Complex sentence: Since Tagatose is a naturally occurring in dairy, it is derived from lactose which is hydrolyzed to glucose and galactose, of which the galactose is isomerized to D-tagatose by adding calcium hydroxide, then purified.
Split into simpler sentences: Since tagatose is naturally occurring in dairy products, it is derived from lactose which is hydrolyzed to glucose and galactose. The galactose is isomerized to D-tagatose by adding calcium hydroxide, then purified.