Oliver Frank

Since the discovery of caffeine in 1819 by Runge, its role in the sensory perception of coffee remains unclear. To further investigate its influence, the caffeine–chlorogenate complex was examined using NMR spectroscopy as well as sensory experiments. It was found that the complex formation between caffeine and chlorogenic acid (CQA) has no impact on the bitter taste of caffeine. The changes of the ¹H NMR resonance signals of caffeine in the coffee matrix compared to an aqueous reference solution suggested that caffeine interacts, besides its complex with CQA, with the high molecular weight melanoidins containing fraction of coffee. After isolating the melanoidins from the coffee beverage, these interactions were confirmed in model studies using ¹H NMR spectroscopy. Sensory studies with caffeine and the melanoidins in their natural concentrations showed a strong reduction in bitter intensity by about half compared to an aqueous caffeine solution of the same concentration.

This study employed activity-guided fractionation to identify the compounds that are responsible for the bitter off-taste of fava bean protein isolates and concentrates. UHPLC-ToF-MS and 1D/2D NMR experiments led to the identification of three known bitter compounds, vicine, convicine, and 3'-O-β-d-glucopyranosyl-L-DOPA. In addition, eight previously unknown vicine and convicine derivatives were identified. The bitter thresholds of the analytes were determined and found to be in the range of 0.10 to 1.44 mmol/L. To assess the taste contribution, the corresponding dose-overthreshold (DoT) factors were calculated, and it was shown that convicine with a DoT > 230 is playing a central role for the bitter off-taste. Furthermore, the analysis of fatty acids and their oxidation products suggests that linolenic-, linoleic-, and oleic acid directly contribute to the off-taste of fava bean protein. In addition, cell-based studies showed activation of bitter receptors TAS2R16 and TAS2R43 by vicine and convicine, respectively.

Potato protein hydrolysates (Solanum tuberosum L.) exhibited a strong bitter off-taste. This study aimed to elucidate the origin of this bitter taste; therefore, literature-known bitter tastants such as amino acids, fatty acids, and fatty acid oxidation products were quantified. Using reconstitution experiments, the bitter taste of hydrolysates with low bitterness was explained by the amino acids l-leucine, l-histidine, l-lysine, l-arginine, l-tyrosine, l-isoleucine, l-phenylalanine, and l-tryptophan, as well as palmitic acid; however, the taste of the hydrolysates with high bitterness could not be fully elucidated utilizing literature-known tastants. Using a sensoproteomics approach, including activity-guided fractionation coupled with untargeted/targeted proteomics, 21 new bitter peptides were identified. These peptides showed bitter recognition thresholds ranging from 48 μmol/L (PAF) to 707 μmol/L (DDKDFLPF), suggesting that they are key contributors to the bitterness of the hydrolysate. Using these peptides, new UHPLC-MS/MS methods can be developed to guide the production of less bitter hydrolysates.

The Sensomics approach, including activity-guided fractionation and taste dilution analysis, was employed to identify the key compounds responsible for the bitter off-taste of sunflower press cake. A combination of liquid chromatography-tandem mass spectrometry, liquid chromatography-time-of-flight-mass spectrometry, one-/two-dimensional nuclear magnetic resonance spectroscopy, and dose-overthreshold factor calculation led to the identification of 9,12,13-trihydroxyoctadec-10-enoic acid, 9,10,11-trihydroxyoctadec-12-enoic acid, 11,12,13-trihydroxyoctadec-9-enoic acid, (10E,12E)-9-hydroxyoctadeca-10,12-dienoic acid, (10E,12Z)-9-hydroxyoctadeca-10,12-dienoic acid, (9E,11E)-13-hydroxyoctadeca-9,11-dienoic acid, (9Z,11E)-13-hydroxyoctadeca-9,11-dienoic acid, (9Z,11E)-13-oxooctadeca-9,11-dienoic acid, α-linolenic acid, linoleic acid, oleic acid, 2-hydroxyoleic acid, palmitic acid, stearic acid, and novel pinocarveol β-d-apiofuranosyl-(1→6)-β-d-(4-O-caffeoyl) glucopyranoside as contributors to the bitterness of sunflower press cake. The findings provide valuable insights into the sensory challenges associated with using sunflower press cake in food applications and offer pathways to enhance its palatability and potential as a sustainable protein alternative to meet future protein demands.

