During the manufacture of Swiss-type cheese varieties, such as Emmental, Gruyère and Comté, a high scald temperature is used, which inactivates the added coagulant, decreasing the primary proteolysis of αs1-casein during ripening and resulting in the development of the rubbery texture characteristic of this type of cheese (Kosikowski & Mistry, 1997). Swiss-type cheese may be made from raw, thermized or pasteurized milk, depending on the type of manufacture and the country (Beuvier et al. 1997).

The suitability of milk for cheesemaking is often assessed from the somatic cell count (SCC) of the raw milk, with increased milk SCC being linked to impaired cheesemaking properties and cheese yield potential (Barbano et al. 1991; Auldist et al. 1996). Late lactation milk, which often has elevated SCC, also yields poor quality cheese (Lucey, 1996). High SCC milk has elevated levels of the milk alkaline proteinase plasmin (EC 3.4.21.7), but its activity is not generally considered to be detrimental during cheese ripening (Farkye & Fox, 1992; Bastian & Brown, 1996). Milk somatic cells are themselves also associated with a number of proteolytic enzymes of differing properties, the activities of which are ill-defined in dairy products (Verdi & Barbano, 1991), with the exception of the acid proteinase cathepsin D, which has a chymosin-like activity towards αs1-casein (McSweeney et al. 1995). Preliminary evidence of cathepsin D activity in Swiss cheese during ripening has been reported (Igoshi & Arima, 1993; Beuvier et al. 1997).

In the present study the effect of milk SCC on the ripening of Swiss-type cheese was examined. Of particular interest was the role of polymorphonuclear leucocytes (PMN), the predominant cell type in high SCC mastitic milk (Heegaard et al. 1994) and late lactation milk, when the frequency of milking varies (Kelly et al. 1998). PMN possess a range of proteolytic enzymes, including cathepsin D (Verdi & Barbano, 1991) and plasminogen activators (PA; Heegaard et al. 1994). The experimental strategy chosen was to examine the effects on the quality and proteolysis during ripening of Swiss-type cheeses of adding high SCC milk, whose cell population is mostly PMN, to creamery milk.

In response to nutrition guidelines recommending a reduction in saturated fats in human diets, the dairy industry has developed new products containing unsaturated fats to satisfy the demand of the more health conscious consumer. The fatty acid composition of milk, naturally rich in saturated fatty acids (SFA), can be modified either through genetic selection of dairy cows or by changing feed composition (Palmquist et al. 1993). For example, a number of dairy products including butter (Wood et al. 1975; Badings et al. 1976), Gouda (Badings et al. 1976) and Cheddar (Wong et al. 1973; Lightfield et al. 1993) containing increased amounts of linoleic acid (18[ratio ]2n–6) have been made from the milk of cows given diets supplemented with unsaturated lipids. However, dairy farmers would prefer to produce milk as cheaply as possible, leaving it to food technologists to modify milk components at the post- production stage (Banks, 1987). Therefore, dairy products made from skim milk combined with a fat mixture could be attractive, but little information is available on this type of modified product. One major problem related to the introduction of unsaturated fats into dairy products is the possible alteration of their properties. Indeed, Badings (1970) reported that butter enriched in polyunsaturated fatty acids (PUFA) has reduced flavour quality and shelf life. It is well known that PUFA are easily oxidized and can form undesirable compounds such as peroxides and aldehydes. Moreover, PUFA are more likely to be oxidized as free fatty acids (FFA) than to be integrated into a triacylglycerol structure. Therefore, when a dairy product is made by recombining skim milk with unsaturated fats, such as the soft ripened cheese in this study, it is important to consider both lipolysis and oxidative stability of the lipid fraction. This was our objective in this study.

The composition of colostrum is quite different from that of mature milk. In particular, colostrum contains relatively high concentrations of proteins responsible for the immunological defence of newborns, such as lactoferrin, lactoperoxidase, lysozyme and immunoglobulins (Farkye, 1992; Cals et al. 1994; Telemo & Hanson, 1996; Wang et al. 1997). Whereas lactoferrin and lactoperoxidase are synthesized in mammary epithelial cells (Cals et al. 1994; Molenaar et al. 1996), the immunoglobulins are derived from blood serum (Larson, 1992). Immunoglobulin A (IgA), predominantly found in milk, participates in the development of the gastrointestinal system and the immune system in newborn infants. It is transported by a system that involves formation of a complex with the polymeric IgA receptor (pIgR) exposed on the basolateral aspect of mammary gland epithelial cells, followed by internalization and release into milk as secretory IgA (Rosato et al. 1995; De Groot et al. 1999). Consequently, expression of pIgR in mammary epithelial cells also contributes to the development of the immune system. These proteins specifically expressed in the early stage of lactation are of interest. However, few studies have been carried out compared with those focusing on the major proteins in mature milk, in part owing to problems of mammary gland availability.

