Production of volatile compounds by Lactobacillus sakei from branched chain α-keto acids.

Lactobacillus sakei belongs to the main flora of raw fermented sausages and is used as starter culture. Bacterial starter cultures can convert amino acids to α-keto acids by aminotransferases. These α-keto acids are the precursors of aroma active aldehydes, alcohols and carboxylic acids. In this study the formation of aldehydes, alcohols and carboxylic acids from leucine, isoleucine, valine and the corresponding α-keto acids are analysed in model fermentations with two different strains of L sakei. In the absence and upon addition of leucine, isoleucine and valine they produced 1 µg/ml 3-methylbutanoic, 0.2 µg/ml 2-methylbutanoic acid and 3 µg/ml 2-methylpropanoic acid, respectively. Upon addition of α-ketoisocaproic acid, α-keto-3-methyl-pentanoic acid or α-ketoisovaleric acid the amount of the corresponding carboxylic acid was increased to 40 µg/ml 3-methylbutanoic acid, 20 µg/ml 2-methylbutanoic acid and 35 µg/ml 2-methylpropanoic acid. The response patterns of the strains and amounts of carboxylic acids produced were similar. This behaviour was typical when compared with other strains of L. sakei and suggests general lack of transaminase activity and a limit in the transport of branched chain amino acids and their conversion to volatiles, some of which can contribute to the aroma of fermented sausages.

Genetic screening of Lactobacillus sakei and Lactobacillus curvatus strains for their peptidolytic system and amino acid metabolism, and comparison of their volatilomes in a model system.

A total of 51 Lactobacillus sakei and 28 Lactobacillus curvatus strains from different origins were screened for their potential to produce biogenic amines (BAs), and for their diversity of peptidolytic systems and specific aminotransferases (AraT, BcaT) that initiate amino acid conversion to volatiles relevant for aroma formation in meat products. The profiles of volatiles formed (volatilomes) were analysed in the headspace of fermentations by solid phase microextraction followed by GC-MS analysis. Tyramine-forming potential was detected only within L. curvatus and was strain-dependent. Histamine decarboxylase (HDC) activity could only be detected in one L. sakei strain, previously described as histidine decarboxylase positive (HDC(+)). Peptide transporters and peptidases were nearly ubiquitous in L. sakei and only a few strains lacked single peptidases. In L. curvatus, differences were detected in the occurrence of peptidase genes detected with PCR primers derived from L. sakei. All strains lacked known aminotransferases specific for branched-chain amino acids (BCAAs) and aromatic amino acids (ACAAs). Although L. sakei is suggested as a genetically very heterogenous species, and relatedness between L. curvatus and L. sakei at the genomic level is rather low, they appeared to be nearly uniform in the genes forming the peptidolytic system. The volatilomes of L. sakei and L. curvatus strains were qualitatively nearly identical. However, slight differences in the formation of single volatile compounds and the interaction with staphylococci may impact upon sausage fermentation which occurs over a period of many weeks. Among the compounds expected to contribute to the aroma were dimethyldisulphide, 3-methyl-1-butanol, acetic acid, 1-butanol and butanoic acid.

A laser-endoscope with direct beam coupling.

An optical element for direct laser-beam coupling into the observation optic of a rigid endoscope was developed. Using this coupling element every suitable endoscope can be modified into a laser endoscope which can be used for laser surgery e.g. in ENT and urology. The requirements for the endoscope and the optical coupling element are discussed.