ANALYTICAL EDITION
October, 1945
The sample size as well as the disk size taken here is for convenience and other sizes work equally well. Precision of the method seems to be in proportion to number of leaves sampled. LITERATURE CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., 1940. (2) Cressman, J . Econ. Entomol.,34,804 (1941). (3) Dawsey, J . Agr. Research, 52, 681 (1936).
Estimation With
623
(4) Dawsey and Hass, Ibid.,46,41 (1933). (5) and Hiley*'bia.l 693 (1937)* DeOng, Knight, and Chamberlin, Hilgardia,2,351 (1927). (6) (7) English, J , A*,., Res-,.&, 41,131 (1930). (8) Griffin, Richardson, and Burdette, Ibid., 34,727 (1927). (9) McCall and Kaw. J. Econ. Entomol..33.905 (1940). Rohrbaughl P1antPhysiols* 9t 699 (ig34)* PRESENTED before the Florida Section of the AMERICAN CHEMICALSOCIETY, Tampa, Fla., May 12, 1945, and in the Meeting-+Print, AMMBICAN CHEYl C A L SOCIETY, September, 1945.
OF
Sucrose and Lactose in Binary Mixtures Particular Application to Sweetened Condensed Milk
H. H. BROWNE, Division of
Dairy Research Laboratories, Bureau of Dairy Industry,
T
H E method described here for estimating sucrose and lactose is based on the fact that sodium bisulfite decreases the optical rotation of aldose sugars (S), and that the optical rotation of a mixture of two sugars, a t least one of which is such a sugar, can be made a linear function of the concentration of each sugar. A method for the determination of maltose and dextrose in a binary mixture has been reported earlier (8'). The method herein described consists of two steps: the determination of the percentage of total sugars by means of the refractometer and the measurement of the optical rotation of a solution containing a known percentage of total sugars in the presence of sodium bisulfite. For the actual determination of these sugars, a curve is first plotted using the data in Table I, the rotation being measured in a Schmidt and Haensch saccharimeter. The data in Table I were obtained by dissolving the sugars in the ratios given in sugar dilution flasks, adding 30 grams of sodium metabisulfite per 10 grams of sugar, making up to the 110mi. mark with distilled water, letting stand (stoppered) about 2 hours a t 20" C., and then reading the rotation. I t was found that when the rotations corresponding to the two 100% points were plotted, and then connected by a straight line, the intermediate points all lay practically on this line, the sucroselactose rotation plot shown in Figure 1.
U. S.
Department of Agriculture, Washington,
D. C.
with distilled water. A concentration greater than 50y0 is difficult to mix thoroughly and filter; a lesser concentration yields inaccurate results. Filter, using a vacuum if necessary. The filtrate will have a pH of about 4.5. Then measure the index of refraction of the filtrate. Since only a drop is necessary the sample need not be large. Convert the index of refraction to sucrose (I), which can be used without great error for other sugars, and hence can be used for total sugars. Compute the size of sample required to give 10 grams of total sugars, then weigh out, dissolve in water, clarify, and make to a volume of 100 ml. in a sugar flask. Now treat 100 ml. of clarified sample with 30 grams of sodium metabisulfite and make to dilution mark with distilled water. After 2 hours filter on a pressure filter (not vacuum), conveniently made with a frittedglass filter (about 50 mm. which will hold about 50 ml.) which will retain the matter suspended in the solution (possibly sulfur). Polarize and take off the corresponding percents, e sucrose (or percentage lactose) from the plot in Figure 1. T%is multiplied by the "per cent total sugars" will give the per cent sucrose (or lactose) in the sample of condensed milk.
LACTOSE I PERCENT I
80
60
0
20
40
I
I
30
2 25 U Y
METHOD
w
In applying the method to sweetened condensed milk, the most commonly occurring mixture of these two sugars, dissolve a known weight of the sample, add a portion of 5% copper sulfate solution (3 ml. per 10 grams are suggested) to clarify; and make up to a volume numerically equal to twice the weight of sample
i
20
z 0 c
' ~
~~
Table 1. Optical Rotation of Sucrose-Lactose (Hydrate) Mixture (10 grams in 30% bisulfite solution a t 20' C. with 200-mm. tube) Sucrose, 7' 0 20 40 50 60 80 1000 Lactose, 100 80 60 50 40 20 0 Sob 8.2 13.1 18.4 21.0 23.2 28.3 33.9" This is a good test for purity of the sucrose; a commercial granulated 3u ar gave a rotation of 32.9.
%
f
Degrees on international sugar scale.
Table
II. Lactose and Sucrose in a Sample of Sweetened Condensed Milk Total Sugars
Lactose/ Sucrose Ratio
% Composition (manufacturer's) 55.62 23.1/76.9 Author'a analysis 56.0 24/76 Calculated as 0.545 X milk-solids-not-fat.
O
Y
I
~~
Lactose
% 12.87' 13.4
Sucrose
5
o
I u2 0
I -
I l 40
I -
SUCROSE
Figure 1,
1 PERCENT
I
I
L 60
u
l SO
I A
I 100
I
Sucrose-Lactose Rotation
In Figure 1 is shown a small inset, covering all sucrose-lactose concentrations in sweetened condensed milk with the corresponding rotations, so that if an enlarged plot is made with these limits, and the plotted section of the locus drawn on it at a steeper angle, a plainer intersection can be made with the rotation with a corresponding increase in accuracy.
%
LITERATURE CITED
42.75 42.6
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., p. 667, 1940. (2) Browne, H.H., IND.ENQ.CHEM.,ANAL.ED.,16,582 (1944). (3) Browne, H. H., J . Org. Chem., 9, 477 (1944).
