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Type Literature review
Authors Irene Delgado
Published 2021
License CC-BY-SA-4.0
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Soil[edit | edit source]

Soil distribution[edit | edit source]

The following maps provide soil distributions over a given area:

Clay[edit | edit source]

Tropical soils[edit | edit source]

Juo, A. Tropical soils : properties and management for sustainable agriculture. (Oxford University Press: New York N.Y., 2003).

  • Clay mineral types
  • Mineral structure
  • Properties of colloids
  • Cation exchange preference
  • Cation exchange capacity
  • Soil taxonomy

Very good source of information on soils in the topical regions and their chemical and physical behaviour of clay minerals.

Composition and mineralogy[edit | edit source]

Velde, B. Origin and mineralogy of clays. (Springer: Berlin ;;New York, 1995).

  • Soil particle behaviour in water
  • Discusses above points as well

Charge characteristics and related D/F of soil colloids as the cause of turbidity[edit | edit source]

Itami, K. & Fujitani, H. "Charge characteristics and related dispersion/flocculation behavior of soil colloids as the cause of turbidity". Colloids and Surfaces A: Physicochemical and Engineering Aspects 265, 55-63 (2005).

Abstract:

Dispersion/flocculation (D/F) behavior of soil colloids is closely related to numerous environmental issues and regulated by the charge characteristics of the colloids. The charge characteristics of five clays commonly distributed in Japan (montmorillonite, sericite, kaolinite, halloysite and allophane) were determined by the modified ion adsorption method using caesium as an index cation and compared with the D/F behavior under the similar conditions. The flocculation effect of Cs on negatively charged clays enabled accurate charge analysis in the high pH and low electrolyte concentration region where measurement had been difficult by conventional methods due to clay dispersion. The dispersion ratio of montmorillonite was evenly high in all pH regions reflecting its large permanent charge. The pH dependence of sericite charge was rather variable than permanent. The dispersion ratio of sericite was increased abruptly above pH 6.5. The pH dependence of kaolinite negative charge was small, indicating the predominance of permanent charge. Both negative and positive charges were detected from halloysite in large pH regions. The dispersibility of kaolinite and halloysite was increased above pH 7. To explain the observed discrepancy between the charge characteristics and the D/F behavior, the significant role of edge surfaces in flocculation was emphasized for sericite, kaolinite and halloysite. In case of halloysite, the reverse imogolite structure curling with the Al-octahedral layers inside was proposed to account for the constant negative charge detected in all pH regions. Allophane showed the charge characteristics typical of variable charge minerals and dispersed in both acid and alkali regions. The difference between the charge characteristics and the dispersibility of allophane was attributed to the heterogeneity of its charge. From the aspect of environmental conservation, keeping the suspension pH below 6–7 so as to prevent edge surfaces from dissociating is likely to be primarily important to control clay dispersion depending on the clay mineralogy. By combining the information derived from this Cs adsorption method with the proper land management, more effective techniques to reduce soil losses and water pollution will be established according to field conditions.

Interaction of a cationic surfactant with bentonite: colloid chemistry[edit | edit source]

Janek, M. & Lagaly, G. "Interaction of a cationic surfactant with bentonite: a colloid chemistry study". Colloid Polym Sci 281, 293-301 (2003).

Abstract:

Cationic surfactants are strongly coagulating agents for clay mineral dispersions. The critical coagulation concentration of cetyltrimethylammonium chloride (CTMAC) is 0.09 mEql-1 for a 0.025% sodium montmorillonite dispersion and increases to 0.35 mEql-1 for a 0.5% dispersion. The very low concentrations are caused by the strong adsorption of the organic cations on the clay mineral surface. The particles aggregate in different ways. Phase diagrams in relation to the mass content of CTMAC and bentonite reveal four domains of different states of the dispersions. As long as the amount of CTMAC added is below the cation exchange capacity, ;, the dispersion consists of fine flocs and separates into a two-phase system of large volume and a clear supernatant (domain I). At CTMAC amounts near ;, the fine flocs rearrange into voluminous flocs (domain II) and at CTMAC amounts of about 2; into compact fine flocs (domain II) as a consequence of recharging of the particles. Unlike type-I dispersions, the type-III dispersions exhibit Newtonian flow, even at relatively high bentonite concentrations (2.5% w/w and above), and the flocs settle to sediments of distinctly smaller volumes.

