THE RELATIONSHIPS BETWEEN TOTAL ACIDITY, TITRATABLE ACIDITY AND ph IN WINE

Transcription

1 THE RELATIONSHIPS BETWEEN TOTAL ACIDITY, TITRATABLE ACIDITY AND ph IN WINE Rger Bultn Assistant Prfessr f Enlgy, Department f Viticulture and Enlgy, University f Califrnia, Davis, Califrnia Presented at the Annual Meeting f the American Sciety f Enlgists, June 24, 1979, Las Vegas, Nevada. Manuscript submitted September 28, Revised manuscript received Octber 26, Accepted fr publicatin Octber 26, The fundamental relatinships that exist between the ttal acidity (the equivalence f the acid anins as measured by spectrphtmetric r chrmatgraphic methds), the titratable acidity (as measured by titratin t a ph endpint with a strng base) and ph in wines are elucidated fr the first time. The acid and mineral analyses f 21 Eurpean wines are used t ABSTRACT cnfirm the first f these relatinships. A pssible mechanism fr the relatinship is suggested. The values f titratable acidity and ph predicted frm the peratin f this mechanism are cmpared with the reprted values fr a number f wines. Acid and mineral analyses f Califrnian wines are used t cnfirm the general nature f the relatinships. The synthesis f rganic acids during the physilgical develpment f grape berries and the micrbial fermentatin f their juice are the principal surces f these cnstituents in wines. The equilibrium between these acids is prbably the mst imprtant f the many physical and chemical systems fund in wines. The acid cmpsitin and ph are f fundamental imprtance nt nly t the perceptin f surness and related rganleptic prperties, but als the micrbial stability (7), the incidence and extent f the mal-lactic fermentatin (8), the slubility f ptassium bitartrate and calcium tartrate (2), the rates f frmatin and hydrlysis f esters, the iniatin (1) and rate f plymeriatin (11) f anthcyanin pigments in red wines and the incidence f prtein instabilities (9) in white wines. In this wrk, ttal acidity is defined as the prtn equivalence f the rganic acid anins. It is the number f prtns that the rganic acids wuld cntain if they were undissciated. It is calculated by measuring the acid anin cncentratins (by spectrphtmetric r chrmatgraphic prcedures), expressing these as mlar quantities and multiplying by the number f prtns that wuld result frm cmplete acid dissciatin. The titratable acidity is defined as the number f prtns recvered during a titratin with a strng base t a specified endpint. It will als be expressed as a mlar quantity. Unfrtunately, many authrs refer t the titratable acidity as the "ttal acidity" while thers use the term "ttal titratable acidity." As will be seen shrtly, the titratable acidity is always less than the ttal acidity, as defined in this wrk, and t use these terms interchangeably is quite misleading. Althugh titratable acidity and ph have been rutinely measured and used as quality parameters fr decades, the general relatinships which describe these measures have eluded many researchers. Tw f the many reprts which have attempted t crrelate varius acidity measures with either mineral and/r rganic acid levels are thse f Wejnar (15) and Smers (12). Wejnar analyed 50 juice samples frm several German cultivars including Riesling, Mfiller- Thurgau and Sylvaner and analyed the crrespnding wines. Subsequent attempts t linearly crrelate 16 different cmbinatins f mineral and acidity measures shwed that thse relatinships which were applicable t the wine analyses were nt equally applicable t the riginal juices. Fr example, the cmparisn f titratable acidity with ph led t crrelatin cefficients f in the juices and in the wines. The mst significant relatinships amng thse btained were fr (a) ptassium with ph and (b) the rati f tartaric acid t ptassium with ph. Substitutin f the cncentratin f free hydrgen ins fr ph in these cmparisns resulted in crrelatin cefficients f and respectively. Smers (12) presented an inverse relatinship between the free hydrgen in cncentratin and the ptassium cntent fr Australian red table wines f tw vintages. The cnstants in the relatinship were dependent n the wir.e type; different values were btained fr wines made frm Cabernet Sauvignn and a grup f blended wines. These empirical relatinships (12,15) are limited t the wines frm which they were derived and are nt generally applicable t ther wines. They appear t be quite cntradictry, but they can be cnsidered t be particular cases f the mre general acidity relatinship (3). The mre general and fundamental apprach t de-, Am. J. Enl. Vitic., Vl. 31, N. 1,

