tions of techniques derived in control engineering for the study of the dynamics of processes. Some recent studies, using a continuous fermentation, where a sinusoidal forcing input function was utilized, have shown that this technique can yield valuable data about transient responses of a fermentation system (4). I t may be possible to utilize even more refined methods of control engineering which d o not require a forcing function input to a system, but rely on random fluctuations of the system for analysis. In fermentations, one is dealing with a unique chemical problem. T h e study of the controllability, dynamics, and opti-
mization of systems involving living organisms is a fascinating one. There is a n urgent need for specialized measuring devices applicable to these complex systems. I n the coming years, great advances can be made in the control of fermentation processes by the application of some of the methods outlined here.
( 3 ) Ellsworth, R., Meakin, L. R. P., Chem. and Znd. (London) 1954, p. 926. (4) Fuld, G. J., Mateles, R. I., Kusmierek, B., Symposium on Continuous Fermentation, SOC.Chem. Ind., London, March 1960. (5) Gaden, E. L., Jr., J . Biochem. Microbiol. Technol. Eng. 1, No. 4 (1959). (6) Shu, P., IND. ENG. CHEM.48, 2204 (1956). (7) Squires, R. W., Hosler, P., Ibid., 50, 1263 (1958).
literature Cited (1) Bungay, H. R., Simons, C. F., Hosler, P., J . Biochem. Microbiol. Technol. Eng., in press.
(2) Dennison, F. W., Jr., West, I. C.: Peterson, M. H., Sylvester, J. C . , IND. END.CHEM.50, 1260 (1958).
GEORGE J. FULD Department of Food Technology, Massachusetts Institute of Technology, Cambridge 39, Mass.
Sterilization of Media for Biochemical Processes P U R E culture fermentations are presumed to require sterile media, yet attainment of true sterility is neither practical nor desirable (7). Although a number of alternatives have been proposed for sterilizing biochemical systems, industrial operations still rely on heat processing. Nevertheless, recent developments have resulted in the use of ionizing radiations for bone sterilization in surgery ( 8 ) )for specialized pharmaceutical applications (4), and potentially in many other situations (72). Agents such as ethylene oxide (74), 8-propiolactone ( 5 ) ,and high frequency sound waves (7 3 ) are of only minor industrial interest a t present. Hence the improvement of heat processing and the possible development of radiation sterilization techniques hold the most promise for improvement of industrial sterilization practice in the immediate future. Perhaps the most notable advance in heat processing during the past decade has been the development of continuous, high temperature processing. This was adapted for fermentations by Pfeifer and Vojonovich in 1952 (769, although it had been well developed for foods a t least a decade earlier (7). T h e technique should receive increasing application in new fermentation installations, because it produces commercially sterile products with less damage to the nutrients ( 7 , 76) and its use can increase the capacity of present units by shortening the down time between fermentations. Both Qlo values and activation energies are used to characterize temperature effects on spore killing rates. However, these expressions are not synonymous, because the former vary with temperature while the latter are relatively constant (2). Nevertheless, both concepts are used industrially. Almost a decade ago Gillespy (70) pointed out that these two bases for calculation produce similar processing schedules a t 250’ F., but a t
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“flash” sterilization temperatures considerable differences develop. These differences may be as much as 35-fold a t 310’ F., the Z value method indicating the longest time. When experimental data for destruction of Clostridzum botulinum spores in phosphate buffer (9) are plotted both as a function of the temperature and as the reciprocal of the absolute temperature, the precision of the data is not adequate for assuming a straight line in either case; neither is it sufficient for the extrapolations that are standard practice for flash sterilization calculations today (3,6, 7, 76). Studies of the use of ionizing radiations for sterilizing fermentation media have been reported only for the lactic acid (7 I), tissue culture (75), and alcohol (77) systems. An extensive literature exists on the use of these rays for sterilization of food (72), biologicals (8), and drugs (4). Basically, two processes are involved, radiopasteurization and radiosterilization. T h e former may become useful in dominant culture, the latter in pure culture fermentations. Heat sterilization is still the method of choice wherever it can be used. Continuous, high-temperature heat sterilization has been successfully adapted to fermentations from previous use in food processing. T h e contradiction between Z value and activation energy calculations has not been resolved; in fact, it has not even been adequately recognized. Hence, improved flash processing schedules should be possible when data become available from spore destruction studies a t higher temperatures. Such studies should also justify one or the other of the calculation methods, if either is valid at high temperatures. Radiation sterilization may become useful in the beverage alcohol and perhaps other fermentations. However, its adaptation to industrial use is contingent
INDUSTRIAL AND ENGINEERING CHEMISTRY
upon development of better and cheaper irradiating equipment. literature Cited (1) Ball, C. O., Olson, F. C. W., “Sterilization in Food Technology,” McGraw-Hill, New York, 1957. (2) Buchanan, R. E., Fulmer, E. I., “Physiology and Biochemistry of Bacteria. Effects of Environment on Bacteria,” Williams & Wilkins, Baltimore, Md., 1930. (3) Charm, S . E., Food Technol. 12, 4-8 (1958). (4) Controulis, J., Lawrence, C. A., Brownell, L. E., J . A m . Pharm. Assoc., Sci. E d . 63, 65-9 (1956). ( 5 ) . Curran, H. R., Evans, F. R., J . Znfectious Diseases 99, 212-18 (1956). (6) Deindoerfer, F. H., Humphrey, A. E., Appl. Microbiol. 7, 256-64 (1959). (7) Zbid., pp. 264-70. (8) DeVries, P. H., Kempe, L. L., Brinker, W. O., Univ. Mich. M e d . Bull. 21, 29-33 (1955). (9) Esty, J. F., Mayer, K. F., J . Infectious Diseases 31, 650-63 (1922). (10) Gillespy, T. G., “Heat Resistance of the Spordiof Thermophylic Bacteria. 111. Thermophilic Anaerobes,” .4nn. Rept. Fruit & Veg. Research Sta., Campden, Endand. 1948. (11) Eillie;, R. A., Kempe, L. L., J . Agr. Food Chem. 5, 706-8 (1957). (12) Hannan, R. S., “Scientific and Technological Problems Involved in Using Ionizing Radiations for the Preservation of Food,” Special Rept. 61, Department of Scientific and Industrial Research, H. M. Stationery Office, London, 1955. (13) Homre, D., J . Bacteriol. 57, 279-95 \
’
(1 , -949’1. .~,(14) Judge, L. F., Pelczar, M. J., Jr., A,b,b1. Microbiol. 3, 292-5 (1955).
5]-Merchant, D. J., Stewart, R. D., Kempe, L. L., Graikoski, J. T., Proc. 506. Exptl. Biol. @ M e d . 86, 128-31 (1 954). G ) Pfeifer, V. F., Vojonovich, C., IND. END.CHEM.44, 1940-6 (1952). 7) Stratton, J. R., Coulter, J. F., Day, W. H., Boruff, C. S., J . Agr. Food Chem. 4, 260-2 (1956).
LLOYD L. KEMPE University of Mich.
Michigan,
Ann Arbor,
