Assessment of swine, bovine and chicken pepsins as rennet substitutes for Cheddar cheese-making

Denaturation of porcine pepsin during Cheddar cheese manufacture

Porcine pepsin was rapidly denatured in phosphate buffers, pH 6·4–6·7, in the temperature range 31–39 °C and was only slightly more stable in milk under similar conditions. However, the enzyme was considerably more stable in Cheddar cheese curd in which the extent of denaturation was very markedly influenced by the pH of the milk at setting. Under normal cheese-making conditions, porcine pepsin was about equally stable with chymosin. Two modifications of the cheesemanufacturing procedure were developed which permit the manufacture of cheese almost free of coagulant and suitable for the assessment of the contribution of starter proteinases to proteolysis during cheese ripening.

The preparation and assessment of a suitableMucor pusillusLindt proteinase–swine pepsin mixture for Cheddar cheese-making

A simple test procedure was used to predict the most suitable mixture of Noury rennet (Mucor pusillusLindt proteinase) and swine pepsin for Cheddar cheesemaking. Duplicate cheeses were made with 6 Noury rennet–swine pepsin mixtures and with each enzyme alone, with calf rennet as the control. There were slight differences in the course of the cheese-making process and in the yields of the cheeses. Sensory evaluation showed that the most acceptable cheeses were those made by coagulant mixtures closely similar to the predicted mixture and these were as acceptable as cheeses made with calf rennet. The survival rates of different groups of bacteria were similar in all cheeses. There were differences in the products of proteolysis in the various cheeses throughout ripening, particularly with respect to products derived from αs1-casein. The Noury rennet was characterized for the proportion of milk clotting activity associated with each of 4 components and its stability in solution was determined.

A comparison of microstructures of Cheddar cheese curd manufactured with calf rennet, bovine pepsin, and porcine pepsin

Calf rennet, bovine pepsin, and porcine pepsin were used to produce cheese curd, using the same milk and lactic culture for each. Specimens were prepared for scanning electron microscope examination by a modified critical-point drying technique.

From examination of the micrographs, the curd made with bovine and porcine pepsin were similar in structure and in orientation of the coagulated protein, whereas the curd produced with rennet was different, having a more compact and organized structure.

Assessment of the suitability for Cheddar cheesemaking of purified and commercial chicken pepsin preparations

Commercial and pure chicken pepsin preparations were compared with rennet as cheesemaking coagulants. From the milk clotting and caseinolytic activities and enzymic stabilities, predictions were made of the relative activities of the chicken pepsins and rennet during cheese ripening. By this means mixtures of chicken and pig pepsins for cheesemaking trials were formulated. These and mixtures of chicken pepsins with rennet were approximately additive in coagulating activity provided they were used immediately. The amounts of chicken pepsins used in laboratory scale Cheddar cheesemaking trials were minimized by acidification and calcification of the milk. The nature of the coagulant did not affect the cheesemaking conditions, yields or cheese composition. Cheeses made with chicken pepsins alone showed faster proteolysis, more intense flavour, off-flavours and bitterness and were softer than those made with rennet. Cheeses made with chicken pepsin/rennet mixtures were intermediate and those made with chicken pepsin/pig pepsin mixtures were similar to those made with rennet. It was concluded that neither of the chicken pepsin preparations was suitable for Cheddar cheesemaking and that the predictive tests had led to correct assessments.

Preparation and properties of rennets from lamb's and kid's abomasa

Lamb and kid rennets were prepared by extraction of dried abomasa with 6% (w/v) NaCl-2% (w/v) H3BO3 and activation of the proenzymes at pH 2·0. Each gave one zone of precipitation on casein-agar gel diffusion, enabling them to be differentiated from calf rennet and pig pepsin. After agarose gel electrophoresis, the proteinase activity of lamb rennet occurred in chymosin and pepsin bands only, whereas kid rennet contained an additional proteinase of intermediate mobility. Relative to their milk-clotting activities, lamb and kid rennets contained less pepsin and were less proteolytic on both haemoglobin at pH 1·8 and casein at pH 5·3 than calf rennet. The milk-clotting activities of lamb and kid rennets increased less with decrease in pH and were more stable to storage at both the pH value of maximum stability and lower pH values than that of calf rennet. Neither cathepsin activity nor lipolytic activity on milk fat was detected in any of the 3 rennets, but lamb rennet caused slight hydrolysis of tributyrin.

