It’s another guest post from Andy Kirk, a MSc (Horticulture) candidate at Lincoln University in New Zealand and alumnus of The Ohio State University! Having grown up in Akron and Canton, he worked under the employ of Arnie Esterer at Markko Vineyard in Conneaut for much of 2011 and 2012. He enjoys New Zealand, but often finds himself relating his viticulture and enology learnings back to the shores of Lake Erie. Many thanks to Andy, and for you, dear readers, enjoy!
Micro-Oxygenation and the Pursuit of Lushness
by Andrew Kirk
Like so many popular wine-trade concepts, the lush tannin has proved difficult to pin down with science. It seems that so many in the wine world, from the New York wine merchant to the aspiring winemaker are fixated on this wine attribute. Truth be told, this is a subject shrouded in as much half-truth and mystery as “Bessie”, the Lake Erie Monster. To this day, there is no broad, universally accepted model of what determines the mouthfeel of a tannin or related polymer. That said, it is important to start asking the right questions and investigating the current theories. Let’s start with what we know.
Flavonoids: A Quick Review
Most, if not all, of the relevant characters in this tale of chemicals are members of the flavonoid group. To the left is a labeled diagram of the basic flavonoid structure. Note the particular pattern of carbon position numbering, as well as the separation of the A and C rings from the B ring. As flavonoids polymerize in various ways, the location and extent of the linkages determine the stability and sensory characteristics of the resulting compound. (Cheynier, Dueñas-Paton et al. 2006)
Within the broader flavonoid class of compounds, there are three main categories of consequence to winemakers: anthocyanin, flavonols, and flavanols (Flavan-3-ol). For the purposes of this discussion, we’ll stick mainly to the anthocyanin and flavanol groups. Anthocyanin, of course, is the main building block of color in red wine. Catechin and epicatechin, flavanols and isomers of each other, are known to be the basic building blocks of tannin. It is worth noting, here, that as tannin molecules polymerize the presence of either catechin or epicatechin subunits impacts the sensory properties of the resulting tannin polymer. For the record, catechin subunits are known to elicit more astringency than epicatechin and other basic flavanols. (McRae and Kennedy 2011)
Believe it or not, there is technically no model of astringency proven beyond a reasonable doubt. That said, there is a generally accepted model that is also, not by coincidence, very neat for conceptual purposes. Without getting too technical, the idea is that astringency is an interaction between wine tannins and salivary proteins, which aggregate with one another and precipitate out. This creates a drying, grainy sensation in the mouth, which is perceived as astringency. It follows that more interaction between the wine tannin and protein molecules is equal to more astringency. This is determined not only by the nature of the wine tannin and polymers, but also by the individual taster. A person with higher amounts of salivary proteins, or a higher rate of saliva flow, will have a weaker astringent sensation. (McRae and Kennedy 2011)
As a quick aside, wine research makes a clear distinction between bitterness and astringency. Astringency, as we have just described, is a tactile, drying sensation. Bitterness is a taste sensation, a result of stimulus upon olfactory and taste receptors, which then activates the signal transduction process in our brain. (Zoecklein, Carey et al. 2003) Interestingly, only molecules of a certain shape and limited size can initiate this transduction process. (McRae and Kennedy 2011) This leads, of course, to the conventional wisdom that large tannin molecules are less bitter and more astringent. With that out of the way, it’s time to get back to astringency.
Tannin Size and Astringency
Well, it’s always nice to state the obvious. In this case, it is that a higher concentration of tannin corresponds to a stronger perception of astringency. (McRae and Kennedy 2011) Likewise, the size, or degree of polymerization, of a tannin has also been shown to correlate positively with astringency. (Gawel 1998, Cheynier, Dueñas-Paton et al. 2006) There are mixed reports as to whether there is a limit to this size effect, with some research noting that the solubility of tannin may be compromised by increasing size and rigidity of structure. (McRae and Kennedy 2011) In that scenario, excessively polymerized tannin would precipitate out, taking its astringency contribution along with it.
