The present method to detect parsnip or turnip adulteration in prepared horseradish uses light microscopy and a starch stain containing iodine in potassium iodide. This method does not work well for potato adulteration because potato and horseradish starch stain similarly. A method has been developed to differentiate potato from horseradish tissue by using a polarizing microscope with crossed polars and a first order red plate. Potato starch grains show interference patterns. This method has been used to qualitatively demonstrate potato starch grains in product packing liquid and to quantitate potato adulteration in horseradish samples at levels of less than 1% potato tissue fragments.