J. W. STEVENS A N D W . E. BAIER, C alifornia F ruit Growers E xchange, O ntario, C alif.
T
HE standard method used in the citrus industry for estimating the total soluble solids content of citrus juices employs the Brix hydrometer. This is the legal method for testing maturity, by determining the ratio of total soluble solids to acid.
During the early development of the citrus by-products industry, the Brix spindle was generally employed in esti
mating the total soluble solids content of juice products, and the standards of composition of these products were based on this method. This was fairly satisfactory so long as simple products such as reamer-extracted juices were involved, but with the development of different types of natural-strength and concentrated juices and compounded products contain
ing various added ingredients, difficulties were encountered and in some cases satisfactory results could not be obtained.
The refractometer offers an alternative method of deter
mining soluble solids in citrus juice. In spite of a number of obvious advantages, this is also subject to certain errors and limitations and does not give the same results as the Brix spindle.
A number of factors are involved in the use of both methods.
The Brix spindle method is based on changes in density of aqueous sucrose solutions with changes in the sucrose con
tent. Actually, solutions of sugars other than sucrose have densities so near those of sucrose solutions that the Brix hy
drometer may be considered accurate in them. Soluble ma
terials other than sugars have variable and in most cases unknown effects on the Brix readings.
In the measurement of degrees Brix it is necessary, for best re
sults, first to check the calibrations of the hydrometer, since many of those on the market are not accurately calibrated. The spindle should be thoroughly clean and dry. Measurements should be made in regulation hydrometer cylinders filled level with the ground-glass rim. The sample_ is preferably deaerated. In the case of finely screened, dilute juices this may not be necessary if the hydrometer is twirled to detach air bubbles as it is lowered into the liquid. A highly concentrated juice must be deaerated, and even then, if it is thick or viscous because of high soluble or insoluble solids or pectin, much time is required for the hy
drometer to assume the equilibrium position. The sluggishness caused by excessive viscosity may result in serious error. Such difficulties may be partially eliminated by use of the dilution method, but this procedure has obvious objections in routine testing. Finally, the temperature should be close to the standard of 20° C., since the usual temperature corrections do not apply equally to sugar and nonsugar solids.Similar precautions are necessary in the use of the refrac
tometer. The prisms must be perfectly clean to be evenly wet by the solution under test and must be perfectly dry to prevent dilution of the test solution. Screening and deaeration of the sample improve the sharpness of the shadow and thereby enhance the precision and accuracy. For accurate results the readings should be made at 20° C. The errors introduced in applying temperature corrections are similar to those occurring with the Brix spindle.
The refractometer, used either with the sucrose scale or the regular refractive index scale and sugar table, and the Brix spindle should give identical results with pure sucrose solutions. Other solutes do not have the same effects as sucrose, however, and consequently when the two methods
448 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 8 are applied to complex solutions, such as citrus juices, different
results are obtained.
In adapting the refractometer to various fruit juice prod
ucts, several workers have recognized that its use is com
plicated by the diverse compositions of these products. Ma
cara (2) recognized that errors are caused by citric acid, glucose solids, and invert sugar and gave data for applying corrections for these components. McRoberts (S) did a large amount of work on the determination of soluble solids in fruit products and showed that the refractometer is satisfactory for the purpose. He also obtained data by which corrections for citric or tartaric acid might be made.
The authors’ work with citrus juices, involving analysis by the two methods of numerous samples of citrus juices and juice products and of prepared solutions of the different juice constituents, has indicated that the Brix spindle gives high values and that the refractometer gives low results. The constituents of citrus juices known to be important in this respect are citric acid, invert sugar, ash, insoluble solids, and essential oils. Since several factors are involved, the relation of the results obtained by the two methods is changed by natural variation in composition among different products.
The greatest single variation is caused by citric acid and, con
sequently, the greatest difference between refractometer and Brix spindle values appears with highly acid products such as lemon juice, concentrated citrus juices, and certain types of beverage bases.
