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Heat Transfer in Foods during Freezing and Subsequent Thawing I-Temperature Changes in Sugar Solutions, Sweetened Fruit Juices, and Other Liquids1 M. A. Joslyn and G. L. Marsh FRUITPRODUCTS LABORATORY, UNIVERSITY OF CALIFORNIA, BERKELEY, CALIP
The rate of temperature change in water, sugar s o h top No. 10 plain tin cans were tions of various concentrations, whole milk, orange determined with copper-contemperature changes juice, dry pectin and dry sugar, a pectin-sugar sol, and stantan thermocouples and a during the freezing and sweetened juices in No. 10 tin cans was determined Leeds and Northrup semipresubsequent thawing of fruits during freezing at -17.8' C. and during subsequent cision portable potentiomeand vegetables is of major thawing at 21.1" C. It was found that heat transfer ter (No. 8662). The workimportance to the industry, in these products occurred primarily by conduction ing junction was sheathed in although little work has been and that only in the later stages of thawing did cona laminated B a k e l i t e tube done on it, especially in convection play an appreciable part. The rate of tempera'/c,X '/4 inch (3.2 X 6.4 mm.) nection with small containers. ture change increased with increase in sugar concenequipped with a small thin Birdseye ( I ) , Diehl (2), and tration and was not appreciably affected by changes brass tip in which the juncNelson and Lang ( 5 ) have rein viscosity. However, there was apparently no simple tion was soldered. The leads ported some observations, but direct relationship between sugar concentration and from the junction were enthe changes have not been rate of temperature change. cased in l/rinch (6.4-cm.) difollowed in detail and genThe chief factors that determined the rate of temameter black gum rubber tuberally not during the thawing perature under the conditions of these experiments were ing sealed to the Bakelite tube stage. It is necessary to found to be the specific heat of the solution, its heat with Vanak. Since it was know how the size and type of conductivity, the temperature at which ice began to desirable to determine t h e container, the nature of the separate, and the amount of ice that separated under temperature without disturbcontents, the type and style the freezing conditions. ing the containers or the room of the packing case, and the Three distinct periods defined by temperature temperature, and since it was kind and temperature of the changes were noted. I t was found that sugar solutions necessary to use the same surrounding medium infludiffered from water in cooling faster with increasing junctions during freezing and ence t h e s e t e m p e r a t u r e sugar concentrations, that their freezing temperature thawing carried out in differchanges before definite recis lower the higher the concentration of sugar, that the ent rooms, the working junco m m e n d a t i o n s c a n be period of fairly constant temperature is less the higher t i o n s c o n n e c t e d to rubmade for the p a c k i n g of the concentration of sugar, and that the cooling of ber-encased copper and confood to be frozen and for frozen sirup is slower than that of ice. stantan leads about 12 feet transportation of the frozen (3.7 meters) long were made product. Outside of the fact that during freezing the heat is trans- detachable. Copper and constantan lugs were soldered to the ferred from the product in the container through the walls of ends of the wires and connections were made to insulated the container to the surrounding air, very little is known about cables entering the freezing room and the thawing room on the exact mode of heat transfer. Which mode of heat trans- a paraffin-impregnated transite panel. The reference juncfer-conduction, convection, or radiation-is the most im- tion was placed in an asbestos-lagged, cotton-filled wooden portant during freezing is not definitely known. If convec- box (17 X 91/2 X 61/2 inches) next to the bulb of an accurate tion currents play as important a part as they do during sensitive calibrated mercury-in-glass thermometer (Figures 1 sterilization by heat, then changes in the viscosity of the prod- and 2). The thermometer bulb and the reference junction uct will influence to a marked degree the rate of freezing. were sufficiently insulated so that the relatively more sluggish If conduction plays the limiting role, then increasing the thermometer indicated fairly closely the temperature changes in the junction. The variation of the reference-junction conductivity and decreasing the specific heat of the producttemperature with room temperature is shown in Figure 3. as, for example, by increasing the sugar content of the fruitshould increase the rate of cooling. The effect of ice crys- A constant temperature ice-water bath would not be as satistallization and the liberation of the heat of formation has not factory, since it would involve reversal of connections when temperatures below 0" C. were measured and was not so been studied. This first report of investigations sponsored by the Paper- adaptable to multiple junction work. Twenty thermoboard Industries Association presents a preliminary study of couples were used and these were connected to the potentiomethe relative importance of the various modes of heat transfer. ter outside the cold-storage rooms through Leeds and Northrup selector switches free from thermal effects. The potentiTemperature Measurement ometer was equipped with an accurate hand-operated cold junction compensator which was arbitrarily set a t 0.500 For the purpose of these investigations temperature millivolt at a reference-junction temperature of 70" F. changes were determined at the center of the contents, which (21.1" C.). When the reference-junction temperature inis the warmest point during freezing and the coldest during creased the setting was decreased at a rate of 0.021 millivolt thawing. The temperatures at the center of the friction- per degree Fahrenheit (0.56' C.) (the slope of the e. m. f.temperature curve at this point) and increased for a decrease 1 Received August 13, 1930.
