Flavors in Malt Whisky: A Review

Characterization of Key Aging Aroma Compounds in Aged Jiangxiangxing Baijiu and Their Formation Influencing Factors during the Storge Process

Long-term storage Baijiu has an appealing sensory property, yet the chemical makeup is rarely reported. This study investigated a 30-year-old Jiangxiangxing (JXX) Baijiu and recognized and measured 69 aroma compounds. 3-Methyl-2,4-nonanedione (2.76 μg/L), 2,5-dimethyl-4-hydroxy-3(2H)-furanone (HDMF, 46.2 μg/L), 2-ethyl-4-hydroxy-5-methyl-3(2H)-furanone (HEMF, 61.7 μg/L), and piperitone (3.66 μg/L) were detected for the first time in Baijiu. Compared with 3-year-old JXX Baijiu, 24 compounds were significantly higher in the 30-year-old, mainly including furans, pyrazines, and aromatics. Notably, 4,5-dimethyl-3-hydroxy-2(5H)-furanone (sotolon), HDMF, HEMF, vanillin, acetovanillone, and alkyl pyrazines in 30-year-old JXX Baijiu were 2-7 times higher than those of a 3-year-old, and they increased steadily during aging for 3, 15, and 30 years, assumed to be associated with the aging aroma. Following 24 months of storing JXX Baijiu under different conditions, the pottery significantly promoted the synthesis of sotolon, HDMF, HEMF, and alkyl pyrazines. These findings suggest that pottery is a potential catalyst for enhancing aged Baijiu.

Investigation of Lactone Chiral Enantiomers and Their Contribution to the Aroma of Longjing Tea by Odor Activity Value and S-Curve

Odor activity value (OAV) and S-curve were used to study the content, proportion, and contribution of lactone chiral enantiomers in Longjing tea. A total of 10 enantiomers were identified in this study, among which (S)-(-)-δ-decalactone (45.4-84.4 μg/L), (S)-(-)-γ-decalactone (31.5-109 μg/L), (S)-(-)-γ-nonanolactone (23.4-72.8 μg/L), and (S)-(-)-γ-undecalactone (21.1-56.2 μg/L) presented the highest concentrations. Furthermore, (R)-(+)-γ-nonanolactone (OAV: 2-7), (S)-(-)-γ-nonanolactone (OAV: 1-5), (S)-(-)-δ-decalactone (OAV: 2-4), (R)-(+)-δ-decalactone (OAV: 1-3), and (R)-(+)-γ-undecalactone (OAV: 1-5) were determined as enantiomeric compounds that play an important role in the perceived aroma of Longjing tea. Compared with the aromatic reconstitution (AR), the threshold increased to different degrees after adding γ-nonanolactone, γ-decalactone, δ-decalactone, γ-undecalactone, and their chiral enantiomers. This finding indicated that these compounds exert significant effects on the overall aroma of the AR. The contribution of racemates and chiral enantiomers to the AR threshold and aroma is completely different. In view of the difference between racemic and enantiomers' aroma characteristics in Longjing tea, the analysis and identification of chiral enantiomers are necessary to enrich and improve the accurate analysis of the flavor profile of Longjing tea.

Distilled beverage aging: A review on aroma characteristics, maturation mechanisms, and artificial aging techniques

The market value of distilled beverage relies on its quality with a major contribution of distinctive and fascinating aromas. The aroma of distilled beverage is built on the basis of chemical components and can be modified through a series of physical and chemical processes such as aging. Revealing the hidden knowledge behind the evolution of numerous chemical components during these physicochemical processes in distilled beverages is not only significant but also challenging due to its complex system. In this review, the trends in the changes of associated aroma compounds over aging are proposed on the basis of understanding the relationship between chemical components and aroma profiles of numerous typical distilled beverages. The different aging systems, both classical platforms from Eastern countries (pottery jars) to Western countries (wood barrels), and modern platforms such as artificial aging technologies are outlined and compared with their respective applications. Optimizing aging processes is a challenging but imperative step, which warrants further fundamental knowledge from targeting aging-related molecules to the exploration of multitude physicochemical reaction mechanisms that occur during this process, such as the formation of potent odorant compounds in specific containers and environments, as well as mass transfer processes between solid and liquid interfaces. Understanding these maturation mechanisms of distilled beverages expressed by chemosensory signature holds promise for major improvements in future aging technologies that can efficiently yield stable and high-quality products.