If this only seems like words on a page, refer back to the diagram of a basic flavonoid and its various bonding sites. From there, remember the ever-critical interaction between these bonding sites and the salivary proteins, resulting in the drying sensation we call astringency. The key concept here is that more, and more active, bonding sites are equal to more astringency. With that in mind, it follows that larger molecules have more binding sites, which results in more protein-tannin interaction. It might be noted here that a polymer reaches equilibrium, loosely speaking, when an anthocyanin joins the end of the chain, thus limiting the size and astringency of the polymer. (Zoecklein, Carey et al. 2003)
Equally important to size is the structural orientation of the tannin molecules in question. What the research refers to as structural flexibility is particularly conducive to increased astringency. (Cheynier, Dueñas-Paton et al. 2006, McRae and Kennedy 2011) In laymen’s terms, the bonding sites of a tannin can find themselves in the right place at the right time to encourage, or discourage, interaction.
Tannin Structure and Astringency
On that note, tannins do not just bind to each other at any position. The “typical” tannin-tannin polymerization, without Micro-Oxygenation, happens at the C4-C8 positions, as seen in the above-right illustration. This linkage is known to be highly reactive with salivary proteins. In the presence of controlled oxidation, however, there is a different story. The coupled oxidation of ethanol by phenolic compounds has the end result of producing acetaldehyde. (Zoecklein, Carey et al. 2003) While generally seen as a foe in winemaking, in this setting it is a key ally.
Acetaldehyde reacts with phenolics such as tannin and anthocyanin to enable polymerization among and between the different phenolic groups. The advantage to winemakers lies in the location of these aldehyde bridges, as they are known. As opposed to the typical C4-C8 linkage, acetaldehyde is known to promote linkages at positions such as C8-C8, C6-C8, and C4-C6. (Zoecklein, Carey et al. 2003) While the sensorial properties of some aldehyde-bridged compounds have not been mapped out, the ones named above have been demonstrated to have a lower level of protein interaction. (McRae and Kennedy 2011)
In case it has not become fully evident already, the idea behind Micro-Oxygenation is to encourage alternative polymer configurations, in hope of limiting the astringency and increasing the stability. This particular discussion has focused on astringency, at the expense of stability. It’s worth noting that the stability of tannin and its polymers is of critical importance to the winemaker, if perhaps out of the scope of this review.
It should also be noted that a form of micro-oxygenation has been in practice for thousands of years. This, of course, is wood barrel aging, which enables the slow exchange of oxygen through pores in the wood. That said, many well-financed wineries have invested in equipment to inject a measured amount of oxygen into a wine from within stainless steel tanks. This is intended, no doubt, to reproduce the benefits of slow oxygen exposure in a more economical manner.
As usual, though, there is just slightly more to the story. Particularly important here is the timing of the oxygen exposure. As implied before, one of the major goals of Micro-Oxygenation is to prevent extensive tannin-tannin polymerization by “capping” these chains with Anthocyanin molecules. To be successful in this, Micro-Oxygenation should be applied early in the maturation process. (Zoecklein, Carey et al. 2003, McRae and Kennedy 2011)
Beyond reducing polymerization, it is necessary to consider the state of the wine’s acetaldehyde if alternative linkages are desired. More specifically, acetaldehyde is known to bind readily with sulfites. To the extent that alternative linkages are sought, sulfites should be delayed as long as possible. (Zoecklein, Carey et al. 2003) That again indicates that oxygenation should begin early in the post-fermentation phase.
Whether or not your winery has state-of-the-art Micro-Oxygenation equipment, it pays to understand the theory behind this practice. In some ways, the most basic winemaking practice of racking operates under a similar, albeit less controlled, principle. For that matter, maybe you balk at the lush tannin craze in favor of a nice sharp tannin chain. Regardless, it’s never too late to start asking the right questions about what’s happening in your phenolic profile. Could they use a breath of fresh air?