Non-Close-Packed Breath Figures via Ion-Partitioning-Mediated Self-Assembly.

We report a one-step method of forming non-close-packed (NCP) pore arrays of micro- and sub-micropores using chloroform-based solutions of polystyrene acidified with hydrogen bromide for breath figure (BF) patterning. As BF patterning takes place, water vapor condenses onto the polystyrene solution, forming water droplets on the solution surface. Concurrently, preferential ion partitioning of hydrogen bromide leads to positively charged water droplets, which experience interdroplet electrostatic repulsion. Self-organization of charged water droplets because of surface flow and subsequent evaporation of the droplet templates result in ordered BF arrays with pore separation/diameter (L/D) ratios of up to 16.5. Evidence from surface potential scans show proof for preferential ion partitioning of HBr. Radial distribution functions and Voronoi polygon analysis of pore arrays show that they possess a high degree of conformational order. Past fabrication methods of NCP structures typically require multi-step processes. In contrast, we have established a new route for facile self-assembly of previously inaccessible patterns, which comprises of only a single operational step.

Comparison of untargeted gas chromatography-mass spectrometry analysis algorithms with implications to the interpretation and putative identification of volatile aroma compositions.

The aroma profiling process requires the identification of the volatile compounds in a sample or its headspace. Typically, the identification of compounds relies on automated feature finding and matching algorithms to (putatively) identify and report compounds based on retention index and mass spectra matching against a compound library. We investigated the use of five different workflows and proposed three metrics (target accuracy A, identification percentage I, uniqueness U) to quantify their impact on generated aroma profiles of a mixture of fragrance standards and a commercial grade essential oil. All workflows accurately identified target compounds (100% in standards, >90% in samples) and reported similar compound identities for major GC-MS features, but beyond that could differ by up to 40-50%. Despite the variances, different workflows did not report conflicting compound identities. Aroma compositions primarily contained unreported or extra (putatively) identified compounds due to variations in mass spectral elucidations within the various workflows. Considering these differences, we show how the proposed metrics, I and U, could be modified to help the analyst interpret and evaluate reported volatile aroma compositions of unknown materials.

Comparison of differences in sensory, volatile odour-activity and volatile profile of commercial plant-based meats.

Descriptive sensory analysis was paired with temporal check-all-that-apply gas-chromatography olfactometry (TCATA GC-O) to compare differences in perceived flavour and volatile odour activity across a series of commercial plant-based meat analogues (PBMAs) versus conventional beef products. Multiple factor analysis separated PBMAs in two clusters along the first principal axis. The first cluster, rated higher in meaty flavour and odour, also showed higher citation proportions of sulfurous odourants. In contrast, the second cluster, higher in off odour and flavour, had higher citation proportions for fatty / legume odourants. Key odourants correlated with meaty flavour and odour were putatively identified as 2-methyl-3-furanthiol, dimethyl trisulfide, and furfuryl mercaptan while compounds correlated to off flavour and odour were putatively identified as (E,E)-3,5-octadien-2-one, 2-undecanol, and (E,E)-2,4-decadienal. No correspondence was found between PBMA odour-activity and source protein, suggesting that volatile flavour production in PBMAs is derived primarily from exogeneous flavouring materials or precursors rather than the base protein material. Contributions of lipid-protein interactions to overall flavour differences is further suggested by the putative discovery of 5,6-dihydro-2,4,6-trimethyl-4H-1,3,5-dithiazine odour activity in several meat samples profiled.

Sustainable production of lipids from cocoa fatty acid distillate fermentation driven by adaptive evolution in Yarrowia lipolytica.

Single cell oil production using oleaginous yeasts is a promising alternative to animal and plant-derived lipids. But substrate costs for microbial fermentation are a major bottleneck. Using side streams as alternative to substrates like glucose, for growing yeast, is a potential cost-effective solution. By combining a previously reported process of growing yeasts on a solid cocoa fatty acid distillate side stream with adaptive evolution techniques, the growth of oleaginous yeast Yarrowia lipolytica was improved by 2-fold. The lipid titre was also boosted by more than 3-fold. Using transcriptomics, key genes were identified that are possibly involved in tailoring of lipid composition, side stream utilisation and enhancement of lipid titres. Candidate genes were also identified that might enable efficient growth and utilization of fatty acids and triacylglycerides found in cocoa fatty acid distillate. In summary, this research has improved the understanding of side stream utilisation for lipid production in oleaginous yeast.

Optimization of extraction conditions for LC-ToF-MS analysis of mevalonate pathway metabolites in engineered E. coli strain via statistical experimental designs.

