Acid production, growth kinetics and aroma profiles of Lactobacillus flora from Stilton cheese.

The effect of Lactobacillus plantarum isolates from Stilton cheese on aroma profiles of milk fermentation was examined. Representative Lb. plantarum isolates were cultured alone and in combination with acid-producing and non-acid producing Lactococcus lactis NCIMB 9918 in UHT milk at 30 & 18 °C for 48 h & 12 weeks, respectively in presence and absence of salt, simulating cheese production and ripening. During long-term ripening, Lb. plantarum grew faster when co-cultured with non-acid producing Lc. lactis in the presence of salt. One isolate of Lb. plantarum produced the highest concentration of alcohols, organic acids and acetoin. Co-culture of Lb. plantarum with acid-producing Lc. lactis enhanced acid and alcohol production, whereas co-inoculation with non-acid producing Lc. lactis increased acetoin synthesis. Lb. plantarum is an incidental organism in cheese and its presence is unpredictable. Occurrence of different genotypes of Lb. plantarum could contribute to batch to batch variation in the cheese aroma characteristics.

Aroma binding and stability in brewed coffee: A case study of 2-furfurylthiol.

The aroma stability of fresh coffee brew was investigated during storage over 60 min, there was a substantial reduction in available 2-furfurylthiol (2-FFT) (84%), methanethiol (72%), 3-methyl-1H-pyrole (68%) and an increase of 2-pentylfuran (65%). It is proposed that 2-FFT was reduced through reversible chemical binding and irreversible losses. Bound 2-FFT was released after cysteine addition, thereby demonstrating that a reversible binding reaction was the dominant mechanism of 2-FFT loss in natural coffee brew. The reduction in available 2-FFT was investigated at different pH and temperatures. At high pH, the reversible binding of 2-FFT was shown to protect 2-FFT from irreversible losses, while irreversible losses led to the reduction of total 2-FFT at low pH. A model reaction system was developed and a potential conjugate, hydroxyhydroquinone, was reacted with 2-FFT. Hydroxyhydroquinone also showed 2-FFT was released after cysteine addition at high pH.

Carbohydrate utilization and the lager yeast transcriptome during brewery fermentation.

The fermentable carbohydrate composition of wort and the manner in which it is utilized by yeast during brewery fermentation have a direct influence on fermentation efficiency and quality of the final product. In this study the response of a brewing yeast strain to changes in wort fermentable carbohydrate concentration and composition during full-scale (3275 hl) brewery fermentation was investigated by measuring transcriptome changes with the aid of oligonucleotide-based DNA arrays. Up to 74% of the detectable genes showed a significant (p

In vivo flavor release from gelatin-sucrose gels containing droplets of flavor compounds.

Gelatin-sucrose gels containing the same amount of flavor compounds present as either suspended droplets or homogenously distributed in the gel (dissolved) were eaten, and the in vivo flavor release was studied using atmospheric pressure chemical ionization-mass spectrometry. The maximum intensity of release was higher from all droplet-containing samples as compared with the dissolved sample (by a factor of 4-2500-fold). When the flavor was dispersed as a greater number of smaller droplets rather than one 1 microL droplet, the intensity of in vivo release was slightly lower. The release of 16 of the flavor compounds varied in their Log P (range 0.26-4.83) and vapor pressure (Log vapor pressure ranged from -1.09 to 1.99). The differences in release for flavors present as either droplets or dissolved in the gel matrix were strongly influenced by both of these factors. This suggested a different mechanism for flavor release from droplets as compared to the classical partition mechanism established for dissolved flavors.

Volatile emission by contest losers revealed by real-time chemical analysis.

