In Alcoholic Fermentation and Yeast, Processing

There is always a major lesson (or many) to be learnt in each harvest. A few years ago after planning space, bottling etc., I was convinced we had enough space at a cellar where I consult to take in the entire crop to spare. Little did I know the owner, with dreams of grandiosity, decided to buy in a few extra hundred tons. After three weeks the cellar was loaded to capacity and we were in full swing harvesting Sauvignon blanc.

For the first two days we could rack the SB. After that we could not even rack. Various options such as storage elsewhere and hiring tankers to stand for a day so we could rack in and out diminished to such a point that we were forced to ferment almost full tanks (1% headspace) on 100% lees.

Our cooling system was chilled water so the minimum temperature we could get was about 12°C. So I decided to inoculate at a rate of only 5 g/hl of Anchor VIN 13, as I knew this yeast gives very little foam. It is aggressive though, so my fear was excessive reductivity.  I hoped to control this by keeping the fermentation temperature as low as possible (12°C), racking off the lees at about 5° Brix was also not an option at this stage.

I added 5 g/hl Anchorferm (inactivated yeast based nutrient), also at well below recommended dosage, so that I could avoid reductive characters. No ammonia was added at the early stages of fermentation. I took some comfort in the fact that the University of Bordeaux recommends high NTU content of about 200 NTU in SB for better formation of mercapto-pentanones, ours was over 400!

The fermentation kicked off at a phenomenal pace and after three days all tanks started H2S formation. I tried to ignore this until we had reached a residual of 7°Brix. At this stage I dosed with ammonia and started praying. (Funny how I always seem to pray when I am in total stress…)

Well, the H2S stopped within a few hours and most tanks finished dry without a recurrence. Copper man did not even have to make an appearance. Most tanks still had to lie on full lees for three to four weeks after dryness before we could get enough space in which to work.

Finally the resultant wines were assessed and my conclusion was that there were higher levels of tropical fruits in the lees fermented wines.  Wines fermented with clear juice had higher ester levels. Our final blend was a combination of the two “styles” for our top label and the 100% lees wine for our second label. (Perhaps the wrong choice, as two years down the line the “lees” wine is showing better and better in the bottle!) No discernible bitterness on the “lees fermented” wines, they also show better mouth feel.

Lessons learnt when fermenting SB on full lees:

Keep yeast inoculation and nutrition at an early stage to a minimum.

Try and rack at 5-6°Brix.

Ferment with a good dose of Bentonite.

Keep the fermenting must cold (12- 13°C max).

Nitrogen addition as late as possible (but do remember nitrogen addition will have no effect if added too late).

The conclusion I came to was that it is possible to get a high quality wine when fermenting on full lees; however temperature control and various other factors must be controlled strictly. Above all remember no one knows everything. So, as said in the Hitchhikers guide to the galaxy, “Take along a towel and don’t panic.”

Mike Dobrovic is the former winemaker of well-known South African wine estate: Mulderbosch. He currently does some winemaking consulting, grows apples on his farm, writes poetry and dwells on all things spiritual.

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