Isoprenoids give rise to many functional products used today such as flavours, fragrances and even pharmaceutical compounds. Mevalonate pathway metabolites are the key intermediates that affect the production yield of isoprenoids. With increasing demand and benefit of isoprenoids, the present study adopts Analytical Quality-by-Design (AQbD) approach to establish an efficacious extraction protocol prior to the determination of mevalonate pathway metabolites in an engineered Escherichia coli model. The statistical experimental design approach, described in this work, has successfully validated an optimised sample preparation method i.e., using acetonitrile: 50 mM ammonium formate (pH 9.5) (7:3) (ACN73) at -20 °C for 10 min without solvent evaporation to retain the targeted mevalonate metabolites in engineered E. coli strain. The study also demonstrates the use of liquid chromatography paired with a Time-of-Flight Mass Spectrometer (LC-ToF-MS) for the quantitative analysis of the mevalonate pathway metabolites in E. coli. The analytical method was validated in accordance with guidelines in Metabolomics Standards Initiative and ICH Q2 (R1) with analyte spike recoveries at 80% and above. In short, the present study overcomes the one-variable-at-a-time (OVAT) limitations in analytical development, minimises metabolite losses and gives better cost and time efficiencies by eliminating the solvent evaporation and swapping process. This work highlights the importance of analytical methods development in microbial metabolomics studies.

Co-culture of Kluyveromyces marxianus and Meyerozyma guilliermondii with In Situ Product Recovery of 2-Phenylethanol.

Production of 2-phenylethanol (2-PE) via Kluyveromyces marxianus is well-established. However, co-culture with other microbes in combination with in situ product recovery (ISPR) yields improved selectivity and volumetric productivity. Fermentation ofK. marxianus (MUCL 53775) with direct inclusion of absorptive polymer Hytrel3548 achieved ISPR, but accumulation of the byproduct phenylethyl acetate (PEA) was strongly favored. Co-culture of K. marxianus (MUCL 53775) with Meyerozyma guilliermondii (MUCL 28072) with ISPR limited PEA production, thereby improving the 2-PE selectivity from 13 to 90%, compared to a pure culture of K. marxianus (MUCL 53775) under similar conditions. This improved the volumetric productivity by 85% compared to 2-PE ISPR with a pure culture of K. marxianus. This is the first report of co-culture in a two-phase fermentation for 2-PE bioproduction and demonstrates that interactions between co-culture and ISPR techniques can modulate bioproduction between 2-PE and byproduct PEA, and this technique will be explored for other strain combinations and for other high-value molecules of interest.

Aromatic Yeasts: Interactions and Implications in Coffee Fermentation Aroma Profiles.

Nontraditional yeasts prevalent in tropical agricultural fermentations such as coffee and cocoa are known to contribute to aroma profiles, yet the functional roles and interactions between the associated microbial consortia in a farm fermentation are unclear. Here, boiled green bean extract (GBE) from green coffee beans was developed as a rich screening medium to deconstruct the microbial consortia and their interactions during the fermentation of dried green coffee beans. When cultivated in coculture with S. cerevisiae on GBE, strain-specific groupings with distinct volatile organic profiles were observed for nontraditional yeasts (e.g., Hanseniaspora spp., Pichia kudriavzevii). Further changes are evident when constructed consortia composed of nontraditional yeast, S. cerevisiae, and Lactococcus lactis var. cremoris were cultured in GBE, and a comparison with abiotically acidified GBE suggests that pH plays a major role in the influence of lactic acid bacteria (LAB) on fermentation aromas. This approach represents a tool for the development of starter culture formulations to create different flavor profiles in coffee fermentation.

Single cell protein and oil production from solid cocoa fatty acid distillates co-fed ethanol.

The use of solid lipid sidestreams have been overlooked as a feedstock for the production of microbial biomass for food and feed applications and little to no recent work has examined the utilization of solid fatty acid distillates (FADs), which are a significant residue from vegetable oil processing. Yarrowia lipolytica and Rhodosporidium toruloides cultivated on cocoa fatty acid distillates (CFAD) generated final cell dry weight values > 40 g/L, with strong productivity (3.3 g/L·h) and rich protein (>45%) and lipid content (>25%). Interestingly, microbial oils were > 65% unsaturated fatty acids, compared < 20% unsaturated content in FAD. Importantly, to overcome mass-transfer limitations associated with bioconversion of solid lipid residues, ethanol was applied as a co-substrate to solubilize FAD residues. Here, FAD residues from cocoa deodorization have been demonstrated to be high energy feedstocks that represent an attractive substrate for the production of both single cell protein and oil (SCPO).

Profiling of Aroma-Active Compounds in Ylang-Ylang Essential Oils by Aroma Extract Dilution Analysis (AEDA) and Chemometric Methods.

