Crystallization of lactulose

Abstract

Lactulose can be obtained by epimerisation of lactose according to the method of Montgomery and Hudson l. The main problem in preparing pure lactulose is the separation from lactose and galactose. Several chemical methods have been used 2-4 for this separation, yielding products with melting points of 158" and 163" 4.A purer product can, however, be easily obtained by recrystallization from boiling methanol (three times), starting from a syrup containing 50 % by weight of lactulose. The snow-white, transparent crystals melted at between 168.5 and 169.0" (0.5-1 .O"/min).In a paper chromatography test with isopropanol-water (80/20) as the mobile phase and triphenyltetrazolium chloride as indicator, the lactulose crystals obtained contained less than 0.1 % by weight of lactose and less than 0.2 % by weight of galactose.A Debye-Scherrer X-ray photo of these crystals showed sharp lines, which indicates well formed crystals.Under the microscope these crystals were transparent, as distinct from the former conglomerates.The method of Botiseken with a solution of boric acid indicated that our product was the a-form.The specific rotation was -51.4" at a concentration of 0.9975 g in 25.0 ml solution, with pH = 4..8 at 20.0". Literature values: -50.6" 3 and -50.4" 4.If the specific rotation was plotted against temperature, straight lines were obtained, as with fructose ' 3.Montgomery and Hudson, J. Am. Chem. SOC. 52, 2101 (1930). Bares et al., Circ. N 440, Bureau of Standards, Washington (1942) 447. Shuposhnikova, et. al., Ukr. Khim. Zhurnal28, 724/5 (1962).