YMPOSI ,'MO N
STERILIZATION OF FOODS J. M. JACKSON AND H. A. BENJAMIN Amdoan Can Company. Maywood, 111. change durinp-9. Recently i n t m d d mtullking teah&,ues designed to incrsm the rate of huting of the food: thin maken possible SuautJ imlsnnmunt. through tha UI of .hort.r times at hipha temperaturea for stedizatien. Furthomnca of tha high temperature dwrt tinu,principl. may be nnwpatdin future development.. Bt.rilintion without heat show qonu poribilitb. for eventual ahanges in s t d l b t i o n pmcdura.
0
NE of tbe principal m e t b h of preservation of foods d s
pen& on thermal stsriliastion to prevent bacterial decomposition and in hexmetic containera to prevent reinfection by q4lage mimmgankm. Thia pmcedure is commonly re. Incidentel to the p-ation fermd to M the canning p.OafoOaS, the starilktion of foods% amwm ofBuardiagagninet
e
food-borneinfectiom. It amno appropriate that food stefilisation should be dismmed at a joint Sympasum of the Agricultural and Food Cheminky and Indusbial and Fhgiwathg chemistry Divisions, ea the prin~plainvolved and the procedures employed sre b a d on integration of ImowMge from a number of 6elds of science and te&dogy. Bropdly m,biologid, pbysid, and cheanid sciencss am m d d in the principlea of food steriliss, tion and e n g k d n g skill i8 required for nu& application of tbeae principlesto wmmemid practice. Altbougb application of h c e and technologv in the canning indmtry is relatively new, p m t b of the indnstry in the United Stska has reaohed a volume of mved billion pounds and a value approximating tbrea billion dolLn in recsnt years (Sa, 43). Public demand for bigb quality, mnwnient, and safe p d a g d foods & for wnsbmt improve ment in industrial operation. Such improvemeht may be effectedby uw of enginmring skills in developing improved mathOds of thermal aterilisation. Ale0 application of the sciencea m the development of other principlae of food steriliastion may evantnallye6ectimprovdfoodp~ationpmcedures. The history of the canning process h a a h well mvered in the
literature (f.416,481. The Frenabman, Nichoka Appert, w m perhapa the 6rat to utilire the atdimtion principle for the preservation of food, early in the 19th century. Bwauae of hia aocomplishment be wasawarded a 12,wO franc prize by the k c h Government and became h o r n M the father of the canning industry. It is doubtful if Appert had any inkling of what be W M doing by boiling hia bottlae of food other than that be wan bringing about conditions wnducive to the p-ation of the food. It was Pasteur who p r o d and established the soundma of Appert'a principle of food preserwtion, about 1860. A die t i m e d contemporary, Samuel Cata Presoott. w one of the 6rat to bring the &ry of bacteria and their role in d f o o d spoilage to the canningindustryin 1895. In 1912, B m m n Barlow, a graduate atudent at the Umvernity of Illinois, d i e c o y 4 that calmed corn which hpd p r e s d l y bean sterilised w w not bacteriologically aterile, but tbat the hest reeistsnt orgmiam wbiob survived wem capable of causing the corn to sou only if the product w w beld at tempsnrtures between 100' and 180' F. B e e a w little or no gin wsa formed, the product became What is h O W n 88 h t WUP, b PIticukrly insidious type of spoilage because the can ends did not bulge and make the spoilage externally detestable. Thus WM bori~the tarm, munmercialy &sile, which carried the wnnotation that living organisms may exist in the can but under n o d storage wnditiom are not capable of e p d h g or otberwiae affectingthepmduot.
.
