COMPARISON OF SOURNESS OF ORGANIC ACID ANIONS AT EQUAL pH AND EQUAL TITRATABLE ACIDITY

The sourness of six organic acids was compared by pair tests in binary acid solutions of equal pH and equal potential hydrogen ion concentration. Monoprotic lactic acid was more sour than three of the four acids against which it was tasted, while triprotic citric acid was less sour than the four other acids against which it was compared. Of the diprotic acid pairs which were evaluated, only one significant difference in sourness was observed; succinic acid was more sour than malic acid. No relationship between buffer capacity or hydrophobicity and intensity of sourness was observed.     

Astringent Subqualities in Acids

Astringency, astringent subqualities (drying, roughing and puckering)and sourness were compared among six acids: hydrochloric, lactic,citric, acetic, fumaric and malic acids. The attribute profilesof organic acids were similar to each other but different fromhydrochloric acid, the only inorganic acid, which was the mostastringent and the least sour. In a second experiment, two inorganicacids (hydrochloric and phosphoric) and two organic acids (citricand malic) were tested at three concentration levels. At approximatelyequal levels of overall sensory impact, the inorganic acidswere alike in astringency and sourness, receiving higher ratingsfor roughing and drying, and lower ratings for sourness thanthe organic acids. Interactions with concentration (differencesin psychophysical functions) for the subquality of drying werenoted, in addition to the differences in the astringent subqualitiesof roughing and drying seen across acids in both experiments.The higher level of astringency for inorganic acids suggeststhat the current model for tannin binding to salivary proteinsas an explanation of astringency needs to be extended to includea direct pH-dependent effect. Chem. Senses 20: 593–600,1995     

INTERACTION OF ASTRINGENCY AND TASTE CHARACTERISTICS1

The perception of astringency and basic taste in mixtures and their interaction effects were investigated by two procedures. In Experiment 1, focused and nonfocused testing procedures were compared using mixtures of low and high concentrations of alum and basic taste solutions. Both procedures yielded taste and astringency intensities that were modality‐dependent. Nonfocused testing was used in Experiment 2 to investigate the interactions of astringent phenolic (tannic acid) and nonphenolic (alum) compounds with each basic taste. Sweetness of sucrose increased with increased concentration with or without alum or tannin present. Changes in salty, bitter, and sour taste intensities were modality‐dependent. Astringency either remained unchanged or decreased with the addition of sucrose, sodium chloride, citric acid, or caffeine depending upon the taste concentration. Bitterness of tannin and alum at high concentrations was suppressed by the addition of sucrose, sodium chloride, or citric acid; sourness also decreased in the presence of sucrose or sodium chloride as well as a high level of caffeine.     

Physicochemical Aspects of Chitosan Dispersibility in Acidic Aqueous Media: Effects of the Food Acid Counter-Anion

Differences in formation of colloidal dispersions of chitosan in aqueous solutions of citric acid or lactic acid (25, 50 or 100 mM) were quantitatively studied. Protonation enthalpies, electrical conductivity and ζ-potential measurements were additionally undertaken, aiming at better understanding these differences at a molecular level. In dispersion kinetics assays, experimental data were well fitted (R²?>?0.9; MAPE?<?4 %) by a first-order kinetics model with two terms - one accounting for the fast, direct dispersion of biopolymers chains and another accounting for the slow dispersion of chains from lumps. In all cases, maximal dispersibility was reached after about 20?30 min of stirring. For both acids, the higher the acid concentration in the medium, the higher was the chitosan dispersibility. At a given acid concentration, chitosan showed higher dispersibility in lactic acid than in citric acid solutions. Protonation of chitosan -NH₂ groups was strongly exothermic, with ΔH values three times higher for citric acid (triprotic) than lactic acid (monoprotic) (ΔH?=??120 kJ?mol^{- 1} and ΔH?=??40 kJ?mol^{- 1}, respectively), indicating that chitosan -NH₂ protonation itself was not dependent on the type of acid. However, the electrical conductivity of suspensions of powdered chitosan in water evolved differently as these systems were titrated with citric or acid lactic. With citric acid, electrical conductivity remained virtually constant for acid concentration?<?of 15 mM, and then increased linearly as the acid concentration increased until 75 mM. Instead, with lactic acid, electrical conductivity progressively increased with increasing of acid concentration from 0 to 75 mM. The ζ-potential of chitosan dispersed particles was +28.5 mV and +52.1 mV in dispersions containing 10 mM of citric and lactic acids, respectively. The conjoint analysis of data from physicochemical analyses suggested that, contrarily to lactate anions, citrate anions bind more strongly on the electrical double layer of protonated, positively charged chains of chitosan, diminishing the inter-chains electrostatic repulsion, thus leading to a lower dispersibility of this polysaccharide in aqueous solutions of citric acid, compared to equimolar solutions of lactic acid.     