Animal interactions often involve chemical exchange but simultaneous evaluation of chemistry and behaviour has been problematical. Here we report findings from a novel method, atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) coupled with manipulation of molecular-mass achieved by rearing organisms on deuterium-enhanced nutrients. This allows real-time monitoring of the occurrence and quantity of volatile chemicals released by each of two interacting individuals, in tandem with behavioural observations. We apply these methods to female-female contests in the parasitoid wasp Goniozus legneri. We show that this species emits the spiroacetal 2-methyl-1,7-dioxaspiro[5.5]undecane. Chemical release is most common in more behaviourally aggressive contests, which occur when prior resource owners successfully resist take-over by similar-sized intruder females. Volatiles released during contests are always emitted by the loser. Aggression in contests is reduced after spiroacetal release. We suggest that the spiroacetal functions as a weapon of rearguard action. We anticipate that APCI-MS, which is rapid, non-intrusive and relatively inexpensive to operate, will be widely applied in studies linking chemistry and behaviour.

Development and validation of an APCI-MS/GC-MS approach for the classification and prediction of Cheddar cheese maturity.

Headspace techniques have been extensively employed in food analysis to measure volatile compounds, which play a central role in the perceived quality of food. In this study atmospheric pressure chemical ionisation-mass spectrometry (APCI-MS), coupled with gas chromatography-mass spectrometry (GC-MS), was used to investigate the complex mix of volatile compounds present in Cheddar cheeses of different maturity, processing and recipes to enable characterisation of the cheeses based on their ripening stages. Partial least squares-linear discriminant analysis (PLS-DA) provided a 70% success rate in correct prediction of the age of the cheeses based on their key headspace volatile profiles. In addition to predicting maturity, the analytical results coupled with chemometrics offered a rapid and detailed profiling of the volatile component of Cheddar cheeses, which could offer a new tool for quality assessment and accelerate product development.

Dynamic headspace analysis of the release of volatile organic compounds from ethanolic systems by direct APCI-MS.

Static equilibrium headspace was diluted with a stream of nitrogen to study the stability of the volatile headspace concentration. The headspace dilution profile of 18 volatile compounds above aqueous and ethanolic solutions was measured in real time using atmospheric pressure chemical ionization-mass spectrometry. Under dynamic conditions the volatiles headspace concentration above water solutions decreased readily upon dilution. The presence of ethanol helped to maintain the volatile headspace concentration when the ethanol solution concentration was above 50 mL/L. This effect was such that under dynamic conditions the absolute volatile concentration above an ethanolic solution was higher than that above an aqueous solution, contrary to results observed in equilibrium studies. The ratio of the headspace concentration of volatiles above ethanolic 120 mL/L and water solutions was correlated to their air/water partition coefficient.

Model studies on retention of added volatiles during breadcrumb production.

Breadcrumb samples were prepared with a range of volatile compounds at known concentrations. The retention of these volatiles was assessed via solvent extraction and quantification by gas chromatography-mass spectrometry. Volatile loss during processing was shown to be substantial and dependent upon the compound's vapor pressure. The influence of initial concentration levels on the retention of volatiles was linear within the bounds of the experimental concentrations (0-300 mg/kg). Comparison of volatile concentration at various stages throughout the production process (by headspace analysis) showed that the greatest losses occurred during the processing stages that involved heat, namely, microwave heating and drying. The production of samples by freeze drying showed an increased average retention of 17% as compared to fluidized bed drying and flat bed drying, which showed the highest volatile losses.

Direct coupling of supercritical fluid extraction to a gas phase atmospheric pressure chemical ionisation source ion trap mass spectrometer for fast extraction and analysis of food components.

Supercritical fluid extraction (SFE) was coupled to a custom-built atmospheric pressure chemical ionization (APCI) source of an ion trap mass spectrometer (ITMS). This method was developed for the monitoring of static or dynamic carbon dioxide extraction of volatile aroma compounds in real time. The gas phase APCI source resulted in simple spectra where the extraction of individual compounds could be monitored. A classical mint extraction was used to demonstrate the possibilities of the method. Influence of cell loading and standard injection possibilities are described. Limit of detection of the system was 0.5 nl or lower for a series of test compounds. Prospect for a fast method for extraction and quantification was raised and discussed.

Effect of beta-cyclodextrin on aroma release and flavor perception.