2241
2242
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40. No. 12
MICROBIOLOGICAL AND PHYSICAL BASIS
CHEMICAL ASPECTS
i s has been mentioned, sterilization of food requires destruclion of potential spoilage microorganisms. Therefore, it is necessary to know the various combinations of times and temperatures required to destroy the contaminating organisms These data are determined by the bacteriologist using techniques described in the literature ( I , 48, 52) These bacteriological data show that the time required for destruction of a population of apoilage bacteria, called the thermal death time, is a function of the temperature and can be expressed h r an equation of the gencia1 form:
C'heniical principles have important applications iii the sterilization of canned foods. The acidity of the food as reflect,edin the hydrogen ion concentration has an important function in food preservation (46). One of the oldest methods of preserving foo(1 is by pickling or by allowing the acidity to increase through thc process of fermentation. As the pH value is decrcased, t)he destruction or inactivation of spoilage bacteria is accomplished af, Iuwer temperatures. Thus, in cst,ablishing a thermal process,. pH is one of the most important factors that, must bc considered. 111general, foods lvhich have pH values greater than 4.5 require processing a t temperatures above 225" ]I". in order that thc stcriliaation can be completed in a reasonable time. Thcse arc' tlcsignatcd as low-acid foods and those with pH below 4.5 arc con.qidercd to be acid foods. Foods with pH values in the range 4 to -4.5, with a few exceptions, are usually commercially sterilined by heating to a temperature of 190" F. Correspondingly l o n w trmperatureu may be sufficient to sterilize foods commercially at lover pH levels. A high concentration of soluble solids in a food frequently enables a moderate heat treatment t o produce a commercially sterile product-that is, one in which any surviving spores of heat rcsistant organisms are incapable of groxvth. Organisms capable of growth in food with high soluble solids are usually killed at relatively low temperatures. An osmotic phenomenon known as plasmolysis apparently is the basis of this method of preservation. Foods such as sweetened condensed milk, jams, and .?irups are preserved in this manner. Control of soluble solids, or per cent moisture, becomes an important analytical function i i i the preservation of foods of this type, Salting is another well known method of food preservation. Commercial sterilization of mmc? foods has been effected by control of the sodium chloride content in corijunction xith a thermal process that, for unsalted food a t the same pH level, would bc inadequate for preservation. This is another example of eornmercial sterilization that is not strictly synonymous with bacteriological sterilization because vinhlc organisms frequently can he recovered from t,he product,. Chemical Evaluation of Quality of Sterilized Foods. Food quality is a composite of a number of factors including appearance, texture, flavor, and nut,ritive value. .ilthi)ugh good quality as rncasured by organoleptic standards is easily recognized by thv layman as well as the tasting expert,, the inability to apply ohjcctive measurements to such factors as flavor and appearance is disappointing to those who are trained in the physical scionces. Factors such as the retention of vitamins and texture and consistency differences are more susceptible to mcasurcmcrrt a,nd inay serve as a partial index of quality, The quality factors in foods are unstable chemical entities and their rate of loss is a function of temperature. The ratc of ical reaction is approximately doubled with an 18" F. incrc temperature. The food quality factors are lost in proportion to the chemical reaction rate as has heen shown by Grecnrvood, ~t ai. (96, 54). Data which they have reported on the destruction of thiamine in pork luncheon meat are shown in Figure I . The relation of time and temperature with respect to the destruct,ion of typical food spoilage bacteria also are shorn it! Figure I. The thermal death t,iine ( T D T ) curve expresses co111binations of time and temperature which are cqually effective it( tlestroyiiig bacteria when no significant time is involvcd in h m t irig to or cooling from the lethal temperature. Tlnder thost, conditions which approach instantaneous heating and cooling, it is obvious that the destruction of microorganisms as illustrated by the thermal death t,irne curve can bc achieved with thc greatest retention of thiamine by the use of high processillg remperatures. Ball (6) has called high Lemperaturc short time ing, and this rlomr!t1chsterilizing operations, high-short pro turc will he rtstainerl in this papei.
