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A pomelo has a thicker peel than a grapefruit, and inside it divides into 11 to 18 segments. At the end of winter the same tree carries clusters of thick petalled white flowers. Peel and flower are two different materials, taken by two different methods, and they age at two different speeds.

One tree, two materials

The pomelo tree carries the botanical name Citrus maxima, recorded in older books as Citrus grandis. It is the citrus with the largest fruit, native to Southeast Asia and southern China. Along with a few other wild species, pomelo is an ancestor of many of the citrus fruits grown today.

Grapefruit is a hybrid of pomelo and sweet orange that appeared in Barbados in the 18th century. Genetic analysis shows that around 63 percent of the grapefruit genome comes from pomelo. The two names are therefore often confused on essential oil labels, even though they refer to two different trees.

From one Citrus maxima tree, perfumery takes two things. The first is pomelo peel oil, from the outer green yellow skin of the fruit. The second is the scent of the flower, the white blossoms that open in late winter and early spring in northern Vietnam. The two materials sit a few months apart on the same branch.

Their scents sit apart too. The peel gives something fresh, bright, faintly bitter at the edge. The flower gives something sweet, dense, with a note of honey. To see why one tree yields two such different smells, one has to look at where each part holds its scent.

The oil glands in pomelo peel

Pomelo peel has two distinct layers. The outer layer is thin, coloured, its surface dotted with small points. The inner layer is white, spongy, thick enough to be peeled away in large pieces. The oil sits not in the white layer but in the coloured layer above it.

Each small dot on the surface of the peel is an oil gland, lying just below the skin. When the peel is folded, pressed or rasped, the gland bursts and the oil sprays out in a fine mist. This is why someone peeling a pomelo in a closed room can fill it with the smell of peel before the first segment is separated.

Because the glands lie so shallow, taking oil from citrus peel does not necessarily need heat. It is enough to rupture the glands and collect the oil. The white layer underneath, by contrast, has almost no smell, and if it enters the process it only adds water and pulp to be separated out.

The thickness of pomelo peel sets it apart from thin skinned citrus. An orange can go whole into a machine that takes juice and oil at the same time. With pomelo, the thick heavy peel is usually handled on its own, and the question becomes whether to press it or to distil it.

Cold pressing and distilling the peel

Cold pressing, or expression, is the general name for any mechanical way of rupturing the oil glands without heat and collecting the oil. ISO 9235:2021 classes pressed oil as a type of essential oil and restricts it to citrus peel. No other material is made this way within the ISO definition.

In Calabria and Sicily the oldest hand method pressed the peel against a sponge and then wrung the oil out. In southern France there was a bowl lined with spikes, the écuelle à piquer: the fruit rolled in the bowl and the spikes pierced the glands. Machines replaced hands from the nineteenth century, but the principle stayed the same. Oil leaves the machine as an emulsion in water, and is then centrifuged apart.

With no heat involved, pressed oil keeps the non volatile part of the gland contents: waxes, pigments, coumarins and furocoumarins. The furocoumarins are why some pressed citrus oils carry a phototoxicity limit. For cold pressed grapefruit oil, the library lists a maximum dermal level of 4 percent under IFRA.

Steam distillation takes another route. The peel is placed in the still and steam carries the volatile molecules across to the condenser. Heavy compounds such as furocoumarins do not reach the condenser, so a distilled oil usually does not carry them. In exchange, heat and time change the smell: some of the fresh peel character is lost, and a certain roundness and haze is added.

Pomelo peel oil on the market may be made either way. Two bottles both labelled Citrus maxima can differ in smell and in safety profile, and the extraction method printed on the label is the first line to read. The piece orange peel under the press goes further into the machines and history of cold pressing.

Limonene and the smell of fresh peel

The volatile part of citrus peel oil is mostly monoterpene hydrocarbons, and at the head of that group stands limonene. Formula C₁₀H₁₆, molecular weight 136.23 g/mol. The library describes its smell as fresh orange, citrus peel, lightly sweet: the defining top note of the whole citrus family.

