Whipping Capacity of Ice Cream Mixes ALAN LEIGHTON AND ABRAHAM LEVITON Bureau of Dairy Industry, U. S. Department of Agriculture, Washington, D. C.
The improved whipping properties of ice cream mixes brought about by aging can be obtained if butterfat and improvers are absent, but cannot if the mix has not been heated. Higher homogenization pressures favor increased stability and greater maximum overruns without affecting the lie of the overrun-temperature line. Higher homogenization temperatures may accelerate aging effects in mixes where they are normally not apparent. Butterfat exerts a marked depressing effect upon overruns and determines the level of whip of ice cream mixes. The use o f butter in ice cream mixes lowers whipping capacity. This difficulty can be
overcome without the use of egg yolk by first reconstituting a cream from butter and using this as normal cream is used in making up the mix. Such mixes whip with abnormal rapidity. Increased sugar content tends to raise overrun in ice cream mixes, particularly at the lower temperatures; this advantage is offset by the detrimental effect of sugar upon stability and maximum overruns obtainable. Gelatin beyond a certain amount decreases overrun. Sodium alginate stabilizes ice cream mixes remarkably. Carob bean extract acts in much the same way as gelatin but increases the load on the freezer.
P
REVIOUS work by the authors,’ which showed that ice cream overrun is a function of temperature and that this relationship, with certain qualifications, can be shown graphically by a straight line, makes possible the exact comparison of the whipping capacities of different ice cream mixes. Using this graphic method of expressing whipping capacity, the authors studied a number of the factors usually incident to ice cream manufacturing procedure in their effect upon overrun and here present the results of this study. This report is limited to experiments conducted with batch freezers, but the study is being continued with a freezer of the continuous type. It is believed that a complete investigation of the factors influencing the whipping capacity in the more simple batch freezers is essential as a preliminary to work with continuous freezers since the data thus obtained are of a fundamental character. The work is also limited to mixes of normal composition, reserving for later investigation mixes of extremely high butterfat and solids content, as well as mixes prepared from special ingredients such as frozen cream, powdered milk, and various milk powders low in their lactose content. It has previously been the custom, in comparing the whipping capacities of ice cream mixes in batch freezers, to record the time interval necessary to obtain a certain overrun. Since this time interval is now known to be dependent upon a number of factors-for instance, the temperature to which the mix is frozen and the speed with which the mix can warm up in the individual freezer, factors that have not been controlled or recorded-satisfactory comparison cannot be made between the data of various investigators. The authors’ previous work1 showed that, if a number of samples of a given ice cream mix are frozen down to different temperatures and then whipped back, and the overrun data are plotted against temperature, portions of each curve follow along a common straight line. This equilibrium line
is the locus of points which represent the highest overruns obtainable with the given mix a t the existing temperatures. This line can be used as a measure of the whipping capacity of the mix. In other words, that work showed that if the ice cream mix is whipped long enough for equilibrium to be reached, the resulting overrun for a given mix is dependent upon the temperature, provided that the mix has not been whipped too long. In a brine-cooled 20-quart freezer, equilibrium is attained in about 12 minutes total time in the freezer; in a direct-expansion ammonia freezer with higher dasher speed, the total time is about 8 minutes. If whipping is carried on too long, the mix either warms close to the melting point and the overrun decreases, or an incipient churning of the mix occurs, also with loss of overrun. The data show clearly that, if the freezing process is so adjusted that the temperature corresponding to the desired overrun is reached a t the time equilibrium is attained, then the whole process of freezing has been carried out in the shortest possible time. If, as has frequently been the custom in manufacturing procedure, the mix is frozen to lower temperatures than this, then it is necessary to wait for the material to warm up before the desired overrun is attained. On the other hand, if the mix in the freezer is held a t higher temperatures than this, the desired overrun can be obtained in less time, but the mix is drawn without the formation of all the ice that can exist at this overrun. Whether or not this causes an impairment of texture has not been determined. Theory clearly indicates that the freezing should be carried out so that equilibrium exists a t the time of drawing. Mixes vary considerably in regard to their ability to resist churning. As might be expected, mixes homogenized a t the higher pressures are the more stable. It is of considerable interest, too, that mixes of identical composition made up from time to time during the course of nearly two years’ work almost always gave identical overrun lines. The whipping capacity of a mix expressed as a function of temperature is therefore a fundamental
J Deirg Sci.. 20, 371 (1937).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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property of the mix, and, once determined for a certain formula, it can be used as a control for other mixes of the same composition. Bearing these facts in mind, we shall now discuss the effects on overrun of the various processes and constituents common to ice cream manufacture. In the course of this
6-4
-
TEMPERATURE C. FIGURE 1. OVERRUN LINES OF STANDARD MIXES A. B. C.