HE investigation of the
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
in reference-junction temperature. Owing to the large external resistance of the cable leading to the working junction, the galvanometer furnished originally with the instrument was not found sufficiently sensitive. A No.
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to enable us to get a large number of readings, it was sufficiently rapid so as not to prolong the experiments. Moreover, although it was sufficiently large to enable us to place the thermocouple tip accurately a t the center of contents, the amount of material used was not excessive. Owing to expansion in volume in water and substances containing water on freezing, the containers could not be filled full without danger from bursting on freezing. The degree of expansion of sugar solutions at 70" F. (21.1' C.) when allowed to freeze a t 0" F. was approximately determined and is shown in Table I. Table I-Expansion of Water and Sugar Solutions o n Freezing SUGAR INCREASE I N V O L U M E Per cent Per cent 0 (water) 8.6 R 7 10 20 8.2 30 6.2 40 5.2 50 3.9 60 None 70 1 . 0 (decrease)
To be perfectly safe the cans were filled only to 90 per cent of their capacity by volume. The brass tip of the working junction was inserted through the top of the can to the geometric center of the contents. The thermocouple sheath was sealed to the top of the can by means of a brass bushing and lock nut soldered to the lid. Freezing Room Figure 1-Method of Adjusting Reference Junction i n Asbestos-Lined Box
20 B. & S. gage insulated stranded copper wire and a No. 26 B. & S. gage single, double cotton-covered tested constantan wire were used and the cable to the freezing room was about 60 feet (18 meters) long and to the thawing room about 40 feet (12 meters) long. A special 10,000-ohm resistance galvanometer was used, but it was not completely critically damped for the conditions of these experiments. The sensitivity of the galvanometer on the freezing-room circuit was 0.010 millivolt per division and on the thawing-room circuit, 0.0096 millivolt per division. On short circuit its sensitivity was about 0.0074 millivolt. By means of a reading lens a displacement of one-tenth of a division could be determined. Leeds and Korthrup guaranteed the instrument to be accurate to 0.1" F. (0.05"C.). The potentiometer was calibrated over the range of -2" to 80" F. (-18.9" to 26.7" C.) against a standardized and tested mercury in glass thermometer having a range of -30" to 100" F. (-34" to 37.8" C.) anddivisions of 0.5"F. (0.28" C,). For this purpose it was found best to use a chilled saturated sodium chloride solution in a %gallon earthenware crock. Thermocouples Kere fastened to the thermometer so that the tip rested a t the bulb and temperatures during warming a t room temperature were determined. The brine was stirred during these tests. A typical calibration curve is shown in Figure 4. The thermocouples were frequently checked against each other and a t various temperatures. It was found that, in making connections on the transite panel, an increase in the pressure on the screw nut beyond that easily accomplished by the hand did not, affect the readings. The temperatures recorded were probably accurate and reproducible to within 0.2" F.(0.11" C.). Containers
The No. 10 plain tin friction-top cans (6 inches in diameter and 6'/8 inches high) were used in these tests because, although the rate of temperature change was sufficiently slow
An experimental freezing storage room cooled to about 0" F. (-17.8' C.) by a direct-expansion ammonia system was used in these experiments. During the f i s t tests the temperature in the room was not constant; later a more careful control of the room temperature was made by the use
Figure 2-Reference
Junction i n Cotton-Packed Air Bath
of a heater. The cans with the thermocouples were staggered on an 8-inch high (20-em.) slatted wooden platform and carried into the room a t the commencement of the run after their initial temperatures were determined. The couples were connected and readings begun within 10 minutes from the time they were brought into the room. Readings were made every 10 minutes for the first hour and every 20 minutes for 22 hours until the rate of temperature change was slow
