The Alliums

TL;DR. Onion and garlic taste of nothing until you cut them. Both store an odourless precursor and a separate enzyme, and damage brings them together to produce the pungent sulphur compounds that define the family, plus the gas that makes you cry. Garlic's allicin is unstable and never reaches your bloodstream intact, which is why supplement chemistry matters and why crushing and waiting ten minutes before cooking measurably increases what you get. The health evidence is modest and real: small blood pressure reductions, small cholesterol effects, and consistent observational associations with lower gastric and colorectal cancer.

1. What they are

Genus Allium, family Amaryllidaceae. Around 900 species; a handful are cultivated.

2. Where they come from

Onion and garlic both originate in Central Asia, in the region between Iran and the Tian Shan. Both are among the oldest cultivated plants: garlic appears in Egyptian tombs (including Tutankhamun's), was rationed to the labourers who built the pyramids according to Herodotus, and is documented in Sumerian, Chinese, and Indian texts from the third millennium BCE.

Garlic occupied a strange cultural position for millennia: valued as medicine and as food for labourers and soldiers, and often disdained by elites for the smell. Ancient Greek and Roman writers, medieval Europeans, and various religious traditions all restricted it.

Production: dry onions around 110 million tonnes, making onion one of the largest vegetable crops on Earth, led by India and China, then Egypt, the US, Iran, and Turkey. Garlic around 30 million tonnes, with China producing roughly 75 to 80 percent of the world's supply, a concentration matched by few other crops.

3. How they are grown

Onions are photoperiod-sensitive, which is the single most important agronomic fact about them and the reason home growers fail. Bulb formation is triggered by day length, and varieties are sold by category:

Plant a long-day onion in Texas and it produces leaves and never bulbs; plant a short-day onion in Scotland and it bulbs prematurely at the size of a marble. Short-day onions are also generally milder and store poorly; long-day onions are pungent and store for months.

Garlic is planted as individual cloves in autumn, needs a cold period (vernalisation, roughly 4 to 8 weeks below 10 °C) to form a proper bulb, and is harvested the following summer. Almost all commercial garlic is sterile and propagated only vegetatively, so every plant of a variety is a clone.

Hardneck garlic produces a flowering stem called a scape, which growers cut off to divert energy into the bulb; scapes are excellent food and increasingly sold. Softneck garlic does not, stores far longer, and is what braided garlic and most supermarket garlic is.

Soil. Both want well-drained, fertile, loose soil at pH 6.0 to 7.0. Both are shallow-rooted and poor competitors against weeds, so weed control is a major cost. Sulphur availability in the soil directly affects pungency: high-sulphur soils produce hotter onions and garlic, which is partly why some regional garlics are notably stronger.

Curing is essential and often skipped by home growers. After lifting, bulbs are dried in a warm, airy, shaded place for 2 to 4 weeks until the necks are tight and the outer skins are papery. Cured onions and garlic store for 6 to 10 months; uncured ones rot within weeks.

Sweet onions (Vidalia, Walla Walla, Maui) are low-pungency varieties grown on low-sulphur soils. Vidalia is a legally protected designation covering only a defined area of Georgia, and the sweetness comes from the local soil chemistry as much as the variety.

4. What is inside them

Per 100 g raw:

Garlic looks nutrient-dense per 100 g and nobody eats 100 g of garlic; a clove is about 3 g. The nutritional contribution of alliums is essentially their sulphur chemistry and their fructans, not their vitamins.

Fructans matter twice. They are prebiotic fibre, selectively fermented by Bifidobacterium (Chapter 14), and onion, garlic, and leek are among the top dietary sources. They are also the single most common FODMAP trigger in irritable bowel syndrome, and they are water-soluble but not oil-soluble, which produces the useful trick in section 8.

Quercetin. Onions, particularly red and yellow, are among the richest dietary sources of this flavonol, concentrated in the outer layers just under the skin. Red onions additionally carry anthocyanins.

5. The chemistry: why cutting matters

In short: Both onion and garlic store an odourless amino acid derivative in one compartment and an enzyme in another, and cutting the cell releases a cascade.