Comparison of the rates of proteolysis during ripening of Cheddar cheeses made with calf rennet and swine pepsin as coagulants

The rates of proteolysis during ripening were followed in cheeses made with either calf rennet or swine pepsin and either starter or δ-gluconic acid lactone (GAL) as a replacement for the starter. A gel-filtration column technique and starch-gel electrophoresis were used for analysis, and bacterial counts were made on all samples. Proteolysis was faster in cheeses made using GAL than in those made using starter and also slightly faster in GAL cheeses made with swine pepsin than in those made with rennet. Further, it was considerably slower in starter-containing cheeses made with swine pepsin than in those made with rennet. It is suggested that these differences were due to the much greater rate of development of acidity in cheeses made with GAL than in those made with starter, which resulted in more of the coagulant being incorporated into the curd in an active state. The rate of proteolysis in starter-containing cheeses appeared to follow a characteristic course, being initially slow, then markedly increasing with a later slow decline. It is suggested that the increase in the rate of proteolysis was due to an increase in the total activity of bacterial proteinases released by lysis of the bacteria. Indications were obtained that the coagulants and bacterial proteinases catalysed broadly similar patterns of protein breakdown in cheese, and that medium-sized peptides (mol. wt 9000–14000) were formed as definite intermediates in the process. The results also showed that rennet and swine pepsin remained active for at least 7 months in GAL cheeses, that rennet contributed significantly to proteolysis in starter-containing cheeses, and that swine pepsin was at least extensively inactivated and possibly completely inactivated during cheese-making with starter.

[Effect of the degree of phosphorylation of bovine pepsin A on its enzymatic activity]

Proteolytic and clotting activities of bovine pepsin A with respect to its degree of phosphorylation were studied on various substrates. The occurrence of phosphate group(s) on bovine pepsin A more or less strongly affects its enzymic properties according to the substrate and its environment. This is particularly obvious as far as kappa-casein is concerned. The specific flocculating activity of unphosphorylated (fA0) as well as dephosphorylated (treated with potato acid phosphatase) bovine pepsin A, determined on a 0.2% kappa-casein solution, is significantly higher than that observed with phosphorylated pepsins, especially after kappa-casein was treated with alpha-D.N-acetyl galactosaminyl oligosaccharidase, while specific milk clotting activity remains unchanged regardless to the level of phosphorylation of bovine pepsin A is. Using haemoglobin as substrate, unphosphorylated pepsin A exhibits the highest specific proteolytic activity and the less acidic pH optimum. Conversely, the amount of phosphate groups does not seem to have any effect on the peptidase activity assayed towards the synthetic chromophoric hexapeptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-OMe. By treating whole bovine pepsin A with potato acid phosphatase during 24 h at 37 degrees C and pH 5.6, using a 1/100 E/S ratio, almost complete dephosphorylation can be reached. The stability of different bovine pepsin A preparations, more or less phosphorylated, treated or not with phosphatase was also investigated. At pH 2.2, phosphorylated bovine pepsin A is twice more stable at 37 degrees C than the dephosphorylated enzymes while dephosphorylated pepsin does not exhibit any degradation at pH 5.6, judging by isoelectric focusing patterns, or loss of activity. Such a result suggests that post-translational phosphorylation might play an essential physiological function by improving the stability and integrity of pepsin in the bovine abomasum, the pH of which is very acidic (between 1.0 and 2.0).