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juice, sweetened lemon juice, and punch sirup prepared by mixing equal quantities of the above three sirups. At the end of the run the sugar solutions and pectin solution were tested with the results shown in Table 11. The relative viscosity was determined by a Stormer viscometer and corrected for air resistance. Discussion of Results
The results are presented in graphical form in Figures 5 to 9. The curves follow closely the actual plotted points and none of these lie appreciably off the curve. The temperature changes during the cooling of sugar solutions (Figure 5 ) in general agree with those that could be predicted from a study of the freezing I 66 1 HOURS I point-solubility relations of sugar soluIO 20 30 40 50 60 70 tions as shown in Figure 10 adapted from Figure 3-Variation of Temperature of t h e Reference Junction with Room Temoerature data in International Critical Tables (4). enough, when readings every 30 minutes were taken. About Although only slight supercooling occurred in the 10 per cent 100 readings were made during freezing on each test and the sugar solution, it was appreciable in-the 20,30,40, and 50 per average elapsed time was about 44 hours. cent solutions, Table 11-Concentration SoLuTrou
Water Sugar solution: 10 per cent 20 per cent 30 per cent 40 per cent 50 per cent 60 per cent 70 per cent 5 per cent pectin plus 10 per cent sugar
Figure 4-Calibration
Curve
Thawing Room
At the end of this period the cans were allowed to stay in the freezing room for several more davs " (over . the week-end) and then removed and the couples re-connected in the thawing room held a t about 70' F. (21.1' C.). Immediately after removal frost formed on the bottom and sides of the cans to within 1 inch (2.5 cm.) of the top and not on the top. The amount of frost and length of time it persisted were found to be a rough index to the rate of thawing. During thawing readings were taken every 5 minutes for 1 hour, then every 10 minutes for 1 hour, and after that every 20 minutes. During a thawing period of 27 hours about 70 readings were made. Materials Used Y
Water, cane sugar solutions of approximately 10, 20, 30, 40, 50, 60, and 70 per cent, dry cane sugar, whole milk, orange juice, dry pectin, 5 per cent pectin and 10 per cent sugar solution, and 50 O Balling grape concentrate, sweetened orange
4
and Viscosity of t h e Solutions Used SOLUBLE SOLIDS VISCOSITYRBBY REFRACBERRBD TO TOMETER WATER Per cent 0 1 00 9 6 19 1 29 1
17 50 70 00 04 92
49 6 59 7 70 6
3s 3
1 1 1 2 3 6 67
14 6
29 5
7
I n Figure 5, especially the curve for water, three distinct periods defined by temperature changes will be noted. During the first stage of the freezing process the center of the can falls in temperature fairly rapidly as the contents of the can lose their heat to the surrounding cold air, the material a t the outside of the can losing its heat directly through the walls of the can to the air and that a t the center losing its heat to that a t the outside. Owing to the insulating effect of the air in the head space, most of the heat was disseminated through the sides and bottom and but little through the top of the can. During the second period the center of the can is at a constant temperature since the material has
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Figure 5-Temperature
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Changes of Sugar Solutions during Freezing
November, 1930
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Upon thawing, the cold ice rapidly reached the freezing point and the temperature a t the center eD remained fairly constant until all the ice melted, in spite of a fairly large temperature gradient between the water a t the side of the can and the ice remaining a t the center on the thermocouple. However, as soon as that ice melted the tempera1 7 ture a t the center rapidly increased. The rate of rise of temperature a t the center of sugar and 70 per cent sugar solution was somewhat slower in the legand G ' initial stages than that of ice, but more rapid thereafter owing to absence of any marked change 3 io% . y o r *',/on -in phase. The more concentrated the sugar solu4 LO?. s!qm ad"/ 5. solu,on tion, in general, the slower was the initial but the 6 40% -or rdulm more rapid was the subsequent rise of temperature. 