Cheese and fermented sausages

INTRODUCTION Cheese was first made about 8,000 years ago in the Middle East, most likely in Mesopotamia, today’s Iraq. Herdsmen filled up their milk in bags made of animal stomachs, these stomachs still contained the coagulating enzyme now known as chymosin. After some time the milk was transformed into the thick curd and the watery whey, while lactic acid bacteria metabolised the milk sugar lactose into lactic acid. The manufacture of cheese is one of the oldest and most natural examples of food preservation. It is an effective way of preventing bacterial spoilage and preserving the nutritious components of the milk. Two classical methods are combined: souring with lactic acid bacteria and transforming milk into a semi-solid mass by separating the whey from the curd, the water activity is further reduced by adding dry salt or dipping in brine. In addition to cheese, the history of fermented sausages and their preservation by fermentation, drying and salting probably also dates back to more than 2,000 years. As in the case for cheese, the two factors primarily responsible for preservation are both low pH and low water activity. The fermentation process of dry fermented sausages such as salami, cervelat and pepperoni involves the conversion of added sugars to lactic acid, thus lowering the pH. Dry salting originated from the AngloSaxon cultures. In Cheddar cheese or cured meat production processes, salt is mixed with the milled curd or meat particles. Traditionally, for the brine-salting process as applied in the production of many cheese types concentrated

brine from seawater was used. In today’s cheese production of for example Gouda cheese, blocks of curd are put into metal or plastic cheese moulds, pressed and put into the brine. In some meat products the brine is injected. After brining a further reduction of the moisture content is achieved during the ripening process. The processing steps described above contribute to the protection against bacterial spoilage and the growth of pathogenic bacteria. However, the surface of cheese and fermented sausages can be considered as a good substrate for fungal growth. Moreover, the products are mostly ripened in open air at a high relative humidity, conditions ideal for mould growth. Although for specific cheeses and sausages both yeasts and moulds are important for the ripening process, in most cases their development has to be prevented. Apart from the economic losses due to spoilage, the presence of fungi can be of considerable concern for human health by their formation of mycotoxins, i.e., fungal metabolites that are toxic. As prevention of mould growth, especially during ripening and storage, is essential in the production of cheese and fermented sausages, preservatives such as natamycin or sorbate are applied. Modern production processes with ripening under higher humidity as well as the development of new products with less salt, enhance the risk of fungal spoilage and require more intensive preservation measures. In this chapter, first mould-ripened cheeses and sausages will be described. The ripening conditions of these products are quite similar to the conditions of cheeses and sausages where fungi cause spoilage. Secondly contami-

nation of cheeses and sausages is addressed, including which fungi may occur on these products and ways to prevent this. In addition the topic of mycotoxins will be discussed. MOULD-RIPENED CHEESES AND SAUSAGES Mould-ripened cheeses are divided into two major types: those varieties ripened by moulds growing inside the structure of the cheese and those ripened using moulds growing on the surface. The best known of internally mouldripened cheeses are Roquefort, Gorgonzola, Stilton and Danish blue, all ripened using different strains of Penicillium roqueforti which forms blue veins within the cheese. The strains used for the production of Gorgonzola are greener in colour and have more proteolytic activity, while for Stilton a highly lipolytic strain of P. roqueforti is used. Blue cheeses are traditionally made from high fat milk and ripened in caves (Roquefort, Gorgonzola) or cellars (Stilton) at low temperature and high humidity. To allow the moulds to grow in the interior of the cheese, aerobic conditions are required. Therefore, open channels in the structure of the cheese are created by spiking. Also gas producing Leuconostoc and yeasts contribute to a more open curd. Finally the use of non-homogenized milk with low acidity gives a less dense and more crumbly-textured curd (Nichol, 2000). Traditional white cheeses such as Camembert and Brie are produced using the white mould Penicillium camemberti, which gives the cheeses their characteristic white rind. It seems to be that till around the end of the nineteenth century also most surface-ripened cheeses were blue-green of colour due to growth of P. roqueforti. Besides the mould-ripened soft cheeses and blue-veined cheeses, there are a small number of semi-hard mould-ripened cheese types. Tome de Savoie and Saint-Nectaire are examples of such cheeses of which the surface is covered with mixed populations which may contain Penicillium, Mucor, Cladosporium,

Geotrichum, Epicoccum and Sporotrichum species. Other examples are Taleggio and Robiola from Italy and Gammelost from Norway (Beresford et al., 2001). For the production of dry-fermented sausages with a white/cream coloured appearance mostly a culture containing spores of Penicillium nalgiovense is applied. In some traditional production processes a spontaneously appearing home flora develops on the surface of the sausages. The fungal mycelium penetrates into the sausages and contributes to the characteristic flavour development through metabolic activities, e.g., by proteolytic and lipolytic activities and by degrading lactic acid resulting in an increase of the pH. The surface mould protects the surface against light and undesirable microorganisms preventing the development of off-flavours. ROQUEFORT AND CAMEMBERT The two most famous mould-ripened cheeses are Roquefort and Camembert (Figure 1). The history of both traditional French cheeses starts with a legend. The myth says that Roquefort cheese was discovered when a young shepherd eating his lunch in a cave saw a beautiful girl in the distance.