Microextraction techniques in the analysis of food flavor compounds: A review

Food flavor compounds due to the complexity of food as a matrix, and usually their very low concentrations in a product, as well as their low odor thresholds, create a challenge in their extraction, separation and quantitation. Food flavor volatiles represent compounds of different polarity, volatility and chemical character, which determine method of extraction for their isolation from food. Microextraction techniques, mainly SPME and SBSE have been used for food flavor compounds analysis for two decades. Microextraction methods other than SPME and SBSE are seldom used despite their analytical potential. The review discusses the nature of food flavor compounds, and different approaches to food flavor analysis. It summarizes the use of microextraction methods in food flavor compounds analysis based on papers published in the last 5 years, and discusses the potential of microextraction methods in this field.     

Enantioselective gas chromatography in flavor and fragrance analysis: strategies for the identification of known and unknown plant volatiles

This review describes current analytical technology for the analysis of chiral constituents in essential oils and other natural volatiles, flavor and fragrance compounds, and covers some important results achieved by natural compound chemists, food chemists, perfumers, and molecular biologists. The technique of enantioselective gas chromatography (GC) is described and applied for assigning absolute configuration of chiral natural compounds, which is strongly connected to differences in odor properties of their enantiomers. In addition, some recent results to facilitate the handling of GC-mass spectrometry data of known and unknown plant volatiles are discussed.     

Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of garlic oil obtained by hydrodistillation

Garlic (Allium sativum L.) is highly consumed worldwide. This crop is mainly known for its flavor and odor, although the many medicinal properties that are attributed to it, including anticarcinogenic, antiatherosclerotic, and antithrombotic potential, among several others, have called the attention of scientists since very early times. It is known that sulfur-containing volatiles are the principal compounds responsible for such properties. The aims of this work were to develop a solventless extraction method for sulfur-containing volatiles from garlic, as well as their chemical characterization. Since garlic volatiles are rather thermolabile, low-pressure hydrodistillation was chosen as the extracting method. The analysis of all compounds was performed on an HP-FFAP chromatographic column mounted in a GC-MS system. For volatile transfer and injection method, solid-phase microextraction was selected, with the use of eight different fibers. The most abundant volatile compound was diallyl disulfide, followed by diallyl trisulfide. Among the 47 totally identified compounds, 18 were linear sulfur-containing volatile compounds, 6 were of non-sulfur nature, and the other 23 were cyclic compounds. However, linear sulfur volatiles accounted for 94% of the total amount.     

Monitoring volatile compounds during dry-cured ham ripening by solid-phase microextraction coupled to a new direct-extraction device

Key flavour volatile compounds were monitored during ripening of dry-cured ham by solid-phase microextraction (SPME) coupled to a new direct-extraction device (DED). DED allows the insertion of the SPME fibre into the core of solid materials with no damage to the fibre. This enables extraction of volatiles from solid foodstuffs while avoiding sample handling. Major groups of volatile compounds extracted with SPME-DED agreed with available scientific literature about dry-cured ham volatiles. Moreover, volatile compounds previously highlighted as quality markers in dry-cured ham, such as 3-methylbutanal or hexanal, were satisfactorily extracted using SPME-DED. Changes in the profile of volatile compounds throughout the processing followed a typical pattern of volatile compounds formation. Therefore, SPME-DED appears as a new and promising method for monitoring ripening of dry-cured hams with no depreciation of the product, which might substitute traditional subjective methods currently used in the ham processing industry. However, the use of the internal standard method is not possible with this technique. Therefore, results using SPME-DED only point out a trend in the volatile profile. Further attempts relating data obtained using SPME-DED in dry-cured hams with sensory and chemical data from the same samples would be necessary for optimising this method as a quality control method in dry-cured ham industries.     

Headspace solid phase microextraction and gas chromatography-quadrupole mass spectrometry methodology for analysis of volatile compounds of marine salt as potential origin biomarkers

