Effect of Radiant Energy on H. H. HALL AND J. C. KEANE1 Bureau of Agricultural Chemistry and Engineering, United States Department of Agriculture, Washington, D. C.
T
BEETSUGAR FACTORY AND BEETSTORAGE BINS
Data are presented to show that the spores of a thermophilic canned food spoilage organism, B. stearothermophilus Donlc, are killed in dry white sugar by radiant energy rays, most of which are in the region of 2537 A., and that enhanced lethal action is obtained by turbulence of the sugar during irradiation. An average of 47.8 per cent of the spores were killed by irradiating eight successive strikes of sugar with twenty-four lamps installed in a sugar granulator. No immediate or delayed physical or chemical changes were noted in the irradiated sugar. The lethal effect of the energy rays occurred during irradiation, and no residual lethal power was retained by the sugar. There was no increase in the spore counts of irradiated samples during storage. The results indicate the possibility of the sterilization of white sugar and other such ingredients of foods by irradiation.
1
Present address, Utah-Idaho Sugar Company, Salt Lake City.
HE recent development of radiant energy ray equipment having germicidal properties has stimulated new interest in methods of food sterilization. The prevention of the growth of microorganisms on bakery products and meats (10) by the use of radiant energy rays raises two fundamentally important questions regarding food sterilization: First, what would be the effect of the energy rays on the bacteria in several of the raw materials of food products, and secondly, could the bacteria be effectively eliminated by exposing the raw materials to the rays of germicidal lamps? Since white sugar is an ingredient common to many prepared food products, it was selected as a test material. Cameron (4), Owen and Mobley (11), and others have demonstrated the presence and significance of thermophilic food spoilage bacteria in sugar and have declared it responsible for carrying mixed bacterial flora into nonacid canned food products. Cameron (4) and Bigelow (9) have proposed bacterial standards which limit the number of thermophilic bacterial spores that may be present in canning grade sugar. The annual reports of the National Canners Association laboratory (6, 7 , 15, 16, 17) show that the bacteriological quality of sugar has been improved since the adoption of standards, and that very few lots are now below standard. Although most sugar manufacturers are able to produce canning grade sugar consistently, there is need in the industry for a simple and inexpensive sterilization method that will assure the manufacturer of the continuous production of bacteria-free sugar. It was thought that irradiation of the dry sugar crystals with ultraviolet rays might result in the production of such sugar. Effect of Rays on B. stearothermophilus Results obtained in this bureau during a study of the production of white sugar of uniform bacteriological quality showed that the spores of Bacillus stearothermophilus Donk (9) was usually the predominant type in contaminated sugar. Because of this fact and of the ease with which they may b e detected and enumerated, the study of the effect of radiant energy rays was made only on these spores. The radiant-energy-producing lamps used were described by James (10) as "a gaseous conductor tube, called the Sterilamp, that generates radiant energy of a particular wave length, 90 per cent of the radiation being in the region of the radiant energy spectrum (2537 B.)which is strongly germicidal. " The Sterilamps used in this study had an effective length of 30 inches (76.2 cm.) with an outside diameter of "16 inch
I.168
Thermophilic Organisms in Sugar (1.43 mm.). Each end of the lamp was equipped with a metal electrode over which was placed a metal clip suitable for holding an electric light wire to supply the current. The operating voltage of the lamp is 475 volts and the operating current is 0.03 ampere. The lamp operates a t approximately room temperature. Preliminary experiments were made by weighing seven 20-gram quantities of each of several selected contaminated samples of sugar into the bottom half of a sterile Petri dish; this quantity formed a layer approximately 4 mm. in depth. The samples mere placed directly under a lamp and 6 inches (15.2 cm.) from its longitudinal axis. The covers were removed from the dishes, and the current was turned on. At the end of 1-, 2.5-, 5-, lo-, 20-, and 30-minute intervals a sample was removed and examined for the number of spores of B. stearothermophilus, according to Cameron’s method (5) for the analysis of sugar. The results are given in Table I.
