The invention is a method of monitoring for the presence of certain constituents of food processing streams by measurement of fluorescence. The constituent monitored can be a fluorescing impurity of a food product, or the food product itself if such naturally fluoresces. Moreover, losses of the constituent to be monitored may be determined by concurrent measurement of fluorescence of a fluorescent material added to the food process stream. The method is applicable to the following food processing streams among others: meat, vegetable oil, sugar beet, sugar cane, grain, poultry, fruit and soybean processing streams. Upon determination of a variation of the fluorescing constituent to be measured, the food process can be adjusted accordingly.