AN INVESTIGATION TO DETERMINE LACTOSE BY USING OXIDATION-REDUCTION REACTION AND ITS APPLICATION TO SOME KINDS OF MILK AND MILK-PRODUCTS

The Lane-Eynon method is the Japanese official method for measuring lactose in milk products containing sucrose. This method includes titration procedure which must be completed within a definite time at boiling temperature, and the visual detection of its end point is also difficult. To remove these defects an improved method was developed. Sample solution (10 ml) is mixed with an excess amount of copper (II) solution containing strong alkali (5 ml of Fehling's soln. A + 7 ml of Fehling's soln. B), and the mixture is boiled for 2 min. The copper (II) which is equivalent to reducing sugar is reduced to copper (I). After the boiling procedure, an excess amount (4 g) of potassium iodide crystal is added to the mixture. To avoid oxidation of potassium iodide by air, sodium carbonate (40 ml, 4%) and hydrochloric acid (8 ml, 6.7 N) are added successively to the mixture. Carbon dioxide thus liberated is effective in removing oxygen from the solution. The mixture is kept in dark for about 5 min. The liberated free iodine which is equivalent to the remaining copper (II) is titrated with sodium thiosulfate solution (0.05 N) by use of starch as an indicator. The blank test is carried out in the same way by using water as a sample. With the difference between the resulting titre of the sample and that of water, the amount of lactose can be known from a calibration curve which is prepared previously by use of samples containing known amounts of lactose. In this method, the operation was more convenient and the visual detection of the end point was easier than the Lane-Eynon method, since the titration was carried out at room temperature and free from time restriction. The method described here is suitable for practical use judge by reproducibility, recovry and accuracy.