5i6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



TABLE 3. The Taste of Salts at Different Concentrations* 



X 

 a. 



NUMBER OF C ATOMS 



-t- 



FIG. 10. pH required to elicit a constant reaction time in 

 the sunfish, for hydrochloric acid and a series of normal ali- 

 phatic acids of increasing carbon chain length. [From ."Mlison 

 (8).] 



isms (fish, frogs, barnacles, mollusks, worms) is 

 probably due to the general sensitivity and thus 

 simpler than in the case of taste. Stimulation by 

 mineral acids in this case is determined by pH, but 

 normal aliphatic acids stimulate at lower hydrogen 

 ion concentrations, the efficacy increasing systemat- 

 ically with increasing chain length (8, g, 54, 55) 

 (see fig. 10). Similarly, the nonpolar parts of the 

 molecule in a series of n aliphatic alcohols add to 

 stimulating efficiency with increasing number of 

 carbon atoms (53). The same holds for a series of 

 alcohols and glycols for taste in man and in insect 

 chemoception (66). In the insect studies a wider 

 sampling of organic molecules was employed. Stimu- 

 lation appeared to be associated with increasing 

 lipoid solubility attendant upon increased chain 

 length and the introduction of functional groups that 

 reduced water solubility (65, 68, 6g). The hydrogen 

 ion is clearly an important determinant of the sour- 

 ness of acids, but it alone does not determine stimu- 

 lating efficiency. 



s.\LTY. All substances with a s<ilty taste are soluble 

 salts composed of positive and negative ions in the 

 solid (crystalline) state which dissolve in water to 

 Droduce a solution of these ions. Sodium chloride is 



the only substance said to possess the 'pure sahy taste' 

 except that the threshold concentrations of this salt 

 taste sweet (see taljle 3). Other salts display the same 

 phenomenon but yield complex salty tastes at supra- 

 thresholcl values. 



Both the anion and cation contribute to the taste 

 quality and to the stimulating efficiency (80). Thus, 

 whereas .04 m sodium chloride is distinctly salty, 

 sodium acetate of the same concentration has no 

 salty taste. In a series of sodium salts, the quality of 

 the taste elicited will vary with the anion. A similar 

 effect can be noted in a chloride series with different 

 cations. In a series of halides of the monovalent 

 alkali metals (plus ammonium) the low molecular 

 weight (below iio) salts are predominantly salty in 

 taste, while the higher molecular weight (over 160) 

 salts are bitter (122). Salts of heavy metals such as 

 mercury have a metallic taste but some lead salts, 

 especially lead acetate (sugar of lead), and beryllium 

 salts are sweet. 



The thresholds for different salts ha\c been vari- 

 ously reported to be equimolar for the cation (92), 

 for halogen salts (84), inversely related to the molecu- 

 lar weight (80), directly related to cation mobility 

 (79). Table 4 shows the median values ba.sed on a 

 sampling of a numljer of different threshold studies in 

 man. 



von Skramlik (198) attempted to specify objectively 

 the complex taste of salts by means of the following 

 taste equation : .'Y = .v.^ -|- yB -\- zC -\- vD in which 



>■' 



and v are molar concentration values and .-1 



stands for sodium chloride; B, quinine sulphate; C, 

 fructose; D, potassium tartrate; and .A' is the molar 

 concentration of the salt being matched. Although 

 indi\idual differences among subjects are clearly 

 apparent in the matches, certain trends or consist- 

 encies can be noted. 



The degree of saltiness of a series of salts is given 

 b> the ratio of m NaCl/M 'salt' required to match the 



