Dominique Le Bars

Ability of Thermophilic Lactic Acid Bacteria To Produce Aroma Compounds from Amino Acids

ABSTRACT Although a large number of key odorants of Swiss-type cheese result from amino acid catabolism, the amino acid catabolic pathways in the bacteria present in these cheeses are not well known. In this study, we compared the in vitro abilities of Lactobacillus delbrueckii subsp. lactis, Lactobacillus helveticus, and Streptococcus thermophilus to produce aroma compounds from three amino acids, leucine, phenylalanine, and methionine, under mid-pH conditions of cheese ripening (pH 5.5), and we investigated the catabolic pathways used by these bacteria. In the three lactic acid bacterial species, amino acid catabolism was initiated by a transamination step, which requires the presence of an α-keto acid such as α-ketoglutarate (α-KG) as the amino group acceptor, and produced α-keto acids. Only S. thermophilus exhibited glutamate dehydrogenase activity, which produces α-KG from glutamate, and consequently only S. thermophilus was capable of catabolizing amino acids in the reaction medium without α-KG addition. In the presence of α-KG, lactobacilli produced much more varied aroma compounds such as acids, aldehydes, and alcohols than S. thermophilus, which mainly produced α-keto acids and a small amount of hydroxy acids and acids. L. helveticus mainly produced acids from phenylalanine and leucine, while L. delbrueckii subsp. lactis produced larger amounts of alcohols and/or aldehydes. Formation of aldehydes, alcohols, and acids from α-keto acids by L. delbrueckii subsp. lactis mainly results from the action of an α-keto acid decarboxylase, which produces aldehydes that are then oxidized or reduced to acids or alcohols. In contrast, the enzyme involved in the α-keto acid conversion to acids in L. helveticus and S. thermophilus is an α-keto acid dehydrogenase that produces acyl coenzymes A.

Specificity of plasmin towards bovine αs2-casein

Etude des points de clivage dans la proteine. La specificite est etudiee par separation des peptides hydrolysees par chromatographie HPLC en phase inverse

Purification and characterization of a cell wall proteinase from Streptococcus lactis NCDO 763.

A proteinase was purified from a cell wall extract of a culture of Streptococcus lactis NCDO 763 grown in skim milk. Being active at a low pH (at pH 4.8 on haemoglobin and pH 6.0-6.5 on casein) and completely inhibited by diisopropylfluorophosphate, it was considered to be a serine proteinase partly inhibited by EDTA; the mol. wt was approximately 80,000.

Glutamate dehydrogenase activity can be transmitted naturally to Lactococcus lactis strains to stimulate amino acid conversion to aroma compounds

ABSTRACT Amino acid conversion to aroma compounds by Lactococcus lactis is limited by the low production of α-ketoglutarate that is necessary for the first step of conversion. Recently, glutamate dehydrogenase (GDH) activity that catalyzes the reversible glutamate deamination to α-ketoglutarate was detected in L. lactis strains isolated from a vegetal source, and the gene responsible for the activity in L. lactis NCDO1867 was identified and characterized. The gene is located on a 70-kb plasmid also encoding cadmium resistance. In this study, gdh gene inactivation and overexpression confirmed the direct impact of GDH activity of L. lactis on amino acid catabolism in a reaction medium at pH 5.5, the pH of cheese. By using cadmium resistance as a selectable marker, the plasmid carrying gdh was naturally transmitted to another L. lactis strain by a mating procedure. The transfer conferred to the host strain GDH activity and the ability to catabolize amino acids in the presence of glutamate in the reaction medium. However, the plasmid appeared unstable in a strain also containing the protease lactose plasmid pLP712, indicating an incompatibility between these two plasmids.

Role of Penicillium roqueforti proteinases during blue cheese ripening

The hydrolysis of isolated α s1 - and β-caseins by Penicillium roqueforti aspartyl proteinase produced comparable quantities of pH 4·6 soluble N. The amount of non-protein nitrogen obtained with β-casein was clearly lower than that obtained with α s1 -casein, showing that few low molecular weight peptides were released when this casein was hydrolysed. Electrophoresis of α s1 -casein hydrolysates produced by aspartyl proteinase showed 5 bands of mobility close to or higher than that of α s1 -casein. β-Casein hydrolysates gave 4 bands, 2 of which (βPrapl and βPrap2) showed low electrophoretic mobility. The products corresponding to βPrapl and βPrap2 were purified from a β-casein hydrolysate and identified as fragments Val 98 -Val 209 and Glu 100 -Val 209 of β-casein respectively. The occurrence of the βPrapl and βPrap2 bands in electrophoretic patterns obtained from sterile curd with aspartyl proteinase and controlled-flora curd, where P. roqueforti was the only micro-organism developing, showed the presence of aspartyl proteinase synthesis and activity in cheese. A band of very low electrophoretic mobility (βPrmpl) was present in electrophoregrams of controlled-flora curd inoculated with P. roqueforti . This band, resulting from the action of the metalloproteinase on β-casein, revealed that this enzyme was both synthesized in and active in cheese.

Characterisation of protected denomination of origin cheeses: relationships between sensory texture and instrumental data

Relationships between sensory and instrumental measurements of cheese texture have been explored through an interlaboratory experiment in the framework of a European FLAIR pro- gramme (SENS, Concerted Action no. 2, contract AG RF 025). Ten very different samples of each of the two cheese varieties, Appenzeller, a Swiss serni-hard cheese, and Parmigiano Reggiano, an Ita- lian very hard cheese, were analysed using biochemicaI, rheological and sensory methods. The rela- tional study is one of the first using hannonised methods. Results have shown thar sensory data can be related to instrumental data. Partialleast square regression appears use fui to relate sensory texturai properties from instrumental data. The approach is recommended for similar investigations of other cheese varieties.