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Understanding Farmstead Odors: An Annotated Review

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Abstract and Figures

There are few issues in animal agriculture today more contentious than odors. Although odors are generally considered a swine problem, all livestock producers may have to address the changing public attitude toward rural air quality eventually. Adding to the problem of farmstead odors is a lack of standardization in odor measurement and description. A review of the literature reveals that there is little consensus among scientists on odor description.
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203
Review: Farmstead Odors
The Professional Animal Scientist 15:203–210
REVIEWS
Abstract
There are few issues in animal
agriculture today more contentious than
odors. Although odors are generally
considered a swine problem, all livestock
producers may have to address the
changing public attitude toward rural air
quality eventually. Adding to the prob-
lem of farmstead odors is a lack of
standardization in odor measurement and
description. A review of the literature
reveals that there is little consensus
among scientists on odor description.
The authors of this paper attempt to
clarify the situation using terminology
employed by the perfume industry to
provide structure to the definition of
farmstead odors. Odor-causing chemicals
called odorants mix to form distinct scents
called odor notes, and odor notes mix to
form complex farmstead odors. Odor
description is accomplished by answering
four questions. How many molecules of
odor-causing chemicals are present in the
air? How bad does the odor smell? How
strong is the odor? How long do the
various components of the odor last?
Among the numerous odor measurements
currently in use, these questions are best
answered using the parameters concen-
tration (as measured by odor units),
character (as measured by degree of
offensiveness), intensity, and persistence.
All of the parameters mentioned
previously can be measured by a trained
panel of sniffers. Using a dynamic
olfactometer to present odor samples to a
panel is becoming the standard for odor
measurement. This article concludes
with a very brief description of the ability
of olfactometers and other analytical
techniques (scentometers, electronic
noses, and gas chromatographs) to
quantify farmstead odors following the
analysis scheme set forth by the authors.
(Key Words: Odors, Odorants, Odor
Notes, Odor Intensity, Odor Measure-
ment.)
Odor Perception
The nose and the brain work
together to create what is perceived as
odor. The sense of smell is activated
when the nose captures odor-causing
chemicals called odorants from the
air. Humans are equipped with two
separate systems for collecting and
processing odorant stimuli. Some
animals (e.g., rats, dogs, cats, horses)
have a third system called the vome-
ronasal organ with receptors located
in the roof of the mouth to define
odors further. As it is, humans smell
using two systems: the olfactory
organ and the trigeminal nerve. The
human olfactory organ uses receptors
located at the top of the nasal
passages to collect odors from the air.
Signals from several receptors are
grouped and preprocessed in the
olfactory bulb before they are sent on
to the brain. Odor messages are
processed in the more primitive part
of the brain known as the limbic
system. The limbic system is also the
seat of memory and emotion in
humans, which helps explain why
smells often release old, long-forgot-
ten memories. The olfactory organ
does not have a monopoly on
smelling odors. The trigeminal nerve
is primarily responsible for feeling in
the face. In mammals, this nerve
also reacts to certain chemical sub-
stances, including smells. The
general function of the trigeminal
nerve is to protect against harmful
external influences, which is why the
perception of a substance such as
ammonia leads to a pain or repellent
response. There is some interplay
between the olfactory and trigeminal
systems. Many substances, such as
alcohol, ammonia, and acetic acid,
are perceived by both systems (32).
Science has identified thousands
of odorants forming the bouquet of
smells known as farmstead odors.
Table 1 is a list of manure odorant
nderstanding Farmstead Odors:
An Annotated Review
D. W. HAMILTON*,1 and J. AROGO
*Department of Biosystems and Agricultural Engineering, Oklahoma State University,
Stillwater, OK 74078-6021 and Department of Biological and Agricultural Engineering,
North Carolina State University, Raleigh, NC 27695-7625
U
1To whom correspondence should be ad-
dressed: dhamilt@okstate.edu
Reviewed by J. T. Brake, D. W. Freeman, F. A.