Onion, and why you cry

Onions store S-alk(en)yl cysteine sulphoxides, principally isoalliin, and separately the enzyme alliinase. Cut the cell and:

1. Alliinase converts isoalliin to 1-propenylsulphenic acid.
2. A second enzyme, lachrymatory factor synthase, rearranges it into syn-propanethial-S-oxide, a volatile gas.
3. The gas reaches your eyes, dissolves in the tear film to form sulphuric acid, irritates the cornea, and your eyes water.

The discovery of lachrymatory factor synthase as a distinct enzyme (published in Nature in 2002 by a Japanese team at House Foods) made a tearless onion possible by suppressing that one gene, and such onions have since been commercialised. Crucially, the flavour compounds come from a different branch, so a tearless onion is not a flavourless one.

What actually reduces crying, in rough order of effectiveness: chill the onion for 30 minutes first (the gas is less volatile cold), use a genuinely sharp knife (less cell damage), cut near a running extractor or fan, cut under running water, and leave the root end (which is highest in the enzyme) attached until last. Contact lenses and goggles work by physical barrier. Bread in the mouth, a candle, and a spoon between the teeth do not.

Garlic, and the allicin problem

Garlic stores alliin (an odourless sulphoxide) and, separately, alliinase. Crush a clove and alliinase converts alliin to allicin within seconds. Allicin is the compound with most of the antimicrobial activity in laboratory tests, and it is the smell of fresh crushed garlic.

Three inconvenient facts follow:

1. Allicin is unstable. It decomposes within hours at room temperature into diallyl disulphide, diallyl trisulphide, ajoene, vinyldithiins, and others. Those breakdown products carry much of the biological activity actually seen in the body.
2. Allicin does not survive digestion and is essentially undetectable in blood after eating garlic. Whatever garlic does systemically, it does not do it via circulating allicin.
3. Heat destroys alliinase before it has acted. Throwing whole or freshly chopped garlic straight into a hot pan denatures the enzyme, so much of the alliin is never converted.

The practical consequence, and it is well demonstrated: crush or chop garlic and wait about 10 minutes before heating it. The enzyme completes its work, and the allicin and its derivatives, unlike the enzyme, are considerably more heat-stable. A 2001 study by Song and Milner found that a 10-minute rest before microwaving preserved the anti-cancer activity in an animal model that immediate cooking destroyed.

Black garlic is whole bulbs held at around 60 to 70 °C and high humidity for several weeks. There is no fermentation involved despite the common description: it is Maillard browning and enzymatic reactions, producing a sweet, sticky, balsamic-flavoured product with almost no pungency, higher antioxidant activity, and no allicin.

Aged garlic extract is garlic aged in dilute alcohol for months, producing S-allyl cysteine, which is stable, absorbable, and odourless, and which is the form used in several clinical trials.

6. What the evidence actually shows

Established: allicin and its relatives have genuine antimicrobial and antifungal activity in vitro. Alliums are among the richest sources of prebiotic fructans and, for onions, quercetin.

Strong: observational associations between high allium intake and lower gastric and colorectal cancer risk are among the more consistent single-food cancer associations, seen across Chinese, Italian, and other populations. Confounded like all such data.

Mixed, and this is where most of the interest is:

• Blood pressure. Meta-analyses of garlic supplement trials find reductions of roughly 6 to 8 mmHg systolic in hypertensive participants, which is a clinically meaningful size, comparable to a low dose of a first-line drug. Effects in normotensive people are small. The trials are mostly small, often use aged garlic extract or standardised allicin-yield products rather than food, and quality is variable.
• Cholesterol. Small reductions in total and LDL cholesterol, in the range of 0.1 to 0.3 mmol/L, inconsistent across trials.
• Common cold. One frequently cited 2001 trial found fewer colds with a garlic supplement. The Cochrane review concluded there was insufficient evidence from a single trial of moderate quality.
• Immune function, where laboratory effects are clear and clinical outcomes are not.

Thin: garlic for blood thinning as a clinical strategy, garlic as an antibiotic substitute (topical raw garlic causes chemical burns, and this is a real and repeated emergency presentation), and garlic for insect repellence or parasite treatment.

7. Who should eat more, who should be careful

Good for: most people, in normal culinary quantities, which also happens to be the amount that makes food taste good.

