Bakers’ yeast is a specialist, not a generalist. Saccharomyces cerevisiae handles a short list of sugars efficiently, works through them in a predictable order, and simply ignores several common ones. Which sugars are present in a wort, must, or dough determines how far fermentation can go and what remains in the finished product.

The Short Answer

S. cerevisiae readily ferments glucose, fructose, sucrose, maltose, maltotriose, and galactose. It cannot ferment lactose, and it cannot break down starch, dextrins, cellulose, or most fiber into anything it can use.

The reason for each case comes down to two questions: can the yeast transport the sugar into the cell, and does it have the enzyme to cleave it once inside.

The Fermentable Sugars

Glucose. The default. A six-carbon monosaccharide that enters glycolysis directly with no preliminary processing. Every fermentation pathway in the cell is built around it.

Fructose. Also a monosaccharide with the same formula as glucose, fermented nearly as readily. It is phosphorylated by hexokinase and joins glycolysis a step later. Fructose makes up roughly half the sugar in ripe grapes, which matters for wine, since yeast consumes glucose preferentially and the last sugar remaining in a slow fermentation tends to be fructose.

Sucrose. A disaccharide of glucose and fructose. Yeast secretes the enzyme invertase, encoded by the SUC2 gene, which splits sucrose outside the cell wall. The resulting glucose and fructose are then transported in. This external hydrolysis is why table sugar ferments so quickly.

Maltose. Two glucose units joined by an α-1,4 bond, and the dominant sugar in malted barley. Fermenting it requires a dedicated transporter and the enzyme maltase, both encoded by the MAL gene family. Not all strains carry the same complement, which is a genuine functional difference between brewing and baking strains.

Maltotriose. Three linked glucose units. Present in significant quantity in beer wort. Some strains ferment it well, others poorly or not at all, and this single trait accounts for much of the variation in how dry a beer finishes.

Galactose. Fermentable, but only after the yeast switches on the GAL gene cluster and routes it through the Leloir pathway. The genes stay switched off while glucose is available, so galactose fermentation begins late.

What Yeast Cannot Ferment

Lactose. Milk sugar, a disaccharide of galactose and glucose. S. cerevisiae fails on both counts: it has neither a lactose permease to carry the molecule inside nor the β-galactosidase enzyme to cleave it. This is why milk does not spontaneously become alcoholic and why brewers add lactose to milk stouts specifically because it survives fermentation and leaves residual sweetness.

Other yeasts do handle it. Kluyveromyces lactis and Kluyveromyces marxianus carry both the transporter and the enzyme, and they are the basis of fermented dairy products such as kefir and koumiss.

Starch and dextrins. Starch is a glucose polymer, and yeast has no amylase to break it apart. In brewing, this work is done ahead of time: malting activates the grain’s own amylase enzymes, and the mash converts starch into fermentable sugars before yeast is ever added. The longer glucose chains that survive mashing, called dextrins, pass through fermentation untouched and contribute body to the finished beer.

Cellulose and dietary fiber. Structurally similar to starch but with β-1,4 linkages that yeast enzymes cannot cleave.

Pentoses. Five-carbon sugars such as xylose and arabinose, abundant in plant biomass, are not fermented by wild-type S. cerevisiae. Engineering strains that can is an active line of work in cellulosic ethanol research, precisely because so much plant material is locked in this form.

Most sugar alcohols. Erythritol, xylitol, and similar polyols largely resist fermentation, which is part of why they behave differently from sugar in baking.