Rapeseed products, such as protein concentrates, hold promise for addressing global protein demands, but their application in food products is limited by their bitter and astringent taste. This study investigates the use of β-glucosidase (BG) and laccase (LAC) enzymatic treatment, individually and combined, to enhance the flavor of rapeseed protein concentrate (RPC). Untargeted metabolomics and sensory analysis reveal that LAC reduces the bitter compound kaempferol 3-O-(2‴-O-sinapoyl-β-D-sophoroside) (K3OSS) as well as a general reduction in other phenolic compounds, which correlates with a significant decrease in bitterness and astringency. In contrast, BG treatment elevates the levels of K3OSS and is accompanied by increased bitterness due to the conversion of precursor compounds to K3OSS. In addition, the synergistic use of both enzymes significantly reduces the concentration of K3OSS, resulting in a lower perception of bitterness. The LC-MS analysis of pure reference compounds treated with LAC and BG confirms that BG-mediated treatment facilitates the breakdown of larger kaempferol glycosides into K3OSS, while LAC treatment promotes polyphenol polymerization. Consequently, LAC treatment seems to be an effective strategy to improve the sensory quality of RPC and make it more suitable for human consumption.

Clostridium ragsdalei was found to produce (S)-2-hydroxyisovalerate (2-HIV) as a novel product in addition to acetate, ethanol, and d-2,3-butanediol in heterotrophic (d-xylose), autotrophic (CO), and mixotrophic (d-xylose + CO) conditions. Mixotrophic batch processes in stirred-tank bioreactors with continuous gassing resulted in improved production of this alpha-hydroxy acid compared to batch processes solely with either d-xylose or CO. The maximal CO uptake rate was considerably reduced in mixotrophic compared to autotrophic processes, resulting in a concomitant decreased total CO₂ production. Simultaneous conversion of 9.5 g L⁻¹ d-xylose and 320 mmol CO enabled the production of 1.8 g L⁻¹ 2-HIV in addition to 1.1 g L⁻¹ d-2,3-butanediol, 2.0 g L⁻¹ ethanol, and 1.8 g L⁻¹ acetate. With reduced initial d-xylose (3.1 g L⁻¹), l-valine production started when d-xylose was depleted, reaching a maximum of 0.4 g L⁻¹ l-valine. Using l-arabinose or d-glucose instead of d-xylose in mixotrophic batch processes reduced the 2-HIV production by C. ragsdalei. Considerable amounts of meso-2,3-butanediol (0.9–1.3 g L⁻¹) were produced instead, which was not observed with d-xylose. The monomer 2-HIV can form polyesters that make the molecule attractive for application as bioplastic (polyhydroxyalkanoates) or new composite material.

Beyond the key bitter compound kaempferol 3-O-(2‴-O-sinapoyl-β-d-sophoroside) previously described in the literature (1), eight further bitter and astringent-tasting kaempferol glucosides (2-9) have been identified in rapeseed protein isolates (Brassica napus L.). The bitterness and astringency of these taste-active substances have been described with taste threshold concentrations ranging from 3.3 to 531.7 and 0.3 to 66.4 μmol/L, respectively, as determined by human sensory experiments. In this study, the impact of 1 and kaempferol 3-O-β-d-glucopyranoside (8) on TAS2R-linked proton secretion by HGT-1 cells was analyzed by quantification of the intracellular proton index. mRNA levels of bitter receptors TAS2R3, 4, 5, 13, 30, 31, 39, 40, 43, 45, 46, 50 and TAS2R8 were increased after treatment with compounds 1 and 8. Using quantitative UHPLC-MS/MS_MRM measurements, the concentrations of 1-9 were determined in rapeseed/canola seeds and their corresponding protein isolates. Depending on the sample material, compounds 1, 3, and 5-9 exceeded dose over threshold (DoT) factors above one for both bitterness and astringency in selected protein isolates. In addition, an increase in the key bitter compound 1 during industrial protein production (apart from enrichment) was observed, allowing the identification of the potential precursor of 1 to be kaempferol 3-O-(2‴-O-sinapoyl-β-d-sophoroside)-7-O-β-d-glucopyranoside (3). These results may contribute to the production of less bitter and astringent rapeseed protein isolates through the optimization of breeding and postharvest downstream processing.