The microflora of Blue Stilton cheese
Blue Stilton is a blue-veined cheese manufactured in a restricted area of the UK, using lactic starter cultures plus a secondary culture of Penicillium roquefotti. The aim of this study was to determine the change in microflora during ripening of the cheese and to investigate potential microbial interactions. Additionally, the volatile compounds present in mature samples of cheeses exhibiting few blue veins were compared with those in good quality cheeses, showing ample blue veining. Experiments on cheeses from a single dairy, monitored during the ripening process, showed that the total Lactobacillus count increased from levels of around 103 cfu g-1 on day one to around 107 cfu g-1 after 8 weeks of ripening. This is comparable to values found in other cheeses including both mould-ripened and non mould-ripened varieties. Yeast counts were generally higher than those found in other cheeses and also increased to levels in the region of 107 g-1. The total viable count (TVC) decreased from around 109 g-1 initially, reflecting the presence of the starter bacteria, to 107 g.1, suggesting a decline in the starter bacteria similar to that found in other cheeses. Mature cheeses always exhibited similar numbers of microorganisms although the species varied between cheeses. High quality, mature, cheeses were compared with sub-standard cheeses from the same production site. The predominant species of lactobacilli in good quality cheeses were Lb. plantarum and Lb. curvatus, whereas in poor quality cheeses Lb. brevis predominated. This corresponded to the results of gas chromatography-olfactometry, which indicated the presence of fruity off flavours in poor quality cheeses. Several strains of these species were isolated, as indicated by differing capabilities in utilisation of a range of carbon sources. Yeast species also varied between good and poor quality cheeses with Candida sphaerlca and C. catenulata predominating in good cheeses and C. famata, C. lipolytica and C. catenulata also occurring in both good and poor quality samples. Strain differences were observed by the biochemical profiles and two strains of C. famata demonstrated inhibitory effects against P. roqueforti when incubated under anaerobiosis. It was concluded that these strains may affect the development of blue veins in Stilton cheese when maturation conditions encourage their proliferation. Comparisons were made between samples of cheeses from several Stilton producers and the results suggested that although the levels of the groups of microorganisms tested were similar, the species of lactobacilli and yeasts present were different. This suggests that the indigenous microflora may have a significant impact on the flavour of cheeses from individual production sites. It was concluded that the microflora of Blue Stilton cheese may have a significant impact on the quality of the product both in terms of flavour and the development of the blue veins.