INDUSTRIAL A N D ENGINEERING CHEMISTRY
3 76
of filtration, it is necessary to establish a practical mean between a poor filtration rate and excessive formation of objectionable substances. It is believed that with the proper supervision most muds can be filtered with a resulting juice having a p H value not exceeding 7.8, and with many muds even less lime can be used. It has also been pointed out that double pressing of muds further increases the impurities entering the process. However, calculated on a solids basis, the quantity of material that may be redissolved in the second filtration is so much less than in the first that the benefits to be derived by careful handling of the muds a t the first press station can be readily seen.
VOl. 20, No. 4
Likewise, the data indicate that, while a portion of the lime is utilized when the filtered juice is returned to the raw juice, the colloidal material (reversible colloids) formed in the filtered juice is not eliminated during redefecation. The quantity of filtered juice is about 15 per cent of the total juice; therefore, when the filtrate is returned to the raw juice the defecation capacity is, of necessity, reduced, and it is a question whether the benefits derived are sufficient to offset this loss in capacity. When, however, with good filtration, the resulting juice is maintained a t a p H near that of the defecated juice, its addition to the defecated juice appears to be the better practice.
Viscosity and the Ice-Cream Mix’ G . D. Turnbow and K. W. Nielson UNIVERSITY O F CALIFORNII, D - ~ v r sCALIF. ,
I
T HAS been considered desirable that an ice cream mix show a viscosity between certain limits and much has been written concerning the proper processing, aging, and methods of handling in order that it should possess and retain this viscosity. Obviously, a certain amount of true viscosity will be derived from the components in solution in the mix, which would affect viscosity only from the points of temperature and concentration. The true viscosity depends largely upon the composition of the mix.
milk solids-not-fat, 15.5 per cent sugar, and 0.35 per cent gelatin. The mixes were pasteurized in a glass-lined vat a t 63” C. and homogenized a t the same temperature and a t a pressure of approximately 1133 kg. per sq. cm. The pressure was varied slightly with the acidity, in order to obtain for a uniform composition a uniform viscosity for the fresh mix. Table I-Relation ACIDITY
Development of Apparent Viscosity
Present standard procedure calls for the aging of the mix from 24 to 48 hours a t 1.1’ to 4.4” C. There develops during this period a colloidal viscosity due to the formation of a gel structure known as “apparent viscosity.” Ice-cream manufacturers believe that this apparent viscosity is necessary for proper yield and smoothness of texture. The apparent viscosity of the mix depends somewhat upon the processing, the amount and quality of gelatin used, and the time a n l temperature that the mix is held in the aging vat. If the mix is held too long it becomes difficult to incorporate air into it during the freezing period, and mixes held for a long time may be quite viscous. It has been assumed, therefore, that the long period allowed for aging led to the development of too great viscosity, making it difficult to incorporate air. Leighton and Williams2 processed a standard mix in the usual way, and found the true viscosity in the fresh mix t o be, for example, 9.11 centipoises; upon aging approximately 24 hours the mix had an apparent viscosity of 50 centipoises. I n this experiment a 6-gallon brine-cooled freezer TI a: fillel with mix, agitated a t 175 r. p. m., and the destruction of the apparent viscosity measured with an Oswald viscometer. After 60 minutes there remained a basic or constant viscosity of 36.31 centipoises. The authors of this paper have obtained repeatedly a much lower basic-viscosity figure-in fact, so low as to indicate that the apparent viscosity developed during the aging period is completely destroyed by agitation -this basic viscosity being equal to the true viscosity in the original mix prior to aging. Reduction of Apparent Viscosity
The mixes were divided into three lots; one was homogenized once, another twice, and the third three times. The average composition was 11 per cent fat, 10.5 per cent 1
2
Received October 31, 1927. .I. P h y s . Chem., S 1 , 596 (1927)
a
of Acidity to Homogenization Pressure, Viscosity Being Constanta PRESSURE
ACIDITY
P e r cent K g . / s q . cm. P e r cent 0.16 1582 0.24 0.18 1474 0.26 0.20 1361 0.28 0.22 1246 0.30 Turnbow and Milner, unpublished data.
PRESSURE Kg./sq. cm.