A Narrative Review of Sulfur Compounds in Whisk(e)y

The production process of whisky consists of malting, mashing, fermentation, distillation and maturation. Sulfur volatile compounds generated during this process have long attracted interest because they influence quality in general. More than forty compounds have been reported: they are formed during malting, fermentation, and distillation, but some may decrease in concentration during distillation and maturation. In sensory analysis, sulfur characteristics are described as sulfury, meaty, cereal, feinty, and vegetable, among others. Their contribution to overall quality depends on their concentration, with a positive contribution at low levels, but a negative contribution at high levels. Chemical analyses of sulfur volatiles have been developed by using sulfur-selective detectors and multi-dimensional gas chromatography to overcome the numerous interferences from the matrix. Formation pathways, thresholds, and contribution have not been elucidated completely; therefore, methods for integrating diverse data and knowledge, as well as novel technical innovations, will be needed to control sulfur volatiles in the future.

Identification of Compounds Contributing to Trigeminal Pungency of Baijiu by Sensory Evaluation, Quantitative Measurements, Correlation Analysis, and Sensory Verification Testing

Pungency is one of the most important mouthfeel characteristics that is primarily related to the sensory quality of distilled spirits. However, the chemical basis of pungency is still unclear. A set of Baijiu samples with different levels of pungency was characterized by sensory analysis and volatile compound analyses. Several esters, aldehydes, and acids significantly correlated with pungency. Ethyl hexanoate, ethyl acetate, 3-methylbutyl hexanoate, acetaldehyde, acetal, and 3-methylbutanal were confirmed to be the strongest contributors to the pungency of Baijiu by the two-alternative forced-choice test. Sensory recombination testing further revealed that the contribution of esters to pungency was much higher than that of the aldehydes, and acid compounds at low concentrations suppress the pungency perception. In this study, the importance of esters in the pungency of distilled spirits is first reported. The results provide an instructive basis for further research into optimizing the quality of products.

Experimental Whisky Fermentations: Influence of Wort Pretreatments

In addition to ethanol yield, the production of flavour congeners during fermentation is a major consideration for Scotch whisky producers. Experimental whisky fermentations can provide useful information to the industry, and this is the focus of this paper. This study investigated the impact of wort pretreatments (boiled, autoclaved, filtered) on fermentation performance and flavour development in Scotch whisky distillates as an alternative to freezing wort for storage. Our study showed that no significant sensorial differences were detected in low wines (first distillates), while the chemical compositions showed clear changes in increased levels of esters and higher alcohols in boiled and autoclaved wort. In contrast, filtered wort comprised overall lower levels of congeners. Regarding alcohol yield, all three pretreatments resulted in decreased yields. In practice, the pretreatment of wort prior to fermentation requires additional process operations, while freezing requires large storage units. The pretreatments adopted in this study significantly influence the composition of the malt wort used for experimental whisky fermentations, and this results in a poorer fermentation performance compared with untreated wort. We recommend the use of fresh or frozen wort as the best options for small-scale fermentation trials.

Production, Characterization, and Sensory Profiling of Novel Pepper Distilled Spirit

This study described the production method of novel whiskey using black pepper berries as raw material including the determination of chemical composition and sensory profile of this distillate. The production process consisted of production of fermentation medium via hydrothermal treatment, followed by fermentation and distillation. The incorporation of hydrothermal extraction process makes this whiskey production process differ from the existing commercial spirit production protocol. Chemical composition analysis showed that there were 12 main volatile compounds that contribute to the aroma profile of this pepper whiskey which consisted of 1 aldehyde group, 3 ester groups, 2 phenolic compounds, and 6 alcohol groups. All of these compounds are presented at different concentrations that are able to give pleasant and signature aroma to this spirit. A 10-member descriptive analysis panel conducted showed that 7 descriptors can be used to define the organoleptic quality of this whiskey, namely, clarity and brilliance (visual), fruity and flora (aroma), spiciness and sweetness (taste), and fruity mouthfeels. In conclusion, pepper berries can be used as raw material for pepper whiskey production and having organoleptic quality acceptable for human consumption.