The establishment of geographic origin chemical biomarkers for the marine salt might represent an important improvement to its valorisation. Volatile compounds of marine salt, although never studied, are potential candidates. Thus, the purpose of this work was the development of a headspace solid phase microextraction (SPME) combined with gas chromatography-quadrupole mass spectrometry (HS-SPME/GC-qMS) methodology to study the volatile composition of marine salt. A 65 μm carbowax/divinylbenzene SPME coating fibre was used. Three SPME parameters were optimised: extraction temperature, sample quantity, and presentation mode. An extraction temperature of 60 °C and 16 g of marine salt in a 120 mL glass vial were selected. The study of the effect of sample presentation mode showed that the analysis of an aqueous solution saturated with marine salt allowed higher extraction efficiency than the direct analysis of salt crystals. The dissolution of the salt in water and the consequent effect of salting-out promote the release of the volatile compounds to the headspace, enhancing the sensitivity of SPME for the marine salt volatiles. The optimised methodology was applied to real matrices of marine salt from different geographical origins (Portugal, France, and Cape Verde). The marine salt samples contain ca. 40 volatile compounds, distributed by the chemical groups of hydrocarbons, alcohols, phenols, aldehydes, ketones, esters, terpenoids, and norisoprenoids. These compounds seem to arise from three main sources: algae, surrounding bacterial community, and environment pollution. Since these volatile compounds can provide information about the geographic origin and saltpans environment, this study shows that they can be used as chemical biomarkers of marine salt.     

Yeast Fermentation at Low Temperatures: Adaptation to Changing Environmental Conditions and Formation of Volatile Compounds

Yeast plays a key role in the production of fermented foods and beverages, such as bread, wine, and other alcoholic beverages. They are able to produce and release from the fermentation environment large numbers of volatile organic compounds (VOCs). This is the reason for the great interest in the possibility of adapting these microorganisms to fermentation at reduced temperatures. By doing this, it would be possible to obtain better sensory profiles of the final products. It can reduce the addition of artificial flavors and enhancements to food products and influence other important factors of fermented food production. Here, we reviewed the genetic and physiological mechanisms by which yeasts adapt to low temperatures. Next, we discussed the importance of VOCs for the food industry, their biosynthesis, and the most common volatiles in fermented foods and described the beneficial impact of decreased temperature as a factor that contributes to improving the composition of the sensory profiles of fermented foods.     

Comparison of solid-phase microextraction and purge-and-trap methods for the analysis of the volatile fraction of butter

The volatile fraction of butter stored at three different temperatures was investigated to monitor quality during commercial shelf-life (90 days). Two different extraction techniques were compared: dynamic headspace (purge-and-trap), and static headspace (solid-phase microextraction, SPME). As expected, the dynamic extraction provided a generally higher amount of volatile compounds than that obtained by SPME, but, with reference to individual compounds, SPME seemed to provide better extraction for volatiles having a higher molecular mass. Despite the different performances, both methods were able to detect volatiles useful for evaluating changes during storage.     

Metabolite profiling on apple volatile content based on solid phase microextraction and gas-chromatography time of flight mass spectrometry

A headspace SPME GC-TOF-MS method was developed for the acquisition of metabolite profiles of apple volatiles. As a first step, an experimental design was applied to find out the most appropriate conditions for the extraction of apple volatile compounds by SPME. The selected SPME method was applied in profiling of four different apple varieties by GC-EI-TOF-MS. Full scan GC-MS data were processed by MarkerLynx software for peak picking, normalisation, alignment and feature extraction. Advanced chemometric/statistical techniques (PCA and PLS-DA) were used to explore data and extract useful information. Characteristic markers of each variety were successively identified using the NIST library thus providing useful information for variety classification. The developed HS-SPME sampling method is fully automated and proved useful in obtaining the fingerprint of the volatile content of the fruit. The described analytical protocol can aid in further studies of the apple metabolome.     

Determination of volatile compounds in cider spirits by gas chromatography with direct injection

Two analytical methods based on gas chromatography with direct injection are described for the quantitative analysis of volatile compounds (acetals, aldehydes, esters, alcohols, and volatile phenols) in cider brandies. Analytes were divided into major, 15, and minor volatile, 24, compounds depending on their usual concentration in samples. Parameters usually tested for method validation are evaluated. Correlation coefficients are calculated to estimate linearity, obtaining values higher than 0.999. Detection limits range between 0.325 mg/L (1-propanol) and 1.663 mg/L (methanol) among the major volatile compounds and between 0.086 mg/L (ethyl 2-methylbutyrate) and 0.332 mg/L (ethyl tetradecanoate) among the minor volatiles. Mean recoveries ranged between 109% (ethyl lactate) and 95% (1-butanol) for major volatiles and between 109% (1-octen-3-ol) and 94% (ethyl 2-methylbutyrate) for minor volatiles, thus confirming the accuracy of both methods. Reproducibility for major volatiles is < 5.4% (furfural) in all cases and < 9.6% (hexyl acetate) for minor volatiles. Moreover, the accuracy of the methods is evaluated by analyzing a certified whisky and five samples from interlaboratory assays, generally obtaining results in accordance with previous values.     