1133 1020 907 816
Viscosity determinations were made immediately after the mix left the cooler, and then a t 5-, 24-, 48-, and 72-hour periods. The measurements were made with a MacMichael viscometer using standard methods. The viscosity of the fresh mix processed once was 0.7921 poise, and the same mix after 5 hours had developed an apparent viscosity of 3.3884 poises (Table 11). The standard commercial mixes when taken from the cooler were drawn into glass containers of uniform size and held a t a constant temperature. At the end of the 5-hour period one of these containers was placed on a shaker in a room of constant temperature and shaken 1400 oscillations, a t the end of which a constant viscosity was obtained. This viscosity was 0.7481 poise, a figure slightly lower than the viscosity of the fresh mix taken directly from the cooler. This slight variation can be explained by experimental error and by the fact that it is impossible to obtain a true temperature of the fre;jh mix. If time were allowe I for the tempering of the sample, a colloidal structure would have started to develop. The mix processed two and three times decreased in apparent viscosity with each processing. The viscosity developed during the aging period regardless of the number of times processed, though the same relationship tends to hold. One fact largely responsible for the decrease in viscosity usually caused by homogenizing two and three times is that there is less clustering of the butter fat. An increased dispersion of the butter-fat clusters was noticed by homogenizing three times instead of twice. An ice-cream mix homogenized once showed marked formation of fat clusters, but these clusters were largely destroyed during the first few minutes in the freezer. The data presented in Table I1 were checked by preparing standard mixes as previously described and determining
INUUSTEIA L AND ENGINEEIZING CHEMISTRY their viscosity directly from the cooler and again at the end of the 24-hour period. A standard commercial 40-quart (381itcr) brine freezer with the dasher operating a t 215 r. p. m. was completely filled, the ordinary outlet being sealed to make sure that no air was let into the freezer, and sufficient brine being used to maintain a constant temperature. The mix i m s agit.atetl for 60 minutes, smiples being withilrau~nat 1-minute intorials for the first, 10 minutes, and a t 5-minute intervals thereafter. It was so arranged that a mix uf constant temperature was allowed to flow into the freezer to replace the sample withdraim. The trne viscosity of the mix fresh from the cooler was 0.7921 poise at 16.7" C.; the mix after aging for 24 hours had an apparent viscosity of 5.6027 poises; a t the end of the 60-minute agitation period t.here remained a true viscosity of 0.8501 poiseat 15.6' C. Theauthors consider that the true riscosity of 0.8801 poise is identical with the original viscosity of 0.7921 poistL.-that of the fresh mix. The temperature of the mix when basic viscosity was determined was approximately 1' C. lower than that at the time the t r u e viscosity was determined. The precision of the results ohtained for the basic viscosity of the fresh mix is influenced by the fact that it was necessary to hold the mix at the temperature at which the viscosity measurements were to be made for 2 hours before making the measurements. During this period a colloidal structure developed which caused a much higher reading. Surface Tension
s
Y " 0
m
a
0
0
..Ne
The surface-tension mearurenients were made with a du Noiiy surface-tension apparatus at 5and 72-hour periods. Considera,blework has been done in studying surface tension of the icecream mix. The surface tension of the mix after aging for 5 hours (Table 11) differs but little from that of mixes aged 72 hours, and is little influenced by processing.
377
Figure I~--MInShowlnp Butter-Fat Cluaterinp as a Result of Homagenlnlne wlfh
B
Single-Stage Valve
three tinres. Eachseries was frozen and whipped in the usual way, and representative data froin the results are tabulated in Table 111. of TOW Freer,lne Process on Mix Proceeeed One,
Table 111-EUect
Two, and Three Times
TIMKS 'rBMPBwmm PROCSEYBD I UnmR
C.
T M ~TO :
Fnasre
Mi". S e i .
Sec.
35 20
0 10
55
8
+1.5
6
2
+1 +I
6
n
TO
WXZP
Miin.
1
3
TOTAL Pna~rrlio TIMS
40
30
1s
AND W X ~ P I N E
TIME
Min. 16 16 15
SCC.
15 50
30
It is impossible to control the yield accurately using standard methods, because an unstable emulsion exists at the end of the freezing and whipping period. The difference in the t.otal freezing time as shown in Table I11 is insignificant in bliat the variation would exist in similar samples regardless of the number of times processed. Therefore, homogenization has little, if any, effect upon the total time required to freeze an ice-cream mix. As previously mentioned, a microscopic study of the ice-cream mix before and after freezing and after homogenizing one, two, and three times indicated that t,here existed a clumping of the fat in the' ice-cream mix after one homogenization, but that all of the finished cream had the same general appearance as far as clusters or smoothness of body or texture were concerned. Summary of Results I~GApparentviscosity increases during the aging of an icwxeam mix. 2--By successive homogenization of an ice-cream mix the apparent, viscosity was decreased and this relationship war maintained during the aging period.
