In Gouda and Cheddar type cheeses the amino acid conversion to aroma compounds, which is a major process for aroma formation, is essentially due to lactic acid bacteria (LAB). In order to evaluate the respective role of starter and nonstarter LAB and their interactions in cheese flavor formation, we compared the catabolism of phenylalanine, leucine, and methionine by single strains and strain mixtures of Lactococcus lactis subsp. cremoris NCDO763 and three mesophilic lactobacilli. Amino acid catabolism was studied in vitro at pH 5.5, by using radiolabeled amino acids as tracers. In the presence of ␣-ketoglutarate, which is essential for amino acid transamination, the lactobacillus strains degraded less amino acids than L. lactis subsp. cremoris NCDO763, and produced mainly nonaromatic metabolites. L. lactis subsp. cremoris NCDO763 produced mainly the carboxylic acids, which are important compounds for cheese aroma. However, in the reaction mixture containing glutamate, only two lactobacillus strains degraded amino acids significantly. This was due to their glutamate dehydrogenase (GDH) activity, which produced ␣-ketoglutarate from glutamate. The combination of each of the GDH-positive lactobacilli with L. lactis subsp. cremoris NCDO763 had a beneficial effect on the aroma formation. Lactobacilli initiated the conversion of amino acids by transforming them mainly to keto and hydroxy acids, which subsequently were converted to carboxylic acids by the Lactococcus strain. Therefore, we think that such cooperation between starter L. lactis and GDH-positive lactobacilli can stimulate flavor development in cheese.

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Microbial contamination of bovine raw milk often occurs at the farm. To acquire a deeper knowledge of the microbiota of farm tank milk, we studied milk from 45 farms situated in 2 geographical areas in Norway. Each farm was visited on 3 different occasions, with at least 2 wk between visits. We combined both bacterial cell counts and a sequence variant inference method of amplicon-based high-throughput sequencing to achieve a high-resolution overview of the microbiota in each sample. Compositional variation of the farm milk microbiota was shown in relation to the 2 areas, between the farms and between the sampling times. Despite the near constant level of bacteria enumerated in milk from each individual farm, the dominant microbiota differed significantly between the samplings. The predominant microbiota was dominated by spoilage genera, such as Pseudomonas and Bacillus, as well as the dairy fermentation genus Lactococcus and mastitis-causing organisms (Streptococcus). Analysis of the identified sequence variants within these genera showed that the populations of Pseudomonas and Lactococcus in milk had similar composition between the farms, but that Bacillus and, in particular, Streptococcus populations changed between collection days from the same farm and between farms and geographical areas. Furthermore, the levels and composition of Bacillus and Paenibacillus were different between the 2 geographical areas. The results presented here provide new insight into the farm milk microbiota and show that this microbiota is a dynamic community highly subject to variation.