Buffer System of Brine Solutions for Pickled Green Olives J
JOSE M. R. DE LA BORBOLLA Y ALCALA, c. GOMEZ HERRERA, AND ROSARIO GUZMAN Instituto de la Grasa, Seccibn de Quimica, Santa Clara, Sevilla, Spain
T
HE production of Spanish-type green olives is acquiring
great importance in various countries. The different phases of this industry, which were practiced originally in a purely empirical manner, have today a firm scientific basis (d), primarily because of the studies carried out by Cruess ( 5 ) and other investigators at the University of California (6). I n this process olives are subjected t o a treatment by a solution of diluted sodium hydroxide. They are then washed with water one or more times to eliminate any excess of lye and are finally placed in casks and covered with a brine solution having a salt content of 10 t o 11%, so t h a t lactic fermentation may take place. I n another paper (3) the existence in these brine solutions of a buffer system, which is of extraordinary importance in the fermentation, has been demonstrated. The maximum value of this system is ordinarily found for p H values of 3.7 to 3.9 (1). However, the changes which this buffer system undergoes in the course of fermentation are of interest because the causes of these changes need to be explained with greater accuracy. Much has been contributed toward a solution of this problem by experiments undertaken on olives of the last harvest (1951) t o study the effects of lye treatment. By means of these studies it was possible to verify the formation of an important quantity of organic acids, both volatile and nonvolatile (4). Contributing t o the buffer system existing in these brine solutions are the following factors:
3. That nonneutralized sodium hydroxide which remains in the olives at the end of the washing 4. Those organic acids formed during the course of fermenta tion. Various aspects of the influence of the buffer system on the process of pickling will be considered briefly. DEVELOPMENT OF THE BUFFER SYSTEM
I n Figures 1 and 2 are presented the buffer index curves of the brine solutions of a cask of Sevillano olives and another of Manzanillo olives in different phases of the fermentation process I n each case the point corresponding t o the state of the brine solutions is indicated at the moment when these curves were constructed. The series of these points allows the tracing of the line representing the evolution of the brine solution during lactic fermentation. These curves show that, logically, the capacity of the buffer system increases as diffusion of the solution through the skin of the fruit and the lactic fermentation are proceeding, The influ-
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1. Those sodium salts of the organic acids originally present in the fruit and not eliminated during - the lye treatment nor during the washes 2. The sodium salts of the acids formed during the lye treatment and subsequent washes not eliminated by this latter operation
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Figure 1. Development of Buffer System ih Brine Solution of SevilIano Olives
ence of those weak acids formed during the lye treatment and the washings is seen clearly; this influence is shown even better in Figure 3, in which are plotted the buffer index curves of pitted Sevillano olives, treated and nontreated with sodium hydroxide. But as the fermentation proceeds, thus incre&ing the quantity of lactic acid in the brine, the buffer system tends to approximate, each time more closely, that formed by the lactic acid and its sodium salt, which has its maximum a t a p H value of 3.7 t o 3.8. The same result is reached with the statistical study of the correlation of total acidity and of the p H in brine solutions of c a s h of the different varieties of olives employed in this industry. 2227
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 3. Buffer Index Curves of Sevillano Olives Treated and Nontreated with Sodium Hydroxide REGULATION O F pH
The line which represents the evolution of the brine solution demonstrates that in the first days the value of the buffer capacity] p, is very small. This fact is of great importance, inasmuch as under such conditions the first portions of acids give rise t o a more or less rapid decrease of pH; this inhibits, undoubtedly, any microbial alterations. As the fermentation progresses, the value of p increases, giving rise t o a growing stabilization of pH-that is, to a fixation of the hydrogen ion activity-in values favorable to the development of lactic bacteria even in the presence of high quantities of acids. On the other hand, if this buffer system did not exist, the first portions of acids would rapidly decrease the p H t o a value too low for good fermentation, Thus, for example, in olives untreated with lye, placed directly in brine solutions of normal concentration, the pH very swiftly reaches a value of 3.7, which according to microbiological study inhibits the development of lactic bacteria. With time the point representative of the brine solution tends to arrive a t the maximum of the buffer index curves. In practice it should not leave this maximum because the buffer capacity under such conditions (from as much as 0.130 equivalent per liter and per unity of pH) is very strong and because the lactic bacteria has slight activity. This maximum may easily be reached with Sevillano olives. With Manzanillo olives, however, this maximum is not easily reached because of the smaller quantity of lactic acid formed during the fermentation. INFLUENCE OF OTHER ACIDS
If, as a result of a defective development of the pickling process, the proportions of acids weakcv than lactic acid (acetic acid, etc.)
