Prediction of total and volatile acidity in red wines by Fourier-transform mid-infrared spectroscopy and iterative predictor weighting.

Fourier-transform mid-infrared (FT-MIR) spectroscopy, combined with partial least-squares (PLS) regression and IPW as feature selection method, was used to develop reduced-spectrum calibration models based on a few IR bands to provide near-real-time predictions of two key parameters for the characterization of finished red wines, which are essential from a quality assurance standpoint: total and volatile acidity. Separate PLS calibration models, correlating IR data (only considering those regions showing a high signal to noise ratio) with each response studied, were developed. Wavenumber selection was also performed applying IPW-PLS to take into account only significant predictors, in an attempt to improve the quality of the final models constructed. Using both PLS and IPW-PLS regression, prediction of the two responses modelled was performed with very high reliability, with RMSECV and RMSEP values on the order of 1% (comparable in terms of accuracy to the results provided by the respective reference analysis methods). An important advantage derived from the application of the IPW-PLS method had to do with the low number of original variables needed for modelling both total acidity (22 significant wavenumbers) and volatile acidity (only 11 selected predictor variables), in such a way that variable selection contributed to enhance the stability and parsimony properties of the final calibration models. The high quality of the calibration models proposed encourages the feasibility of implementing them as a fast and reliable tool in routine analysis for the determination of critical parameters for wine quality.

Robustness test of a headspace solid-phase microextraction method for the determination of chloroanisoles and chlorophenols related to cork taint in wine using experimental design.

A robustness test of a solid-phase microextraction-based method optimised for the simultaneous determination of chloroanisoles and acetyl-chlorophenols implicated in the presence of corky taste in wine has been carried out using a hybrid experimental design. The influence of small changes around the nominal level of four factors (Vs/Vt ratio, extraction temperature, exposure time and sample incubation time) on the measured response were evaluated in order to indicate if the method is robust for the experimental range considered. Moreover, it was also necessary to identify the critical parameters in the validated model in order to keep them under strict control. Experimental design provides an effective approach for robustness testing as a part of the analytical method validation.

Monitoring of substrate and product concentrations in acetic fermentation processes for onion vinegar production by NIR spectroscopy: value addition to worthless onions.

Wastes and by-products of the onion-processing industry pose an increasing disposal and environmental problem and represent a loss of valuable sources of nutrients. The present study focused on the production of vinegar from worthless onions as a potential valorisation route which could provide a viable solution to multiple disposal and environmental problems, simultaneously offering the possibility of converting waste materials into a useful food-grade product and of exploiting the unique properties and health benefits of onions. This study deals specifically with the second and definitive step of the onion vinegar production process: the efficient production of vinegar from onion waste by transforming onion ethanol, previously produced by alcoholic fermentation, into acetic acid via acetic fermentation. Near-infrared spectroscopy (NIRS), coupled with multivariate calibration methods, has been used to monitor the concentrations of both substrates and products in acetic fermentation. Separate partial least squares (PLS) regression models, correlating NIR spectral data of fermentation samples with each kinetic parameter studied, were developed. Wavelength selection was also performed applying the iterative predictor weighting-PLS (IPW-PLS) method in order to only consider significant spectral features in each model development to improve the quality of the final models constructed. Biomass, substrate (ethanol) and product (acetic acid) concentration were predicted in the acetic fermentation of onion alcohol with high accuracy using IPW-PLS models with a root-mean-square error of the residuals in external prediction (RMSEP) lower than 2.5% for both ethanol and acetic acid, and an RMSEP of 6.1% for total biomass concentration (a very satisfactory result considering the relatively low precision and accuracy associated with the reference method used for determining the latter). Thus, the simple and reliable calibration models proposed in this study suggest that they could be implemented in routine applications to monitor and predict the key species involved in the acetic fermentation of onion alcohol, allowing the onion vinegar production process to be controlled in real time.

Raman spectroscopy for wine analyses: A comparison with near and mid infrared spectroscopy.

