Can yeast metabolize lactose? (5 Recent Developments)

In this brief guide, we will answer the question, can yeast metabolize lactose? We will discuss why yeast cannot metabolize lactose and a simple experiment to prove it. We will also shed light on a technology that has allowed the yeast to metabolize lactose. 

Can yeast metabolize lactose?

Scerevisiae has been an essential component of human civilization because of its extensive use in food and beverage fermentation in which it has a high commercial significance. In the European yeast industry, 1 million tonnes is produced annually, and around 30% of which is exported globally. The global market’s annual growth rate was 8.8% from 2013 to 2018 (1).

Yeast that is known as brewers yeast cannot metabolize lactose. The enzyme needed to metabolize lactose called lactase is absent in yeast. There are many species of yeast, one of which includes saccharomyces cerevisiae which has wide application in baking, cooking, and brewing. However, some yeast can metabolize lactose, including Kluyveromyces lactis (2).

Yeast Saccharomyces cerevisiae cannot metabolize lactose because it does not have lactase permease or B-galactosidase enzyme. However, yeast has a high tolerance for ethanol and sugar. 

Saccharomyces cerevisiae is exploited in cooking and baking as it respires and produces carbon dioxide. However, to metabolize and rise, it needs sucrose or glucose and will not break down lactose. Lactose is the primary sugar in milk and milk-based products only.

If you would like to carry out a process where you need yeast to metabolize lactose, you can make use of a pill called Lactaid. The Lactaid capsule would break down the lactose sugars into glucose and galactose, which will then be used by yeast to produce alcohol and carbon dioxide. Lactaid is an over-the-counter lactase supplementation. Some patients incubate lactose-containing milk overnight with a lactase enzyme. Adding 5 to 10 drops of lactase preparation to a glass of milk hydrolyzes about 70% to 100% of the lactose (3).

Located in the enterocytes of the small bowel on the brush border membrane, lactase (D-galactosidase glycohydrolase) selectively acts on lactose, cleaving it into the monosaccharides glucose and galactose (3). Lactose is made of two sugars galactose and glucose, both of which can be metabolized by yeast. 

How is yeast metabolism demonstrated as a lab experiment?

A common experiment makes use of yeast suspension, lactose, and water to prove if yeast will metabolize lactose. Balloons are used to prove the absence or presence of carbon dioxide gas produced as a result of fermentation. 

The experiment can be carried next to a sucrose solution, which is digestible by yeast (4). 

As the yeast uses sucrose and breaks it down, it produces carbon dioxide as a result.

 Balloons placed over the test tubes or bottles that hold the suspensions inflate, as carbon dioxide gas is produced. 

As sucrose is a disaccharide, yeast breaks it down with help of the enzyme called invertase and produces fructose and glucose. The glucose is then metabolized by the yeast by fermentation and glycolysis. The by-product of the fermentation process is carbon dioxide and alcohol. S. cerevisiae can metabolize sucrose in two ways. In the first and predominant mechanism, sucrose is hydrolyzed by an extracellular invertase, yielding glucose and fructose, which enter into the cell by facilitated diffusion via hexose transporters encoded by members of the HXT gene family. In the second mechanism, sucrose can be actively transported into the cells by a proton-symport mechanism and hydrolyzed intracellularly (5).

The test tube that had lactose and yeast, will not cause the balloon to inflate. As the yeast is not capable of digesting lactose, it cannot turn it into carbon dioxide gas and alcohol (4). 

How does modern technology allow the yeast to metabolize lactose?

Saccharomyces Cerevisiae is the species of yeast with the widest application of all, but other species of microorganism can metabolize lactose that Saccharomyces Cerevisiae cannot.  

Metabolic engineering of S. cerevisiae has been done to create strains capable of lactose fermentation. Metabolic engineering is defined as “the improvement of cellular activities by manipulations of enzymatic, transport and regulatory functions of the cell with the use of recombinant DNA technology” (2).

Research has allowed the introduction of genes from one microorganism to the other; hence, introduce a new set of characteristics in the former species. 

In this instance, another yeast called Kluyveromyces Lactis can metabolize lactose. 

Other microorganisms also allow obtaining Lactose metabolizing genes such as from E. coli, the yeast K. Lactis, or the filamentous fungi A. niger. 

The gene expression process is different for each species of microorganism. Moreover, the efficiency of conversion from lactose to ethanol also varies depending on the source and technique used. 

By introducing the B-galactosidase and permease genes into Saccharomyces cerevisiae, the yeast can use lactose. The permease enzymes take up the lactose while B-galactosidase breaks the cells down. 

Another yeast called Aspergillus Niger can also be genetically engineered to express the lactase enzyme. 

Another technology has made a strain of yeast able that grow on lactose and express lactase

(beta-galactosidase) internally by using a marker gene. 

Yeasts produced in labs can be genetically engineered to digest lactose by secreting the lactase enzyme. With the advancement in gene technology, recombinant fungal lactase has been made that can metabolize lactose. 

Why do we need yeast to metabolize lactase?

The driving force behind trying to get Saccharomyces cerevisiae to digest lactase is related to making use of whey. As whey is a by-product that is left after cheese and yogurt are produced, it contributes to waste that needs to be reutilized. A great portion of milk byproduct is whey that remains in huge quantities after dairy products have been produced. 

Whey has plenty of other uses and is utilized for food and non-food application. However, its surplus quantities require that there need to be more applications. Whey streams could be used as an abundant and renewable raw material for microbial fermentations, with lactose providing the carbon source. Lactose fermentation to bioethanol is one of the possibilities.

As there is still a lot to go around, a prospective use is producing bio-ethanol fuel. When lactase-metabolizing yeasts make use of the leftover whey, a sufficient amount of fuel can be produced. 

However, natural lactose-fermenting microorganisms, such as the yeast Kluyveromyces marxianus, cannot ferment efficiently (i.e., rapidly and with high conversion yields) such high concentrations of lactose. Saccharomyces cerevisiae is the organism of choice for bioethanol production. However, this yeast is not able to metabolize the sugar lactose. Thus, strain development programs through metabolic engineering of S. cerevisiae are required for the implementation of lactose-to-ethanol processes with increased productivity (2).

Other FAQs about Yeast that you may be interested in.

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Conclusion

In this brief guide, we answered the question, can yeast metabolize lactose? We discussed why yeast cannot metabolize lactose and, a simple experiment to prove it. We also shed light on a technology that has allowed the yeast to metabolize lactose. 

Citations

  1. Parapouli, Maria, et al. Saccharomyces cerevisiae and its industrial applications. AIMS microbiol, 2020, 6, 1.
  2. Domingues, Lucília, Pedro MR Guimarães, and Carla Oliveira. Metabolic engineering of Saccharomyces cerevisiae for lactose/whey fermentation. Bioengin bugs, 2010, 1, 164-171.
  3. Zarbock, S. D., et al. Lactose: the hidden culprit in medication intolerance?. Orthopedics, 2007, 30, 615-617.
  4. Demonstration: can yeast digest lactose? 2020 California State University. Department of Education.
  5. Basso, Thiago O., et al. Engineering topology and kinetics of sucrose metabolism in Saccharomyces cerevisiae for improved ethanol yield. Metab engin, 2011, 13, 694-703.