In the Iberian Peninsula, the proteinases present in the flowers of members of the Cynara genus, C. cardunculus, C. humilis and C. scolymus, have for many years been successfully used in the manufacture of traditional cheeses from ovine and/or caprine milk on individual farms (Vieira de Sá & Barbosa, 1972; Trujillo et al. 1994). In Portugal, C. cardunculus is the species most frequently employed. Although commercial thistle was tentatively assumed to be pure in taxonomic terms, accurate analyses have shown that the flowers of C. cardunculus are often mixed with flowers of C. humilis (Pires et al. 1994). The clotting activity of C. humilis is due to an aspartic proteinase, currently designated cardosin A and similar to another enzyme obtained from C. cardunculus. This enzyme is similar in specificity and activity to chymosin (Pires et al. 1994).

The action of cardosin A from C. cardunculus upon ovine and caprine caseins has been reported recently (Ramalho-Santos et al. 1996; Simo4es, 1998; Sousa & Malcata, 1998), but as yet there is no information on the proteolytic activity of the proteinase from C. humilis upon caseins from milks other than bovine. Caseins from small ruminants' milks are the usual substrates of cardosin during milk coagulation and cheese ripening, and sodium caseinate represents an intermediate system between isolated caseins and the cheese matrix that is free from interference by fat. Thus ovine and caprine caseinates may be useful substrates for investigating the proteolytic activity of cardosin.

The aim of the present study was to compare the action of pure cardosin A, obtained from C. humilis, on caprine and ovine caseinates, and to assess the in vitro contribution of this enzyme to the overall proteolytic action of thistle rennet.