Yeast is a one-celled highly complex creature. Its major enzymes maltase, invertase, and zymase help convert sugars into alcohol and carbon dioxide for yeast activity, human baking and brewing on industry and artisan levels.
The extraordinary qualities of yeast are due to its enzymes. Made from proteins, enzymes are biological catalysts. They speed up chemical reactions. Maltase, invertase, and zymase are major operators.
Yeast, a fungus, loves sugar. It happily gobbles up saccharides like maltose, glucose and mannose for energy through fermentation. Then, it exudes the byproducts carbon dioxide and alcohol, possibly releasing fragrant esters.
In nature this is part of the decomposition process. Yeasts gather where sugars are abundant, as in ripe or rotting fruit. They're noticed on grapes at harvest. Many brewers use wild strains to start the fermentation process.
In the human world, yeast's fermenting ability is invaluable in applications like baking bread, making wine and brewing beer. Maltase, invertase, and zymase transform complex sugars to simplified forms yeast can use.
Enzymes: Biological Catalysts
Enzymes are biological catalysts. These proteins dramatically accelerate the rate of biochemical reactions within living organisms.
Without them, these reactions would occur far too slowly to sustain life. Enzymes can do this by lowering the activation energy needed for a reaction to happen.
Enzymes reduce the activation energy by attaching to reactant molecules. They position the molecules to facilitate the processes of breaking and forming chemical bonds.
1. Maltase: The Maltose Enzyme
Creation
Maltase, also known as α-glucosidase, is produced by yeast cells. They react especially when maltose, a disaccharide composed of two glucose molecules, is present in their environment.
The presence of maltose is an inducer, triggering the expression of genes necessary for maltase production. The yeast only produces the enzyme when it’s needed.
In brewing, maltase is produced when yeast cells encounter maltose derived from grains during the malting process. In humans, maltase performs a similar function.
It turns maltose from ingested food into glucose, which humans use for energy. The body can produce about 300 grams of glucose from maltose in a single meal.
Function in Yeast
Maltase's primary function is to hydrolyze the disaccharide maltose into two glucose molecules. This is important for yeast, as glucose is the preferred sugar for fermentation.
By breaking down maltose, yeast can access glucose as a source of energy and carbon for growth. Sugar is about 42% carbon.
Function in Other Organisms
Maltase is found in other organisms, including humans, where it resides primarily in the small intestine. Here, it digests dietary maltose derived from earlier starch digestion by other microbes.
This enzyme occurs in the saliva of mammals, where it helps digestion. Human saliva has about 30% maltase activity.
The production of maltase in yeast is often influenced by the strain of yeast and environmental conditions. Some strains are more efficient at producing maltase than others, influencing fermentation.
Brewing
In beer production, maltase is crucial for converting malt sugars into alcohol. Beers with higher maltase levels may have more alcohol content.
2. Invertase: Splitting Sucrose
Creation
Invertase, also known as sucrase or β-fructofuranosidase, is produced by yeast and secreted into the surrounding environment. Like maltase, its production is influenced by the presence of sucrose (table sugar).
Invertase is made in the cytoplasm of yeast cells. It hydrolyzes sucrose into monosaccharides glucose and fructose used by yeast during fermentation. Invertase raises the fermentation rate when sucrose is the primary sugar source.
Function in Yeast
Invertase catalyzes the hydrolysis of disaccharide sucrose into simple sugars glucose and fructose. The breakdown of sucrose is important for yeast fermentation.
Many yeast strains can't directly transport sucrose into the cell. By breaking it down externally, they can readily absorb both glucose and fructose for energy production.
Function in Other Organisms
Invertase is widely distributed in plants and insects. In plants, helps in sucrose metabolism, ensuring the availability of glucose and fructose for cellular processes.
This enzyme appears in the chemistry of honeybees. A forager bees uses her proboscis, a straw-like tongue, to collect nectar. As she transports the nectar, she introduces invertase.
The invertase starts converting sucrose into glucose and fructose in the honey stomach or crop. The forager then passes the nectar to house bees, who add more enzymes.
The name "invertase" comes from the hydrolysis of sucrose. It changes the rotation direction of polarized light. Sucrose is dextrorotatory, rotating light to the right. The mix of glucose and fructose is levorotatory, rotating light to left.
Invertase is not unique to yeast; it is also found in plants and some microorganisms. The glucose and fructose created by invertase are about 1.5 times sweeter than sucrose. This makes it popular in the candy industry.
Too high a sucrose level can overwhelm and inhibit invertase activity.
3. Zymase - Fermentation
Creation
Zymase isn't a single enzyme, but a complex mixture of enzymes found in yeast cells. This system drives the multi-step process of fermentation.
The enzymes are continually produced within the yeast cell. They're always present regardless of the external environment.
Zymase is formed in the presence of sugars, especially active when oxygen levels are low. The enzyme complex also includes alcohol dehydrogenase and phosphofructokinase, which help sugar breakdown and fermentation.
Function in Yeast
Zymase orchestrates the anaerobic breakdown of sugars, primarily glucose and fructose, into ethanol (alcohol) and carbon dioxide. This is the core process of alcoholic fermentation.
Each enzyme in the zymase complex has a specific function in the complex pathway. These range from initial sugar phosphorylation to the final production of ethanol.
Function in Other Organisms
While zymase is predominantly associated with yeast, similar enzymatic pathways exist in other organisms. It's used by bacteria and some animal cells, with different end products.
For example, muscle cells during intense exercise can perform anaerobic glycolysis, a process analogous to fermentation. This produces energy when oxygen supply is limited.
The discovery of zymase by Eduard Buchner in 1897 is a groundbreaking moment in biochemistry. It demonstrates fermentation can occur outside living cells, earning Buchner the Nobel Prize in Chemistry in 1907.
Anaerobic vs. Aerobic: In oxygen-rich environments, yeast breaks down sugars differently, primarily through respiration. In contrast, zymase is essential in anaerobic conditions, making it fundamental for processes like brewing. Overall yeast prefers oxygen-free environments.
Ethanol Production: The fermentation powered by zymase is responsible for the alcohol content in beverages. Wines can vary in alcohol content from 8% to over 15%, largely depending on zymase activity.
Yeasts also go dormant in alcohol concentrations of 10% to 20%. They sink to the bottom to become part of the sediment.
Some brewers recycle yeast, repitching it up to three times. In wine making, the lees is made largely of fallen yeast, and produces specialty wines.
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