Why spices behave as they do

Vanilla pods are harvested green and nearly odourless, because their vanillin is bound to glucose as glucovanillin. Curing — killing the pod with hot water, sun or oven; weeks of alternating sweating in blankets and sun drying; then months of conditioning — breaks down the compartments that keep glucovanillin apart from the enzymes that split it, and free vanillin is released, while oxidation and browning reactions build the hundreds of minor compounds that distinguish cured vanilla from vanillin alone. The usual account credits the pod’s own β-glucosidase, and the measurements are less tidy: one study could detect none of that enzyme after blanching or through the sweating in which most of the vanillin formed; another concluded that the breakdown of cell compartments, not the amount of enzyme, limits the conversion; and Bacillus that colonise the curing bean produce a β-glucosidase of their own.

The finished bean is dark, supple and intensely aromatic, and its aroma is far broader than vanillin: the minor compounds formed in curing are what separate a bean from a synthetic vanilla flavouring.

Curing is most of what the price of vanilla pays for, and it is the reason vanilla cannot be cheaply grown and sold fresh. It is also why the difference between real and imitation vanilla is most audible in uncooked and gently cooked preparations — custards, ice cream — and least in long baking, where the minor compounds fade and vanillin carries most of the flavour either way.

Fresh turmeric rhizomes are boiled or steamed — traditionally in water, sometimes lightly alkaline — before sun drying. The heat kills the living tissue, gelatinises the starch and lets the curcuminoid pigment diffuse evenly through the rhizome rather than staying in scattered cells, which gives the dried spice its uniform deep colour. Cured rhizomes also dry faster and more evenly, reducing the time in which mould can grow. The dried fingers are then polished to remove the rough skin.

Cured, dried turmeric is evenly orange-yellow throughout, earthy and faintly bitter, and has lost most of the fresh root’s gingery, citrus aroma to the process.

Fresh and dried turmeric are different ingredients rather than two strengths of one: fresh is brighter, more aromatic and less bitter, and is grated into pastes; dried is the colour-and-earth spice of the blend. The bright colour of dried turmeric is a product of curing, which is also why an unusually vivid powder is a reason for caution rather than reassurance — see the notes on authenticity.

Fresh ginger’s pungency is gingerol. During drying and storage, and during prolonged heating, gingerol loses a molecule of water and becomes shogaol, which is roughly twice as pungent and harsher. Fresh ginger contains little shogaol; dried ground ginger contains a great deal. At the same time drying drives off much of the volatile citral and other light terpenes that give fresh ginger its lemony top note.

Dried ginger is hotter, drier and more peppery, and has lost the bright, juicy citrus of the fresh root. The two are distinct flavours rather than stronger and weaker versions of one.

Fresh and dried ginger are not substitutes at any fixed ratio, because the swap changes the kind of heat as well as its strength. Use dried ginger where sustained warmth is wanted — baking, spice blends, long braises — and fresh where brightness is, added towards the end. Where a recipe gives only one, keep to it.

Fresh saffron stigmas contain picrocrocin, a bitter glucoside, and very little of the aroma compound safranal. Drying splits the glucose from picrocrocin — through the stigma’s own enzymes and through heat — and the aroma molecule released is safranal. The drying method therefore sets the aroma of the finished spice: fast, hot drying, as in the Spanish toasting tradition — recorded at 75 to 121 °C for about half an hour to an hour — and slow drying in sun or shade at room temperature, as in Iran, give saffron with different balances of the three compounds ISO 3632 measures. Saffron dried hot has no active enzymes left, so its safranal comes from heat alone.

The hay-like, honeyed, faintly metallic smell of saffron appears on drying; the bitterness it retains is the picrocrocin that was not converted.

The aroma is the fragile part: safranal is volatile and is lost to long boiling and to months of storage while the colour survives both. Steep threads in warm liquid, add them late enough that the smell survives the cooking, and judge a jar by its smell rather than by how strongly it still colours water.

Living woodruff holds its coumarin bound to sugar, in glycosidic precursors that have no smell. As the cut herb wilts or dries its cells break down, an enzyme splits the sugar off, and the precursor cyclises to free coumarin — the sweet, hay-and-vanilla compound also found in tonka bean and cassia. The aroma is made after harvest, as it is in vanilla and saffron.

