Selective oxidation of glucose–galactose syrup to gluconic and galactonic acids

The global food industry, inclusive of dairy, is searching for sustainable upgrading of the large volumes of inedible waste they produce. Presented here is the selective oxidation of Greek yoghurt acid-whey derived glucose–galactose syrup (GGS) to gluconic and galactonic acids. Aldonic acids are high value biproducts with many applications. Our straightforward approach combines efficient catalytic oxidation and simple separation to upgrade a large dairy waste product to higher value products under base free conditions, using an Au/Al$_2$O$_3$ catalyst with 73% and 55% selectivity towards gluconic and galactonic acids, respectively, under conditions of 2.5 bar O$_2$ for 24 h at 80 °C with a low Au : GGS ratio of 1 : 1000. A selective separation method was developed allowing the separation of the two epimeric acids based solely on their solubilities with high purity.