Martz, and E. F. Wheeler.
204
Hamilton and Arogo
groups compiled from the literature.
Under each group is a list of indi-
vidual chemical compounds that are
commonly found in manure odors.
The two numbers listed next to the
odorants in Table 1 are concentra-
tions at which the brain makes
decisions about the odorant. The
first number, the detection level, is
the concentration at which the
average, healthy person first notices
an odor. People cannot recognize the
odor at the detection level, but they
know they smell “something”. For
example, when the concentration of
ammonia in air reaches 17 ppb, the
brain detects a smell, but it does not
recognize the smell as ammonia. The
second number listed in Table 1 is the
recognition level. At this concentra-
tion, the brain begins to recognize
the odorant as a distinct scent. The
average human recognizes the scent
of ammonia cleanser when the
concentration of ammonia reaches
37,000 ppb in air.
Values for detection and recogni-
tion levels can be difficult to compre-
hend. Consider the following. A
person is standing on the floor of the
Louisiana Superdome. The
Superdome is a very large, domed
stadium containing 3.5 billion L of
air space. Ammonia is released into
the Superdome. A person would
detect “something” if 17 ppb, or 57
g, of ammonia gas were mixed with
clean air in the Superdome. The
observer would begin to recognize a
faint ammonia smell once the
concentration reached 37,000 ppb, or
100 kg, of ammonia gas mixed in the
airspace.
Most odorants are not gases.
Skatole is a N-containing compound
largely responsible for causing ma-
nure to smell like manure. Skatole is
a solid below 98oC, but it is soluble in
both polar and non-polar liquids.
Solutions of skatole release minute
amounts of the compound at the air-
liquid interface. It only takes a
minute amount to create an effect
because the detection level of skatole
is 1.2 ppb (<30 g in the Superdome
airspace). The recognition level of
skatole is much higher (470 ppb);
therefore, the brain would not
recognize a manure-like smell in the
Superdome until 10 kg of skatole were
released.
The large range between detection
and recognition levels for most
odorants may explain the ambivalent
feeling humans have about odors.
The Superdome observer smells
“something” in the air at low doses
of skatole. He cannot describe the
smell, but he knows he does not like
it. As skatole concentration in-
creases, the observer waivers on the
edge of recognition. Finally, he
recognizes the scent, and it brings
back unpleasant memories (e.g., dirty
diapers, the outhouse at grandma’s,
an open sewer).
Odor Notes
Farm odors are never pure samples
of one odorant but a mixture of
many different odorants. When
chemists analyzed an air sample
taken from a German hog building,
they measured at least 11 different
organic acids, but none of the
individual acids were present at
detectable levels (10). Because the
individual odorants were below the
detection level does not mean that
they could not be smelled. Stimuli
from many receptors are combined as
a group in the olfactory bulb. Be-
cause of this grouping, the brain
TABLE 1. Components of manure odorsa.
Groups and individual Detection Recognition Odor
odorants levelblevelcdescription
(ppb)
Organic acids
Acetic acid 10.2 1000 Vinegar
Propionic acid 3.6 300
Butyric acid 1.1 1 Sour meat
Iso-valeric acid 1.2
Valeric acid 20
Alcohols, aldehydes, ketones
Methanol 100,000 Sweet
Formaldehyde 1000 Straw, pungent
Acetylaldehyde 210 Fruity, pungent
Acetone 4.0 100,000 Sweet, pungent
Methyl ethyl ketone 10,000 Sweet
Phenolic compounds
Phenol 5.7 1000 Medicinal
p-Cresol 8.0
N compounds
Ammonia 17 37,000 Sharp, pungent
Methylamine 2.1 Fishy, pungent
Dimethylamine 37 37 Fishy, pungent
Diethylamine 500 Fishy, pungent
Indole 1.0 Fecal
Skatole 1.2 470 Fecal, pungent
S compounds
Hydrogen sulfide 0.5 4.7 Rotten egg
Methyl mercaptan 0.5 2.1 Rotten cabbage
Dimethyl sulfide 1.1 1.1 Rotten vegetable
Diethyl sulfide 6.0 6.0 Rotten vegetable
aLiterature citations: 1, 11, 13, 14, 16, 29, 33.
bLevel at which an odorant is detected as an unidentified smell.
cLevel at which an odorant is recognized as a distinct scent.
