Yeast Nutrients


The objective of this post is to give a useful summary about 
(i)            Yeast nutrients
(ii)           Their role in fermentation
(iii)          How to calculate nitrogen requirement of a yeast
(iv)          What to supplement the ongoing fermentation with, when the must/wort is low on nitrogen.

At the end of this post, I hope you get a better understanding of how the nutrients work. I have included links and references at the end. If you think the post is getting too geeky just skip to part III. The information in boxes is something extra too, skippable if you are not into it.

Part I. About Yeast Nutrients

Yeast metabolises sugars in an alcoholic fermentation to generate ethanol. For this it needs to first grow in a healthy fashion so as to produce minimum off-flavours. And for this, it requires nutrients. As a mead-maker it falls to us to make sure it has all that it needs to turn that must into delicious glass of mead.

Here’s a chart of the nutrients all the yeasts need




Macronutrients
Almost all macronutrients have 2 roles in the yeast cell, biochemical and structural. Both roles for each component are briefly given below

Carbon: Carbon comes in form of sugars and higher carbohydrates. Sugars are used for making energy and ethanol; while higher carbohydrates are used to make structural components of the yeast cell like the cell wall, cell membrane and inner skeleton of the cell that holds its shape (cytoskeleton).

Nitrogen: Nitrogen comes in form of inorganic nitrogen (ammonium or ammonia) and organic nitrogen (amino acids and short peptides). Nitrogen forms integral part of all the yeast proteins (enzymes and structural proteins) and nucleic acids (DNA and RNA).


Organic nitrogen that can be taken in by the yeast cell, is in the form of amino acids or very small peptides which are chains of 2-3 amino acids. Hence this nitrogen is called free amino nitrogen, or FAN. Inorganic nitrogen that the yeast can take in is in the form of ammonia gas (NH3) or ammonium ion (NH4+). It is almost always supplied as ammonium ionin form of di ammonium phosphate (DAP) or ammonium sulphate. Together, the organic and inorganic nitrogen present in the must that can be taken in by the yeast cell is called Yeast Assimilable Nitrogen or YAN

Phosphates: The main energy storage molecule in any living form including yeast is Adenosine tri phosphate (ATP). There are several other phosphates required for a normal cell cycle by the yeast. Structurally they form an important part of the nucleic acids as well.

Sulphur: It is present in 2 important amino acids cysteine and methionine. These amino acids are crucial in giving any protein it’s 3D structure. It is also a minor component of lipids and other cell fluids.

Oxygen: Oxygen is a macronutrient alright, but the way we provide it, is by aeration of the must. Oxygen is used mainly for breaking down sugars into COand H2O. The yeast in its growth phase takes in the oxygen and converts the sugars with the help of other nutrients into COand H2O. Structurally oxygen is used to synthesise the lipid component of the yeast cell membrane, which I have explained a bit more in the section microprotectors below

Micronutrients

These are required in small amounts by the yeast cell but in no way are any less important in their role for a healthy fermentation. These are
1.    Vitamins – biotin, pyridoxine, thiamine
2.    Minerals – magnesium, manganese, zinc, copper, potassium, iron and calcium

They are catalysts for all the metabolic functions and make life easy for the yeast (and us) by keeping the fermentation speedy and healthy. Their functions range from acting as co-enzymes during the ethanol synthesis to maintaining regular housekeeping in the yeast cell to buffering the inside of the cell environment to keeping the cell membrane healthy and fluid. We can go in as much detail as we want with each and every one of them.

Most of the micronutrients are bioavailable, which means they can be readily taken up by the cell. Which also means they can be taken up by the unwanted competition that may be present in the must, hence they should be added early, a lot of mead and wine makers add them at the yeast rehydration stage itself when they can be readily assimilated by the yeast cells and provide maximum advantage in gearing it up for a robust fermentation that is coming their way.

Microprotectors
Polyunsaturated fatty acids (PUFA) and sterols form the lipid component in the yeast cell. These are present in the cell membrane that protects the inside of the cell from the everchanging outer environment which is, at first, rich in fermentable sugars and at a comfortable pH but through the fermentation changes to low sugar, depleted nutrients, high acidity and alcohol.