Comparison of Different Volatile Extraction Methods for the Identification of Fishy Off-Odor in Fish By-Products

This study was conducted to analyze volatile odor compounds and key odor-active compounds in the fish soup using fish scarp and bone. Five extraction methods, including solid-phase microextraction (SPME), dynamic headspace sampling (DHS), solvent-assisted flavor evaporation (SAFE), stir bar sorptive extraction (SBSE), liquid-liquid extraction (LLE), were compared and SPME was finally selected as the best extraction method for further study. The volatile odor compounds were analyzed by gas chromatography-olfactometry-mass spectrometry (GC-O-MS) and comprehensive two-dimensional gas chromatography-olfactometry-mass spectrometry (GC × GC-O-MS) techniques, and the key odor-active compounds were identified via aroma extract dilution analysis (AEDA) and relative odor activity value (r-OAV) calculation. A total of 38 volatile compounds were identified by GC-O-MS, among which 10 were declared as odor-active compounds. Whereas 39 volatile compounds were identified by GC × GC-O-MS, among which 12 were declared as odor-active compounds. The study results revealed that 1-octen-3-one, 2-pentylfuran, (E)-2-octenal, 1-octen-3-one, hexanal, 1-octen-3-ol, 6-methylhept-5-en-2-one, (E,Z)-2,6-nondienal and 2-ethyl-3,5-dimethylpyrazine were the key odor-active compounds in the fish soup.     

Headspace solid-phase microextraction–gas chromatography–mass spectrometry for profiling free volatile compounds in Cabernet Sauvignon grapes and wines

The complex aroma of wine is derived from many sources, with grape-derived components being responsible for the varietal character. The ability to monitor grape aroma compounds would allow for better understanding of how vineyard practices and winemaking processes influence the final volatile composition of the wine. Here, we describe a procedure using GC–MS combined with headspace solid-phase microextraction (HS-SPME) for profiling the free volatile compounds in Cabernet Sauvignon grapes. Different sample preparation (SPME fiber type, extraction time, extraction temperature and dilution solvent) and GC–MS conditions were evaluated to optimize the method. For the final method, grape skins were homogenized with water and 8 ml of sample were placed in a 20 ml headspace vial with addition of NaCl; a polydimethylsiloxane SPME fiber was used for extraction at 40 °C for 30 min with continuous stirring. Using this method, 27 flavor compounds were monitored and used to profile the free volatile components in Cabernet Sauvignon grapes at different maturity levels. Ten compounds from the grapes, including 2-phenylethanol and β-damascenone, were also identified in the corresponding wines. Using this procedure it is possible to follow selected volatiles through the winemaking process.     

Analysis of virgin olive oil volatile compounds by headspace solid-phase microextraction coupled to gas chromatography with mass spectrometric and flame ionization detection

The efficiency of headspace solid-phase microextraction (SPME) was evaluated for the qualitative and semi-quantitative analysis of virgin olive oil volatile compounds. The behaviour of four fibre coatings was compared for sensitivity, repeatability and linearity of response. A divinylbenzene-Carboxen-polydimethylsiloxane fibre coating was found to be the most suitable for the analysis of virgin olive oil volatiles. Sampling and chromatographic conditions were examined and the SPME method, coupled to GC with MS and flame ionization detection, was applied to virgin olive oil samples. More than 100 compounds were isolated and characterised. The presence of some of these compounds in virgin olive oil has not previously been reported. The main volatile compounds present in the oil samples were determined quantitatively.     

Volatile flavoring substances in foodstuffs

The odor of most foods is produced by mixtures of many volatile flavoring substances. Over the past century, numerous food flavors have been analyzed in sufficient detail to allow a survey of the classes of substances responsible, the formation of individual flavoring substances, and their fixation in the foods. In addition to special biosynthetic pathways, the breakdown of fats and the Maillard reaction have been found to be particularly important to the formation of flavoring substances.     

Investigation of volatile biomarkers in liver cancer blood using solid-phase microextraction and gas chromatography/mass spectrometry

Solid-phase microextraction (SPME) followed by gas chromatography/mass spectrometry (GC/MS) was used for the detection of liver cancer volatile biomarkers. Headspace SPME conditions (fiber coating, extraction temperature and extraction time) and desorption conditions were optimized and applied to the determination of volatiles in human blood. Between the liver cancer group (n = 19) and the normal group (n = 18), positive rates of 19 volatile compounds among the total of 47 detected were found to be different with statistical significance (p < 0.05, chi-squared test). We suggested hexanal, 1-octen-3-ol and octane, of the 19 compounds, as biomarkers of liver cancer with clinical diagnostic value. The sensitivity and specificity of 94.7% and 100% for hexanal, 84.2% and 100% for 1-octen-3-ol, and 89.5% and 100% for octane were obtained, respectively, after the cutoff values had been properly established. These results show that SPME-GC/MS is a simple, rapid and sensitive method for the investigation of volatile disease markers in human blood.     

