PASTEURIZATION EQUIVALENTS OF HIGH-TEMPERATURE SHORT-TIME HEATING WITH ICE CREAM MIX

J . TOBIAS, O. W. KAUFMANI#, A~D P. It . TRACY Department of Food Technology, University of Illinois, Urbana

Previous studies (7) have indicated that ice cream mix may be pasteurized at temperatures above the boiling point with no deleterious effect on flavor. This is accomplished by heating rapidly to the desired temperature, keeping the hold- ing time to a minimum, and cooling immediately. It is necessary in such a system to know the lowest temperature which, with no intended holding time, will result in bacterial destruction comparable to that obtained by accepted pasteurization methods. The flow diversion valve control may be set at this temperature to comply with minimum pasteurization requirements, even though the product may be processed at much higher temperatures at the option of the operator.

The primary purpose of this study was to determine what processing tem- perature with no intended holding time is necessary to cause destruction of selected heat~resistant bacteria equivalent to that obtained by laboratory pas- teurization at 155 ° F. for 30 minutes.

EXPERIMENTAL PROCEDURE

Description of Roswell heater. The Roswell heater shown in Figure 1 was used in these studies. I t consists of a single heating unit, two cooling units, and two regeneration units. The heating unit consists of a hollow stainless steel tubular element and the chamber housing it. The tubular element, shown par-

1 2

FIG. 1. F ron t view of Roswell heater with accessory equipment. A- -cab ine t cooler; B- - re - generator ; C- -hea te r ; D- -surge tank ; E--f low diversion value (par t ia l ly h idden) ; F - - s am- pl ing port for raw mix; Cr---sampling port for heated mix with no intended holding t ime; H - - sampling port for heated mix held for 3 seconds.

FIG. 2. Heat ing element showing spiraled heat ing surface par t ia l ly withdrawn from the heat ing chamber.

ReCeived for publication Janua ry 24, 1955.

959

960 a. TOBIAS ET AL

tially wi thdrawn from the chamber in F igure 2, is 68 in. long and 4 in. in diame- ter. The element and chamber are both par t of the heat ing system, as high pressure desuperheated steam is present inside the hollow element as well as in the chamber wall. In this manner heat is supplied to the product from both sides. The single-element heater used in this s tudy had a heating capacity of 600,000 to 750,000 B.t.u. per hour.

To prevent " s t r a igh t t h r o u g h " flow, the element is machined in such a manner that a 1/s-in. ridge is spiraled around i t ; this directs the 1/s-in. film of flowing product in a spiral fashion between the two heating surfaces. The liquid travels a total distance of 22 ft. in passing through the heating unit, which pro- vides 11 sq. ft. of heating surface. The average time required for a part icle of liquid to pass through the heating uni t was found to be 1.8 seconds when the equipment was operated at a rate of 6,000 lb. per hour. The tolerances incor- porated in the design of the heating uni t are such that the volume of product making up the film within the heating uni t is 1.5 qt.

The cooling and regeneration units of the heater are of the mix-to-water-to- mix type. They consisted of four tubular elements and chambers similar in design to the heating unit. F igure 3 shows the flow of product through the cool- ing and regeneration system. Although the regenerative uni t is a s tandard par t of this heater, in order to simplify the procedure in the experiments reported this feature was not used. Because of the various temperatures used in this study, the flow diversion valve was operated by manual control.

To determine accurately the processing temperatures, a copper-constantan thermocouple lead wire was permanent ly inserted in the T-joint connected direct ly to the outlet of the heating unit. The lead was installed in such a manner that the temperature was determined at the center of the flowing stream of mix at a point 3 in. f rom the heater outlet. A 3-second holding tube was installed for special study.