and was probably due to the low rate of penetration of the rays into the mass of sugar. Spores trapped within the crystals or those on the faces of the crystal away from the source of energy rays would also be more difficult to kill. Protection was also afforded to a few spores in sugar in which the crystals were massed together or were lumpy, as shown by sample E in which there was no further reduction of spores after 2.5 minutes of exposure. The results show that the energy rays do kill the spores but that more rapid killing might be obtained by agitation of the crystals during exposure. TABLE 11. EFFECT OF RAYSON SPORES WITH SUGAR IN MOTION UnSample exposed A 25 B 45 C 60
D E
TABLEI. EFFECT OF RAYS ON SPORES IN SUGAR IN OPEN CONTAINER, ~ - M MLAYER . -Spores Sample
a
Unexposed 36 55 215
-
1 35 50 60
per 10 Grams of Sugar Min. of exposure 2.5 5 10 5
0
20
30
n
Sample was lumpy.
The most pronounced killing of the spores was during the first minute of exposure. Subsequent killing was much slower
2;:
Spores per 10 Grams of Sugar No. of exposures 1 0
2 0 0 135
2 3 4 5 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 65 10 10 5 0
7 0
0 0 0 0
8 0 0 0 0 0
9 0 0 0 0
10
0
0
0 0 0 0
I n order to study the influence of the rays on spores on sugar crystals in motion, three lamps were arranged horizontally one above another a t 6-inch intervals and with the lower lamp 6 inches above the table surface. Approximately 1 kg. of the test sample was sifted from a sterile cardboard over the lamps a t such a rate that 20 seconds were required to expose the entire quantity of sugar. Each sample tested was exposed to the rays a total of ten times. Immediately after each exposure the current was turned off, and a small quantity of sugar was removed for bacteriological examination. The results obtained by this method are given in Table 11.
SUGAR BOILINQ PANS 1169
INDUSTRIAL AND ENGINEERING CHEMISTRY
1170
Rapid killing of the spores was obtained by exposing moving crystals of sugar to the rays. One or two exposures (20 to 40 seconds) caused complete killing of spores in samples contaminated with less than 85 spores per 10 grams of sugar. Five exposures (100 seconds) caused complete killing in a sample containing 295 spores per 10 grams of sugar. These results, in contrast to those of Table I, show definitely that an increased rate of killing is obtained by agitation of the crystals.
Irradiation of Sugar during Manufacture A study was made of the use of radiant energy rays for the irradiation of the entire output of sugar in a factory where there was pronounced bacterial contamination of the sugar. Since it was noted that turbulence of the sugar was a necessary factor for the rapid killing of bacteria, the lamps were installed in the sugar granulator where constant motion of the crystals could be obtained. A circular rig was made and installed in the sugar outlet end of a granulator and twenty-four 30-inch lamps were mounted in the rig parallel to the longitudinal axis of the granulator. Installation was made in such a manner that the dry granulated sugar would spill over the entire length of the lamps, The pitch of the granulator caused the sugar to progress toward the outlet end a t the rate of 3 inches (7.62 cm.) per revolution of the granulator and resulted in ten exposures of the sugar to the zone of energy rays, which required 3 to 4 minutes. The granulator was equipped with a steam drum heater, and the temperature of the irradiated sugar was in excess of 70" C. The first strike of sugar was exposed to the rays of twelve lamps, and twenty-four lamps were used for subsequent strikes. Samples from each strike were obtained from the wet sugar box before irradiation and from the dry sugar spout after irradiation. I n order to preclude possible effect on the remaining viable spores of residual lethal action of radiant energy rays, the examination of all samples was begun within 2 to 24 hours after they were obtained. The results of the examination of nonirradiated and irradiated 'sugar are given in Table 111. TABLE 111. Strike NO.
