Plasmin activity and proteolysis in high pressure-treated bovine milk

Abstract

-In this study, proteolysis resulting from the action of the indigenous milk proteinase, plasmin, in high pressure (HP)-treated raw skim bovine milk during storage was examined. Plasmin activity was reduced by treatment at pressures ≥400 MPa and decreased further throughout 28 d of storage at 5°C. In untreated milk or milk treated at 100 MPa plasmin activity increased during the first 3 d of storage at 37°C, indicating activation of plasminogen; considerably less activation occurred in milk treated at 200-600 MPa. However, considerable decreases in plasmin activity, probably due to autolysis, were apparent in all samples on storage at 37°C for >3 d. Proteolysis, as measured by increases in the level of pH 4.6-soluble N and decreases in the level of β-casein, was very limited on storage of milk at 5°C, with little difference between untreated and HP-treated samples. Proteolysis on storage of milk at 37°C was influenced only slightly by treatment of milk at a pressure ≤250 MPa and was reduced considerably in milk treated at 600 MPa for 30 min, but in milk treated at 300-400 MPa, proteolysis was more extensive than in untreated milk, possibly as a result of HP-induced disruption of casein micelles. Overall, HP can either induce or reduce proteolysis in milk, and may therefore have implications for products made from such milk.High pressure / milk / proteolysis / plasmin / casein micelle Résumé -Activité de la plasmine et protéolyse dans un lait de bovin traité par hautes pressions. Dans cette étude, nous nous sommes intéressés à la protéolyse résultant de l'activité d'une protéinase indigène du lait, la plasmine, au cours de l'incubation d'un lait de bovin frais écrémé et traité par hautes pressions. L'activité de la plasmine était réduite par un traitement à des pressions ≥400 MPa et diminuait progressivement au cours des 28 j de stockage à 5°C. Pour un lait non traité ou traité à 100 MPa, l'activité de la plasmine augmentait pendant les trois premiers jours d'incubation à 37°C, indiquant l'activation du plasminogène ; l'activation était bien moins importante pour un lait traité à des pressions comprises entre 200 et 600 MPa. Cependant, des diminutions considé-rables de l'activité de la plasmine, probablement due à une autolyse, étaient observées pour tous les échantillons incubés à 37 °C pour des durées de stockage >3 j. La protéolyse, mesurée par les augmentations du taux d'azote soluble à pH 4,6 et par les diminutions du taux de caséine β résiduelle, était très limitée pour un stockage du lait à 5°C, avec une petite différence entre les échantillons non traités et ceux traités aux hautes pressions. La protéolyse durant l'incubation du lait à 37°C était seulement légèrement influencée par le traitement du lait à des pressions ≤250 MPa et était considérablement réduite pour un lait traité à 600 MPa pour 30 min, mais pour un lait traité à 300-400 MPa, la protéolyse était plus importante que dans un lait non traité, cela étant probablement dû à la désintégration des micelles de caséine induite par les haut...

High pressure-induced changes in milk proteins and possible applications in dairy technology

“…HP treatment of milk affected proteolysis to a greater extent on storage at higher temperatures, closer to the optimum temperature of plasmin activity. Thus, at 37 1C proteolysis in milk treated at 300-400 MPa for 30 min was more extensive than in untreated milk, suggesting an increased availability of substrate to plasmin (Huppertz et al, 2004g;Scollard et al, 2000a). Proteolysis was reduced considerably in milk processed at 600 MPa for 30 min at 20 1C, which is consistent with the reduced plasmin activity in those samples (Huppertz et al, 2004g;Scollard et al, 2000a).…”

“…Thus, at 37 1C proteolysis in milk treated at 300-400 MPa for 30 min was more extensive than in untreated milk, suggesting an increased availability of substrate to plasmin (Huppertz et al, 2004g;Scollard et al, 2000a). Proteolysis was reduced considerably in milk processed at 600 MPa for 30 min at 20 1C, which is consistent with the reduced plasmin activity in those samples (Huppertz et al, 2004g;Scollard et al, 2000a). Similarly, the application of 400 MPa for 15 min, at temperatures from 40 to 60 1C, effectively reduced proteolysis during subsequent storage at 37 1C.…”

“…It has been reported that a S1 -and b-caseins from milk treated at 200 and 400 MPa for 15 min were digested by added plasmin (the main native milk proteinase, Bastian & Brown, 1996) at a faster rate than those from untreated milk (Garcı´a-Risco et al, 2003). However, Garcı´a-Risco et al (2003) did not observe an enhanced proteolytic degradation when pressurized and unpressurized milk were compared during refrigerated storage, while Huppertz, Fox, and Kelly (2004g) found that milk treated at 400 MPa for 30 min showed slightly faster proteolysis than untreated milk at 5 1C. HP treatment of milk affected proteolysis to a greater extent on storage at higher temperatures, closer to the optimum temperature of plasmin activity.…”

Factors affecting the hydrolytic action of plasmin in milk

“…1). This decrease could be attributed to the autolysis (self-hydrolysis) of plasmin, as observed by Ueshima et al (1996) and Huppertz, Fox, and Kelly (2004b). On storage of SM at 5 1C, there was a progressive but less extensive decrease in plasmin activity (Fig.…”

Towards the classification of cheese variety and maturity on the basis of statistical analysis of proteolysis data—a review

“…This may include standardization of the casein and fat levels to achieve the proportions required for a particular cheese type, and pasteurization to help control the cheese microflora, or, alternatively, raw milk may be used so that the milk microflora can impart particular sensory characteristics (Grappin & Beuvier, 1997). Recently, high pressure treatment has been suggested as a means to control the cheese microflora (O'Reilly, Kelly, Murphy, & Beresford, 2001;Huppertz, Fox, & Kelly, 2004a), to increase cheese yield (O'Reilly et al, 2001;Huppertz, Fox, & Kelly, 2004b) or to influence subsequent plasmin activity and proteolysis (Huppertz, Fox, & Kelly, 2004c). The milk may be pre-acidified using acidulants such as lactic, acetic, citric and malic acids or glucono-d-lactone (Metzger, Barbano, Kindstedt, & Guo, 2001;Guinee, Feeney, Auty, & Fox, 2002), and CaCl 2 may be added to increase curd firmness.…”