The Main Chemical Constituents of Garlic Explained

January 8, 2026 •

3 min read

Garlic (Allium sativum) is a widely cultivated plant with a history of use spanning over 4,000 years, prized for its culinary flavor and medicinal properties. Its pungent aroma and potent bioactivity are primarily attributed to its diverse and reactive array of organosulfur compounds, which are key chemical constituents of garlic. This complex chemistry is responsible for the health-promoting effects observed in this bulbous plant.

Quick Summary

Garlic's distinctive flavor and health benefits stem from organosulfur compounds like alliin, which is converted to allicin by the enzyme alliinase upon crushing. These unstable molecules break down into numerous derivatives, such as diallyl sulfides and ajoene, which offer antioxidant, anti-inflammatory, and antimicrobial properties depending on how the garlic is prepared.

Key Points

Alliin: An odorless, sulfur-containing amino acid present in whole garlic that is the precursor to most of its active compounds.
Allicin: Formed when alliin reacts with the enzyme alliinase upon crushing or chopping, this unstable compound gives fresh garlic its pungent aroma and is a potent bioactive agent.
Reactive Breakdown: Allicin quickly breaks down into a range of organosulfur compounds, including diallyl sulfides (DAS, DADS, DATS) and ajoene, which offer various health benefits.
Preparation Matters: The method of preparing garlic—raw, cooked, or aged—significantly alters its chemical profile and the resulting health benefits due to the stability of different sulfur compounds.
Diverse Compounds: Beyond sulfur, garlic contains other beneficial compounds such as antioxidants (polyphenols) and minerals (manganese, selenium) that contribute to its overall nutritional value.
Health Impact: The main chemical constituents of garlic are responsible for its antioxidant, anti-inflammatory, antimicrobial, and cardiovascular protective effects.

The Transformative Chemistry of Garlic

Garlic’s unique biochemical profile is a defense mechanism developed by the plant, which is activated upon cellular damage. In its intact, raw state, the bulb contains an odorless, sulfur-based amino acid called alliin. This dormant state is fundamentally transformed when the garlic is cut, crushed, or chewed. The damage to the plant's cells causes the enzyme alliinase to mix with alliin, catalyzing a rapid chemical reaction.

Allicin: The Key Bioactive Intermediate

This enzymatic reaction produces allicin, a highly reactive and unstable organosulfur compound responsible for the characteristic sharp odor and taste of fresh garlic. Allicin acts as a defense substance for the plant and is known for its strong antimicrobial, antioxidant, and anti-inflammatory properties. However, its instability means it quickly degrades into other sulfur-containing compounds.

Allicin's Reactive Breakdown Products

As allicin breaks down, it forms a variety of other bioactive compounds. This breakdown process is influenced by factors such as heat and solvents, which is why different garlic preparations—raw, cooked, or aged—have distinct chemical compositions and health effects. Some of the most notable derivatives include:

Diallyl Sulfides (DAS, DADS, DATS): A group of oil-soluble organosulfur compounds that contribute significantly to garlic's health benefits, including cardiovascular protection and anticancer activity.
Ajoene: This compound, formed when allicin reacts in oil-based preparations, is a potent antioxidant and may have antifungal and antithrombotic effects.
Vinyldithiins: These compounds are also derived from allicin and have demonstrated significant antioxidant properties.
S-allyl Cysteine (SAC): Found in aged garlic extract, SAC is a water-soluble compound that is highly stable and bioavailable, contributing to garlic's antioxidant and neuroprotective effects.

Beyond Sulfur: Other Active Compounds

While organosulfur compounds receive the most attention, garlic contains a broad spectrum of other valuable phytochemicals and nutrients. These include:

Polyphenols: Including flavonoids like quercetin and phenolic acids like caffeic acid, which provide additional antioxidant capacity.
Saponins: A class of plant compounds with known cholesterol-lowering properties.
Minerals: Garlic is a good source of manganese, vitamin C, vitamin B6, selenium, and phosphorus.

Comparison of Garlic Compound Bioactivity


Compound	Source/Form	Stability	Key Health Benefits
Alliin	Intact raw garlic	Stable	Precursor to allicin; limited direct bioactivity until converted.
Allicin	Freshly crushed/chopped garlic	Highly Unstable	Potent antimicrobial, antioxidant, anti-inflammatory; responsible for pungent aroma.
Diallyl Sulfides (DAS, DADS, DATS)	Cooked, dried, or garlic oil	Relatively Stable	Antioxidant, anticancer, and cardioprotective effects.
Ajoene	Oil macerates of crushed garlic	More Stable than Allicin	Antioxidant, antifungal, and antithrombotic properties.
S-allyl Cysteine (SAC)	Aged garlic extract	Highly Stable	Water-soluble antioxidant, neuroprotective, and modulates immune function.

How Preparation Affects Garlic's Chemistry

The way garlic is prepared fundamentally changes its chemical composition and resulting health benefits. Crushing or chopping raw garlic is crucial for the formation of allicin, but cooking rapidly degrades this compound, converting it into more stable, less pungent derivatives. Aged garlic extract, on the other hand, is a preparation that intentionally removes the unstable, pungent compounds, leaving behind stable, water-soluble ones like S-allyl cysteine. This makes aged garlic a powerful source of long-lasting antioxidant benefits, although it lacks the sharp, direct antibacterial punch of freshly crushed raw garlic.

Conclusion

The main chemical constituents of garlic are a fascinating blend of dynamic and stable compounds, predominantly centered around its unique organosulfur chemistry. The initial conversion of odorless alliin into volatile allicin upon cellular damage sets off a cascade of chemical reactions that yield a wide array of bioactive molecules like diallyl sulfides and ajoene. These transformations explain why raw, cooked, and aged garlic products offer different health benefits and possess unique flavors. By understanding these key chemical components, consumers can appreciate the complex science behind garlic’s storied reputation as a flavorful spice and potent natural remedy. For further reading on the pharmacological effects of these constituents, you can visit the National Institutes of Health (NIH) website.

Frequently Asked Questions

While garlic contains many active compounds, allicin is considered the most significant. It is responsible for the characteristic odor and many of garlic's initial health-promoting effects, though its rapid decomposition leads to a cascade of other important compounds.

No, cooking garlic changes its chemical makeup. Heat rapidly degrades the unstable allicin into other, more stable sulfur compounds. This process mellows the flavor and alters the range of health benefits, with some compounds decreasing and others becoming more prominent.

The strong smell of crushed garlic is due to the formation of allicin. The enzyme alliinase is activated when the garlic is damaged, converting the odorless alliin into the pungent allicin.

Organosulfur compounds are organic molecules that contain sulfur. In garlic, these include alliin, allicin, and its breakdown products like diallyl sulfides, which are responsible for its distinctive smell and most of its medicinal properties.

Aged garlic extract (AGE) is a supplement made by aging raw garlic in an alcohol-water solution over a long period. This process removes the pungent, unstable compounds like allicin, leaving behind stable, water-soluble antioxidants such as S-allyl cysteine (SAC), which offers distinct health benefits.

Yes, garlic also contains important non-sulfur compounds. These include polyphenols (like quercetin and caffeic acid) and saponins, which contribute to its antioxidant capacity and cholesterol-lowering effects.

The chemical composition is influenced by various factors, including the garlic's variety, growing conditions (temperature, irrigation), and post-harvest handling. Processing methods like drying or maceration also significantly impact the final profile of bioactive compounds.