Yeast Nutrition: From Surviving to Thriving

By Rox Tiburolobo and Elena Fossati

Often as distillers, we find that one of the most overlooked aspects in creating quality whiskey or grain spirits is the nutrition needed to maintain healthy Saccharomyces yeast. Part of this is a lack of understanding of the more complex aspects of yeast metabolism and nutrition. Another part is because developing a proper nutritional strategy can often feel difficult, and in the short term, seem costly. As a result, many distillers take a “good enough” approach to yeast nutrition, hitting bare minimum requirements and leaving valuable ethanol and flavor congeners behind. In an increasingly competitive market, this can end up making or breaking a small distillery.

With this in mind, let’s look at an overview of what yeast need to flourish during whiskey and grain spirits fermentation, the pros and cons of various solutions to enhance the nutritive profile of common grains, the biochemical pathways of congener and off-flavor formation that are affected by nutrition, and how all of this can be used in your process to produce better yields of higher quality grain spirits.

What Is Yeast Nutrition?

In the case of whiskey and grain-based spirits, when we talk about nutrition we tend to focus on carbohydrates. We all know that yeast consume the sugars released from starches via enzymatic activity, and from this they produce ethanol, carbon dioxide, more yeast biomass, and flavor congeners. However, we often neglect the vitamins, inorganic ions, and nitrogen that are all essential in the complex chemical pathways that produce the latter two. These nutrients contribute to the overall health of yeast and result in fermentations that help produce greater yields and flavor complexity.

Less understood, but extremely important, aspects of yeast nutrition are vitamins and inorganic ions. Vitamins serve as important regulators, acting as coenzymes in numerous metabolic processes. B-complex vitamins in particular play vital roles as electron and functional group carrier molecules in a variety of oxidation/reduction and carboxylation/decarboxylation reactions as part of their metabolic functions. Additionally, they play roles in fatty acid, amino acid, carbohydrate, and choline metabolism, and serve a structural function as components of membrane phospholipids.

Similarly, inorganic ions in the form of various minerals and metals play a variety of roles as essential macro- and micronutrients with diverse functions that include cellular signaling (calcium), osmoregulation (potassium), enzyme cofactors (magnesium, zinc, manganese), and as building blocks for nucleic and amino acids (sulfur, phosphorus).

Maybe the most important, but often underutilized, element of yeast nutrition is nitrogen. Nitrogen has two crucial roles in yeast metabolism: an anabolic (building) role in the biosynthesis of protein, enzymes, and nucleic acids, and a catabolic (recycling) role in the synthesis of higher alcohols and esters. While Saccharomyces cerevisiae cannot fix nitrogen directly from the air, it can assimilate a variety of organic and inorganic nitrogen sources. Together these are referred to as yeast assimilable nitrogen (YAN).

In grains, nitrogen exists as organic nitrogen: nitrogen sources that are part of a carbon-containing molecule. These include free amino acids and small peptides, which are referred to as free amino nitrogen (FAN). Generally, most of this organic nitrogen is bound up in proteins and larger peptides that must be hydrolyzed to be assimilable to yeast. Inorganic nitrogen, which is not present in grain feedstocks, refers to simpler compounds without carbon, such as ammonia or ammonium ions, which yeast can readily utilize. Inorganic nitrogen is often incorporated into α-keto acid intermediates to generate amino acids. In both instances, amino acids are not only the building blocks for yeast proteins, but also the primary means of nitrogen transport in yeast metabolism.

The composition and potential deficiencies of these nutrients in various grain feedstocks can vary widely depending on the type and quality of grain being used. Grain mashes, particularly those of unmalted grains, generally contain sufficient concentrations of vitamins, but are often deficient in trace minerals and FAN. The amount of nitrogen needed to finish a fermentation is yeast strain dependent, but as a rule 200–250 ppm (mg/L) of YAN is required by most S. cerevisiae for growth and efficient starch-based fermentations. Usable FAN levels in common grains can average as low as 60 mg/L in corn mashes to upward of 500 mg/L in some malted barley mashes and thus generally require some sort of supplementation unless dealing with a pure malt feedstock. Additionally, some trace minerals and inorganic compounds may be present in sufficient amounts in the feedstocks themselves, or in mash water, while others will require exogenous additions. It’s important to consult your grain COAs and understand your water composition to get a baseline for the current nutritive content of your mashes.

Regulating Nutrition in Mashes

Now that we have an understanding that a combination of nitrogen, inorganic ions, and vitamins is important for yeast metabolism, we can work on filling the gaps in our nutrition profile with supplementation. This can come in a variety of forms, both endogenous and exogenous. Here are some of the most common and readily accessible:

• Ammonium and Phosphate Salts

Grains used in spirits production contain insignificant amounts of inorganic nitrogen, so this would be added exogenously and most commonly in the form of ammonium-containing phosphate salts such as monoammonium phosphate (MAP) and diammonium phosphate (DAP). While inorganic nitrogen is very good at quickly initiating fermentation, it is often consumed too readily, leaving yeast deficient and stressed toward the later part of fermentation. This can lead to sluggish fermentations and/or off-flavor formation. On the contrary, organic nitrogen in the form of amino acids and small peptides is assimilated gradually throughout the fermentation and thus it is often associated with better fermentation performance.

• Exogenous Proteases

With the limitations of inorganic nitrogen, proteases can provide more consistent nutrition throughout the fermentation process. As previously mentioned, nitrogen is naturally present in starch-based feedstocks, but its content is often insufficient or not readily available to yeast. Proteases are enzymes that cleave the proteins and polypeptides present in grain mashes giving a gradual release of FAN, resulting in faster kinetics throughout the course of the entire fermentation. Additionally, sugars are bound in the protein matrices of many grains, and hydrolyzing these matrices can release sugars that would otherwise be inaccessible to yeast. This can result in small increases in yield that may not be immediately noticeable but add up over time.