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Buffer Index Curve of Brine Solution of a Zapatero Cask
Vol. 44, No. 9
were such that the maximum of the buffer index curves stood a t a p H value higher than 4.2, the brine solution would tend to establish itself a t the p H corresponding t o this maximum. In such cases it is not possible for the p H to reach an adequate value, even though total acidity as determined by titration might be relatively elevated; this results in important inconveniences foi the good preservation of olives, inasmuch as under such conditions dangerous microbial alterations. such as zapateria, may occur. Such a possibility may be observed experimentally in various instances, one of which is indicated in Figure 4, which shorn s the buffer index curve of the brine solution of a zapatero cask. In consequence of these results, experiments arp being carried out on the displacement of the buffer system Llith promising results INFLUENCE OF SODIUM HYDROXIDE QUANTITY
Inasmuch as the maximum values of the buffer capacity are a growing function of the concentration of the salt of a weakacidand strong base, undoubtedly the one factor which directly affects the development of fermentation is the quantity of sodium hydroxide, which, free or combined with organic acids, is retained by olives after washing. To study this effect, semi-industrial experiments have been carried out by adding different quantities of sodium hydroxide to brine solutions of olives, which are otherwise equally treated with lye and washing. The buffer index curves after 3 days showed the formation in brine solutions of acids wealter than lactic acid, their proportion being greater whm more sodium hydroxide was added to the brine. Moreover, the formation of acids in the fermentation indicates the existence of an optimum zone of values for the concentration in brine of the sodium salts of organic acids, above and below which the acidity attained is inferior. STABILIZATION OF pH AT CONCLUSION OF FERMENTATION
Because of rebrining, the concentratioii of the sodium salts of organic acids decreases dowly but continually. The total acidity decreases in the same proportion. Therefore] if the brine has a strong buffer capacity, the dilution should not appreciably change the p H value, which will not alter appreciably during the whole of the normal period of the preservation of the olives. This also should occur when the sorting and grading takefi place as soon as fermentation is completed, because in this case when the cask is filled with fresh brine, the relation between the concentration of organic salts and that of their respective acids is not varied. CONCLUSION
The considerations outlined here amply demonstrate the importance of the character of the buffer system of brine solutions in the course of lactic fermentation, as well as in the subsequent, preservation of olives. LITERATL-RE CITED
( I ) Horbolla y Alcalb, J. hl. R. de la, “Problemas en el adereeo de aoeitunas,” Sevilla, Spain, Asociaci6n de Exportadores de Aceitunas Sevillanas, 1951. ( 2 ) Horbolla y Alcalb, J . M. 1%.de la, and G6mee Herrera, C., Rei:. C i e n c . A p l i c a d a (iWad?id), 3, 120-32 (1949). ( 3 ) Rorbolla y Alcalb, J. M. R. dela, B6mez Herrera, C., and Gutierrea G-Quijano, R., Anales fis. 1/ qzcim. ( M a d r i d ) , 47B, 515-22 (1951). (4) Rorbolla y AlcalB, J. M. R. de la, G6mee Herrera, C., and Guembrl, Rosario, paper presented at Congress of Asociaci6n EspaAola para el Progreso de las Cienoieas, MBlaga, 1951. (5) Cruess, U‘.V., Calif. Agr. Exp. Sta., BUZZ. 498, 3-42 (1930). (6) Vaughn, R. H., Douglas, H. C., and Gililland, J. R . , Calif. Agr. Exp. Sta., BUZZ.678,3-82 (1943). RECEIVED for review March 4, 1952.
ACCEPTEDApril 21, 1952.
