The products resulting from the non-enzymatic browning in processed diets are known to exert negative effects
on the digestive enzymes of vertebrates. In addition, browning heavy products (BHP's) are known to co-precipitatewith
proteins depending on the pH and ionic strength of the mediumand on the isoelectric point of the protein.
As the manufacture of aquafeeds as well as aquafeed ingredients frequently implies heating processes, the
effects of BHP's from a well-known model mixture (glucose + glycine) on the gastric proteases of the rainbow
trout Oncorhynchus mykiss were investigated in the present work. The results support that BHP's interact with
gastric proteases most probably to form a co-precipitate, thus potentially reducing the protease activity in the
fish stomach. The maximal loss of activity found in the experiments was above 20%. The interaction is pH and
ionic strength-dependent and relatively stable against mechanical perturbation. In the range of BHP concentration
500–1000 μg/mL, the interaction was dose-dependent at pH 3.5 with a maximumabove 20%, and apparently
independent of the dose at pH 4.0. Above an ionic strength of 100mMdue to monovalent ions (NaCl), the intensity
of the interaction is reduced. On the other hand, divalent cations such as Ca2+ and Mg2+ in the range 30–
80 mM disturb the interaction between BHP's and gastric proteases even at ionic strengths below 100 mM,
thus alleviating the inactivating effect of BHP's.