205
Review: Farmstead Odors
analyses the multiple organic acids as
one fatty or sour smell.
Perfumers call a mixture of
odorants making a distinct scent an
odor note. A single note may con-
tain hundreds of different odorants.
Returning to the Superdome analogy,
our observer is standing in the
middle of the football field; he
cannot hear one person way up in
the stands blowing softly into a
plastic trumpet. If a hundred people
blow into plastic, brass, and tin
trumpets all at the same time, he
hears a note, and he perceives it as
a general blaring sound, not a mosaic
of individual trumpets. Add cowbells
and cheering, and the situation
becomes analogous to perfume or
farmstead odors.
Odor Concentration
Farmstead odors always occur as
mixtures of notes. Notes, in turn, are
mixtures of odorants. It is difficult
and expensive to measure the concen-
tration of each odorant in a sample.
Instead, odor scientists measure the
concentration of odors as a whole by
presenting a sample to a panel of
trained sniffers. The sample is
diluted with odorless gas until one-
half of the panel can no longer smell
anything. When 50% of the sniffers
can no longer detect an odor, the
sample has been diluted to the
detection threshold. Detection
threshold is similar to detection level,
which was discussed in the odor
perception section. Detection thresh-
old is the detection level of a mixture
of odorants under specific experimen-
tal conditions.
The ratio of odorless gas to sample
volume is called the dilution factor.
Dilution factor is a good measure of
odor concentration. The odor thresh-
old standard used by the European
Union (5) assigns odor concentration
at the detection threshold the arbi-
trary value of one odor unit (OU) per
cubic meter. For example, if an air
sample is taken inside a dairy barn
and diluted 100 times so that one-
half of an odor panel can no longer
detect an odor, then the air inside
the dairy barn has an odor concen-
tration of 100 OU/m3. Other odor
threshold standards do not designate
odor concentration in OU per
volume. Because the dilution factor
is a ratio, it has no units; therefore,
the inverse of dilution factor is
simply assigned OU (3). No matter
what standard is used, the concept is
the same: dilute the sample to the
detection threshold, then use the
inverse of the dilution factor as odor
concentration.
Odor Character
The term character describes
what an odor smells like. In general,
odor character does not change with
concentration. Ammonia at 10 OU/
m3 smells the same as ammonia at
100 OU/m3. Of course, there are
always exceptions. At high doses,
humans are likely to sense ammonia
through the trigeminal system, and
the response is more a feeling of
repulsion than a true smell. The
character of some odorants changes
dramatically with concentration. At
concentrations usually detected in
farmstead odors, indole smells like
feces. At very low concentrations,
however, indole has the scent of
flowers (4, 32).
The fourth column of Table 1 lists
identifying terms used to describe the
character of selected odorants.
Because farmstead odors are mixtures
of many odor notes, it is hard to
quantify their character based on
simple descriptors. Two descriptors
used to assign numerical values to the
character of complex odors are
hedonic tone and offensiveness.
Hedonic Tone. Hedonic tone is a
measure of the relative pleasantness
of an odor. Odor panelists compare
an unknown sample with a set of
known odorants. The panel decides
which of the known odorants best
describes the odor. The odor is
assigned a rating based on the
comparisons. Pleasant odors have
positive hedonic tones, and negative
hedonic tones indicate unpleasant
odors (7). Table 2 lists hedonic tones
for common agricultural odors as
well as those for some of the odor-
ants listed in Table 1. If an apple has
a hedonic tone of 2.61, and rotten
fruit has a hedonic tone of 2.76, it is
safe to assume that most people find
rotting fruit more offensive than a
fresh cut apple.