Freshly picked woodruff smells faintly green; the same sprigs left to wilt for a few hours smell strongly of new-mown hay, vanilla and almond.

Pick the herb, leave it to wilt for a few hours, and only then infuse it — briefly — in wine, punch or syrup before removing it. Brief infusion of a small bunch is the traditional use, and it keeps the coumarin that carries the flavour at the level the practice has always used rather than extracting all of it.

None of these spices is smoky of itself. Each is dried over a wood fire, and the burning wood’s lignin breaks down into phenolic compounds — guaiacol and syringol chief among them — that condense onto the drying pods or fruit. The smoke adds flavour rather than transforming the spice’s own chemistry, and it is why the same plant dried two ways gives two different spices: sweet paprika and pimentón, a ripe jalapeño and a chipotle.

A campfire, bacon-like, sometimes medicinal smokiness that sits on top of the spice’s own aroma and can dominate it. The balance between guaiacol’s sharper smoke and syringol’s softer one depends on the wood.

Treat smoke as its own ingredient. A smoked spice cannot be replaced by its unsmoked counterpart without losing the smoke, and the smoke cannot be removed; used where it is not wanted, it takes the dish over. Smoke phenols are fat-soluble, so smoked paprika and chipotle carry furthest in oil and fat.

The boiled, dehulled locust bean seeds are heaped, covered and left for a few days, during which Bacillus bacteria — chiefly Bacillus subtilis and its relatives — dominate. Their enzymes break the seed protein down into peptides and free amino acids, including glutamate, and release ammonia, which pushes the pH up well into the alkaline range. The same fermentation, on different seeds, makes Nigerian ugba, Burkinabé bikalga, Nepali kinema and Japanese natto. It is the opposite of the acid fermentations behind most pickles, and the high pH is also what keeps most spoilage organisms out.

A seed with almost no flavour becomes intensely savoury — the free amino acids supply umami — and powerfully pungent and ammoniacal, a smell that softens greatly once it is cooked into a soup.

Use dawadawa as a savoury base, crumbled or pounded into a soup or stew early enough that it cooks through, where it does the work fish sauce or miso does elsewhere; the raw ammonia note is not the flavour of the finished dish. A stock cube supplies the savouriness and none of the aroma, which is why it has replaced dawadawa in many kitchens and does not taste the same.

White pepper is made by soaking ripe berries in water for one to two weeks until the outer skin rots loose — retting — and then rubbing it off and drying the seed. The skin is where most of black pepper’s volatile terpenes sit, so removing it leaves the pungency of piperine with far less aroma. The soaking is a microbial fermentation, and when it is poorly controlled, bacteria produce compounds with faecal, cheesy and farmyard smells; analytical work has identified skatole and cresols among the contributors to that atypical aroma.

White pepper is hot and musty-earthy with little of black pepper’s citrus and pine; a poor lot has an unmistakable animal note that clean, decorticated or mechanically processed white pepper lacks.

Where white pepper is wanted for its own sake — Chinese and Thai cooking, where its mustiness is part of the flavour — a good one is worth seeking out; where it is wanted only to keep a pale sauce unspeckled, use it sparingly, because its character is not a milder black pepper. Smell a new jar before cooking with it.

Intact mustard seed, horseradish root and wasabi rhizome contain no pungency. They hold a glucosinolate — sinigrin in black and brown mustard, horseradish and wasabi — and, in separate cells, the enzyme myrosinase. Crushing or grating breaks the cells, and in the presence of water the enzyme cleaves the glucosinolate to release allyl isothiocyanate, the volatile, sinus-clearing pungent compound. Yellow mustard differs: its glucosinolate, sinalbin, yields a non-volatile isothiocyanate, which is why it bites on the tongue without reaching the nose. The enzyme is inactivated by heat and slowed by acid — and it is not one enzyme across the mustards: in one study the enzyme extracted from yellow mustard kept no activity after ten minutes at 70 °C, where that of brown mustard kept about a third and that of black mustard more than half.

A dry mustard powder smells faintly of nothing; ten minutes after mixing with cold water it is fiercely hot. Freshly grated wasabi and horseradish peak within minutes of grating and fade within the half hour as the volatile compound escapes.

Mix mustard powder with cold water and let it stand for about ten minutes before adding vinegar or salt, which stop the enzyme once the heat has formed; mixed straight into vinegar it stays mild. Grate horseradish and wasabi to order. And whole mustard seeds fried in hot oil, as in a South Indian tempering, never develop pungency at all — the heat destroys the enzyme before any water reaches it, and the seed turns nutty instead.