Influence of electrolytes, SAR, and disturbance on dispersed clay size and critical flocc. concentration[edit | edit source]

Panayiotopoulos, K.P., Barbayiannis, N. & Papatolios, K. Influence of Electrolyte Concentration, Sodium Adsorption Ratio, and Mechanical Disturbance on Dispersed Clay Particle Size and Critical Flocculation Concentration in Alfisols. Communications in Soil Science and Plant Analysis 35, 1415-1434 (2004).

Abstract:

Clay dispersion affects both soil productivity and environmental quality through its effect on structure degradation. The effect of solution concentration (C), sodium adsorption ratio (SAR), and mechanical disturbance on clay dispersion, particle size of dispersed clay and critical flocculation concentration (CFC) was investigated in four Greek Alfisols equilibrated with NaCl/CaCl2 solutions of different C (= 5, 10, 50, and 100 mmol/L) and SAR [= 0, 5, 10, 20, and 40 (mmol/L)(1/2)]. Suspensions of equilibrated soil samples in solutions or in deionized water received a minimum (30s) or a prolonged (16 h) shaking. After shaking, the dispersed clay fractions < 2 and < 1 mum were determined by measuring the optical density of the Suspensions. It was found that both clay dispersion and CFC were increased with SAR. Relative dispersed clay of any size obtained in deionized water was always higher than in any NaCl/CaCl, solution. Prolonged shaking resulted in much higher relative dispersed clay of any size, in greater CFC and in lower coarse (1-2 mum) to fine (< 1 mum) clay ratio than minimum shaking. Relative dispersed coarse clay was always higher (p < 0.05) than relative dispersed Fine clay. The coarse clay Fraction displayed higher CFC than the fine clay fraction. The influence of clay content and mineralogy, organic matter, Fe2O3, pH, and mechanical disturbance on the differences in clay dispersion found between the soils used is discussed. In order to avoid clay dispersion in soils containing large-sized clay particles and/or receive intensive mechanical disturbance, it is suggested to keep the Soil Solution at a higher concentration.

URL: http://www.informaworld.com/openurl?genre=article&doi=10.1081/CSS-120037555&magic=crossref%7C%7CD404A21C5BB053405B1A640AFFD44AE3

Settling velocity[edit | edit source]

Gibbs, R.J. Settling Velocity, Diameter, and Density for Flocs of Illite, Kaolinite, and Montmorillonite. SEPM Journal of Sedimentary Research Vol. 55 (1985).

Abstract:

The relation between floc settling velocity and diameter was measured for illite, kaolinite and montmorillonite. The curves between floc diameter and settling velocity show a non-Stokes' relation that indicated the floc density decreased or drag properties increased as the floes increased in size.

URL: http://search.datapages.com/data/doi/10.1306/212F860C-2B24-11D7-8648000102C1865D

Coagulation rates of clay minerals and natural sediments[edit | edit source]

Gibbs, R. Coagulation Rates of Clay Minerals and Natural Sediments. SEPM Journal of Sedimentary Research 53 (1983).

Abstract:

Coagulation experiments conducted using illite, kaolinite, montmorillonite, and four natural sediment samples in blade and Couette reactors with solutions of various salinities yielded collision-efficiency factors significantly higher than those in the literature, indicating that coagulation begins at lower salinities (1/2-1 per mil). The natural sediments do not generally behave as might be predicted for a mixture of the standards of the minerals of which the natural sediment samples were comprised. The halftimes for coagulation of the particles indicate that, in nature, the coagulation process would probably be completed in the first few parts-per-thousand salinity

Behaviour of clay with NaCl[edit | edit source]

Flocculation of clay in NaCl[edit | edit source]

Joseph, A.F. & Oakley, H.B. "The Anomalous Flocculation of Clay." Nature 119, 673-673 (1927).

  • This letter outlines the results of experiments performed to investigate the flocculating nature of clay in sodium and calcium chlorides and hydroxides. The clay was a 0.1% purified clay suspension. --> What kind of clay?? Find original article.
  • The results for sodium show that at a a concentration 0.9 normal, sodium chloride took 18 minutes for flocculation; at 0.5 normal, 17 minutes; at 0.2 normal, 14 minutes; at 0.1 normal, 13 minutes; and at 0.05 normal, 13 minutes.
  • A preliminary experiment with a 0.1% suspension of highly purified amorphous silica showed that no flocculation occurred, even over a 10 hour period in 1.0 normal solution of sodium chloride.

