Summary

As we were breaking new ground from several perspectives, the project took some unexpected turns. At this point, we have completed several goals. First, we have created a table of expected wine pigments based on known reactions between anthocyanins and proanthocyanidins, goals 2a and 2b. These results were published in two papers, the first describing how to enumerate all possible proanthocyanidins that could be distinguished by mass spectrometry, up to a degree of polymerization (DP) of 10 (Cave and Waterhouse 2019), and the second, taking that list and “reacting” those tannins with the pigment to create a list of all possible wine pigments. This was a list of over 1 million structures. (Cave et al. 2019)

Having that list, we then compared the masses of those proposed structures with mass spectral data from two sources, QToF and ICR. In the case of the QToF, we could match about 10% of the several hundred observable compounds (Cave et al. 2020). However, the ICR is much more sensitive and precise, and here we could observe about 18,000 signals, and of those could match over 20% of the signals to entries in our table (Cave et al. 2020). Unfortunately, many signals could be represented by more than one structure, so additional study is needed to determine which of the options is present. These results addressed goals 1a, 1b and 2c. The data we now have will require further mass spectral analysis to discriminate between a number of redundant structures.

The pathway of wine oxidation, as currently understood, encompasses the cascade of reactions incited by the oxidation of phenols in the presence of oxygen, eventually coming to the conversion of ethanol into acetaldehyde. While it is evident that wines consume oxygen over time and acetaldehyde becomes increasingly apparent with age, the rate of oxidation can vary unpredictably among different wines, and it was hypothesized that different phenolic structural features, particularly those of oak ellagitannins, are the reason for such variability in oxidation. It was originally proposed that ellagitannins and other phenols with distinct functionalities be studied for their effects on oxygen consumption and acetaldehyde production. However, in light of recent studies conducted by our laboratory demonstrating that the input of oxygen does not guarantee the output of acetaldehyde, it was decided that attempting to study the pathway of oxidation in its entirety, from oxygen to acetaldehyde, would not be an effective approach.

Given the complexity of wine oxidation, a more sensible strategy would be to study the effects of phenolic structure on individual reactions rather than the pathway as a whole. In the first step of wine oxidation, the oxidation of phenols is coupled to the reduction of oxygen by iron, which acts as a shuttle for electrons between phenols and oxygen. A more specific hypothesis now is phenolic structure affects their reactivity with iron, subsequently affecting oxygen consumption and the remainder of the wine oxidation pathway. The initial reactions of wine oxidation may be characterized by the redox cycling of iron between its two oxidation states: Fe(II) and Fe(III). The addition of electrons to oxygen occurs with the oxidation of Fe(II) to Fe(III), and in the opposite direction, the loss of electrons from phenols takes place with the reduction of Fe(III) to Fe(II). The ratio of Fe(II) to Fe(III) should thus depend on the relative reaction rates of Fe(II) with oxygen and that of Fe(III) with phenols.

A quick and simple spectrophotometric method for iron speciation, employing the complexing agent Br-PADAP, is currently being optimized and validated, to be used not only to assess differential rates of iron reduction by structurally diverse phenols, but also by the industry to more generally measure the “redox status” of their wines. Our laboratory’s modified version of the Br-PADAP assay is simple and inexpensive, requiring a sample volume of only 200 μL and a reaction time of 10 min, and is done directly in a cuvette. Validation of the assay is currently underway; difficulty lies in the fact that there does not exist a standard method for iron speciation to compare, thus alternative methods of validation are being considered. This research would not only improve management of oxidation, but also furnish a more complete understanding of phenolic oxidation, with the ultimate goal being the prediction of wine aging based on phenolic content and composition.

The objectives of this proposal have been to do the following:

In a companion project being submitted to the Agricultural Research Institute (California State University research initiative)

II. Conduct extended sensory studies with the University of California, Davis, on a subset of the above wines to determine the relationship between sensory and instrumental analysis of red wine mouthfeel.

These objectives are consistent with the highest priority research objective as outlined by the American Vineyard Foundation.