Because this error may amount to several per cent, it is necessary to apply some form of correction if the refractometer is to be used for routine analysis. There is some question as to whether this correction should be applied to give true sol
uble solids or degrees Brix, but since the use of the hydrometer method is well established, it appears desirable to correct to degrees Brix.
The over-all corrections to make refractometer sucrose scale values comparable with Brix values may be made in at least three ways: (1) by applying separate corrections for each of the above factors, determined on synthetic mixtures; (2) by applying empirical corrections determined for a large num
ber of representative products with some constituent such as citric acid as an index of the magnitude of the correction; or (3) by applying a correction for citric acid only, this being the greatest single factor.
When these three methods and modifications of them were considered, it became obvious that none would be entirely
satisfactory because of the wide natural variation in compo
sition of citrus juices. Method 1 would necessitate a fairly complete analysis of every sample before the correct solids could be ascertained and this would defeat the very purpose of the use of the refractometer. Methods 2 and 3 have been used experimentally, applying corrections based on data obtained on a large number of samples of orange, lemon, and grapefruit juices of different types and on synthetic solutions of sugar and citric acid. The corrections were used in both tabular and nomographic forms. This experience led the authors to adopt method 3 as tabulated herein for all routine work with citrus juice products, other than maturity testing.
Proced ure an d D ata
In obtaining data for correction tables a number of solutions of pure citric acid were prepared and the soluble solids, as sucrose, of the solutions were determined with the refractome
ter and the Brix spindle.
The citric acid was prepared by recrystallization of the ordi
nary U. S. P. grade of acid (from lemons). The acid was recrystallized four times from water by vacuum concentration in Pyrex glass apparatus at about 35° C. The final crystals, after being rinsed with cold water and drained as free of liquor as pos
sible by suction, were dried to about 3.5 per cent moisture at about 40° C.Solutions of different strengths, as shown in Table I, were pre
pared from this recrystallized acid with boiled, carbon dioxide- free distilled water. The strength of these solutions in terms of anhydrous citric acid by weight was determined by titration with 0.1 Ar sodium hydroxide solution which had been standardized with National Bureau of Standards benzoic acid according to di
rections furnished with the acid.In making these analyses, a portion of the acid solution con
taining approximately 1.64 grams of anhydrous acid was weighed in a weighing bottle on the analytical balance, transferred to a 500-ml. volumetric flask, and diluted to the mark with boiled distilled water. Several 50-ml. aliquots of this final solution were titrated in 150-ml. Erlenmeyer flasks with 0.1002 N sodium hy
droxide solution, using 3 drops of neutralized 1 per cent phenol- phthalein solution as the indicator. About 25.5 ml. of sodium hydroxide solution were required to neutralize each portion.
As a further check, duplicate 1.64-gram portions of the citric acid dried to constant weight under vacuum at 60° C. were weighed out and made into 500 ml. of solution as described above. Portions of these solutions were titrated as above and it so happened that the average of the two determinations gave the exact theoretical value. (The error of a single determination was indicated as not greater than 1 in the third decimal place.)The refractometer (Zeiss Abbe type with refractive index and sucrose scales) and the Brix spindles used in determining the soluble solids of the citric acid solutions were checked for accu
racy. Boiled distilled water and solutions of highly purified moisture-free sucrose of 1, 5, 10, 20, and 30 per cent theoretical strength, respectively, were used in checking the refractometer.
The Brix spindles were checked with solutions of pure sucrose at points on the scale coinciding very closely with tnose used with the citric acid solutions. These readings were made at 20° C.
with the room temperature approximately the same.
Each of 24 citric acid solutions, varying in strength from 0.545 to 30.07 per cent, was checked with the refractometer and Brix spindle with results as shown in Table I. At least five readings were made on each of three or more portions, on two or more different days, with the refractometer, at 20° C.
Duplicate checks of five readings each were made with the Brix spindle also at 20° C. The room temperature was 19° to 22° C. when most of the determinations were made.
