T H E BASIC VISCOSITY O F ICE-CREAM M I X E S BY ALAN LEIGHTOS A N D OWEN E. WILLIAMS*
The purpose of this paper is to demonstrate that there exists in normal icecream mixes a measurable basic viscosity, that is, a viscosity independent of changes in mechanical structure which take place when the mix is allowed to stand. This paper also shows that the change in value of this basic viscosity with variation in water concentration, a t constant temperature, follows closely a modified form of the equation given by Arrhenius,’ log q = 0 c . When freshly prepared ice-cream mixes are allowed to stand at low temperatures they show a marked increase in viscosity. I n commercial practice freshly prepared mixes are held for a period of at least twenty-four hours before being frozen, as it is the common belief that this increased viscosity is desirable. This procedure is known technically as ripening, and is sometimes called ‘aging.’ A study of the viscosity of such a ripened mix shows that any mechanical agitation reduces the viscosity, and also that, with successive passages of the mix through a viscometer of the Ostwald type, the time required for the passage of the liquid gradually decreases to a value dependent upon the instrument itself. These phenomena are aptly described in an abstract2 of a paper by WeissenbergerJ dealing with the structure of disperse systems. Portions of this abstract are quoted: “Dispersoids, more particularly emulsoids, whose concentration exceeds a certain limit, tend to form uniform secondary aggregates of the primary particles of the disperse phase, probably as the result of a special hindering of the free movements of the particles. The primary particles, which are ultramicroscopic aggregates of molecules, are very stable and reproducible. . . I n turn, secondary structures may unite to form aggregates of still higher orders. , . . These structures of higher orders are easily broken down, for example, by mechanical agitation, by shaking, or by forcing the solution through capillaries. If the viscosity of such an emulsion is measured in an Ostwald viscometer, it is bound to fall off with successive passages through the capillary until a constant value is obtained, the magnitude of which depends upon the size of the capillary. If the dispersoid is then allowed to rest, the viscosity slowly increases again as the structures of higher order are again formed.” From the foregoing it is apparent that as soon as ripened ice-cream mix enters the freezer the dashers will reduce the existing viscosity. If the in-
.
*Dairy Research Laboratories, Bureau of Dairy Industry. United States Department of Agriculture. Medd. Kobe1 Inst., 8, Xo. 13, 1-20 (1916), J. Chem. SOC., 112 11, 130. * Chem. Abs., 15, 3777 (1921). Kolloid-Z., 29, 113-24 ( 1 9 2 1 ) .
THE BASIC VISCOSITY OF ICE-CREAM MIXES
597
creased viscosity of this ripened mix is due to the development of a mechanical structure, when this structure is broken down the viscosity obtained should be a basic value, independent, within certain limits, of the rate of stirring. For clearness, this paper will refer to the viscosity of the unagitatcd ripened mix as the apparent viscosity and to that obtained upon agitation as the basic z~iscosity. The existence of this basic value was demonstrated as follows: An unflavored ice-cream mix was made up, as is customary, from unsweetened evaporated milk, cream, cane sugar, and water, with the addition of a small amount of gelatin. This mix was pasteurized at 63"C., for one-half hour, homogenized a t a pressure of z joo pounds per square inch while still hot, immediately cooled, and stored for a day a t a temperature of about 2 O C . The total solids content of this mix was 36.3 per cent. Its viscosity when freshly prepared (measured a t oo C.) was found to be 9.11 centipoises (cp). The next day the apparent viscosity was > 50 cp. A portion of the mix was then run into a horizontal-type, brine-cooled freezer of six gallons capacity. The freezer was filled completely so that air could not be whipped into the mix by the dasher. When the mass was brought to a temperature of o"C., the dashers were started at a speed of 130 R. P. M., the normal speed of the freezer. At intervals of five minutes for a period of one hour samples wcre draivn from the freezer into small flasks, packed with snow, and carried to the viscometer, which was immersed in a constant temperature bath held at 0°C. The viscosity value was found to be constant for this period of time at 36.3 I cp. The basic viscosity was therefore reached quickly in the freezer. To check this value, portions of the same mix were placed in small side-necked filter flasks, the flasks evacuated of air at the pump and then shaken on a shaking machine of two-inch stroke at a rate of about 17 j strokes per minute for various periods up to one hour, care being taken to prevent the mix warining appreciably. The same constant basic viscosity value was obtained as in the freezer. As a third check, about 400 cubic-centimeter portions of the mix were placed in 600 cubic-centimeter beakers, set in snow, and stirred rapidly with a small laboratory stirrer. The basic viscosity of these samples remained constant up to periods of one hour a t the same value given by the other experiments. These results have been repeatedly obtained, although it has been found that the structural viscosity of more concentrated mixes could not be completely broken up by the shaking machine or the stirrer. S o w Arrhenius, has shown for colloidal solutions: log
7) =
ec
where 7 is the viscosity (in centipoises), c the concentration (molecular concentration, or with substances of high molecular weight, parts per I O O of solvents), and 8 a constant. This equation is that of a straight line passing through the origin. I n other words, if the concentration of the solution under investigation becomes zero, the logarithm of viscosity is zero, or the viscosity value of the solvent is one, that of viater a t zoo C . The experiments herein described were carried out at 0°C where the viscosity of water is 1.796. I t seems best therefore to express the equation as
598
ALAN LEIGHTON AND OWEN E. WILLIAMS
log 7 = 0 c
+ k.
