The common yeast used in baking and brewing, Saccharomyces cerevisiae, does not metabolize starch. This yeast is highly effective at fermenting simple sugars like glucose and fructose, or even disaccharides such as maltose. Starch, a complex carbohydrate found in grains and potatoes, presents a significant biological challenge that standard yeast strains cannot overcome, which is why human intervention is typically required to prepare starchy materials for fermentation.
The Starch Molecule and Yeast’s Dietary Preference
Starch is a polysaccharide, meaning it is a long chain constructed from numerous individual glucose molecules. It consists of two main polymers: amylose, which is a linear chain of glucose units, and amylopectin, a highly branched structure. Amylopectin typically makes up the majority of natural starch, with its branches created by different chemical bonds.
This large, complex structure is the reason standard yeast cannot consume starch directly. The starch molecule is far too massive to pass through the yeast cell wall and membrane, which are designed to admit only small, simple sugar molecules. For yeast to use starch as a food source, the long, coiled chains of amylose and the highly branched structure of amylopectin must first be broken down outside the cell.
The Missing Enzyme: Why Standard Yeast Fails to Metabolize Starch
Standard Saccharomyces cerevisiae lacks the necessary exoenzymes to initiate the breakdown of starch. The process of converting starch into fermentable sugars is called hydrolysis, a chemical reaction where water is used to break the bonds linking the glucose units. This task requires a cooperative set of specialized enzymes, primarily alpha-amylase and glucoamylase.
Alpha-amylase randomly cuts the long starch chains into smaller fragments known as dextrins and oligosaccharides. Following this initial fragmentation, glucoamylase then works systematically to snip off individual glucose molecules from the ends of these smaller chains. Only once the starch has been completely hydrolyzed into simple sugars can it be transported across the cell membrane and utilized by the yeast. In traditional processes like brewing or distilling, humans must perform this breakdown, often called “mashing,” by adding external enzymes or heating the mash to activate enzymes naturally present in malted grains.
Specialized Strains That Can Metabolize Starch
While the common baking and brewing yeast cannot process starch, certain specialized strains of Saccharomyces and other yeasts can. One natural exception is Saccharomyces cerevisiae var. diastaticus, a strain known in the brewing industry for its ability to cause “super-attenuation.” This strain possesses genes that encode for and allow the secretion of extracellular glucoamylase.
These genes enable S. diastaticus to break down dextrins and, to a lesser extent, starch, allowing it to ferment sugars that other yeast strains leave behind. The ability of yeast to consume starch has also been achieved through genetic engineering, particularly for industrial applications like bioethanol production. Scientists introduce genes for enzymes like alpha-amylase and glucoamylase from other organisms, such as fungi, into industrial S. cerevisiae strains. This modification creates an amylolytic yeast that can perform both the hydrolysis of starch and the subsequent fermentation of the resulting sugars in a single step, significantly lowering the cost and complexity of the production process.