8 per cent butterfat 12 per cent butterfat 16 per cent butterfat
work the mix was usually prepared, pasteurized, homogenized, and cooled immediately on a surface cooler. One portion of the mix was taken from the cooler and frozen immediately, another portion was frozen after 4 hours, and the rest was frozen after aging for one day a t a temperature of 35" F. Only one portion was taken from the cooler because the whipping capacity was usually changing so rapidly a t the time that duplicate measurements could not be made. Overruns were recorded every 2 minutes during the freezing process, the recordings usually starting 4 minutes after the mix entered the freezer. Temperatures of the overrun samples were recorded. As shown in the previous report,' a t equilibrium overrun plots a straight line against temperature. In the graphs presented in this paper this line only will be shown, its upper extremity being the maximum overrun obtained. The line will be referred to in the text as the overrun line and represents the whipping capacity of the mix. The overrun line of aged mixes is usually a plot of the data from three to five separate
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10 per cent milk-solids-not-fat, and 14 per cent sugar; the third contains 16 per cent butterfat, 8 per cent milk-solidsnot-fat, and 14 per cent sugar. Figure 1 shows the overrun lines for these mixes when made up without gelatin, homogenized a t 2500 pounds per square inch pressure, and aged one day. These overrun lines are taken as the standard for comparison in presenting the data of this report.
Aging Although comparison of aged and unaged mixes will be made throughout the report, a brief discussion of the effects of aging upon overrun is perhaps not out of place here. An unaged mix is usually less stable to the action of the beaters, possesses lower whipping capacity, and cannot be whipped to as high a maximum overrun as a mix that is aged one day. In respect to these properties, a 4-hour-aged mix may approach the day-old mix closely, but more often it is intermediate between the fresh and the day-old mix. The improved whipping properties of a mix can be obtained through aging even if no improver is present and also if butterfat is absent. They cannot be obtained unless the mix has been heated as in the pasteurization process. It seems evident, then, that the effects of aging concern the serum solids and are the result of the heating process. The stability of mixes to the action of the beaters also increases with aging when butterfat is present. This effect is concerned primarily with the condition of the butterfat and may be the result in part simply of the solidification of the fat particle.
---
3. EFFECT OF HOMOGENIZATION ON WHIPPING CAPACITY FIGURE
A . 1500 pounds per square inch B . 2500 pounds per square inch C . 3500 pounds per square inch
p 'j ....
Unoged - _... - Aqed 4 k
500
4 00
T€MPERATURE
C.
FIGURE 2. OVERRUN LINES OF STANDARD MIXESAFTER DIFFERENT AGING PERIOD^
A. B. C.
Homogenization Temperature and Pressure
8 per cent butterfat
12 per cent butterfat 16 per cent butterfat
freezings. The lines representing freezings upon unaged and mixes aged 4 hours are the data from but a single freezing for each separate mix, and the uppermost point is not necessarily the highest overrun that could be attained with these mixes. During most of this work two freezers were used, the overrun line witb the direct expansion freezer usually lying slightly above the one obtained with the brine freezer. The data presented are those obtained with the brine freezer. Most of the work reported here has been carried out with three characteristic ice cream mixes. The first contains 8 per cent butterfat, 11 per cent milk-solids-not-fat, and 14 per cent sugar; the second contains 12 per cent butterfat,
As will be evident from data to be presented subsequently, excess gelatin or other stabilizer may cause a decrease in whipping capacity upon aging. This is invariably caused by heavy gelation. Figure 2 gives overrun lines for aged and unaged standard mixes.
-6 -4 -2 0
TEMi?
'c.
FIQURE 4. EFFECT OF BUTTERFAT ON
WHIPPING CAPACITY
A.
B. C.
D. E.
12 per cent butterfat No butterfat 0 . 5 Der cent butterfat 1.0 per cent butterfat Butter a8 fat source, 12 per cent butterfat
Increases in homogenization pressure above 1500 pounds per square inch do not alter the position of the mix temperature-overrun equilibrium line but do increase markedly its stability to the action of the beaters as well as the maximum overruns obtainable in both the aged and the unaged mixes. For example, Figure 3 gives the overrun lines of three normal 12 per cent butterfat mixes made up a t different times and homogenized a t different pressures. The lie of the overrun lines
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INDUSTRIAL AND ENGINEERING CHEMISTRY
is the same for the three mixes, but stability and maximum overrun are raised by increasing the homogenization pressure. Similar relationships were found to exist with the mixes containing 8 and 12 per cent butterfat. An increa,se of homogenization temperature may increase whipping capacity of certain high-fat mixes. As will be shown, the process of aging seems to receive an impetus. I n mixes of high gelatin content higher temperatures of horn ogeni z a t ion may decrease whipping capacity, probably because of accelerated gelatin. Bearing these facts in mind, we can now consider the effects of ice cream constituents upon whipping 6 0 ncapacity.
