THE BASIC VISCOSITY AND PLASTICITY OF I C E C R E A M MIXES ALAN LEIGHTON AND FLOYD ERVIN
KURTZ*
Two important papers dealing with the plasticity of milk and its products' have recently been published from the laboratories of the Dairy Department of Cornell University. Exact measurements showed that even skim milk was very slightly plastic. Interesting data were also given concerning the plasticity of the normal unstirred ice-cream mix. Casual unpublished measurements made by one of the authors of this paper had shown previously that the stirred out or basic viscosity of an ice-cream mix could for all practical purposes be considered a true measurement. I n view of the Cornell work it seemed desirable to check these measurements] particularly since i t had been shown in these laboratories that simple relationships do exist between basic viscosity, concentration, temperature] and whipping capacity of the mix, and since it might develop that certain corrections would have to be made in the expression of these relationships. At the same time it seemed desirable to obtain in a general way data showing which constituents of the mix most markedly affect the plastic properties] Le., yield-value and consistency of the mix. It is not safe to assume that a stirred ice-cream mix is plastic even if skim milk exhibits these properties] since the presence of sugar and gelatin in the liquid phase of the mix markedly alters the physical properties of the system as a whole. The basic viscosity and plasticity measurements were made in the same general way as described in previous papers; i.e., the liquid under measurement was drawn by suction against gravity through the capillary tubes of the viscometers. This method was used so that in the case of the basicviscosity measurements no time would be lost in filling the viscometer and bringing it t o the proper temperature. The complete procedure of a measurement follows: First the air pressure within a 5-gallon reservoir was reduced to the proper value for the experiment. This tank was connected to the house vacuum line, a mercury pressure regulator being inserted between the vacuum cock and the tank, which permitted air to bubble into the system through a predetermined depth of mercury. The reduced pressure as read on a mercury manometer was maintained very accurately in this way, provided the cock in the vacuum line was closed to the point which permitted but a small stream of air to pass steadily through the mercury of the regulator. The pressure tank was connected to the viscometer by means of pressure tubiug and a glass stop-cock. If the basic viscosity of a mix were to be measured, the mix was poured into a specially constructed 2-quart freezer and stirred vigorously for one* Research Laboratories, Bureau of Dairy Industry, U. S.Department of Agriculture. J. Agr. Research, 36, 647; J. Dairy Sei., 11, 380 (1928).
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ALAN LEIGHTON AND FLOYD ERVIN KURTZ
half hour while the freezer was immersed in an accurately regulated thermostat at zero degrees centigrade. Care was taken to fill the freezer completely so that no air would be incorporated into the mix. If the unstirred viscosity, or plasticity, were to be measured a z-quart can of the mix which had been standing since preparation a t approximately zero degrees centigrade was placed in a special rack within the thermostat and allowed to stand for one-half hour to reach the temperature of the bath.
Shearing Force in Dynes FIG.I A--No B-63" C-43'
gelatin Homogenization Homogenization
To make the actual determination, the viscometers, made by sealing capillary tubing onto pipette bulbs, were immersed in the mix to a definite depth and connected to the low pressure reservoir. The interval of time required to fill them was noted with the aid of a 1 / 1 0 second stop watch. 111 the course of this work two viscometers of the dimensions given in Table I were used.
TABLE I Dimensions of viscometers A and B
Viscometer A Viscometer B
Volume
Capillary radius
cc
Capillary length
centimeters
centimeters
8.223
0,05044
8.492
24,440
0.04957
8.808
BASIC VISCOSITY AXD PLASTICITY O F ICE-CREAM MIXES
I487
The degree of vacuum in the reservoir was measured on a mercury gauge. Five different pressures were used in this work I n determining the shearing force a t the walls of the capillary tubing, the pressure correction for the height of the liquid column in the viscometer was evaluated for each viscometer by standardizing the instrument with a viscous cane-sugar solution of known dEnsity at all five pressures, plotting the straight line flow against reservoir pressure, and extrapolating this line to the pressure axis. The pressure above zero a t the line intersection gives the proper correction for a liquid of the same density as the cane-sugar solution used in the standardization, and gives data from which the average hydrostatic pressure of any liquid in the viscometer may be calculated. The capillary dimensions of the viscometers were purposely chosen of fairly large radius in order that the time of flow of liquid be not overlong, since in the stirred out viscosity determinations it seemed desirable to make the measurement before there could be an appreciable recovery in the viscosity of the mix. There Fould also be less danger of trouble from clogging. The accuracy under these conditions is all that could reasonably be desired. For absolutely accurate results, however, smaller capillaries should probably be used, and, as Sharp' has pointed out, the possible effect of capillay size needs further study.
Experimental The first experiments were conducted to ascertain whether there was an appreciable yield-value in ice-cream mixes which had been stirred. Out of twenty-five to thirty mixes only two demonstrated a measurable yieldvalue, and these had been standing for about one week and were on the verge of spoiling. I n Table I1 and Fig I are given data of a number of typical experiments. The basic viscosity measurements are given for a mix without gelatin and for mixes containing a good grade of gelatin homogenized a t different temperatures. The plasticity data for corresponding unstirred portions of the experimental mixes are also given. As has already been pointed out, the measurements upon the mixes which had been stirred exhibited no plastic properties detectable within the limits of experimental error. The conclusion is reached, therefore, that the basic viscosity measurement may still be considered as the true measurement of a physical property. Cnstirred ice-cream mixes, even without the presence of gelatin, do, however, show a slight degree of friction when run through the capillary tubes of the viscom6ters. The most striking effects, however, are produced by the presence of gelatin. It is seen a t once that the gelatin not only imparts an appreciable yield-value to the mix, which value is in all probability a measure of the 1
Private communication.
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ALAN LEIGHTOS A S D FLOYD ERVIN KURTZ
TABLE I1 Basic viscosity-plasticity data on three stirred and unstirred typical mixes Force
dynes 446 .7 2 . 562.8 3 . 653.4 4 . 775.8 j . 886.3 I .
Mix I No gelatin
I .
446.7
Mix 2 Gelatin homogenized at 43OC.
562.8 3 . 653.4 4 . 775 8 j. 886.3
Mix 3 Gelatin homogenized a t 63'C.
446 .i 562.8 3 . 653.4 4 . 775.8 5 . 886.3
2 .
I.
2 .
Basic viscosity Plasticity of structure Delivery Viscosity Delivery consistency
cc.
_-
CP.
cc.
dynes
0.2167
0.2932
0.2724
0.4001
0.3213 0.3807
0.4556
0.4422
0 .I I. ;;. 0.1455 0.1714
0,04033
0.3470
Yield value
'9.43
19.71
20
-
38.62
0.2041
0 . 0 6 9 8 ; 69.72
Ijj
-
0.2311
0.10270
0.1269 o.rjj9 0.1900 36.07 0.2194 0.2474
0.04809 0.08299 j 9 . 2 0
170
0.12IOO
structural strength of the gel, but the consistency of both the stirred and unstirred mix with gelatin is markedly greater than would be the case if gelatin were not present. The conclusion is reached, therefore : That the basic viscosity value of an ice-cream mix should be considered a true measurement. This conclusion is made in spite of the fact that the viscosity of the mix is markedly affected by gelatin which gives both a yield-value and greater consistency to the unstirred mix.
