Time course and specificity of lipolysis in Swiss cheese

Controlling lipolysis in cheese is necessary to ensure the formation of desirable flavor. To get a better understanding of the mechanism of lipolysis in Swiss cheese, cheeses were manufactured with and without (control) the addition of Propionibacterium freudenreichii. Products of lipolysis were quantified throughout ripening. Half of the free fatty acids (FFA) released in milk (3.66 mg/g fat), in particular the short-chain FFA, were lost in the whey during curd drainage, whereas diglycerides and monoglycerides were retained within the curd. P. freudenreichii was responsible for the release of most FFA during ripening (10.84 and 0.39 mg/g fat in propionibacteria-containing and control cheeses, respectively). Indices of lipolysis displayed low specificity. All types of FFA were released, but butyric and palmitic acids more significantly, which could be due to a low sn-1,3 regioselectivity. All glycerides were hydrolyzed in the following order: monoglycerides>diglycerides>triglycerides. The results of this study show the quantitative and qualitative contributions of the different lipolytic agents to Swiss cheese lipolysis.

The complete genome of Propionibacterium freudenreichii CIRM-BIA1, a hardy actinobacterium with food and probiotic applications

BACKGROUND

Propionibacterium freudenreichii is essential as a ripening culture in Swiss-type cheeses and is also considered for its probiotic use. This species exhibits slow growth, low nutritional requirements, and hardiness in many habitats. It belongs to the taxonomic group of dairy propionibacteria, in contrast to the cutaneous species P. acnes. The genome of the type strain, P. freudenreichii subsp. shermanii CIRM-BIA1 (CIP 103027(T)), was sequenced with an 11-fold coverage.

METHODOLOGY/PRINCIPAL FINDINGS

The circular chromosome of 2.7 Mb of the CIRM-BIA1 strain has a GC-content of 67% and contains 22 different insertion sequences (3.5% of the genome in base pairs). Using a proteomic approach, 490 of the 2439 predicted proteins were confirmed. The annotation revealed the genetic basis for the hardiness of P. freudenreichii, as the bacterium possesses a complete enzymatic arsenal for de novo biosynthesis of aminoacids and vitamins (except panthotenate and biotin) as well as sequences involved in metabolism of various carbon sources, immunity against phages, duplicated chaperone genes and, interestingly, genes involved in the management of polyphosphate, glycogen and trehalose storage. The complete biosynthesis pathway for a bifidogenic compound is described, as well as a high number of surface proteins involved in interactions with the host and present in other probiotic bacteria. By comparative genomics, no pathogenicity factors found in P. acnes or in other pathogenic microbial species were identified in P. freudenreichii, which is consistent with the Generally Recognized As Safe and Qualified Presumption of Safety status of P. freudenreichii. Various pathways for formation of cheese flavor compounds were identified: the Wood-Werkman cycle for propionic acid formation, amino acid degradation pathways resulting in the formation of volatile branched chain fatty acids, and esterases involved in the formation of free fatty acids and esters.

CONCLUSIONS/SIGNIFICANCE

With the exception of its ability to degrade lactose, P. freudenreichii seems poorly adapted to dairy niches. This genome annotation opens up new prospects for the understanding of the P. freudenreichii probiotic activity.

A genomic search approach to identify esterases in Propionibacterium freudenreichii involved in the formation of flavour in Emmental cheese

Background

Lipolysis is an important process of cheese ripening that contributes to the formation of flavour. Propionibacterium freudenreichii is the main agent of lipolysis in Emmental cheese; however, the enzymes involved produced by this species have not yet been identified. Lipolysis is performed by esterases (carboxylic ester hydrolases, EC 3.1.1.-) which are able to hydrolyse acylglycerols bearing short, medium and long chain fatty acids. The genome sequence of P. freudenreichii type strain CIP103027^T was recently obtained in our laboratory.

The aim of this study was to identify as exhaustively as possible the potential esterases in P. freudenreichii that could be involved in the hydrolysis of acylglycerols in Emmental cheese. The proteins identified were produced in a soluble and active form by heterologous expression in Escherichia coli for further study of their activity and specificity of hydrolysed substrates.

Results

The approach chosen was a genomic search approach that combined and compared four methods based on automatic and manual searches of homology and motifs among P. freudenreichii CIP103027^T predicted proteins. Twenty-three putative esterases were identified in this step. Then a selection step permitted to focus the study on the 12 most probable esterases, according to the presence of the GXSXG motif of the α/β hydrolase fold family. The 12 corresponding coding sequences were cloned in expression vectors, containing soluble N-terminal fusion proteins. The best conditions to express each protein in a soluble form were found thanks to an expression screening, using an incomplete factorial experimental design. Eleven out of the 12 proteins were expressed in a soluble form in E. coli and six showed esterase activity on 1-naphthyl acetate and/or propionate, as demonstrated by a zymographic method.

Conclusion

We were able to demonstrate that our genomic search approach was efficient to identify esterases from the genome of a P. freudenreichii strain, more exhaustively than classical approaches. This study highlights the interest in using the automatic search of motifs, with the manual search of homology to previously characterised enzymes as a complementary method. Only further characterisations would permit the identification of the esterases of P. freudenreichii involved in the lipolysis in Emmental cheese.

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.

Comparison of amplified ribosomal DNA restriction analysis, peptidoglycan hydrolase and biochemical profiles for rapid dairy propionibacteria species identification

Species of dairy propionibacteria are used as cheese-ripening cultures as well as probiotics. However, no rapid identification methods are currently available. With this in mind, the present study compared three methods, (i) carbohydrate fermentation, (ii) ARDRA (amplified ribosomal DNA restriction analysis) and (iii) peptidoglycan hydrolase (PGH) activity profiles to improve the identification of Propionibacterium thoenii, Propionibacterium jensenii, Propionibacterium acidipropionici and Propionibacterium microaerophilum. The species Propionibacterium freudenreichii and Propionibacterium cyclohexanicum have previously been shown to be easily distinguishable from the other species. Principal component analysis of the carbohydrate fermentation profiles of 113 P. thoenii, P. jensenii, P. acidipropionici and P. microaerophilum strains correctly classified 85% of the strains based on the fermentation of seven carbohydrates. Regarding PGH profiles, optimized conditions of PGH-renaturing SDS-PAGE were applied to 34 of the strains. The PGH profiles of P. acidipropionici and P. microaerophilum were indistinguishable from one another, but were easily distinguished from P. jensenii and P. thoenii. However, four strains exhibited atypical profiles. Hence, in general, the PGH profiles were shown to be conserved within a species, with some exceptions. Four endonucleases were tested for ARDRA and the four species differentiated by combining the profiles obtained with MspI and HaeIII. P. freudenreichii and P. cyclohexanicum profiles were also performed but showed wide differences. Consequently, ARDRA was shown to be the most appropriate method for rapidly distinguishing strains of propionibacteria. Carbohydrate fermentation and peptidoglycan hydrolase activity profiles are useful as complementary identification tools, since about 15% of the 34 strains tested showed atypical profiles.