Expanded
Summary
Assessing taste and visual perception of Mn(II)
and Mn(IV)
A MAN DA E. SAIN, A SH L E Y GRIF F IN, A ND A ND R EA M. D I ET R I C H
http://dx.doi.org/10.5942/jawwa.2014.106.0003
The US Environmental Protection Agency (USEPA)
secondary maximum contaminant level (SMCL) for
manganese (Mn) is 0.05 mg/L, reportedly because of bitter
metallic taste, black–brown particles in water, and black–
brown discoloration of fixtures and laundry. This study
revisited visual and taste thresholds for Mn(II) and Mn(IV)
using one-in-five, one-in-three, and triangle tests for 0.005–
506.3 mg/L. Results showed that Mn(II) and Mn(IV) do
not taste significantly different from distilled water at the
SMCL. The population taste threshold for Mn(II) from
manganese sulfate (MnSO4) is > 1,000 times the SMCL;
the threshold concentration is 75–100 mg/L Mn(II) and is
likely influenced by both sulfate and Mn(II) taste
contributions. Aqueous Mn(II) is colorless at 0.05 mg/L
and remains visually undetectable at concentrations as high
as 506 mg/L; Mn(IV) is visually detectable in water at
concentrations as low as 0.005 mg/L. Findings suggest that
consumers cannot detect Mn(II) concentrations above the
SMCL by taste or sight and that the bitter and metallic
descriptors for 0.05 mg/L Mn are unwarranted.
Manganese is ubiquitous in drinking waters and is
regulated as a secondary standard at 0.05 mg/L (USEPA,
2012). The SMCL for manganese reportedly is based on
a bitter metallic taste, black–brown particles in water, and
undesirable black or brown color of fixtures and laundry,
but there are limited data establishing sensory thresholds
for manganese ions. A single previous taste test (Cohen
et al, 1960) showed the taste threshold for manganese to
be well above the SMCL, but the methods used had a
high chance of guessing correctly and made use of trained
panelists who may not reflect the tasting ability of the
general population. Historically manganese has been
known to cause off-colors in drinking water at levels
lower than the SMCL. In order to minimize complaints
caused by aesthetic issues, it is important to know the
concentrations at which manganese can be detected by
the public, both by taste and visually.
(BET) and population threshold as determined by linear
regression analysis were calculated.
• Examine the reaction of manganese with human
saliva by measuring lipid oxidation using thiobarbituric
acid reactive substances (TBARS) method with a 1:1 ratio
of human saliva to a 9.84-mg/L-Mn(II) solution.
• Determine visual detection of Mn(II) and Mn(IV) at
and below the SMCL of 0.05 mg/L. The one-in-five AFC
method was adapted to determine visual perception of
Mn(II) and Mn(IV).
STUDY OBJECTIVES
A total of 98 subjects (30% women) participated in
sensory tests throughout the course of this project. The
research objectives were fourfold:
• Determine whether Mn(II) or Mn(IV) is detectable
by taste at the SMCL of 0.05 mg/L for Mn(II) and Mn(IV)
and at 10 times the SMCL for Mn(IV). Triangle taste tests
were used to determine whether there is a detectable difference between waters containing manganese and deionized water controls.
• Determine a population taste threshold for Mn(II)
using the one-in-five alternate forced choice (AFC) tests.
Mn(II) concentrations ranged from 0.005 to 506.3 mg/L
Mn as MnSO4 and were compared with deionized water
as the control. A 50% population best estimate threshold
RESULTS AND DISCUSSION
Taste testing. Triangle test results indicated there was
no detectable difference between samples containing
Mn(II) at 0.05 mg/L and Mn(IV) at 0.05 and 0.5 mg/L,
compared with the deionized water controls. Because the
Mn(IV) samples were highly colored at the USEPA SMCL
(discussed subsequently), no further taste tests were conducted for Mn(IV).
Results from the subjects who participated in the one-infive AFC test for Mn(II) are shown in Figure 1. Thresholds
were determined for individuals, and the calculated BET
was 101 mg/L Mn(II). Lawless (2010) recommended using
the linear portion of data for threshold analysis to perform
logistic regression; therefore the taste threshold was calculated using concentrations of 0.8 mg/L Mn(II) and 8.8–
337.5 mg/L Mn(II), for which all subjects tasted all concentrations. The resulting logistic regression of the data
indicated that the 50% population taste threshold for
Mn(II) was 75.4 mg/L, which is significantly higher than
the SMCL. At threshold concentrations, the taste solutions
would contain 131–175 mg/L sulfate (SO4–2) and 206–275
mg/L total dissolved solids (TDS). These SO4–2 levels are
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approaching the aesthetic guideline (250 mg/L SO4–2 ). The
TDS values are below the aesthetic guideline (< 450 mg/L)
but above the 85-mg/L TDS level reported to taste better
than distilled water, indicating subjects may be responding
to the taste of TDS rather than manganese (Bruvold, 1968).