6 =
1oma-i
0 = thermal death time at tempeiatuie, t , and m and u are constants established by the particular renditions of type and strain of organisms, number, and chemical environment. 15 heir
21
1
E f f e c ' of T e r n p e r c t u r e I on R a t e s of D e s t r u c t i o n of T h i a m i n e c n d T y p i c a i B a c t e r i a l Spores
I
I I'
2OC
210
140 Temperature -'F
220
230
250
260
The thermal death tiiiic equtttloll hits been used (4,8, 11, together with a heating curve obtained by temperature Inca$urernents a t the can center, t o establish a so-called lethal. rate curve which is integrated to determine the necessary time and temperature conditions for sterilization v, ith respert t o the particular bacteria in question. The heating eurve is usually obtained by inserting a thcrmocouple into the can d t h the hot junction at the point of slowest heating. For products heating by conduction, a curve can uwallv be fitted to an equation having the general form-
10'" = c rb
- t)
xherc z = tune, t = teinpeiature, and b, e, and n are constant. established by the particular conditions of container dimensions, thermal diffusivity of the product. initial temperature of the product, and heating temperature. Elementary physical conslderdtiorls and experimental data (33, 359 indicate that foods of heavy consistency uil1 heat by
conduction in sealed containers; consequently, the outer portions of the food must be subjected to more Severe heat treatmcnt than that required for sterilization of the center portion. The effect of such severe heat treatment is beneficial to some foods but deleterious t o others. Heat transfer by convection of thin fluid food components provides rapid and nearlv uniform heating
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY ENGINEERING ASPECTS
Sterilization of food in commercial practice involves a number of engineering operations. Not the least important of these is the unit operation of heat transfer. The conventional sterilizing treatment consists of heating the sealed containers in water or a steam atmosphere until sterilization is achieved and then cooling in water or air to a suitable storage temperature. Heat transfer is a unique unit operation in food sterilization. As has been mentioned, the rate of heat transfer into a can is usually measured with thermocouples and the time-temperature schedules are established in accordance with analysis of such data. Heat transfer into a can is an unsteady state because of the continuously variable can temperature during the sterilization process (38). There are a great many other heat transfer problems in connectionwith the sterilization of canned foods. Some of the newer methods of continuous sterilization which will be described later involve heating the fluid product in plate or tubular type heat exchangers. There is a considerable need for this type of equipment in the food industry and there is need for more basic data concerning the physical characteristics such as specific heat, viscosity, and thermal conductivity of food materials for use in engineering calculations involving heat exchange. It is desirable from quality considerations t o cool food products as rapidly as possible after sterilizing. Water cooling is practiced for most canned foods and this operation requires application of engineering methods for unsteady state heat transfer. I n the case of the newer presterilizing techniques, steady state heat transfer is involved in cooling of such products to filling temperatures. Regardless of the method of sterilization, instrumentation finds wide application in this field. Simple to complex temperature and/or pressure controlling systems have been developed for retorts and other heat exchange devices used for food sterilization. Conventional Still Retorts. The conventional means of sterilizing, or processing, is to place the closed containers in a retort or autoclave which can be closed and filled with hot water or steam and held at sterilizing temperature. It is extremely important that a unifo'rm temperature be maintained in all parts of the retort so that all cans in the load be given a uniform heat treatment. This accomplishment requires a knowledge of good venting practice t o eliminate air from the retort, and a knowledge of steam flow and its control. Proper piping of the retort and effective instrumentation are necessary for the control of a sterilizing process. Although application of the high-short principle usually requires specialized equipment, simple application in the ordinary still retort may be made in the case of foods which heat rapidly by convection within the hermetic container. The Blair process (18)for the retention of chlorophyll in canned peas depends on the addition of traces of alkalies to control the pH; thus chlorophyll is stabilized during storage. By the use of a high-short retort process, loss of chlorophyll is minimized during the sterilizing process. The No. 2 and smaller cans may be processed 8 minutes a t 260" E'. in the still retort; this is bactericidally equivalent
Table I.