--> What does this mean for my work?? Need to find the original article and compare these types of clay to the soil distribution in areas of interest.

--> This article is very old.

Sedimentation of two commercial bentonites in NaCl[edit | edit source]

Akther, S., Hwang, J. & Lee, H. Sedimentation characteristics of two commercial bentonites in aqueous suspensions. Clay Miner. 43, 449-457 (2008).

Abstract:

The sedimentation characteristics of two commercial bentonites, Tixoton (organically treated) and Montigel-F (untreated), were investigated using a 3% w/v clay suspension at different concentrations (1, 3.5 and 10%) of NaCl and pH values (2, 7 and 12). Settling rates, floc diameters and sediment volumes were derived from changes in light transmittance using a Turbiscan Ma 2000 instrument.

Both bentonite suspensions were unstable (flocculated) in NaCl solutions. The settling rate increased with increasing concentration of NaCl and was directly related to floc diameter. The sediment volume reduced with increasing NaCl concentrations, a result of greater double layer compression caused by increased ionic strength. At comparable salt concentrations, the organically-treated bentonite (Tixoton) settled at a much slower rate and had a greater sedimentation volume. The suspensions of both organically-treated and untreated bentonites were stable (dispersed) above pH 7 and unstable in acidic conditions. The settling rate for Tixoton under acid conditions was much smaller than that for the Montigel-F. Differences in sedimentation characteristics between the two bentonite samples are probably due to the presence of an anionic polymer (carboxymethyl cellulose: CMC) in Tixoton.

The viscosity of the bentonite suspensions was also studied. The viscosity of the clay suspension is closely related to clay dispersivity in solution. The CMC was highly effective in increasing the viscosity of the bentonite suspensions, but only under neutral and alkaline conditions.

NOTES:

  • Good, but how does this information affect my experimental design? ie. types of bentonites used, and how do these compare to soil distribution?
  • Effects of pH? How does this compare to typical field conditions?

Kinetic determination of CCC for Na- and Ca-montmorillonite in NaCl and CaCl2[edit | edit source]

García-García, S., Wold, S. & Jonsson, M. "Kinetic determination of critical coagulation concentrations for sodium- and calcium-montmorillonite colloids in NaCl and CaCl2 aqueous solutions". Journal of Colloid and Interface Science 315, 512-519 (2007).

Abstract:

The stability of the sodium and calcium forms of montmorillonite was studied at different NaCl and CaCl2 concentrations. The aggregation kinetics was determined from the decrease in particle concentration with time at different electrolyte concentrations. The DLVO theory defines the critical coagulation concentration (CCC) value as the electrolyte concentration that balances the attractive and repulsive potential energies between the particles, making aggregation diffusion-controlled. Therefore CCC values were obtained by extrapolation of the aggregation rate constants measured as a function of ionic strength to conditions where the rate constant value is determined by diffusion only. When the electrolyte was CaCl2, the CCC value was found to be approximately two orders of magnitude lower than the CCC values obtained using NaCl as electrolyte.

Influence of shear rate, organic matter content, pH and salinity on mud flocculation[edit | edit source]

Mietta, F., Chassagne, C., Manning, A.J. & Winterwerp, J.C. "Influence of shear rate, organic matter content, pH and salinity on mud flocculation". Ocean Dynamics 59, 751-763 (2009).

Abstract:

The purpose of this paper is to establish a relation between a few measurable quantities (the so-called ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud. The results obtained with small-scale flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The mud used for all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. From this study, it was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions with different pH and salt concentrations. The size of flocs at a given shear rate depends on the properties of the suspension, which affect the electrokinetic properties of the sediment; these can be described by means of the ζ potential. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the salt concentration and the ζ potential increase. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. (3) The mean floc size increases with increasing organic matter content. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little. The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. This suggests that a general trend can be established for different and complex types of clays and mud. This systematic study can therefore be used for further development of flocculation models.

Application to water treatment[edit | edit source]

Effect of salt on the flocculation behavior of nanoparticles in oil sands fine tailings[edit | edit source]

Kotylar, L., Schutte, R. & Sparks, B. "Effect of salt on the flocculation behavior of nanoparticles in oil sands fine tailings". Clays a 44, 121-131 (1996).

