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.
Various parts of dead yeasts can be used as a source of nutrition / alcohol tolerance for live yeast cells during fermentation. Sometimes they are used in combination with inorganic nitrogen such as DAP and sometimes they are used on their own. Here is a short explanation of the different types:
Inactivated yeast – the whole yeast cell has been killed by heat. It contains the cell wall, the cell membrane and the whole inside of the yeast. Inactivated yeasts are a source of vitamins, minerals and to a lesser extend amino acids. As the alcohol content of the must increases during fermentation the dead yeasts’ cell membranes become more and more “leaky” and more of the inside goodies of the dead yeasts leak out to the must where the live yeasts can take it up. This does not happen to live yeasts since live ones can regenerate their cell membranes. They can keep it together so to speak. Inactivated yeasts are found in products such as Fermaid K and E (Lallemand) and Maxaferm and Nutrivin (Oenobrands).
Yeast autolysate – the whole yeast cell is killed and then exposed to lytic enzymes at 45°C for a certain time period. The result is that the cell wall, that contains glucans, is partially degraded and the cell membrane and the “soluble inside” of the yeast are more exposed, and therefore more available, to the hungry fermenting yeasts (cannibals) lurking around for a bite. Commercial examples are Natuferm (Oenobrands), Go-FermProtect (Lallemand) and Dynastart (Laffort). These products are usually added separately from inorganic nitrogen. They are applied for specific purposes and they are more effective than normal inactivated yeast. Apart from the normal role of nutrition they also do the following: Natuferm = aroma enhancement, especially esters; Go-Ferm Protect = source of sterols and Dynastart = aroma enhancement, especially thiols.
Yeast cell walls /hulls / ghosts – this is the insoluble yeast cell wall fraction of yeast autolysate after centrifugation. Depending on the washing process used during the manufacturing of yeast hulls, they may or may not contain parts of the cell membrane. Commercial examples are Extraferm (Oenobrands) and Springcell (Bio-Springer). Technically they are not nutrients, they detoxify the must from medium chain fatty acids produced by sluggish yeasts.
Yeast extract – the supernatant of yeast autolysate or in plain English: the soluble insides of yeast cells once the insoluble cell walls and cell membranes have been removed. This is found in Superfood (Vinotec).
Specific yeast fractions – e.g. mannoproteins. Mannoproteins are a specific cell wall constituent and production thereof requires further processing of yeast cell walls. Technically they are not nutrients either. They assist with tartrate stabilisation and mouthfeel. Commercial examples are Claristar (Oenobrands) and Mannostab (Laffort).
For more detailed info on the topic go the technical article with the same title.
Karien O’Kennedy is the Online Communications Manager of Oenobrands and knows the odd thing or two about winemaking and fermentation.
Now you probably wonder what old Jack Sparrow had to do with this…Interestingly enough, Stinkpots, favorite weapons of pirates, were malodorous concoctions made from saltpeter, limestone, asafetida (a vile-smelling gum resin), and decayed fish that were packed into earthenware jugs, ignited, and hurled onto an enemy ship…
Most of us had to manage a stinkpot brewing in our cellar at some stage of our lives, whether it was a result of a power failure (load sharing for South African winemakers), too little nitrogen, or just a general struggling yeast population. And then of course, if the H2S turns into mercaptans… it will just about kill any odorous component remotely smelling of fruit.
H2S contains sulfur in its most reduced form. Mercaptans are common organic compounds. When aerated, mercaptans can be oxidized to disulfides, which slightly change their sensory threshold and character. The oxidized forms are usually less of a stinkpot. These reductive components have unpleasant odor descriptors, and these can increase post-fermentation. Some of these, such as H2S and mercaptans, react with copper and can be removed by copper additions, usually in the form of cupric sulfate. In some wineries additions of cupric sulfate is a standard procedure, sometimes with an addition of inactivated carbon. All of us are aware of course that such an addition, particularly on Sauvignons, can reduce the concentration of aromatic thiols with up to 20%… I personally believe a slight element of H2S might actually contribute to the complexity of Sauvignon blanc.
But what to do? Well, the magic potion arrived in the form of inactivated yeasts impregnated with copper – Reduless®. It can be suspended with water and added to the tank directly after primary fermentation. The wine is racked off the lees after 72 hours. No residues, no potential risk of copper casse forming, but only a fresh smelling, wonderful wine. It has been developed by Lallemand for red and white wines to reduce H2S, DMS, DES and other stinkpot related off-flavors in wine.
Recent crackdowns on doping in sport have made all of us more aware of the effects of performance enhancing supplements. This got me thinking. What if winemakers could come up with a legal magic potion for yeast? Like the magic potion that enabled Asterix and Obelix to defeat the Romans time after time. Something that would give mere mortal yeasts super human (rather super yeast) qualities?
An increasing worldwide trend is longer “hang-time”. Delaying harvest might increase berry aroma and decrease acidity, but it creates a unique problem for our little athletes. Increased sugar leads to increased alcohol levels in wine made with these grapes. Fermenting yeast thus run the risk of being smothered in the alcohol they produce as a result of them snacking on sugar. Fortuitously, there is a magic potion that you can give your yeast to boost their viability during fermentation.
First, let’s look at the definition of sterols: “Any of various alcohols having the structure of a steroid, usually with a hydroxyl group (OH) attached to the third carbon atom. Sterols are found in the tissues of animals, plants, fungi, and yeasts and include cholesterol and ergosterol.” Here comes the interesting part. Sterols and unsaturated fatty acids (UFA’s) are survival factors during fermentation, but oxygen is needed for the synthesis of said survival factors. With insufficient amounts, the yeast cell membrane functions poorly, especially during highly anaerobic conditions and especially with increasing ethanol levels. Inadequate sterol concentrations around flux controlling proteins in the yeast cell membrane cause damage to the cell membrane and ultimately results in cell death (read: stuck or sluggish ferment!). The key role between oxygen and sterols now becomes evident. Simply put, controlled and timely oxygen addition = more sterol synthesis = better ethanol resistance = happy yeast = happy winemaker.
In my previous life, I’ve found it useful to add oxygen to red ferments anytime from a third of the way through alcoholic fermentation, up to halfway. This roughly corresponds with the end of the cell growth phase and research has shown that an oxygen addition of five to ten mg/L has a very positive effect on cell viability. Another trick is to combine oxygenation and nutrient addition with a pump-over or punchdown. Complex yeast nutrients contain inactivated yeast, which is a good source of sterols. The abovementioned trick also counteracts reductivity, which every winemaker deals with at some stage.
Research is ongoing to gain more insights into how yeast sterol uptake and synthesis affects cell viability. Ergosterol is one of the main compounds being studied, but I’ve also read a paper which outlines the addition of cholesterol to a fermentation! Fermenting yeast are just as happy with cholesterol as they are with ergosterol, but I seriously doubt if winemakers will be chucking cholesterol by the bucketful into their wholesome red wines!
Bernard Mocke is a technical consultant for Oenobrands.