Understanding the发酵机制：蔗糖与乳糖在酵母中的不同命运

Why is Sucrose Fermented by Yeast While Lactose is Not?

Introduction

The fermentation of sugars by yeast, particularly Saccharomyces cerevisiae, is a complex process that hinges on the type of sugar and the specific enzymes present within the yeast. This article delves into the fermentation processes of sucrose and lactose, explaining why sucrose is fermented by yeast but lactose is not.

Sucrose Fermentation

Composition

Sucrose, a common sugar found in fruits and vegetables, is a disaccharide composed of glucose and fructose. Its chemical structure makes it a prime target for yeast fermentation.

Enzymatic Action

Yeast possess the enzyme invertase, also known as sucrase, which catalyzes the hydrolysis of sucrose into its constituent monosaccharides, glucose, and fructose.

Fermentation Process

Once sucrose is broken down into its constituent monosaccharides, yeast can utilize these sugars through glycolysis and fermentation pathways. This process results in the production of ethanol and carbon dioxide, the key components in beer and wine production.

Lactose Non-Fermentation

Composition

Lactose, another disaccharide, is composed of glucose and galactose. Unlike sucrose, lactose is not easily fermented by most strains of yeast, including common brewing yeasts like Saccharomyces cerevisiae.

Lack of Enzymes

The primary reason yeast cannot ferment lactose lies in the absence of the enzyme lactase, which is necessary to hydrolyze lactose into glucose and galactose. Without lactase, yeast cannot break down lactose, making it an impermeable obstacle for the fermentation process.

Summary

Sucrose is readily fermented by yeast because it can be hydrolyzed into fermentable monosaccharides. In contrast, lactose is not fermentable by most yeasts due to the absence of lactase, a prerequisite for lactose breakdown.

Notable Exceptions: Fermentation of Lactose by Yeast

While the majority of yeast species lack the ability to ferment lactose, some microorganisms, such as certain bacteria like Lactobacillus, possess the necessary enzymes to perform this task. However, it is intriguing to note that some yeasts, such as Kluyveromyces marxianus, have also been found to ferment lactose.

Kluyveromyces marxianus plays a significant role in the fermentation of whey, a byproduct of cheese manufacturing. This yeast ferments lactose to produce alcohol, which can be used in alcoholic beverages, such as Bailey's Irish Cream. The production of alcohol in products like Bailey's is a prime example of how some yeasts have adapted to utilize lactose as an energy source.

Conclusion

Understanding the fermentation processes of sugars by yeast not only provides insights into biological chemistry but also highlights the diverse applications of these microorganisms in the food and beverage industries. The ability of certain yeasts to ferment lactose, like Kluyveromyces marxianus, showcases the adaptability of microbial life and opens up new avenues for industrial use.