Routine wine analysis are commonly employed to ensure the quality and safety standards, and to meet consumers' demands and legal requirements. In the last decades, efforts have been done in order to replace the traditional analytical techniques by vibrational spectroscopic techniques such as near infrared (NIR) and mid infrared (MIR) spectroscopy. The potential of these techniques has already been proved by several studies that revealed their ability for the determination of several wine parameters with high levels of precision and accuracy. Raman spectroscopy, (which is also a vibrational technique), was much less explored in the wine industry. In this work, the ability of Raman spectroscopy for routine wine analysis was evaluated and compared to NIR and MIR spectroscopy. Several calibration models were developed aiming the quantitative assessment of alcoholic strength, density, total acidity, volatile acidity, total sugars and pH in white wines. For this purpose, partial least squares (PLS) regression was employed, enabling the correlation between reference results and spectral information obtained by NIR, MIR and Raman spectroscopy. Results revealed the better performance of MIR spectroscopy for the measurement of alcoholic strength (R2P = 0.99, RMSEP=1.77%, and RER=56.41), and total acidity (R2P = 0.98, RMSEP=2.02%, and RER=49.46). Raman spectroscopy was pointed out as the most suitable for the determination of total sugars (R2P = 0.97, RMSEP=5.12%, RER=19.52), and pH (R2P = 0.90, RMSEP=4.92%, RER=20.34). The three techniques presented similar results in what referred the assessment of density (R2P = 0.96, 0.98, and 0.97, RMSEP=4.72%, 3.90%, and 3.80%, for Raman, MIR, and NIR respectively). None of the three techniques seemed to be suitable for the accurate determination of volatile acidity (R2P <0.78, RMSEP>14.32%, and RER<6.98).

Multiple headspace solid-phase microextraction for eliminating matrix effect in the simultaneous determination of haloanisoles and volatile phenols in wines.

This paper proposes a multiple headspace solid-phase microextraction (MHS-SPME) method coupled to gas chromatography-tandem mass spectrometry detection (GC/MS/MS) for the simultaneous determination of 2,4,6-trichloroanisole, 2,3,4,6-tetrachloroanisole, pentachloroanisole, 2,4,6-tribromoanisole, 4-ethylphenol, 4-ethylguaiacol, 4-vinylphenol and 4-vinylguaiacol in wines. These compounds are involved in the presence of "cork taint" and Brett character in wines. The MHS-SPME method is a modification of SPME developed for quantitative analysis that avoids possible matrix effects based on an exhaustive analyte extraction from the sample. After demonstrating the existence of matrix effect in the analysis of the target compounds by HS-SPME with a divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fibre, the MHS-SPME method was developed and validated. The proposed method showed satisfactory linearity, precision and detection limits, all below the odour detection thresholds of the compounds in wine matrices. Good recoveries were observed for all compounds, always above 90%, and the repeatability obtained was considered acceptable, ranging between 2 and 11%. After checking the applicability of the method by comparing the results recorded with those obtained with the standard addition method, the method was applied successfully to the analysis of wine samples. To our knowledge, this is the first time that MHS-SPME combined with GC/MS/MS has been applied to simultaneously determine haloanisoles and volatile phenols in wine.

Determination of Brett character responsible compounds in wines by using multiple headspace solid-phase microextraction.

This study presents a method based on the use of multiple headspace solid-phase microextraction (MHS-SPME) for the quantitative analysis of 4-ethylphenol, 4-ethylguaiacol, 4-vinylphenol and 4-vinylguaiacol. MHS-SPME is a modification of SPME that implies several consecutive extractions from the same sample and avoids possible matrix effects. This study demonstrates the existence of a matrix effect in the analysis of compounds responsible for Brett character in wine when an HS-SPME based method is used with a carbowax/divinylbenzene (CW/DVB) fibre. For this reason, MHS-SPME is proposed as an alternative technique with respect to HS-SPME. The method proposed was validated and the detection limits obtained were 0.06 microg/l for 4-ethylguaiacol and 4-ethylphenol and, 0.20 microg/l for 4-vinylguaiacol and 0.12 microg/l for 4-vinylphenol. These detection limits are below the odour detection thresholds of the compounds in wine matrices. The repeatability obtained, in terms of relative standard deviation (RSD), was considered acceptable, ranging from 1 to 12%. To evaluate the applicability of the proposed MHS-SPME method, concentration results were compared with those obtained with the standard addition method, and the results were similar with both methods. Furthermore, the new method was satisfactorily applied to a number of commercial red, white and rosé wines. Therefore, MHS-SPME can be considered as an alternative to avoid the matrix effect in wine samples.

