Curtis M. Kalua

Evolution of Volatile Compounds during the Development of Cabernet Sauvignon Grapes (Vitis vinifera L.)

The evolution of volatile compounds was explored in grape berries at fortnightly intervals from fruit-set to late ripening to identify when biosynthetic pathways may be targeted for enhancement of grape and wine aroma. Stepwise linear discriminant analysis (SLDA) fully recognized patterns in berry physiological developmental stages with most of the variance (>99.0%) explained. The preveraison berry developmental stage was identified as a transition stage for volatile compound biosynthesis when most compounds were potentially sequestered to nonvolatile conjugates and berries lost their potential to synthesize esters and terpenes. Terpenes (predominantly eucalyptol, beta-caryophyllene, and alpha-humulene) characterized early berry development, whereas benzene derivatives (2-phenylethanol and 2-phenylethanal) appeared toward late ripening. Furthermore, C(6) volatile compounds changed from acetate esters to aldehydes and finally to alcohols during early, middle, and late berry developmental stages, respectively. The dominance of alcohols in the late stages of berry development, preceded by aldehydes, offers an opportunity for alcohols to aldehydes ratios to be used in the prediction of harvest timing for enhanced grape and wine aroma. The evolution of volatile compounds during berry development suggests a greater dependency on enzyme activity and specificity than extent of fatty acid unsaturation. The dependence of the stage of berry development on the accumulation of the products of alcohol dehydrogenase (ADH), alcohol acetyl transferase (AAT), and enal isomerase enzyme activity from the lipoxygenase pathway raises possibilities for the manipulation of aroma profiles in grapes and wines.

Grape contribution to wine aroma: production of hexyl acetate, octyl acetate, and benzyl acetate during yeast fermentation is dependent upon precursors in the must.

Wine is a complex consumer product produced predominately by the action of yeast upon grape juice musts. Model must systems have proven ideal for studies of the effects of fermentation conditions on the production of certain wine volatiles. To identify grape-derived precursors to acetate esters, model fermentation systems were developed by spiking precursors into model must at different concentrations. Solid-phase microextraction-gas chromatgraphy mass spectrometry analysis of the fermented wines showed that a variety of grape-derived aliphatic alcohols and aldehydes are precursors to acetate esters. The C6 compounds hexan-1-ol, hexenal, (E)-2-hexen-1-ol, and (E)-2-hexenal are all precursors to hexyl acetate, and octanol and benzyl alcohol are precursors to octyl acetate and benzyl acetate, respectively. In these cases, the postfermentation concentration of an acetate ester increased proportionally with the prefermentation concentration of the respective precursor in the model must. Determining viticultural or winemaking methods to alter the prefermentation concentration of precursor compounds or change the precursor-to-acetate ester ratio will have implications upon the final flavor and aroma of wines.

Two O-methyltransferases involved in the biosynthesis of methoxypyrazines: grape-derived aroma compounds important to wine flavour

Methoxypyrazines (MPs) are volatile, grape-derived aroma compounds that contribute to the distinct herbaceous characters of some wines. Although the full pathway leading to MP production has not been elucidated, there is strong evidence that the final step involves the methylation of non-volatile hydroxypyrazine (HP) precursors. Two cDNA encoding O-methyltransferases (OMTs) that have homology to an enzyme previously purified and shown to catalyse the methylation of HPs were isolated from Cabernet Sauvignon. Recombinant protein from the cDNAs (VvOMT1 and VvOMT2) was produced in E. coli and activity assays demonstrated that both encode OMTs able to methylate HPs to produce MPs, however both showed greatest activity against the flavonol quercetin. VvOMT1 has higher catalytic activity against isobutyl hydroxypyrazine compared to isopropyl hydroxypyrazine, whereas the converse is true for VvOMT2. The timing of the expression of VvOMT1 in the skin and the flesh of developing Cabernet Sauvignon grape berries was associated with the period of MP accumulation in these tissues, while VvOMT2 expression was greatest in roots, which were found to contain high levels of MPs. The MP composition of these tissues also reflects the relative levels of expression of these genes and their substrate preference. The identification of genes responsible for MP production in grapevine will help in understanding the effect of different viticultural and environmental factors on MP accumulation.