Cell membrane is responsible for controlling what goes in the cell from the outer environment based on the requirements of the yeast cell for its survival and growth. The cell membrane is not rigid and fixed structure but fluid and dynamic. This fluidity is an attribute of PUFA. For biosynthesis of PUFA and sterols, oxygen is required. 

At the end of fermentation when the surrounding becomes harsh due to low pH and high ABV, the role of cell membrane becomes even more crucial in order to maintain the inside of the cell healthy so that no off flavours are produced by the strained yeast.1

If you want to geek out more on roles of macro and micronutrients, there is a very good article written on this topic by one and only Ken Schramm in Zymurgy several years ago.2


Part II. Which nutrient is needed when

According to Scottlab 2016 handbook different nutrients are critical at different phases in the fermentation.3
Onset of fermentation: Vitamins and minerals. 
Mid-point of fermentation: Nitrogen and oxygen
End of fermentation: Protectors such as polyunsaturated fatty acids (PUFA) and sterols.

There is a practical aspect to which type of nitrogen source is to be used when too. Inorganic (DAP) and organic (FAN) are the two types of nitrogen sources that we can make available to the yeast. But which to use when?

Now imagine that you are sitting home all day without burning much energy and I give you a box of sweets to finish. With no avenue to burn all that sudden influx of energy in your blood, you will go on a sugar high. That’s what DAP does to a yeast cell. Whereas, if you munch on some low glycaemic index food like beans or whole grains, the blood sugar level will be better managed by your body, it won’t spike suddenly, and you won’t go jumping around the house hysterically. That’s what FAN does to the yeast cell.

DAP is super easy to take up for the yeast cell and has instant bioavailability leading to speed bursts in fermentation. FAN on the other hand is taken up more slowly and doesn’t cause bursts in the fermentation and spikes in temperature but leads to a steady fermentation for longer duration. 

Conversely, if you are running a marathon and I give you a protein bar, you will use the remaining energy to hit me and ask for glucose solution for instant energy. Similarly, if in an ongoing fermentation nitrogen is depleted (the clue is eggy smell) it calls for instant action and that is when DAP comes in handy.

BOX 1
The amino acids cysteine and methionine are the sulphur containing amino acids. 

When the nitrogen in the must/wort is depleted and there’s still some fermentable sugar remaining, the yeast cell starts chewing up these amino acids to make some new ones to ferment the leftover sugar. 

This results in release of H2S, that gives the fermenting musts a characteristic eggy or chemistry lab smell.

BOX 2
It is observed that as the alcohol content goes up in an ongoing fermentation, the cell membrane tightens its control over what can enter the yeast cell to protect it from alcohol toxicity. This also affects the uptake of DAP assimilation negatively. 
At this point, FAN is still able to enter the cell. Thus, once the fermentation reaches above 9% ABV level, it is pretty useless to add DAP to the must, so it is crucial to make sure that the must has enough nitrogen in form of FAN to last the fermentation if you are making meads more than 9% ABV.

Part III. Yeast nutrient dosing in fermentations

So now, how do we make use of all this information to our advantage?
A.   Find out in what concentration each of these components makes the best impact on the ongoing fermentation
B.   Provide a ready concoction based on the initial brix and available YAN in the must by adding yeast nutrients like Fermaid K/O/A
C.   Make your own concoction by adding a few easily available resources here in India.

This is where I talk about yeast extract.

It is safe to assume a yeast cell that is living a healthy life will have all the nutrients it needs present inside it. So why not use that directly as a single source of all that a growing yeast needs?

Yeast extract (YE) is made by growing yeast biomass using a good growth media where they grow and make most of the micronutrients, amino acids, vitamins themselves. These healthy cells are then slowly dried to a temperature where the cell wall cracks, the cell membranes and organelle membranes lose their integrity allowing the contents of the cells, which were earlier compartmentalised, to mix with each other. Some degenerating enzymes break down the proteins and peptides into smaller pieces and alpha amino acids. When the cell wall breaks, it releases the vitamins and minerals that the yeast has made. After this process this biomass is dried into a powder called yeast extract. It is available at your local laboratory supply shop. Go for any brand of yeast extract powder for bacteriology and you are good. Good brands are HiMedia, Qualigen are some well-known and good brands, but there are other local suppliers who may be good too. I have had good experience from the stuff I bought my stuff from this guy (7).

