Two essential components are indispensable for winemaking. One is grapes, the raw material. The other is yeast.
Yeast is utilized in the production of fermented beverages such as wine, beer, and sake. It is also employed in the fermentation of bread dough. Consequently, yeast is a relatively familiar entity to most people. Yeast is also referred to as “yeast” in English terminology.
However, when asked “What is yeast?”, can you explain it without hesitation? This article reviews the fundamentals of yeast.
What Is Yeast?
Yeast is a type of microorganism generally categorized as “fungi.” From a microbiological perspective, yeast is the collective term for unicellular eukaryotic organisms with cell walls, classified as fungi. Within our living environment, yeast is recognized as a microorganism used in fermentation.
The terms “yeast” are frequently used as standalone words, creating the impression that they refer to a specific entity. In fact, in everyday conversation, the term “yeast” most commonly refers to Saccharomyces cerevisiae, a particular species of yeast.
Wine yeast, beer yeast, sake yeast, and bread yeast are all fundamentally the same Saccharomyces cerevisiae. However, more than 500 species across 60 genera of yeast are known to exist. Furthermore, even within the same Saccharomyces cerevisiae, yeast strains with differing characteristics exist.
Note: Saccharomyces cerevisiae denotes yeast of the genus Saccharomyces, species cerevisiae.
For those who wish to understand yeast classification in greater depth, specialized literature exists. Such resources are entirely unnecessary for those seeking detailed knowledge of wine yeast, but are mentioned here for reference.
Ubiquitous Yeast
Dry Yeast and Wild Yeast
When hearing the term yeast, one might envision a special-looking powder packaged in small sachets. Yeast that has been processed into powder form through the selection of specific yeast types, individually packaged, and commercialized is termed “dry yeast.”
This dry yeast also originates from yeast that existed in nature. The various types of yeast present in the natural environment are collectively referred to as “wild yeast” or “natural yeast.”
From wild yeast, strains suitable for winemaking, brewing, sake production, or bread making are selected. These are then isolated from other yeast types and processed for ease of storage and transport, resulting in dry yeast.
When wild yeast is termed “natural yeast,” dry yeast may seem artificially created. However, dry yeast is also a form of natural yeast.
Yeast Habitat
Yeast exists ubiquitously in natural environments. Yeast drifts in the air and inhabits soil. Among all locations, “places where sugar-containing liquids are present” are particularly conducive to finding yeast.
Sugar-containing liquids may seem somewhat obscure, but tree sap, flower nectar, and fruits correspond to this description. The surface of fruits with damage where juice has seeped out constitutes an optimal habitat for yeast. Sweet fruit juice serves as an excellent energy source for yeast.
The number of yeast on fruits increases proportionally with fruit ripeness.
Wild Yeast and Insect Relationships
In winemaking contexts, wild yeast is frequently discussed in association with terroir. The argument posits that using yeast indigenous to the vineyard where grapes were harvested, rather than selected dry yeast sourced from elsewhere, produces wine that more distinctly expresses the character of that specific vineyard.
Yeast does indeed exist in vineyard soil. Therefore, the claim that wild yeast represents terroir is not entirely unfounded.
Conversely, most yeast is vectored by insects. Insects moving from flower to flower transport yeast attached to their legs and mouthparts across extensive areas.
When such insects contact ripened grape berries, yeast first adheres to the grape skin surface. Subsequently, as insects consume fruit flesh or extract juice, yeast enters the fruit interior via their mouthparts.
Severely damaged grape berries are eliminated during sorting. However, berries with microscopic perforations invisible to the human eye are not removed. Yeast attached to these remaining berries begins proliferation upon contact with abundant juice when grapes are crushed within the winery.
Is Yeast Necessary?
The Role of Yeast in Fermentation
In winemaking, yeast plays an indispensable and critically important role: fermentation.
In extreme terms, wine can be produced without a winemaker. However, wine cannot be produced without yeast. Truly, “no yeast, no wine.”
Incidentally, are you aware of the number of yeast involved in winemaking? This refers not to yeast types, but to cell count.
During the initial fermentation stage, approximately 10⁶ yeast cells exist per mL. As fermentation progresses, this number increases to approximately 60 × 10⁶ cells per mL. At peak maximum values, 10⁸ cells per mL are reported to exist.
An astronomical number of yeast work to transform grape juice into wine.
Fermentation as Yeast Energy Metabolism
In winemaking contexts, fermentation specifically refers to yeast converting sugars contained in grape juice into ethanol and CO₂.
Many perceive “fermentation = alcohol acquisition.” However, strictly speaking, this is inaccurate. The objective of fermentation is for yeast to acquire necessary energy, with alcohol production being merely a by-product.
Since alcohol production is not the primary objective, the quantity of alcohol produced from the same amount of sugar varies among yeast types. Depending on the yeast type, some produce almost no alcohol.
However, alcohol is necessary in winemaking. Efficient production is required. Yeast selected for this purpose is wine yeast of the genus Saccharomyces.
Characteristics of Saccharomyces Genus Yeast
Saccharomyces genus yeast exhibits high alcohol productivity. Within the Saccharomyces genus, species such as bayanus also exist. Among these, cerevisiae species is particularly valued for theoretically converting 51% of sugar to alcohol.