Evaluation of two commercial solid-phase microextraction fibres for the analysis of target aroma compounds in cooked beef meat

The aroma profile of cooked beef meat has been investigated by solid-phase microextraction (SPME) combined with gas chromatography-mass spectrometry (GC-MS). Out of more than 200 volatile compounds, 36 key odour-active compounds were selected for analysis. Several extraction times, desorption times, temperature conditions and fibre types were tested to achieve a fast and reproducible extraction, and a representative analysis of the aroma profile of cooked beef. Extraction conditions and fibre type significantly affected the majority of the target compounds. Divinylbenzene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS) fibre presented a better reproducibility at all extraction times and extracted more efficiently the less volatile compounds than the carboxen-polydimethylsiloxane (CAR-PDMS) fibre. The high molecular weight compounds seemed to achieve faster the equilibrium between the headspace and DVB-CAR-PDMS fibre. The use of SPME was shown to be a simple, sensitive, selective, representative, fast, and low-cost method for the evaluation of key odour-active compounds in cooked beef meat, even if further research on quantitative analysis of volatiles using SPME on solid samples has to be done.     

Melt Extrusion Encapsulation of Flavors: A Review

Encapsulation of flavor and aroma compounds has been largely explored in order to meet appraisal demands from consumers by improving the impact of flavor during the consumption of food products. Even though several techniques have been used for encapsulating volatile compounds, i.e., spray drying, fluidized bed coating, coacervation, and melt extrusion, those most frequently used in the food industry are spray drying and melt extrusion. In this article, the different techniques of encapsulation of flavors and fragrances in polymer-based matrices by extrusion are reviewed and partly re-defined, emphasizing the differences between the various techniques reported so far and the role of matrix types, additives, and operative conditions. Also, the role of water as a key parameter for controlled release and shelf stability of the delivery system will be discussed.     

Solid-phase extraction and determination of trace aroma and flavour components in cider by GC-MS

Summary Minor volatile compounds are responsible for the aromas of cider. A simple technique for the analysis of these components is described based on solid-liquid phase extraction followed by quantitation by gas chromatography-mass spectrometry (GC-MS). The method is quantitative for analysis of alcohols, esters, lactones, phenols, and medium and long chain-length fatty acid.     

Optimization and Preparation of Tallow with a Strong Aroma by Mild Oxidation

This study was performed to extract and separate the volatiles with solid-phase microextraction (SPME), and was conducted to analyze volatile odor compounds qualitatively and quantitatively in the production of a strong aroma tallow by mild oxidation. A total of 51 odor compounds were detected in the tallow smelted under different conditions. It was found that the high proportion of aldehydes was an important feature of the aroma components in the oxidized melted tallow, such as 1-hexanal, heptanal, nonanal, octanal, benzaldehyde, etc. Through the determination of various indicators, sensory evaluation, and the gas chromatography-olfaction–mass spectrometry (GC-O–MS) analysis and, in combination with response surface methodology, the optimal process parameters for oxidative smelting of tallow were determined as follows: a reaction temperature of 149.61 °C, a reaction time of 31.68 min, and an airflow rate of 97.44 L/h. The accelerated oxidation test further verified the quality of the oxidized tallow.     

Assessment of flavor volatiles of modified Iranian rice cultivars during the gelatinization process

A combined GC-MS with the headspace solid-phase microextraction (SPME) method has been employed for the analysis of the flavor volatiles of two modified Iranian rice cultivars during gelatinization. In order to optimize the different experimental parameters, the effect of fiber composition, water content of the rice samples, and equilibrium time were investigated. As a result, while gelatinization progresses, the amount of volatile compounds would increase as well. Therefore, a broad range of the flavor volatiles of rice could be extracted, concentrated, and identified. Altogether, 54 and 66 components were identified for HD5 and HD6 rice samples, respectively, of which 33 unique compounds were not detected previously. The identified volatile components in the modified cultivars belong to the chemical classes of aldehydes, ketones, alcohols, and heterocyclic compounds, phenolic compounds, and hydrocarbons.