2 FUNDAMENTAL RELATIONSHIPS- 77 fining the acidity relatinships in wine presented here, makes a clear distinctin between the dependent variables (titratable acidity and ph) and the independent variables (cncentratins f rganic acids) n which they depend. This apprach has led t the predictin f a prtn/mineral exchange mechanism nt previusly reprted r suggested in grapes, and general relatinships which are nt limited t particular wines, cultivars r cultural cnditins. THEORY The key t understanding the acid equilibrium in wines is the realiatin that nt all f the hydrgen ins expected frm the acids f grape juice r wine are fund during the determinatin f titratable acidity. It is pstulated that the cause f this phenmenn is the stichimetric exchange f hydrgen ins frm the acids in the berry with mnvalent metal catins such as ptassium and sdium. This exchange is thught t ccur acrss the membranes f berry cells. It is prpsed that a specific enyme system is respnsible fr this exchange and details f its expected activity are the subject f anther reprt (4). It is sufficient t say that this enyme has a strng preference fr ptassium ver sdium and ther mnvalent metal catins. Further, it is cnsidered that this is the nly mechanism by which these ins enter the cells f the berry. One test f this exchange hypthesis lies in the predictin that the ptassium and sdium fund in the wine are there in place f the acid derived prtns. If this is s, the sum f the ptassium, sdium and titratable hydrgen ins shuld be numerically equal t the number f hydrgen ins expected frm the ttal acid cmpsitin, and the prpsed acidity relatinship is: Ptassium ins Prtns + Sdium ins = expected frm (I) + Titratable prtns acid cmpsitin. A secnd test f the hypthesis invlves a cmparisn f experimentally determined values f ph with thse f the acid mixture whse prtns were remved in equal numbers t the ptassium and sdium ins. The prpsed ph relatinship is" ph = f [Acid cncentratins, ptassium and (II) sdium cncentratins and ethanl cntent]. MATERIALS AND METHODS The calculatin prcedure was applied t 21 Eurpean wines whse published analyses were sufficiently cmprehensive t test the prpsed relatinships. Specifically, these were nine Algerian (6), eight French (10), tw German (14) and tw Italian (13) wines. Independent analyses f three Califrnian wines were als used. These wines were made frm several grape cultivars and include white, red and rs~ styles with varius degrees f sweetness. The general acidity relatinship: The acid cn- centratins f each wine were cnverted t mlar quantities. Then the crrespnding number f hydrgen ins expected frm these acids during titratin with a strng base, referred t hereafter as the "expected prtns" was calculated. The "expected prtns" is a measure f ttal acidity. The titratable acidity, ptassium and sdium cncentratins were als expressed as mlar quantities. The numerical equivalence f the ppsite sides f equatin (1) was then tested by a linear regressin analysis using a Tektrnix 4051 Cmputer. A crrelatin which mitted the cntributin due t minerals (that is, f titratable prtns versus the expected prtns) served as a reference. Similarly the cntributin due t sdium alne was als tested by the missin f data representing this element. The predictin f titratable acidity: Transfrmatin f equatin (1) prvided a frmula fr predictin f titratable acidity frm the acid cncentratins and the ptassium and sdium cntents, vi. Titratable prtns = expected prtns - (ptassium ins + sdium ins). This calculatin was perfrmed fr each f the nine Algerian wines. The predictin f ph: The ph f each Algerian wine was predicted frm the iterative slutin f the iniatin equatins fr the seven majr acids using the same cmputer, details f this slutin prcedure are reprted elsewhere (5). The acids cnsidered in this study were tartaric, malic, lactic, succinic, citric, acetic and sulfurus. RESULTS The general acidity relatinship: The first relatinship cnsidered was that between the titratable acidity and the expected prtns (Fig. 1). The slpe, r regressin cefficient, the intercept and the crrelatin cefficient are presented in Table 1. Als included in Table 1 are the standard errr estimates f the slpe and intercept values and these are shwn in parentheses. Table 1. Statistical summary f varius acidity relatinships fr 21 Eurpean wines. Y variable a Regressin Y intercept Crrelatin (mm/l) cefficient (mm/l) cefficient Titratable acidity b (0.059) c (6.535) Titratable acidity ptassium and sdium (0.018) (1.968) Titratable acidity and ptassium (0.023) (2.537) a X variable = expected cncentratin f prtns (mm/l). b End pint ph = 7. c (Standard errr estimate.) d p < "**d 0.996*** 0.993*** The secnd relatinship, which tested the validity f equatin (1), is shwn in Fig. 2. The regressin parameters and errr estimates are presented in Table 1. The details f a third relatinship in which the sdium cntent was excluded are als presented in Table 1. Am. J. Enl. Vitic., Vh 31, N. 1, 1980