Be careful if:

• You have IBS. This is the big one. Onion and garlic fructans are the most common identified FODMAP trigger in irritable bowel syndrome, and eliminating them is one of the first steps in a low-FODMAP protocol. It is also the hardest, because they are in almost everything. See section 8 for the workaround.
• You take anticoagulants or are having surgery. Garlic has mild antiplatelet activity, and high-dose garlic supplements alongside warfarin, aspirin, or clopidogrel have a plausible bleeding risk. Standard surgical advice is to stop garlic supplements 7 to 10 days before an operation. Culinary garlic is not a concern.
• You take HIV protease inhibitors. Garlic supplements have been shown to significantly reduce saquinavir levels, which is a clinically important interaction.
• You have reflux. Onion and garlic are common triggers.
• You are applying garlic to skin. Raw garlic applied to skin, a folk remedy for verrucas, acne, and fungal infection, causes chemical burns. Emergency departments see these regularly.
• You own a dog or cat. All alliums are toxic to them, causing haemolytic anaemia, with garlic more potent than onion by weight. Onion powder in leftovers and gravy is a common route.
• You are infusing garlic in oil. Garlic in oil at room temperature is a genuine botulism risk. The anaerobic, low-acid, moist environment is exactly what Clostridium botulinum spores need. Home-made garlic oil must be refrigerated and used within a few days, or acidified. There have been documented outbreaks and deaths.

Ages. Fine from weaning onwards in cooked form. Alliums in the maternal diet do transfer to breast milk and amniotic fluid, and there is evidence that this early exposure influences later acceptance of those flavours, which is an argument for eating varied food during pregnancy and breastfeeding.

8. When and how to eat them

• Crush garlic and wait 10 minutes before cooking. This is the single most useful technique in the chapter.
• The finer the cut, the stronger the flavour. A whole clove simmered in a stew is mild; the same clove crushed to a paste is aggressive. Cell damage is the variable.
• Cooking transforms onions completely. Raw onion is pungent and sulphurous; slowly cooked onion is sweet, because heat destroys the sulphur compounds and caramelises the fructans and sugars. Proper caramelisation takes 30 to 45 minutes on low heat, not the "10 minutes" that recipes routinely claim.
• Do not burn garlic. It turns acridly bitter within seconds, and there is no recovery. Add it later than the onion.
• Soaking raw onion in cold water for 10 to 15 minutes removes much of the harshness for salads, because the offending compounds are water-soluble.
• The FODMAP workaround, which is genuinely useful: fructans are water-soluble and not oil-soluble. So infusing garlic or onion in oil, then removing the solids, gives you the flavour without the fructans. Garlic-infused oil is permitted on a low-FODMAP diet for exactly this reason. The same logic makes the green parts of spring onions and leeks low-FODMAP while the white bulbs are high. Asafoetida (hing) is also used as an allium substitute in low-FODMAP cooking.
• Use the leek greens. The dark green tops are usually discarded and are perfectly good in stock and slow-cooked dishes.
• Garlic scapes are excellent: mild, green, and good in pesto or stir-fries.

9. Choosing and storing

Onions and garlic want the opposite conditions from most produce: cool, dark, dry, and ventilated, not the fridge.

• Choose: firm, heavy, dry papery skins, no soft spots, no green shoots, no mould at the neck or base. Garlic should be tight and heavy with no give.
• Store at 10 to 15 °C in a dark, ventilated place: a mesh bag, a basket, a paper bag with holes. Cured onions keep 2 to 6 months; garlic 3 to 6 months.
• Not the fridge, where the humidity causes mould and cold triggers sprouting in garlic. The exception is peeled or cut onion and garlic, which must be refrigerated and used within a few days.
• Keep onions away from potatoes. Potatoes emit moisture and onions emit ethylene; together they spoil each other faster.
• Spring onions, leeks, and chives are different: these are leaf crops and belong in the fridge, wrapped, for a week or two.
• Sprouting garlic is still usable; the green germ in the centre is slightly bitter and can be removed. Soft, brown, or dusty garlic has gone.
• Freeze surplus: chopped onion freezes well for cooking; garlic can be frozen as whole cloves or as paste.