Preparation of inocul~em and bacteriological procedures. Micrococcus sp. MS 102 (1) was used as the test organism to determine the equivalence point. The

TO ENIZER ANDHGC~)4~)GLL ER

REATER

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t ~ ' coou. L . . . . . . . . . . . . . "1 I MIX

WATER

/ r ' l ~ - - ~ nEGE.ERATIO, i 1~ t-_- . . . . . . . . . . . ! l _ i

. . . . . = - - - . . . . J l ~ T I I -,- I

| L . . . . . . . . . ,-i* . . . . . _ _ _ , . i I

FIG. 3. Flow diagram showing flow of mix from mixing vat to homogemzer cooling water from city water supply line to coolers and regenerators.

and cooler and

NO-HOLD PASTEURIZATION O~ ICE CREAM MIX 961

organism was grown on N-Z-Case 1 agar slants at 32 ° C. for 24 hours and stored at 40°-45 ° C. for 48 hours. Large slants of this medium were inoculated f rom the re f r igera ted stock culture and incubated at 32 ° C. for 22-26 hours. The cells were harvested in sterile distilled water and filtered through cotton to remove agar clumps. The filtered suspension was made up to 200 ml. with cold water and mixed in a chilled War i ng blendor for 1 minute. To aid dispersion in the mix, sufficient water was added to make 1 l i ter of inoculum.

S tanda rd bacteriological procedures were used in making plate counts. The initial 1 : 10 dilution of ice cream m~x was p repa red by weighing 11 g. of mix into a dilution blank containing 99 ml. of water. All dilutions were plated in duplicate (runs 1-5) or in quintuplicate (runs 6-11) with N-Z-Case medium. Plates were incubated at 32 ° C. for 72 hours.

Operating procedure during experimental trials. Four teen hundred pounds of ice cream mix was used in each exper imental trial, The mixes contained 12% bu t t e r f a t and 38-40% total solids. Speck et al. (4) have shown tha t var ia t ions in total solids of this magni tude do not exert any significant effect on the inacti- vat ion ra te of Micrococcus sp. MS 102. In prepara t ion , the mix was preheated to 120°-130 ° F. and 1 1. of inoculum was added. The inoculum was dispersed by means of a funnel p u m p of the type used to reconsti tute d ry milk solids. I t required approx imate ly 10 minutes of agi tat ion and recirculat ion to obtain a un i fo rm dispersion of the test organism. An a t tempt was made in each t r ia l to process mix at t empera tures above and below tha t necessary to give bacterial destruct ion equivalent to tha t obtained by labora tory pasteurizat ion at 155 ° F. for 30 minutes. The mix was processed at the highest t empera tu re first.

Samples for bacteriological p la t ing were taken simultaneously at three differ- ent locations with the sampling device described by K a u f m a n n et al. (2). Sam- pling por ts were located as shown in F igure 1. The raw mix sample was taken at F; a sample of the heated mix with no intended hold was obtained at G, and mix held for 3 seconds at the processing t empera tu re was sampled at H. The dura t ion of the sampling period was s tandardized to insure uni formi ty . The raw mix sample was divided into two por t ions; one was used to determine the initial count of the unheated mix ; the other was labora tory pasteurized at 155 ° F. for 30 minutes. Since homogenization took place a f te r pasteur izat ion (F igure 3), all samples represent unhomogenized mix.

F o r labora tory pasteurization, 13 ml. of mix was placed in a sterile screw-cap test tube 5 in. long and 0.75 in. in diameter. Care was taken to minimize splash- ing on the sides of the tube above the liquid level. Mix which contaminated the lip of the tube dur ing the p ipe t t ing process was charred by flaming before re- placing the cap. To insure reproducibi l i ty of the heat ing and cooling process, ten tubes were always included in the labora tory pasteur izat ion procedure, blanks containing water or mix being used to main ta in the quota when the num- ber of test samples was less than ten. One test tube containing mix was used as

1 Y e a s t e x t r a c t , 1 g. ; N-Z-Case , 0.5 g. ; g lucose , 0.5 g . ; K2 t tPO~, 0.4 g. ; KI=LPO~, 0.1 g . ; agar, 1.5 g.; water, 100 ml.

