The present invention demonstrates the ethanol extracts of fresh rhizome of turmeric (Curcuma longa) strongly inhibit xanthine oxidase, a key biological enzyme responsible for the production of uric acid and superoxide radicals. The dose-response data demonstrate the turmeric extracts prepared with 40% and 50% ethanol are much stronger inhibitors of xanthine oxidase than curcumin. Based on the enzyme-inhibitory data (IC50 values), a biochemical method is developed to standardize turmeric extracts/products as Enzyme-Inhibitory Units (EIU), which has many commercial and biological applications, including, identification and development of pharmacologically effective turmeric products; use of turmeric extracts/products with EIU values >50 per mg to prevent the production of harmful uric acid and superoxides in diseases caused by hyperuricemia and gout; determination of the stability (shelf-life) of turmeric products stored at the retail stores by comparing their EIU values.