Vivian Marlowe
Sucrose is a sugar composed of a glucose linked to a fructose. It is the major carbon source used by Saccharomyces cerevisiae during the production of baker's yeast, fuel ethanol, and several distilled beverages. The enzyme sucrase catalyzes the hydrolysis of sucrose into glucose and fructose. Sucrose fermentation proceeds through extracellular hydrolysis of the sugar, producing glucose and fructose that are transported into the cells and metabolized. The rate of fermentation depends on the concentration of the sugar and yeast. Sucrose has been used as a substrate in biorefineries for several decades, particularly in tropical countries such as Brazil, due to its low price compared to other substrates. Sucrose fermentation plays a crucial role in the production of alcoholic beverages, ethanol fuel, and bread dough rising.
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What You'll Learn
Sucrose fermentation
Sucrose is a sugar composed of a glucose molecule linked to a fructose molecule. In the first step of alcoholic fermentation, the enzyme invertase cleaves the bond between the glucose and fructose molecules. Each glucose molecule is then broken down into two pyruvate molecules in a process known as glycolysis. Finally, pyruvate is converted to ethanol and carbon dioxide in two steps, regenerating the oxidized NAD+ needed for glycolysis. This reaction is catalysed by alcohol dehydrogenase.
The rate of sucrose fermentation depends on the concentration of yeast rather than the concentration of sugar. Increasing the amount of yeast will speed up the reaction. Additionally, the fermentation rate is influenced by the ingredients used, such as the amounts of sugar and salt.
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Sucrose as an industrial substrate
Sucrose, a disaccharide consisting of glucose and fructose, is commonly found in sugarcane and sugar beets. It has been used as a sweetener and in the food and beverage markets. However, its role as an industrial substrate is also significant. Sucrose is an abundant, readily available, and inexpensive substrate for industrial biotechnology processes, particularly in the production of fuel ethanol. The use of sucrose in industrial processes has been explored for the microbial production of various compounds or products, with the yeast itself sometimes being the desired product.
In tropical countries like Brazil, sucrose obtained from sugarcane has been a longstanding substrate in biorefineries. The fuel ethanol industry in Brazil showcases the cost-effectiveness of cane sugar. Despite advancements in second-generation fuel ethanol production, sucrose remains a preferred substrate for yeast fermentations due to its low price and abundance. The Saccharomyces cerevisiae (S. cerevisiae) strain of yeast is particularly efficient at consuming sucrose and is therefore a popular choice for industrial applications.
The enzyme sucrase plays a crucial role in sucrose fermentation. It catalyzes the hydrolysis of sucrose, breaking it down into glucose and fructose. This process is essential for the production of ethanol and carbon dioxide through fermentation. The hydrolysis reaction can be represented as:
> C12H22O11 + H2O → 2C6H12O6
The fermentation of sucrose by S. cerevisiae has been extensively studied, and it has been found that the physiology of S. cerevisiae during sucrose-limited chemostats is similar to that observed on glucose. However, more research is needed to verify if this behaviour holds true for different strains and dilution rates.
In addition to fuel ethanol production, S. cerevisiae is used for the production of heterologous proteins, pharmaceuticals, and bulk and fine chemicals. It is also an attractive host for the production of recombinant proteins due to its advantages in correct protein folding, post-translational modifications, and efficient protein secretion. The versatility of sucrose as an industrial substrate contributes significantly to the advancement of various industries, including food and beverage, pharmaceutical, and biofuel production.
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Yeast enzymes
The enzyme invertase, produced by yeast, plays a key role in the first step of alcoholic fermentation. Invertase cleaves the glycosidic linkage between the glucose and fructose molecules in sucrose. This cleavage results in the extracellular hydrolysis of sucrose, forming glucose and fructose. These monosaccharides are then transported into yeast cells and further metabolised. The hydrolysis of sucrose can also be catalysed by the enzyme sucrase, which is not a yeast enzyme but is present in some yeast formulations.
