Flavor 1 - Principle of food chemistry

Flavor 1 - Principle of food chemistry

INTRODUCTION Flavor has been defined by Hall (1968) as follows: "Flavor is the sensation produced by a material taken in the mouth, perceived principally by the senses of taste and smell, and also by the general pain, tactile and tem- perature receptors in the mouth. Flavor also denotes the sum of the characteristics of the material which produce that sensation." This definition makes clear that flavor is a property of a material (a food) as well as of the receptor mechanism of the person ingesting the food. The study of flavor includes the composition of food com- pounds having taste or smell, as well as the interaction of these compounds with the receptors in the taste and smell sensory organs. Following an interaction, the organs produce signals that are carried to the cen- tral nervous system, thus creating what we understand as flavor. This process is proba- bly less well understood than the processes occurring in other organs (O'Mahony 1984). Beidler (1957) has represented the taste process schematically (Figure 7-1). Although flavor is composed mainly of taste and odor, other qualities contribute to the overall sensation. Texture has a very definite effect. Smoothness, roughness, granularity, and viscosity can all influence Figure 7-1 Schematic Representation of the Taste Process. Source: From LM. Beidler, Facts and Theory on the Mechanism of Taste and Odor Perception, in Chemistry of Natural Food Fla- vors, 1957, Quartermaster Food and Container Institute for the Armed Forces. flavor, as can hotness of spices, coolness of menthol, brothiness or fullness of certain amino acids, and the tastes described as metallic and alkaline. TASTE It is generally agreed that there are only four basic, or true, tastes: sweet, bitter, sour, Flavor CHAPTER 7 TASTE SENSATIONS BRAIN NEURAL PATTERNS OF ACTIVITY TACTILE TONGUE PAIN WARM TASTE COLD and salty. The sensitivity to taste is located in taste buds of the tongue. The taste buds are grouped in papillae, which appear to be sen- sitive to more than one taste. There is undoubtedly a regional distribution of the four kinds of receptors at the tongue, creat- ing areas of sensitivity—the sweet taste at the tip of the tongue, bitter at the back, sour at the edges, and salty at both edges and tip (Figure 7-2). The question of how the four types of receptors are able to respond this specifically has not been resolved. Accord- ing to Teranishi et al. (1971), perception of the basic taste qualities results from a pattern of nerve activity coming from many taste cells; specific receptors for sweet, sour, bit- ter, and salty do not exist. It may be envi- sioned that a single taste cell possesses multiple receptor sites, each of which may have specificity. The mechanism of the interaction between the taste substance and the taste receptor is not well understood. It has been suggested that the taste compounds interact with spe- cific proteins in the receptor cells. Sweet- and bitter-sensitive proteins have been reported. Dastoli and Price (1966) isolated a protein from bovine tongue epithelium that showed the properties of a sweet taste recep- tor molecule. Dastoli et al. (1968) reported isolating a protein that had the properties of a bitter receptor. We know that binding between stimulus and receptor is a weak one because no irre- versible effects have been observed. A mech- anism of taste stimulation with electrolytes has been proposed by Beidler (1957); it is shown in Figure 7-3. The time required for taste response to take place is in the order of 25 milliseconds. The taste molecule is weakly adsorbed, thereby creating a distur- bance in the molecular geography of the sur- face and allowing an interchange of ions across the surface. This reaction is followed by an electrical depolarization that initiates a nerve impulse. The taste receptor mechanism has been more fully described by Kurihara (1987). The process from chemical stimulation to trans- mitter release is schematically presented in Figure 7-4. The receptor membranes contain voltage-dependent calcium channels. Taste compounds contact the taste cells and depo- larize the receptor membrane; this depolar- ization spreads to the synaptic area, activating the voltage-dependent calcium channels. Influx of calcium triggers the release of the transmitter norepinephrine. The relationship between stimulus concen- tration and neural response