Offensiveness. It is difficult to
describe exactly what farmstead odors
smell like, and, in the final analysis,
the exact description of the smell
may not matter. People know if they
enjoy the smell or not. Offensiveness
is an attempt to add degrees of good
and bad to odor phenomena. Offen-
siveness is measured by an odor
panel. A series of samples is diluted
to equal odor intensity. Panelists are
asked to rank the offensiveness of
each sample on a scale of 0 to 5 (0 =
inoffensive to 5 = strongly offensive)
(22). Because the relative badness
of a farmstead odor is more impor-
tant than the exact description of the
odor, agricultural scientists have
generally adopted offensiveness as a
measure of character for farmstead
odors.
Odor Intensity
Offensiveness indicates how bad
an odor smells, and OU can be
correlated to the number of odorant
molecules floating in the air, but
neither character nor concentration
provides a measure of how strong an
odor smells. A third measurement is
requiredodor intensity. Odor
intensity is the direct measurement of
a persons reaction to an odor. To
measure odor intensity, scientists ask
a panel to describe the strength of an
unknown odor without knowing the
odor concentration or dilution
factor. A commonly used scale ranks
intensity between 0 and 6 (0 = no
odor to 6 = extremely strong odor).
Odor intensity is standardized by
having the panel compare the
unknown odor with a reference
odorant of known concentration.
Methods approved by the American
Society for Testing and Materials (2)
206
Hamilton and Arogo
and the European Committee for
Standardization (5) use n-butanol as
a reference. With n-butanol as a
reference, intensity is reported as a
concentration of butanol, regardless
of the actual odorants present.
Studies have related odor intensity
to concentration using equations of
the form: I = K (C)n or log(I) = logK
+ n log(C), where I is odor intensity,
C is odor concentration, and K and
n are empirical constants (33). A
recent study conducted in England
(19) provided relationships between
odor concentration and odor inten-
sity for broiler house exhaust and
land-applied hog slurry. The results
of that study, shown in Figure 1,
demonstrate three concepts needed to
understand intensity.
First, every mixture of odorants
has its own relationship between
concentration and intensity. Similar
mixtures of odorants, such as five
different samples of broiler house
exhaust, have similar relationships
between concentration and intensity.
Second, intensity versus concentra-
tion is not a one-to-one relationship.
Diluting an odor sample in half will
not diminish odor intensity by one-
half. To diminish a strong broiler
house odor (I = 4) to a faint odor (I =
2) requires an eightfold dilution (40
OU/m3 diluted to 5 OU/m3).
Third, intensity and character are
not related. According to the results
shown in Figure 1, if odor concentra-
tions were held equal, the panel
would perceive broiler house odor to
be more intense than hog slurry
odor. This does not mean the broiler
house smells worse than the hog
slurry; it only means the broiler
house smells stronger than the hog
slurry. To describe an odor com-
pletely, you must measure both
intensity and character. Perfumes
emit high intensity odors, but these
odors are not considered offensive.
An apple pie baking in the oven
smells both strong and pleasant.
Skunks release strong and offensive
odors. A glass of water smells neither
strong nor offensive because it has
no smell at all.
Odor Persistence
Perfumers are masters in the art of
blending many odors to form com-
plex mixtures known as perfume. If
one distinct odor is a note, then a
mixture of many odors is a chord.
Perfumers also recognize that a chord
of odors can change with time (4,
27). Perfumers group notes according
to their relative volatility or persis-
tence. The most persistent odors are
base notes or fixiants. The least
persistent odors are top notes. Odor-
ants with medium persistence are
called middle notes or modifiers.
When perfume is placed on the skin,
the first scent smelled is the top note.
Because top notes are made of vola-
tile or short-lived odorants, they fade
with time. Base notes remain long
after the top notes have faded.
Middle notes give the perfume lift
or body throughout the life of the
scent.