962 J. TOBIAS ET AL

a thermometer well. The tubes were placed in a rack to faci l i tate handling. The tubes were immersed in a 4-in. deep water ba th at 130 ° F. unt i l the mix in the control tube reached this tempera ture . The rack and tubes were then immersed to the same level in a water ba th at 160 ° F. unt i l the t empera tu re reached 155 ° F. This required approx imate ly 2 minutes. The tubes were then t r ans fe r red to a constant t empera tu re water ba th adjusted to 155 ° F. and submerged to the base of the screw-cap for 30 minutes. Rap id cooling was obtained by placing the tubes in an ice water bath. Dur ing cooling the tubes were inverted several times.

RESULTS AND DISCUSSION

Results shown in Table 1 indicate tha t a processing t empera tu re of 187.2 ° F . with no intended holding time yields bacterial destruction of Micrococcus sp. M8 102 equivalent to tha t obtained by labora tory pasteur izat ion at 155 ° F. for 30 minutes. A tempera tu re of 181.3 ° F. with a 3-second holding t ime gave the same results. The equivalent tempera tures were obtained in a similar manner as de- scribed by Tobias etal. (5); the process is i l lustrated in F igure 4, which is a plot of log log of the plate count (log of count plot ted on semi-log paper ) versus tem- pera tu re at constant time. On this line were located points which corresponded to the log log of the plate count of the labora tory pasteurized samples and corre- sponding tempera tures were read f rom the graph. Each run was t reated sepa- rately.

I-- I000 g e

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o 4' 1 I i i ~ i I 180 182 184 186 188 190 192 r-

TEMPERATURE ( °F )

~Pm. 4. L o g log o f bac ter ia l coun t s versus ~em- p e r a t u r e p lo t ob ta ined a f t e r h e a t i n g wi th no in- I t ended ho ld ing t ime ( d a t a f r o m the las t r un in Tab le 1) . Equ iva l en t t e m p e r a t u r e s y ie ld ing the same des t ruc t ion of MS 102 as l abo ra to ry pas teur i - za t ion a t 155 ° F . fo r 30 m i n u t e s were de te rmined 0.1 i t 1 ! , i , i

f r o m these types o f plots . 150 160 170 180 190 200 TEMPERATURE ( 'F)

Fro. 5. ( R i g h t ) G r a p h showing po i n t s of c o n s t a n t popu l a t i on of MS 102 a t va r ious t ime a n d t e m p e r a t u r e combina t ions . L ines p a s s t h r o u g h 155 ° F . fo r 30 m i n u t e s (') and 160 ° F . for 30 m i n u t e s (b).

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N O - H O L D P A S T E U I ~ I Z A T I O N O F I C E C R E A M M I X 965

F r o m these considerations it is evident tha t 155 ° F. for 30 minutes, 181.3 ° F. for 3 seconds, and 187.2 ° F. with no intended holding t ime are points of constant bacterial populat ion with respect to the test organism. Therefore, a plot of t empera ture versus log t i m e should yield a s t ra igh t line whose slope is the z value of the organism. However, before a plot of these three points could be made, it was necessary to determine the effective holding t ime when processing was pe r fo rmed with no intended hold. This effective holding t ime includes the combined lethal effects of heat-up time to processing t empera tu re (0.5 second), discharge t ime f rom the heater (0.2 second), sample collection time, and cooling time (0.1 second), all expressed in terms of exposure to the processing tempera- ture. I t is obvious tha t the effective hold can never be zero, and the shorter it is the more difficult it becomes to ascertain its exact extent. I n these studies it was estimated tha t the total effective hold was approximate ly 0.8 second (0.5 + 0.2 + 0.1). The plot in F igure 5 was made with this value as the t ime of exposure. The z value so obtained was found to be 9.8 ° F. Barber (1) repor ted a z value of 9.7 ° F. for the same organism. Since the effective holding t ime does not lend itself to direct measurement, i t was thought desirable to calculate the z value, assuming an error of ± 5 0 % in the est imated hold. Under these conditions the z values obtained are 8.9 ° and 10.2 ° F., as compared with 9.8 ° F.

