Herbs

Leaves that season a dish

A culinary herb is a plant used for the flavour and aroma of its leaves, leafy stems, shoots and sometimes flowers. A handful can change the character of a much larger food without adding much substance to it. The word names a use, not a botanical group, and it covers plants of very different habit: soft annuals such as basil and coriander, woody shrubs such as rosemary, and low perennials such as thyme.

The line between herb and spice follows the plant part. Coriander and dill supply herbs and spices from the same crop: their leaves are traded as herbs and their seeds as spices, and coriander leaf is called cilantro in some markets. A recipe that asks for coriander leaf is asking for a different ingredient from one that asks for coriander seed. Herbs reach a kitchen as a bunch of soft stems, a few sprigs, a young plant in a pot, a jar of dried leaves, or, less often, as an extract. Naming the plant and the form tells a cook far more than the word herbs.

The principal herbs and what each brings

Most familiar herbs belong to two plant families. The mint family, Lamiaceae, includes some 236 genera and about 6,900 to 7,200 species, among them basil, mint, thyme, rosemary, sage, oregano and marjoram. The carrot family, Apiaceae, supplies parsley, coriander, dill, chervil and fennel. A kitchen grouping brings these together because their leaves do related work at the table, not because the plants are related.

Each herb carries its own aroma chemistry. Thyme (Thymus vulgaris) is dominated by thymol, about 42 percent of the volatiles in one analysis, with p-cymene at about 28 percent. Spearmint (Mentha spicata) is built on carvone, about 46 percent, and peppermint (Mentha piperita) on menthol and menthyl acetate, each about a third. Rosemary (Rosmarinus officinalis) gives eucalyptol, alpha-terpineol and camphor in similar amounts of 14 to 22 percent, and sage (Salvia officinalis) alpha-thujone at about 31 percent with camphor. Lemon balm (Melissa officinalis) is two-thirds citral, which accounts for its lemon scent. These are relative shares of the volatiles that rise from the leaf, and they vary with the plant, the place and the harvest.

Sweet basil, Ocimum basilicum, shows how much one species can vary. The Genovese basil of the Ligurian coast is rich in linalool, 47 to 54 percent of its essential oil, with eugenol at 10 to 21 percent, and it lacks estragole, the compound that gives other basils a mint or aniseed note. Asian and Pacific basils are associated with camphor, limonene, beta-selinene and methyl cinnamate. In one reading of the medieval estate list, parsley, chervil and chives are salad herbs and sage, dill and coriander physic herbs, a reminder that cooking and medicine shared the same garden.

Where the aroma comes from

The smell of a herb is carried by essential oils, mixtures of small volatile molecules, mostly terpenes and aromatic ethers, that evaporate from the leaf at room temperature. The Lamiaceae analysis counted 74 volatiles across twelve herbs: 37 terpenes, 23 alcohols, 7 esters, 3 aldehydes and 3 ketones. A cook perceives the mixture, not a single molecule, and the balance shifts between cultivars and growing conditions.

Volatility is also the herb's weakness. Essential oils decline in storage, and a leaf can look marketable after it has lost some of the aroma it was bought for. Drying removes moisture but too much heat carries off the oils, and drying too slowly lets the crop deteriorate or go mouldy. The task in processing is to remove the water while keeping the colour and aroma of the particular plant, which explains why a jar of dried herb and a fresh bunch are different ingredients and not two forms of one thing.

Herbs in the old kitchen and garden

Written cookery shows herbs moving between garden, pot and table for two thousand years. The Roman collection translated by J. D. Vehling as Apicius includes a cereal-and-pulse dish with chopped green herbs, coriander, dill and fennel, in which leaves and crushed seasonings enter the same pot in different forms. In the ninth century a Frankish royal ordinance, the Capitulare de villis, listed 88 plants to be grown on the king's estates, including parsley, chervil, chives and celery as salads, sage, coriander, dill, fennel, rosemary and lovage as physic herbs, and savory and mint as pot-herbs. It is a list of intentions without planting instructions, and some of its plant names are uncertain, but it shows what an administrator expected a working garden to hold.

John Evelyn's Acetaria: A Discourse of Sallets, first published in 1699, shows the seasonal logic of a later cook. He lists the young leaves, shoots, tops and flowers fit for the salad bowl by month, with chervil, parsley, tarragon, mint and other aromatics among them, and he describes aromatic flowers infused in vinegar and mint used for tea. Growth stage mattered as well as the plant's name: a tender shoot and a mature leaf were different salad materials.

Where the plants came from is a botanical question separate from where they are cooked. Coriander, Coriandrum sativum, is native from the eastern Mediterranean to western Pakistan. Sweet basil is native to tropical and subtropical Asia and northern Australia, according to the botanical record, and is now grown worldwide, with the Mediterranean and Italy as the centre of its traditional use. Genovese basil has had an EU protected designation of origin since 2010. A plant that has spread around the world by trade and cultivation gives no clue to its native range from the places that cook with it.