Farmstead odors are also chords of
many notes. Odorants in the ma-
nure chord can be grouped in notes
based on relative volatility. Table 3
classifies common manure odorants
into top, middle, and base notes (D.
Hamilton, unpublished data). Know-
ing that all notes do not have the
same persistence can explain why the
strength and character of farmstead
odors change over time. Consider
the results of a series of land applica-
tion experiments shown in Table 4.
Different types of swine waste were
applied 0.2 inches deep to soil inside
a wind tunnel. Samples of air were
collected and presented to a panel to
determine odor offensiveness, con-
centration, and intensity immedi-
ately after spreading. Air sample
collection and panel presentation
were repeated 4 to 6 h after applica-
tion.
The panel determined that raw
manure was definitely offensive.
Figure 1. Relationship between odor concentration and intensity for broiler house exhaust
and land-applied hog slurry (19). OU = odor unit.
207
Review: Farmstead Odors
Odor intensity was extremely strong
immediately after application, and
intensity remained extremely strong 6
h after application. The odors
released by raw manure exposed at
the soil contained persistent, strong-
smelling odorants. Using perfume
terminology, raw waste is replete with
base notes.
Anaerobically digested manure
presents a different picture. Anaero-
bically digested manure was described
by the panel as faintly offensive.
Initial intensity was extremely strong
and similar to raw manure. Odors
from anaerobically digested manure
were not persistent, however. Six
hours after application, the panel
only smelled faint odors in the
samples; therefore, the anaerobically
digested manure contained more top
notes and fewer base notes than raw
manure. These results are consistent
with the chemistry of anaerobic
digestion. During digestion, base
notes (e.g., organic acids, skatole,
large organic sulfides) are converted
to top notes (e.g., hydrogen sulfide,
ammonia) and odorless gases (e.g.,
carbon dioxide, methane).
Treating raw manure by aeration
reduced odors even further than
anaerobic treatment. The panel
described screened manure aerated at
1 to 2 mg/L of dissolved oxygen as
inoffensive. Odor intensity increased
from no odor immediately following
application to a faint odor 4 h after
application. Why did land applica-
tion odors increase with time?
Aerobic bacteria were produced as raw
manure was aerated, creating a large,
living biomass. A portion of the
aerobic biomass died when exposed
to a new environment by land
application. The biomass decayed
anaerobically, giving off odorants
similar to anaerobically digested
manure.
Measuring Farmstead
Odors
Direct measurement of odor
phenomenon with a sensory panel is
quickly becoming the standard
method of odor measurement. The
sensory panel gives immediate mea-
sures of odor concentration, charac-
ter, and intensity. With a time series
of observations, the panel can
determine odor persistence as well.
There are a number of problems
associated with odor panels, however.
Experimenters must overcome indi-
vidual bias in observations. It is also
important to consider the phenom-
enon of odor fatigue (i.e., prolonged
exposure to an odor lessens an
individuals ability to distinguish the
odor). Olfactometers and
scentometers are two instruments
used to measure human odor sensory
response. Other methods devised to
overcome the problems associated
with sensory panels include electronic
noses and gas chromatographs.
Odor Collection. The first diffi-
culty an experimenter must overcome
is capturing a representative odor
sample. Farmstead odors are transi-
tory in nature. They are constantly
changing in character, intensity, and
composition. There are two ways to
sample odor phenomena: take
measurements directly as the phe-
nomenon is occurring or collect
representative samples of the odor
event. Measuring phenomena
directly from the environment is
called dynamic sampling. Static
sampling involves grabbing represen-
tative samples of the odor. The most
common method of static sampling
is to gather whole air samples in non-
reactive bags made of TeflonÒ,
MylarÒ, or KedlarÒ (30) (E. I. Du Pont
de Nemours and Company, Wil-
mington, DE). Another static odor
collection method is to trap or
adsorb odors onto a surface or
chemical reactant. Adsorption
creates additional bias because
individual odorants can be selected
based on the characteristics of the
adsorbent. A method of capturing
odors on swatches of cotton cloth
was shown to measure adequately
those odor character and intensity
changes caused by a packed column
air scrubber (18). More detailed
analysis of the cotton swatch method
showed that it did not correlate well
with observations made on whole air
samples, particularly at high odor
concentrations. The authors sug-
gested that the fabric became satu-
rated with odorants at lower concen-
trations, and higher concentrations
did not register above the saturated
levels (21). The European Committee
for Standardization considers only
whole air samples in its olfactometry
standard (5).
