In other articles, I’ve noted how nitrogen is one of the key compounds yeast need to turn apple juice into hard cider. It’s essential for protein synthesis and protein is needed to transport sugar into the yeast cell. Sugar creates the energy, ATP, needed for cell function and reproduction or what we prefer to call fermentation. While I have covered the types, role, and amount of nitrogen that is needed for effective fermentation, I thought it would be interesting to explore some of the nuances on the impact nitrogen or the lack of it can have on your fermenting cider. Let’s look at the following condition and impacts.
- Nitrogen and Heat
- Nitrogen and Aroma
- Nitrogen and Nutrients
Nitrogen and Heat
To yeast cells, not all nitrogen sources are the same and the reference to heat is not really the ambient temperature of your cider when fermenting or at least not directly. When yeast process sugar, they release and capture energy. The released energy is heat. When yeast process a lot of sugar in a short period of time, they produce a lot of heat (on a relative basis). Sugar is processed in the highest quantities during yeast reproduction or what cider makers call fermentation. When all this happens quickly, it can result in a temperature spike or what is called a Heat Peak(1). While your ambient temperature may be low or your juice may not appear to get hot, a heat peak can cause damage to your yeast because the heat is concentrated around the multiplying yeast. Even a cool juice could experience a heat peak condition. This is because the yeast simply utilize the available sugar too quickly. The reproduction process creates more yeast cells and these cells process even more sugar. This creates a concentrated area of heat or a Heat Peak. This can result in damage to the yeast cell. While yeast have numerous pathways and means to protect themselves and survive, they also create toxins and conditions that can cause harm. Ethanol and heat produced during reproduction/fermentation are just two examples.
So why might yeast reproduce too quickly? The usual answer is nitrogen. Yeast need ammonia and they get it from nitrogen sources or what is called Yeast Assimilated Nitrogen (YAN). Unlike grapes, most apple juice has enough naturally occurring YAN. For grape must, its high sugar content often requires nitrogen supplementation, usually in the form of diammonium phosphate (DAP) or what is often just labelled yeast nutrient. Adding excessive amounts of easily processed nitrogen will cause yeast to go into a hyper-reproductive mode. This can generate the concentrated heat pockets or Heat Peaks resulting in damage to the yeast cell.
The Moral: While nitrogen is essential for fermentation, too much can make yeast overact and cause heat related damage to the yeast cells. More is not always better.
Nitrogen and Aroma
While all yeast need nitrogen, the preferred source for nitrogen can vary from strain-to-strain. Also, not all sources of nitrogen can be assimilated by yeast. For example, proline is a nitrogen source that is common in juice but, most yeast can’t process it. Given a choice, most yeast will process inorganic nitrogen, like what comes from ammonia (DAP), over organic nitrogen sources, like what comes from amino acids. This presents an interesting aspect regarding yeast and how that impacts aroma.
The aroma of your cider is highly dependent on amino acids. Yeast process amino acids for nitrogen and other compounds that it needs to function and reproduce. Different pathways within the yeast cell take up the amino acids and extract the needed compounds. That can be nitrogen or other nutrients needed by the yeast cells. The left over compounds are often precursors to aroma. Fusel or higher alcohols are one common result of the breakdown of amino acids. Hydrogen sulfide (H2S) is another example of an aroma that can come from the processing of amino acids. That is not to say that all aromas produced by the processing of amino acids are bad. Research has shown that even bad aromas in small amounts can actually have a positive effect and that the aromas of a cider are generally better when organic nitrogen (amino acids) is used for yeast nutrition over inorganic nitrogen (DAP). The main point is that what nitrogen is available to the yeast will impact the aroma.
If you add inorganic nitrogen (DAP) to your cider before or with your yeast addition, the yeast will quickly absorb the inorganic nitrogen and ignore the organic nitrogen unless absolutely necessary. This means you limit the potential aroma development in your cider, good and bad. This happens because as yeast grow and begin fermenting their ability to process amino acids reduces(1). Alcohol weakens yeast cell membranes, which increases permeability of hydrogen (H+) ions into the yeast cell and impacts the acid balance of the cell. Since amino acids release hydrogen ions when processed, yeast will process fewer amino acids as alcohol levels increase. The ability to unlock aroma precursors and proteins that can positively impact your cider decreases.
The Moral: Adding inorganic nitrogen, especially early, can negatively impact aromas in your cider. Add organic nitrogen if your YAN is too low or to increase aroma potential. Use inorganic nitrogen later if you notice negative aromas like Hydrogen sulfide forming (H2S) as the yeast will quickly switch from processing amino acids.
Nitrogen and Nutrients
While nitrogen is generally considered the most important nutrients for yeast and fermentation, its not the only one. Yeast also need vitamins, minerals, and other compounds like proteins and lipids. In fact, it even needs oxygen. Yeast just need these compounds at much lower levels than nitrogen. Inorganic nitrogen generally will only contain nitrogen and a binder like phosphate. Organic nitrogen sources usually contain much more than just nitrogen. That is because the most common source for organic nitrogen is a yeast derivative product (YDP). These are inactivated yeast and usually contain many compounds like amino acids, vitamins, minerals, and even lipids. All of these are beneficial to yeast. Yeast use these to ensure their growth and reproduction (i.e. fermentation). We like these compounds because they help yeast create aromas, taste, and other organoleptic aspects as it turns juice into hard cider. But, it is the yeast’s health and the link between nitrogen and nutrients that is important.
In the aroma section above, I talked about how alcohol reduces the uptake of amino acids and how this can impact aroma. How yeast get the nitrogen it needs to function also plays a big role in how healthy it is and its ability to function and survive. The yeast membrane and cell wall is its primary means of protection and survival. A healthy cell wall requires many compounds or nutrients like vitamins, minerals, lipids, proteins, amino acids, and their derivatives. It also needs oxygen. The fermentation environment we put yeast into is a stressed environment. By cutting off oxygen, we force yeast to process sugar in a way to create ethanol. This isn’t its preferred pathway for processing sugar and reproducing but, it takes what is available and works with it. Its success depends on how healthy it is. If it has access to oxygen and nutrients when it also has large amounts of nitrogen available, it uses these nutrients to makes its membrane and cell structure healthy and strong. If it has access to lots of nitrogen but limited other nutrients and oxygen, it will grow and develop but the yeast will be weak and unhealthy. The result with be a yeast that struggles and has a reduced ability to ferment.
The Moral: It is important to initially provide oxygen and nutrients of all kinds along with nitrogen. If not, your yeast may not be healthy enough to fully ferment. This is where using a yeast starter for a harvested batch of yeast can be beneficial.
(1) K. O’Kennedy and G. Reid, Yeast nutrient management in winemaking, The Australian & New Zealand Grapegrower & Winemaker, Issue 537, pop. 92-100, October 2008
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