Fermentation chemistry: ethanol, lactic acid and living systems

The chemistry of fermentation explains how microorganisms turn sugars into useful products, from ethanol in beer and wine to lactic acid in yoghurt, kimchi and sourdough. Although the word often brings to mind alcohol, fermentation is a broader set of anaerobic or oxygen-limited metabolic pathways that help cells release energy when respiration cannot proceed normally.

For chemistry students and researchers in Australia, fermentation connects classroom equations with familiar examples: a loaf of sourdough in Melbourne, yoghurt in a Brisbane fridge, wine from the Barossa Valley, or a small-batch beer sold through a local bottle shop. Understanding the reactions makes it easier to interpret flavour, acidity, gas production, preservation and analytical results.

How cells extract energy without oxygen

During glycolysis, one glucose molecule is split into two molecules of pyruvate. This pathway produces a small amount of ATP, the usable energy currency of the cell, along with reduced NADH. If oxygen is unavailable, the cell must regenerate NAD⁺ so glycolysis can continue. Fermentation performs this recycling role.

In alcoholic fermentation, pyruvate first loses carbon dioxide to form ethanal, also called acetaldehyde. Alcohol dehydrogenase then reduces ethanal to ethanol:

[ \text{Glucose} \rightarrow 2\text{ ethanol} + 2\text{ carbon dioxide} + \text{energy} ]

Yeasts such as Saccharomyces cerevisiae are especially effective at this process. In a warm Queensland kitchen, rapid fermentation may produce visible bubbles quickly, while cooler conditions in Tasmania or a refrigerated laboratory slow the reaction and alter the final aroma.

Ethanol production and its chemical variables

Brewing and winemaking depend on controlling sugar concentration, temperature, pH, yeast strain and nutrient availability. Maltose from malted barley supports beer fermentation, while grape juice supplies glucose and fructose for wine. As ethanol accumulates, it becomes toxic to yeast, so fermentation eventually slows even when sugar remains.

Carbon dioxide is another important product. In beer, some carbon dioxide is retained under pressure; in sparkling wine, it dissolves in the liquid before being released as bubbles. In bread dough, the gas expands gluten networks rather than producing an alcoholic beverage. Most ethanol evaporates during baking, although the amount depends on recipe, temperature and cooking time.

Australian producers work across very different climates, from the cooler Yarra Valley to warm inland regions. Temperature management is therefore central to consistent fermentation. The alcohol content measured by volume is commonly reported as ABV, while analytical laboratories may use gas chromatography to separate and quantify ethanol, methanol and volatile flavour compounds.

Lactic acid fermentation and acid balance

Lactic acid bacteria use pyruvate as an electron acceptor, converting it into lactate while regenerating NAD⁺:

[ \text{Glucose} \rightarrow 2\text{ lactate} + \text{energy} ]

Some species produce mainly lactic acid, known as homofermentative metabolism. Others generate lactic acid alongside ethanol, carbon dioxide or acetic acid, giving heterofermentative products a more complex flavour. The acid lowers pH and can inhibit many competing microorganisms.

Yoghurt production commonly involves Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These bacteria convert lactose into lactic acid, causing milk proteins to coagulate and form a gel. In sourdough, lactic acid bacteria coexist with yeasts. Their balance influences whether the bread tastes creamy, sharply sour or mildly tangy.

Fermentation in food, health and everyday life

Lactic fermentation is used in sauerkraut, kimchi, pickled vegetables and some traditional Australian food experiments influenced by Asian and European methods. The salt concentration encourages desirable microorganisms while drawing water from plant tissues. In a home kitchen, temperature, salt percentage and oxygen exposure strongly affect safety and texture.

Kombucha involves a mixed culture often called a SCOBY, containing yeasts and bacteria. Yeasts produce ethanol from sugar, while bacteria oxidise some ethanol to organic acids such as acetic acid. The result is acidic, lightly carbonated and variable from batch to batch. “Probiotic” claims require care: the presence of live cultures does not automatically demonstrate a specific health benefit.

Food safety remains essential in Australia, where commercial products must meet national standards and home fermenters still need good hygiene. Uncontrolled fermentation can permit pathogens, excessive alcohol or unwanted toxins. A clean vessel, measured ingredients, reliable refrigeration and pH monitoring are more dependable than judging safety by smell alone.

Measuring fermentation with chemistry

Analytical chemistry helps identify what microbes have produced. pH meters track acidification, while titratable acidity estimates the total amount of acid capable of reacting with a base. These measurements are related but not interchangeable: two samples can have similar pH values and different total acid concentrations.

High-performance liquid chromatography can measure glucose, fructose, lactic acid, acetic acid and ethanol in a single sample. Gas chromatography is useful for volatile compounds, including ethanol and aroma molecules. Spectrophotometric assays may monitor NADH-linked enzyme reactions, although calibration, matrix effects and sample preparation must be controlled.

For students at the University of Sydney, Monash University or a regional TAFE laboratory, a simple comparison can demonstrate the principles: ferment identical glucose solutions with baker’s yeast and a yoghurt culture under suitable conditions. Record mass loss from carbon dioxide, pH change and sugar concentration over time, then relate each result to the metabolic pathway rather than treating fermentation as a single reaction.

Fermentation is best understood as a negotiated chemical process between cells and their environment. Ethanol pathways preserve glycolysis by producing alcohol and carbon dioxide; lactic pathways do the same by producing lactate and lowering pH. The products shape food, beverages, biotechnology and laboratory measurements across Australia, from a Barossa cellar to a home ferment in a Perth suburb.

Begin with two sterile, labelled glucose samples—one inoculated with baker’s yeast and one with a yoghurt culture—and record their pH and mass at regular intervals for 48 hours.