The effect of proteolytic enzymes from somatic cells on cheese quality was studied. In preliminary experiments, milk and two sodium caseinate systems (pH 6.5 and pH 5.2, the latter in the presence of 5% NaCl) were used as substrates to investigate the proteolytic activity of somatic cells recovered from mastitic milk. Urea-polyacrylamide gel electrophoretograms of hydrolysates suggested that somatic cell extracts contributed directly to proteolysis both in buffer and in milk, but that such activity was reduced by batch pasteurisation (63 degrees C for 30 min). Sodium caseinate was readily hydrolysed by somatic cell extracts; hydrolysis of alpha(s1)-casein was greater at pH 5.2 and increased with level of somatic cells, suggesting that somatic cells contain proteolytic enzymes which are more active at acidic pH values. Subsequently, miniature Cheddar-type cheeses were made from batches of milk to which somatic cells were added (at levels of levels of 3 x 10(5) or 6 x 10(5) cells mL(-1)), either before or after pasteurisation. Proteolysis during ripening of cheese (as measured by levels of pH 4.6-soluble nitrogen) increased with somatic cell addition, although this effect was reduced by pasteurisation after cell addition. Somatic cells may also have directly influenced cheese moisture content, which has been established as a principal indicator of quality of Cheddar-type cheese. Proteolytic enzymes of somatic cells from milk were shown to contribute directly to proteolysis in milk and cheese. (c) 2004 Elsevier Ltd. All rights reserved.