Reference Table

What Saccharomyces cerevisiae can and cannot ferment, and why. "Gets into the cell" matters more than people expect: sucrose is split outside the cell, maltose is carried in whole.
Sugar Structure Gets into the cell Enzyme required Fermented?
Glucose Monosaccharide HXT hexose transporters, facilitated diffusion None — enters glycolysis directly Yes — used first
Fructose Monosaccharide HXT transporters None Yes — alongside glucose, more slowly
Sucrose Glucose–α1,2–fructose Split outside the cell, then the hexoses enter Invertase (SUC2), in the cell wall Yes
Maltose Glucose–α1,4–glucose Carried in intact by MAL/AGT1 transporters Maltase (α-glucosidase), inside the cell Yes — but only after glucose is gone
Maltotriose Three glucose, α1,4 AGT1 transporter only α-glucosidase Strain-dependent — often left behind
Galactose Monosaccharide GAL2 permease, which must be induced first Leloir pathway (GAL1, GAL7, GAL10) Yes, after a lag while genes switch on
Melibiose Galactose–α1,6–glucose — α-galactosidase (MEL1) Lager yeast yes, ale yeast no
Raffinose Galactose–glucose–fructose Invertase clips off the fructose outside the cell Invertase, plus α-galactosidase for the rest Only partly — ale yeast takes about a third
Lactose Galactose–β1,4–glucose No transporter for it at all β-galactosidase — absent No
Dextrins Short glucose chains Too large to cross the membrane Glucoamylase — absent No — they stay behind as body
Starch Glucose polymer Far too large to cross the membrane Amylase — absent No — must be malted or mashed first
Xylose Pentose (5 carbons) No native transporter No native pathway to use it No — only genetically engineered strains
The order matters. Yeast works through these in sequence, not all at once: glucose, then fructose, then maltose, then maltotriose. Glucose actively represses the MAL genes, so maltose uptake cannot begin until glucose is nearly exhausted. A stuck fermentation that stalls with maltotriose left in the vessel is usually a strain problem, not a temperature one.

The Order Yeast Works Through Them

Yeast does not consume available sugars simultaneously. Glucose suppresses the genes needed for the others, a regulatory behavior called carbon catabolite repression. While glucose remains, the maltose and galactose systems stay switched off.

The practical sequence in a typical wort is glucose, then fructose and sucrose, then maltose, then maltotriose if the strain can manage it. This produces the characteristic fermentation curve, with a fast early phase and a slower tail as the yeast reorganizes its metabolism for the harder sugars.

Brewers quantify the endpoint as apparent attenuation, the percentage of the original extract consumed. Most beers land between 65% and 85%. What remains is largely dextrins and other unfermentables, and a brewer targeting a fuller body will mash to leave more of them behind.

Fermentable Sugars: Key Numbers

  • Fermentable by standard bakers’ yeast: glucose, fructose, sucrose, maltose, maltotriose, galactose
  • Not fermentable: lactose, starch, dextrins, cellulose, pentoses, most sugar alcohols
  • Sucrose is cleaved outside the cell by invertase (SUC2)
  • Typical apparent attenuation for beer: 65-85%
  • Wine grapes at harvest hold roughly 150-250 g of sugar per liter, split close to evenly between glucose and fructose

Sources:

Frequently Asked Questions

What sugars can yeast ferment? Saccharomyces cerevisiae ferments glucose, fructose, sucrose, maltose, maltotriose, and galactose. Sucrose is split into glucose and fructose by invertase before uptake. Maltose and maltotriose require specific transporter and enzyme genes that vary between strains, and galactose is fermented only after glucose is exhausted.

Why can’t yeast ferment lactose? Standard bakers’ yeast lacks both the transporter needed to move lactose into the cell and the β-galactosidase enzyme needed to split it into galactose and glucose. Other species, notably Kluyveromyces lactis and K. marxianus, possess both and are used to ferment dairy.

Which sugar does yeast prefer? Glucose. While glucose is present, it represses the genes required to metabolize maltose and galactose, so those sugars are consumed only after glucose is largely depleted. Fructose follows glucose closely, then maltose, then maltotriose.

Can yeast ferment starch? Not directly. Yeast produces no amylase, so starch must be broken into fermentable sugars first. In brewing this happens during malting and mashing, where the grain’s own enzymes convert starch to maltose and other short chains before the yeast is pitched.

What is left over after fermentation? Unfermentable material: dextrins, any lactose added deliberately, minerals, proteins, and whatever fermentable sugar the yeast could not finish before alcohol tolerance or nutrient limits stopped it. In beer this residual extract is what apparent attenuation measures.