Optimisation of a headspace solid-phase microextraction with on-fiber derivatisation method for the direct determination of haloanisoles and halophenols in wine.

A solid-phase microextraction (SPME) procedure for the determination of four haloanisoles (2,4,6-trichloroanisole, 2,3,4,6-tetrachloroanisole, pentachloroanisole and 2,4,6-tribromoanisole), as well as their precursor halophenols (2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol, pentachlorophenol and 2,4,6-tribromophenol), involved in the presence of cork taint in wine, was developed. Firstly, analytes were concentrated on a SPME fiber, and then halophenols were derivatised using N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA). The compounds were desorbed for 5 min in the gas chromatography injector port and then determined with an electron capture detector. The influence of different parameters on the efficiency of extraction (volume of sample, type of fibre coating and time) and derivatisation (time, temperature and volume of MSTFA) steps was evaluated. Polyacrylate (PA) was selected as the extraction fiber, optimised parameters for SPME were 10 ml of wine, temperature 70 degrees C and extraction time 60 min. The optimal conditions identified for the derivatisation step were temperature 25 degrees C, reagent volume 50 microl and extraction time 25 min. Under optimal conditions, the proposed method showed satisfactory linearity, precision and detection limits. The method was applied successfully to the analysis of red wine samples. To our knowledge, this is the first time that headspace (HS) SPME combined with on-fiber derivatisation has been applied to determine cork taint responsible compounds in wine.

Optimisation of a microwave-assisted extraction method for the simultaneous determination of haloanisoles and halophenols in cork stoppers.

This study presents a method based on the use of microwave-assisted extraction (MAE) for the quantitative analysis of 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), pentachloroanisole (PCA), 2,4,6-tribromoanisole (TBA), 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP), pentachlorophenol (PCP) and 2,4,6-tribromophenol (TBP) in cork stoppers. The influential parameters of the MAE procedure (extraction time, temperature and solvent volume) were optimised using a central composite experimental design combined with desirability functions. The optimal conditions identified were temperature 170 degrees C, solvent volume 35 mL and extraction time 90 min. MAE extracts were concentrated and derivatised prior to separation and quantification by gas chromatography with electron capture detection. To evaluate the applicability of the proposed MAE method, recovery results were compared with those obtained with the Soxhlet extraction method; the results were similar with both extraction methods. The new method was also satisfactorily applied to real cork stopper samples.

Merging vibrational spectroscopic data for wine classification according to the geographic origin.

The wine making procedure is no longer a secret and it is nowadays well described and repeated around the world. Nevertheless, wines present unique features, strongly associated with their geographic origin. Classification systems were developed to catalogue wines according to the provenance, and are currently established by official authorities in order to ensure wine authenticity. The use of near-infrared (NIR), mid-infrared (MIR) and Raman spectroscopy for tracing the origin of wine samples, has been reported with different levels of success. This work evaluated and compared the performance of these techniques, as well as their joint use, in terms of geographic origin classification. NIR, MIR and Raman spectra of wine samples belonging to four Portuguese wine regions (Vinhos Verdes, Lisboa, Açores and Távora-Varosa) were analyzed by partial least squares discriminant analysis (PLS-DA). Results revealed the better suitability of MIR spectroscopy (87.7% of correct predictions) over NIR (60.4%) and Raman (60.8%). The joint use of spectral sets did not improve the predictive ability of the models. The best models were achieved by combining MIR and NIR spectra resulting in 86.7% of correct predictions. Multiblock partial least squares (MB-PLS) models were developed to further explore the combination of spectral data. Although these models did not improve the percentage of correct predictions, they demonstrated the higher contribution of MIR spectroscopic data, in the development of the models.

