Biologically active peptides are of particular interest in food science and nutrition because they have been shown to play physiological roles, including opioid-like features, as well as immunostimulating and anti-hypertensive activities, and ability to enhance calcium absorption. Hidden or inactive in the amino-acid sequence of dairy proteins, they can be released or activated in vivo during gastrointestinal digestion, or upstream during food processing via specific, enzyme-mediated proteolysis. Caseins, in either milk or dairy products (e.g. cheese), are important sources of those peptides; their biological significance, their impact on human health and the manufacture of novel functional food ingredients therefrom have been subject to intensive research, which will be briefly presented and critically discussed in this review. r
Studies encompassing variation of milk clotting time with the concentration of proteases extracted from Cynara cardunculus were performed; milk clotting time dependence is not linear, and a model was postulated that fits well the experimental data obtained, and hence may be useful in predicting changes during the cheesemaking process. Parallel studies were also conducted pertaining to proteolysis of a mixture of ovine and caprine caseins, in attempts to investigate the stability of the aforementioned coagulating enzymes in crude or in pure form, with or without previous incubation for a certain time under typical ripening conditions. The enzymes exhibited an increase in activity whenever previous incubation had taken place. Moreover, the extent of enzyme-mediated proteolysis was always higher on caprine than on ovine milk caseins.
The proteolytic activity of cardosin B, an aspartic proteinase from the thistle, Cynara cardunculus, on ovine a s -caseins and bcaseins (independently or present together in sodium caseinate) was followed by urea polyacrylamide gel electrophoresis and reversed phase high performance liquid chromatography. This enzyme degraded both types of caseins, but not to the same degree. In sodium-caseinate, by 10 h at 30 C, a s -caseins were more susceptible to proteolysis by cardosin B than b-casein whereas, in isolated form, the reverse was observed. Sequencing of the peptides produced by hydrolysis of Na-caseinate showed that the major cleavage sites in a s1 -casein were Leu156-Asp157 and Trp164-Tyr165 whereas, in b-casein, they were Leu127-Thr128, Leu165-Ser166 and Leu90-Tyr191. The bonds Trpl64-Tyr165 and Leu165-Ser166 were the most susceptible to cardosin B when this enzyme acted upon isolated a s1 -and b-casein, respectively. #
A comparative study was developed on the clotting activities and gelling properties of cardosins A and B, extracted from dried flowers of Cynara cardunculus, and chymosin on cow's skim milk, at various pH values. The determination of the total milk-clotting activity was performed following an international standard, whereas a rheometer was employed to measure the viscoelastic properties of the gels subsequently formed: the evolution of the complex modulus (GÃ) and the phase angle (d) was monitored with time. The GÃ values of the milk gels were higher for cardosins than for chymosin at pH 6.6, but the reverse held at pH 6.4 and 6.2. The d values were identical for all three enzymes tested. Chymosin exhibited the highest specific milk clotting activity, followed by cardosin B. The clotting activity of chymosin seems to be more influenced than that of cardosins by the pH of milk.
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