Ionic liquids as an alternative to the separation of bioactive ketoses from aldoses in isomerization mixtures

Abstract

Lactulose is obtained from lactose by isomerization in basic media using different catalytic systems, and yield ranges from 20 to 80 % dry extract, including variable amounts of lactose and small contents of other sugars. On the other hand, tagatose is a stereoisomer of D-fructose and normally obtained from galactose by an isomerization process. In US is considered a GRAS (Generally Recognized As Safe) product and its use in foods and beverages has been approved in several countries. Both carbohydrates are considered as prebiotics and their purification is of especial interest for the food industry taking into account that these processes represent the 90% of costs of food production. Current commercial processes for purification of carbohydrates are usually based on the use of chromatographic techniques involving ion-exchange resins or activated charcoal. Room temperature ionic liquids (RTILs) are non-molecular ionic solvents and classified as environmentally friendly and provide a safe alternative to the use of traditional organic solvents which produce volatile compounds. In a previous study, we have evaluated the solubilities of aldoses (glucose, galactose, and lactose) and ketoses (fructose, tagatose, and lactulose) in different RTILs at 26 and 45 ºC, detecting significant differences depending on the carbohydrate structure. Therefore, the possibility of using RTILs for the selective separation of binary aldose/ketose mixtures and mixture of synthesis of lactulose from lactose has been evaluated. The best results for the selective separation of the binary mixtures of lactulose - lactose and tagatose - galactose were achieved using [EMIM][DCA] and [BMIM][MeSO4], respectively. The ketose/aldose molar ratios were enriched from 1 to 2.9 in the pair lactulose/lactose and to 2.53 in tagatose/galactose. When these results were applied to the mixture of synthesis, concentration of lactose was notably reduced and the ratio lactulose/lactose raisedfrom 0.5 to1.47. Consequently, these results are the first evidence of the ILs selectivity which can beusefulfor the efficient separation of bioactive ketoses from their corresponding aldoses.