Some municipal pasteurizat ion s tandards require heat ing to 160 ° F. for 30 minutes to pasteurize ice cream mix. The equivalent tempera tures for this more rigorous s tandard also m a y be determined f rom the data collected in this study, as i l lustrated in F igure 4. A line passing through 160 ° F. for 30 minutes was drawn paral lel to the one passing through 155 ° F. for 30 minutes. Theoretically, this line shows all t ime and t empera tu re relationships giving the same destruction of Micrococcus sp. MS 102 as 160 ° F. for 30 minutes. At the 0.8-second and 3.8- second intercept the corresponding equivalent tempera tures obtained f rom the plot are 192.7 ° and 186 ° F., respectively. An examinat ion of the experimental data in Table 1 indicates an equivalent destruction at a t empera tu re of approxi~ mate ly 194 ° F. at 0.8-second and 187 ° F. at 3.8-second hold; these values are in reasonably good agreement with those obtained f rom the plot. The reason an in- direct approach was used to determine the t empera tu re equivalents to 160 ° F. for 30 minutes is tha t the la t ter process allows less than 0.1% of the test organisms to survive. The ice cream mix used in these studies was not sterile pr ior to inocu- lation with the test organisms and general ly contained some spore-formers, which, in low dilutions (1 to 10), exhibited spreader tendencies and inhibi tory effects on Micrococcus sp. MS 102. I t was difficult, therefore, to detect a few test or- ganisms in the presence of an equal or a larger number of these spore-formers.

In order to determine the safety marg in afforded by the equivalent tempera- tures found in this study, the lethali ty in terms of Mycobacterium tuberculosis was determined as proposed by Tobias et al. (5). The results are shown in Table 2. I t can be seen tha t the H T S T s tandard of 175 ° F. for 25 seconds has a lower lethal i ty in terms of Myc. tuberculosis than the long-hold s tandard but still fur - nishes a good marg in of safety. Theoretically, a value of one or over indicates that the heat t rea tment is of sufficient magni tude to destroy Myc. tuberculosis;

9 6 6 J. TOBIAS ET AL

TABLE 2 Lethality of various time and temperature combinations in terms of 1Ylyc. tuberculosis (1, 8)

Temp. Time Lethality Temp. Time Lethality

(° F.) (seo.) (° r . ) (seo.) 155.0 1800 57.6 160.0 1800 144.0 175.0 25 32.5 177.5 c 30 60.0 187.2 a 0.8 8.6 194.0 ~ 0.8 32.8 181.3 ~ 3.8 12.2 187.0 a 3.8 39.9 194.0 b 0.8 32.8 198.0" 0.8 69.0 186.0 b 3.8 37.2 190.0 ~ 3.8 72.2 Time and temperature combinations giving destruction of )/IS 102 equivalent to that ob-

tained by laboratory pasteurization at 155 ° F. for 30 minutes, taken from Table 1. b Proposed time and temperature combinations. c Time and temperature combination recommended (6, 8) as giving destruction of ~IS 102

equivalent to that obtained by laboratory pasteurization at 160 ° F. for 30 minutes. a Time and temperature combinations calculated to give destruction of MS 102 equivalent to

that obtained by pasteurization at 160 ° F. for 30 minutes. Proposed time and temperature combinations.