Spent Grain from Malt Whisky: Assessment of the Phenolic Compounds

In order to extract antioxidant phenolic compounds from spent grain (SG) two extraction methods were studied: the ultrasound-assisted method (US) and the Ultra-Turrax method (high stirring rate) (UT). Liquid to solid ratios, solvent concentration, time, and temperature/stirring rate were optimized. Spent grain extracts were analyzed for their total phenol content (TPC) (0.62 to 1.76 mg GAE/g SG DW for Ultra-Turrax pretreatment, and 0.57 to 2.11 mg GAE/g SG DW for ultrasound-assisted pretreatment), total flavonoid content (TFC) (0.6 to 1.67 mg QE/g SG DW for UT, and 0.5 to 1.63 mg QE/g SG DW for US), and antioxidant activity was measured using 2,2-diphenyl-2-picrylhydrazyl (DPPH) free radical (25.88% to 79.58% for UT, and 27.49% to 78.30% for UT). TPC was greater at a high stirring rate and high exposure time up to a certain extent for the Ultra-Turrax method, and at a high temperature for the ultrasound-assisted method. P-coumaric acid (20.4 ± 1.72 mg/100 SG DW for UT, and 14.0 ± 1.14 mg/100 SG DW for US) accounted for the majority of the phenolic found compounds, followed by rosmarinic (6.5 ± 0.96 mg/100 SG DW for UT, and 4.0 ± 0.76 mg/100 SG DW for US), chlorogenic (5.4 ± 1.1 mg/100 SG DW for UT, and non-detectable for US), and vanillic acids (3.1 ± 0.8 mg/100 SG DW for UT, and 10.0 ± 1.03 mg/100 SG DW for US) were found in lower quantities. Protocatechuic (0.7 ± 0.05 mg/100 SG DW for UT, and non-detectable for US), 4-hydroxy benzoic (1.1 ± 0.06 mg/100 SG DW for UT, and non-detectable for US), and caffeic acids (0.7 ± 0.03 mg/100 SG DW for UT, and non-detectable for US) were present in very small amounts. Ultrasound-assisted and Ultra-Turrax pretreatments were demonstrated to be efficient methods to recover these value-added compounds.

Characteristics of the Microbial Community in the Production of Chinese Rice-Flavor Baijiu and Comparisons With the Microflora of Other Flavors of Baijiu

Rice-flavor baijiu is one of the four basic flavor types of Chinese baijiu. Microbial composition plays a key role in the classification of baijiu flavor types and the formation of flavor substances. In this study, we used high-throughput sequencing technology to study the changes of microbial community in the production of rice-flavor baijiu, and compared the microbial community characteristics during production of rice-, light-, and strong-flavor baijiu. The results showed that the species diversity of bacteria was much higher than that of fungi during the brewing of rice-flavor baijiu. The bacterial diversity index first increased and then decreased, while the diversity of fungi showed an increasing trend. A variety of major microorganisms came from the environment and raw rice materials; the core bacteria were Lactobacillus, Weissella, Pediococcus, Lactococcus, Acetobacter, etc., among which Lactobacillus was dominant (62.88-99.23%). The core fungi were Saccharomyces (7.06-83.50%) and Rhizopus (15.21-90.89%). Temperature and total acid content were the main physicochemical factors affecting the microbial composition. Non-metric multidimensional scaling analysis showed that during the fermentation of rice-, light-, and strong-flavor baijiu, their microbial communities formed their own distinct systems, with considerable differences among different flavor types. Compared with the other two flavor types of baijiu, in the brewing process of rice-flavor baijiu, microbial species were fewer and dominant microorganisms were prominent, which may be the main reason for the small variety of flavor substances in rice-flavor baijiu. This study provides a theoretical basis for the production of rice-flavor baijiu, and lays a foundation for studying the link between baijiu flavor formation and microorganisms.

Comparison of Three Approaches to Assess the Flavour Characteristics of Scotch Whisky Spirit

This study compared the use of three sensory and analytical techniques: Quantitative Descriptive Analysis (QDA), Napping, and Gas Chromatography-Mass Spectrometry (GC-MS) for the assessment of flavour in nine unmatured whisky spirits produced using different yeasts. Hierarchical Multiple Factor Analysis (HMFA) showed a similar pattern of sample discrimination (RV scores: 0.895–0.927) across the techniques: spirits were mostly separated by their Alcohol by Volume (ABV). Low ABV spirits tended to have heavier flavour characteristics (feinty, cereal, sour, oily, sulphury) than high ABV spirits, which were lighter in character (fruity, sweet, floral, solventy, soapy). QDA differentiated best between low ABV spirits and GC-MS between high ABV spirits, with Napping having the lowest resolution. QDA was time-consuming but provided quantitative flavour profiles of each spirit that could be readily compared. Napping, although quicker, gave an overview of the flavour differences of the spirits, while GC-MS provided semi-quantitative ratios of 96 flavour compounds for differentiating between spirits. Ester, arenes and certain alcohols were found in higher concentrations in high ABV spirits and other alcohols and aldehydes in low ABV spirits. The most comprehensive insights on spirit flavour differences produced by different yeast strains are obtained through the application of a combination of approaches.