Sugar metabolism by Saccharomyces cerevisiae is represented more specifically by the following equation:
Glucose + 2ADP + 2P → 2 Ethanol + 2 CO₂ + 2ATP + 25 Kcal
Fermentation is fundamentally metabolism for yeast to acquire energy. Therefore, the “2 ATP” portion of the equation is most important for yeast. However, for humans, that portion is relatively inconsequential, with the “2 Ethanol” portion being critical.
As an aside, temperature increase during fermentation results from the 25 Kcal heat generated as a consequence of yeast metabolism.
Regarding Malolactic Fermentation
Lactic fermentation, also termed MLF (malolactic fermentation), is called “fermentation” despite the absence of alcohol production. However, yeast is not involved in MLF. MLF is executed by bacteria called lactic acid bacteria.
Though less widely known, cases exist where Saccharomyces genus yeast is involved in malic acid degradation itself.
Yeast Produces More Than Alcohol
Fermentation By-Products
The primary reason for using yeast in winemaking is alcohol acquisition. Conversely, yeast also produces substances other than alcohol during metabolism. These are termed fermentation by-products.
Fermentation by-products are strictly classified into primary and secondary by-products. However, this involves considerable complexity and will be omitted here.
Fermentation by-products include the following: pyruvic acid, aldehydes, ketones, glycerol, lactic acid, acetic acid, other organic acids, esters, higher alcohols, methanol, among others.
Esters, higher alcohols, and aldehydes influence wine aroma. Glycerol affects mouthfeel. These fermentation by-products can exert positive influences on wine. However, they may also produce negative effects.
Yeast-Mediated Varietal Aroma Expression
Beyond alcohol production, yeast plays a critically important role in aroma influence independent of fermentation by-products.
In certain grape varieties, variety-specific aromatic compounds are present within the fruit. However, these aromas are completely imperceptible prior to fermentation.
The absence of aroma occurs because aromatic compounds such as monoterpenes and norisoprenoid derivatives exist in juice as glycosides—glycosidically bound states where these compounds are conjugated with sugars. These aromatic compounds become aromatic only after dissociation from sugar.
In other words, to incorporate aromatic compounds in glycosidic form appropriately into wine as proper aromas, the sugars linked to these aromatic compounds must be cleaved. This cleavage is performed by yeast.
Upon hearing that yeast resolves glycosides, one might assume that since yeast metabolizes sugar as a substrate for metabolism, it similarly metabolizes sugars bound as glycosides. Such understanding is acceptable on a superficial level. However, the actual mechanism underlying glycoside resolution is considerably more complex.
Yeast aptitude for glycoside resolution also varies. Consequently, yeast possessing properties better suited to such mechanisms have been selected and commercialized as dry yeast.
Yeast Requirements
Fermentation is yeast metabolic activity, suggesting it proceeds autonomously without intervention. Such thinking is not entirely incorrect. However, efficiency becomes relevant here.
Like humans, yeast performs more efficiently and produces higher quality work when environmental conditions are optimized. Environmental optimization requires sugar, nutrients, temperature, and oxygen.
Sugar is naturally necessary as substrate for metabolism. Other factors are manageable without, but constitute requirements for yeast to work without excessive stress.
Temperature and Oxygen Requirements
In wine production, commonly heard terms include “low-temperature fermentation” and “reductive condition.” These terms suggest that fermentation temperature should be as low as possible and oxygen should be minimized.
However, yeast cannot function in excessively cold conditions. Additionally, efficiency deteriorates when substantial energy is required but respiration for energy acquisition is impossible.
Under these circumstances, even low-temperature fermentation is advised to maintain approximately 15°C. Moreover, during proliferation when yeast increases in number, incorporation of adequate oxygen is essential.
Generally, individual yeast cells undergo 5-7 rounds of division and proliferation. Fermentation metabolism is reported to commence around the fourth generation onward. During fermentation, yeast can metabolize under anaerobic conditions. Therefore, oxygen supply is required until that point.
Critical Nutrients | Nitrogen, Ammonium, and Sulfur Compounds
The Role of Nitrogen
It is frequently stated that nitrogen is the necessary fermentation supplement in winemaking. Indeed, nitrogen constitutes critically important nutrition for yeast in fermentation. However, ammonium and sulfur compounds are also indispensable nutrients.
In sake production, such minerals are reportedly supplied via water. However, in winemaking, nutrients other than nitrogen are typically present in sufficient quantities within grapes. Consequently, external addition of these nutrients is unnecessary. This appears to contribute to their natural oversight as “necessary nutrients.”
Nitrogen is utilized both for yeast proliferation and metabolism. Statements asserting that nitrogen quantity determines the amount of sugar yeast can metabolize are occasionally encountered. However, this is slightly inaccurate.
Nitrogen quantity initially determines yeast cell count. Resultantly, the maximum quantity of metabolizable sugar is influenced by cell count. Therefore, while it could be argued that nitrogen determines sugar metabolism quantity when consolidated, attention must be paid to the sequence.
Misunderstanding this relationship leads to errors in nitrogen supply timing.
Nitrogen Requirements
Nitrogen requirements vary substantially depending on yeast type. However, based on yeast cell counts during fermentation, a minimum of 200 mg/L is required.
Incidentally, nitrogen quantity is known to increase production of pyruvic acid, ketones, esters, glycerol, and higher alcohols primarily through deficiency. Nevertheless, indiscriminate excess addition is not advisable. Appropriate supply of required quantities is critical.