the m a r g i n of s a f e ty is g iven b y the d i f ference be tween the ac tua l l e t h a l i t y va lue a n d one. The equ iva l en t t e m p e r a t u r e s o b t a i n e d f r o m cons ide ra t i on of the da t a in Table 1 have lower l e tha l i t i e s t h a n 175 ° F . fo r 25 seconds. The r eason is t ha t Myc. tuberculosis has a z va lue of 12.6 ° F . as c o m p a r e d w i th 9.8 ° F . f o r Micro- coccus sp. MS 102. Thus , a t some t e m p e r a t u r e h i g h e r t h a n a n y cons ide red in th i s s t udy , the p lo t s of log t ime versus t e m p e r a t u r e a t c o n s t a n t p o p u l a t i o n in te r - sect . I n o r d e r to m a i n t a i n the same l e t h a l i t y as a f fo rded b y the H T S T s t a n d a r d , the fo l lowing t ime a n d t e m p e r a t u r e combina t i ons shou ld be u s e d : 194 ° F . wi th an effective ho ld of 0.8 second o r 186 ° F . w i th a 3-second ho ld or a t o t a l effective ho ld of 3.8 seconds. S i m i l a r l y , where the 160 ° F . fo r 30 minu t e s p a s t e u r i z a t i o n s t a n d a r d is enforced , 198 ° F . or 190 ° F . w i th an effective ho ld of 0.8 second o r 3.8 seconds, r e spec t ive ly , shou ld p rove s a t i s f a c t o r y .

The use of tes t o rgan i sms in e v a l u a t i n g the a d e q u a c y of a n y p a s t e u r i z a t i o n process u t i l i z i n g e x t r e m e l y h igh t e m p e r a t u r e s was f o u n d to be sub j e c t to a num- be r of l imi t a t ions . H i g h t e m p e r a t u r e s exe r t ed a de le t e r ious effect on the g r o w t h of the s u r v i v i n g o rgan i sms , a n d i t was neces sa ry to inc rease the i n c u b a t i o n pe r iod f r o m 2 to 3 days in o r d e r to ob ta in m a x i m u m counts .

A n o t h e r p r o b l e m assoc ia ted w i th H.TST p a s t e u r i z a t i o n ar ises f r o m a consid- e r a t i on of the z va lue of Micrococcus sp. M S 102. This va lue is sma l l e r t h a n t ha t of some p a t h o g e n i c b a c t e r i a w i th the r e s u l t t h a t equ iva lence po in t s a t h igh t e m p e r a t u r e s do no t a f ford the same m a r g i n of sa fe ty . I t wou ld be unwise to p ropose a p a s t e u r i z a t i o n t e m p e r a t u r e w i th a sma l l e r m a r g i n of s a f e t y t h a n is p r o v i d e d b y the accep ted s t a n d a r d s u n t i l pos i t ive d i r e c t evidence is p r e s e n t e d to i nd i ca t e t h a t the s t a n d a r d s a re too r igorous . A new tes t o r g a n i s m which m a y overcome some of these l i m i t a t i o n s is now u n d e r s t u d y in th is l a b o r a t o r y .

A n eva lua t i on of the p rocess ing p r o c e d u r e b y m e a n s of l e t h a l i t y in t e r m s of Myc. tuberculosis has r e s u l t e d in a more r i g o r o u s r e q u i r e m e n t t h a n t h a t de ter - m i n e d f r o m the equ iva l en t d e s t r u c t i o n of Micrococcus sp. MS 102. Th is t echn ique is a p p l i c a b l e to a n y p a s t e u r i z a t i o n process a n d f u r n i s h e s a n u m e r i c a l i n d e x o f the m a r g i n of sa fe ty . Thus, i t is poss ib le to select the de s i r e d s a f e ty f a c t o r a n d

NO-HOLD PASTEURIZATION OF ICE 0REAM MIX 967

to calculate the temperature required to obtain it. Although this technique pre- sents an excellent means of checking the adequacy of any heating process, it does not eliminate the need for experimentally evaluating various types of heaters, preferably with test organisms, to ascertain whether or not the heating charac- terietics are in accord with those theoretically postulated.