is not a simple one. As the stimulus concentration increases, the response increases at a decreasing rate until a point is reached where further in- crease in stimulus concentration does not produce a further increase in response. Beidler (1954) proposed the following equa- BITTER Figure 7-2 Areas of Taste Sensitivity of the Tongue SWEET Figure 7-4 Diagram of a Taste Cell and the Mechanism of Chemical Stimulation and Transmitter Release. Source: Reprinted with permission from Y. Kawamura and M.R. Kare, Umami: A Basic Tale, © 1987, Marcel Dekker, Inc. Release of transmitter (norepinephrine) Taste nerve Ca influx Activation of voltage-dependent Ca channel Receptor potential Adsorption Receptor membrane Electric current Synapse Figure 7-3 Mechanism of Taste Stimulation as Proposed by Beidler. Source: From L.M. Beidler, Facts and Theory on the Mechanism of Taste and Odor Perception, in Chemistry of Natural Food Flavors, 1957, Quartermaster Food and Container Institute for the Armed Forces. NERVE ACTION POTENTIALS SENSE CELL DEPOLARIZATION CELLULAR CHANGES STRUCTURAL CHEMICAL PHYSICOCHEMICAL CHANGES SPATIAL ARRANGEMENTS CHARGE DENSITIES BINDING SITES PROTEINS LIPIDS HYDRATED IONS tion relating magnitude of response and stim- ulus concentration: £ - — — R " W s + KR s where C = stimulus concentration R = response magnitude R s = maximum response K = equilibrium constant for the stimulus- receptor reaction K values reported by Beidler for many sub- stances are in the range of 5 to 15. It appears that the initial step in the stimu- lus-receptor reaction is the formation of a weak complex, as evidenced by the small values of K. The complex formation results in the initiation of the nerve impulse. Taste responses are relatively insensitive to changes in pH and temperature. Because of the decreasing rate of response, we know that the number of receptor sites is finite. The taste response is a function of the proportion of sites occupied by the stimulus compound. According to Beidler (1957), the threshold value of a substance depends on the equilib- rium constant and the maximum response. Since K and R x both vary from one substance to another and from one species to another, the threshold also varies between substances and species. The concentration of the stimu- lus can be increased in steps just large enough to elicit an increase in response. This amount is called the just noticeable difference (JND). There appear to be no significant age- or sex-related differences in taste sensitivity (Fisher 1971), but heavy smoking (more than 20 cigarettes per day) results in a deteriora- tion in taste responsiveness with age. Differences in taste perception between individuals seem to be common. Peryam (1963) found that sweet and salt are usually well recognized. However, with sour and bit- ter taste some difficulty is experienced. Some tasters ascribe a bitter quality to citric acid and a sour quality to caffeine. Chemical Structure and Taste A first requirement for a substance to pro- duce a taste is that it be water soluble. The relationship between the chemical structure of a compound and its taste is more easily established than that between structure and smell. In general, all acid substances are sour. Sodium chloride and other salts are salty, but as constituent atoms get bigger, a bitter taste develops. Potassium bromide is both salty and bitter, and potassium iodide is predominantly bitter. Sweetness is a property of sugars and related compounds but also of lead acetate, beryllium salts, and many other substances such as the artificial sweeteners saccharin and cyclamate. Bitterness is exhib- ited by alkaloids such as quinine, picric acid, and heavy metal salts. Minor changes in chemical structure may change the taste of a compound from sweet to bitter or tasteless. For example, Beidler (1966) has examined saccharin and its sub- stitution compounds. Saccharin is 500 times sweeter than sugar (Figure 7-5). Introduc- tion of a methyl group or of chloride in the para position reduces the sweetness by half. Placing a nitro group in the meta position makes the compound very bitter. Introduc- tion of an amino group in the para position retains the sweetness. Substitutions at the imino group by methyl, ethyl, or bromoethyl groups all result in tasteless compounds. However, introduction of sodium at this loca- tion yields sodium saccharin, which is very sweet. The compound 5-nitro-otoluidine is sweet. The positional isomers 