Abstract:

Currently, two commercial plants, operating in the Athabasca region of Alberta, produce approximately 20 percent of Canada's petroleum requirements from oil sands. Surface mined oil sand is treated in a water based separation process that yields large volumes of clay tailings with poor settling and compaction characteristics. Clay particles, suspended in the pond water, interact with salts, dissolved from the oil sands ore, to produce mature fine tailings (MFT) containing only 20 to 50 w/w% solids. As a result, large sedimentation ponds are required to produce enough process water to recycle for the plant. Tailings pond dykes can only be constructed during a short summer season. Consequently, the capability to predict production rate and final volume of MFT is essential for mine planning and tailings disposal operations. Previous research has demonstrated that a small fraction of nano sized clay particles (20 to 300 nm) effectively controls the bulk properties of MFT. These particles are present in the original ore and become mobilized into the water phase during the oil separation process. In this work, the nano sized particles have been separated from the bulk tailings and subjected to a fundamental study of their flocculation behavior in model tailings water. Photon correlation spectroscopy and a deuterium NMR method were used to follow particle flocculation and gelation processes. These results were correlated with particle settling data measured under the same conditions. It was determined that the nano particles form fractal flocs that eventually interact to give a thixotropic gel. The ultimate sediment volume produced is almost entirely dependent on the original concentration of nano particles while the rate of water release is governed primarily by electrolyte concentration.

Low-tech water purification by bentonite clay flocculation[edit | edit source]

Madsen, M. & Schlundt, J. "Low technology water purification by bentonite clay flocculation as performed in Sudanese villages: bacteriological examinations". Water Research 23, 873-882 (1989).

Abstract:

The effects of a water purification method traditionally used in Sudan to treat turbid waters were studied with respect to removal of faecal indicator bacteria as well as selected enteric bacterial pathogens. Water treatment was performed with natural bentonite clays (rauwaq) from the banks of the Nile, and the technique employed corresponded closely to that used to clarify Nile water in Sudanese villages. Employing various types of waters a primary bacterial reduction of 1–3 log units (90–99.9%) was obtained within the first 1–2 h of flocculation. During the 24 h observation period bacterial multiplication in the water phase occurred consistently for Vibrio cholerae and test organisms belonging to the Enterobacteriaceae group, but not for Streptococcus faecalis and Clostridium perfringens. Some of the conditions influencing the hygienic effects obtained were examined. The potential and limitations of the method as a local alternative in water improvement are discussed.

URL: http://linkinghub.elsevier.com/retrieve/pii/0043135489900122

Effects of Cl-based coagulants on electrochemical oxidation of textile wastewater[edit | edit source]

Kim, T., Park, C., Shin, E. & Kirm, S. "Effects of Cl-based chemical coagulants on electrochemical oxidation of textile wastewater". Desalination 155, 59-65 (2003).

Abstract:

As a supporting electrolyte and the source of chloride reactant, NaCl is generally added for the electrochemical oxidation process. In this study, Cl-based chemical coagulation was employed as the pretreatment step for the preremoval of suspended and colloidal solids which impede electrochemical oxidation. It was adopted for the purpose of providing the source of the chloride reactant for the electrochemical oxidation. It was ultimately intended to omit the artificial addition of electrolyte solution and to decrease the pollutant loading efficiently on the post electrochemical oxidation process in order to improve the performance of organics removal. PAC and FeCl3, Cl-based chemical coagulants, were successfully employed as the pretreatment step of electrochemical oxidation. PAC and FeCl3, were able to achieve sufficient removal efficiency of organics as well as to exclude the artificial addition of a supporting electrolyte and the source of a chloride reactant.

Effects of polyelectrolytes with bentonitic clay on contaminants removal[edit | edit source]

Rebhun, M., Narkis, N. & Wachs, A. "Effect of polyelectrolytes in conjunction with bentonitic clay on contaminants removal from secondary effluents". Water Research 3, 345-355 (1969).

Abstract:

The following paper describes investigations in the use of polyelectrolytes in combination with bentonitic clay in the tertiary treatment of municipal wastewater. Besides causing the flocculation of fine suspended solids, cationic polyelectrolytes react with soluble organic matter having anionic functional groups. The reaction products are colloidal and do not settle readily. Bentonitic clay, when used by itself, causes removal of some organic compounds, mostly nitrogenous organics.