781
into a cream diluted with skim milk, overruns were greater than normal (Figure 5, E ) . It would seem, therefore, that the difficulties of low overrun encountered when butter is the source of milk fat in ice cream may be overcome by the simple expedient of preparing a cream of skim milk and butterfat by homogenization and using this cream as normal cream is used in the preparation of the mix.
Butterfat "-
A mixture of sugar and milk-8 -6 - 4 -2. 0 solids-not-fat of the water conTEMPERATURE C. centration normally used in ice cream mixes whips readily upon FIGURE 5. EFFECT OF THE BUTTERFAT freezing. It is known that overON WHIPSURFACE run is decreased by the addition of PING CAPACITY butterfat. The exact magnitude A . Mix made from of this decrease had not previously normal cream and also from butter been measured. Figure 4 shows cream E . Mix made from the overrun lines of a mix conbutter taining different amounts of butterC. Butter mix with egg yolk added fat. D . Butter mix from butter cream made The nature of the surface of with conoentrated the fat particle is probably also skim milk, E. Butter mix made of importance in its effect upon from butter cream f r o m dilute skim overrun. Line E (Figure 4) repremilk sents the same mix containing 12 per cent butterfat, but in this case butter was used as the source of milk fat. This overrun line lies markedly below the others, which indicates that the surface of the fat particle is important.
a
Butter
120 3
Q
100 Q
Butter has long been used as an 41 80 L emergency source of fat in ice 0 cream manufacture; one drawback - B -6 - 4 -2 60e is the poor whipping quality of TEMP "C. 6. coMmixes in which it is used. This has been overcome in part by the PARATIVE RATEOF addition of egg yolk. A (Figure 5) W H I P P I N GO F is the overrun line of the normal MIXESMADEFROM mix made from cream; B represents the same mix in which the butterfat was supplied by butter. C is the same butter mix with egg yolk added. In the preparation of these mixes the butter was simply melted in the pasteurization kettle and homogenized with the mix. There seemed to be the possibility that the surface of the butter particle might be coated so heavily with protein as to mask the effect of the butter particle. In order to prove this point, butter was melted and homogenized with the condensed skim milk (approximately 35 per cent total solids) that was to be used. This cream was then used in making up the mix, and the whole mass was homogenized as if fresh cream were being employed. The overruns from this mix (Figure 5 , D) were markedly lower than those obtained when plain butter was used. The butter was then homogenized in skim milk, and this cream was used in the preparation of the mix; the normal overrun was obtained (Figure 5 , A ) . If the butter was made
u 0 u 4 0 u 4 0 - 8 -6 -4 -2 - 8 - 6 -4 - 2 -8 -6 -4 -2
TEMPERA TURE "C. FIGURE 7. WHIPPING CAPACITIES OF MIXES CONTAINING (above) 16 PER CENT CANE SUGARAND (below) 16 PER CENT SUGAR OF WHICH4 PERCENTIs DEXTROSE A. B.
C.
8 per cent butterfat 12 per cent butterfat 16 per cent butterfat
Another observation of interest is the speed with which these mixes made from butter cream whipped. No mixes other than these have been encountered in nearly two years' work in which temperature-overrun equilibrium was reached in the brine freezer in less than 10 to 12 minutes total time in the freezer. As Figure 6 shows, the equilibrium curve is approached from below in the case of the normal mix. The butter mix whips so rapidly, however, that equilibrium is approached from above. Equilibrium was reached with the butter mix in 6 minutes, with the cream mix in 12 minutes. The butter mix could have been drawn from the freezer completely frozen and whipped in 6 minutes. Two facts have therefore been demonstrated: (1) The whipping capacity of a mix can be varied by the simple expedient of varying the fat particle, and (2) the speed of whipping can be altered by the same process. This presents opportunity for further work which should be of considerable value.
