Yeasts are lactose intolerant; but for every rule there are exceptions. These three species are able to use lactose, or milk sugar, as a major energy source. They're found in products such as whey, wine and wheels of cheese.
Powerful microscopic workers of the natural world, yeasts metabolize any sugar but lactose. They lack the enzyme lactase to break down lactose into usable components glucose and galactose.
However a few yeast species can convert lactose to energy. These include Kluyveromyces marxianus, Kluyveromyces fragilis, and Lachancea thermotolerans (formerly Kluyveromyces thermotolerans).
Yeasts can't move on their own, but they have ways to get around. For instance as soon as fruit is ripe, boom! there is the whitish film of yeast, which shows best on grapes or plums. Perhaps they were there all along.
Lactose Intolerance
Lactose, a disaccharide or double sugar, is made of glucose and galactose. Most organisms, including the majority of yeast, can't directly absorb lactose.
They need the enzyme lactase to split lactose into its constituent sugars, which they can then process for energy. As a rule yeast species lack this crucial enzyme, making lactose a useless source of yeast food.
In the wild, yeast is often found with lactic acid bacteria, who break down lactose, and acetic acid bacteria, who produce the active component of vinegar. These two lower the pH to provide acidic conditions yeast loves, and contribute to flavors as in yogurt.
Unlike their lactose-intolerant yeast relatives, the three milketeers below have a natural affinity for lactose. In whey for instance, K. marxianus multiplies like crazy.
Kluyveromyces marxianus: Denizen of Dairy
Kluyveromyces marxianus is primarily found in dairy products, especially those fermented or leftover, like cheese and whey, the watery part of milk after curdling. K. marxianus is used in the dairy industry.
It also colonizes decaying fruits, vegetables, and soil. Like most yeast species its adaptability is remarkable and almost entirely sugar-driven. It can travel on airborne motes, human clothes and feet of bees.
K. marxianus is important to fermentation of various food sources and processes such as decomposition and nutrient cycling within its environment. Its presence contributes to complex microbial communities.
This yeast recycles lactose in dairy waste, facilitating waste management. Its fermentation talents support renewable energy with bioethanol. Ethanol and CO2 are common byproducts of yeast sugar metabolism.
Kluyveromyces marxianus has an efficient lactose uptake system involving permeases, proteins enabling lactose transport across the cell membrane.
It has a gene which codes for lactase, allowing it to produce the enzyme.
Inside the yeast cell, lactose is broken down into sugars glucose and galactose, which are metabolized through glycolysis to produce energy. This resourceful yeast is a common inhabitant of high-lactose environments.
Kluyveromyces fragilis - Beloved by Artisans
Kluyveromyces fragilis is known for rapid lactose fermentation, crucial for yogurt and cheese production. It adapts to various substrates, maintaining the balance of microbial populations in fermented foods.
Habitat: Similar to K. marxianus, K. fragilis thrives in dairy products, specifically whey, and decaying plant matter. It's a common inhabitant of various agricultural environments.
Essential Functions in Nature: This yeast is a busy fermenter, contributing to the breakdown of organic material and the cycling of nutrients in its environment. It's often found in partnership with other microbes.
Lactose Conversion Ingenuity: K. fragilis also possesses the vital lactase gene, enabling it to actively metabolize lactose. Similar to K. marxianus, it intracellularly hydrolyzes lactose into usable monosaccharides.
Like other yeasts, Kluyveromyces fragilis converts lactose into both ethanol and carbon dioxide. This dual conversion is useful in mixed fermentation processes, as in producing artisan cheeses.
K. fragilis has been used in traditional making of kefir and other fermented dairy products. Kefir, a fermented milk, is promoted for a number of health benefits. Its name is from Turkish keyif, or “good feeling” after drinking it.
Kluyveromyces fragilis has potential for exploitation in bioremediation, effectively metabolizing organic compounds. This has possibilities for use in cleaning up the huge amount of environmental pollutants.
Lachancea thermotolerans (Kluyveromyces thermotolerans): Heat-Loving Fermenter
Lachancea thermotolerans is recognized for its fermentation profile, producing diverse flavors and aromatic compounds. It's desirable in the beverage industry, especially in wines and ciders.
Wild yeast and local strains impart regional flavors. L. thermotolerans can also improve fermentative efficiency, and contributes to flavor complexity.
Lachancea thermotolerans uses unique transporters and intracellular enzymes to convert lactose into ethanol and organic acids. Its metabolic flexibility helps it outcompete less tolerant strains in warm fermentation.
Habitat: Unlike the previous two, Lachancea thermotolerans prospers in warm environments and higher temperatures. It's often found in grape must, or the crushed grapes used for winemaking, and other plant matter.
Functions in Nature: This yeast is particularly significant in winemaking. It's known to enhance the complexity of wine flavors and aromas through its fermentation activity. It can also spoil food, depending.
Lactose Conversion Ingenuity: While not predominantly adapted to lactose-rich habitats, L. thermotolerans can use lactose via intracellular lactase when present. This is useful in environments of dairy byproducts.
Because it prioritizes other sugars when available, the yeast’s lactose conversion is often slower. Although lactose is not its primary food source, L. thermotolerans's tolerance of this sugar makes it an effective fermenter.
It's also known to produce lactic acid during fermentation. This makes it suited for applications such as malolactic fermentation in wine production.
Malolactic conversion, also malolactic fermentation (MLF), is a common winemaking process. The naturally occurring tart malic acid in grape must is transformed to "softer" lactic acid.
Its fermentation properties help achieve desired flavors while intensifying the wine's overall complexity. It's a competitive edge for winemakers wanting to craft unique high-quality beverages.
Kluyveromyces marxianus, Kluyveromyces fragilis, and Lachancea thermotolerans exemplify the natural versatility of yeasts. Their ability to thrive in high-lactose environments enriches ecology and industry.
Genetic Engineering
Saccharomyces cerevisiae is the first and most widely engineered yeast species in milk sugar experiments, largely due to its adaptable nature. Several strains of S. cerevisiae have been mutated to produce lactic acid.
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