Astringency of Organic Acids is Related to pH

Astringency and sourness of lactic, acetic and citric acids,each adjusted to pH 3, 5 and 7, were evaluated in two experiments,one starting at equal concentrations in wt/vol before neutralizationand the second starting at equal molarity. Astringency and sournessdecreased with increasing pH. However, acids were differentiallysour at equal pH, consistent with previous findings. In contrast,the tactile attributes associated with astringency (drying,roughing of oral tissues and puckery/tightening sensations)were similar across acids; pH was the major influence on astringency.Strong dependence on pH suggests that astringency of these acidsis a direct result of their acidic properties, and not solelydue to the hydrogen bonding mechanisms previously suggestedas an explanation of astringency in tannin interactions withsalivary proteins. Chem. Senses 21: 397–403, 1996.     

Sensory evaluation of acids by free-choice profiling

The technique of free-choice profiling was applied in orderto characterize the sensory properties of some common organicand inorganic acids. Analysis of panelists' scores by generalizedProcrustes Analysis (GPA) provided information on the relationshipsamong samples and assessors for both the consensus and individualconfigurations. Results indicated that on a weight basis (w/vor v/v), acids differed in their flavor and taste dynamics.Acids were described differently by individual panelists. The GPA resulted in three important principal axes (PA). Thefirst PA had astringency/mouthfeel as the most important factor,while bitterness and sourness were the most important for thesecond and the third PAs, respectively. At 0.08% (w/v or v/v),the inorganic acids, hydrochloric and phosphoric, were moreastringent than sour. The bitterness of succinic (S) was intenseas was the sourness of fumaric, malic and the combinations offumaric:malic (FM), citric:malic (CM) and citric:fumaric (CF).The sensory characteristics of adipic and quinic were perceivedto be very weak at this concentration. The relationship betweenastringency and pH was more evident than was the relationshipbetween pH and sourness.     

Effect of sweetener type and of sweetener and acid levels on temporal perception of sweetness, sourness and fruitiness

Sweetness, sourness and fruitiness of 18 orange-flavored solutions,with three levels of citric acid (0.75, 1.5, 2.25 g/1) and threeequi-sweet levels of either sucrose (80, 100, 120 g/l) or aspartame(0.6, 0.7, 0.8 g/l), were evaluated by time–intensitymethodology. At these concentrations, a larger range in sournessintensity than in sweetness was produced, resulting in greatersuppression of sweetness by increasing acid levels than of sournessby increasing sweetener levels. Although aspartame samples hada longer duration of sweetness and fruitiness, sucrose and aspartamedid not interact differently with the sourness of citric acid.Fruitiness intensity and duration was enhanced by both sweetnessand sourness, but to a greater extent by sourness. Whether thisenhancement is attributable to a cognitive association of sweetnessor sourness with fruitiness or is due to the inability of thesubjects to separate sweet and sour tastes from orally perceivedfruity flavor cannot be concluded from this study.     

The influence of acid on astringency of alum and phenolic compounds

Astringency of aqueous solutions of phenolic compounds (grape seed tannins, tannic acid, catechin and gallic acid) increased upon addition of citric acid, whereas the astringency of alum was reduced. Astringency of alum was decreased equivalently by addition of equi-sour levels of lactic acid, citric acid or hydrochloric acid. The difference between alum and the phenolic compounds is speculated to result from chemical modifications affecting binding of the astringents with oral proteins rather than cognitive differences. Chelation of the aluminum ion in alum by acids reduces its availability for interacting with salivary proteins or epithelial proteins. In contrast, the increased astringency produced upon acidification of phenolic compounds is speculated to result from the pH driven increase in the affinity of the phenols for binding with proteins. These results suggest that alum cannot be used interchangeably with phenolic astringents in psychophysical studies.      