10. The varieties worth knowing

Onions: yellow/brown (the all-purpose cooking onion, best storage, strong raw), red (milder, anthocyanins, good raw), white (sharper, common in Mexican cooking), sweet (Vidalia, Walla Walla, Cévennes; low sulphur, poor storage, good raw), and the tiny pickling and cipollini types.

Garlic: softneck (silverskin, artichoke; long-storing, mild, supermarket standard, braidable) versus hardneck (rocambole, porcelain, purple stripe; more complex flavour, larger easier-to-peel cloves, produces scapes, stores less well, more cold-hardy). Regional varieties with real reputations include Solent Wight, Lautrec pink, and Spanish Morado.

Elephant garlic is a leek, has huge mild cloves, and is not a substitute for garlic in flavour terms.

Shallots are worth using where onion would dominate: milder, sweeter, more complex, and the classic base for French sauces. Banana shallots are larger and easier to prepare.

11. Myths and confusions

Don't be confused: garlic supplements are not garlic. Products differ enormously: garlic powder (variable allicin potential), enteric-coated tablets standardised to "allicin yield," aged garlic extract (no allicin, contains S-allyl cysteine), and garlic oil (contains breakdown products). Trials using one cannot be generalised to another, which is a large part of why the literature is so inconsistent.

• "Garlic is a natural antibiotic." It has real in vitro antimicrobial activity and does not treat an infection in your body. Do not substitute it for antibiotics, and do not put it on your skin.
• "Garlic thins the blood so it prevents heart attacks." Mild antiplatelet activity is real; it is not an aspirin substitute and no trial supports that use.
• "Cooking destroys all the benefit of garlic." It destroys the enzyme if you cook immediately. Crush and wait, and much survives.
• "Sweet onions are a different vegetable." They are low-sulphur varieties grown on low-sulphur soil.
• "Black garlic is fermented." It is not; it is a slow Maillard reaction.
• "Putting a cut onion in the room absorbs germs during illness." An old folk belief with no mechanism and no evidence.
• "Onions absorb bacteria and become toxic if stored cut." Also false; cut onion stored refrigerated is fine for a few days.

12. The bottom line

• Alliums taste of nothing until you cut them. Damage brings a stored precursor and an enzyme together, producing the sulphur compounds that define the family.
• Crush garlic and wait ten minutes before heating. Immediate cooking destroys the enzyme before it has worked.
• Garlic's allicin never reaches your bloodstream. Its breakdown products, and the compounds in aged extracts, are what the clinical trials are actually testing, and they are not interchangeable.
• The health evidence is modest and real: 6 to 8 mmHg systolic reductions in hypertensive people in supplement meta-analyses, small lipid effects, and consistent observational associations with lower gastric and colorectal cancer.
• Onion and garlic fructans are the commonest FODMAP trigger in IBS, and because fructans are water-soluble but not oil-soluble, garlic-infused oil gives the flavour without the trigger.
• Store them cool, dark, dry, and ventilated. Never the fridge, never next to the potatoes, and never garlic in oil at room temperature.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Onion photoperiod classes and bulbing requirements follow horticultural extension guidance; curing and storage follow USDA Handbook 66. Garlic vernalisation, hardneck versus softneck, and scape removal follow extension publications. The alliin-alliinase-allicin system and allicin instability follow Block's Garlic and Other Alliums, 2010. Lachrymatory factor synthase identification is Imai et al., Nature, 2002, from House Foods, which enabled tearless onions. The crush-and-wait finding is Song and Milner, Journal of Nutrition, 2001. Garlic and blood pressure follows Ried's meta-analyses in the Journal of Nutrition and Experimental and Therapeutic Medicine; the Cochrane review on garlic for the common cold found a single moderate quality trial. Gastric and colorectal cancer associations follow Chinese and Italian case-control literature and pooled analyses. The saquinavir interaction is Piscitelli et al., Clinical Infectious Diseases, 2002. Fructan content and the oil-infusion workaround follow the Monash University low-FODMAP programme. Allium toxicity in dogs and cats follows veterinary toxicology references. Botulism risk from garlic in oil follows FDA and FSA guidance.

Open questions. Garlic supplement trials are notoriously heterogeneous because the preparations differ chemically, which makes the blood pressure meta-analyses hard to interpret as a single effect.

👉 Next: the root vegetables.