3-nitro-o-toluidine and 3-nitro-p-toluidine are both tasteless (Figure 7-6). Teranishi et al. (1971) pro- vided another example of change in taste resulting from the position of substituent group: 2-amino-4-nitro-propoxybenzene is 4,000 times sweeter than sugar, 2-nitro-4- amino-propoxybenzene is tasteless, and 2,4- dinitro-propoxybenzene is bitter (Figure 7-7). Dulcin (p-ethoxyphenylurea) is extremely sweet, the thiourea analog is bitter, and the 0-ethoxyphenylurea is tasteless (Figure 7-8). Just as positional isomers affect taste, so do different stereoisomers. There are eight amino acids that are practically tasteless. A group of three has varying tastes; except for glutamic acid, these are probably derived from sulfur-containing decomposition prod- ucts. Seven amino acids have a bitter taste in the L form or a sweet taste in the D form, except for L-alanine, which has a sweet taste (Table 7-1). Solms et al. (1965) reported on the taste intensity, especially of aromatic amino acids. L-tryptophan is about half as bitter as caffeine; D-tryptophan is 35 times sweeter than sucrose and 1.7 times sweeter than calcium cyclamate. L-phenylalanine is about one-fourth as bitter as caffeine; the D form is about seven times sweeter than sucrose. L-tyrosine is about one-twentieth as bitter as caffeine, but D-tyrosine is still 5.5 times sweeter than sucrose. Some researchers claim that differences exist between the L and D forms of some sug- ars. They propose that L-glucose is slightly salty and not sweet, whereas D-glucose is sweet. There is even a difference in taste Figure 7-5 The Effect of Substitutions in Saccharin on Sweetness. Source: From L.M. Beidler, Chem- ical Excitation of Taste and Odor Receptors, in Flavor Chemistry, I. Hornstein, ed., 1966, American Chemistry Society. Sweet Tasteless Tasteless Tasteless Sweet Sweet Sweet Sweet Bitter Sweet Figure 7-6 Taste of Nitrotoluidine Isomers SWEET TASTELESS TASTELESS between the two anomers of D-mannose. The a form is sweet as sugar, and the (3 form is bit- ter as quinine. Optical isomers of carvone have totally different flavors. The D+ form is characteris- tic of caraway; the L- form is characteristic of spearmint. The ability to taste certain substances is genetically determined and has been studied with phenylthiourea. At low concentrations, about 25 percent of subjects tested do not taste this compound; for the other 75 percent, the taste is bitter. The inability to taste phen- ylthiourea is probably due to a recessive gene. The compounds by which tasters and nontasters can be differentiated all contain the following isothiocyanate group: S Ii -C-N- These compounds—phenylthiourea, thio- urea, and thiouracil—are illustrated in Figure 7-9. The corresponding compounds that contain the group, O Il -C-N- phenylurea, urea, and uracil, do not show this phenomenon. Another compound con- taining the isothiocyanate group has been found in many species of the Cruciferae fam- ily; this family includes cabbage, turnips, and rapeseed and is well known for its goitrogenic effect. The compound is goitrin, 5-vinyloxazolidine-2-thione (Figure 7-10). Sweet Taste Many investigators have attempted to relate the chemical structure of sweet tasting com- pounds to the taste effect, and a series of theo- ries have been proposed (Shallenberger 1971). Shallenberger and Acree (1967, 1969) pro- TASTELESS BITTER SWEET Figure 7-8 Taste of Substituted Ethoxybenzenes BITTER TASTELESS SWEET Figure 7-7 Taste of Substituted Propoxybenzenes posed a theory that can be considered a refine- ment of some of the ideas incorporated in previous theories. According to this theory, called the AH,B theory, all compounds that bring about a sweet taste response possess an electronegative atom A, such as oxygen or nitrogen. This atom also possesses a proton attached to it by a single covalent bond; there- fore, AH can represent a hydroxyl group, an imine or amine group, or a methine group. Within a distance of about 0.3 nm from the AH proton, there must be a second electrone- gative atom B, which again can be oxygen or nitrogen (Figure 7-11). Investigators have recognized that sugars that occur in a favored chair conformation yield a glycol unit confor- mation with the proton of one hydroxyl group at a distance of about 0.3 nm from the oxygen of the next hydroxyl group; this unit can be considered as an AH,B system. It was also found that the K bonding cloud of the benzene ring could serve as a B