*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!
Pre-fermentation heat treatments have been shown to increase the extraction of monomeric anthocyanin, which together with fermentation management, can result in deeper and more stable wine color.
The denaturing of proteins and enzymes can prevent oxidative browning and the production of compounds perceived as wine faults.
Unfortunately, this also inactivates pectinase, and potentially other beneficial enzymes as well.
You may have heard already, but occasionally Winegrowers in the Eastern US & Canada have less than ideal conditions at harvest. Or perhaps the weather has been fine, but yields were stretched a bit this year, and color is lagging behind. If this sounds familiar, know that you are not alone. Winegrowers, old and new world alike, are familiar with this situation and have developed a technique over the years to make the most out of a rough vintage. The technique alluded to, known as thermovinification, consists of heating the skins (or must) to somewhere between 140°F (60°C) to 158°F (70°C) for a few minutes. This process damages the hypodermal cell membrane of the grape skin, thus facilitating the release of anthocyanin, the principal component of red wine color. After the heating process, a rapid cooling usually follows, at which point the skins are either pressed or left for maceration and fermentation. (Sacchi, Bisson, & Adams, 2005) The effects of thermovinification fall into two main categories, which should aid our understanding of this technique and its applications: Color and Sensory Profile.
Simply put, color is the main incentive to do thermovinification. Gao et al. (1997) found that in Pinot Noir, the end result of thermovinification was a wine with 2.5 times the amount of monomeric anthocyanin. Monomeric anthocyanin is (at most wine pH levels) pigmented in its own right, but its most critical role may be as a building block for the formation of polymeric pigments, which are deeper and more stable than their monomeric counterparts. The other clear impact of thermovinification is to denature enzymes such as polyphenol oxidase (PPO) and laccase, which are responsible for oxidative browning in wines. (Gómez-Plaza, Gil-Muñoz, López-Roca, Martínez-Cutillas, & Fernández-Fernández, 2001; Sacchi et al., 2005)
Inconvenient though it may be, it is often difficult to separate the effects of thermovinification from the effects of other winemaking variables, particularly fermentation temperature. Gao et al. (1997) demonstrated that skins that were heat-treated and cooled prior to fermentation do not necessarily yield more intense color when subsequently fermented at low temperatures. This is due to increasing levels of favan-3-ol (tannin and its building blocks) extraction at higher temperatures. Higher levels of flavn-3-ols, particularly during the early part of fermentation, bind with anthocyanin in a process known as co-pigmentation. (Sacchi et al., 2005) These structures essentially store anthocyanin until it can be polymerised into larger, more stable forms. (Boulton, 2001) The timing is particularly critical, as post-heat treatment monomeric anthocyanin is known to sharply decrease after the first few days of fermentation. (Gao, Girard, Mazza, & Reynolds, 1997) Interestingly, the combination of thermovinification and fermentation on the skins has been shown to yield higher levels of flavan-3-ols than either method alone. (Netzel et al., 2003) To summarize: Without a mindful eye on the fermentation temperature , the color benefits of heating the skins are lost.
Effects on Sensory Profile
We know color goes a long way to impress at the cellar door, but how does the wine actually taste and smell? It is no secret that thermovinification, perhaps unfairly, does not have the best reputation here. One criticism is that a heat treatment will surely lead to a cooked characteristic in the resulting wine. This claim has been shown to be overzealous, if not completely unsubstantiated. (Girard, Kopp, Reynolds, & Cliff, 1997; Ough & Amerine, 1967) In fact, as was the case above, much of the literature is unable to isolate the effects of pre-fermentation heat treatments, when it comes to the production of flavour and aroma compounds. (de Andrade Neves, de Araújo Pantoja, & dos Santos, 2014; Fischer, Strasser, & Gutzler, 2000; Girard et al., 1997) That said, there are a few things we can say with some confidence.
It may go without saying by now that many elements of taste perception are functions of flavan-3-ol extraction. While flavan-3-ols are largely unaffected by pre-fermentation heat treatment, an increased base level of anthocyanin may encourage the formation of polymeric phenols, and most notably tannins. (Sacchi et al., 2005) Polyphenols have been shown to increase in astringency as they expand and move upward in molecular weight. Bitterness, on the other hand, has not been shown to follow a clear pattern with regards to its molecular structure. (Preys et al., 2006) However, there is some support for the commonly held notion that polymeric compounds are less bitter than their monomeric counterparts. (Robichaud & Noble, 1990)
As noted earlier, a number of enzymes are deactivated by heat treatment. Among these is lipoxygenase, which is responsible for the production of C6 alcohols in wine, known as higher alcohols. (Girard et al., 1997) Higher alcohols are usually perceived as wine faults, and have a number of associations within the “green” or “unripe” spectrum. Another bane in the existence of the red wine grower is iso-butyl methoxypyrazine (IBMP), which is often likened to green pepper. Research has shown that thermovinification can evaporate IBMP at a temperature of 50°C (122°F), leaving no trace of the pesky compound. Wild yeasts are also a source of unpredictable wine aroma, and temperatures as low as 52°C (125°F) are enough to kill some yeast strains. (McAlister & Finkelstein, 1980)
This brings us to our next point. What are we potentially losing with thermovinification? The strongest argument against the practice is that much is lost by the denaturing of proteins and enzymes with the intense heat treatment. For instance, while wild yeast are in many cases responsible for off-odours in wine, they can also produce a variety of favourable effects. Hypothetically, beneficial proteins might be denatured as well, resulting in a less favourable perception of mouthfeel. This an oft-cited criticism of thermovinification, but evidence to support it is surprisingly lacking.
One very legitimate concern is the heat inactivation of pectolytic enzymes, which are responsible for breaking down grape skins to enable the extraction of compounds and the clarification of juice. Work on apple pectolytic enzymes has identified 50°C (122°F) as a critical point in the deactivation of these enzymes. (Ceci & Lozano, 1998) Likewise, some difficulties with clarification and filtration have been noted after a thermovinification treatment, due to the lack of pectinase. (Jackson, 2008) An easy solution here would be to simply add pectinase back into the system after the heat treatment.
In a perfect world, this technique would probably not have been invented. Instead, we live in a world where thermovinification is widely used in cool climate wine regions throughout the world, as a means of salvaging a difficult vintage. There are drawbacks, as with any miracle elixir, but many of these are poorly understood and seem ideologically driven at times. The benefits, on the other hand, would seem difficult to ignore in a challenging year. For that matter, reducing the risk of oxidative browning due to microbial enzymes has appeal in any year. That, together with the potential for higher extraction of anthocyanin, makes this technique certainly worth a look.