Comparison of the acidifying activity of Lactococcus lactis subsp. lactis strains isolated from goat's milk and Valdeteja cheese C. Alonso-Calleja 1 , J. Carballo 2 , R. Capita 1 , A. Bernardo 1 and M.L. Garcõ Âa-Lo  pez 1 1 Departamento de Higiene y TecnologõÂa de los Alimentos. Facultad de Veterinaria, Universidad de LeoÂn. Campus Universitario de Vegazana, s/n. 24071, LeoÂn. Spain and 2 A  rea de TecnologõÂa de los Alimentos. Facultad de Ciencias de Orense, Universidad de Vigo. 32004, Orense, Spain 2001/200: received 5 July 2001, revised 15 November 2001 and accepted 21 November 2001 C. ALONSO-CALLEJA, J. CARBALLO, R. CAPITA, A. BERNARDO AND M.L. GARCõ ÂA-LO  PEZ. 2002. Aims: This work was carried out to study the acid production by Lactococcus lactis subsp. lactis strains isolated from goat's milk and goat cheese (Valdeteja variety) in order to select a suitable starter culture for industrial goat cheese manufacturing. Methods and Results: The titrable acidity of 45 Lactococcus lactis subsp. lactis strains isolated from a home-made batch of Valdeteja cheese with excellent sensory characteristics was measured over a period of 18 h. The strains were divided into two groups depending on the acid production rate: 20 fast acid producer (F) strains and 25 slow acid producer (S) strains. The kinetic parameters (lag phase, maximum acid production rate and value of upper asymptote curve) of the acid production curves for F and S strains were signi®cantly (P <0á001) different. Conclusions: Signi®cant (P <0á001) differences between titrable acidity of F and S strains were observed after the second hour of incubation. Signi®cance and Impact of the Study: An F strain acetoin producer (Lactococcus lactis subsp. lactis 470Ch2) was selected as autochthonous starter culture for industrial Valdeteja goat cheese manufacturing. INTRODUCTION Valdeteja is a traditional hard cheese variety produced in Leo Ân (north-west Spain) from whole raw goat's milk. The poor microbiological quality of the milk (Gutie Ârrez et al. 1988; Alonso-Calleja et al. 2002) together with the absence of pasteurization and of starter cultures (the fermentation process depends of the natural contamination with acid lactic bacteria), yield cheeses without overall standardized quality and are potentially dangerous because of the short ripening period (about four weeks). In view of the growing interest on the part of the organized dairies for large-scale manufacturing of goats' milk cheeses, the selection of suitable starter cultures for production of Valdeteja cheese from uniformly high quality is necessary. For most purposes, the ability to produce acid at a high and predictable rate is essential in cheese starter strains (Stad- houders 1974, 1986; Sandine 1979, 1985; Farrow 1980; Nu Ân Äez et al. 1981; Cogan and Daly 1987; Oliver et al. 1988; Fox et al. 1990; Kandarakis et al. 1998). The acidifying activity of different starter micro-organisms has been studied world- wide. However, there is little published data on starter cultures for goats' milk cheese. The majority of studies carried out also refer to the ®nal acid production and in only a few are the acid production rate over a long period of time studied. Lactococci constitute the dominant population through- out ripening in many cheeses, including goats' milk varieties (Tornadijo et al. 1995; Za Ârate et al. 1997). Lactococcus lactis subsp. lactis is the only subspecies isolated in the lactococci group of Valdeteja cheese in M-17 and MSE media (Alonso- Calleja 1991). Correspondence to: C. Alonso-Calleja, Departamento de Higiene y TecnologõÂa de los Alimentos, Facultad de Veterinaria. Campus Universitario de Vegazana s/n. 24071, LeoÂn, Spain (e-mail: dhtcac@unileon.es). ã 2002 The Society for Applied Microbiology Letters in Applied Microbiology 2002, 34, 134±138