The general concept here is to add sugar, nutrients and potassium carbonate to the wash, incrementally over time to reduce stress on the yeast. It’s advantages are:
*Reduces negative osmotic pressure on the yeast cells
*Keeps the Ph in an optimum range
*Control the rate of fermentation, reducing overheating
*Achieves a higher sugar to alcohol conversion
This information applies to those who are interested in vodka distillation. Achieving a higher alcohol content of the wash means less time distilling and there is no reason this can't be done while avoiding off flavors or stuck fermentations. In the wine industry high alcohol washes are common with Zinfandel averaging 15.5%.
Sugar and Osmosis
Increasing the amount of sugar is correlated to higher levels of yeast activity up to a 10% sugar threshold, at which point yeast growth becomes inhibited. The problem is that the high concentration of sugar creates a negative osmotic pressure on the yeast cells depriving them of water and nutrients essential to their reproductive cycle. The osmotic properties of a yeast cell are due to selective permeability of the cell wall with regard to solutions. This selectivity plays an important role in controlling the movement of nutrients into a cell. Nutrients are present in the wash in the form of ions, sugar, and amino acids. The permeability of the cell wall also permits the release of alcohol and carbon dioxide from the cell during fermentation. Yeast then is very sensitive to the osmotic conditions it finds itself in. This sensitivity varies with different yeast strains, with some being better suited than others for fermenting. I wonder if “high alcohol resistant” yeasts, like turbo, aren’t really Osmotolerant yeast something the baking industry has been using for years.
Rehydration
Re-hydration of yeast in water is recommended because of the principles of osmosis. Simply tossing dry yeast into a high sugar solution won’t help the yeast get off to a healthy start. In a wash with a high concentration of dissolved sugar, the water that the yeast needs cannot be drawn across the cell membrane to wet it. The water is instead locked up in the wash, hydrating the sugars. Scott Labs makes a rehydrating solution called Go-Ferm. Here is what they say: "Go-Ferm® is a natural yeast rehydration nutrient containing a balance of micronutrients. It was developed to enhance kinetics and thereby potentially avoid problem fermentations. Suspend Go-Ferm in the rehydration water before adding the selected active dried yeast culture. The yeast soak up the valuable bio-available micronutrients as they rehydrate. Infusing yeast with these essential nutrients arms them against ethanol toxicity and optimises nutrient availability to the rehydrating yeast culture. The result is fermentations that finish stronger. "
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Ph and carbonic acid
The next problem with high sugar concentration washes is the production of carbonic acid from CO2 . More sugar means more CO2 which in turn produces more carbonic acid. The excess CO2 generated will cause a rapid drop in Ph. Since there is no buffering material present in the sugar solution, the pH will drop from an initial pH of about 5.0 to 5.5 down to as low as 2.7. This low pH will stress the yeast and it may never recover. Numbers like this can lead to a die off of the yeast.
Rate and excessive heat
Higher sugar solutions can lead to excessive heat and are another source of yeast die off thus several manufactures of yeast nutrients often recommend metering in several doses appropriate nutrients to control the rate of fermentation. Flavor compound formation is affected quite heavily by the fermentation temperatures; higher temperature fermentations produce less esters and more higher alcohols. Yeast microbes contain biological catalysts called enzymes. These enzymes speed up the conversion of sugar to alcohol. Most enzymes are proteins, and can be destroyed (denatured) by heat. So adding the nutrients along with the sugar seemed to make sense in that the supplements are there when the yeast are ready for them.
Nutrients and substrates
Two common yeast supplements are Fermaid K and DAP.
Information on Fermaid K: http://www.scottlab.com/product-104.aspx" onclick="window.open(this.href);return false;" rel="nofollow
DAP (Di-Ammonium Phosphate): Source of yeast assimilable nitrogen to inhibit the possible formation of hydrogen sulphide (rotten egg gas) during fermentation, and to assist in obtaining complete fermentation of the sugar. Using DAP in conjunction with Fermaid K is a common practice with some sugar washes.
Substrates
I tested Fermaid K and DAP in a stand alone fashion and didn’t get the high conversion rates Alcotec or turbo gets. I then tried using a substrate. By substrate I mean a organic material the supplies nutrients among other things, to the yeast. I don’t think that a substrate is ultimately needed but I tested several and they all outperformed washes without them. From all the reading I have done on this and other sites I chose tomato paste. Tomato paste contains all the nutrients listed in other supplements but I don’t think that is the reason they work so well. It may be that they contain phytochemicals, or it could be something we haven’t thought of....
I looked at several, apple including the pulp and barley also did well.
Test Equipment
I was totally dismayed at how inaccurate the various methods of measuring alcohol content were so I built a ebulliometer. This turns out to be very accurate especially towards the end of a ferment. More information on the one I built can be found here: http://homedistiller.org/forum/viewtopi ... =2&t=43241
Conversion of sugar to alcohol
The convention for estimating potential alcohol is that the expected alcohol content of a sugar solution is 16.5 to 17.2 grams of sugar per 1% alcohol per liter, by total volume. In practice, ethanol yields observed are 90 to 95% of theoretical. I use a goal of 90%. Using those numbers then at 90% efficiency we should be able to get 16-17% from a total volume of 3.1 liters containing 1kg sugar.
The tests:
Set 1: Alcotec yeast did a amazing job fermenting up to 16.5% in a very short time. According to their advertising it should have gone higher. I am concerned about the rate going from1% to 14% in three days. A plain sugar wash or a sugar wash with some nutrients was not enough.
Finally there are the taste tests. These fell into two categories. Tasting the wash, then distilling and blind tasting the results.
Wash tasting:
I was surprised to find #3 and #4 Tasted similar to a diluted wine with a yeasty note, like fresh bread on the nose. No off taste.
Vodka tasting:
The wash was distilled to 95.6% purity. The taste test is done blind...the same way I have conducted wine tastings over the years, I included 3-4 high end products i.e. Absolute, Gray Goose. etc. I had four different tasters. This product came out the winner over all of those.
Finally this is not a scientific effort so don’t take it as fact. The principals of metering nutrients and adjusting Ph during the ferment are fairly common and well established. Metering sugar creates osmotic conditions favorable to the yeast so I think it’s worth further exploration.
Cheers,
Max
So by metering sugar into the wash over a period of several days you get higher alcohol concentration without the off tastes. I will try it myself but will use rice bran as my substrate.