An intact garlic clove smells mild because its pungent compound does not yet exist. The clove stores the odourless amino acid alliin and, in separate compartments, the enzyme alliinase. Cutting or crushing mixes them, and within seconds alliinase converts alliin into allicin — the sharp, pungent compound of raw garlic. The finer the damage, the more cells are broken and the more allicin forms. Heat inactivates alliinase, so garlic cooked before it is cut makes very little.

Crushed or microplaned garlic is hot and acrid; sliced garlic is milder; a whole roasted clove is sweet and soft, with almost none of raw garlic’s bite.

The cut sets the character before the heat does. Crush or grate for a raw dressing or a sharp finish; slice for a gentler result; roast whole heads for sweetness. Letting crushed garlic stand for a few minutes before it meets the heat allows more allicin to form, which is why some cooks rest it — the effect on flavour is real, whatever is claimed for it beyond the kitchen.

A whole onion contains none of the compound that draws tears. It stores an odourless sulphur amino acid, isoalliin, and cutting brings the enzyme alliinase to it. Alliinase cleaves isoalliin to a reactive sulfenic acid. In garlic the matching acid simply condenses into thiosulphinates such as allicin; the onion has a second enzyme, lachrymatory factor synthase, that rearranges its sulfenic acid into propanethial S-oxide, a small volatile molecule. The rearrangement was once thought to happen by itself, but its energy barrier is too high at room temperature, and onions in which the second enzyme was silenced made very little of the compound.

The familiar irritation of the eyes when an onion is chopped. Onions with the enzyme silenced did not cause it, and their cut tissue sent more of the same intermediate into thiosulphinates and other sulphur volatiles instead.

Nothing read for this profile tested a way of preventing onion tears, and none is offered. What the chemistry establishes is narrower: the compound is made only where the bulb is cut or bruised and does not exist beforehand, and it is one branch of the same breakdown that gives the onion its flavour, which boiling and frying each take in a different direction.

Most of a spice’s aroma is a mixture of small, volatile terpenes and related compounds held in oil cells inside the seed, berry or bark. Whole, the spice is sealed: the aroma can escape only slowly through an intact coat. Grinding breaks the cells and multiplies the exposed surface many thousands of times, so the volatiles evaporate and the remaining oils meet oxygen. The lightest, brightest notes go first, which is why stale spice smells flat and woody rather than simply weak.

Freshly ground cardamom, pepper or nutmeg smells markedly brighter than the same spice ground months earlier; the difference is largest in the spices whose character rests on the lightest compounds.

Buy whole where the spice is used often enough to grind, grind close to use, and buy ground spices in small quantities. Pepper and nutmeg in particular reward grinding to order. Staleness is a quality matter: an old jar is flat, not dangerous.

Milling generates heat, and a seed’s volatile oil begins to evaporate as soon as its cells are broken and warmed. Grinding with the mill chilled by liquid nitrogen keeps the material cold and brittle, so it fractures cleanly without the temperature rise. In two Indian cumin genotypes, cryogenic grinding recovered 33.9 and 43.5 per cent more volatile oil than conventional grinding, and cuminaldehyde rose from 48.2 to 56.1 per cent of the oil in one of them. The two grinds were not done on the same machine, a domestic mixer grinder against a chilled pin mill, so the comparison is of two ways of grinding and not of temperature alone.

Cryogenically ground cumin keeps more of the volatile oil that carries its aroma, and a larger share of it is cuminaldehyde, cumin’s defining note.

Industrial cold milling is not a kitchen technique, but the same mechanism applies at home: grinding that warms the spice drives off aroma, so short bursts and a cool mill or mortar should lose less; no kitchen grinder was tested. The finding is from cumin; it is the mechanism, not the figures, that carries to other seeds.

Dry heat does two things to a seed spice at once. It drives off part of the lightest, most volatile aroma compounds — the green, sharp top notes — and it starts browning reactions between the seed’s sugars and amino acids, which form new roasted, nutty and toasty aroma compounds that the raw seed never had. The result is not the same spice made stronger but a different balance: less bright, more rounded and roasted.