SOURNESS–SWEETNESS INTERACTIONS IN DIFFERENT MEDIA: WHITE WINE,ETHANOL AND WATER*

The aim of this work was to study the sourness–sweetness interactions in water, white wine and alcoholic environment to interpret sweet/sour perception in low concentrations within the range normally encountered in white wine. Nine trained assessors rated sweetness and sourness intensity in mixtures of fructose (11.1, 25.0 and 38.9 mM) and tartaric acid (pH 3.0, 3.4 and 3.8) in water and wine (experiment 1) or ethanol solutions at 2.0, 4.0 and 12.0% v/v (experiment 2). The range of quantitative responses was larger for sourness than for sweetness in the three media. The global sourness intensity perception in wine mixtures was significantly lower than in water and ethanol mixtures, indicating the effect of other wine components. The suppressive effect of tartaric acid on fructose sweetness was stronger than the suppressive effect of fructose on tartaric acid sourness.     

SELECTION OF AN ASTRINGENCY REFERENCE STANDARD FOR THE SENSORY EVALUATION OF BLACK TEA

Astringent and bitter sensations are characteristic sensory qualities of black tea. Three different classes of potential astringent reference standards (two concentrations each of alum and tannic acid and three fruit juices) were evaluated in this study. The perceived astringency, bitterness and sourness of each were profiled using computerized time-intensity and compared with the astringent intensity of a standardized brew of black tea. The differences in temporal profiles of potential reference standards across taste attributes were evident and intensity ratings were found to be dependent upon the stimulus and its concentration. Both concentrations of tannic acid were evaluated as the highest in perceived bitterness. For the juices, a strong sour taste was perceived in addition to astringency. It was concluded that the best reference standard for the astringency of black tea is a solution of 0.7 g/L alum as it is low in perceived bitterness and sourness.     

Perceptual Properties of Benzoic Acid Derivatives

Sensory properties of equimolar concentrations of benzoic acidderivatives were examined using time-intensity procedures. Significantdifferences in maximum intensities (P < 0.005) were foundfor astringency, bitterness, prickling, sourness and sweetness.Although these compounds differed only in the number and positionof the hydroxy groups, they exhibited quite different profiles.Gentisic acid had the highest sourness and bitterness maximumintensity, salicylic and gentisic acids were highest in astringency,and m-hydroxybenzoic acid was the sweetest sample. Benzoic acidhad the highest intensity of prickling feeling which lasted20 s longer than salicylic acid and 40 s longer than the othersamples which elicited lowest intensity of prickling sensation.Chem. Senses 20: 393–400, 1995.     

Relationships among taste qualities assessed with response-context effects

Psychophysical judgments often depend on stimulus context. For example, sugar solutions are judged sweeter when a tasteless fruity aroma has been added. Response context also matters; adding a fruity aroma to sugar increases the rated sweetness when only sweetness is considered but not when fruitiness is judged as well. The interaction between stimulus context and response context has been explored more extensively in taste-odor mixtures than in taste-taste mixtures. To address this issue, subjects in the current study rated the sourness of citric acid mixed with quinine (bitter), sodium chloride (salty), and cyclamate (sweet) (stimulus context). In one condition, subjects rated sourness alone. In another, subjects rated both sourness and the other salient quality (bitterness, saltiness, or sweetness) (response context). Sourness ratings were most sensitive to response context for sour-salty mixtures (i.e., ratings of sourness alone exceeded ratings of sourness made simultaneously with saltiness) and least sensitive to context for the sour-sweet mixtures (sourness ratings made under the 2 conditions were essentially identical). Response-context effects for the sour-bitter mixture were nominally intermediate. The magnitudes of these context effects were related to judgments of qualitative similarity between citric acid and the other stimuli, consistent with prior findings. These types of context effects are relevant to the study of taste-taste mixture interactions and should provide insight into the perceptual similarities among the taste qualities.     

