Off-flavor in soy drink: Development, optimization, and validation of an easy and fast method to quantify the key odorants
Graphical abstract
Introduction
Soy drink is becoming a popular substitute for dairy milk in Western countries due to its plant-based high-quality protein and essential fatty acids [1,2]. The traditional beverage was typically consumed in Eastern countries, where the intense green and beany flavor of soy drink was well known and accepted, but it suffered from consumer rejection in Western countries [3].
A detailed understanding of the aroma profile of soy drink was an essential step to link the consumer preference and to optimize the flavor quality. To date, quantification data of odorants in soy drink were limited in literature [4,5]. Researchers mainly analyzed the concentrations of hexanal, hexanol and other green aldehydes [3,4,6,7], whereas other compounds were determined as the key aroma compounds [5]. Over the years the molecular sensory science concept has been developed to analyze aroma profiles in depth [8]. Following this concept, odor-active compounds in samples are firstly analyzed by means of gas chromatography-mass spectrometry-olfactometry (GC-MS-O) after suitable extraction of volatile substances using solvent and/or solvent-free extraction techniques. Solvent-free isolation techniques are faster and more environment-friendly compared to solvent-using extraction techniques. Next, key aroma compounds are studied by diluting the samples and determining flavor dilution (FD) factors, which are the ratio of the concentration in the initial solution to the concentration in the highest dilution where it is still detectable [8]. Lastly, the assigned key aroma compounds are quantified, and odor activity values (OAVs) are calculated [8].
Different methods to quantify aroma compounds are known. Internal standard method (IS) is a fast and easy method with high precision. Yet, mostly the concentrations are only estimated. This is due to the fact that the compound concentrations are estimated against a single standard compound, which can be from a different chemical class, and consequently have different extraction efficiencies resulting from sample preparation and/or different responses in detection signal. Further, each authentic standard compound should be prepared in a suitable concentration as the detector response is different depending on the mass unit of the analyte. Standard addition (SA) eliminates matrix effects and instrumental errors, but is time-consuming as it requires a calibration curve for each sample [9]. External calibration (EC) can measure a large throughput of samples if the calibration is set, however matrix effects must be taken into account. Stable isotope dilution assay (SIDA) is highly specific as it is based on ion diagnostics, with which high levels of accuracy and precision can be reached. But labeled standards are not always available and/or very expensive [9].
In this study, the aroma profile of freshly produced soy drink was comprehensively analyzed. Subsequently, four commercially available soy drinks were characterized in their aroma properties and Western consumers preferences were linked by sensory tests. In detail, key odorants of a freshly produced soy drink with intense green and beany off-flavor were identified using solvent-free extraction techniques (i.e. headspace-solid phase microextraction (HS-SPME), HS-sorptive extraction (HSSE) and direct immersion-stir bar sorptive extraction (DI-SBSE)) coupled with gas chromatography-mass spectrometry-olfactometry (GC-MS-O). Then, four quantification methods (standard addition, external calibration, internal standard, and stable isotope dilution assay) after application of DI-SBSE-GC-MS-O were investigated, aiming to select a fast and reliable method for quantification of all assigned key odorants in soy drinks. In the end, commercially available soy drinks were analyzed and compared in their compositions of key odorants and Western consumer preference was correlated to the molecular profile.
Section snippets
Materials
Authentic standard compounds with purities of 85%–99.8% were purchased from Alfa Aesar (Ward Hill, United States); aromaLAB GmbH (Planegg, Germany); Carl Roth GmbH (Karlsruhe, Germany); Cayman chemical company (Ann Arbor, United States); Fluorochem (Pune, India); J&K Scientific GmbH (Marbach a. N., Germany); Merck KGaA (Darmstadt, Germany); Sigma-Aldrich (St. Louis, United States); and Th. Geyer GmbH & Co. KG (Renningen, Germany). A freshly produced soy drink was prepared in laboratory scale,
Gas chromatography-olfactometry analyses of soy drink
In this study, odor-active compounds in soy drink were studied after the application of different solvent-free extraction techniques. The complex matrix of soy drink contains protein, fat, and mineral salts, all of which influence instrumental aroma analysis (Supplementary Table 1). Solvent-free extraction methods are based on an equilibrium influenced by the partition coefficients of the analytes between the sorptive material and the aqueous/gaseous phase. The partition coefficient of
Conclusion
In this study the aroma profile of soy drinks was analyzed. Key aroma compounds were determined by revised aroma dilution analyses after the application of two solvent-free extraction methods, i.e. HS-SPME and DI-SBSE, coupled with GC-MS-O analysis. The assigned key odorants were quantified by different developed and validated quantification approaches (internal standard method (IS), external calibration (EC), standard addition (SA), and stable isotope dilution assay (SIDA)). All approaches had
Author contribution
Ann-Kathrin Nedele: Conceptualization, Writing – Original Draft, Methodology, Visualization, Validation. Nicole Mayer: Visualization, Investigation. Natalie Feller: Visualization, Investigation. Jörg Hinrichs: Writing- Reviewing and Editing. Yanyan Zhang: Conceptualization, Project Administration, Supervision, Writing- Reviewing and Editing.
Funding
A. Nedele thanks the “Landesgraduiertenförderung Baden-Württemberg” for the scholarship of individual financial support.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Christian Schmidt for revising the manuscript.
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