Proposed Theoretical Water Activity Values at Various Temperatures for Selected Solutions to Be Used As Reference Sources in the Range of Microbial Growth

419

Proposed Theoretical Water Activity Values at Various Temperatures for Selected Solutions to be Used as Reference Sources in the Range of Microbial Growth

SILVIA L. RESNIK1* and JORGE CHIRIFE Downloaded from http://meridian.allenpress.com/jfp/article-pdf/51/5/419/1658009/0362-028x-51_5_419.pdf by guest on 26 September 2021 Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Republic of Argentina

(Received for publication March 26, 1987)

ABSTRACT based on the thermodynamic properties of strong electro lyte aqueous solutions, and was applied either to satu This paper compiles recent data on the theoretical prediction rated or unsaturated solutions at different temperatures. of the water activity (a ) of selected saturated salt solutions, w The system of equations developed by Pitzer (7) and his unsaturated NaCl and LiCl solutions, and H2S04 solutions. These results are presented in tabular form in such a way that associates (8,9,14) was successfully used to predict the they can be safely used as reference sources for aw determina aw of selected saturated salt solutions, unsaturated NaCl tion in the range of microbial growth (i.e. about 0.57-0.97) and and LiCl solutions (2,6), and H2S04 solutions (13). at different temperatures. It is the purpose of the present work to compile all these results in such a way that they can be safely used as reference sources for aw determinations in the range Adjustment of the water activity (aw) of foods is an of microbial growth (i.e. about 0.57-0.97) and at differ important method of controlling microbial spoilage (14). ent temperatures. This has been recognized by regulatory agencies in sev eral countries who have incorporated the aw principle in EXPERIMENTAL METHODS the definition of standards for various preserved foods. The U.S. Food and Drug Administration has adopted A brief explanation is given on how the predicted aw values were obtained. the water activity specification in the definition of low- The osmotic coefficient (<)>) is given by the equations de acid food (5) as has the FAO/WHO Codex Alimentarius veloped by Pitzer (7) as, Comission (4) and the European Community for the trade of meat products (3). The reliability of most aw measur 2 x

JOURNAL OF FOOD PROTECTION, VOL. 51, MAY 1988 420 RESNIK AND CHIRIFE

In the case of H2S04 solutions the equations also considered RESULTS AND DISCUSSION + = the dissociation HSO"4 -* i H + SO 4 (9). Values of pa The predicted aw of selected saturated salt solutions in (0) (,) rameters B , B and the approximate range 0.75-0.98 and between 10 and CMX for the solutes of interest to this 37CC is shown in Table 1; it is believed that these values work are reported in the literature (8,14). The parameters 6<0), are "accurate" to about 0.002-0.003 aw (6,11). Results 8(1) and CMX are described by arbitrary functions of temperature for other saturated salt solutions commonly used in this f(T) (6,14). The osmotic coefficient was cowerted to a accord- w range, such as NaBr and KN03 are not given because ing to of some discrepancies between prediction and measured -55.51 In aw [4] 4> = vm values.

Table 2 shows the aw of unsaturated NaCl solutions at any temperature between 15 and 50CC. As shown by where v is the number of particles into which the solute of Chirife and Resnik (2), the a of NaCl solutions in the molality m dissociates. To calculate the a of saturated salt sol w w microbiologically important range, 15C-50CC, may be utions, the molality of solute at saturation (ms) was used in equation [4]. considered almost equal; maximum error is about 0.002 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/51/5/419/1658009/0362-028x-51_5_419.pdf by guest on 26 September 2021 aw. Of course, NaCl solutions are useful as standards

only for values of aw above = 0.76.