7- 50% sqor eddm It is interesting to note that, because of the slow8 60% Srqor 60hhOn _ _ ~ _ _ _ _ _ _ _ _ _ 9 70% s v p r v I . ~ v ~ I M _ _ ness with which the frozen sugar solutions warmed Io - R a m Yem,wOkre - 20 in their initial stages, the 10 and 20 per cent sugar i I novra solutions showed a lower temperature than did I6 I0 20 IO I2 2 4 6 8 4 even water. This may have been due to convecFigure 6-Temperature Changes of Sugar Solutions during Thawing tion currents, which probably accounted for the cooled to its freezing point and heat is liberated by change of rapid rise of temperature in water in the later stages. That there is apparently no direct relation between sugar water to ice. The material a t the outside reaches the freezing point first and forms a sheet of ice surrounding the contents, concentration and rate of temperature change is shown in the ice being first formed along the sides and bottom and later Figures 11 and 12. During cooling, in the initial stages there is a slow increase on top. As long as water 2o in the rate up to 50 per continues to separate as cent, then a fairly rapid ice the temperature reincrease to dry s u g a r . m a i n s constant. Dur- ~1 The 60 per cent and 70 ing the third period, representing the cooling of c per cent sugar solutions show marked deviation the frozen product, the from this behavior which temperature a t the center -1 may be due, especially falls to room temperau. in the 70 per cent soluture. tion, to the crystallizaIf convection currents do play an i m p o r t a n t t i o n of s u g a r . I n the l a t e r s t a g e s , approxirole in the process, then mately at the middle of they should exert their , 3 oranoepce the constant-t e m p e r a f u l l e f f e c t in the first s t a g e ; cooling a f t e r ture period for ice, there -a is a rapid increase in the separation of ice occurs is i n a l l p r o b a b i l i t y r a t e of t e m p e r a t u r e LO 24 K8 32 36 a conductivity change with sugar 'OnFigu:e 7-T:mpera:ure Cianges in Some Food Products during Freezing phenomena. The temcentration to 60 per cent perature changes will depend not only upon the heat transfer and a slow increase from 60 to 100. That this relation is occurring, but also upon the specific heat of the material. even more complex during thawing is shown in Figure 12. There are verv few mblished data on the specific heat and In the early stages there is a rapid decrease in rate of temheat conduct&ity ofsugar solutions. The specific heat of sugar ( 3 ) is 0.3005; of 43.2 per cent sugar solution, 0.7558 (3); and of 4.5 per cent sugar solution, 0.9742 (3). There is thus a t most a threefold change in the specific heat of water to 70 per cent sugar solution, but a t least a 68-fold change in viscosity a t room temperature. The sugar solutions, however, differ from water in that the rate of cooling is slightly faster with increasing sugar concentration in the first stage, the freezing temperature is lower the higher the concentration of sugar; that the period of fairly constant temperature is less the higher the concentration; and that the cooling of frozen sirup is slower than that of ice. I n the sugar solutions, as in water, ice forms first near the outside of the can, but the separation of ice on the outside increases the sugar concentration in the center to a point where it does not freeze, and consequently further separation of ice does not take place without a decrease in tempera0 4 6 8 a I2 U 16 18 ep ture. Figure 8-Temperature Changes in Some Food Products during Thawing I
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Vol. 22, No. 11