Sample preparation optimization in wine and grapes: Dilution and sample/headspace volume equilibrium theory for headspace solid-phase microextraction

Most headspace solid-phase microextraction (HS-SPME) volatile analysis methods have been developed for aqueous samples and have been either adapted or applied to complex matrices. This study examines sample/headspace equilibrium based on realistic (non-spiked) concentration levels in real complex sample matrices (grapes and wine) with a systematic multivariate statistical approach. The presence and absence of matrix effects are explained through exponential and linear relationships, respectively. The potential of over- and underestimating volatile compounds in a diluted sample is illustrated and the common dilution equation (C1V1=C2V2) is shown to not always apply to headspace volatile analysis. Additionally, sample dilution was shown to be more sensitive to matrix effects than sample/headspace volume variations with the latter showing analyte dependency. An optimum sample size of 6.9-8.6g in a 20mL vial without dilution was observed. This study shows that sensitivity and limit of detection (LOD) can be improved to a limit with a subsequent loss - an extension to existing theory. The study further illustrates that in trying to bring an analyte within linear range through sample dilution, sensitivity and LOD can be lost with a probable shift in optimum ranges and sample/headspace equilibrium.

Volatile Compounds in Australian Olive Oils

Publisher Summary The volatile component of olive oil is a complex mixture of chemicals that determines the oils aroma and influences quality and consumer acceptability. The relationship between number and quantity of volatiles and the aroma is not simple and depends on synergistic and antagonistic effects. Moreover, exogenous volatile compounds such as aromatic hydrocarbons and halogenated solvent residues may occur in an edible oil such as olive oil consequential to the extraction procedure. The situation is complicated, as many volatile compounds and particularly hydrocarbons can be present in virgin olive oils either naturally or as contaminants. Moreover, volatile compounds may also be lost from an oil either to the atmosphere or due to absorption by packaging materials. This chapter examines the differences between Australian and other oils, principally European, in terms of the volatile fraction. This follows a brief discussion of the measurement of the volatile fraction. The latter is important, as any differences in the volatile profile, quantitative or qualitative, may be artifactual and relate to methodological differences in the measurement step.

Volatile Compounds in Australian Olive Oils: How Different Are They From Other Oils

Publisher Summary The volatile component of olive oil is a complex mixture of chemicals that determines the oils aroma and influences quality and consumer acceptability. The relationship between number and quantity of volatiles and the aroma is not simple and depends on synergistic and antagonistic effects. Moreover, exogenous volatile compounds such as aromatic hydrocarbons and halogenated solvent residues may occur in an edible oil such as olive oil consequential to the extraction procedure. The situation is complicated, as many volatile compounds and particularly hydrocarbons can be present in virgin olive oils either naturally or as contaminants. Moreover, volatile compounds may also be lost from an oil either to the atmosphere or due to absorption by packaging materials. This chapter examines the differences between Australian and other oils, principally European, in terms of the volatile fraction. This follows a brief discussion of the measurement of the volatile fraction. The latter is important, as any differences in the volatile profile, quantitative or qualitative, may be artifactual and relate to methodological differences in the measurement step.

Fermentation of grapes throughout development identifies stages critical to the development of wine volatile composition

Background and Aims Many variables affect the volatile profile of wine during production, and grape composition is an important source of varietal characters and metabolites for yeast during fermentation. In order to manage wine style through changes in grape composition, the knowledge of when important changes occur in the berries is essential. This study sought to identify stages of berry development that are most critical in defining wine volatile composition. Methods and Results Cabernet Sauvignon and Riesling grapes at several stages throughout development were fermented, after normalising sugar content, and the volatile compounds in the resulting wines were profiled. For both cultivars, the compounds were grouped into six clusters, which best described the changes in concentration of the volatile compounds in the wines. Some varietal compounds showed significant differences in wines made from berries at the various developmental stages; however, many fermentation-derived volatile compounds, especially esters, were also significantly affected by berry developmental stage. Conclusions Dynamic changes occur in the volatile profile of wines made from grapes at several stages of development, but a few common patterns of change are seen for several volatile compounds. Significance of the Study Several distinct phases of berry development associated with either an increasing or decreasing concentration of wine volatile compounds were identified, which will inform the timing of future strategies designed to alter wine composition through interventions in the vineyard.

Chapter 22 – Volatile Compounds in Australian Olive Oils: How Different Are They From Other Oils

Publisher Summary The volatile component of olive oil is a complex mixture of chemicals that determines the oils aroma and influences quality and consumer acceptability. The relationship between number and quantity of volatiles and the aroma is not simple and depends on synergistic and antagonistic effects. Moreover, exogenous volatile compounds such as aromatic hydrocarbons and halogenated solvent residues may occur in an edible oil such as olive oil consequential to the extraction procedure. The situation is complicated, as many volatile compounds and particularly hydrocarbons can be present in virgin olive oils either naturally or as contaminants. Moreover, volatile compounds may also be lost from an oil either to the atmosphere or due to absorption by packaging materials. This chapter examines the differences between Australian and other oils, principally European, in terms of the volatile fraction. This follows a brief discussion of the measurement of the volatile fraction. The latter is important, as any differences in the volatile profile, quantitative or qualitative, may be artifactual and relate to methodological differences in the measurement step.