Binding and release of volatile compounds to and from beta-cyclodextrin were measured in model aqueous systems using static equilibrium headspace and dynamic headspace dilution. Beta-cyclodextrin decreased the static equilibrium headspace for some volatiles (e.g., ethyl octanoate and decanone) due to binding, but dilution studies demonstrated that binding was readily reversible. Dynamic release of hydrophobic volatile compounds was similar to that observed from emulsions. When beta-cyclodextrin was added to fat free yogurt, the release of a commercial lemon flavoring was modified and was similar to release from a regular fat yogurt. Sensory difference testing confirmed the release results. The data demonstrate that beta-cyclodextrin can be used to modify flavor delivery in both model and real systems; the effects in the latter are sensorially significant.

Effects of hydrocolloid thickeners on the perception of savory flavors.

The perceived intensities of savory flavors in hydrocolloid-thickened solutions were investigated using sensory paired comparison tests between two distinct thickener concentrations (high and low viscosities). The perceived saltiness of 3.5 g/L NaCl was found to be significantly reduced (P < 0.01) at the higher thickener concentration of both hydroxypropylmethyl cellulose (HPMC) and lambda-carrageenan, relative to the lower concentration. Mushroom flavor (8 ppm of 1-octen-3-ol with 3 g/L NaCl) was perceived as significantly more intense (P < 0.05) in 1.7 g/L lambda-carrageenan as compared with the same concentration of flavoring in 10.2 g/L lambda-carrageenan. Garlic flavor (2.5 ppm of diallyl disulfide with 2 g/L NaCl) was perceived to be significantly more intense in 2 g/L HPMC (P < 0.01) than in 10 g/L HPMC. However, when the NaCl concentration in the more viscous sample was increased to 3 g/L, the garlic flavor intensities of the two systems were not significantly different, suggesting a perceptual interaction (enhancement) between salt taste and garlic flavor. In vivo aroma release measurements from the same samples, using API-MS, showed that hydrocolloid concentration did not significantly alter the amount of mushroom or garlic aromas released when solutions were consumed. It was concluded that changes in perceived saltiness were driving the reduction in savory flavor perception even though the aroma stimulus was unchanged (a taste-aroma interaction). These findings parallel previous results in sweet hydrocolloid-thickened solutions.

Real-time monitoring of thermal flavor generation in skim milk powder using atmospheric pressure chemical ionization mass spectrometry.

The feasibility of monitoring volatile flavor compounds formed by thermal treatment of skimmed milk powder in real time by atmospheric pressure chemical ionization mass spectrometry (APCIMS) was established. Skim milk powder samples were heated isothermally (70 to 120 degrees C) at different moisture contents (2.2 and 12.7 g water/100 g dry solids). Headspace was sampled and analyzed continuously in full scan mode (30-180 amu) by APCIMS. The identity of the volatile compounds monitored by APCIMS was confirmed by coupled GC-EI-APCIMS. The concentration measured by the APCIMS was the net effect of three processes, namely formation of the compound, partition from the skim milk powder into the gas phase, and dilution due to the headspace sampling method used. Preliminary experiments established that the technique could follow the effects of heating temperature and moisture content on the formation of selected compounds from skim milk powder.

Modeling the partition of volatile aroma compounds from a cloud emulsion.

Parameters determining the partitioning behavior of volatile compounds between a cloud emulsion and the gas phase were measured under static equilibrium headspace conditions, using volatiles (e.g., ethyl hexanoate, cymene, and octanol) representing different volatilities and different degrees of hydrophobicity. The significant factors were the molecular characteristics of the volatile and the concentration of the oil phase. The nature of the lipid (C8 and C12 triglycerides), particle size, and emulsifier type (modified starch and gum arabic) did not significantly alter volatile partitioning. An empirical model based on the partition behavior and physicochemical parameters of 67 volatile compounds was produced. This predicted the partition of volatiles (R(2) = 0.83) in cloud emulsions as a function of lipid content. The significant terms (P < 0.05) in the empirical model were Log P, Log solubility, the dipole vector, and the oil fraction.