The aroma-active compounds in the extra, first, and third grades of ylang-ylang essential oils (YYEO) from Comoros and Madagascar were identified by gas chromatography-mass spectrometry with olfactometry (GC-MS/O) using an aroma extract dilution analysis (AEDA) technique. In the previous study, the authors investigated differences in volatile compound profiles between YYEO of different grades and regions using GC coupled with a flame ionization detector (FID) and GC-MS. This study follows up with identification of the aroma-active compounds present in YYEO of various grades from both origins and to profile the aroma of those oils. For the first time, principal component analysis (PCA) on AEDA logarithmic flavor dilution (LFD) data was performed, in comparison with the corresponding PCA on GC-FID-MS data. Based on AEDA data, 21 aroma-active compounds were found across all samples and grades of YYEO, with 8 common ones previously identified by GC-FID. Linalool had the highest odor activity and is the major component of YYEO, followed by geraniol, although the latter only appeared as a much smaller peak in the chromatogram. Other trace compounds such as eugenol and vanillin were also found to be significant to the aroma of YYEO. Using PCA on resulting LFD data, YYEO from Comoros were found to have spicier odor qualities as compared to those from Madagascar. The main contributors that determine the difference in a spicy aroma profile of Comoros and Madagascar oils are vanillin, methyl eugenol, and trans-cinnamyl acetate.

Real-time mechanistic study of carbon nanotube anion functionalisation through open circuit voltammetry.

The mechanism of the functionalisation of reduced single walled carbon nanotubes with organobromides was monitored by open circuit voltammetry throughout the reaction and further elucidated through a series of comparative reactions. The degree of functionalisation was mapped against the reagent reduction potential, degree of electron donation of substituents (Hammett parameter), and energies calculated, ab initio, for dissociation and heterolytic cleavage of the C-Br bond. In contrast to the previously assumed reduction/homolytic cleavage mechanism, the reaction was shown to consist of a rapid association of carbon-halide bond to the reduced nanotube as a complex, displacing surface-condensed countercations, leading to an initial increase in the net nanotube surface negative charge. The complex subsequently slowly degrades through charge transfer from the reduced single-walled carbon nanotube to the organobromide, utilizing charge, and the carbon-halide bond breaks heterolytically. Electron density on the C-Br bond in the initial reagent is the best predictor for degree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Both the mechanism and the new application of OCV to study such reactions are potentially relevant to a wide range of related systems.

Rectification and negative differential resistance via orbital level pinning.

A donor-acceptor system, 4-thiophenyl-azafulleroid (4TPA-C60), is investigated at the point of HOMO/LUMO resonance and beyond to understand how negative differential resistance (NDR) features may be observed in such systems. Our previous investigation showed that charge transfer between the occupied and unoccupied states at resonance hindered crossing of the HOMO and LUMO levels, thus preventing the formation of an NDR feature. In this work, it is shown that the negative differential resistance feature of 4TPA-C60 can be tailored based on the couplings at the metal/molecule interface. Ab initio calculations show that limited charge extraction from atomically sharp contacts results in a HOMO-LUMO pinning effect which delays the onset of the NDR feature. Subsequent unpinning of the states can only occur when additional charge extraction channels enter the bias window, highlighting an important role which non-frontier states play in charge transport. The proposed charge transfer mechanism is then exploited by introducing a fluorine atom into the C60 cage to tune the energies of the acceptor, and narrow the width of the current peak. These findings not only demonstrate the importance of the metal/molecule interface in the design of molecular electronic architectures but also serve to inform future design of molecular diodes and RTDs.

HOMO-LUMO coupling: the fourth rule for highly effective molecular rectifiers.

Three rules for creating highly effective unimolecular rectifiers that utilize asymmetric anchoring groups have been proposed by Van Dyck and Ratner [Ratner et al., Nano Lett., 2015, 15, 1577-1584]. This study investigates their proposed rectification mechanism in a functionalised azafullerene system (4TPA-C60) and identifies a fourth rule. NEGF-DFT shows that 4TPA-C60 fulfills the three design rules and finds that a saturated bridge is not required to fulfil the third rule, contrary to previous belief. Instead a twisted-π bridge decouples the donor and acceptor states whilst still providing a high conductance pathway. The molecular junction has a calculated rectification ratio of 145 at a bias of ±1 V and the U-type rectification mechanism is driven by the pinning of the HOMO to the LUMO when the device is forward biased, but not when reverse biased. The switching behaviour is a result of a charge dipole forming at different interfaces for different bias directions. An additional design rule is thus proposed: charge transport should allow bias dependent coupling of filled to unfilled states. The findings in this work not only help in understanding charge transport in molecular rectifiers, but also have wider implications for the design of molecular resonant tunneling devices.