Multiple response optimisation based on desirability functions of a microwave-assisted extraction method for the simultaneous determination of chloroanisoles and chlorophenols in oak barrel sawdust.

A microwave-assisted extraction (MAE) method was optimised for extracting 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), pentachloroanisole (PCA), 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP) and pentachlorophenol (PCP) from oak barrels. The method was optimised by using a central composite experimental design with extraction time, temperature and solvent volume as influential parameters. A desirability function was then employed in addition to the simultaneous optimisation of the compounds. The optimal conditions identified were temperature 130 degrees C, solvent volume 35 mL and extraction time 50 min. The compounds were determined by gas chromatography with electron-capture detection. MAE was compared with conventional Soxhlet extraction; the results obtained with MAE were in good agreement with those obtained by Soxhlet extraction.

A simple and sensitive vortex assisted liquid-liquid microextraction method for the simultaneous determination of haloanisoles and halophenols in wines.

A vortex assisted liquid-liquid microextraction (VALLME) method was developed and optimised for the determination of the main compounds that can cause cork taint in wines, 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), 2,4,6-tribromoanisole (TBA) and pentachloroanisole (PCA); and their corresponding halophenolic precursors. Target compounds were determined by gas chromatography combined with a micro-electron capture detector (GC-µECD) system. Halophenol extraction and derivatisation processes were performed at the same time. To optimise the VALLME method, the extraction solvent was selected. Then, the other parameters of influence, such as volume of extraction solvent and derivatisation agent, salt addition and vortex time were optimised using a central composite design combined with desirability functions. Once the optimal conditions had been determined, the method was validated, showing satisfactory linearity (with correlation coefficients over 0.983), repeatability (below 10.0%) and reproducibility (below 11.2%). Detection limits obtained were lower than the olfactory threshold of the studied compounds, being similar or even lower than previously reported with the advantage of reducing the extraction time. The analysis of real wine samples demonstrated the applicability of the method. To our knowledge, this is the first time that VALLME has been applied for the simultaneous determination of haloanisoles and halophenols in wine.

Experimental design approach to evaluate the impact of oak chips and micro-oxygenation on the volatile profile of red wines.

A chemometric strategy based on combining an experimental design approach and response surface methodology was applied to gain further knowledge on the influence of chip maceration and micro-oxygenation related factors (oxygen doses, chip doses, wood origin, toasting degree and maceration time) on the volatile profile of red wines during the accelerated ageing process. The results obtained indicated that the volatile profile of wines could be modulated by applying different combinations of factor conditions. Thus, these results would be used to obtain wines with specific volatile profiles that would lead to particular olfactory attributes according to consumers' preferences. Moreover, it was shown that combining wood from different origins helped enhance the quality of the elaborated wines. To the best of our knowledge, this is the first time that an experimental design methodology has been applied to simultaneously evaluate the influence of five different ageing parameters on the volatile profile of red wines.

Modulation of the phenolic composition and colour of red wines subjected to accelerated ageing by controlling process variables.

The aim of the present work was to evaluate the effect of the main factors conditioning accelerated ageing processes (oxygen dose, chip dose, wood origin, toasting degree and maceration time) on the phenolic and chromatic profiles of red wines by using a multivariate strategy based on experimental design methodology. The results obtained revealed that the concentrations of monomeric anthocyanins and flavan-3-ols could be modified through the application of particular experimental conditions. This fact was particularly remarkable since changes in phenolic profile were closely linked to changes observed in chromatic parameters. The main strength of this study lies in the possibility of using its conclusions as a basis to make wines with specific colour properties based on quality criteria. To our knowledge, the influence of such a large number of alternative ageing parameters on wine phenolic composition and chromatic attributes has not been studied previously using a comprehensive experimental design methodology.

Classification of Spanish extra virgin olive oils by data fusion of visible spectroscopic fingerprints and chemical descriptors.