The high taste threshold for manganese could be related to
its lack of lipid oxidation, which is necessary to produce
the strong metallic flavor sensation and retronasal odors
for ferrous, cupric, and cuprous ions (Ömür-Özbek et al,
2012). When tests were conducted using the TBARS
method, 9.84 mg/L Mn(II) was found to have no significant
difference from the control.
Visual testing. Visual test findings suggested that elevated Mn(IV) would likely be detected before consumption. Mn(IV) concentrations 10 times lower than the
SMCL were accurately seen by subjects in this study,
compared directly with the deionized controls (Table 1).
However, Mn(II) was not found to be visually detectable
at high concentrations.
CONCLUSION
Human senses cannot be relied on to detect Mn(II) in
drinking water. The 50% population taste threshold for
Mn(II) was estimated to be more than 1,000 times the
current USEPA SMCL of 0.05 mg/L. In addition, Mn(II)
is visually undetectable in drinking water, even at concentrations much greater than typical drinking water concentrations. Manganese concentrations in disinfected and
oxidized drinking water typically are lower than the
SMCL because of oxidation of Mn(II) to Mn(IV) and
therefore should not result in off-flavors. However,
groundwater or surface water that is not treated with an
oxidant could lead to ingestion of water with high Mn(II)
concentrations. Mn(IV) could not be distinguished from
the control by taste at 10 times the SMCL, so taste alone
would not result in detection of Mn(IV) in drinking water.
FIGURE 1
Individual Mn(II) taste thresholds, n = 18
Individual Mn(II) Threshold—mg/L
Age
pH
70
200
180
160
140
120
100
80
60
40
20
0
0.0
4.0
REFERENCES
Bruvold, W.H., 1968. Scales for Rating the Taste of Water. Journal of
Applied Psychology, 52:3:245.
Cohen, J.M.; Kamphake, L.J.; Harris, E.K.; & Woodward, R.L., 1960.
Taste Threshold Concentrations of Metals in Drinking Water.
Journal AWWA, 52:660.
Kohl, P.M. & Medlar, S.J., 2007. Occurrence of Manganese in Drinking
Water and Manganese Control. AWWA, Denver.
Lawless, H.T., 2010. A Simple Alternative Analysis for Threshold Data
Determined by Ascending Forced-choice Method of Limits.
Journal of Sensory Studies, 25:3:332.
Ömür-Özbek, P.; Dietrich, A.M.; Duncan, S.E.; & Lee, Y., 2012. Role of
Lipid Oxidation, Chelating Agents, and Antioxidants in Metallic
Flavor Development in the Oral Cavity. Journal of Agricultural and
Food Chemistry, 60:9:2274.
Sly, L.; Hodgkinson, M.; & Arunpairojana, V., 1990. Deposition of
Manganese in a Drinking Water Distribution System. Applied and
Environmental Microbiology, 56:3:628.
USEPA (US Environmental Protection Agency), 2012. Secondary
Drinking Water Regulations: Guidance for Nuisance Chemicals.
water.epa.gov/drink/contaminants/secondarystandards.cfm
(accessed Mar. 15, 2013).
Corresponding author: Andrea M. Dietrich is a professor
of civil and environmental engineering at Virginia Polytechnic Institute and State University, 413 Durham Hall, 1145
Perry St., Blacksburg, VA 24061 USA; [email protected].
TABLE 1
Visual detection of Mn(II) and Mn(IV) from
one-in-five test
Concentration—mg/L Mn
pH
2.0
The USEPA SMCL of 0.05 mg/L is protective of taste
of Mn(II) or Mn(IV). The SMCL may not protect against
Mn(IV) color, however, depending on water depth and
such viewing characteristics as the size and color of the
container. In this study, Mn(IV) concentrations 10 times
lower than the SMCL were visually detected. Given this
result, the current research and previous studies suggest
that a value of 0.01–0.02 mg/L Mn may be a more appropriate concentration to protect against off-color in water
(Kohl & Medlar, 2007; Sly et al, 1990).
6.0
8.0
10.0
Percent Detected*
Mn(IV), Oxidized
0.005
100
0.01
97
0.03
97
0.05†
97
Mn(II), Reduced
0
20
Age—year
40
60
0.05†
10
0
0
Mn—manganese, n—number, SMCL—secondary maximum contaminant level,
USEPA—US Environmental Protection Agency
Mn—manganese, n—number
n = 31 for all tests
Results showed no correlation with age (R2 = 0.0252) or salivary
pH (R2 = 0.1772).
*Percentages represent number of subjects positively detecting manganese over
total number of subjects.
†USEPA SMCL = 0.05 mg/L
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