Storage of lactose-hydrolysed dried milk: effect of water activity on the protein nutritional value

The retention of the protein nutritional value during storage of lactose-hydrolysed dried milk at different water activities (aw) was studied and compared with that of ordinary milk. In the lactose-hydrolysed milk biologically available lysine decreased much more rapidly than in the ordinary milk at all the different aw studied. Thus, at conditions normally accepted for ordinary dried milk (aw approximately 0.2; moisture approximately 4%) there was an available lysine loss in the hydrolysed milk of about 25% after 2 months and about 40% after 6-months storage at room temperature. This occurred without any visible browning. It is concluded that drying to very low aw (less than or equal to 0.11) is necessary to obtain good stability of the protein nutritional value (PNV) in lactose-hydrolysed milk. The drying, however, must be done so that losses in nutritional value are minimal during that process. It should also be noted that fat oxidation might be a problem at such low aw. A chemical method for available lysine assay (guanidination and assay of homoarginine) gave values in good agreement with the biological evaluations with rats.