Sugar In spite of the fact that sugar has long been known as an overrun deterrent, we should expect that sugar, by lowering the freezing point of a mix, would cause higher overruns a t given temperatures. Figure 7 shows that the stability of mixes and maximum overruns are lowered with increase in sugar content; a t lower temperatures, particularly with mixes containing corn sugar, there is a marked increase in the overruns under equilibrium conditions over the corresponding 14 per cent sugar content mixes. If cane sugar in increasing amounts is substituted for the corn sugar until mixes of the same freezing points are obtained, the overrun curves are practically identical with the lines of the corn sugar mixes; thus the effect is largely the result of lowering the freezing point. It is known that creams containing sugar churn more readily. It is likely that the instability of ice cream mixes high in sugar content is the result of this action of sugar in accelerating churning.
~'~~~~~&(pB',,
Stabilizers Of the so-called stabilizers, gelatin is one of the most commonly used. I n proper amounts it favors the formation
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782
A
60
80 60
120
120
IW
/oo
80 60 40
40
60
120
80
80
80
60
60
60
TEMPERATURE
"c.
FIQURE 8. EFFECTO F GELATINUPON WHIPPINQ CAPACITY OF MIXES CONTAINING (above) 8 PER CENT, (center) 12 PER CENT,AND (below) 16 PERCENTBUTTERFAT A . No gelatin B . 0.1 per cent C. 0.2 per cent D . 0.3 per cent E . 0.4 per cent F. 0.5 per cent
gelatin gelatin gelatin gelatin gelatin
of small ice crystals and increases the melting resistance without the formation of a mass that will not melt. As Figure 8 shows, an increase in the amount of gelatin decreases the whipping capacity of 8 per cent butterfat mixes. This effect is not as marked with 12 per cent butterfat mixes and is entirely absent in the mixes containing 16 per cent butterfat (Figure 8). I n comparing these curves later with the data on sodium alginate, it must be remembered that the lines for unaged and 4-hour aged mixes are those of single freezings and that the highest overruns recorded are not necessarily the highest that could be obtained. The solid line indicating the whipping capacity of aged mixes is the composite of from three to five freezings, and its uppermost point indicates the highest overrun obtainable. -6 - 4 - 2 0 At a later date it was found that aging effects could be produced in the TEMI? OC. high-butterfat mixes by homogenizaFIGURE 9. EFtion a t higher temperatures. This FECT OF HIQHTEMPERATURE will be referred to again in the exH O M O G E N I Z A - periments with sodium alginate but TION ON THE is illustrated in Figure 9. The solid AGING OF 16 line of the aged mix is realized only PERCENTBUTTERFAT MIXES if the mix is frozen a t a comparaA . Unaged and tively high temperature. Otherwise low-temper a the lower overrun line results. The ture frozenaged mix thus has two equilibrium lines. B . Higher temSodium alginate in a form especially perature aged prepared for the purpose is another stabilizer recommended for ice cream. The claim is made that with its use aging is unnecessary. Actually, aging does improve the whipping properties of the mixes containing sodium alginate. However, sodium alginate does have the remarkable property of immediately stabilizing the butterfat to the action of the beaters so that freshly prepared mixes can easily be brought to the desired overruns. It is thus a double stabilizer; it stabilizes the structure (as does gelatin) and the particles in addition.
5
'
TEMPERATURE 'C. FIGURE 10. EFFECT OF SODIUM ALGINATEUPON THE WHIPPINGCAPACITY OF MIXES CONTAININQ (above) 8 PERCENT (center) 12 PER CENT,AND (below) 16 PERCENTBUTTERFAT A . No sodium alginate B . 0.1 per cent sodium C. 0.2 per cent sodium D. 0.3 per cent sodium E . 0.4 per cent sodium
alginate alginate alginate alginate
These effects are shown in Figure 10 for the mixes of varying butterfat content. Of particular interest are the high overruns obtainable under certain conditions with both unaged and aged mixes. As mentioned previously, the aging effects produced in the 16 per cent fat content mixes were found to be caused by the 120 high temperature of pasteurization and homogenization (71" C.) 100 2 recommended for use when sodium alginate is employed, rather than 802 6op: to any specific effect of the al-
E!
_._ Unaged 40 $ __ _._. Aged 4hr3. w
ginate. A few experiments were carried out to study the use of a certain carob bean extract, as an improver -6 -4 - 6 - 4 - 2 in ice cream. This material acts TEMP "C. in many respects as does gelatin. FIGURE 11. WHIPI t s effect upon whipping and agPING CAPACITY OF A ing is shown in Figure 11. In 12'%BuTTERFATMIX spite of the fact that the overrun of these mixes is similar to WITH A sIMIthe overrun of gelatin mixes, the LAR MIX CONTAINpresence of this product was ING GELATIN shown to increase markedly the A . Gelatin0.3percent load on the freezer. Where the B . Carob bean extract, 0.2 per cent gelatin mix of the same composition would cause the motor to draw 6 amperes a t a given temperature, the mix containing the extract would draw 7 amperes a t the same point.