The potential of visible fingerprints and physical-chemical parameters in combination with multivariate data analysis was examined to classify extra virgin olive oils (EVOOs) from different Spanish regions according to their geographical origin. Firstly, spectral and quality parameters matrices were processed separately and subsequently were joined to evaluate the effect of synergy on the information obtained from the different methodologies. Linear discriminant analysis (LDA) and partial least squares discriminant analysis (PLS-DA) were performed as classification methods. The results revealed a perfect discrimination between the defined categories after performing PLS-DA on the Fused matrix, reaching 100% of correct classifications and showed a clear improvement in the overall prediction rates (92.5%), so that the effect of synergy was confirmed. These accurate results emphasise the feasibility of the proposed strategy and encourage the development of similar approaches based on visible spectroscopy in olive oil quality and traceability determination.

Simultaneous determination of cork taint and Brett character responsible compounds in wine using ultrasound-assisted emulsification microextraction with solidification of floating organic drop.

In this study, ultrasound-assisted emulsification microextraction combined with solidification of floating organic drop method (USAEME-SFOD) has been proposed as a novel approach for the sensitive determination of haloanisoles and volatile phenols in wines. For this purpose, the influence of the different parameters affecting the procedure (type and volume of extraction solvent, temperature, time and ionic strength) was evaluated in order to optimise the efficiency of the process. Subsequently, the linearity, detection and quantification limits, precision, recoveries and applicability to real samples were studied, obtaining excellent method performance results. Moreover, USAEME-SFOD was compared with other liquid-liquid microextraction methods such as dispersive liquid-liquid microextraction (DLLME) and ultrasound-assisted emulsification microextraction (USAEME). This comparison study proved the suitability of USAEME-SFOD as an alternative to previously reported methods for the simultaneous determination of cork taint and Brett character responsible compounds in wines.

Optimisation of ultrasound-assisted emulsification microextraction method with solidification of floating organic drop for the analysis of cork taint responsible compounds in wine.

This paper presents the optimisation of a method based on ultrasound assisted-emulsification method (USAEME) combined with solidification of floating organic drop (SFOD) for the sensitive determination of cork taint responsible compounds in wine. Haloanisoles (2,4,6-trichloranisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), 2,4,6-tribromoanisole (TBA) and pentachloranisole (PCA)) and their halophenolic precursors were determined by gas chromatography with electron capture detection (GC-ECD). The derivatisation of the halophenols was performed simultaneously with the extraction. To perform the optimisation, the main parameters affecting both extraction and derivatisation were exhaustively studied in different steps. Once the extraction solvent and the pH and time conditions had been determined, experimental design was applied for the simultaneous study of the volumes of acetic anhydride and extraction solvent, temperature and ionic strength conditions. Then, the analytical characteristics of the optimised method were evaluated achieving satisfactory linearity (with correlation coefficients over 0.984), repeatability (below 10.7%) and inter-day precision (below 10.6%) for all target analytes. Moreover, detection limits found were similar or even lower than the olfactory threshold of the studied compounds. Finally, the applicability of the proposed method was tested by analysing real wine samples. To the best of our knowledge, this is the first time that USAEME-SFOD method has been used to determine cork taint responsible compounds in wine.

Recognition of volatile compounds as markers in geographical discrimination of Spanish extra virgin olive oils by chemometric analysis of non-specific chromatography volatile profiles.

Chromatographic profiles obtained by headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography (GC) were processed as continuous and non-specific signals through multivariate analysis techniques in order to select and identify the most discriminate volatile marker compounds related to the geographical origin of extra virgin olive oils. The blind analysis of the chromatographic profiles was carried out on several steps including preliminary mathematical treatments, explorative analysis, feature selection and classification. The results obtained through the application of stepwise linear discriminant analysis (SLDA) method revealed a perfect discrimination between the different Spanish geographical regions considered (La Rioja, Andalusia and Catalonia). The assignment success rate was 100% in both classification and prediction by using cross validation procedure. In addition, it must be noted that the proposed strategy was able to verify the geographical origin of the samples involving only a reduced number of discriminate retention times selected by the stepwise procedure. This fact emphasizes the quality of the accurate results obtained and encourages the feasibility of similar procedures in olive oil quality and traceability studies. Finally, volatile compounds corresponding to the predictors retained were identified by gas chromatography-mass spectrometry (GC-MS) for a chemical interpretation of their importance in quality virgin olive oils.