moiety. This explains the sweetness of benzyl alcohol and the sweetness of the anti isomer of anisaldehyde oxime, as well as the lack of sweetness of the syn isomer. The structure of these compounds is given in Figure 7-12. The AH,B system present in sweet compounds is, according to Shallenberger, able to react with a similar AH,B unit that exists at the taste bud receptor site through the formation of simultaneous hydrogen bonds. The relatively strong nature of such bonds could explain why the sense of sweetness is a lingering sensation. According to the AH,B theory, there should not be a dif- ference in sweetness between the L and D iso- mers of sugars. Experiments by Shallenberger (1971) indicated that a panel could not distin- guish among the sweet taste of the enantio- morphic forms of glucose, galactose, man- nose, arabinose, xylose, rhamnose, and gluco- heptulose. This suggests that the notion that L sugars are tasteless is a myth. Phenylthiourea Thiourea Thiouracil Figure 7-9 Compounds Containing the Differentiated Group by Which Tasters and Nontasters Can Be S Il -C-N- Table 7-1 Difference in Taste Between the L- and D-Forms of Amino Acids Amino Acid Asparagine Glutamic acid Phenylala- nine Leucine Valine Serine Histidine lsoleucine Methionine Tryptophane Taste of L lsomer Insipid Unique Faintly bitter Flat, faintly bitter Slightly sweet, bitter Faintly sweet, stale after- taste Tasteless to bitter Bitter Flat Bitter Taste of D lsomer Sweet Almost taste- less Sweet, bitter aftertaste Strikingly sweet Strikingly sweet Strikingly sweet Sweet Sweet Sweet Very sweet Figure 7-10 5-Vinyloxazolidine-2-thione Spillane (1996) has pointed out that the AH,B theory appears to work quite well, although spatial, hydrophobic/hydrophilic, and electronic effects are also important. Shallenberger (1998) describes the initiation of sweetness as being due to a concerted intermolecular, antiparallel hydrogen-bond- ing interaction between the glycophore (Greek glyks, sweet; phoros, to carry) and receptor dipoles. The difficulty in explaining the sweetness of compounds with different chemical structures is also covered by Shal- lenberger (1998) and how this has resulted in alternative taste theories. The application of sweetness theory is shown to have important applications in the food industry. Extensive experiments with a large num- ber of sugars by Birch and Lee (1971) sup- port Shallenberger's theory of sweetness and indicate that the fourth hydroxyl group of glucopyranosides is of unique impor- tance in determining sweetness, possibly by donating the proton as the AH group. Ap- parently the primary alcohol group is of lit- tle importance for sweetness. Substitution of acetyl or azide groups confers intense bitterness to sugars, whereas substitution of benzoyl groups causes tastelessness. As the molecular weight of saccharides increases, their sweetness decreases. This is best explained by the decrease in solubility and increase in size of the molecule. Appar- ently, only one sugar residue in each oli- gosaccharide is involved in the interaction at the taste bud receptor site. The relative sweetness of a number of sug- ars and other sweeteners has been reported by Solms (1971) and is given in Table 7-2. These figures apply to compounds tasted sin- gly and do not necessarily apply to sugars in foods, except in a general sense. The relative sweetness of mixtures of sugars changes with the concentration of the components. Synergistic effects may increase the sweet- ness by as much as 20 to 30 percent in such mixtures (Stone and Oliver 1969). Sour Taste Although it is generally recognized that sour taste is a property of the hydrogen ion, there is no simple relationship between sour- ness and acid concentration. Acids have dif- ferent tastes; the sourness as experienced in the mouth may depend on the nature of the acid group, pH, titratable acidity, buffering SWEET COMPOUND RECEPTOR SITE Figure 7-11 The AH,B Theory of Sweet Taste Perception effects and the presence of other compounds, especially sugars. Organic acids have a greater taste effect than inorganic acids (such as hydrochloric acid) at the same pH. Infor- mation on a number of the most common acids found in foods and phosphoric acid (which is also used in soft drinks) has been collected by Solms (1971) and compared with hydrochloric acid. This information is presented in Table 7-3. According to Beatty and Cragg (1935), rel- ative sourness in unbuffered