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Yeasts & Winemaking…Part II

Fermentation Phases:

So what’s this “Lag Phase” we referred to previously?   That’s the brief period of  time after inoculation [putting the yeast mixture into the must] that it takes for things to start humming [“humming” is a scientific term].  Briefly, there are four phases:

Lag phase:  The time it takes for the yeast to acclimate to its new environment. Sort of like a human moving into a new apartment or home. Some of the things it acclimates to are high sugar level, sulfur dioxide – if used, pH, and temperature.

Exponential Growth Phase: The phase in which the yeast multiplies exponentially. It will – in a healthy fermentation – reach about a 100 million cells per cubic centimeter. Sugars decline rapidly in this phase.

Stationary Phase: Yeast density is at its greatest. Growth of the yeast population is basically halted.

Decline Phase:   Nutrients are scarce here, and the yeast is struggling to make it through the day. Toxic byproducts are high – sort of like living in Los Angeles in the 70’s and 80’s, or Beijing today. The number of healthy yeast cells is decreasing gradually, and things are settling down.

Nutrients:

Like us, yeasts need nutrients to perform at their best.  Nutrients are commercial products that offer all the good stuff needed by any yeast.  Each contains micro nutrients, nitrogen – organic and inorganic, sterols, as well as growth and survival factors.  Some of these products are used when re-hydrating the yeast before putting it into the must. As strange as it may seem, yeasts need to be re-hydrated prior to adding them to the must. That’s an easy, but necessary process.  Basically, the winemaker calculates how much nutrient is needed for a given amount of yeast, He or she adds heated water to the nutrient, lets it hydrate for 20 minutes or so, until the temperature is “yeast friendly” then adds the yeast to the nutrient mixture.  That mixture now sets for a specific amount of time, during which – more often than not – one can see the yeast beginning to become active.  After about 12-15 minutes, juice from the must is added to jump start the yeast mixture. After this, the yeast and nutrient mixture are added to the must.

Sugar Levels (Brix)

The sugar level in the must has an effect on yeast performance. [To know the beginning sugar level in the must one takes a Brix reading (measured in density) using a simple instrument called a hydrometer. A Brix reading measures all the solid materials in the must – sugar, acids, salts, proteins, pigments, etc]. During the “Lag Phase” [See Above] the optimum yeast growth is between 15 and 20 Brix regardless of the starting Brix.  In essence, too much sugar can inhibit yeast growth and reproduction, or result in the death of the yeast by what we – unscientifically – refer to as “sugar overdose”.  As sugars are converted to alcohol, and the alcohol levels increase, eventually the yeast cells begin to die as they cannot survive past certain alcohol levels.  If you have heard of a “stuck fermentation”, you are basically hearing about a fermentation that stopped because the nascent wine contained either too much alcohol, or other debris that causes the yeasts to become injured or die.  Briefly, some of these other fermentation killers are too high a temperature during fermentation, too little aeration in the beginning stages of fermentation, increased fatty acids or all of the above.  Alone or in combination these factors can cause major problems.

pH or Acidity

The pH level of must is important. In our winery we always target a pH of 3.4 or 3.5 as the optimal pH for a red wine.  A white wine is usually more acidic with pH values of 3.1 to 3.3.  Fermentation slows at higher acids so white wines often take longer to ferment in general. This is not always the case – so don’t make it a rule!  pH coupled to fermentation temperatures are very important relative to whether or not a winemaker will have to deal with a stuck fermentation. [We have, and it’s no fun!]

Fermentation Temperature

In general modern commercial yeast can function in the temperature range 41-100 degrees F. We are not great believers in high temperature fermentations for a number of reasons. Foremost is that above 85-90 degrees F. we fear yeast death, and a stuck fermentation. Secondly at high temperature, alcohol is lost, and so are volatile small molecules that contribute greatly to the flavor and sensory aspect of a wine.  Some winemakers – and authors – insist that higher temperatures mean better color extraction, and more intense resultant wine color. We disagree.  Frankly, our wines are rich in color, often quite dark and eye catching, and we ferment at about 84-86 degrees, although with larger tanks we do have to deal with 90-93 degrees F. for a few days. How we do that is not the subject of this article, but basically we stir, and pump over, the wines to cool them.

White wines are fermented at lower temperatures (45-70 degrees F) as color extraction is not usually a major concern. Maintaining the fruity character in white wines is a major objective, and cool fermentation temperatures is one way to do that. Fermenting anaerobically is another. We – De Angelis Wines – have done the latter in our 2010 Sauvignon Blanc, and while labor intensive, the wine is distinct from most Sauvignon Blanc one finds anywhere…

So now that you understand how these little fellows need to be handled, do you think that’s it?  No – it’s not.  Along the fermentation road the yeasts need additional nutrients so that they can keep doing their job, and doing it efficiently.  Approximately 20-26 hours after inoculation of the must, an addition of a yeast nutrient is added at half the recommended dose. These nutrients – almost all of them – provide inactive yeast cells, free amino acids – organic nitrogen source – , sterols, and a variety of key nutrients.  This same nutrient is added in the same dose at about 1/3 sugar depletion. Sometimes when we are near the end of fermentation and it looks and feels tired, we add a small amount of additional nutrient to take it over the finish line.

Actually by this time the yeast that was so viable for so long is now just a ghost of itself!

Done! Fini with Part II!

 

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