The use of a mixture of bentonite and cationic polyelectrolytes improves the efficiency of removal of organics and lowers the required flocculant dose. The mechanisms of the reactions between organics and polycationic flocculants, and between the resulting polycationic-organic complexes and the clay, are discussed in the last section of the paper.

URL: http://linkinghub.elsevier.com/retrieve/pii/0043135469900876

Salt[edit | edit source]

Perceived taste[edit | edit source]

Perceived taste of NaCl and acid mixtures in water and bread[edit | edit source]

Hellemann, U. "Perceived taste of NaCl and acid mixtures in water and bread". International Journal of Food Science and Technology 27, 201-211 (1992).

Abstract:

The perceived intensity of saltiness, sourness, and overall taste of aqueous solutions containing NaCl, acetic, or lactic acid and rye bread samples was rated by 14 assessors. Pleasantness of the bread samples was also rated. Samples were prepared combining four levels of NaCl (0-1.6%), and four levels of acetic or lactic acid (0-0.9%) in water, and three levels of NaCl (0.5-1.7%) and three levels of acid (0-1-0.9%) in rye bread, yielding a total of 16 samples in all in both water series and a total of 9 samples in both bread series. Acetic-lactic acid mixtures (two ratios) were used as the acid component in the bread samples.

The perceived saltiness increased as the acid concentration was raised with low levels of NaCl in water and with all levels of NaCl in bread samples. Saltiness decreased with increasing acid concentration with high levels of NaCl in water. Sourness was depressed with increasing concentration of NaCl in water. The reduction of sourness by NaCl was more prominent in lactic than in acetic acid solutions. In contrast, sourness was not affected by NaCl in bread samples. The acid component was slightly dominant in terms of overall taste. This, there were noteworthy differences between the two media and the acids in the strength and in the type of inter-relationships with NaCl. Direct transformation of results from one medium to another may be misleading. It may be possible to achieve marked reduction of NaCl in solid food such as rye bread without a decrease in perceived saltiness or pleasantness by increasing the level of acidity through the addition of acids or through natural fermentation.

Materials and methods:

Aqueous solutions containing NaCl (0, 0.4, 0.8, 1.6% w/v) and either acetic or lactic acid (0, 0.1, 0.3, 0.9% v/v) were prepared in distilled water with no off-taste. The pH was determined in duplicate with standard procedure: pH of the acetic acid was 6.1, 3.2, 3.0, 2.8, respectively. Chemicals used were NaCl (J.T. Baker Chemicals. 100% p.a.), acetic acid (Merck, 100% p.a.). Concentrations selected represented actual concentrations found in real foods such as rye bread.

Results::br> Increasing concentrations of acid increased saltiness at the lowest concentrations and decreased saltiness at the highest concentrations of NaCI, so that the overall effect was not apparent. Thus this also resulted in small F ratios (not significant) for acid concentration. This cross-over effect of acid on perceived saltiness was somewhat more marked in acetic acid than in lactic acid solutions.Increasing concentrations of acid in water showed a cross-over effect by increasing the perceived saltiness at low NaCl levels (0.4%) and decreasing it at high NaCl levels (1.6%). Pangborn & Trabue (1967) have suggested that low acid levels (range 0.025-0.4%) enhanced and high levels depressed saltiness of NaCl (0.03-0.24%). Saltiness (NaC1 up to 1.5%) was increased by citric, tartaric, and malic acids with all concentrations used in water (up to 0.73%) (Kamen et al., 1961; Marum, 1986).

URL: http://onlinelibrary.wiley.com.proxy.queensu.ca/doi/10.1111/j.1365-2621.1992.tb01196.x/abstract;jsessionid=E5615F24613B9881F01AC91EF1F23F0A.d03t02

Perception of sweet and salty flavors in different population groups[edit | edit source]

González Carnero, J., de la Montaña Miguélez, J. & Míguez Bernárdez, M. "Perception of sweet and salty flavors in different population groups". Nutr Hosp 17, 256-258 (2002).