EFFECT OF IRRADIATION ON SPORES DURING
No. of Lamps
Nonirradiated
135 12 155 24 145 24 155 24 24 90 24 50 7 24 30 24 85 9 24 320 Weighted av. reduction of 24 lamps
s
Studies of Irradiated Sugar A study was made to determine the cause of the reduction of the bacterial spore content of irradiated sugar. Roux (IS)concluded that the failure of anthrax spores to germinate in culture media exposed to sunlight was caused by materials formed as a result of the oxidation of the carbohydrate fractions. Coblentz and Fulton (8),Blank and Arnold (S), Woodrow, Bailey, and Fulmer ( I C ) , and Proks (12) showed that the ability of carbohydrate media to support the growth of microorganisms was diminished if the medium was irradiated with ultraviolet energy rays. Baumgartner (1) investigated the influence of ultraviolet irradiation on carbohydrate-containing bacterial culture media and determined that inhibition of growth was due to increased acidity. A drop from pH 7.0 to 3.5-4.0 was noted after 2-hour irradiation of a one per cent sucrose solution. The pH values were determined for several strikes of irradiated sugar that showed marked spore reductions and were compared with those of the same nonirradiated sugar. The pH values of all samples that were tested ranged from 6.8 to 7.0, and there were no samples that showed increased acidity as the result of irradiation. There were no other physical or chemical changes noted in the samples even after prolonged storage. To demonstrate further that killing was due to a direct action on the spores, several lots of sugar that were freed of flat sour type spores by irradiation were mixed with an equal quantity of the same nonirradiated sugar. Flat sour type spore counts were made in duplicate immediately after mixture, and 16 and 94 hours after storage a t 25' C. The following average results based on the spore counts per 10 grams of sugar are typical of those obtained: Oricinal aount
295 0 148 165 175 166
The results indicate that the killing of spores by irradiation in dry white sugar is obtained only during irradiation and that there is no lethal action retained by the sugar. Also, these results and others obtained by the periodic examination of irradiated samples demonstrate that permanent inhibition of the activity of the spores is obtained.
GRANULATION Irradiated
S p O T 8 S / l O 8. SUQW
1 2 3 4 5 6
VOL. 31, NO. 9
Reduction
%
135 70 90 75
0 54.8 37.9 51.6 16.6
35 20 50 125
30.0
75
33.3 41.1 60.9 47.8
There was no reduction of the spore count when twelve lamps were used, but there was a reduction of 16.6 to 60.9 per cent of the spores when twenty-four lamps were used. The weighted average reduction of spores in eight successive strikes (about 200 tons of sugar) was 47.8 per cent when twenty-four lamps were used. An accumulation of sugar dust on the lamps, which later became a sticky sirup film, or an insufficient number of lamps probably accounted for the failure to obtain greater reductions in the spore counts. Installation of the lamps in a cooling drum or over a specially prepared sugar conveyor where they could be kept free of dust would probably result in increased efficiency. The hazard of breakage by lumps of sugar falling on the lamps would also be eliminated by installation a t some point other than in the granulator.
Acknowledgment The authors wish to acknowledge the cooperation of the Farmers and Manufacturers Beet Sugar Association, Saginaw, Mich.; Michigan Sugar Company, Saginaw; and The Sterilight Company, Detroit, who have made this work possible.
Literature Cited Baumgartner, J. G., J. Bact., 32, 75 (1936). Bigelow, W.D., et al., Canner, 72,No. 16,19 (1931). Blank, I. H.,and Arnold, William, J. Bact., 30, 507 (1935). Cameron, E.J., Canner, 70, No. 13, 17 (1930). Cameron, E.J., J. Assoc. OficiaE AUT.Chem., 19,438 (1936). Cameron, E. J., Yesair, J., and Williams, C. C., Canner, 78, No. 10,76 (1934). Ibid., 80,No. 9, 110 (1935). Coblenta, W.W.,and Fulton, H. R., Natl. Bur. Standards, Sci. Paper 19, 641 (1924). Donk, P.J., J. Bact., 5 , 373 (1920). James, R. F.,Food Industries, 8, 295 (1936). Owen, W.L.,and Mobley, R. L., IND.ENG.CHEM.,24, 1042 (1932). Proks, J., Lait, 13, 331 (1933). Roux, E.,Ann. inst. Pasteur, 1, 445 (1887). Woodrow, J. N.,Bailey, A. C., and Fulmer, E. I., PEant Physiol., 2, 171 (1927). Yesair, J., and Cameron, E. J., Canner, 82, No. 11, 107-8 (1936). Yesair, J., and Reed, J. M., Ibid., 86,No. 12,61-2 (1938). Yesair, J., and Rhymes, F., Ibid., 84, No. 12, 13 (1937).