solutions of acids is not a function of molarity but is pro- portional to the amount of phosphate buffer required to bring the pH to 4.4. Ough (1963) determined relative sourness of four organic acids added to wine and also preference for these acids. Citric acid was judged the most sour, fumaric and tartaric about equal, and adipic least sour. The tastes of citric and tar- taric acids were preferred over those of fumaric and adipic acids. Pangborn (1963) determined the relative sourness of lactic, tartaric, acetic, and citric acid and found no relation between pH, total acidity, and relative sourness. It was also found that there may be considerable differ- ences in taste effects between sugars and acids when they are tested in aqueous solu- tions and in actual food products. Table 7-2 Relative Sweetness of Sugars and Other Sweeteners Compound Relative Sweetness Sucrose 1 Lactose 0.27 Maltose 0.5 Sorbitol 0.5 Galactose 0.6 Glucose 0.5-0.7 Mannitol 0.7 Glycerol 0.8 Fructose 1.1-1.5 Cyclamate 30-80 Glycyrrhizin 50 Aspartyl-phenylalanine 100-200 methylester Stevioside 300 Naringin dihydrochal- 300 cone Saccharin 500-700 Neohesperidin 1000-1500 dihydrochalcone Source: From J. Solms, Nonvolatile Compounds and the Flavor of Foods, in Gustation and Olfaction, G. Ohloff and A.F. Thomas, eds., 1971, Academic Press. SWEET TASTELESS Figure 7-12 Anfr'-Anisaldehyde Oxime, Sweet; and Syrc-Anisaldehyde Oxime, Tasteless Buffering action appears to help determine the sourness of various acids; this may explain why weak organic acids taste more sour than mineral acids of the same pH. It is suggested that the buffering capacity of saliva may play a role, and foods contain many sub- stances that could have a buffering capacity. Wucherpfennig (1969) examined the sour taste in wine and found that alcohol may decrease the sourness of organic acids. He examined the relative sourness of 17 organic acids and found that the acids tasted at the same level of undissociated acid have greatly different intensities of sourness. Partially neutralized acids taste more sour than pure acids containing the same amount of undis- sociated acids. The change of malic into lac- tic acid during the malolactic fermentation of wines leads to a decrease in sourness, thus making the flavor of the wine milder. Salty Taste The salty taste is best exhibited by sodium chloride. It is sometimes claimed that the taste of salt by itself is unpleasant and that the main purpose of salt as a food component is to act as a flavor enhancer or flavor poten- tiator. The taste of salts depends on the nature of both cation and anion. As the molecular weight of either cation or anion— or both—increases, salts are likely to taste bitter. The lead and beryllium salts of acetic acid have a sweet taste. The taste of a num- ber of salts is presented in Table 7-4. The current trend of reducing sodium intake in the diet has resulted in the formula- tion of low-sodium or reduced-sodium foods. It has been shown (Gillette 1985) that sodium chloride enhances mouthfeel, sweet- ness, balance, and saltiness, and also masks Table 7-3 Properties of Some Acids, Arranged in Order of Decreasing Acid Taste and with Tartaric Acid as Reference Properties ofO.OSN Solutions Acid Hydrochloric Tartaric Malic Phosphoric Acetic Lactic Citric Propionic Taste +1.43 O -0.43 -1.14 -1.14 -1.14 -1.28 -1.85 Total Acidg/L 1.85 3.75 3.35 1.65 3.00 4.50 3.50 3.70 pH 1.70 2.45 2.65 2.25 2.95 2.60 2.60 2.90 lonization Constant 1.04 x 10~ 3 3.9X10" 4 7.52 x 1Q- 3 1.75 x 10~ 5 1.26 x 1Q- 4 8.4 x 1Q- 4 1.34 x 10~ 5 Taste Sensation Hard Green Intense Vinegar Sour, tart Fresh Sour, cheesy Found In Grape Apple, pear, prune, grape, cherry, apricot Orange, grapefruit Berries, citrus, pineapple Source: From J. Solms, Nonvolatile Compounds and the Flavor of Foods, in Gustation and Olfaction, G. Ohloff and A.F. Thomas, eds., 1971, Academic Press. [...]