Table 3 shows the aw of LiCl solutions between 5 and Accuracy of the predictions 45CC (6). In this instance, the available data extend to The thermodynamic properties of most common inorganic the lower range of microbial growth (about 0.6 aw). Most electrolyte solutions have been exhaustively determined over the of these values may be considered "accurate" to 0.002

past 50 years. Dozens of workers in the physical-chemistry area aw; only at low aw (i.e. below about 0.7 aw) the error determined the activity of water in solutions of NaCl, KC1, may be somewhat higher (i.e. up to about 0.005 aw). LiCl, H2S04, and the like, and reported their data as osmotic It is to be noted that at water activity values above about coefficients for the whole range of concentration and at various C 0.90 the effect of temperature (between 5 and 45 C) on temperatures. Experimental determinations were based on pre a of LiCl solutions does not exceed the 0.002 a level. cise and accurate methods, mainly isopiestic ones. From time w w to time well known researchers critically evaluated the literature At lower water activities the effect of temperature be data on measurement of the osmotic coefficient of most com comes more significant. Finally, Table 4 (13) shows the mon electrolytes and published "best" values of <(> for rounded predicted aw of sulfuric acid solutions between 0 and molalities. Platford (10) critically reviewed the accuracy of 55°C. In this instance, the data cover the entire range isopiestic determinations and concluded that most of this compi of microbial growth. These data may be considered "ac lations are in general accurate to about 0.001 in <(>, which curate" to the 0.002-0.003 level. As observed with LiCl

means that aw values may be considered in most instances to solutions, the effect of temperature on the aw of H2S04 be accurate to about 0.001-0.002 aw. solutions becomes more noticeable at lower water ac As discussed later, the predicted values were compared to tivities (i.e. more concentrated solutions). these tabulated values (compilations) and the deviation between The data in Tables 1 to 4 are thus recommended as predicted and tabulated (measured) values were used as an indi reference sources of a at various temperatures in the cation of the "accuracy" of the predictions. Admittedly, this is w not accurate unless the measured values were absolutely accu range of microbial growth. The accuracy of these pro rate which is not true; however, since the accuracy of the tabu posed values fulfill the actual needs for aw measuring in lated values largely exceeds the needs in food microbiology re food microbiology research and development. It is ex search (i.e. 0.005 aw), we arbitrarily named "accuracy" to the pected that this proposal may contribute to reach a "uni deviation between preditions and tabulated compilations. versal" agreement on the values to be assigned to solu

tions used as reference sources of aw.

TABLE 1. Predicted aw of selected saturated salt solutions between 10°C and 37°Ca Temp. (°C) NaCl (NH4)2S04 KC1 BaCl2 K2S04 10 0.754 0.809 0.867 0.913 0.980 15 0.753 0.808 0.859 0.910 0.979 17 0.753 0.806 0.856 0.909 0.978 19 0.752 0.805 0.852 0.907 0.977 21 0.752 0.804 0.849 0.906 0.977 23 0.751 0.803 0.846 0.905 0.976 25 0.751 0.803 0.842 0.903 0.975 27 0.750 0.802 0.840 0.902 0.975 29 0.750 0.801 0.836 0.900 0.974 31 0.750 0.800 0.833 0.899 0.973 33 0.749 0.799 0.830 0.898 0.973 35 0.749 0.798 0.827 0.895 0.972 37 0.748 0.797 0.823 0.894 0.971 aData at I0°C are from Pollio et al. (11 ); data at the other tempera tures are from Kitic et al. (6).

JOURNAL OF FOOD PROTECTION, VOL. 51, MAY 1988 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/51/5/419/1658009/0362-028x-51_5_419.pdf by guest on 26 September 2021

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21.0 0.649 0.652 0.654 0.657 0.659 0.662 0.664 0.666 0.671 22.0 0.621 0.623 0.626 0.629 0.632 0.634 0.637 0.640 0.645 23.0 0.591 0.594 0.597 0.600 0.603 0.606 0.608 0.612 0.618 aFrom Kitic et al. (6).

