Acetaldehyde is a volatile organic compound. It is a colorless liquid or gas with a pungent odor, though at low concentrations, it contributes a fruity aroma to many foods and beverages. It occurs widely in nature and is also manufactured for industrial use. The International Agency for Research on Cancer (IARC) classifies acetaldehyde, when derived from alcoholic beverages in the body, as a Group 1 human carcinogen.
Acetaldehyde Generated by the Human Body
The most significant source of acetaldehyde in the human body is the metabolism of ethanol, the alcohol found in alcoholic drinks. When alcohol is consumed, the liver enzyme Alcohol Dehydrogenase (ADH) rapidly converts ethanol into acetaldehyde. This intermediate metabolite is highly reactive and toxic, responsible for many of the unpleasant physical symptoms associated with a hangover.
The body relies on another enzyme, Aldehyde Dehydrogenase (ALDH), primarily the mitochondrial ALDH2 form, to quickly break down acetaldehyde into harmless acetate. However, certain genetic variations, especially the ALDH2\2 allele common in some populations, can result in a deficient ALDH2 enzyme. This deficiency causes acetaldehyde to accumulate significantly in the blood and saliva after drinking, greatly increasing exposure to the toxic compound.
Acetaldehyde is also produced by microbial communities in the human gastrointestinal tract. Various bacteria and yeasts, including Helicobacter pylori and species in the colon, contain their own ADH enzymes. These microbes can convert ethanol from alcoholic beverages into acetaldehyde, especially in the large intestine where the protective ALDH enzyme activity of the body’s cells is relatively low.
The gut microbiota can also produce acetaldehyde by fermenting carbohydrates and sugars, even without alcohol consumption. This microbial activity contributes to the overall acetaldehyde burden. Small amounts are also generated as a byproduct of normal cellular processes, such as amino acid metabolism and lipid oxidation.
Sources in Food and Beverages
Acetaldehyde is a natural component in many ingested products, often contributing to flavor and aroma. Fermentation processes are major contributors to its presence, resulting in varying concentrations in alcoholic beverages. Wine and spirits typically contain some of the highest concentrations, with levels in wine reaching up to 290 parts per million (ppm) and spirits up to 104 ppm.
The compound is also present in non-alcoholic fermented foods that utilize bacteria or yeast. Yogurt, for instance, contains measurable amounts, with concentrations reported up to 17 milligrams per kilogram (mg/kg). Vinegar can contain extremely high concentrations, sometimes exceeding 1 gram per kilogram (g/kg).
Acetaldehyde naturally occurs as an intermediate in the respiration process of plants. This means it is found in many fruits and vegetables, particularly those that are ripe, where it acts as a volatile flavor compound. For example, concentrations in fruits and fruit juices can range from 0.2 to 230 ppm.
Processing and heat treatment affect acetaldehyde levels in food items. Roasted coffee beans, bread, and certain processed dairy products contain the compound. Additionally, it is intentionally added to some processed items, such as soft drinks and candies, to enhance flavor profiles.
Environmental and Product Exposures
Exposure to acetaldehyde occurs through inhalation from environmental and consumer sources. It is a significant component of smoke generated by combustion, which allows for exposure both indoors and outdoors. Tobacco smoke is a major source, containing the compound in concentrations as high as 1,400 micrograms per cigarette.
Acetaldehyde is also released into the air from other burning materials, including wood smoke from fireplaces and stoves. In urban environments, vehicle exhaust, particularly from gasoline and diesel engines, contributes to ambient air pollution. A substantial portion of the acetaldehyde found in the atmosphere is also formed indirectly through the photochemical oxidation of other hydrocarbons.
The compound is present in consumer and industrial products due to its role as a chemical intermediate. Indoor air can contain acetaldehyde released from certain building materials, such as rigid polyurethane foams. Other common household items like adhesives, coatings, inks, and nail polish remover may also release the volatile chemical.