Monitoring chemical and physical changes during thermal flavor generation.

On-line techniques were developed to monitor chemical and physical changes occurring during the heating of skim milk powder (SMP). Atmospheric pressure chemical ionization mass spectrometry (APCIMS) followed the generation and release of volatile compounds from SMP in a packed-bed reactor. Operating conditions were optimized to avoid condensation of high boiling compounds such as maltol, and the system was highly reproducible (CV < 7%). Differential scanning calorimetry (DSC) of SMP identified a potential glass transition at an onset temperature of 67.9 degrees C and a series of exothermic events that were related to different stages of the Maillard reaction. No lactose crystallization was found after heating. Using a heated stage reflectance FTIR device, spectra were obtained at different temperatures. Analysis of the data showed a correlation between the intensity ratio at wavenumbers 1017 and 1064 cm(-1) and the glass transition measured by DSC. This FTIR system was not sensitive enough to detect Maillard intermediates. Combining data from the three techniques provides a fuller picture of the physical changes during the Maillard reaction and their effects on the chemical reactions.

Flavour generation during commercial barley and malt roasting operations: a time course study.

The roasting of barley and malt products generates colour and flavour, controlled principally by the time course of product temperature and moisture content. Samples were taken throughout the industrial manufacture of three classes of roasted product (roasted barley, crystal malt and black malt) and analysed for moisture content, colour and flavour volatiles. Despite having distinct flavour characteristics, the three products contained many compounds in common. The product concentrations through manufacture of 15 flavour compounds are used to consider the mechanisms (Maillard reaction, caramelisation, pyrolysis) by which they were formed. The use of water sprays resulted in transient increases in formation of certain compounds (e.g., 2-cyclopentene-1,4-dione) and a decrease in others (e.g., pyrrole). The study highlights rapid changes in colour and particularly flavour which occur at the end of roasting and onwards to the cooling floor. This highlights the need for commercial maltsters to ensure consistency of procedures from batch to batch.

Diversity and activities of yeasts from different parts of a Stilton cheese.

Blue cheeses are very complex food matrices presenting significant spatial differentiation between sections and the Stilton variety also has a hard brown crust making its matrix even more complex. The mycobiota communities in the three sections (blue veins, white core and outer crust) of a Stilton blue cheese were studied by employing culture-independent (TRFLP, DGGE) and culture-dependent analyses. Yeasts isolated from the cheese were studied for aroma production in a dairy model system with and without the starter Lactococcus lactis and filamentous fungus Penicillium roqueforti using SPME GC-MS. Significant qualitative and quantitative differences were observed in the yeast communities between the cheese sections with all the techniques. Yarrowia lipolytica presented strong synergistic activity with P. roqueforti enhancing the production of ketone aroma compounds, characteristic of blue cheeses. Culture techniques allowed the observation of the presence and uneven distribution of two different morphological groups of Debaryomyces hansenii in the different sections and of Trichosporon ovoides but failed to isolate Candida catenulata which dominated some parts of the cheese in the culture-independent analysis. This suggests that this species may be an important early coloniser but fails to survive into the final cheese. The study indicated that the yeast flora in the cheese sections differ including isolates that could affect their aroma profiles.

Study of the influence of yeast inoculum concentration (Yarrowia lipolytica and Kluyveromyces lactis) on blue cheese aroma development using microbiological models.

Yarrowia lipolytica and Kluyveromyces lactis occur as part of Stilton cheese microflora yet are not controlled during production. This study investigated the influence of their inoculum concentration on aroma production. Models of Y. lipolytica and K. lactis, with Penicillium roqueforti, were analysed using instrumental and sensory analysis. Different concentrations of Y. lipolytica produced important changes in the aroma profiles of microbiological models, analysed by solid-phase microextraction (SPME GC-MS). Sensory analysis with discrimination tests showed differences were detectable via human perception but did not concern the similarity to blue cheese odour. Increasing the inoculum concentration of K. lactis resulted in decreased variation between replicates. Partial least squares (PLS) regression on Flash profile data showed models inoculated with low concentrations of K. lactis exhibited blue cheese-related attributes, associated with increased ketone production. Results suggest that controlling the amount of Y. lipolytica and K. lactis during production offers potential to manipulate blue cheese aroma development.