to the 35 minutes at 240' F. still retort process generally used for conventionally canned peas. The comparative retention of green color as measured by Munsell hue values is illustrated in Table I. Agitating Retorts. Sterilizing retorts which contain a cage or framework for supporting and revolving cans during the heating cycle have been used for a number of years, especially in the evaporated milk industry. Recently (41) retorts of this type have been used for the processing of vacuum packed vegetables with a marked increase in the heat penetration rate; this made possible a reduction in sterilizing times. The rate of heat penetration expressed in terms of equivalent process time for different types of can rotation and for vacuum packed peas and vacuum packed corn is shown in Table 11.
Table 11. Relative Heating Rates in Vacuum Packed Vegetables Agitated during Processing-307 X 400 Cans
Product Sweet peas
Type Agitation End over end Continuous axial Intermittent axial Still Corn End over end Continuous axial Intermittent axial Still After Ball (6).
Reel Speed R.P.M. 23
23 48
..
23 23 48
..
Sterilizing VrslueQFa. Min. 7.4 7.4 7.4 7.4 9.8 9.8 9.8 9.8
Calod. Processing Time at 250° F., Min. 9.6 9.9 13.2 20.0 12.6 11.8 14.0 35.0
Q
The shorter time processes which are made possible by agitation of vacuum packed cans during the sterilizing process result in a striking improvement in the color and flavor of corn This quality improvement is manifested also in the relative degree of retention of the heat labile vitamin, thiamine. Thiamine retention data in vacuum packed corn subjected t o bactericidally equivalent still and agitating sterilizing processes are shown in Figure 2 (18). There is a significant improvement in thiamine retention as a result of the accelerated heat penetration and reduction in heating time. Further improvement might be obtained by the use of higher retort temperatures and correspondingly shorter times but would introduce problems of can strain and process control.
.
Figure 2
Retention o f Thiamine in Vacuum Pack White Corn Processed by Agitated Can and Conventional Methods
loo]
307x306
603 x 408 cans
71
/I m
In liquid from cans In solids from agitated cans In solids from conventional cans
Effect of Sterilizing Conditions on Color of Peas Packed by Blair Procedure-No. 1 Cans
Munsell Hue Value" Blair treated Conventional Freshly pem peas Sterilizing Process blanched stored 1 month stored 1 month Min. O F. pea8 at 40' F. at 40° F. 35 240 .. 32.1 27.9 8 260 a5 28.5 None 36:2 .. .. Loss in green color is measurable by reduced hue value on the Munsell scale (67). ( I
2243
Continuous Retorts. Several years ago a helical track continuous retort was developed for continuously and automatically sterilizing canned foods. The cans are valve'd into a pressure chamber and caused to roll in a steam atmosphere for several minutes before being valved out for cooling. I n some products the rate of heat penetration is increased and the use of steam temperatures of 260" to 265" F. makes sterilization of the product
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
2244
FIGURE 3
Conventtonot and Flos'l
Ster~li2n*!on. AsePliC Filling cnd S e c i i l g P r o c e d u r e s for L o r A c i d Fluid F o o d P r o d u c I L
Co-rei'8onal Prcced-;e
possible in much shorter times. -1 better quality product is obtained and operating costs are lower because of the higher production per unit labor cost. Continuous cookers of this typcx have been used successfully for such products as evaporated milk, pureed vegetables, whole kernel corn, peas, mixed vegetables, dog food, and various meat product,s. Some products such as asparagus are damaged by the agitation and cannot be pro(.essed in continuous agitating cookers. Some continuous cooker. have been made with a so-called nonagitating reel which arc) used specifically for those product)s subject to damage by thc agitating cook. Thc relative rate of heat penetration i n an agititting type coiltinuous cooker for vacuuni packed peas and corn is illustrated in Table I1 for the conditiori of intermittent axial rotation. These are experimental data obtained under simulated agitated can conditions; they have been confirmed by others ( 4 7 ) using equipment duplicating the rotation given cans by continuous agitating cookers. Continuous Open Bath Spinner Cookers. Another type of food sterilizing equipment, which has been used rather widely for fruit juices, causes the containers t o spin rapidly by mechanical means such as by conveying them on rotatirig rollers, while they are sprayed with or immersed in hot matcr. This apparatus causes a fast rate of heat penetration into canned liquid products and permits relatively short process tinle schedules (9). K i t h atmospheric pressure cookers t,he use of hot u-ater as the heating medium limits the use of these machines to products having pH values of 4.5 or less because temperatures in excess of 212" F. necessary for sterilizing low acid products cannot be attained. The use of higher boiling liquids may extend the usc of thew cookers to the IOT acid products. This procedure ~ o u l diiivolve a number of problems such as clingage t o containers, container strain from unbalanced pressures, effect of heating medium 011 container materials, and effect of fumes on cannery worliers. There is a definite need for engineering knowledge in considering the design of such a process. Presterilization Procedures. Application of the high-short principle of food sterilization to hermetic containers of practical dimensions is complicated by the lag in penetration of heat to the center of the food, particularly in viscous foods. Effcctive application requires nearly uniform and rapid heating of the ent'ire portion of food to accomplish sterilization within a fen- minutesor preferably aitliin a few seconds. Engineering ingenuity ill be required to develop practical procedures for application of the high-short sterilization principle. Theoretically, there aro a number of methods for effecting rapid uniform heating of food products to sterilizing temperat'urcs. These have been classified and discussed in detail in a comprehensive paper by Ball (6).
Vol. 40, No. 12
A procedure which may be applied to fluid products and which appears to be the most feasible of these methods depends on prrsterilization of the food in a heat exchanger prior t o filling into the hermetic cuntainer. This procedure may bo applied to either acid or low acid products. The acid products such as fruit juices may be sterilized a t temperatures below 212" F. and filled hot; the heat of the product is expected to destroy spoilage bacteria which may re-enter the product during filling and closing. The low acid product,s, such as vegetable pur6es and dairy products, must be heated rapidly to temperatures in the range of 260" to 300" F. to effect sterilization in an interval of a few seconds. Sterilization is followed bv cooling " ragid - in a heat exchanger to stop thc deleterious action of the heat, as soon a.s possible. The sterilized food then must, be filled and sealed in sterilized containers under aseptic condit,ions because air- or equipment-borne microorganisms capable of growth in low acid products arc not, sterilized readily a t temperatures below 212" F. Final co(11ing of the sealed containers is ordinarily required before storing. Diagrammatic representation of this procedure appears i n Figure 3 (Adapted 40). The superior thiamine retention obtained by experimental application of this procedure to cream style corn is exemplified in Figure 4. The vitamin assay data are from the product, as canned in 12-ounce metal cans (18,19). Figure 4 E f f e c t s o f High- Short and Conventlonal Processing on Vitamins in Cream Style Yellow Corn T hia rn ine
Niacin High- short process Convent ionol process
number of engineering problcnis are involved both in the presterilization and in the filling, sealing, and cooling operations for either acid or low acid products. Both tubular and plate types of heat exchangers may be used for t,he acid products with either steam or water on the hot side of the exchanger. I n either case the area of heat exchange surface required must be calculated from applicable heat transfer data; adequate allowance inus1 be made for reduction in heat transfer by the coating of tho exchange surface with solids from the food product. The higher temperatures required for sterilization of low acid foods precludes the use of available plate type exchangers. Tubular exchangers impose requirements for high pumping pressures in order to obt,ain the turbulence required for good heat transfer and for minimizing the coating of exchange surfaces at high temperature. Obviously a knowledge of the theoretical anti
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2245