From garden crop to market bunch

Herbs are grown in household gardens, on small farms and in commercial systems, and the product handled differs with the market. Basil can supply several cuttings from plants grown in spaced rows; cilantro for bunching may be cut just above or below the soil line and tied. Most culinary herbs grow adequately in the tropics. Sage, moss-curled parsley, tarragon and thyme do better in temperate climates, while basil, coriander and rosemary can grow at least as well in the tropics given irrigation, higher altitude and cooler temperatures. The popular fresh herbs in trade include basil, chives, cilantro, dill, mint, parsley and rosemary. Fresh herbs can also be processed into pastes such as pesto, which loses less flavour and aroma than drying does.

The fresh market sells leafy stems, developing leaves, flowers and young plants. Dried herbs are sold as whole leaves, broken fragments or powder, with the amount of stalk depending on cleaning. Essential oils and oleoresins are separate products made by extraction. Essential oils are steam-distilled from aromatic crops. Oleoresins are made by macerating the flavour compounds in a solvent, such as hexane or liquid carbon dioxide, and evaporating the solvent under vacuum to leave a concentrate. Both concentrate selected components and are not a spoon-for-spoon version of the leaf. Harvest, cleaning, drying, grading and packaging each add handling, and each can protect or damage the aroma.

Herbs in fermented and preserved foods

In fermented foods herbs are usually flavourings, and the fermenting is done by the organisms already in the brine or the culture. A fermented dill pickle shows the pattern. A tested home recipe uses 8 lb of pickling cucumbers with either 2 tablespoons of dill seed or 4 to 5 heads of fresh or dry dill, optional garlic and pickling spices, and a salted, slightly acidified brine; the jar is held for 3 to 4 weeks at 70 to 75 degrees F (about 21 to 24 degrees C). Temperatures of 55 to 56 degrees F are acceptable but take 5 to 6 weeks, and above 80 degrees F the pickles soften. The dill gives its flavour to the brine and the lactic acid bacteria do the souring. Fresh cultured cheeses such as quark take herbs at the end, stirred into the drained curd with a little cream and salt, so the seasoning does not interfere with the culture or the rennet.

Herbs also carry antimicrobial chemistry, and this is where they are most often misread. Thymol and carvacrol from thyme and oregano, eugenol, and menthol damage microbial cell membranes, and in laboratory media the oils of these herbs are strong. In real food the effect shrinks: the oils dissolve into fat, bind to protein and starch, evaporate, and meet food with its own pH and water activity. The concentrations that inhibit microbes often exceed what people find acceptable in mild foods such as dairy, drinks and fish. Rosemary oil added to refrigerated fermented thick cream, for example, gave the best sensory scores at only 0.13 percent; at 0.24 and 0.34 percent the aroma was excessive and bitter. At that dose the oil slowed the rise in acidity and the growth of aerobic bacteria over 35 days, in one study with a single harvest of rosemary. A fragrant herb therefore flavours a ferment far more reliably than it protects it.

Processing changes herbs as well. Basil for pesto is typically heated at 95 degrees C for a minute to stabilise its enzymes; acid and heat convert chlorophyll to olive-brown pheophytins, and the polyphenol oxidase that browns a bruised leaf is the other enemy of the colour. Pesto is an oil-in-water emulsion, so the fat is oxidised at the droplet surface, with iron and copper speeding it.

Choosing, preparing and storing herbs

Good fresh herbs have bright leaves, a fresh smell and crisp stalks, and the leaves are not dried out. Yellow or brown leaves and black spots mean the bunch has aged or been damaged, and those leaves are discarded. Herbs are rinsed gently under cool running water, which is as effective as a produce cleanser, and kept apart from raw meat and seafood. Soaking them in a sink invites contamination from the sink itself.

Storage follows the plant. Most fresh herbs are kept unwashed in the refrigerator, with the stalk ends trimmed, in a re-sealable bag in the crisper, for up to five days; for the freezer, they are washed, patted dry and bagged. Basil is the exception: it is kept unwashed, uncovered and unrefrigerated, because cold can turn its leaves black. The postharvest work agrees. Basil showed chilling injury at 5 degrees C or below, with leaf necrosis after 1 day at 0 degrees C or 3 days at 5 degrees C, and its shelf life was longest at 12 to 15 degrees C, about 12.5 days, against 3.2 days at 5 degrees C and 1.6 days at 0 degrees C, in air of 90 to 95 percent humidity. Shiso is also chilling-sensitive. Damaged leaves discolour and decay faster and water loss lowers quality, so a bunch is sorted before it is stored. Dried herbs are kept away from moisture and pests, and their aroma fades with time.

Hygiene and allergen notes

Fresh herbs are usually eaten raw. Sampling by the US Food and Drug Administration of basil, cilantro and parsley found pathogens in some samples, including Salmonella and the parasite Cyclospora cayetanensis, which is why clean growing water, handling and transport matter for a garnish as much as for a salad. Dried aromatic herbs have also been linked to outbreaks, and low moisture and strong aroma do not make them sterile.

A herb mixture is only as clear as its label. In European Union food labelling, celery and mustard are among the allergens that must be declared when used as ingredients, and a seasoning blend may contain either. Read the formulation of any blend, including ingredients that contribute only a small part of its flavour.