The microstructure and distribution of micro-organisms within mature Serra cheese

The distribution of micro-organisms in mature Serra, a traditional Portuguese cheese made from unpasteurised ewes' milk without added starter culture, was examined by light microscopy and electron microscopy. Four populations of micro-organisms were recognized according to their position within the cheese: (i) those present as apparently axenic colonies within the curd matrix; (ii) bacteria growing along curd junctions; (iii) yeasts and bacteria present in the smear on the surface of the cheese and (iv) bacteria found in cracks which penetrated the outer part of the cheese from the rind. Two types of crystals were observed, together with contaminants of vegetable origin and somatic cells originating from the milk.