Another vintage and the usual suspect samples, and more, arrived in our laboratories. The main feature of this vintage that will be remembered is the highest atmospheric temperatures ever recorded in Southern Australia. The temperatures in the high 40s on the Celsius scale that were experienced in February affected fruit throughout South East Australia with many vineyards having fruit that was burnt or shriveled. The high temperatures also contributed to the tragic Victorian bushfires that caused loss of life and property. Many vineyards were also affected with fruit that was tainted from smoke exposure.
Without doubt the main issue we have been involved in this vintage has been smoke taint, predominantly in those regions of Victoria that were affected by the horrendous bushfires that started in early February. The fires took many weeks to be controlled and eventually extinguished. During this time much fruit throughout a number of regions was exposed to bushfire smoke.
We have analysed hundreds of samples of grapes for smoke taint since February with varying results of the marker compounds guaiacol and 4 methyl guaiacol (G/4MG) that are used as an indicator of smoke taint. Table 1 provides a summary of these results. The threshold value where many winemakers reject affected fruit is 1 part per billion (ppb) of guaiacol.
As can be seen from Table 1 the incidence of guaiacol levels above 1ppb varied by grape variety. It is not possible to tell from our data if this relates to a particular variety being more prone to picking up smoke taint or if the sample came from a vineyard that was more affected by the smoke because of its locality.
A list of the major varieties received, the approximate relative amount of each variety received, and the approximate percentage incidence of recorded guaiacol levels >1ppb.
|% of total smaples
Variety % of total samples % Guaiacol >1ppb Cabernet Sauvignon 5 40 Chardonnay 20 30 Merlot 5 5 Pinot Noir 30 75 Sauvignon Blanc 10 55 Shiraz 10 65 Other 20 35
Fortunately many samples were not tainted and so were used for winemaking. Smoke taint is an area of active research by several groups in Australia with many questions still outstanding.
Some of the main questions still to be answered are:
- how much G/4MG is acceptable in the fruit?
- what level of G/4MG is acceptable in finished wine?
- does this differ between white and red wine?
- what is the relationship between the level in fruit to that in finished wine?
- are some varieties more prone to the taint than others?
The usual suspect stuck ferment samples also turned up in our labs. Numerous reports of ferments that generated hydrogen sulfide and those that slowed down or got stuck were received. As is usually the case many of these could have been prevented with correct preparation.
One important aspect of yeast fermentation relates to the nutritional requirements of the yeast used. For many years we have been offering Yeast Assimilable Nitrogen (YAN) analyses and also selling YAN kits so that winemakers can perform their own YAN analyses on their in-house spectrophotometers. In spite of the fact that winemakers can now easily measure the amount of YAN and decide how much yeast nutrient they need to add, we still receive some ferments that become stuck simply for lack of nutrient additions.
One sample we received of Cabernet Sauvignon had become stuck at 23 grams per litre of sugar. This wine had 22 grams of fructose and 1 gram of glucose, giving a fructose: glucose ratio of 22. A rule of thumb for restarting stuck ferments is that if this ratio is greater than 10 then the ferment becomes very difficult to restart.
Upon discussing the situation with the winemaker he declared that he never adds any nitrogenous nutrient to a ferment and has never had any problems before. It is well known that the vast majority of grape juices do not have enough nitrogen in the form of YAN (a combination of ammonia and amino acid nitrogen) to support Saccharomyces cerevisiae getting the must to dryness. To say this winemaker has been lucky to not have had any stuck ferments in the past is an understatement!
As well as juice and must analyses, we receive a large number of wine samples during vintage. Leading in to vintage, in order to prepare the winery for harvest, many wineries bottle wines that have been in barrel. At this point in the winemaking process some interesting examples of what can go wrong occasionally turn up.
One such wine submitted had a slightly yellow deposit that had developed in small amounts in the base of bottles of a white wine after bottling.
Microscopic analysis shows some of the magnificent specimens (see Figure 1) that had been deposited in the wine. Unfortunately, even though the crystals were striking in appearance, their preferred place is obviously not in the bottom of a bottle of wine.
Figure 1: Microscopic analysis of yellow deposit
These crystals were identified as being a form of calcium sulfate. An excess of calcium is one of the causes of these types of problems. The winemaker was not aware of any additives he had used that contained calcium, however he was going back through his winery records to double check that he was not using calcium-based additives.
Brettanomyces in a Chardonnay
This was an unusual sample; in fact in all the years we have been analyzing wine for the Brettanomyces marker compounds 4 ethyl phenol and 4 ethyl guaiacol (4EP/4EG), we cannot remember ever seeing a Chardonnay badly affected by Brettanomyces before.
The wine had been bottled and upon tasting by another winemaker with a more sensitive palate than that of the original winemaker’s, it was decided to submit a sample for 4EP/4EG analysis. This winemaker thought he detected 4EP but wanted some confirmation of his suspicions.
The results for this wine were 390 ppb 4EP and 260 ppb 4EG. Although the winemaker had detected 4EP upon tasting, it was actually the 4EG that was of a high order. The ratio of 4EP/4EG was also very low – the 4EG was higher relative to 4EP than is normally encountered. Normal sensory threshold levels of 4EP/4EG are around 400 and 80 respectively.
This situation has only just come to light so we are not sure of the cause of the infestation. However, we suspect that the Brettanomyces had previously been present in the barrels that were chosen for the maturation of the Chardonnay. Normally the lower pH and the greater effectiveness of sulfur dioxide in white wines prevent this from developing. This case simply illustrates that it is not only reds that should be monitored for Brettanomyces and that the usual good winemaking practices should be followed in all wines.
A number of interesting samples have been tested by our labs this vintage. The incidence of smoke taint in Victoria was quite marked, as would be expected from a severe bushfire season. Other more typical, but generally preventable winemaking issues emerged in the samples that we analysed over the 2009 vintage period.
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