ACID PENETRATION INTO LIVING TISSUES

The threshold concentrations for sourness of nine acids have been determined with an accuracy of about 8 per cent, and the H⁺ ion concentration of these acids measured. Calculations have been made of the relative concentration gradients of the undissociated acid across the cell membrane for a series of acids having equal sourness and also for a series of acids having equal penetration velocity as determined from experiments by Crozier on Chromodoris and on Allolobophora. For solutions of equal pH a high degree of sourness has been found to be associated with a high penetration velocity of the undissociated acid or of the anion. A comparison of these gradients with the results of adsorption experiments on charcoal indicates that the acids are taken into the tissues by an adsorption process. Polar groups such as OH and Cl and Br are found to have a very marked effect in reducing the ability of organic acids to penetrate living tissues. The important rôle of optical activity of the acids in determining their physiological action has been noted.     

Co-mapping studies of QTLs for fruit acidity and candidate genes of organic acid metabolism and proton transport in sweet melon (Cucumis melo L.)

Sweet melon cultivars contain a low level of organic acids and, therefore, the quality and flavor of sweet melon fruit is determined almost exclusively by fruit sugar content. However, genetic variability for fruit acid levels in the Cucumis melo species exists and sour fruit accessions are characterized by acidic fruit pH of <5, compared to the sweet cultivars that are generally characterized by mature fruit pH values of >6. In this paper, we report results from a mapping population based on recombinant inbred lines (RILs) derived from the cross between the non-sour 'Dulce' variety and the sour PI 414323 accession. Results show that a single major QTL for pH co-localizes with major QTLs for the two predominant organic acids in melon fruit, citric and malic, together with an additional metabolite which we identified as uridine. While the acidic recombinants were characterized by higher citric and malic acid levels, the non-acidic recombinants had a higher uridine content than did the acidic recombinants. Additional minor QTLs for pH, citric acid and malic acid were also identified and for these the increased acidity was unexpectedly contributed by the non-sour parent. To test for co-localization of these QTLs with genes encoding organic acid metabolism and transport, we mapped the genes encoding structural enzymes and proteins involved in organic acid metabolism, transport and vacuolar H+ pumps. None of these genes co-localized with the major pH QTL, indicating that the gene determining melon fruit pH is not one of the candidate genes encoding this primary metabolic pathway. Linked markers were tested in two additional inter-varietal populations and shown to be linked to the pH trait. The presence of the same QTL in such diverse segregating populations suggests that the trait is determined throughout the species by variability in the same gene and is indicative of a major role of the evolution of this gene in determining the important domestication trait of fruit acidity within the species.     

Generalized model of the effect of pH on lactate fermentation and citrate bioconversion in Lactococcus lactis ssp. Lactis biovar. diacetylactis

An aroma-imparting mesophilic lactic starter (Lactococcus lactis ssp. lactis biovar. diacetylactis) was studied in batch culture in medium with 50 g·l^–1 lactose and 2 g·l^–1 citrate. The effect of pH on the physiology of growth and the production of flavour compounds was investigated with a mathematical model. The specific rates of growth and of lactose fermentation obeyed a law of non-competitive inhibition by lactic acid produced, inhibition increasing as the pH of the medium decreased. The pH thus acted indirectly by increasing the proportion of non-dissociated lactic acid, identified as the inhibiting form of lactic acid. The generalized model, taking into account the effect of pH, was tested using fermentations at pH controlled at different values (4.5–6.5), as well as with a fermentation conducted at non-regulated pH. These simulations supported the working hypotheses. The effect of pH on the fermentation of citric acid resulted in an increase in the maximal specific rate of citrate utilization, in the bioconversion yield, and in the constant of diacetyl and acetoin reduction at acid pH. The production of flavour compounds is a complex phenomenon resulting from the interaction of pH, citric acid concentration, and the physiological state of the cells. These results are discussed with respect to the use of this strain in the preparation of manufactured dairy products.     