Olfactometer. An olfactometer is
a laboratory device that distributes
sample dilutions to odor panelists.
There are a number of variations on
the olfactometer depending on
method of sampling and analysis,
but all devices perform essentially the
same function, to present samples of
odors to a panel of sniffers. Flexibil-
ity and direct measurement of hu-
man sensations are the main advan-
tages of olfactometry. Disadvantages
are expense of operation and over-
coming human bias (6, 8, 9, 12).
Olfactometers are classified into two
major categories by method of
analysis. Suprathreshold referencing
olfactometers present air samples to
panelists for comparison with known
quantities of n-butanol. Sometimes
called butanol olfactometers,
suprathreshold referencing olfacto-
meters are used to measure odor
intensity. Dilution-to-threshold
olfactometers, commonly referred to
as dynamic olfactometers, present
mixtures of odorous air samples and
odor-free gases at known dilutions to
a panel. The dynamic olfactometer
may be operated in yes/no, choice,
forced-choice, triangle forced-choice,
or forced-choice/probability mode,
depending on method of delivery (5,
30). Dilution-to-threshold olfactome-
ters are used to measure concentra-
tion, intensity, and character.
Scentometer. A scentometer is a
simple, hand-held, dilution-to-
threshold device used to measure
odor concentration and intensity in
the field. The person taking measure-
ments holds the device up to his nose
and breaths through the scentometer.
Gases can either pass directly to the
nose or pass through an activated
carbon filter. The analyst chooses
208
Hamilton and Arogo
dilution factor by selecting the size of
the hole passing unfiltered air.
Advantages of the scentometer are
that it is portable, simple to use, and
gives immediate values for odor
concentration and intensity. It is
particularly useful for measuring
intensity of odor sources (31). The
main disadvantage is that it is hard
to overcome the analysts personal
bias in measurement. Odor fatigue is
also a problem with repeated analyses
(8).
Electronic Nose. Electronic noses
mimic the mammalian olfactory
system using metal oxide or organic
polymer sensors to represent olfactory
receptors in the nose and an elec-
tronic neural network and a micro-
processor to perform the functions of
the olfactory bulb and brain (25, 26,
28). The main use of an electronic
nose is to differentiate between two
odors or to compare an unknown
odor with a known odor. They have
been used effectively for this purpose
in the food processing industry.
Researchers have used an electronic
nose to determine successfully the
onset of estrus in dairy cattle by
sampling perineal odors (15). Elec-
tronic noses have shown a limited
ability to measure odor concentra-
tion and to differentiate between
groups of some odorants (17, 20).
The greatest potential for electronic
nose technology is to measure subtle
changes in odor character. Electronic
noses are also very adaptable to
dynamic, real-time sampling and
sampling in hazardous environ-
ments. Despite recent advances in
electronics, the electronic nose is no
match for the human nose. The
human sense of smell, as used in
olfactometry panels, is 10,000 times
more sensitive than current electronic
analogs (17). Electronic noses be-
come less sensitive in humid condi-
tions because sensor materials re-
spond to water vapor as well as
volatile organics. Electronic nose
technology is likely to improve in
two directions: devices with a limited
number of sensors chosen to identify
selected odorants and devices with a
large number of sensors of varied
composition to determine a wide
spectrum of odors. Devices of the
second type, coupled with improve-
ments in electronics based on recent
discoveries in animal olfactory
systems, will be required if higher
order analyses such as intensity and
character are to be measured (25).
