Tham khảo tài liệu 'scientific, health and social aspects of the food industry part 10', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Yeast the Man s Best Friend 259 In rural areas in the north of Portugal a corn and rye bread is still prepared using a piece of dough usually kept in cool places covered with a layer of salt. Prior to bread making this piece of dough is mixed with fresh flour and water and when fully developed serves as the inoculum for the bread dough. This starter dough is a natural biological system characterized by the presence of yeast and lactic acid bacteria living in complex associations in a system somewhat similar to that existing in sourdough. In a survey carried in 33 dough samples from farms mainly located in the north of Portugal 73 yeast isolates were obtained belonging to eight different species. The predominant species was S. cerevisiae but other yeasts also occurred frequently among which Issachenkia orientalis Pichia membranaefaciens and Torulaspora delbrueckii were the most abundant being present in about 40 of the doughs examined. Only six of the doughs contained a single yeast species. Associations of two species were found in 48 of the bread doughs 30 presented three different species and the remainder consisted of a mixture of four yeast species. Associations of S. cerevisiae and T. delbrueckii I. orientalis and or P. membranaefaciens were the most frequent. All mixed populations included at least one fermentative species with the exception of the association between P. anomala P. membranaefaciensand I. orientalis which was found in one of the doughs Almeida Pais 1996a . Apparently this dough is somehow similar to the San Francisco sour dough in which maltose-negative S. exiguus is predominantly found and the fermentation may be carried out by lactic acid bacteria Sugihara et al. 1971 . In another Portuguese study in which besides sourdough maize and rye flour were examined the most frequently isolated yeasts were S. cerevisiae and C. pelliculosa Rocha Malcata 1999 . In conclusion yeasts and lactic acid bacteria LAB are often encountered together in the .