Imamura et al. (1996) reported the isolation of mRNA from bovine mammary epithelial cells derived from colostrum and showed that it is possible to detect αs1-casein mRNA, predominantly expressed in the bovine mammary gland, through the application of reverse transcriptase polymerase chain reaction (RT-PCR). We have applied this procedure to porcine colostrum, and successfully analysed the coding sequence of pIgR. In this paper we describe the possibility of detecting gene expression in mammary epithelial cells present in colostrum and the porcine pIgR sequence obtained is compared with those of other animal species.

Several lactic acid bacteria produce exopolysaccharides (EPS), either attached to the cell wall or excreted into the environment as slime material. EPS produced by Lactobacillus delbrueckii subsp. bulgaricus (Lb. bulgaricus) and Streptococcus thermophilus play an important role in improving the texture and stability of yogurt and preventing syneresis (Cerning, 1990; Nakajima et al. 1990). The amount and composition of the EPS produced by lactic acid bacteria are dependent on a number of factors, such as temperature, initial pH, carbon source and the availability of minerals, vitamins and other medium components.

In previous work it was shown that the production and sugar composition of the EPS from Lb. bulgaricus NCFB2772 are affected by the carbohydrate source (Grobben et al. 1995, 1996). In a simplified defined medium, from which several vitamins and trace elements were omitted, EPS production by Lb. bulgaricus significantly increased, although growth of the strain was reduced (Grobben et al. 1998).

Milk-clotting enzymes are used during the production of cheese to coagulate the casein, allowing the formation of a three-dimensional network that entraps the milk fat. Commercially available milk-clotting enzymes differ with respect to source, specificity, optimum pH and thermostability. All are acid proteinases that can cleave κ-casein resulting in the coagulation of milk. Chymosin (EC 3.4.23.4) is specific for the Phe–Met bond in κ-casein at the natural pH of milk (6·7). Recombinant chymosin is available commercially from a variety of sources and has a maximum activity at 40 °C. Recombinant chymosins are purified from the fermentation of recombinant strains of Aspergillus niger, Asp. oryzae or Kluyveromyces marxianus. These enzyme preparations are chemically and functionally identical to calf chymosin. Rennets are purified from the abomasum of bovines and can contain from 60 to 100% chymosin with the remainder being primarily bovine pepsin (Wigley, 1996). Microbial proteinases (EC 3.4.23.6) are generally more proteolytic than chymosin, with varying heat stability. These enzymes liberate more non-protein N from casein and can cleave α- and β-casein as well as κ-casein at the natural pH of milk. Acid proteinases from Cryphonectria parasitica are more heat labile than those from Rhizomucor miehei, which are characterized as thermostable (Ernstrom & Wong, 1974).

The objective of this research was to characterize milk-clotting enzymes with respect to thermal inactivation in skim milk. This information has applications in milk and whey processing.

One of the characteristics of Roquefort cheese is the presence of irregularly shaped openings. Although many factors affect the development of opening in blue-veined cheeses (Martley & Crow, 1996), the limiting step in the case of Roquefort cheese is the production of CO2 by lactic acid bacteria (J.-P. Reverbel, pers. comm.). Most of the opening occurs after moulding; the process is difficult to control and many manufacturing runs result in cheeses with an insufficient opening. Concentrated suspensions of Leuconostoc strains are used to increase the production of CO2 (Devoyod & Muller, 1969), but it would clearly be useful to have microorganisms that produce larger quantities of the gas. McKay & Baldwin (1974) isolated a spontaneous mutant of Lactococcus lactis that produced more acetoin and CO2 than the parent strain. This mutant was lactate dehydrogenase (LDH)-deficient, which favoured the conversion of pyruvate into end products other than lactate. Following nitrosoguanidine mutagenesis, we isolated three Lc. lactis mutants whose LDH activities were reduced to varying extents and which produced varying amounts of CO2 (Boumerdassi et al. 1997). Subsequent work showed that these mutants were unstable on successive subculture in milk or synthetic broth (El Attar et al. 2000).

The aim of our current work was to select a large number of Lc. lactis mutants producing excess CO2. This would increase the probability of selecting stable mutants and also provide a collection of strains with differing gas production activities. Currently available screening methods are, however, unsuitable for processing large numbers of mutants, which is why we have developed an improved screening method.