Quantitative measurements on the acid taste and their bearing on the nature of the nerve receptor

Summary Quantitative determinations of the threshold concentrations for sourness of about 40 organic acids have been determined with an accuracy of about 6 per cent. Hydrogen ion concentrations have been obtained by use of the cell Hg/HgCl/KCl sat./organic acid/H₂. It is assumed that only undissociated acid molecules can cross the phase boundary between the aqueous solution and the nerve receptor, and that the stimulation of the sensation of sourness is due solely to hydrogen ion. All acids tasting equally sour should have the same pH within the nerve receptor. From the experimental data, relative concentration gradients for the nonionized acids have been calculated. It is shown that the addition of polar groups, such as OH, Cl, C=C, C=O, COOH, NH₂ to the organic acid make it from 20 to 500 times more difficult for the organic acid to penetrate into the nerve receptor. On the other hand, lengthening the carbon chain by addition of CH₂ groups makes penetration easier. The phenomena can be largely explained on the hypothesis that these organic acids are adsorbed into a tissue which is essentially like fat rather than like protein.     

Electrophysiological studies of salty taste modification by organic acids in the labellar taste cell of the blowfly.

Using the labellar salt receptor cells of the blowfly, Phormia regina, we electrophysiologically showed that the response to NaCl and KCl aqueous solutions was enhanced and depressed by acetic, succinic and citric acids. The organic acid concentrations at which the most enhanced salt response (MESR) was obtained were found to be different: 0.05-1 mM citric acid, 0.5-2 mM succinic acid and 5-50 mM acetic acid. Moreover, the degree of the salt response was not always dependent on the pH values of the stimulating solutions. The salt response was also enhanced by HCl (pH 3.5-3.0) only when the NaCl concentration was greater than the threshold, indicating that the salty taste would be enhanced by the comparatively lower concentrations of hydrogen ions. Another explanation for the enhancement is that the salty taste may also be enhanced by undissociated molecules of the organic acids, because the MESRs were obtained at the pH values lower than the pKa(1) or pKa(2) values of these organic acids. On the other hand, the salty taste could be depressed by both the lower pH range (pH 2.5-2.0) and the dissociated organic anions from organic acid molecules with at least two carboxyl groups.     

Relationship of Cell Sap pH to Organic Acid Change During Ion Uptake

Excised roots of barley (Hordeum vulgare, var. Campana) were incubated in KCl, K₂SO₄, CaCl₂, and NaCl solutions at concentrations of 10⁻⁵ to 10⁻² n. Changes in substrate solution pH, cell sap pH, and organic acid content of the roots were related to differences in cation and anion absorption. The pH of expressed sap of roots increased when cations were absorbed in excess of anions and decreased when anions were absorbed in excess of cations. The pH of the cell sap shifted in response to imbalances in cation and anion uptake in salt solutions as dilute as 10⁻⁵ n. Changes in cell sap pH were detectable within 15 minutes after the roots were placed in 10⁻³ n K₂SO₄. Organic acid changes in the roots were proportional to expressed sap pH changes induced by unbalanced ion uptake. Changes in organic acid content in response to differential cation and anion uptake appear to be associated with the low-salt component of ion uptake.     

The effective diffusion coefficient and the distribution constant for small molecules in calcium-alginate gel beads

The effective diffusion coefficient, D(e), and the distribution constant, K(i), for selected mono- and disaccharides and organic acids were determined in homogeneous calcium-alginate gel with and without entrapped bacteria. Results were obtained from transient concentration changes in well-stirred solutions of limited volume, in which the gel beads were suspended. The effective diffusioncoefficients and the distribution constants were estimated by fitting mathematical model predictions to the experimental data using a nonlinear model fitting program (MODFIT). Both single solute diffusion and multiple solute diffusion were performed. A small positive effect was obtained onthe values of D(e) for the system of multiple solute diffusion; however, the values of K(i) were not significantly influenced. For the nine solutes tested, D(e) for 2% Ca-alginate gel beads was found to be approximately 85% of the diffusivity measured in water. The effects on D(e) and K(i), for lactose and lactic acid were determined for variations of alginate concentration, pH, temperature, and biomass content in the beads. D(e) decreased linearly for both lactose and lactic acid with increasing cell concentration in the Ca-alginate gel. K(i), was constant for both lactose and lactic acid with increasing cell concentration. D(e) was significantly lower at pH 4.5 than at pH 5.5 and 6.5 for both lactose and lactic acid. Furthermore, D(e) seemed to decrease with increased alginate concentration in the range of 1% to 4%. The diffusion rate increased with increasing temperature, and the activation energy for the diffusion process for both lactose and lactic acid was constant in the temperature range tested. (c) 1995 John Wiley & Sons Inc.