Analysis of Volatile Compounds, Amino Acid Catabolism and Some Technological Properties of Enterococcus faecalis Strain SLT13 Isolated from Artisanal Tunisian Fermented Milk

Abstract

Aims: Enterococcus faecalis strain SLT13, isolated from the Leben microbiota was investigated for their technological properties. Place and Duration of Study: The study was undertaken in the LETMi laboratory, Tunis, Tunisia with collaboration with CWBI, Gembloux, Belgium, and at INRA (Unité de Biochimie Bactérienne), Jouy-en-Josas, France. The duration of the study was during the period of January 2012 to February 2013. Methodology: Enterococcus faecalis SLT13 was identified biochemically and by ITS rDNA gene sequencing and then characterized for acidifying activity after growth in milk. Solid Phase Microextraction (SPME) and Gas Chromatography (GC) were used for analyzing the volatile profile of fermented milk obtained with SLT13. Catabolism of pheneylalanine and Leucine were studied through using radiolabelled amino acids as tracers. The rheological behavior of the fermented milk was determined using a viscosimeter. Results: The final pH and acidity after growth of the strain SLT13 on milk were 4.29±0.1 and 75°D±2 respectively. The volatiles analyzed by SPME-GC-MS, included a wide range of volatile compounds: ketones, acids, alcohols, aldehydes, sulphur compounds and hydrocarbons. The aminotransferase activity towards Leucine (290±95 nmol min-1 mg-1) was higher than those towards Phenylalanine and Aspartic acid (63±29 and 37±13 nmol min-1 mg-1 respectively). In fermented milk containing radiolabelled Leucine, hydroxyisocaproate was the only metabolite produced by SLT13. The main Phenylalanine degradation product was phenyllactate and small amounts of phenylethanol and phenylacetate. The milk fermented by Enterococcus faecalis SLT13 presented a consistency coefficient K= 0.156 and the flow behaviour index n=0.09. This strain was suited for pilot-scale production and downstream processing with an important growth rate µmax=0.9 h-1, high viable count (1.19 1010 CFU ml-1) and an important survival rate after freeze-drying (82%). Conclusion: Enterococcus faecalis SLT13 exhibited remarkable technological properties and seems also suitable for pilot-scale production and downstream processing.