The refractive indices were recorded to the nearest fourth decimal as indicated by the average reading. The sucrose values, based on Schonrock’s data (1), were recorded as in
dicated by the average. The Brix values shown are the av
erage of the different readings.
The values of Table I were plotted on a large-scale graph (20 mm. = 1 per cent on each ordinate) to obtain data for pre
paring a correction table. In the case of the refractometer data,
AUGUST 15, 1939 ANALYTICAL EDITION 449
T a b l e II. C o r r e c t i o n s f o r O b t a i n i n g 0 B r i x o r A c t u a l S o l u b l e S o l i d s f r o m R e f r a c t o m e t e r R e a d in g s
(Based on citric acid content of citrus juices or other acid-containing sugar solutions)
C itric Acid, Correction to B e Added to C itric Acid, Correction to B e Added to C itric Acid Correction to B e Added to Anhydrous, Refractom eter Sucrose Value Anhydrous, Refractom eter Sucrose Value Anhydrous, Refractom eter Sucrose Value
7°
To obtain T o obtain true % T o obtain To obtain true % T o obtain To obtain trueby W eight ° B rix soluble solids b y W eight ° Brix soluble solids b y W eight ° Brix soluble solids
0.0 0.00 0.00 11.0 2.10 1.33 22.0 4.05 2.73
0.2 0.04 0.02 11.2 2.14 1.36 22.2 4.09 2.75
0.4 0.08 0.04 11.4 2.18 1.38 22.4 4.13 2.77
0.6 0.12 0.06 11.6 2.21 1.40 22.6 4.17 2.79
0.8 0.16 0.08 11.8 2.24 1.43 22.8 4.20 2.81
1.0 0.20 0.11 12.0 2.27 1.46 23.0 4.24 2.84
1.2 0.24 0.13 12.2 2.31 1.49 23.2 4.27 2.87
1.4 0.28 0.16 12.4 2.35 1.51 23.4 4.30 2.89
1.6 0.32 0.18 12.6 2.39 1.54 23.6 4.34 2.92
1.8 0.36 0.21 12.8 2.42 1.56 23.8 4.38 2.95
2.0 0.39 0.23 13.0 2.46 1.59 24.0 4.41 2.97
2.2 0.43 0.26 13.2 2.50 1.61 24.2 4.44 2.99
2.4 0.47 0.29 13.4 2.54 1.64 24.4 4.48 3.02
2.6 0.51 0.31 13.6 2.57 1.67 24.6 4.51 3.04
2.8 0.54 0.33 13.8 2.61 1.70 24.8 4.54 3.07
3.0 0.58 0.35 14.0 2.64 1.72 25.0 4.58 3.10
3.2 0.62 0.38 14.2 2.68 1.74 25.2 4.62 3.13
3.4 0.66 0.40 14.4 2.72 1.76 25.4 4.66 3.15
3.6 0.70 0.42 14.6 2.75 1.79 25.6 4.69 3.18
3.8 0.74 0.45 14.8 2.78 1.81 25.8 4.73 3.20
4.0 0.78 0.47 15.0 2.81 1.84 26.0 4.76 3.23
4.2 0.81 0.49 15.2 2.85 1.86 26.2 4.79 3.25
4.4 0.85 0.52 15.4 2.89 1.89 26.4 4.83 3.28
4.6 0.89 0.54 15.6 2.93 1.92 26.6 4.86 3.30
4.8 0.93 0.56 15.8 2.97 1.95 26.8 4.90 3.33
5.0 0.97 0.59 16.0 3.00 1.97 27.0 4.94 3.35
5.2 1.01 0.62 16.2 3.03 1.99 27.2 4.97 3.38
5.4 1.04 0.64 16.4 3.06 2.02 27.4 5.00 3.40
5.6 1.07 0.66 16.6 3.09 2.04 27.6 5.03 3.43
5.8 1.11 0.69 16.8 3.13 2.07 27.8 5.06 3.45
6.0 1.15 0.71 17.0 3.17 2.10 28.0 5.10 3.48
6.2 1.19 0.73 17.2 3.21 2.13 28.2 5.14 3.50
6.4 1.23 0.76 17.4 3.24 2.16 28.4 5.18 3.53
6.6 1.27 0.78 17.6 3.27 2.18 28.6 5.22 3.55
6.8 1.30 0.81 17.8 3.31 2.21 28.8 5.25 3.58
7.0 1.34 0.84 18.0 3.35 2.23 29.0 5.28 3.60
7.2 1.38 0.86 18.2 3.38 2.25 29.2 5.31 3.63
7.4 1.42 0.88 18.4 3.42 2.27 29.4 5.35 3.66