3 78- FUNDAMENTAL RELATIONSHIPS E v f- rv 1. hj Q3 --~ 80 < I--.< E H EXPECTED PROTONS (mm) Fig. 1. The relatinship between titratable prtns and prtns expected frm the acid anin cntent. 2O ' 160 t,-~" 8,. < d 4 i [ EXPEC'IED PROTONS (mm) Fig. 2. The relatinship between the sum f Na+, K + and titratable prtns and prtns expected frm the acid anin cntent. The predictin f titratable acidity: The predicted titratable acidities are cmpared with the measured values in Table 2. The predictin f ph: Table 3 presents the cmparisn f the measured ph values with thse predicted frm cnsideratins f mineral exchange and acid equilibria. Table 4 presents a cmparisn f the predicted titratable acidities and ph fr the Califrnian wines with their measured values. DISCUSSION The ttal acidity f a wine differs appreciably frm Table 2. Cmparisn f predicted and reprted titratable acidities f Algerian wines.a, b Predicted Reprted a Expressed as grams f tartaric acid per liter. b Data f Bremnd (6). Table 3. Cmparisn f predicted and reprted ph values f Algerian wines, a Predicted a Data f Bremnd (6). Reprted Table 4. Cmparisn f predicted and measured titratable acidity and ph f three Califrnian table wines. Blended white GewQrtraminer Gamay Predicted withut exchange a (2.28) (2.25) (2.30) Predicted with exchange (3.00) 81.9 (3.21) 83.0 (3.33) Measured 97.3 (3.13) 78.6 (3.39) 86.6 (3.24) a Expressed as millimles per liter. that f a juice in that lactic, succinic, citric, acetic and sulfurus acids can be, and generally are, at significant cncentratins. The tartaric and malic acid levels are nt accurate indicatrs f either ttal r titratable acidities. The imprtance f lactic and succinic acids frmed during the alchlic fermentatin, has ften been verlked, but cannt be verstated. The cnditins influencing their frmatin certainly warrant further attentin. The cntributin f sulfur dixide t the titratable acidity will depend n the frm in which it was added. If added as a sulfurus acid slutin, tw prtns per sulfite in will result, but if added as a bisulfite salt, nly ne prtn per in will result. The extent t which the bisulfite ins are bund t acetaldehyde and similar cmpunds will affect the ph equilibria, but nt the titratable acidity. The crrelatin between the titratable acidity and the ttal acidity (expressed as the expected prtns) shws cnsiderable scatter. The slpe f the line is indicating that nly 74% f the expected prtns are recvered by titratin. It must be remembered that these analyses were titrated t a ph endpint f 7.0, as is the Eurpean practice. Based n the study by Wng and Caputi (16) the crrespnding value fr a ph end- Am. J. Enl. Vitic., Vl. 31, N. 1, 1980

4 FUNDAMENTAL RELATIONSHIPS- 79 pint f 8.2 is between six t eight percent higher. Even with this crrectin, nly abut 80% f the expected prtns are recvered by titratin. When the relatinship based n the exchange hypthesis is tested, the slpe becmes and the scatter, as indicated by the standard errr estimate in the slpe, is substantially reduced. When crrected t the higher endpint, essentially all f the expected prtns are accunted fr. The nn-er intercept in this crrelatin is due t the accumulated errrs f the analyses and des nt invalidate this cnclusin. The imprved crrelatin cefficient and errr estimates f the prpsed relatinship are significant in that they have ccurred in spite f the inclusin f tw additinal quantities, the ptassium and sdium cntents. This indicates that deviatins in titratable acidity are cmpensated fr by similar, but ppsite deviatins in the ptassium and sdium cntents. Hence, the prpsed relatinship is cnfirmed, suggesting that there is indeed direct exchange f these species in the grape berry. The inclusin f sdium in the acidity relatinship is based n the belief that the enyme respnsible fr the exchange will transfer any mnvalent metal catin, but that it has a strng preference fr ptassium and a weaker preference fr sdium. The inclusin f sdium is supprted statistically by the imprved crrelatin cefficient and errr estimates when cmpared with ptassium alne (Table 1). It is supprted numerically by the mre cmplete recvery f expected prtns. The predicted titratable acidity values (Table 2) have an uncertainty due t the accumulatin f the errrs in the cncentratins f the individual acids, ptassium and sdium. If an uncertainty f five percent is assumed in each f these quantities, the uncertainty in the predicted titratable acidity is at least five percent and mre likely seven percent. There is agreement within this range fr all f the wines cnsidered. Similar errr cnsideratins apply t the predictin f ph, but additinal uncertainty arises frm the limited infrmatin cncerning dissciatin cnstants in ethanl slutins and due t cmplexing f certain species invlved in the equilibria, particularly bisulfite and bitartrate ins. In all cases the predicted value is within 0.2 ph units f the measured value and this crrespnds t apprximately 2 x 10-4 mles per liter f free hydrgen ins. The