Production of branched-chain aroma compounds by Propionibacterium freudenreichii: links with the biosynthesis of membrane fatty acids

AIMS

Short branched-chain fatty acids (BCFAs) are cheese flavour compounds, which result from the conversion of branched-chain amino acids (BCAAs). In Swiss cheese, the production of short BCFAs is mainly performed by Propionibacterium freudenreichii and is strain dependent. Our aim was to investigate the possible links between the biosynthesis of short BCFAs and membrane BCFAs in P. freudenreichii.

METHODS AND RESULTS

Short and membrane BCFAs were analysed by gas chromatography-mass spectrometry. Two strains differing in their capacities to release short BCFAs were selected. Tri-deuterated-labelled leucine was used in both strains as a precursor of short extracellular iso-BCFAs and of membrane iso-BCFAs. The proportions of anteiso : iso BCFAs synthesized varied as function of the BCAAs provided in the growth medium, from 72 : 28 to 100 : 0, with leucine and valine, and with isoleucine as sole BC precursors, respectively. The branching pattern of short BCFAs exactly matched that of membrane BCFAs, whatever the exogenous BCAAs provided.

CONCLUSIONS

The biosynthesis of short BCFAs is closely related to that of membrane BCFAs in P. freudenreichii.

SIGNIFICANCE AND IMPACT OF THE STUDY

The biosynthesis of short BCFAs in P. freudenreichii depends more on the strain than on the presence of exogenous BC precursors.

Efficient mechanical disruption of Lactobacillus helveticus, Lactococcus lactis and Propionibacterium freudenreichii by a new high-pressure homogenizer and recovery of intracellular aminotransferase activity

Microbiological studies often involve bacterial cell fractionation, which is known to be difficult for Gram-positive as compared to Gram-negative bacteria. Our purpose was to test the breaking efficiency of a new high-pressure pilot homogenizer for three Gram-positive species involved in dairy technology and to assess the activity of an intracellular aminotransferase. Varied pressures (50, 100 and 200 MPa) were applied to concentrated bacterial suspensions (1.2 mg dry weight/ml) of Lactobacillus helveticus, Lactococcus lactis and Propionibacterium freudenreichii. Breaking efficiency was estimated by decreases in optical density at 650 nm, cellular dry weight and viability. The proteins released were quantified and the residual intracellular aminotransferase activity was estimated using leucine as substrate. One run at 50 MPa was sufficient to break 80% of lactobacilli cells whereas 200 MPa were required for the same efficiency for L. lactis and P. freudenreichii. Whatever the pressure, leucine aminotransferase activity was recovered in the supernatant after cell breaking. This new high-pressure pilot homogenizer can allow rapid (20 s/run), easy, continuous and highly efficient cell breaking for intracellular enzyme recovery or other purposes. As the species tested were not phylogenetically related, and had different morphologies and cell wall compositions, we conclude that most Gram-positive bacteria may be broken efficiently by this new device.

Conversion of amino acids into aroma compounds by cell-free extracts of Lactobacillus helveticus

AIMS

Lactobacillus helveticus is an essential starter in Swiss-type cheeses such as Emmental. This study was to determine whether cell-free extracts of Lact. helveticus were able to convert free amino acids into neutral volatile aroma compounds at the pH and temperature occurring in cheese.

METHODS AND RESULTS

A mix of branched-chain (Leu, Ile, Val), aromatic (Tyr, Phe) and sulphur (Met) amino acids was incubated for 7 days, at pH 5.7 and 24 degrees C, with cell-free extracts of six strains. The amino acids were all transaminated into the corresponding keto acids when an amino group acceptor (alpha-ketoglutaric acid) was provided. Phe and Tyr were transaminated the most efficiently, followed by Leu, Met, Ile and Val. Three major volatile compounds were detected by GC-MS: benzaldehyde, dimethyl disulphide and 2-methyl propanol. Whatever the strain, benzaldehyde was produced in the highest quantity (0.25-1 micromol l(-1) mg(-1) protein).

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

Lactobacillus helveticus intracellular enzymes could significantly contribute to the production of aroma compounds from amino acid catabolism.