SUMMARY AND CONCLUSIONS

1. Equipment for heating and pasteurizing with no intended holding t ime possesses some effective holding time which is derived from the lethal effect o f the heat-up time, discharge time, and cooling time. I t was estimated that the Roswell heater used in these studies had an effective hold of 0.8 second at the processing temperature.

2. The destruction of Micrococcus sp. MS 102 equivalent to that resulting" f rom laboratory pasteurization at 155 ° F. for 30 minutes was obtained at 187.2 ° 1~. or 181.3 ° F. with effective holding times of 0.8 second or 3.8 seconds, re- spectively.

3. The destruction of Micrococcus sp. MS 102 equivalent to that resul t ing from pasteurization at 160 ° F. for 30 minutes would be obtained at 194 ° F. o r 187 ° F. with effective holding times of 0.8 second or 3.8 seconds, respectively, as shown by calculation.

4. Plot t ing log time of 1,800 seconds versus temperature at 155 ° F., 3.8 sec- onds at 181.3 ° F., and 0.8 second at 187.2 ° F. (points at which the population of Micrococcus sp. MS 102 is constant) yields a straight line with a z value o f 9.8 ° F.

5. Because of the smaller z value of Micrococcus sp. MS 102 compared to that of Myc. tuberculosis (1, 3), the margin of safety progressively decreases as~ higher temperature equivalents are obtained. A numerical index of the safety margin may be obtained from a calculation of the lethality in terms of Myc. tuberculosis by means of which time and temperature combinations may be postulated with any desired margin of safety.

6. A temperature of 194.0 ° F. for 0.8 second or 186.0 ° F. for 3.8 seconds: yields the same lethality in terms of Myc. tuberculosis as 175.0 ° F. for 25 seconds. 2~ temperature of 198.0 ° F. for 0.8 second or 190.0 ° F. for 3.8 seconds gives a lethality equivalent to that obtained at 177.5 ° F. for 30 seconds, conditions which have been found to give results comparable to pasteurization at 160 ° F. for 30 minutes (6, 8).

ACKNOWLEDGMENTS

The authors wish to express appreciation to the Chicago Stainless Equipment Corp. for financial aid in this study. The assistance of the following persons in planning the project and evaluating the results is also acknowledged: Harold Wainess, Wainess Associates, Chicago; P. E. Riley, Illinois Dept. of Public Health; S. ft. Conway, J. A. Meany, Harry Cohen, and E. ]~. King, Chicago Board of Health; H. B. Robinson, James Westbrook, and L. C. Peckham~ U. S. Public Health Service.

968 J. TOBIAS ~T AL

R E F E R E N C E S

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(3) OLD~NBUSH, C., FROBISHRR, M., JR., AND SHRADER, J . H. Thermal Death Points of Pathogenic Bacter ia in Cream and Ice Cream. Am. J. Pub. Health, 20: 615. 1930.

(4) Se~.CK, lYI. L., GROSCSE, C. A., LUCAS, H. L., JR., AND HA~KI~, L. Bacteriological Studies on High-Temperature Short-Time Pasteurizat ion of Ice Cream Mix. J. Dairy Sci., 37 : 37. 1954.

(5) TOBIAS, J., H E ~ m , E. O., A~I) ORz).¢5, Z. J . A Study of Milk Pasteur izat ion a t High Temperatures. J. Dairy Sci., 36: 356. 1953.

(6) TOBIAS, J., AND T~CY, P. H. Continuous Pasteur izat ion of Ice Cream l~Iixes. Ice Cream Trade J., 48 (9) : 116. 1952.

(7) T ~ c Y , P. H., TOBIAS, J., AND HRRP~ID, E. O. Applicat ion of the Vaereator and i~Iallorizer for High-Temperature Short-Time Heat ing of Ice Cream Mix. Ice Cream Trade J., 47 ( 4 ) : 76. 1951.

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