... Taste of Some Selected Peptides Taste Flat Sour Bitter Sweet Biting Composition of Peptides L-Lys-L-Glu, L-PhE-L-Phe, GIyGIy-GIy-GIy L-Ala-L-Asp, y-L-Glu-L-Glu, GIyL-Asp-L-Ser-Gly L-Leu-L-Leu, L-Arg-L-Pro, L-VaIL-VaI-L-VaI L-Asp-L-Phe-OMe, L-Asp-LMet-OMe y-L-Glutamyl-S-(prop -1 -enyl)-Lcystein Source: From J Solms, Nonvolatile Compounds and the Flavor of Foods, in Gustation and Olfaction, G Ohloff and... 8x10 8 ular configurations The odor potency of vaEthyl mercaptan 1 x 10 9 rious compounds ranges widely Table 7 -1 1 1, 3-Xylen-4-ol 2x10 12 indicates a range of about eight orders of ji-Xylene 2x10 1 2 magnitude (Teranishi 19 71) This indicates that volatile flavor compounds may be present Acetone 6x10 1 3 in greatly differing quantities, from traces to Source: From K.B Doving, Problems in the Physiol-... the aroma of heated foods is the furanones Teranishi (19 71) summarized the findings on several of the furanones (see Figure 7-2 3) The 4-hydroxy-2,5-dimethyl3-dihydrofuranone (1) has a caramel or burnt pineapple odor The 4-hydroxy-5-methyl-3dihydrofuranone (2) has a roasted chicory root odor Both compounds may contribute to beef broth flavor The 2,5-dimethyl-3dihydrofuranone (3) has the odor of freshly... 1, 10-dihydronootkatone have One of these, the y-lactone with a total chain a grapefruity flavor (Figure 7-2 4) Several length of 10 carbons, has peach flavor The other related compounds have a woody flaa-hydroxy-p-methyl-y-carboxy-A^-y-hexvor The odor character of stereoisomers eno-lactone occurs in protein hydrolysate may be quite different The case of menthol and has very strong odor and flavor of beef has already... has a scent reminiscent of chrysanthemum The 2-trans-6-cis Ethanol 10 0,000 nonadienal smells of cucumber and is quite Butyric acid 240 different from the smell of the 2-trans-6Nootkatone 17 0 trans isomer (nonadienal, CHO-CH=CHHumulene 16 0 (CH2)2-CH=CH-CH2-CH3) Lengthening Myrcene 15 of the carbon chain may affect odorous n-Amyl acetate 5 properties The odor of saturated acids A7-Decanal 0.0 changes remarkably... single class of compounds, odor potencies showed a range of eight orders of magnitude equal to that of the widely varying compounds listed in Table 7 -1 1 The compounds examined by Seifert et al (19 70) are listed in Table 7 -1 3 2-methoxy-3-isobutylpyrazine appears to be the compound responsible for the green pepper odor Removal of the methoxy- or alkyl- groups reduces the odor potency by 10 5 to 10 6 times,... zine are needed to distinguish 10 ,000 odors on a 6 2-methoxy-3-propylpyrayes-or-no basis, but more than 20 might be zine required to respond rapidly and without 2 2-methoxy-3-isopropylpyraerror Many attempts have been made to claszine 400 sify odors into a relatively small number of 2-methoxy-3-ethylpyrazine 4000 groups of related odors These so-called pri2-methoxy-3-methylpyramary odors have been... (about 20 mL) of air would be 1 x 10 10 molecules Obviously, only a fraction of these Odor Threshold 12 would interact with the receptors, but (Parts perl O undoubtedly numerous interactions are reParts of Water) Compound quired to produce a neural response 1 2-methoxy-3-hexylpyrazine Dravnieks (19 66) has indicated that accord2-methoxy-3-isobutylpyra2 ing to information theory, 13 types of sensors zine... amounts ogy of Olfaction, in Symposium on Foods: The Chemistry and Physiology of Flavors, H.W Schultz et al., The musks are a common illustration of eds., 19 67, AVI Publishing compounds with different structures that all difference between the cis- and trans- forms of 3-hexenol (CH2OH-CH2-CH=CH-CH2CH3) The ds-isomer has a fresh green odor, Odorant Threshold (\ig/L of Water) whereas the frans'-isomer has... in Symposium on Foods: The Chemistry and Physiology of Flavors, H.W Schultz et al., eds., 19 67, AVI Publishing Co MOLECULAR 2 CROSS-SECTION AL AREA (A.) Next page (J T OAVlES ) -AG0/w(CALORIES MOLE"') Figure 7-3 0 Plot of Molecular Cross-Sectional Area Versus Free Energy of Adsorption for Davies' Theory of Olfaction DESCRIPTION OF FOOD FLAVORS The flavor impression of a food is influenced by compounds . Solutions Acid Hydrochloric Tartaric Malic Phosphoric Acetic Lactic Citric Propionic Taste +1. 43 O -0.43 -1. 14 -1. 14 -1. 14 -1. 28 -1. 85 Total Acidg/L 1. 85 3 .75 3.35 1. 65 3.00 4.50 3.50 3 .70 pH 1 .70 2.45 2.65 2.25 2.95 2.60 2.60 2.90 lonization Constant 1. 04 x 10 ~ 3 3.9X10" 4 7. 52. proposed (Shallenberger 1 9 71 ). Shallenberger and Acree (19 67, 19 69) pro- TASTELESS BITTER SWEET Figure 7- 8 Taste of Substituted Ethoxybenzenes BITTER TASTELESS SWEET Figure 7- 7 Taste of Substituted. Solutions Acid Hydrochloric Tartaric Malic Phosphoric Acetic Lactic Citric Propionic Taste +1. 43 O -0.43 -1. 14 -1. 14 -1. 14 -1. 28 -1. 85 Total Acidg/L 1. 85 3 .75 3.35 1. 65 3.00 4.50 3.50 3 .70 pH 1 .70 2.45 2.65 2.25 2.95 2.60 2.60 2.90 lonization Constant 1. 04 x 10 ~ 3 3.9X10" 4 7. 52 x 1Q- 3 1 .75 x 10 ~ 5 1. 26 x 1Q- 4 8.4 x 1Q- 4 1. 34 x 10 ~ 5 Taste Sensation Hard Green Intense Vinegar Sour, tart Fresh Sour, cheesy Found