predicted titratable acidity and ph values fr the blended white, Gewfirtraminer and Gamay wines frm Califrnia (Table 4) are als within the abve errr limits. This further demnstrates the generality f these relatinships. The metal catins, calcium, magnesium, inc and cpper, are cnsidered t play n rle in either the prpsed exchange r the ph equilibrium. The ther acids fund in wines such as glucnic, glyxylic, glucurnic, pyruvic, the phenlic acids and the amin acids are present in very lw mlar cncentratins and cntribute little t bth the titratable acidity and ph. Anins such as sulfate, nitrate and chlride have n effect as bases because their crrespnding acids are cmpletely dissciated at wine ph. The phsphate cntent culd, hwever, influence the titratable acidity and ph t a minr extent depending n the frm in which it enters the berry. This crrectin is cnsidered t be f secndary imprtance nly, and is limited by ur present understanding f the phsphate transprt mechanism in the vine. The influence f varius wine treatments n the validity f these relatinships warrants sme cmment. Any treatment which remves r adds species frm ne side f the acidity relatinship by a different amunt than thse frm the ther side will lead t deviatins frm these relatinships. Treatments such as the carbnate deacidificatin, either by a single salt r a duble salt precipitatin, wuld nt influence the relatinship if the lss f prtns was equal t the crrespnding lss f acid anins. Hwever, the prtn lss will be essentially immediate and cmplete, whereas saturatin must be exceeded befre the acid salts precipitate. With catin exchange treatments, the exchange f ptassium fr either sdium r hydrgen wuld nt affect the relatinship if an ideal interchange ccurred. Hwever, the remval f calcium and magnesium during this treatment will lead t deviatins frm the relatinships. The precipitatin f ptassium bitartrate wuld nt nrmally be f cncern, but the remval f tartrate ins as the calcium salt r the nnstichimetric remval f ptassium will intrduce errrs int the relatinships. The additins f varius acids r their acid salts t a wine will nt affect the relatinships. Fr example, the additin f ptassium salts f metabisulfite r srbate will make such cntributins t the ptassium levels, but these will be cmpensated fr prvided the anins are als included in the calculatins. CONCLUSION The general relatinships fr titratable acidity and ph in wines have been described and cnfirmed. A hypthesis that ptassium and sdium ins enter grape berries by direct exchange fr prtns frm the fruit acids is prpsed. The predicted values f titratable acidity and ph based n the prpsed relatinships are in clse agreement with measured values fr wines f varius cuntries, cultivars and types. LITERATURE CITED 1. Berg, H. W. Stabilisatin des anthcyannes. Cmprtement de la culeur dans les vins ruges. Ann. Technl. Agric. 12: (1963). 2. Berg, H. W., and R. M. Keefer. Analytical determinatin f tartrate stability in wine. Am. J. Enl. 9: (1958); 10:105-9 (1959). 3. Bultn, R. B. The general relatinship between ptassium, sdium and ph in juices and wines. Accepted fr publicatin. Am. J. Ent. Vitic. 4. Bultn, R. B. A hypthesis fr the presence f a hydrgen/ ptassium adensine triphsphatase in grape tissue. (In preparatin). Am. J. Enl. Vitic., V!. 31, N. 1, 1980

5 80- FUNDAMENTAL RELATIONSHIPS 5. Bultn, R. B., and R. A. Ward. An algrithm fr the predictin f titratable acidity and ph in fruit juices and wines. (In preparatin). 6. Bremnd, E. Cntributin a l'~tude analytique et physicchimique de l'acidit~ des vins. p. 96. Carbnel, Algiers. (1937). 7. Dupuy, P. Les facteurs du develppment de l'acescence dans le vin. Ann. Technl. Agric. 6: (1957). 8. Frnachn, J. C. M. The ccurrence f mal-lactic fermentatin in Australian wines. Aust. J. Appl. Sci. 8:120-9 (1957). 9. Mretti, R. H., and H. W. Berg. Variability amng wines t prtein cluding. Am. J. Enl. Vitic. 16:69-78 (1965). 10. Peynaud, E. Analyses cmpletes de huit vins dux naturels. Ann. Agrn. 1:382-8 (1950). 11. Smers, T. C. The plymeric nature f wine pigments. Phytchemistry 10: (1971). 12. Smers, T. C. A cnnectin between ptassium levels in the harvest and relative quality in Australian red wines. OIV. Int. Symp. Quality Vintage. p (1977). 13. Tarantla, C. Studi chimic e fisic-chimic dell' Asti spumante e dell spumante Italian. Ann. Sper. Agrar. 7: (1932). 14. Vn der Heide, C., and W. J. Baragila. Beitr~ige ur chemie und analyse des weines. Untersuchung eines weines, Kritik der verfahren und deutung der ergebnisse. Landwirtsch. Jahrb. 39: (1910). 15. Wejnar, R. Etude de l'influence de l'acide tartarique et de l'acide malique sur le ph du vin. Cnnais. Vigne Vin 5(4): (1971). 16. Wng, G., and A. Caputi, Jr. A new indicatr fr ttal acid determinatin in wines. Am. J. Enl. Vitic. 17:174-7 (1966). Am. J. Enl. Vitic., Vl. 31, N. 1, 1980