Toasted cumin is nuttier and deeper, with less of raw cumin’s sharp, sweaty edge; toasted fenugreek loses much of its raw bitterness and moves towards maple and burnt sugar. Taken too far, the browning turns to burning and bitterness within seconds.

Toast whole seeds in a dry pan until fragrant and a shade darker, tip them out at once — they keep cooking in a hot pan — and grind after cooling. Toast when the recipe wants depth, as in roasted cumin powder for raita or chaat; leave raw when it wants brightness. Ground spice toasts, and burns, several times faster than whole seed.

The colour compounds of these spices — curcumin in turmeric, capsanthin in paprika and chilli, bixin in annatto — dissolve readily in fat and barely in water, and so do most of the aroma compounds of spices generally. A brief fry in hot oil draws them out of the spice and disperses them evenly through the fat, which then carries them through the dish. Added to a watery liquid instead, the same spices leave specks and uneven colour. The oil must not be too hot: when chilli powder was fried for ten minutes, 97 per cent of its capsanthin reached the oil at 130 °C and only 15 to 37 per cent survived at 150 to 190 °C. Nor is every spice colour fat-soluble — saffron’s crocins dissolve in water and hardly at all in oil.

Bloomed turmeric colours a dish evenly and loses its raw, dusty edge; bloomed paprika turns the oil brick red and tastes sweeter and fuller; annatto oil carries its colour with almost no flavour.

Bloom colour spices in the fat of the dish at the start, briefly — and watch paprika closely, because its sugars scorch in hot oil within seconds and turn it bitter and brown. Take the pan off the heat or add the next ingredient as soon as the oil colours. Annatto seeds are infused in warm oil and then discarded.

Capsaicin is almost insoluble in water and dissolves readily in fat and in alcohol. Infused in oil, chilli gives up its capsaicin to the oil, which is why chilli oils and fat-rich chilli dishes carry heat so efficiently. In the mouth the same property decides what helps: water spreads capsaicin around without dissolving it, while milk fat and the milk protein casein bind and carry it away. Capsaicin is also far more durable in heat than any aroma compound, though not untouched by it: boiling has been measured to remove between about one per cent and a third, and frying at 190 °C most of it.

Heat that lingers and spreads after a sip of water, and that dairy damps. The burn itself is TRPV1, the receptor for painful heat, being opened by capsaicin.

Only part of the heat cooks out of a chilli, and not dependably, so control it by quantity and by what is removed: the pale placental ribs, where capsaicin is made, carry more of it than the flesh. Serve yoghurt, raita or coconut milk alongside hot food rather than water. And expect a chilli oil to be hotter than the same chillies in a stock.

Allicin is unstable. Over hours at room temperature, and within minutes in a hot pan, it breaks down into a family of sulfur compounds of which diallyl disulfide is the most important, together with smaller amounts of other sulfides. These carry the savoury smell of cooked garlic without the pungency of allicin. In oil, allicin also forms other sulfur compounds, which is part of why garlic-infused oils taste different from garlic cooked in water.

The sharp, throat-catching heat of raw garlic gives way to a deeper, rounder, more savoury garlic flavour; pushed too far, towards browning, it turns bitter.

Add chopped garlic to a pan after the onions rather than with them, because garlic browns — and turns bitter — far faster than onion softens. For a mellow garlic backbone cook it gently in oil; for bite, add some raw at the end. The two are different ingredients, chemically, and many dishes want both.

Zingerone, a far less pungent compound with a sweet, spicy aroma, is absent from dried ginger and forms when gingerol or the oleoresin is heated or treated with alkali; the reaction is usually described as a reverse aldol splitting of the gingerol molecule. How much forms in a cooked dish is less clear: gingerol heated in water was found to turn mainly into shogaol, and neither source measured zingerone in a dish. Long cooking therefore moves ginger towards shogaol and a little zingerone, while the volatile citrus notes are lost.

Ginger simmered for an hour contributes a mellow, rounded warmth rather than a bite, and none of the fresh root’s zest.

Split the ginger when a long-cooked dish should still taste of it: some at the start for background warmth, some grated in at the end for brightness. The same logic holds for garlic.

A wet paste of onion, garlic, ginger and spices is held near the boiling point of water for as long as it contains water, so it steams: the sharp, volatile sulfur compounds of the raw alliums and ginger are driven off, but little browning happens. Once the water has evaporated the temperature rises well above 100 °C and Maillard reactions and caramelisation begin, building the deeper, sweeter, roasted flavours of a cooked base. The fat that was dispersed through the watery paste separates out when the water that held it has gone, which is why its reappearance marks the change.