where k is a constant and, theoretically, should,equal log 1.796 or .z544. As further proof that there is a true fundamental viscosity, measurements were made of the basic viscosity of ice-cream mixes of varying water content. Since it can be shown1 that probably not more than half the water of an icecream mix is ever changed to ice in the freezer, the limiting concentration was that of a two-to-one mixture. Three mixes of different relative fat, milk solids and sugar contents were chosen, as shown in Table I. I n the case of mix KO.I four separate portions, of the different water concentrations in1Ooo 800
600 500
400
‘ri 300
82
2000
$100
3
BO
i.60 50
$ ‘O 990
EO
10
50 60
70
80 90 100 110 120 130 140 150
coNce-NmAr/oN TO
O F MMES IN mer3 S O L ~ T PART^ wcrre
/oo
FIQ.I Logarithmic plotting of basic viscosity against direct plotting of the concentration of ice-cream mixes.
dicated, (a, b, c, and d) were prepared directly. These were stirred separatelv in the freezer at 0°C. until a constant value for basic viscosity was obtained. I t took 90 minutes to reach the basic value of sample d. In the case of mixes Eo. z and No. 3 only the most concentrared sample (d) was prepared. After the basic viscosity had been determined for this concentration (d), the mix was drawn from the freezer, weighed and diluted with cold water to the next concentration (c) and the process repeated to the lower concentrations (b and a). The table gives not only the concentrations as determined by analysis but also the basic viscosities and the calculated value of 0, the hrrhenius constant, as well as the constant k . In Fig. I the viscosities are plotted logarithmically against the actual concentration. Leighton: “The Calculation of the Freezing Point of Ice Cream Mixes.” In press, J. Dairy Sci.
THE BASIC VISCOSITY OF ICE-CREAM MIXES
599
TABLE I Basic Tiscositg and Arrhenius' Constant (0)of Ice-Cream Mixes of Varying Composition and Concentration Visc (cp) e S O . Conc'n* S o . I . Composition Parts 3 6 . 2 1 . 0 1 6jg Fat 12.0 a 57 6 b 76.8 Sugar 14.0 77.18 ,016jo 96.0 C Milk Solids 147.70 ,01630 326. j o ,016j8 115.2 S o t Fat 10.0 d Gelatin .3
Water S o . 2. Composition
Fat Sugar Milk Solids Xot Fat Gelatin
Water
36.3 63.7 Parts
8.0 14.0 12.0
Conc'n*
a b C
64.5 75.2 89.4
110. j
d
110.0
246.8
43.34 65.04
e ,01660 . 01660 ,01652 ,01660
.3
34.3 65 7
KO.3 Composition
Parts
Fat Sugar Milk Solids Not Fat Gelatin
16.0 14.0
IVater
Visc (cp)
X O
6.0 .3
KO.Conc'n* a 56.3 b 66.6 c 82.8 d 109. j
Vise (cp)
e
35.9 70.5 175.4 881.4
.02600
,02637 ,02600 ,02605
36.3 63.7
'Expressed in number parts total solids to
IM)
parts water.
From the table and plot it is seen that straight line curves result, showing, as would be expected from the Arrhenius formula, that there is a definite relation between the logarithm of the viscosity and the concentration of the mixes. The viscosity values calculated for zero concentration (the values of k ) vary from the theoretical. These variations are probably to be explained by the fact that in the mixes under experiment the weight concentration is only a rough approximation of the true molecular concentration. I t seems reasonable, therefore, to conclude that the basic viscosity is a true fundamental value. We have already stated that the viscosity of a freshly prepared mix has a lower value than the basic viscosity of the mix that has stood overnight. (9.11 cp to 36.21 cp). Additional experiments have shown that the basic viscosity of a mix shows no further change after standing for one week at
600
ALAN L E I G H T O S AND OWEN E . WILLIAMS
so C., although, as would be expected, there is a marked increase in the apparent viscosity. When the mix is pasteurized it is, of course, heated above the melting point of the fats. If these do not harden rapidly upon cooling, it would be expected that the viscosity of the fresh mix would be low. The basic viscosity of a mix standing one day would therefore be higher than the viscosity of the freshly made mix. This increase can be explained in only one other way, which is to assume, at this stage, a certain degree of hydration of the milk solids, but Dahle and Caulfield’ have measured the “bound water” of an ice-cream mix when freshly prepared and at intervals of 24, 48, and 7 2 hours. They find a constant low value. They conclude that the hydration of the protein of an ice-cream mix is complete before the ripening process is begun. If this is the case then the increase in the basic viscosity of the mix on standing, over that of the freshly prepared mix, must be due, not t o the hydration of the protein, but to the solidification of the fats. The apparent viscosity must be the result of mechanical structure. Conclusions The existence of a measurable basic viscosity of ice-cream mixes has been demonstrated. The change of value of this basic viscosity with concentration has been shown to follow the Arrhenius equation.
* Unpublished data, kindly transmitted
to authors of this paper.