Effect of Multi-Processing upon Whipping Properties of Ice-Cream Mix Several mixes of identical comuosition were divided into three iots; One lot was homogenized once, another twice, and the third
Figure >Same MIXas In Figure I except Ilomoeenized w i t h a TwoSfape valve. Note Freedom of Clustering
INDUSTRIAL AND ENGINEERING CHEMISTRY
3i 8
3-By sufficient agitation of an aged ice-cream mix the apparent viscosity may be reduced to the viscosity of the fresh mix from the cooler. 4 S u r f a c e tension of an ice-cream mix is not affected by aging. 5-Surface tension of an ice-cream mix tends to increase slightly after each successive homogenization. &During the freezing process the apparent viscosity approaches the true viscosity of the fresh mix obtained from the cooler. 7-Repeated homogenization of an ice-cream mix does not
Vol. 20, No. 4
materially influence the total time for freezing and whipping the mix. Conclusion
From the foregoing data it would appear that the benefits obtained by aging an ice-cream mix are not due to the development of viscosity. However, it is known that an icecream mix manufactured under present conditions is improved in quality by aging. This is probably due to some change in the proteins caused by the presence of certain components in the normal mix.
Effect of Alkali Solutions on Bacteria Found in Unwashed Milk Bottles’ C. S. Mudge and B. M . Lawler ~ N I V E R S I T YOF CALIFORNIA, COLLEGE OF
AGRICULTURE, DAVIS,C A L I F .
The action of alkali solutions upon bacteria was both the efficiency of a bottle studied. The bacteria in the wash water from forty bottle washer has rewasher and the effect of soludirty bottles was used as being typical of conditions c e n t l y met with intions of sodium h y d r o x i d e encountered in practice. creasing favor in the dairy inand sodium carbonate on a The pH of the solution is a determining factor in the dustry. Its p r i n c i p l e of test organism, a bovine B. germicidal action. The relation of time holding to the operation is the continuous coli. He concluded that 5 concentration of alkali is established. “Isolethal” passage of the bottles through per cent solution of sodium lines or zones are shown which represent equal effects a series of compartments conhydroxide a t 37.7’ C. deof the varying factors of time, temperature, and containing alkali solutions which strovs B. coli in 2 minutes. centration of alkali. c l e a n s e the surface of the He “found sodium carbonate bottle. At present there are less efficient both when used several of these machines on the market, in which dif- alone and in combination with sodium hydroxide. Informaferent solutions are used (1) in varying concentrations, (2) tion of this sort is of great importance, but the use of a a t different temperatures, and (3) for varying periods of fresh broth culture of any organism is far removed from the time during which the bottles are in contact with the solution. conditions actually encountered in washing and sterilizing It is also generally true that in these machines no attempt bottles. is made to sterilize the bottle, reliance for the sterilizing Experimental Procedure action being placed entirely on the cleansing operation. Doubtless there is a wide range of concentration of alkali, It seemed to the writers that it would be better to study of temperature, and of holding times within which the exthe action of the alkali solutions on the bacteria actually pected lethal effect may take place; although a serious found in milk bottles returned to the plant. Their methods, problem in public health would arise should it ever be found therefore, have been somewhat different. A liter of sterile that the washing of the bottle does not at the same time water was taken to the plant. Forty of the dirtiest bottles sterilize it. I n some cities regulations are in force in an were selected and into each of these 10 to 15 cc. of the sterile attempt to limit these variations. water was poured. The bottles were then shaken to loosen the milk material from the sides and bottom after which Previous Work this wash water was poured again into the flask, which now There is little in the literature dealing specifically with contained a milky white suspension of casein, fat, and bacthe germicidal action of alkalies. Some textbooks report teria. This was taken to the laboratory, where a series of concentrations and time action which are effective for the alkali solutions had been previously prepared. killing of some specific organism, but the extremely high Tes‘s with Sodium Hydroxide Solutions concentration mentioned is surprising. At best, however, the information gained is of little value. Recently Sherman2 I n the experiments first reported the concentrations of reported that the action of alkali solutions is directly correlated with the alkalinity of the solution as measured in pH. alkali chosen were 0.1, 0.3, 0.5, 0.7, and 1.0 per cent. The He states that a solution of pH 12 will destroy non-spore-form- stock solutions were made 10 per cent stronger than deing bacteria in 5 minutes, while one of pH 11 requires an in- sired, so that when 9 cc. of the stock solution was mixed crease in temperature to get the same lethal effect. The with 1 cc. of the wash water the desired concentrations mentioned above were obtained. Five sets of test tubes were writers’ results seem to substantiate Sherman’s findings. set up, each containing an alkali series. To each tube 1 Another approach to the problem was taken by Parker,3 who studied the action of alkalies with reference to the phenol cc. of the wash water was added and all were placed in a coefficient. He quotes Hamilton as giving the phenol co- water bath at 48.8’ C. A set of tubes was removed and efficient of sodium hydroxide as 12. Parker himself studied placed in ice water a t the end of each minute for 5 minutes. Duplicate plates were then made from each tube by placing 1 Received November 9, 1927. 1-cc. portions of the solution in a series of Petri dishes suitably 2 Milk Dcalcr, 16, 52 (1927). marked. To avoid the possibility of the alkali of the sam8 I b i d . , p. 28.
HE “soaker” type of
T
’
