Plasmina

ARTICLE IN PRESS

International Dairy Journal 16 (2006) 619–622 www.elsevier.com/locate/idairyj

Short communication

An investigation of the plasmin–plasminogen system in caprine milk and cheese
G. Cortellino, F. Locci, M. RampilliÃ
C.R.A—Istituto Sperimentale Lattiero Caseario, Via A. Lombardo, Lodi 26900, Italy Received 19 April 2005; accepted 10 November 2005

Abstract Thevariability in activities of plasmin (PL) and plasminogen (PG) in bulk milk samples from different breeds of goats was investigated. The mean PL activity was higher (19.5674.72 U gÀ1) than in milk from other ruminant species, while PG-derived activity was, surprisingly, lower (12.8475.31 U gÀ1). No significant differences in PL and PG levels were observed among goat breeds, but PL activity and PL/PG ratioincreased in late lactation compared with early lactation. PL activities in pilot-scale curds (acid or rennet) and in some Italian cheeses made from caprine milk were also measured. Generally, acid curds and cheeses had lower residual PL and PG-derived activities than rennet curds and semi-hard cheeses. In addition, the PL/PG ratio was greatly reduced in rennet curds, due to a higher extent ofPG-derived activity. Measurement of proteolysis of b-casein in curds confirmed that more extensive proteolysis occurred in rennet curds, which had higher residual PL activities. r 2005 Elsevier Ltd. All rights reserved.
Keywords: Plasmin; Plasminogen; Milk; Goat; Cheeses

1. Introduction Plasmin (PL) is the main indigenous proteolytic enzyme in milk and its role in dairy product quality has beenwell studied (Bastian & Brown, 1996; Fantuz & Baldi, 1997). In milk, PL is mainly associated with casein micelles, which are its substrate (Bastian & Brown, 1996). It is responsible for the hydrolysis of b- and as2-caseins (Aslam & Hurley, 1997; Trujillo, Guamis, & Carretero, 1997), cleaving mainly bonds of the type Lys-X and Arg-X. PL occurs in milk together with its inactive zymogen, plasminogen(PG). The chain of reactions leading to PG activation is regulated by a complex network of molecular interactions between plasminogen activators (PAs) (tissue-type and urokinase type) and at least three types of specific PA inhibitors (Politis, 1996). Most of the existing studies on the PG activation system have been carried out on bovine milk, and available information is scarce for milk of otherspecies; in particular, there is almost no information available regarding PL activity of goats’ milk.
ÃCorresponding author. Tel.: +39 3714 5011; fax: +39 3713 5579.

The objective of this study was to further the knowledge on the distribution of PL and PG in milk from different breeds of goat. Moreover, the effect of cheese-making technology was evaluated in experimental pilot-scale curds (acidor rennet) and in some commercial cheeses.

2. Materials and methods 2.1. Milk and cheese samples The bulk milks from different breeds of goat were sampled at the beginning of lactation; 14 herds (Camosciata, Saanen and Frisa breed) were located in the south of the alpine region and five herds (half-breed Sarda) were in Sardinia. Some pilot-plant curds (acid or rennet) produced at a regionalfarm and obtained from bulk milks of Camosciata breed, sampled at early (n ¼ 4) and late lactation (n ¼ 10), were also analyzed. The acid curds (n ¼ 6) were prepared by adding lyophilized lactic bacteria starter (Lyofast MS 0,62 CM, Sacco, Como, Italy) as suggested by the supplier and 0.3 mL (131 IMCU mLÀ1) calf liquid rennet (Chr Hansen) to 100 kg pasteurized milk (72.5 1C for 15 s);

E-mailaddress: mrampilli@ilclodi.it (M. Rampilli). 0958-6946/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2005.11.012

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the curds were put in a fine net for whey drainage overnight at 5 1C. The rennet curds (n ¼ 8) were obtained by adding a lyophilized lactic bacteria...