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Direct Analysis of Lactose in Milk and Serum¹

Division of Applied Biology, National Research Council, Ottawa, Canada

ABSTRACT

Barnett and Tawab (1) showed that the colorimetric phenol-sulfuric acid method of Dubois et al. (2) could be used for the direct determination of lactose in milk and cheese. The authors stated that the amount of phenol added was not critical, provided that the same quantity was used throughout. However, variations which occurred in our replicate analyses of standard lactose solutions were traced to minute differences in the amount of phenol reagent added. No information appeared to be available concerning the effect of sulfuric acid concentration on color development. The effect of both reagents on the determination of lactose was, therefore, studied.

Intensity of the color obtained with various concentrations of phenol (Figure 1-A) was maximum between 5 to 12 mg/ml of total arithmetic volume. In comparison, the concentrations used by Barnett and Tawab (1) and Dubois et al. (2) were approximately 18.5 and 15.5 mg/ml, respectively; these levels are outside the optimum range and can cause variation in color intensity if not made very accurately. The concentration of sulfuric acid also influenced the intensity of the color (Figure 1-B). Maximum intensity was obtained when the volume of sulfuric acid comprised from 70 to 75% of the total arithmetic volume, a proportion similar to that recommended by previous authors (1, 2).

¹ Manuscript issued as N.R.C. No. 5285.

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