Abstract: In order to evaluate the protein quality of milk products commercially available in Heilongjiang, protein and non-protein nitrogen contents in sweetened whole milk powder, yoghurt, milk beverage, and sterilized pure milk were determined by micro-Kjeldahl method and trichloroacetic acid precipitation method, respectively, and protein and non-protein nitrogen indexes were calculated. The results showed that it was feasible to evaluate protein quality of milk products based on protein and non-protein nitrogen indexes. Four models for protein and non-protein nitrogen indexes in milk products were established. The protein nitrogen indexes of certified sweetened whole milk powder, yoghurt, milk beverage, and sterilized pure milk were in the range of 0.990 to 0.995, 0.870 to 0.956, 0.402 to 0.633, and 0.929 to 0.943, and the non-protein nitrogen indexes were in the range of 0.005 to 0.010, 0.084 to 0.130, 0.367 to 0.598, and 0.069 to 0.071, respectively. On the whole, protein content of commercially available milk products in Heilongjiang is generally consistent with the national standard, although part of them needs to be improved due to poor protein quality.

Key words: milk product, protein quality, evaluation, protein nitrogen index, non-protein nitrogen index