7.6 1.46 0.91 18.6 3.46 2.30 29.6 5.39 3.69
7.8 1.50 0.94 18.8 3.49 2.33 29.8 5.42 3.71
8.0 1.54 0.96 19.0 3.53 2.35 30.0 5.46 3.73
8.2 1.58 0.98 19.2 3.56 2.37 30.2 5.49 3.76
8.4 1.62 1.01 19.4 3.59 2.40
8.6 1.66 1.03 19.6 3.63 2.43
8.8 1.69 1.05 ■ 19.8 3.67 2.45
9.0 1.72 1.08 20.0 3.70 2.47
9.2 1.76 1.11 20.2 3.73 2.49
9.4 1.80 1.13 20.4 3.77 2.52
9.6 1.83 1.16 20.6 3.80 2.54
9.8 1.87 1.18 20.8 3.84 2.56
10.0 1.91 1.21 21.0 3.88 2.59
10.2 1.95 1.24 21.2 3.91 2.62
10.4 1.99 1.26 21.4 3.95 2.64
10.6 2.03 1.28 21.6 3.99 2.67
10.8 2.06 1.30 21.8 4.02 2.70
the true per cent of anhydrous citric acid, as shown by titra
tion, was plotted as ordinate against the corresponding re
fractometer sucrose value as abscissa. This resulted in prac
tically a straight line and was plotted as such. The acid- degrees Brix relationship plotted in a similar manner gave a slight curve, bending upward. A 45° angle straight line was drawn to represent true or theoretical per cent of solids.
This line fell between the other two but was nearer that of degrees Brix.
The data of Table II were obtained from this graph. This shows for each per cent of citric acid by titration the correc
tion to be added to the corresponding refractometer sucrose scale reading to obtain the proper Brix value. Table II also includes a column of data for correcting refractometer sucrose scale values to true soluble solids or, in the case of pure acid solutions, the true per cent of acid, as would be shown by ti
tration.
D iscu ssio n
The use of the correction data of Table II is simple.
For example, a sample of lemon juice, containing 5.60 per cent of anhydrous citric acid by titration, may give a refractometer
sucrose scale or sucrose table value of 8.20. By referring to the table the correction to be added for 5.60 per cent acid is found to be 1.07, when it is desired to correct the refractometer reading to degrees Brix. The'approximate Brix value of the juice is, thus, 8.20 + 1.07 or 9.27. If it is desired to correct the refractometer reading to true per cent of soluble solids, the correction is 0.66 and the final corrected value becomes 8.20 + 0.66 or 8.86.
Table II may also be used to correct Brix readings to true solids when the acid content is known by subtracting the corresponding correction in the second column and adding that in the third column.
Tests made with solutions containing both citric acid and sucrose verified the corrections for acid shown in Table II.
In general the corrected refractometer sucrose values on citrus juices are slightly lower than the corresponding Brix spindle values.
L iteratu re C ited
(1) Assoc.' Official Agr. Chem., Official and Tentative Methods of Analysis, 2nd ed., rev., pp. 429-32 (Schonrock’s table), 1925.
(2) Macara, T., Analyst, 56, 391-6 (1931).
(3) McRoberts, L. H„ J. Assoc. Official Agr Chem.. 15, 375-89 (1932).