Chemical Methods. Chemical
methods are used to determine the
actual concentration of individual
odorants in the air. The most com-
mon instrumentation used in odor-
ant analysis is a gas chromatograph
with a mass spectrometer detector.
Instrumentation can be set up to
analyze ambient air dynamically or
to analyze samples taken from the
field. Similar to the electronic nose,
a gas chromatograph distinguishes
compounds by comparison with a
reference standard. The main disad-
vantage of chemical methods is the
shear number of potential odorants
required for analysis in the sampling
of farmstead odors. The greatest
advantage of chemical methods is the
removal of human bias in the deter-
mination of the presence of odorants
in the air. This advantage is also a
downfall, however, because it is very
difficult to predict human reactions
to odors based on the chemical
constituents in a sample.
Implications
Controlling farmstead odors is a
major concern of todays livestock
producers. A literature search reveals
a lack of consensus among scientists
on the proper methods of measuring
and quantifying agricultural odors.
It makes sense, therefore, to return to
the terminology of the perfume
industry. Perfumers have had little
trouble communicating with each
other for centuries. In perfumery
terms, odor-causing chemicals are
called odorants. Several odorants
combine to form a distinct scent
called an odor note. Complex
farmstead odors are mixtures of odor
notes. Odor notes can be classified
to some extent by grouping com-
pounds in the same chemical family
TABLE 2. Hedonic tonea of
common agricultural odors.
Odor Hedonic tone
Strawberry 2.93
Apple 2.61
Hay 1.30
Grain 0.63
Mushroom 0.52
Iso-valeric acid 1.57
Butyric acid 1.77
Mercaptans 2.30
Ammonia 2.47
Rotten fruit 2.76
Urine 3.34
Manure 3.36
Dead animal 3.75
aHedonic tone is a measure of the
relative pleasantness of an odor.
Pleasant odors have positive hedonic
tones. Unpleasant odors have
negative hedonic tones. Values for
hedonic tone listed in this table were
determined by Dravnieks et al. (7).
TABLE 3. Likely grouping of manure odor notes based on relative
volatility of odorants.
Top notes Middle notes Base notes
Hydrogen sulfide Aldehydes Organic acids
Ammonia Alcohols Phenolic compounds
Ketones Indole and skatole
Amines Organic sulfides
Mercaptans (>5 carbons)
Organic sulfides Dust-borne odorants
(2 to 4 carbons)
209
Review: Farmstead Odors
into three notes based on odorant
volatility. Highly volatile compounds
form top notes. Persistent com-
pounds form base notes. Middle
notes, derived from compounds with
medium volatility, complete the odor
chord.
Once classified through a system
of notes, farmstead odors may be
described using two primary mea-
sures: intensity and character. These
two descriptors measure how strong
the odor smells and what the odor
smells like at any point in time.
Describing the character of farmstead
odors may be simplified using offen-
siveness scales. Mass of odorants in
the air, measured as odor concentra-
tion, and the changing nature of an
odor chord, measured as odor persis-
tence, add depth to the odor descrip-
tion.
Currently, a human sensory panel
tested using some form of olfactome-
ter is the most widely accepted
method of measuring intensity,
character, concentration, and persis-
tence. A hand-held scentometer is
another sensory device shown to
measure odor intensity and concen-
tration. Electronic noses mimic the
mammalian olfactory organ and
have been proven to measure differ-
ences in odor character. Direct
measurement of odorant concentra-
tion is achieved through gas chroma-
tography analysis.
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Highest measured odor Highest measured
concentration (OUb/m3) odor intensity
Type of waste Offensiveness Initial After 4 to 6 h Initial After 4 to 6 h
Raw manure Definitely Extremely Extremely
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Raw manure
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Raw manure
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Screened manure
aerated at 1 to 2 mg/L dissolved oxygen Inoffensive 60 61 No odor Faint odor
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