A new method for determining digestible reactive lysine in foods

The study aimed to develop a bioassay for determining digestible reactive lysine in processed feedstuffs. The approach combines the determination of reactive lysine (guanidination method) in diets and digesta with a true ileal lysine digestibility assay. Using optimized reaction conditions, reactive lysine was determined in unheated casein and heated lactose/casein using both guanidination and FDNB methods. Similar values were obtained with both methods (0.53 and 0.32 mmol (g of casein)(-1) for casein and heated lactose/casein, respectively). Conventional lysine analysis overestimated (0.38 mmol g(-1)) the reactive lysine in the heated mixture. The true ileal lysine digestibility in casein was 98.8%. The corresponding value in the heated lactosa/casein was 70.5%, whereas the true ileal reactive lysine digestibility was 85.9%. The traditional approach to determining the digestible lysine in processed foods appears to considerably underestimate lysine digestibility. The new method, which allows determination of digestible reactive lysine, circumvents some problems inherent in the traditional approach.