An improved method to determine whether apple juice had been added to purple grape juice has been published. The two markers for apple juice are the flavonoid phlorizin, and sorbitol. The authors validated the hplc method, and tested some Brazilian market grape juices, which indicated that 4 out of 39 samples were adulterated.
Read the abstract at: http://www.sciencedirect.com/science/article/pii/S0956713516301748