YE has about 8-12% amino nitrogen. So 1g/L of YE will roughly provide 100 ppm of FAN.

You will find a lot of information on calculating nitrogen requirements online.4

The section YAN part II, calculating YAN requirement in this white paper on reddit is my favourite way.5


BOX 3
Once your recipe is set and you know how much sugar and nitrogen are present in the must, and if you are using yeast nutrients like Fermaid O, go to this calculator.6

Put in the details as asked and then get a number on how much Fermaid O is to be used. 

But if you plan to use YE like I suggest, we have to do further calculations:

Let’s look at a mead must of OG 1.070 being fermented with a low nitrogen requiring yeast like D47. (The highlighted cells in the chart below. For a 20 litre must the TOSNA 2.0 calculator gives you 13.47 g of Fermaid O.
This means 13.47/20= 673 mg per litre of Fermaid O

Now,
1 g/L of Fermaid O ≈ 40 ppm nitrogen (According to Lallemand)
673 mg/L of Fermaid O ≈ 40X673/1000 = 26.94 ppm Nitrogen

Now we know that the YE we use has 8-12% nitrogen. Let’s consider an average of 10%, which means
1 g/L of YE is equivalent to 100 ppm nitrogen
Hence, to achieve 26.94 ppm nitrogen in the must, we must add
269 mg/L of yeast extract. 

I have given below a chart that I made by using the calculations in the box 3 for different gravities ranging from 1.05 to 1.13. You can totally skip reading the box, and use the chart for reference, but I just gave it in case someone is interested in knowing why these numbers. 

In addition to the YE, I recommend adding a small dose of DAP (50% of YE) to get the tempo going as well. The yeast extract should provide enough vitamins and minerals, but DAP also provides phosphate in addition to the nitrogen. 

Magnesium ions are a requirement for many synthase enzymes and supplementing the must with 0.2 g/L of MgCl2has always shown me faster ferments. Apart from very light honeys, other macro and micronutrients usually don’t pose a lot of problems at homebrew level, but if you are enthusiastic, there’s a lot more available on vitamin additions to the fermentation online too.

˚Brix
Gravity
Low N requirement
Medium N requirement
High N requirement


YE (g/L)
DAP (g/L)
YE (g/L)
DAP (g/L)
YE (g/L)
DAP (g/L)
12.4
1.050
0.20
0.1
0.24
0.12
0.33
0.17
17.1
1.070
0.27*
0.14
0.33
0.17
0.45
0.23
21.6
1.090
0.34
0.17
0.41
0.2
0.57
0.29
25.9
1.110
0.41
0.2
0.50
0.25
0.68
0.34
30.2
1.13
0.48
0.24
0.58
0.29
0.81
0.40
*Used to explain the calculations in the box above.

Once you have these calculations ready, for staggered nutrient additions, follow these steps:

1.    Prepare the must as per the recipe
2.    Rehydrate the yeast and pitch in the must
3.    At t=24 h dissolve 1/3rdof all the above nutrients (YE, DAP and MgCl2) in a small amount of boiled cooled water and dump this nutrient concentrate in the fermenter. 
4.    Shake vigorously to dissolve oxygen.
5.    Degas/aerate every day till the 1/3rdsugar break is reached
6.    At 1/3rdsugar break, add 2ndround of the nutrients, follow same procedure as step 3.
7.    At 2/3rdsugar break add the remaining nutrients just like the earlier 2 lots.

Please note, that these calculations assume negligible nitrogen present in the musts. If you are doing fruit additions, please adjust for the nitrogen present in fruit to avoid over dosing the musts with nutrients.

Some additional tips:
Always store all the chemicals in a cool place, in air tight containers and use before the expiry date if any.
I usually write the date I opened the seal on the bottle, that gives me an idea of how much is my usage and when to refresh my stocks.
Magnesium chloride is best bought in the smallest available packaging size, cuz it is very hygroscopic (absorbs moisture) and becomes unusably watery pretty soon.
Ensure that you get the food grade chemicals or be mindful of the percent impurities mentioned on the bottles.

Mic drop!

References:



1 comments:

  1. Hats off to your passion towards brewing and sharing such info to all so generously.

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