Dextrose alone at 0° C holds in solution about one-fifth of its weight of levulose (in the form of levulate). Other nonlevulose sugars are derived from inulin, which upon acid hydrolysis yields, in addition to about 92 percent of levulose, 3 percent of dextrose and about 5 percent of a group of nonreducing difructose anhydrides [14]. It is due mainly to these latter substances that the waste water (line 13) from the plant juices contains so much more levulose than that from the mixture of pure sugars. Other nonlevulose sugars may be derived from too prolonged hydrolysis of the juices, which causes the formation of the levulosans described by Pictet and Chavan [15], or from a too alkaline medium for defecation, which may cause the Lobry de Bruyn-van Ekenstein transformation.

Table 46 shows in general the compositions of products and byproducts which would probably be found in a levulose plant of any magnitude.

The levulate cake, which contains in average about 22 percent of levulose and 10 percent of calcium oxide, is carbonated by adding it in small portions to a violently agitated quantity of cold water into which carbon dioxide is passed, preferably by some device such as the Doherty carbonator [16] which disperses the gas into a fine state of division. Carbonation is continued until the solution is slightly acid to phenolphthalein. The mixture at this point necessarily contains a considerable quantity of calcium bicarbonate which requires neutralization, since, if carried into the filtrate from the calcium carbonate, it would be decomposed during the subsequent evaporation by volatilization of carbon dioxide. When calcium bicarbonate is decomposed in this manner, it imparts a pH of 8 to 9 to the levulose solution. This alkalinity at the temperature of evaporation impairs the sugar seriously.

The most satisfactory method of neutralizing calcium bicarbonate is to add a thin suspension of milk of lime or calcium levulate until a minimum electrical conductivity is reached. The calcium carbonate, which at the low temperature sometimes crystallizes with six molecules of water of crystallization, is filtered and washed. The filtrate is now slightly alkaline, being saturated with calcium (and if present, magnesium) carbonate. It is again adjusted to a minimum electrical conductivity by addition of dilute oxalic acid.

The filtrate is evaporated at reduced pressure to a sirup. At some point during the evaporation there usually occurs a separation of inorganic salts, which are removed by filtration with 1 or 2 percent of active carbon. The resulting solution should have a levulose purity in excess of 99 percent, being contaminated mainly by inorganic salts in which magnesia usually predominates.

(5) CRYSTALLIZATION OF LEVULOSE.-Crystallization of the sugar can be conducted in aqueous or in aqueous alcoholic solution. For laboratory preparations, aqueous alcohol is the more convenient solvent. The solution is evaporated to a sirup and crystallized, after seeding, under the conditions illustrated in table 47. These conditions of course may be considerably varied.

Crystallization is accomplished in a crystallizer or in a tumbling machine or any apparatus which keeps the crystallizing mass in slow continuous motion. Frequently the addition of alcohol causes a separation into two liquid layers which, however, coalesce as crystallization progresses. The crystals are separated on a Büchner filter or by centrifugal drainage and thoroughly washed with 95-percent alcohol.

For the preparation of levulose of highest purity the substance can be recrystallized in the same manner. It sometimes occurs that the inorganic impurities persist. These can be removed by determining the ash content and, considering it as calcium carbonate, adding the stoichiometric equivalent of nitric acid. In this case the final washing with alcohol must be very thorough.

Levulose can also be crystallized from aqueous solution in spite of its high solubility in water. The solution is evaporated at diminished pressure to about 90 percent of solids, placed in a crystallizer at 50° to 55° C, seeded with crystals, and allowed to crystallize in slow motion by letting the temperature drop very gradually over a period of 1 to 3 or 4 days, depending upon the purity of the solution and the rate of growth of the crystals.

4. REFERENCES

21 (1912).

[1] F. J. Bates and R. F. Jackson, Bul. BS 13, 633 (1916) S293. [2] H. F. Bauer, Orig. Comm. 8th Int. Cong. App. Chem. 13, p. [3] E. O. von Lippmann, Die Chemie der Zuckerarten 2, 1051 (F. Vieweg & Sohn Braunschweig, 1904).

[4] This Circular (p. 768).

[5] F. J. Bates and R. F. Jackson, Bul. BS 13, 67 (1916) S268.

[6] F. L. Dunlap, J. Am. Chem. Soc. 28, 397 (1906).

[7] E. Berner, Ber. deut. chem. Ges. 64, 842 (1931).

[8] B. Tollens V. H. Elsner, Kurzes Handbuch der Kohlenhydrate (J. A. Barth, Leipzig, 1935).

[9] J. R. Katz and A. Weidinger, Rec. trav. chim. 50, 1133 (1931). J. R. Katz and J. C. Derksen, Rec. trav. chim. 50, 248 (1931).

[10] H. Pringsheim, Ber. deut. chem. Ges. 62, 2378 (1929); 63, 2636 (1930). H. Vogel, Ber. deut. chem. Ges. 62, 2980 (1929).

[11] H. D. K. Drew and W. N. Haworth, J. Chem. Soc. 1928, 2690.

[12] R. F. Jackson, C. G. Silsbee, and M. J. Proffitt, BS Sci. Pap. 20, 587 (1926) S519.