practical aspects of fluid flow is necessary in applications of this nature. Recent work in this laboratory has indicated t h a t a revolving scraper within a tube type of heat exchanger will help t o overcome these problems. This equipment employs scraper blades mounted on a revolving mutator shaft in a steam or water jacketed tube. The product is pumped a t relativelv 10x7 pressure through the annular space between the mutator and the tube. Rotation of the scraper blades provides artificial turbulence and maintains the heat exchange surface relatively free of film (31). Heating b y direct steam injection involves a number of problems such as cleanliness of steam and control of solids content in the final product. However, this procedure is applicable t o some processes, especially in the dairy field (999). Presterilization of Tomato Juice. Figure 5. Commercial Aseptic Filling and C l o s i n g M a c h i n e for Metal C a n s Tomato juice and tomato-vegetable juice mixtures present a n interesting special case. several methods proposed for this sterilizing procedure. Some Although these are acid products, they support growth of the moderately heat resistant organisms B . thermoacidurans (IO)and involve treatment in a n enclosed chamber under pressure (8, $0-22). Others involve injection through some type of valve CZ. pasteuriunum (49) which arc not destroyed by conventional in the container closure which is subsequently sealed (5, 25, 24). retort processes. It is possible t o sterilize these organisms bv A number of problems, such as a strength of container, uniformusing a presterilizing procedure, usually holding the juice a t ity of heating, and control of solids content of the sterilized prod250' F. for approximately 0.7 minute, then cooling to approximately 190' F. for hot filling, sealing, and subsequent cooling. uct are encountered in these procedures which do not appear to This filling and sealing procedure, of course, is preaicated on the be in commercial usage at present. Electrical Heating. Electrical methods of heating have been assumption t h a t contamination with these resistant organisms considered as means of effecting rapid and uniform heating of will not occur in the filling and sealing operations (46, 51). food products. There appear t o be possibilities both of resistance Aseptic Filling and Sealing. The aseptic filling and sealing of heating, involving direct contact of electrodes with the food, the sterilized product in sterilized containers presents a difficult and of inductive or dielectric heating, involving placing the food engineering problem which has been approached in several ways. in a suitable high frequency electromagnetic field (32). Either One procedure which has been used commercially depends on type of heating might conceivably be applied to the food in bulk, steam retort sterilization of the empty containers and caps, followed by aseptic filling and sealing in sterilized containers, followed by filling and sealing with conventional equipment in a or might be applied t o t,he food after sealing in special nonconsterile room. Strict bacteriological control is required in which ducting containers. the operators follow hospital operating room technique. The air is filtered continuously and ultraviolet radiation is employed The many problems encountered in attempting t o apply any liberally throughout the room. method of electrical heating t o the sterilization of foods far exceed those encountered in simple cooking of food and furnish a Another procedure which has been applied commercially inreal challenge t o the engineer. Among these problems are nonvolves filling and sealing the sterilized containers in a n atmosuniformity of heating, difficulty of carrying out operations under phere of live steam confined in a n enclosed chamber (7). Rosdperatmospheric pressure, and cooling of t h e sterilized food after tary sealing valves are used t o admit empty containers t o high electrical heating. Economic considerations suggest t h a t any pressure steam sterilizing chambers, t o transfer the sterilized successful electrical procedures might be limited t o high cost containers t o the filling and sealing chamber, and t o discharge nonseasonal foods. the filled sealed containers. The problems involved in this procedure are largely mechanical encompassing the operation and STERILIZATION WITHOUT HEAT lubrication of conveying, filling, and sealing equipmept inside of the closed chambers. Maintenance of high pressure steam in the The sterilization of foods without application of heat appears container sterilizing chambers and low pressure steam in the fillto offer the possibility of supplying prescrved foods of fresh food ing and sealing chamber involves well standardized steam engiquality without special storage facilities. Chemical consideraneering and instrumentation. A comniercial aseptic filling and tions indicate t h a t inactivation of enzymes would he required closing machine for metal cans is illustrated in Figure 5 . in addition to sterilization and t h a t some nonenzymic chemical Experimental trials of another ascptic filling and closing prochanges during storage might render the stored products inferior cedure have recently been reported ( 2 ) . This procedure involves t o the fresh. Biological considerations indicate t h a t either use of superheated steam a t atmospheric pressure to