed to produceequivalent odor intensities in the seven class-es. Odor intensity, as referred to in Table 1, isthe strength of the odor sensation as measuredon a psychological reaction scale and is not aconcentration. Complex odors result from theconcurrent stimulation of two or more typesof receptors. This implies that a single chemi-cal can occupy more than one receptor site.


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The use of the seven primary odor classes iswidely cited. However, it is unlikely that thislist actually represents the true primary sensa-tions of smell. More than 50 single substanceshave been identified in odor blindness stud-ies, suggesting that there may be 50 or moresensations of smell.

A more flexible way of presenting the pri-mary odors to clarify the idea of complexodors is through the use of Henning's odorprism (Fig. 2). Six primary odors are locatedat the corners of the prism. All other odorsare mixtures of the primary odors and located

on the surfaces and edges of the prism. Thus,odors consisting of two components would be

represented on the edges of the prism, three-component mixtures occupy the triangularsurfaces, and four-component mixtures oc-cupy the square surfaces.

Odor interactionsUsually, an odorous stimulus is a combinationof many scents. This is certainly the case inanimal production facilities. The effect of oneodor on another may be related to differencesin the water solubility of the two odors result-ing in a number of possible outcomes. Flow-ery, fruity odorants tend to have higher mo-lecular weights. Aldehydes, esters, alcohols,ethers, halogens, phenols and ketones havemore pleasant aromas than the lower mo-lecular-weight carboxylic acids, nitrogenous

compounds (not associated with oxygen), andsulfur-containing compounds. Blending of thetwo odors may occur, producing an odor withproperties of both the original and propertiesunique to the newly-developed odors.

One odor may dominate another, or at leastperiodically, or the two odors may be smelledconcurrently as individual odors. The complexnature of how odorants interact with eachother is the primary challenge in determining

how best to prevent odor formation. However,understanding that manure odors form as theresult of incomplete breakdown of excretedproducts, and that many of these productsare the result of excess protein in the diet canserve as the basis for odor management.

ResourcesThis publication, along with PM 1963a,Science of Smell Part 1: Odor perception and

physiological response;PM 1963c, Science of Smell Part 3: Odordetection and measurement (after 9/1/04)PM 1963d, Science of Smell Part 4: Principlesof Odor Control (after 9/1/04)can be found on the Air Quality and Animal

Agriculture Web page at:http://www.extension.iastate.edu/airquality.

Table 1. Concentrations of the seven primaryodor classes required to produce equal odorintensity.

Odor Compound Concentration

(ppm)

Ethereal Ethylene Dichlor 800Camphoraceous 1,8 Cineole 10Musky Pentadecanlacton 1Floral Phenylethylmethyl 300

ethylcarbinolMinty Methone 6Pungent Formic acid 50,000Putrid Dimethyl disulfide 0.1

Figure 2. Hennings odor prism.

Putrid

Burned

Spicy

Fragrant

Resinous

Ethereal


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. .. and justice for allThe U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin,gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to al l programs.) Manymaterials can be made available in al ternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture.Stanley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.

ReferencesPowers-Schilling, W.J. 1995. Olfaction:chemical and psychological considerations.Proc. of Nuisance Concerns in AnimalManagement: Odor and Flies Conference,Gainesville, Florida, March 21-22.

Table and figures from Water EnvironmentFederation. 1978. Odor Control for Waste-water Facilities. Manual of Practice No. 22.

Water Pollution Control Federation, Washing-ton D.C.

Prepared by Wendy Powers, extensionenvironmental specialist, Department of

Animal Science, and edited by MarisaCorzanego, extension communicationsintern, Communication Services, Iowa StateUniversity.

File: Environmental Quality 4-1

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Olfactory.the Science of Smell Issue 2 PDF