The overall purpose of this proposal is to determine the role of grape and wine production practices on tannin structure and perception.  In combination, tannin activity will be monitored by high performance liquid chromatography (HPLC) so that a comparison can be made between human perception and HPLC measurement.

Following the second year of activities, excellent progress has been made.  The major activity during the second year of this study has been to monitor the development of berries in approximately 75 Napa Valley blocks.  Monitoring the tannin activity in these blocks is expected to provide information on the role that grape production practices can have in overall grape and corresponding wine tannin.

Seven vintages of UC Davis Oakville Cabernet Sauvignon spanning 35 years was selected for analysis by a complimentary suite of mass spectrometric techniques. The vintages from 1974 to 2009 selected by informal sensory evaluation and basic wine chemistry to be of a representative vertical style were: 1974, 1981, 1988, 1994, 2001, 2003 and 2009. All wines were produced at UC Davis for research purposes and stored in the UC Davis cellar together. This sample was determined to be the most controlled and uniform sample of wines varying by vintage only as could be obtained for the project. Al basic wine chemistry was obtained, pH, ethanol concentration etc. as were various assays for comparison to mass spectrometric data.

The following objective were submitted for the grant proposal under which this work was peformed.

1) Observe the evolution of pigmented tannin throughout aging

a. Employ our method of complimentary mass spectrometric techniques (ICR, QTOF, QTrap) for comprehensive identification of wine matrix compounds.

b. Observe the changes in relative abundance, depletion and accumulation in pigmented tannin composition (35 Year Vertical)

c. Correlate pigmented tannin structural analysis with well-known molecular characteristics (mDP, mass recovery) and newly developed sensory representative analyses (“grippiness”) to better convey the impact of these discoveries

2) Apply existing synthetic technologies.

a. Development of a library of standards for quantitation and calibration.

b. Postulate wine pigment precursors for examination of mechanistic pathways.

c. Employ the standards to quantitate the classes of polymeric pigment in wine.

Wine Research Assays

Phloroglucinolysis was performed on the wines used for the study (Table 1) as well as their extracts (Table 2). The mass recovery demonstrates typical value dropping off well below 80%after 7 years. Molecular mass after 50%elution also follows expected patterns of increase with age. This indicates overall larger tannin in the older wines as supported by McRae et al 2012. The molecular mass is a measure of intact tannins, as opposed to the mDP which measures only those tannins which could be cleaved by phloroglucinol. The inconsistency between these two values is reconciled by the mass recovery, indicating that more complexity of the tannin structure is a factor in the older wines, for instance additional interflavan bonds preventing phloroglucinol attack.

The task we are undertaking is fundamental. There is currently no quantitative method for determining tannin and pigmented tannin. It is not understood to what extent certain compounds contribute to the overall color of aged (> 2 years old) wine, nor the relative abundances of those compounds. Furthermore, there are many compounds that comprise the molecular basis of pigmented tannin whose structures are unknown. This investigation aims at using mass spectrometry as a tool for assessing the extent of color contribution due to these compounds, identifying new compounds, and providing a means of identifying the relative abundances for determining which compounds are most highly associated with quality parameters. Once we better understand the molecular basis of pigmented tannin we can provide tools for winemakers to improve the quality of their product. Pursuant to our goals for this year of the project we have (1a) identified many compounds by mass spectrometry which comprise the wine matrix, and many yet which have not been observed. Our FT-ICR experiment for determination of the relative abundance, depletion and accumulation of particular compounds (1b) will be performed in April with our collaborator Professor Nikolai Kuhnert at his laboratories and the Bruker research laboratories in Bremen Germany. Development of synthetic standards (2abc) is in progress as we are still assessing the appropriate standards to synthesize. We have defined the compound classes which comprise the majority of molecular peaks and will be performing further iterative fragmentation of these compounds to determine their structural characteristics in March with fellow anthocyanin researcher Professor Colin Kay at the University of East Anglia. All in all we are on track to accomplish our objectives and maintain pace for the upcoming year of data analysis and subsequent experimentation.