TABLE 4. Predicted aw of sulfuric acid solutions between 0 and 55°C

Concen. aw at °C (%) 0 5 10 15 20 25 30 35 40 45 50 55 0.50 .998 .998 .998 .998 .998 .998 .998 .998 .998 .998 .998 .998 1.00 .996 .996 .996 .996 .996 .996 .996 .996 .996 .996 .996 .996 1.50 .994 .994 .994 .994 .994 .994 .995 .995 .995 .995 .995 .995 2.00 .992 .992 .992 .992 .993 .993 .993 .993 .993 .993 .993 .993 2.50 .990 .990 .991 .991 .991 .991 .991 .991 .991 .991 .991 .991 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/51/5/419/1658009/0362-028x-51_5_419.pdf by guest on 26 September 2021 3.00 .988 .988 .989 .989 .989 .989 .989 .989 .989 .989 .989 .989 3.50 .986 .986 .987 .987 .987 .987 .987 .987 .987 .987 .987 .987 4.00 .984 .984 .984 .985 .985 .985 .985 .985 .985 .985 .985 .985 4.50 .982 .982 .982 .982 .982 .983 .983 .983 .983 .983 .983 .983 5.00 .980 .980 .980 .980 .980 .980 .981 .981 .981 .981 .981 .981 5.50 .978 .978 .978 .978 .978 .978 .978 .979 .977 .979 .979 .979 6.00 .976 .976 .976 .976 .976 .976 .976 .976 .976 .977 .977 .977 6.50 .973 .973 .973 .974 .974 .974 .974 .974 .974 .974 .974 .975 7.00 .971 .971 .971 .971 .971 .971 .972 .972 .972 .972 .972 .972 7.50 .968 .969 .969 .969 .969 .969 .969 .969 .969 .970 .970 .970 8.00 .966 .966 .966 .966 .966 .967 .967 .967 .967 .967 .967 .968 8.50 .963 .964 .964 .964 .964 .964 .964 .964 .964 .965 .965 .965 9.00 .961 .961 .961 .961 .961 .961 .962 .962 .962 .962 .962 .963 9.50 .958 .958 .958 .958 .959 .959 .959 .959 .959 .959 .960 .960 10.00 .955 .955 .955 .956 .956 .956 .956 .956 .957 .957 .957 .957 10.50 .953 .953 .953 .953 .953 .953 .953 .954 .954 .954 .954 .955 11.00 .959 .950 .950 .950 .950 .950 .950 .951 .951 .951 .951 .952 11.50 .947 .947 .947 .947 .947 .947 .947 .948 .948 .948 .949 .949 12.00 .944 .944 .944 .944 .944 .944 .944 .945 .945 .945 .946 .946 12.50 .940 .940 .940 .941 .941 .941 .941 .942 .942 .942 .943 .943 13.00 .937 .937 .937 .937 .937 .938 .938 .938 .939 .939 .939 .940 13.50 .934 .934 .934 .934 .934 .934 .935 .935 .935 .936 .936 .937 14.00 .930 .930 .930 .930 .931 .931 .931 .932 .932 .932 .933 .933 14.50 .927 .927 .927 .927 .927 .927 .928 .928 .929 .929 .929 .930 15.00 .923 .923 .923 .923 .923 .924 .924 .925 .925 .925 .926 .926 15.50 .919 .919 .919 .919 .920 .920 .920 .921 .921 .922 .922 .923 16.00 .915 .915 .915 .915 .916 .916 .917 .917 .918 .918 .919 .919 16.50 .911 .911 .911 .911 .912 .912 .913 .913 .914 .914 .915 .915 17.00 .907 .907 .907 .907 .908 .908 .909 .909 .910 .910 .911 .911 17.50 .903 .902 .902 .903 .903 .904 .904 .905 .906 .906 .907 .907 18.00 .898 .898 .898 .898 .899 .899 .900 .901 .901 .902 .903 .903 18.50 .894 .893 .893 .894 .894 .895 .896 .896 .897 .898 .898 .899 19.00 .889 .888 .889 .889 .890 .890 .891 .892 .893 .893 .894 .895 19.50 .884 .884 .884 .884 .885 .886 .886 .887 .888 .889 .889 .890 20.00 .879 .878 .879 .879 .880 .881 .882 .882 .883 .884 .885 .886 20.50 .873 .873 .873 .874 .875 .876 .877 .878 .878 .879 .880 .881 21.00 .868 .868 .868 .869 .870 .870 .871 .872 .873 .874 .875 .876 21.50 .862 .862 .862 .863 .864 .865 .866 .867 .868 .869 .870 .871 22.00 .857 .856 .857 .858 .859 .860 .861 .862 .863 .864 .865 .866 22.50 .851 .850 .851 .852 .853 .854 .855 .857 .858 .859 .860 .861 23.00 .844 .844 .845 .846 .847 .848 .850 .851 .852 .853 .855 .856 23.50 .838 .838 .839 .840 .841 .842 .844 .845 .847 .848 .849 .850 24.00 .831 .831 .832 .833 .835 .836 .838 .839 .840 .842 .843 .845 24.50 .825 .825 .826 .827 .829 .830 .832 .833 .835 .836 .838 .839 25.00 .818 .818 .819 .820 .822 .824 .826 .827 .829 .830 .832 .833 25.50 .810 .811 .812 .814 .816 .817 .819 .821 .823 .824 .826 .827 26.00 .803 .804 .805 .807 .809 .811 .813 .814 .816 .818 .819 .821 26.50 .795 .796 .798 .800 .802 .804 .806 .808 .810 .811 .813 .815 27.00 .788 .788 .790 .793 .795 .797 .799 .801 .803 .805 .807 .808

