The joy of cooking often stems from adding aromatic components from Allium plants, such as garlic, onions, leeks, chives, scallions, and green onions. The core characteristic of these pungent seasonings is that their aromatic components generally contain the same element—sulfur. What’s particularly remarkable is that these aromas are not present in the plant’s original state, but rather are biosynthesized through enzymatic reactions under conditions of cellular disruption (pressing, cutting, frying, boiling, etc.). For example, a whole clove of garlic and an uncut onion neither emit a pungent odor nor cause eye irritation.
I. Biosynthetic Mechanism of Sulfur-Containing Compounds in Allium Plants
In the case of garlic, in the intact clove cells, the sulfur-containing alliin (S-allyl-L-cysteine sulfoxide) and the catalytic enzyme alliinase are separated in different organelles, existing independently and therefore not reacting. When garlic cloves are pressed and chopped, their cell structure ruptures, releasing alliinase, which rapidly catalyzes the hydrolysis of alliin. Under the action of alliinase, alliin loses pyruvate and ammonia, generating an unstable sulfenic acid intermediate (allyl sulfenic acid).
This sulfenic acid intermediate is extremely unstable and spontaneously undergoes a dimerization dehydration reaction, ultimately forming the core active substance with a strong, pungent odor—allicin (diallyl thiosulfinate). Allicin is the core source of garlic’s flavor and bioactivity; its molecular structure contains the characteristic thiosulfinate (-S(O)-S-) functional group, which is the chemical essence of garlic’s unique aroma.
Besides allicin, garlic undergoes subsequent rearrangement and polymerization reactions to generate a series of structurally diverse sulfur-containing compounds, encompassing various sulfur-containing functional groups:
– Thioethers (RSR’): such as diallyl sulfide (DAS), diallyl disulfide (DADS), and diallyl trisulfide (DATS), are the main sources of garlic’s flavor after heating;
– Sulfoxides (RS(O)R’): such as allicin and diallyl disulfide sulfoxide, are the core of garlic’s fresh flavor and antibacterial activity;
– Disulfide compounds (RSSR’): such as diallyl disulfide, are stable products of allicin decomposition;
– In addition, there are thiosulfonates, cyclic sulfides, and other derivatives, which together constitute garlic’s complex flavor system.
The aroma generation mechanism of onions, leeks, and other Allium species is highly homologous to that of garlic: Whole onions contain isoalliin (S-1-propenyl-L-cysteine sulfoxide), which, after being cut, is catalyzed by alliinase to produce a sulfenic acid intermediate, which then rearranges into syn-propanethial S-oxide. This is the “tear-inducing factor” that causes tears when cutting onions; the sulfoxide functional group in its molecule is the core source of its irritant effect.
II. Bioactivity and Medicinal Value of Sulfur-Containing Compounds
Sulfur-containing compounds in Allium species are not only sources of flavor but also possess a wide range of bioactivities. The medicinal value of some components has been verified by modern medicine:
1. Antibacterial Activity: Allicin is a broad-spectrum antibacterial agent that can significantly inhibit various pathogenic bacteria such as Staphylococcus aureus, Escherichia coli, Vibrio cholerae, and Mycobacterium tuberculosis by disrupting the thiol bonds in bacterial cell membranes and inhibiting sulfhydryl enzyme activity. Before the advent of modern antibiotics, garlic preparations were widely used to treat infectious diseases such as typhus, cholera, dysentery, and tuberculosis, serving as a traditional natural antibacterial agent.
2. Cardiovascular Protective Effects: Components in garlic, such as diallyl disulfide (DADS) and allicin, can reduce endogenous cholesterol synthesis and lower blood total cholesterol and triglyceride levels by inhibiting HMG-CoA reductase activity. Simultaneously, they inhibit arachidonic acid metabolism in platelets, preventing platelet aggregation and thrombosis, thereby reducing the risk of atherosclerosis and coronary heart disease.
3. Anti-tumor Effects: Epidemiological studies have confirmed a significant negative correlation between garlic consumption and gastric cancer incidence in areas with a high incidence of gastric cancer. The mechanism is related to the antioxidant properties of sulfur-containing compounds, their inhibition of carcinogen activation, their induction of tumor cell apoptosis, and their enhancement of the body’s immune function. Among these, diallyl disulfide (DADS) is one of the most extensively studied anti-tumor active ingredients.
4. Antioxidant Effects: Sulfur-containing compounds such as allicin and DATS can scavenge free radicals in the body, enhance the activity of antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), reduce oxidative stress damage, and delay cell aging.
Allium plants have evolved these sulfur-containing compounds, essentially a chemical defense mechanism: when plants are eaten by insects or infected by microorganisms, the sulfur-containing compounds released during cell rupture can exert insect-repelling and antibacterial effects, protecting themselves from harm.
III. Metabolic and Physiological Effects of Garlic Odor
The most well-known side effect of garlic is its unpleasant breath, but the source of this odor is not the garlic residue left in the mouth, but rather the metabolism of sulfur-containing compounds in the body: sulfur-containing components such as allicin are absorbed into the bloodstream through the gastrointestinal tract, circulate to the lungs, and are expelled from the alveoli during respiration, thus forming “garlic breath.” Simultaneously, some metabolites are excreted through the kidneys in urine, remaining in the urine for 3-4 days, which explains the relatively long duration of body odor and bad breath after garlic consumption.
Allicin possesses extremely strong skin permeability, a characteristic highly similar to that of the classic transdermal absorption solvent dimethyl sulfoxide (DMSO). Allicin’s lipid-soluble sulfoxide structure can penetrate the stratum corneum of the skin and enter the bloodstream. Therefore, even if garlic is simply rubbed on the feet, allicin can be absorbed through the skin and ultimately expelled through respiration, resulting in a garlic odor in the mouth. This phenomenon has been experimentally verified.
IV. Background and Practical Knowledge
1. The Effect of Heating on Sulfur-Containing Compounds: The optimal temperature for alliinase is around 37℃. High temperatures (>60℃) will inactivate the enzyme. Therefore, letting freshly chopped garlic stand for 10-15 minutes allows alliin to be fully converted into allicin before heating, maximizing the retention of active ingredients. Direct high-temperature heating will inactivate alliinase, preventing the formation of allicin and reducing the health benefits of garlic. Simultaneously, allicin decomposes into more stable diallyl disulfide and diallyl trisulfide upon heating, reducing irritation and resulting in a milder flavor.
2. The Chemical Principle of Removing Garlic Breath: Casein in milk can bind with sulfur-containing compounds, reducing their volatility; polyphenol oxidase in apples can oxidize and decompose allicin; catechins in green tea can neutralize the odor of sulfur-containing compounds. These methods essentially reduce the release of volatile sulfur-containing components through chemical reactions or physical binding.
3. Commonalities and Differences of Sulfur-Containing Compounds in Allium Plants: All Allium plants use cysteine sulfoxide as a precursor, which is catalyzed by alliinase to produce sulfur-containing flavor compounds. However, the precursor structures differ among species, resulting in diverse flavors: garlic is predominantly allyl-substituted, giving it a strong odor; onions are predominantly 1-propenyl-substituted, possessing both pungent and sweet qualities; leeks are predominantly methyl-substituted, resulting in a fresh odor. The core difference lies in the different carbon chain structures of the substituents.
The unique flavors of garlic, leeks, and onions all come from the sulfur compounds that volatilize when they are cut. These small-molecule sulfides are not only the soul of cooking flavor, but also carry the defensive wisdom of plants and the health value for humans. They are typical examples of the deep integration of chemistry with food and medicine.
