Ester synthesis by lactic acid bacteria and propionibacteria isolated from Argentinean dairy products

ABEIJÓN MUKDSI, M.C.; THIERRY, A.; MEDINA, R.B.

San Miguel de Tucumán

Simposio; III Simposio Internacional de Bacterias Lácticas; 2009

CERELA-CONICET

Esters are part of the aroma array of several cheeses, being short-chain fatty acid (SCFA) esters of C2-C10 important for the development of the characteristic “fruity-type” flavor. Lactic acid bacteria (LAB) and propionibacteria (PAB) are used as starter and/or adjunct cultures in Swiss-type cheese fermentation, and during ripening they can produce SCFA esters via their esterase systems. Esterases can catalyze ester synthesis by esterification (reaction of an acid and an alcohol) and alcoholysis (reaction of an ester and an alcohol). The aim of the present work was to evaluate the ester synthesis ability of mesophylic facultative heterofermentative lactobacilli and propionibacteria strains isolated from argentinean milk and cheeses. Cells were cultured in MRS (for LAB) and LAPTg broth (for PAB) at 37 °C, harvested at early stationary phase, and disrupted by using a French pressure cell. In order to elucidate the mechanisms of ester synthesis involved, cell-free extracts (CFE) were incubated in different reaction mixtures containing as substrates: 1) butanoic acid + ethanol, 2) hexanoic acid + ethanol, 3) tributyrin + ethanol and 4) tricaproin + ethanol. After 24 incubation at 37 °C ethyl esters (C2 to C10) were extracted and determined by gas chromatography. Results showed that evaluated strains were able to synthesise esters of SCFA, mainly ethyl butanoate (EtC4) and ethyl hexanoate (EtC6). A great variability amongst strains was observed. When CFE were incubated in the assay mixture containing butanoic acid and ethanol only EtC4-synthesising activity was observed in PAB strains (P. freudenreichii CRL 757: 53.78 U/mg; P. freudenreichii CRL 758: 18.62 U/mg). In the assay mixture containing hexanoic acid and ethanol, EtC6, EtC8 and EtC10-synthesising activities were observed in PAB. P. freudenreichii CRL 757 showed the highest ester-synthesising activity among PAB strains evaluated (371.69 U/mg for EtC6). In LAB strains EtC4 and EtC6-forming activities were detected when both butanoic or hexanoic acids were used as substrates. In general, ester-synthesising activities by esterification were higher in LAB than in PAB strains. Regarding ester synthesis by alcoholysis, LAB and PAB strains only showed significant amounts of EtC2-synthesising activity when CFE were incubated in the presence of tricaproin and ethanol (P. freudenreichii CRL 757: 24.77 U/mg, L. plantarum ETC17: 57.79 U/mg). In the assay mixture containing tributyrin and ethanol only significant EtC4-forming activities were detected. The highest activity was observed in P. freudenreichii CRL 758 (114.17 U/mg). LAB strain activities ranged from 11.21 to 49.18 U/mg. These results show that LAB and PAB strains evaluated in this work are able to synthesise SCFA ethyl esters, which are desirable key odours in Swiss-type cheeses. A deeper understanding of ester synthesis mechanisms by LAB and PAB would allow to control fruity flavor development in these cheeses.