Development of a dispersive liquid-liquid microextraction method for the simultaneous determination of the main compounds causing cork taint and Brett character in wines using gas chromatography-tandem mass spectrometry.

A novel dispersive liquid-liquid microextraction (DLLME) method, coupled to gas chromatography-tandem mass spectrometry (GC-MS/MS), was developed for simultaneously determining the main compounds responsible for cork taint (2,4,6-trichloranisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), 2,4,6-tribromoanisole (TBA) and pentachloranisole (PCA)) and Brett character (4-ethylguaiacol (EG), 4-ethylphenol (EP), 4-vinylguaiacol (VG) and 4-vinylphenol (VP)) in wines. Optimisation of DLLME procedure was performed by evaluating the type of disperser and extraction solvents and the temperature and salt addition effects. The volumes of disperser and extraction solvents were also optimised by means of a central composite design combined with desirability functions. Under optimum conditions, 5 mL of wine were extracted with an extraction mixture consisting of 1.43 mL of acetone, and 173 µL of chloroform at room temperature. The analytical characteristics of the method were evaluated. Satisfactory linearity (with correlation coefficients over 0.992), repeatability (below 11.6%) and between-days precision (below 11.0%) were obtained for all target analytes. Detection limits attained were at similar levels or even lower than the olfactory threshold of the studied compounds. Finally, the developed method was successfully applied to the analysis of wine samples. To our knowledge, this is the first time that DLLME has been applied to simultaneously determine the compounds responsible for cork taint and Brett character in wine.

Ultrasound-assisted emulsification-microextraction for the sensitive determination of Brett character responsible compounds in wines.

In this study, a method based on ultrasound-assisted emulsification-microextraction (USAEME) was optimised for the determination of Brett character responsible compounds (4-ethylguaiacol (EG), 4-ethylphenol (EP), 4-vinylguaiacol (VG) and 4-vinylphenol (VP)) in wines. For this purpose, once the extraction solvent had been selected, the influence on the extraction efficiency of ratio between extraction solvent and sample volumes, temperature, ionic strength and time was studied using experimental design methodology. The optimum conditions found for USAEME of 5mL of wine were 160µL of chlorobenzene at 60°C during 5min without salt addition. Then, the analytical performance of the optimised USAEME procedure was evaluated. For all the studied compounds, the proposed method showed correlation coefficients over 0.984. Satisfactory repeatability (below 10%) and inter-day repeatability (below 11%) were obtained at different concentration levels and the method achieved detection limits similar or even lower than previously reported for these compounds in wines. Moreover, the applicability of the method was evaluated by the analysis of real samples. To our knowledge, this is the first time that USAEME has been applied to the determination of Brett character responsible compounds in wine.

Optimisation of a simple and reliable method based on headspace solid-phase microextraction for the determination of volatile phenols in beer.

A simple, accurate and sensitive method based on headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography-tandem mass spectrometry (GC-MS/MS) was developed for the analysis of 4-ethylguaiacol, 4-ethylphenol, 4-vinylguaiacol and 4-vinylphenol in beer. The effect of the presence of CO2 in the sample on the extraction of analytes was examined. The influence on extraction efficiency of different fibre coatings, of salt addition and stirring was also evaluated. Divinylbenzene/carboxen/polydimethylsiloxane was selected as extraction fibre and was used to evaluate the influence of exposure time, extraction temperature and sample volume/total volume ratio (Vs/Vt) by means of a central composite design (CCD). The optimal conditions identified were 80 degrees C for extraction temperature, 55 min for extraction time and 6 mL of beer (Vs/Vt 0.30). Under optimal conditions, the proposed method showed satisfactory linearity (correlation coefficients between 0.993 and 0.999), precision (between 6.3% and 9.7%) and detection limits (lower than those previously reported for volatile phenols in beers). The method was applied successfully to the analysis of beer samples. To our knowledge, this is the first time that a HS-SPME based method has been developed to determine simultaneously these four volatile phenols in beers.