A female-emitted pheromone component is associated with reduced male courtship in the parasitoid wasp Spalangia endius.

During courtship interactions, the courted individual may not always be prepared to mate. For example, mating or courtship may be detrimental to its fitness and resistance is expected under these circumstances. As such, various resistance strategies have evolved, from physically fending off courting individuals to producing behavioural signals of unreceptivity. In the parasitoid wasp Spalangia endius, females rarely re-mate and mated females are avoided by males in favour of virgin females. Further, mated females appear to advertise their mating status by the release of a pheromone component (methyl 6-methylsalicylate), but direct evidence of the nature of this release is lacking. Here we used real-time chemical analysis to track the emission of the pheromone component during courtship interactions between virgin males and either virgin or mated females. We found that females actively release methyl 6-methylsalicylate when courted and that significantly greater concentrations are released by previously mated females. Further, high concentrations of this component are associated with both the prevention and termination of courtship.

Effects of ethanol and long-chain ethyl ester concentrations on volatile partitioning in a whisky model system.

Ethanolic atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was used to analyze the headspace concentrations of a test set of 14 whisky volatile compounds above a series of aqueous ethanolic solutions differing in alcohol content (5-40% ABV) and with regard to concentration of ethyl hexadecanoate (0-500 mg/L). The latter was selected to represent the long-chain ethyl esters found at various concentrations in new-make spirit. Headspace ion intensities were modeled against ethanol and ethyl hexadecanoate concentrations as factors. A separate model was prepared for each compound. Not surprisingly, ethanol content in the range of 5-40% ABV had a significant effect (P < 0.0001) on headspace volatile concentrations of all volatile compounds, whereas the ethyl hexadecanoate concentration had a selective effect of reducing headspace concentrations of the more hydrophobic compounds (log P > 2.5). This finding is discussed in terms of the "structuring" effects of ethyl hexadecanoate when present above critical micelle concentration, leading to the selective incorporation of hydrophobic volatile compounds into the interior of micelle-like structures. Data presented illustrate that dilution of whiskies to 23% ABV for "nosing" in the presence of long-chain ethyl esters is likely to change the balance of volatile compounds in the headspace and thus the perceived aroma character.

Aroma release from wines under dynamic conditions.

Aroma release from wines and model ethanolic solutions during dynamic headspace dilution was measured in real time using atmospheric pressure chemical ionization-mass spectrometry. Model ethanolic solutions maintained the headspace concentration of volatile compounds close to equilibrium values during gas phase dilution over 10 min. Wine samples (with the same ethanol content) did not maintain the headspace concentration of volatiles to the same extent. Wine components and acidity ((+)-catechin, glycerol; pH 3.6) in model ethanolic solutions (120 mL/L) had no effect on the volatile headspace concentration during dynamic headspace dilution. However, in the presence of certain proteins (beta-lactoglobulin, beta-casein, bovine serum albumin), the model ethanolic solutions failed to maintain their volatile headspace concentration upon headspace dilution, but other proteins (thaumatin, mucin, lysozyme) had no effect. Thermal imaging of the model ethanolic samples (with and without beta-casein) under dynamic headspace dilution conditions showed differences in surface temperatures. This observation suggested perturbation of the ethanol monolayer at the air-liquid interface and disruption of the Marangoni effect, which causes bulk convection within ethanolic solutions. Convection carries volatile compounds and warm liquid from the bulk phase to the air-liquid interface, thus replenishing the interfacial concentration and maintaining the gas phase concentration and interfacial surface temperature during headspace dilution. It is postulated that certain proteins may exert a similar effect in wine.