A fluorimetric method was used to determine the o-phthaldialdehyde (OPA) reactivity of amino groups for the assessment of non-enzymatic browning reactions in products with a prolonged shelf-life. The studies were conducted in four commercial infant cereals: wheat-based '7-cereals' and '8-cereals', and gluten-free 'rice cereal' and 'rice-corn cereal'. The cereals were monitored at 28 degrees C for 4 or 16 weeks, or under high water activity (0.65) conditions at 25 degrees C or 55 degrees C for 1, 2, 3 or 4 weeks. OPA reactivity gradually decreased during storage under all temperature, time and water activity conditions, with especially marked losses under conditions of greatest heat intensity and high water activity. According to the present results, the determination of OPA reactivity offers a simple, rapid and reliable method to evaluate non-enzymatic browning.