Journal of Applied Microbiology 1998, 85, 839–848 Use of small ruminants’ milk supplemented with available nitrogen as growth media for Bifidobacterium lactis and Lactobacillus acidophilus A.M.P. Gomes and F.X. Malcata Escola Superior de Biotecnologia, Universidade Cato ´ lica Portuguesa, Porto, Portugal 6628/03/98: received 11 March 1998, revised 3 June 1998 and accepted 9 June 1998 A.M.P. GOMES AND F.X. MALCATA. 1998. Growth of, and acid production by Bifidobacterium lactis and Lactobacillus acidophilus using ovine and caprine milk as media were evaluated for their potential use in cheese-making. A protein hydrolysate (MHP, obtained from incubation of bovine milk with protease) or a mixture of free amino acids (FAA, similar to the amino acid fraction of MHP) was added as a nitrogen enrichment source. Bifidobacterium lactis and Lact. acidophilus were inoculated at 50 ml l −1 and incubated at 37 °C with growth supplements added at ratios in the range 25–50 ml l −1 . The maximum viable counts of Bif. lactis were lower in plain ovine and caprine milk than in nitrogen-enriched milk, and MHP was a better growth promoter than FAA. A similar trend was observed with the acidity values developed, and attempts to correlate growth with acidity were successfully performed. The highest uptake rates of amino acids in ovine milk were observed for lysine, isoleucine, leucine and proline, but only isoleucine was taken up at a similar rate in caprine milk. Final bacterial viable counts of Lact. acidophilus in the plain and enriched forms of ovine milk did not differ greatly from each other, although FAA was statistically a better growth promoter than MHP. Unlike results in ovine milk, cultures of Lact. acidophilus in caprine milk exhibited drops of 1–1·5 log cycles in viable cell counts by 24 h of fermentation, irrespective of the nature of the nitrogen source. Parallel studies indicated that the excess of fatty acid residues in caprine milk could be responsible for the poor growth of Lact. acidophilus. INTRODUCTION Advances in the physiological and technological knowledge pertaining to lactic acid bacteria (LAB) have suggested appli- cation of various properties, e.g. inhibitory, flavouring and texturing properties, as well as a potential role as health promoters, for food quality improvement. Among these properties, the ability of some LAB, particularly Bifido- bacterium spp. and Lactobacillus acidophilus, to destroy or inhibit growth of food-borne pathogens and spoilage micro- organisms (Weerkamp et al. 1996) is of unique importance and has accordingly been exploited by many researchers via incorporation of those strains as additives in fermented milk products (Reuter 1990; Blanchette et al. 1996). However, the Correspondence to: Dr F.X. Malcata, Escola Superior de Biotecnologia, Rua Dr Anto ´nio Bernardino de Almeida, 4200 Porto, Portugal. © 1998 The Society for Applied Microbiology development of a mixed starter composed solely of Bif. lactis and Lact. acidophilus would require a growth medium that (i) supported growth at reasonably high rates and (ii) allowed for exponential growth over a wide range of cell densities. Bifidobacteria, in general, are known as nutritionally fas- tidious micro-organisms because of their numerous growth requirements for certain amino acids and vitamins (Reiter and Oram 1962; Poch and Bezkorovainy 1988, 1991). Conse- quently, it is difficult to use plain milk as a propagation medium for starter culture optimization because it is deficient in free amino acids and peptides (despite its relatively high casein content). As one possible approach to clarify the bio- chemical basis of these multiple nutritional requirements in Bif. lactis and Lact. acidophilus, studies have been undertaken on their growth activity in the presence of two milk hydrol- ysates characterized by several degrees of hydrolysis (Gomes