A properly fried paste tastes sweet, deep and rounded; an under-fried one raw, sharp and grainy, with a smell of uncooked onion and ginger that no later simmering fully removes.

Fry the paste over moderate heat, stirring, until it darkens and the oil pools glossily at its edges — bhuna in North India, pecah minyak in Malay kitchens — before adding liquid. Add ground dry spices once the paste is cooked and the pan has a little oil, not into a dry pan, and add a splash of water if the paste catches before it is done.

Held over a flame or on a very hot dry surface, a fresh chilli’s skin dries and rises far above the boiling point, where browning reactions and then pyrolysis produce roasted, smoky and slightly bitter compounds. The flesh underneath, still full of water, stays near 100 °C and steams, softening and sweetening, while the skin separates from it. The capsaicinoids sit in the flesh and the placenta, which stay near boiling point, and a study of fresh Mexican peppers grilled at 210 °C measured more capsaicinoid after grilling than before, where boiling had lowered it.

Charred chillies taste sweeter, softer and smoky, with a little bitterness from the blackened skin; their heat is not reduced.

Char fresh chillies until blistered and blackened in patches, sweat them briefly in a covered bowl, then rub off most of the skin — leaving a few flecks for flavour. Do not treat dried chillies the same way: they have no water to protect them and burn in seconds; they are toasted briefly, not charred.

When garlic is damaged in an acidic environment — pickled, or crushed into vinegar or lemon juice — the sulfur compounds formed on cutting react with amino acids to form small pyrrole molecules, which link into blue and green pigments. Stored or older garlic, which has built up more of the precursor, is more prone to it. The reaction is the one deliberately exploited in the Chinese Laba garlic, steeped in vinegar in winter until it turns jade green.

A colour change only: greened garlic tastes as the rest of the batch does.

Blue or green garlic in a pickle or a vinegar-based sauce is a harmless chemical colour change, not spoilage or contamination. Blanching the cloves briefly before pickling reduces it by inactivating the enzyme; very fresh garlic greens less than garlic that has been stored cold.

Piperine, the pungent compound of pepper, is sensitive to light: exposure converts part of it into geometric isomers with much weaker pungency. Ground pepper, with its exposed surface, is far more affected than whole berries, and this happens alongside — and independently of — the evaporation of pepper’s volatile terpenes. Old ground pepper therefore loses both halves of its character: the aroma and the bite.

Pre-ground pepper that has stood in a clear shaker tastes dusty and mild; freshly cracked pepper is both aromatic and sharply pungent.

Keep pepper whole and grind to order; store any ground pepper in an opaque, closed container. Pepper added at the end of cooking keeps more of its aroma than pepper cooked for hours, though most of its pungency survives: boiling for ten to twenty minutes has been measured to remove between a sixth and a third of the piperine.

The tingle comes from the sanshools — above all hydroxy-α-sanshool — which act on touch-sensitive nerve fibres by blocking a family of potassium channels rather than by triggering the heat receptor. The sanshools are unsaturated and degrade on exposure to air, light and sustained heat, while the citrus and floral terpenes that make up the aroma fade on a different timetable. The two properties of the spice therefore separate in storage.

A jar can smell fresh and lemony and produce no tingle at all — the aroma has survived and the sanshools have not.

Judge Sichuan pepper by tasting a husk rather than by smelling the jar: fresh husks numb the lips within a minute. Buy it recently, keep it whole and sealed, toast lightly and grind just before use, and add it late or as an infused finishing oil rather than cooking it for long.

The red of paprika and chilli powders is capsanthin and related carotenoids, long chains of alternating double bonds that absorb light — and that are attacked by oxygen, accelerated by light and warmth. As the chains break, the colour fades from red towards brown, and the oils in the ground fruit oxidise alongside, flattening the flavour. Because colour is what paprika is measured and sold on, this is the spice whose loss in storage is most visible.

Faded paprika is brownish and dull, colours food weakly and tastes dusty rather than sweet.

Buy paprika in small quantities, keep it in an opaque, closed container away from the stove — or in the fridge, sealed — and replace it when it browns. The colour is a fair guide to its condition, which is not true of every spice.