[13] R. F. Jackson and J. A. Mathews, J. Research NBS 15, 341 (1935) RP832. [14] R. F. Jackson and S. M. Goergen, BS J. Research 3, 27 (1929) RP79. R. F. Jackson and E. J. McDonald, BS J. Research 6, 709 (1931) RP299. [15] A. Pictet and J. Chavan, Helv. Chim. Acta 9, 809 (1926).

[16] E. P. Clark, BS Sci. Pap. 17, 608 (1922) S432.

XXVI. PURITY

1. CALCULATION

The "coefficient of purity" is the percentage of sucrose in the solids or dry substance of a sugar product and may be expressed by the formula

Purity=

percent sucrose.
percent solids

X100

(131)

It is also known as the "purity quotient" or more generally as the "purity." If accurate methods are used in the determinations, such as the Clerget for sucrose, and drying for solids, the above ratio is known as the "true purity." This value is used in special research and for purposes of checking or comparing products at different stages of manufacture and should always be recorded and referred to as true purity.

Of more general use for purposes of process control in individual factories is the "apparent purity," the value of which is found by dividing the direct polarization of the product in solution by the degrees Brix and multiplying the quotient by 100. Although the apparent purity is only an approximation, it is of great value for comparative purposes because of the rapidity with which the determinations may be made. It is important, therefore, that the analyses for each type of product be performed in exactly the same manner.

The procedure for apparent purity is applied directly to raw or thin juices without weighing or measuring. Solids or massecuites must be dissolved and thick liquors diluted. The solution is diluted with water to a convenient density between 12 and 20 Brix, thoroughly mixed, and allowed to stand in the cylinder until all air has escaped. The removal of air bubbles may be hastened by connecting the cylinder to suction by means of a one-hole stopper and suitable tubing. The degrees Brix is then read using standard hydrometers and thermometers, the reading being corrected to 20° C.

To a portion of the solution (150 to 200 ml) roughly measured, a sufficient amount of anhydrous basic lead acetate is added to clarify. About a teaspoonful of dry diatomaceous earth is added and the whole is thoroughly mixed, and filtered on a rapid filter paper. The clear filtrate may be polarized directly in a 200-mm tube or, if it is deeply colored, in a 100-mm tube. In cane products, varying amounts of invert sugar are present and the negative rotation of the levulose constituent is reduced by the presence of basic lead acetate. The positive rotation of the dextrose is not thus affected, so that the net result is a plus error in rotation.

When the amount of invert sugar is high, the effect of the excess lead may be obviated in either of two ways: (1) After addition of the dry lead salt and shaking, dry powdered oxalic acid is added, a little at a time, until the leaded solution is faintly acid to litmus. The whole is then thoroughly mixed and filtered. The clear filtrate is polarized as above. (2) The filtrate from the solution treated with lead alone (100 ml in a 110-ml flask) is treated with dilute acetic acid until the reaction of the solution to litmus paper is slightly acid. Dilution to the 110-ml mark is completed with water, and the solution is mixed and polarized. The polarization is corrected by adding one-tenth of the observed reading.

In beet products little or no invert sugar is normally present. A solution of basic lead acetate (sp gr 1.25, 20°/20°) is commonly used for clarification instead of the dry salt. To 100 ml of the solution (after determining the Brix) contained in a 100-110 ml flask is added the proper amount of lead solution from a burette. The solution is mixed, a drop or two of ether being added to the flask to disperse the foam, if present, and the volume is completed to 110 ml with water. The solution is then filtered and polarized and the reading corrected by the addition of one-tenth of its observed value. The apparent purity of a sugar solution may be calculated by means of the formula

Apparent purity=Factor X direct polarization.

(132) A formula for obtaining the "factor" to be used in the second term of the above equation was originally elaborated by Cassamajor and was based upon the Mohr cubic centimeter at 17.5° C and the normal weight, 26.048 g. An equation based upon the modern units, milliliters at 20° C and the normal weight of 26.00 g as published by Osborne [1], was derived as follows:

Let

D=true sp gr of solution at 20°/20° (table 109).

D' apparent sp gr of solution at 20°/20° (table 114, p. 632).
B =degrees Brix of solution, then

D' D+0.001 (D-1).

Factor=

26.00X100X1.1
99.72XBD'

=

28.680
BD'

(133)

This equation includes the correction for one-tenth dilution and by its use a table of factors was calculated for each 0.1 Brix from 0 to 25, which may be applied in eq 132.

The formula of Rice [2] omits the one-tenth dilution factor and is expressed as follows:

Factor=

26.00 X 100 99.718Xsp grX Brix

(134)

By means of this equation, Rice calculated a table of factors in increments of 0.1 extending from 0 to 25 Brix. This table (table 146, p. 702) may be used for calculating apparent purity from experimental values of the Brix and polarization.

A convenient table of purity values, expanded from Horne's table as calculated by means of Rice's equation 134 is given by Meade [3]. Here the purity may be read directly from the Brix and polarization of the solution. This table is arranged in intervals of 0.2° S from polarization equals 15 to polarization equals 87.0.

It has already been pointed out that in the determination of "true purity" the sucrose is obtained by the Clerget method and the solids by drying, while in the determination of "apparent purity" the direct polarization and Brix are employed. The above methods are the ones in general use.

However, it is occasionally convenient to use the values for solids obtained from the refractive index in conjunction with either the percentage of sucrose or the polarization. Thus, there are six possible combinations. Noel Deerr [4] suggests the use of the terms "true