sterilize the chemical additives or some type of irradiation might be used as containers and maintain aseptic conditions at the filling and seala substitute for heat in the preservation of foods. ing stations; thus, the necessity of providing sealed chambers is Chemical Agents. Chemical agents might be either bactericidal avoided. or bacteristatic. I n either case, the condition of commercial Heating in Containers Prior to Sealing. Another procedure for sterility would have to be attained t o prevent the growth of achieving high-short processing depends on injection of steam spoilage microorganisms. Of course, it is conceivable that some into the product prior to sealing the container There have been substance or combination of substances might not only prevent
2246
INDUSTRIAL AND ENGINEERING CHEMISTRY
the growth of microorganisms, but also inhibit enzymic action. Since it is necessary also thaL the preservative can be ingested in limited quantities continuously and without ill effect t o the consumer, it is obvious that the search for substances satisfying the requirements is difficult and perhaps impossible. The preservative action of benzoate of soda is well known, but the quantities permitted in foods ( 1 7 ) will only preserve those of high acidity or high soluble solids content. Other compounds which have been investigated as chemical preservatives either have not met the requirements of the Food and Drug Administration or they have been not sufficiently effective. Development of a number of antibiotic substances may open new fields for sterilization of food without heat. Experiments with penicillin have not been encouraging ( I d ) , but more recently developed antibiotics should be given further study. Irradiation. Recent work has indicated that high velocity electron streams and high voltage x-rays are capable of sterilizing food products with respect to many of the common spoilage organisms ( I S , 16). The x-rays produced a t high voltages, about 3 megavolts, of course have great penetrating power and show lethal effects on a wide variety of microorganisms including heat resistant spore formers. However, the roentgen dosages required for complete kill of bacterial cultures require considerable irradiation time. High voltage electron streams (cathode rays) give indications of more rapid lethal eflects and appear to hold real promise for sterilization of foods without heat. Energies of the order of several megavolts are required for a few centimeters penetration in food products and penetration is even more limited by the greater density of container materials (50). Thus, it is obvious that highly specialized equipment would be required to generate rays capable of sterilizing food products on a practical basis. In addition to further studies of the bactericidal effects of x-rays and cathode rays, studies of the effects on enzyme activity, vitamin stability, and other chemical changes may be anticipated with interest. There have been reports of extra thermal bactericidal effects from radio waves of certain frequencies as well as sonic or ultrasonic vibrations (25, 27, 28,SO, 42, 44). However, as yet there does not appear to be evidence of complete sterilization of sporeforming food spoilage organisms by either of these types of irradiation. T h e possibility of sterilization by suitable radiation resulting from atomic fission is intriguing but obviously involves many problems which must await further fundamental study. Engineering Aspects of Sterilization without Heat. Should natural or synthetic additives be developed for successful preservation of foods, their use might be adopted with little engineering development. However, the use of any type of irradiation would require much application of engineering skill, both in the development of suitable commercial scale generating equipment and in application of the irradiation t o the foods. Such irradiation might be applied t o food in sealed containers, or applied to food in bulk followed by aseptic transfer to sterilized containers. Further consideration of such engineering problems logically should await more specific development of fundamental knowledge of methods for sterilization without heat.
SUMMARY Heat sterilization of food, which is the basis for the canned food industry, requires application of the scientific principles of biology, physics, chemistry, and engineering. Through application of these principles the quality of food preserved by sterilization in hermetic containers has been improved continually.
ACKNOWLEDGMENT The authors are indebted to many of their associates for much of the work involved in the preparation of this manuscript.
Vol. 40, No. 12
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