• Malted Barley

Malt contains its own proteases and some trace minerals that may be deficient in cereal grains. During the natural germination processes, proteases are produced to hydrolyze seed storage proteins, allowing the delivery of essential amino acids for embryo growth and development. These enzymes remain present in malt after the cessation of the germination process and are available to free up amino acids for yeast nutrition. Malt proteases have a narrow optimal range of activity between 113–131°F (45–55°C) and pH range between 5.0–5.2, meaning most proteolytic activity will occur before the start of fermentation but will continue, albeit more slowly, throughout the process if the barley is not added at the high cooking temperatures used to gelatinize some cereal grains. A major downside is that protein rests take time, between 15–30 minutes, which can add up when doing multiple batches during a day.

• Prepared Nutrient Blends

The nitrogen, ions, and vitamins previously mentioned can be added independently, in house, as a nutrient blend. The creation of on-site nutrient blends can be labor intensive, so it may be advantageous to a distiller to invest in a commercially available pre-prepared blend. Many commercial blends are designed to supplement a variety of nutritional deficiencies. These are very consistent, so the distiller can get reliable fermentation performance. For the best results, you should identify where potential weaknesses in your current mash exist as an appropriate blend selection should be based on the unique issues of your distillery and mash.

The Impact of Nutrition

Most of the focus on ethanol yield in distilled spirits has been on the conversion of complex carbohydrates, but simple sugar sources are not all that’s needed for faster and more complete fermentations. Nitrogen is limiting in most fermentations, with timely and adequate delivery of nitrogen being one of the fundamentals in efficient fermentation. The majority of stuck, sluggish, or incomplete fermentations can be attributed to deficiencies in nitrogen since YAN content has a huge influence on fermentation speed, yeast biomass and viability, and sugar transport kinetics during fermentation. Moreover, the process by which organic nitrogen is freed by proteases also releases bound sugars, increasing yield. The key to consistent fermentations is nitrogen management, with higher nitrogen levels corresponding to faster kinetics up to about 300 ppm of YAN after which there is no added efficiency.

With increased yield efficiency and speed of fermentation also comes the added benefit of decreased risk of infection. Healthy Saccharomyces are excellent competitors, but insufficient nutrition can often lead to slow starts and extended lag phases that can give bacteria, wild yeasts, and other competitive organisms ample opportunities to colonize a mash before our yeast has even had an opportunity to establish itself. A healthy yeast population allows for the completion of fermentation before other organisms can get a foothold. Given proper nutrition, yeast is also less likely to “stall out” before full attenuation, thus leaving little opportunity for competitors to establish themselves and cause unwanted loss of alcohol and off-flavor formation.

As distillers, we know flavor is just as important as yield, and many of the congeners and precursors that yeast produce are essential for the complexity of our spirits. The balance between organic and inorganic nitrogen particularly impacts flavor and aroma congener production. Higher alcohols can be desirable in some quantities, depending on the style and desired profile of finished spirit. These are produced through the Ehrlich pathway, which is used by yeast to degrade amino acids when nitrogen is scarce. Depending on the quantity and type of nitrogen present, this pathway can either be activated or suppressed to produce greater or lesser amounts of higher alcohols.

Nitrogen also plays a crucial role in the formation of esters by directly impacting yeast growth and metabolism. Sufficient nitrogen availability results in increased ester production, particularly through the synthesis of necessary precursors like acetyl-CoA, a crucial molecule involved in the formation of acetate esters. The type of nitrogen source can also influence ester production, with inorganic nitrogen being more readily assimilable by yeast, but organic nitrogen often leading to a wider range of ester profiles. With this in mind, nitrogen supplementation can be manipulated to produce a desired flavor congener profile.

In addition to producing desirable flavor compounds, proper nutrition is also essential for controlling off-flavor formation. Some of the most common congeners associated with poor nutrition are sulfur compounds and diacetyl. Sulfur compounds are often associated with raw materials but are also formed by yeast metabolism. Hydrogen sulfide and sulfur dioxide are intermediates in the formation of amino acids like cysteine and methionine, as well as some coenzymes. In the absence of adequate nitrogen, the synthesis of these compounds slows, and hydrogen sulfide and sulfur dioxide accumulate in the fermentation media, creating a characteristic rotten egg odor. These can be further oxidized to produce other undesirable congeners such as dimethyl sulfide (DMS) or dimethyl disulfide (DMDS), associated with unpleasant vegetal odors.

Diacetyl is a common off-flavor produced from α-acetolactate as a result of oxidation. Diacetyl has a pungent, buttery smell that can’t be removed during distillations due to its similar volatility to eth anol. However, its formation can be controlled by nutrition as increasing late viability of cells helps to prevent accumulation of α-acetolactate as well as reduce diacetyl levels in the final wash. Using nitrogen-liberating enzymes such as proteases or supplying fermentation with sufficient levels of nitrogen can help to increase late cell viability by supplying essential amino acids and peptides throughout the entire fermentation.
Hopefully, this article has given you some practical insights to build upon and enhance your own fermentations. While not an exhaustive guide, it highlights how balanced yeast nutrition can lead to cleaner flavors, faster fermentations, higher yields, consistent quality, and greater profitability. Using these principles as a foundation in your fermentation practice, you can refine your process and unlock the full potential of your spirits.