JOURNAL OF FOOD PROTECTION, VOL. 51, MAY 1988 WATER ACTIVITY VALUES 423

27.50 .780 .781 .783 .785 .788 .790 .792 .794 .796 .798 .800 .802 28.00 .771 .773 .775 .778 .780 .783 .785 .787 .789 .791 .793 .795 28.50 .763 .765 .767 .770 .773 .775 .778 .780 .782 .784 .786 .788 29.00 .754 .756 .759 .762 .765 .768 .770 .773 .775 .777 .779 .781 29.50 .746 .748 .751 .754 .757 .760 .763 .765 .768 .770 .772 .774 30.00 .737 .739 .743 .746 .749 .752 .755 .758 .760 .763 .765 .767 30.50 .730 .730 .734 .738 .741 .744 .747 .750 .753 .755 .757 .760 31.00 .718 .722 .725 .729 .733 .736 .739 .742 .745 .747 .750 .752 31.50 .709 .713 .717 .721 .724 .728 .731 .734 .737 .739 .742 .744 32.00 .699 .703 .708 .712 .716 .720 .723 .726 .729 .731 .734 .736 32.50 .690 .694 .699 .703 .707 .711 .714 .718 .721 .723 .726 .728 33.00 .680 .685 .690 .694 .699 .702 .706 .709 .712 .715 .718 .720 33.50 .670 .675 .680 .685 .690 .694 .697 .701 .704 .707 .709 .712 34.00 .660 .665 .671 .676 .681 .685 .688 .692 .695 .698 .701 .704 34.50 .649 .656 .661 .667 .671 .676 .679 .683 .686 .689 .692 .695 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/51/5/419/1658009/0362-028x-51_5_419.pdf by guest on 26 September 2021 35.00 .639 .646 .652 .657 .662 .666 .670 .674 .677 .681 .684 .686 35.50 .629 .636 .642 .648 .653 .657 .661 .665 .668 .672 .675 .678 36.00 .618 .629 .632 .638 .643 .648 .652 .656 .659 .663 .666 .669 36.50 .608 .615 .622 .628 .633 .638 .642 .646 .650 .653 .657 .660 37.00 .597 .605 .612 .618 .624 .628 .633 .637 .640 .644 .647 .650 37.50 .586 .595 .602 .608 .614 .619 .623 .627 .631 .634 .638 .641 38.00 .575 .584 .592 .598 .604 .609 .613 .617 .621 .625 .628 .631 38.50 .565 .574 .581 .588 .593 .598 .603 .607 .611 .615 .618 .622 39.00 .554 .563 .571 .577 .583 .588 .593 .597 .601 .605 .609 .612 39.50 .543 .553 .560 .567 .573 .578 .583 .587 .591 .595 .599 .602 40.00 .532 .542 .550 .556 .562 .568 .572 .577 .581 .585 .588 .592 'From Resnik et al. (13).

