Sunday, 08 March 2015 00:00

Winemaking practices on colour, phenolics

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Photo supplied by NZW. Photo supplied by NZW.

In my previous article (NZ Winegrower issue 88) I outlined the various phenolic characters in red wine production and what their role is from fermentation to final wine. That was the easy part!

The next step is looking at how winemaking practices influence final phenolic outcomes.

The picture of phenolic extraction is a murky one and significant papers in the past have contradicted each other on the outcomes. Given this, are there any definite “rules” to the phenolic game? I believe so.

Let us first consider what it is we want to achieve from tannin and colour extraction. Organoleptically: A red wine with a good stable colour and enough grip to help the wine age but that does not overwhelm the fruit, and to provide a good accompaniment to food.

Chemically: a wine with a high degree of polymeric anthocyanins (stable colour) and optimal conditions to keep binding the monomeric anothocyanins to a stable form; manage the extraction of skin/seed phenolics to press off at the optimal balance between the two.

So if we dive right into the most obvious winemaking differences, let’s see if we can find a rule of thumb………

The most obvious first choice is how to process the fruit. A full de-stem and crush allows earlier seepage of anthocyanins from the broken skin cells leading to a good colour in the must and a high level of monomeric anthocyanins or anthocyanin (A+A) complexes (self-association) which is not very stable. However, without a huge amount of phenolic material to stabilize them, this colour could be very transitory.  Full de-stem but no crush (ie whole berries) changes the extraction dynamics and slows the process down but theoretically is the same as 100% crushed fruit. With the addition of whole bunches into de-stemmed or crushed fruit you are, in essence, adding a potentially very useful source of both flavanols and flavonols from the stems. Flavanols strongly bind with anthocyanins to form stable colour compounds. Flavonols also bind with anthocyanins but this is a weaker bond and is only a temporary fix for colour stability. 

Okay, given that, what if we throw pre-ferment soaks in? This is probably the most controversial of topics and it certainly has been a question that has plagued researchers for many years. I don’t believe we can talk about cold soaks without specifying variety. As I understand, Pinot is the most likely variety for cold soak preferment. We know that phenolic development in Pinot wines is very different to other varieties. During fermentation, anthocyanins can be extracted, and peak before a good level of skin and seed phenolics are extracted, leaving the colour potentially vulnerable to losses or oxidation. However, Bordeaux varieties and Syrah extract colour and phenolics at roughly the same time during ferment. So, theoretically, if you are going to cold soak Pinot, adding some element of whole bunch to provide a phenolic substrate for the colour stability makes good sense, whereas, other red varieties already have a great potential pool of phenolics to stabilize the colour.

There have been a number of papers investigating varying additions of sulphur, tannin, enzymes, acids, sugar, dry ice, oak chips and other varieties to co-ferment all in an attempt to provide better, more stable colour and pleasing phenolics. A lot of these papers contradict each other or show that, after time, these effects are negligible. Some papers only look at results straight after ferment and others report a consistent loss of colour or phenolics long term (eg adding tannin). It is also important to consider the size of the ferments in the experiment – can we scale up from coffee plungers to commercial reality and get the same effect? So, in my opinion, these areas are best left to trial commercially in your winery to determine if it suits your winestyle, philosophy and vintage logistics.

The effects of cap management during ferment also is a pretty difficult topic to research and provide good solid guidelines on. Pre-ferment and during fermentation, cap management is a matter of common sense – keeping the cap wet to prevent volatility, distributing heat throughout the tank and optimising phenolic extraction from the skins and seeds. There are a number of papers which have tried to compare differing managements but many of them are………..contradictory! Again, it is very difficult to compare results as things like temperature, length of pumpover or plunging, the technique, how thorough the mixing was, were not specified.  So, no real rule of thumb here in my opinion. 

Which leaves us finally with maceration post ferment. This, in my opinion, is where the biggest changes occur and where the most decision making needs to happen. What is happening to the phenolics and colour after the tumult of ferment? It is accepted that the increase in alcohol and high temperatures are required in order to extract the skin and seed phenolics.  There are many debates about the ideal temperatures and alcohol levels – again with few concrete rules of thumb. There is a general consensus that during extended maceration, we see an increase in the extraction of seed phenolics. There is quite often a loss of colour, which could be due to adsorption of anthocyanins on the yeast lees. Some papers have reported that with very long maceration (greater than 30 days) there is a loss of skin phenolics and colour, but although the mechanism for this has not been adequately explained, it is likely to involve precipitation of tannins from yeast proteins. One thing of note, is  varietal difference, specifically between Pinot and other red grapes.  It is well recognised that Pinot Noir grapes have a higher ratio of seed to skin phenolics, which also carries through into the wine. However, other red grapes have a lesser quantity of seed phenolics and in addition, fewer of these phenolics are extracted during fermentation. In essence, Pinot has a double hit! This has the potential to create unbalanced and bitter Pinot Noir wines and needs very careful management. Once you have extracted high levels of catechin, it is very difficult to ‘refine’ the wine during aging and prebottling.

All the research to date has only provided a starting point for explaining phenolic behaviour in wines. In the US many winemakers will use regular testing of phenolics to provide them with empirical data on their specific sites and wines. While this is of some use, it cannot be directly translated to our wines due to the real differences in site, soils, climate and varieties.

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