~
~
~
~
Discussion In the foregoing a survey has been made of factors influencing the overrun of ice cream mixes. It is noteworthy that 100 per cent overrun, the highest usually considered desirable to take, can be obtained with practically all of the mixes used and reported here. It is a matter of so adjusting the freezing process that the required temperature is reached in the
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783
INDUSTRIAL AND ENGINEERING CHEMISTRY
freezer within the necessary time interval before incipient churning starts. The realization of this fact should do much to clarify the confusion that exists concerning the comparative whipping properties of various mixes and the procedure that must be followed to obtain the higher overruns. Since the work here reported was concerned primarily with overrun, no reference has heretofore been made to ice cream quality. Other things being equal, the ice cream that can be drawn from the freezer with the greatest amount of water in the form of ice has the best texture because the ice
crystals will not grow so much in the hardening room. (This is probably the reason ice creams made from aged mixes are superior.) Therefore, those mixes that can be drawn from the freezer a t the desired overrun with the greatest amount of water in the form of ice present are to be preferred. Moreover, ice cream should be drawn from the freezer while the temperature-overrun equilibrium exists, because a t this point the greatest amount of water possible a t the chosen overrun has been frozen to ice and the best possible texture is therefore to be had.
Enthalw-Concentration Charts from Vapor Pressure Data WILLIAM HALTENBERGER, JR. Villanova College, Villanova, Penna.
T
HE minimum thermal data required for the construction of an enthalpy-concentration chart for a liquid solution are the heats of dilution a t one temperature, and heat capacities over the entire temperature and concentration range to be covered; or else, heat capacities a t one concentration, and heats of dilution over the entire temperature and concentration range (6). Experimental data on either heat capacities or heats of dilution a t higher concentrations and temperatures are scarce; consequently enthalpy-concentration charts have been prepared for only few systems. However, information is available on the equilibrium properties (notably vapor pressures) of many liquid solutions, from which heats of dilution a t higher concentrations and temperatures can be calculated. The enthalpy-concentration chart for the system can then be constructed by combining the calculated heats of dilution with experimental heat capacities a t low concentrations. The relations involved in one of the several possible conversions of this nature are derived and illustrated here. The experimental data required for the construction of an enthalpy-concentration chart by the method under consideration are ( a ) heat capacities of the solution a t any one concentration, over the entire temperature range and ( b ) vapor pressures of the solution over the concentration and temperature range to be covered. The method is applicable to all systems containing a single volatile constituent. The accuracy of the calculated values is discussed a t the end of the paper. All methods based on pure thermodynamics for the calculation of thermal properties from equilibrium properties include the differentiation of the equilibrium data. The differentiation can be greatly simplified if the equilibrium data can be converted to a linear or nearly linear form. The most convenient of the several possible “linear” relations for the present purpose is the Duhring plot, since it has to be constructed anyway for calculations on most processes involving solutions. Brown (3) showed how the Clapeyron relations for a liquid and for a solution containing that liquid as solvent can be combined to give a relation in which the differential term is the slope of the Duhring line. The slopes of the Duhring lines are, by all present evidence, constant a t low concentra-
tions and vary only slightly at higher concentrations. relation is : dt’ _ - AHAV’T‘
dt - A H ‘ A V T
-
The (1)
where at a given concentration,
t’
=
T’ = t =
satur:tion
temp. of solution at pressure P , C. F. satpration temp. of solution at pressure P , K or or
R. saturation temp. of solvent at pressure P , or
C.
O F .
T = saturation temp. of solvent at pressure P, K or R. D = slope of Duhring line AH = latent heat of solvent at temp. t and pressure P AH‘ = latent heat of solvent in solution at temp. t’and pressure P AV = difference between gas and liquid volumes of solvent at temp. t and pressure P AV’ = difference between gas and liquid volumes of
solvent at temp. t’ and pressure P
This is an exact relation, applicable to all systems containing a single volatile constituent. To simplify future discussion, it will be assumed that the volatile constituent is water, and English units will be used. This will make all quantities in Equation 1 and future equations, steam table quantities. Equation 1 can be simplified, without introducing a serious error, by neglecting the volume of the liquid phase. Then AV = V,,,, denoted by V hereafter. Furthermore, for ideal gases a t the same pressure, T’V’ -
T V -
(F)’
If 4, a measure of deviation from the ideal gas law, is defined as
Equation 1 can be thrown into a convenient form for calculations : AH‘
=
T[(F) AH
+@I
(3)