thawing curve. It is possible that this was due to some clumping of the very hygroscopic pectin around the thermocouple which was not perfectly dry. A comparison of the t e m p e r a t u r e ,o changes in a 50 per cent sugar solution and in grape concentrate, punch sirup, and in sweetened orange and lemon juice of the same density is shown in Figure 9. Although the latter were all more viscous than the sugar solution and the orange, lemon, and punch contained pulp from the flesh of the orange and lemon, they lo cooled at about the same rate. The sugar solution cooled more slowly than the others. The sweetened orange and lemon cooled somewhat faster and at about the same rate. The punch sirup cooled more rapidly and the grape concentrate the most The freezing points Of Figure 9-Comparison of Temperature Changes i n 50 Per Cent Sugar Solution and FruitJuice Sirups and Concentrates of Same Density orange, lemon. and wnch were lower than- that of 'the &gar solution; that perature rise from water to 50 per cent sugar solutions, a of the grape concentrate was lowest of the five. The rapid increase from 50 to 70 per cent, and a slow increase to grape juice concentrated to 50" Balling had a lower freezing 100 per cent. However, in the subsequent stages there is a point than the other sirups owing to its higher content of gradual shifting of the minimum point from 50 per cent to 10 acids and salts. During thawing the relationships were, in per cent sugar. general, similar to those during freezing. As in all other cases, there was a greater difference between the different materials than during freezing. 20
Conclusions
The heat transfer during freezing and thawing of sugar solutions and other liquids is apparently, to a large extent, transfer of heat by conduction. It is only in the later stages of thawing that convection plays an appreciable part. This does not necessarily mean that convection currents do not occur during the initial stages of the cooling process, but, that if they do occur heat transfer by that means is limited and overshadowed in importance by heat transfer by conduction. Owing to the relatively low temperature differencesheat transfer by radiation does not occur to any appreciable extent. The specific heat of the solution, heat conductivity, the tempera-
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Figure 10-Freezing Point-Solubility Relations of Sucrose
The temperature changes in whole milk during freezing and thawing lie between those of water and 10 per cent sugar solution, and its freezing point is about 31" F. (-0.56" C.) (Figures 7 and 8). The temperature changes in the pectin solution lie between those of the 10 and 20 per cent sugar solution and in spite of its high viscosity it behaves very much like a 15 per cent sugar solution. The orange juice (11.0' Balling) cooled much more slowly than any solution tested, and its freezing point is lower than the 10 per cent sugar solution and about that of the 20 per cent solution. During thawing the temperature changes in orange juice fall between those of 10 and 20 per cent sugar solution. This behavior of orange juice is due in part to its fiber content, since the juice was extracted from oranges by burring and was not filtered. Dry pectin cooled somewhat more slowly than dry sugar and showed a distinct change of slope and a break in the curve at 30" F. This was also evident in the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1930
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of temperature change occurs with increase in sugar concentration. Moreover, the rate of heat transfer is not a p preciably affected by the viscosity of the liquid. Acknowledgment
.T e m p e r d u n ~n ( e n t m oftrr .T e n ~ e r o l u Rjn i e n k r d t c r
3. Temprrotvn
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Figure 12-Effect of Sugar Concentration on Rate of Temperature Change d u r i n g Thawing
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The writers gratefully acknowledge the aid of A. W. Luhrs in securing funds for these investigations from the Paperboard Industries Association. Thanks are due to the American Can Company, especially Mr. Culver, for kindly furnishing the cans used. Literature Cited (1) Birdseye, IND.ENG.CHEM., 21, 414, 573 (1929). (2) Diehl, U. S. Dept. Agr., Tech. Bull. 148 (1930); Glass Packer, 3, 179, 190 (1930). (3) Evans, Physico-Chemical Tables, p. 204, London, 1902. (4) International Critical Tables, Vol. 11, p. 346 (1927). (5) Nelson and Lang, Food Industries, 2, 184 (1930).
