Conversion of amino acids into aroma compounds by cell-free extracts of Lactobacillus helveticus N. Klein 1 , M.-B. Maillard 2 , A. Thierry 2 and S. Lortal 2 1 DSM ± Food Specialities B.V., Delft, The Netherlands, and 2 INRA, Laboratoire de Recherches de Technologie Laitie Áre, Rennes, France 650/11/00: received 17 November 2000, revised 2 March 2001 and accepted 4 April 2001 N. KLEIN, M.-B. MAILLARD, A. THIERRY AND S. LORTAL. 2001. 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 5á7 and 24°C, with cell-free extracts of six strains. The amino acids were all transaminated into the corresponding keto acids when an amino group acceptor (a-ketoglutaric acid) was provided. Phe and Tyr were transaminated the most ef®ciently, 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 (0á25±1 lmol l ±1 mg ±1 protein). Conclusions, Signi®cance and Impact of the Study: Lactobacillus helveticus intracellular enzymes could signi®cantly contribute to the production of aroma compounds from amino acid catabolism. INTRODUCTION Lactobacillus helveticus is an essential starter in Swiss-type cheeses such as Emmental (400 000 tons per year produced worldwide). This species contributes to milk and curd acidi®cation by consuming galactose, to proteolysis by its cell-wall-located proteases, and to the production of free amino acids by the activity of its intracellular peptidases released after lysis (Valence et al. 2000). Free amino acids can then be converted into cheese aroma compounds or, sometimes, into off-¯avour compounds, by bacterial enzymes and/or non-enzymatic reactions (Keeney and Day 1957; Christensen et al. 1999). Decarboxylation, deamination and transamination have frequently been cited as possible mechanisms. Recently, the conversion of branched-chain, sulphur and aromatic amino acids by lactococci was shown to be essential in cheese ¯avour development (Yvon et al. 1997, 1998; Gao et al. 1997; Rijnen et al. 1999b). Several aminotransferases of cheese- related micro-organisms, such as lactococci, Brevibacterium linens and mesophilic lactobacilli have been characterized (Lee and Desmazeaud 1985; Yvon et al. 1997, 2000; Gao et al. 1998; Rijnen et al. 1999a). In contrast, the potential contribution of Lact. helveticus to the production of ¯avour compounds has rarely been studied. Some authors reported that Lact. helveticus contributed particularly to the burned and nutty ¯avours of Swiss cheeses (Biede et al. 1979; Paulsen et al. 1980; Scolari et al. 1985; Kowalewska et al. 1985). When cultured on milk, Lact. helveticus can produce high quantities of various ketones, aldehydes and sulphur compounds, but the metabolic pathways remain unknown (Imhof et al. 1995). Regarding amino acid conversion by Lact. helveticus, the production of acetaldehyde from threonine has been described previously (Hickey et al. 1983), as the production of methanethiol from methionine (Dias and Weimer 1998). The formation of potential off- ¯avours from tryptophan catabolism by this species was recently highlighted (Gummala and Broadbent 1999). As lysis of Lact. helveticus in Swiss cheeses has been demonstrated previously (Valence et al. 1998, 2000), the aim Correspondence to: S. Lortal, INRA, Laboratoire de Recherches de Technologie Laitie Áre, 65 rue de St Brieuc, 35042 Rennes Cedex, France (e-mail: lortal@labtechno.roazhon.inra.fr). ã 2001 The Society for Applied Microbiology Journal of Applied Microbiology 2001, 91, 404±411