Abstract:

The flavour perceived by humans when eating varies depending on age, gender, habits, emotional status, etc. The present study reflects the changes in the perception of sweet and salt flavours among different population groups depending on age, with an assessment, for each flavour, of the threshold concentration for the detection of these flavours. Triangular discrimination sensorial tests were performed in three groups, with thirty members in each, classified to represent young, adult and elderly age groups. With regard to sweet flavours, the groups of young people and adults distinguished the different sample at 0.1% of sugar for 95% and 99% significance levels, whereas the elderly required the concentration to reach 1% at both levels before they could distinguish the sugar solution from water. In the case of salt flavours, young people are able to detect the different sample at the lowest concentration level, for both levels of significance. Adults significantly distinguished the sample containing 0.05% of salt, at the 95% significance level, whereas the elderly needed a concentration of 0.1% for both levels of significance. Age-dependent variations in response were observed. As age increases, greater concentrations are required in order to distinguish the salt or sweet solutions from the samples containing only water.

URL: http://www.ncbi.nlm.nih.gov/pubmed/12428303

Salt taste threshold and salt good taste sensitivity in a community of students in Puerto Rico[edit | edit source]

Vélez, H., Santiago, A., Bredy, R., Magraner, M. & Benítez, P. "Salt taste threshold and salt good taste sensitivity in a community of students of the southern area of Puerto Rico". Bol Asoc Med P R 98, 294-299 (2006).

Abstract:

Sodium appetite reflects the importance of sodium homeostasis. The sodium ion is one of the most important risk factors in the development of hypertension. Humans, for various reasons, seem to have a specific preference for salt which is consumed in excess of need and this has been characterized as an important contributor to hypertension. Salt intake is related to the salt taste sensitivity threshold and the salt good taste level. Gustatory sensibility responds to various physiological mechanisms and salt taste is directly modified by cultural and socio-economical factors. We measured the salt taste sensitivity threshold and salt good taste level of a young student population. Air popped popcorn sprayed with different Molar concentrations of salt where given to students to taste and a questionnaire to evaluate diet salt intake preferences. Both salt taste sensitivity threshold and salt good taste level graph patterns are different from each other. Salt taste sensitivity threshold has a bell shape distribution with different molar salt concentrations. The major tendency of the salt umbral sensitivity of our population was the 0.5 M concentration. Salt good taste level has an exponential shape distribution with different molar salt concentrations. The tendency for the good taste level of our population was 3 M. Smoking does not seem to modify the salt taste sensitivity thresholds or the salt good taste level graphs. Also, salt shaker use does not seem to modify salt taste sensitivity thresholds or salt good taste level graphs in our population. Salt taste sensitivity threshold is probably associated to morpho-physiological factors. Salt good taste level is mainly associated with the cultural environment. The majority of subjects have a tendency to prefer foods with higher concentrations of salt increasing the possibility of exposure to the salt intake risk factor.

URL: http://www.ncbi.nlm.nih.gov/pubmed/19610571

Taste stimuli thresholds[edit | edit source]

Purves, D. Neuroscience. (Sinauer Associates: Sunderland Mass., 2001). At <http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A1035>

Most taste stimuli are nonvolatile, hydrophilic molecules soluble in saliva. Examples include salts such as NaCl needed for electrolyte balance; essential amino acids such as glutamate needed for protein synthesis; sugars such as glucose needed for energy; and acids such as citric acid that indicate the palatability of various foods (oranges, in the case of citrate). Bitter-tasting molecules include plant alkaloids, such as atropine, quinine, and strychnine, that may be poisonous. Placing bitter compounds in the mouth usually deters ingestion unless one "acquires a taste" for the substance, as for quinine in tonic water.

The taste system encodes information about the quantity as well as the identity of stimuli. In general, the higher the stimulus concentration, the greater the perceived intensity of taste. Threshold concentrations for most ingested tastants are quite high, however. For example, the threshold concentration for citric acid is about 2 mM; for salt (NaCl), 10 mM; and for sucrose, 20 mM. Since the body requires substantial concentrations of salts and carbohydrates, taste cells may respond only to relatively high concentrations of these essential substances to promote an adequate intake. Clearly, it is advantageous for the taste system to detect potentially dangerous substances (e.g., bitter-tasting plant compounds) at much lower concentrations. Thus, the threshold concentration for quinine is 0.008 mM, and for strychnine 0.0001 mM. As in olfaction, gustatory sensitivity declines with age. Adults tend to add more salt and spices to food than children. The decreased sensitivity to salt can be problematic for older people with electrolyte and/or fluid balance problems. Unfortunately, a safe and effective substitute for NaCl has not yet been developed.

URL: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A1035

Allowable limits[edit | edit source]