ACKNOWLEDGMENTS 7. Pitzer, K. S. 1973. Thermodynamics of electrolytes. 1. Theoretical basis and general equations. J. Phys. Chem. 77:268-277. The authors acknowledge financial support from Consejo Nacional 8. Pitzer, K. S., and G. Mayorga. 1973. Thermodynamics of electro de Investigaciones Cientificas y Tecnicas and Secretaria de Ciencia y lytes. 2. Activity and osmotic coefficients for strong electrolytes Tecnica (Programa Nacional de Tecnologia de Alimentos). with one or both ions univalent. J. Phys. Chem. 77:2300-2308. 9. Pitzer, K. S., R. N. Roy and L. F. Silvester. 1977. Ther REFERENCES modynamics of electrolytes. 7. Sulfuric acid. J. Am. Chem. Soc. 99:4930-4936. 1. Chirife, J., G. Favetto, C. Ferro Fontan, and S. L. Resnik. 1983. 10. Platford, R. F. 1979. Experimental methods: Isopeistic. In R. M. The water activity of standard saturated salt solutions in the range Pytkowicz (ed.), Activity coefficients in electrolyte solutions. CRC of IM foods. Lebensm. Wiss. Technol. 16:36-38. Press, Florida. 2. Chirife, J., and S. L. Resnik. 1984. Unsaturated soutions of 11. Pollio, M. L., D. Kitic, G. J. Favetto, and J. Chirife. 1987. Pre sodium chloride as reference sources of water activity at various diction and measurement of the water activity of selected saturated temperatures. J. Food Sci. 49:1486-1487. salt solutions at 5°C and 10°C. J. Food Sci. (in press). 3. EEC. 1976. Meat products, Regulations Nr. 77/99, Dec. 21. 12. Resnik, S. L., G. Favetto, J. Chirife, and C. Ferro Fontan. 1984. Economic European Community, Bruxelles, Belgium. A world survey of water activity of selected saturated salt solutions 4. FAO/WHO. 1979. Recommended International Code of Hygienic used as standards at 25°C. J. Food Sci. 49:510-513. Practice for Ground Nuts (Peanuts), CAC/RCP 22-1979. Food and 13. Resnik, S. L., J. Chirife, and C. Ferro Fontan. 1985. Sulfuric acid Agriculture Organization of the United Nations, World Health Or solutions as reference sources of water activity at various tempera ganization, Rome, Italy. tures. Z.F.L. 6:428-432. 5. FDA. 1973. Federal Register 38(16), Part III, 2398. Food & Drug 14. Silvester, L. F., and K. S. Pitzer. 1978. Thermodynamics of elec Administration, Washington, D.C. trolytes. X. Entalphy and the effect of temperature on the activity 6. Kitic, D., D. C. Pereira Jardim, G. Favetto, S. Resnik and J. coefficients. J. Solution Chem. 7:327-337. Chirife. 1986. Theoretical prediction of the water activity of stan 15. Trailer, J. A., and J. H. B. Christian. 1978. Water activity and dard saturated salt solutions at various temperatures. J. Food Sci. food. Academic Press, New York. 51, 1037-1041.

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