The subsequent steps of alcoholic fermentation involve the breakdown of glucose molecules through a process called glycolysis. Each glucose molecule is converted into two pyruvate molecules, which are then transformed into ethanol and carbon dioxide. This conversion is facilitated by the enzyme pyruvate decarboxylase, which decarboxylates pyruvate to produce acetaldehyde. The final step is catalysed by alcohol dehydrogenase (ADH1 in baker's yeast), converting acetaldehyde into ethanol and regenerating the NAD+ cofactor required for glycolysis.
The efficiency of yeast enzymes in alcoholic fermentation can be influenced by various factors. The concentration of yeast affects the rate of fermentation, with higher yeast amounts speeding up the reaction. Additionally, the specific yeast strain and fermentation conditions play a role in the final outcome. For instance, immobilising yeast cells on certain supports, such as calcium alginate or gluten pellets, can enhance fermentation rates and ethanol productivity, as seen in wine production.
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Alcoholic fermentation
Sucrose is a sugar composed of a glucose molecule linked to a fructose molecule. In the first step of alcoholic fermentation, the enzyme invertase cleaves the bond between the glucose and fructose molecules. Each glucose molecule is then broken down into two pyruvate molecules in a process known as glycolysis. Finally, pyruvate is converted to ethanol and carbon dioxide in two steps, regenerating the oxidized NAD+ needed for glycolysis. This reaction is catalysed by alcohol dehydrogenase (ADH1 in baker's yeast).
Sucrose is the major carbon source used by Saccharomyces cerevisiae during the production of baker's yeast, fuel ethanol, and several distilled beverages. Sucrose obtained from sugarcane has been used as a substrate in biorefineries for several decades, particularly in the fuel ethanol industry. Sucrose is preferred due to its low price compared to other substrates.
The rate of alcoholic fermentation depends on the concentration of yeast rather than the concentration of sugar. The addition of a larger amount of yeast can speed up the reaction. The fermentation rate is also influenced by the ingredients used, such as the amounts of sugar and salt.
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Fermentation rate determinants
Type of Sugar
Sucrose, a disaccharide composed of glucose and fructose, has been shown to readily undergo mass loss and thus fermentation. This is due to the enzyme invertase, which cleaves the glycosidic linkage between the glucose and fructose molecules. Lactose, another disaccharide, does not ferment as easily because it first needs to be broken down into glucose and galactose by the enzyme lactase. In addition, galactose does not undergo yeast fermentation, further slowing down the fermentation of lactose.
Amount of Yeast
The amount of yeast present can impact the rate of fermentation. Increasing the amount of yeast can speed up the reaction.
Yeast Strain
Different strains of yeast may have varying fermentation rates. For example, in wine fermentation, the majority of yeast in the initial phase of spontaneous fermentation are those that contribute positively to the final sensory quality of the wine. However, once the concentration of ethanol reaches 4-5% v/v, their growth is inhibited, and Saccharomyces yeasts, which are more resistant to ethanol, take over and complete the fermentation.
Temperature
The temperature at which fermentation occurs can also affect the rate. Yeast cells immobilized on orange peel, for instance, achieved high ethanol productivity and low fermentation times at temperatures ranging from 15°C to 30°C.
Dough Ingredients
In the case of bread fermentation, the fermentation rate is influenced by the ingredients of the dough, including the amounts of sugar and salt used in its preparation.
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Frequently asked questions
Sucrase is an enzyme that catalyzes the hydrolysis of sucrose into glucose and fructose, which can then be fermented by yeast into ethanol and carbon dioxide. Sucrose is a major carbon source for yeast fermentation, and its fermentation proceeds through extracellular hydrolysis of the sugar, producing glucose and fructose that are transported into the cells and metabolized.
The rate of alcoholic fermentation can be affected by various factors, including the amount of yeast, the concentration of sugar, and the temperature. Increasing the amount of yeast or raising the temperature can speed up the reaction, while higher sugar concentrations can lead to higher total ethanol production.
Sucrase plays a crucial role in the production of alcoholic beverages, such as wine, beer, and distilled spirits. It is also used in the food industry for bread-making, as well as in the production of biofuels, chemicals, and pharmaceuticals. Additionally, sucrose obtained from sugarcane has been used as a substrate in biorefineries for ethanol production in countries like Brazil.