The Initial Corrosion Rate of Steels’ H. 0.Forrest, B. E. Roetheli, and R. H. Brown DEPARTMENT OY CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
HE rapid development
Recent publications on the corrosion-resistant propsteels should be of the same and increased producerties of chromium steels have laid considerable stress order of magnitude as for ortion of comosion-rc+ on t h e oxide films as a preventive for corrosion. Studdinary steels, decreasing very &ant steels by alloying iron ies of t h e initial period, during the first few minutes, rapidly to a negligible value. with chromium and nickel in If one of the additional ashave shown that the corrosion rates of an 18 per cent varying proportions have led sumptions is granted-that chromium-8 per cent nickel steel and of a 14 per cent to numerous studies of the in neutral distilled water the chromium steel are of the same order of magnitude as single potentials of the steel those of an ordinary steel. The actual values of oxygen reasons for their b e h a v i o r . Many theories have been adused in the above order are 0.0095, 0.0145, a n d 0.0192 cc. alloys and steel are of the vanced, that which seems to per liter per square decimeter per cubic centimeter of same order of magnitude, or that the rate of diffusion of offer the most plausible exoxygen present per minute. After a 10-minute period p l a n a t i o n being the wellthe rate for ordinary steel decreases only slightly, while oxygen to the metal surface known “oxide film” theory those for the high-chromium steels become essentially controls the reaction-t h en zero. These results give further evidence of the exthe initial corrosion rates of which, in essence, postulates the building up of an intreme importance of films i n corrosion processes. r e s i s t a n c e s t e e l s should s o l u b l e oxide film on the be not only of t h e s a m e surface of the metal. This film serves to prevent diffusion order of magnitude but essentially the same as for low-carbon and, therefore, further attack. The same mechanism may steel. The present research was undertaken for the primary be said to take place in a greater or lesser degree on all iron surfaces which are attacked in media in which an purpose of developing a method of determining initial coroxide is insoluble. I n the case of the resistant steel the film rosion rates of steels in order to ascertain the effect of film formed is supposedly dense, adherent, and not penetrated by formation. As may be obvious, the design of such an experithe corroding medium. mental apparatus entailed many difficulties, but it was finally Many facts have been presented in the literature in recent developed to a point where consistent check values could be years which seem to substantiate this theory. Among the regularly obtained, and where the possibility of results being most important of these are: due to any mechanism other than oxidation of the metal sample was eliminated. Studies were then made of the initial (1) The inability of “air-oxidized” iron to Plate out copper corrosion rates of oxide-free metal surfaces of annealed lowfrom a copper sulfate solution ( 6 ) . (2) The resistance to corrosion imparted to iron by immersion carbon, 14 per cent chromium, and 18 per cent chromium-8 in solutions of oxidizing salts such as chromates (1,2). per cent nickel steels in distilled water. (3) The resistance to the flow of current through iron, nickel, and cobalt electrodes after anodic oxidation (7). Experimental Method (4) The inactivity of metals having basic oxides in neutral or alkaline media and their subsequent activity in acids in which The apparatus used in this investigation was designed to the solubilities of the oxides are greater (3). permit the measurement of the corrosion rates in water of (5) The actual presence of tough, thin, transparent oxide films on “air-passive~~ materials as determined by their isolation active metal surfaces by means of the decrease in the oxygen from carbon and stainless steels by selective dissolution of the content of the water for definite periods of immersion. For underlying metal (4, 5 ) . successful operation the apparatus was required to permit the following processes: (i)treatment of metal wit6 acid, If the assumption is made that an oxide is necessary to (2) washing free of acid with oxygen-free water, (3) introducprevent corrosion, then the initial rate of corrosion of stainless tion of oxygenated water, and (4) determination of the decrease of oxygen concentration of corroding medium. 1 Received August 9, 1930.
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