Synthesis Of Flavor Compounds Research Articles

Metagenomics-based insights into the microbial community profiling and flavor development potentiality of baijiu Daqu and huangjiu wheat Qu

Daqu and wheat Qu are saccharification and fermenting agents in Chinese huangjiu and baijiu production. This study aimed to investigate the difference between Daqu and wheat Qu in physicochemical indices, microbial communities, functional genes, and the metabolic network of key microbes responsible for flavor synthesis by whole-metagenome sequencing and metabolite analysis. Herein, physicochemical indices indicated that compared with wheat Qu, Daqu exhibited higher protease and cellulase activity and acidity, and lower glucoamylase and amylase enzyme activity. Metagenomic sequencing reveals that although Daqu and wheat Qu community composition have significant differences at species level, they have similar functional genes. Daqu were enriched in Pediococcus pentosaceus, Weissella paramesenteroides, Rasamsonia emersonii and Byssochlamys spectabilis (22.48% of the total abundance), while wheat Qu harbored greater abundances of Saccharopolyspora (54.78%, Saccharopolyspora rectivirgula, Saccharopolyspora shandongensis, Saccharopolyspora hirsuta, Saccharopolyspora spinose, and Saccharopolyspora erythraea). From a functional perspective, the important functions of Daqu and wheat Qu are both amino acid metabolism and carbohydrate metabolism. Meanwhile, a combined analysis among microbiota, functional genes, and dominant flavors indicated S. shandongensis, S. rectivirgula, and S. spinose might be the main contributor to the synthesis of flavor compounds in wheat Qu, while R. emersonii, W. paramesenteroides, Leuconostoc citreum, Leuconostoc mesenteroides, Weissella cibaria and P. pentosaceus may make the greatest contribution to flavor compounds synthesis in Daqu. This study reveals the microbial and functional dissimilarities of Daqu and wheat Qu, and helps elucidating different metabolic roles of microbes during flavor formation.

Flavour precursor peptide from an enzymatic beef hydrolysate Maillard reaction-II: Mechanism of the synthesis of flavour compounds from a sulphur-containing peptide through a Maillard reaction

This study aims to investigate the Maillard reaction products (MRPs) generated between enzymatic beef hydrolysated sulphur-containing oligopeptides and xylose subjected to heat treatment. The priority of the meat aroma contribution is Cys-Gly-Val > GSH > Leu-Cys, whereas Val-Met is relatively Maillard inert. Two peptide-specific aroma compounds, namely, 2-Isobutylpyrazine and 4-butyl-2,5-dimethylthiazole, were produced in a Leu-Cys Maillard reaction system at pH 5.5, showing that the dipeptides with Leu in the N-terminus exhibited high Maillard reactivity. The carbon source and generation pathway of the aroma-active compounds were demonstrated by carbohydrate module labelling (CAMOLA). A new pyrazine generation pathway with xylose-contributed C2 was proposed using this method specifically. Ultra-performance liquid chromatography-electrospray ionization/quadrupole-time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF MS/MS) was then used to analyse the peptide-specific MRPs to reveal the peptide cleavage, cyclization, and cross-linking with other reactants and intermediates. The main results of this study include that cross-linking between reactants and their intermediates largely occurred in a peptide-specific Maillard reaction; its oligopeptide-specific MRPs were first demonstrated; and the cross-linking products from cyclopeptides and two xyloses and/or their fragments were observed for the first time.

Intracellular metabolite profiling of industrial yeast and the synthesis of flavour compounds in beer

Beer quality is largely guaranteed by a rational ratio of different flavour compounds. In this study, we present intracellular metabolic profiles in the light of lager beer flavour compound synthesis on an industrial scale. A total of 62 intracellular metabolites were identified and quantitatively analysed. Among these, amino acid was identified as the most dominant category of metabolites. We also report gene expression profiling of relevant genes encoding six representing amino acids, including serine, alanine, lysine, valine, leucine and asparagine. Nine genes involved in the amino acid synthesis – ALT1, ALT2, ASN1, ASN2, BAT1, SER2, SHM1, SHM2 and LYA1 – displayed significantly higher level of expression at various fermentation stages in an industrial lager fermentation. However, concentrations of the recovered amino acids decreased towards the end of fermentation. Our results suggested that the defined amino acids could be assimilated by the yeast to be utilized for synthesis of aroma-active metabolites at different stages in fermentation. Copyright © 2017 The Institute of Brewing & Distilling

Selected dehydrogenases in Yarrowia lipolytica JMY 861: their role in the synthesis of flavor compounds.

The presence of selected dehydrogenases, including alcohol dehydrogenase (ADH-YL) and aldehyde dehydrogenase (ALDH-YL), in Yarrowia lipolytica JMY 861, and their potential role in flavor synthesis were investigated. The experimental findings showed that using reduced form of nicotinamide adenine dinucleotide (NADH) as cofactor, the ADH-YL activity in vitro was 6-fold higher than that with reduced form of nicotinamide adenine dinucleotide phosphate (NADPH); however, under the experimental conditions used in this study, an ALDH-YL activity was not detected. The in situ hexanal reduction reaction was found to be instantaneous; however, when the yeast cells suspension was diluted 150 times, the initial relative hexanal concentration was increased by 84.1%. The chromatographic analyses indicated the conversion, in situ, of linoleic acid hydroperoxides (HPODs) into volatile C6-compounds after 60min of HPODs addition to the yeast cells suspension.

Monitoring of the production of flavour compounds by analysis of the gene transcription involved in higher alcohol and ester formation by the brewer's yeastSaccharomyces pastorianususing a multiplex RT-qPCR assay

Lager yeast produces a broad range of aroma-active substances during fermentation, of which higher alcohols and volatile esters are vital for the complex flavour of beer. Lager yeast contains several genes that encode the proteins responsible for flavour production. Investigation of gene expression in the hybrid Saccharomyces pastorianus is meaningful as it combines genes from both S. cerevisiae (Sc-type) and S. bayanus (Sb-type). In this study, we developed a GeXP analyser-based multiplex polymerase chain reaction assay to simultaneously assess the transcription of BAP2, BAT1, BAT2, ATF1, ATF2, EEB1, EHT1 and IAH1, involved in higher alcohol and volatile ester biosynthesis, within the S. pastorianus genome from both sub-genomes. GeXP proved to be an effective method of performing a gene expression study by providing a continuous profile of the expression of multiplex genes. This offers the possibility of using this type of data to monitor and control fermentations. Profiles of higher alcohol and volatile ester formation, and the transcription of specific genes, indicated that the synthesis of flavour compounds appears to correlate, at least in part, to the transcription of certain genes. As well as the differential expression of various genes, the differential expression of orthologous genes was also observed during fermentation, suggesting that the Sc-type and Sb-type genes may have different functions. Our results will further the understanding and manipulation of the phenotype of commercially important lager yeasts. Copyright © 2014 The Institute of Brewing & Distilling