The share of limonene in citrus peel oils is usually very high. In cold pressed grapefruit oil, limonene makes up 84.8 to 95.4 percent. In sweet orange the figure is 83.9 to 95.9 percent. Pomelo peel oil also belongs to the limonene rich group, though the exact share varies with cultivar, growing region and extraction.

If limonene fills nearly the whole bottle, why does orange oil not smell like pomelo oil? The answer lies in the small remainder. The character of each fruit comes from components present at a few percent or less: aldehydes such as citral in lemon, esters such as linalyl acetate in bergamot, and in grapefruit nootkatone, only 0.1 to 0.8 percent yet carrying a very particular bitterness.

Limonene is therefore more a bright ground than a signature. It opens the scent, leaves quickly, and hands the smaller molecules the task of saying which fruit this is. On a blotter, pomelo peel oil is brightest in the first few minutes, then fades at the pace of limonene's evaporation.

Limonene is also on the EU list of fragrance allergens, to be named on the label above 0.001 percent in leave on products. Fresh, its skin data are fairly mild. What changes that profile is time and air.

Pomelo flower: an absolute sweet as honey

Pomelo flowers open in February and March in the north, white, thick petalled, strong enough that a single branch can scent a whole room. In Hanoi and the Red River Delta the flowers scent sweet soups and cakes and are distilled for pomelo flower water. This is household practice, not industrial production.

The petals are far thinner and more fragile than the peel. The scent of the flower is not stored in large oil glands, and long heat tends to make a floral smell cooked and heavy. For flowers, perfumery therefore tends to use solvents rather than pressing. The method recorded in the library is to extract the flowers with a solvent to a concrete, then wash it with ethanol, chill, filter and remove the alcohol to obtain an absolute.

The library describes pomelo flower absolute as white citrus blossom, sweet with a light honey, with facets of green pomelo peel and leaf. It sits close to the neroli of bitter orange blossom but fresher and less bitter. In perfume it suits colognes, white bouquets and tea accords.

One point needs stating plainly. A commercial source of pomelo flower absolute has not been confirmed, and published composition data for pomelo blossom are scarce compared with bitter orange blossom. On the international market, the name pomelo flower is sometimes attached to synthetic bases. The library page therefore lists no specific constituents for pomelo flower, and this piece gives no figures either.

On regulation, pomelo flower absolute has no dedicated standard in the 51st Amendment of IFRA. The furocoumarin standard applies to citrus oils, and the furocoumarin content of pomelo flower should be confirmed by the supplier. Where linalool and limonene are present, they must still be labelled under the EU thresholds.

Pomelo peel and oxidation

A carbon carbon double bond is a weak point in the face of oxygen. Limonene has two such bonds, and because it makes up nearly all of a citrus peel oil, it makes this whole group the most perishable of essential oils. Oxygen in the air attacks the double bonds, forming hydroperoxides, then alcohols, aldehydes, epoxides and ketones. The process cannot be reversed.

The change is not only in the smell. According to the library data, oxidation products of limonene such as limonene-2-hydroperoxide and limonene-1,2-oxide are far stronger skin sensitisers than fresh limonene; pooled data suggest oxidation may raise allergenicity roughly tenfold. An old bottle of pomelo peel oil therefore does not only lose its scent, it changes its safety profile.

Temperature sets the pace. A lemon oil sample kept at 5 °C, its cap opened three minutes a month, shows almost no change in monoterpene content after 12 months. The same sample at 25 °C, opened three minutes a day, falls from 97 percent to under 31 percent monoterpenes over the same period.

The library's guide to storing essential oils places opened citrus oils at 1 to 2 years under good conditions, and advises that cold pressed citrus oils be refrigerated and used within 6 to 12 months of opening. Dark glass, a tight cap, an ideal temperature of 5 to 15 °C. When the oil falls below 30 percent of the bottle, decant it into a smaller one to reduce the air above it.

Flower and peel therefore age at two speeds. Absolutes are complex in composition but generally more stable when kept cold, at 3 to 5 years in the library's table. Pomelo peel needs to be used while it is fresh. The piece a bottle opened after five years continues the story of time inside a bottle of perfume.

The peel holds its scent in glands, the flower in its petals.

One needs force, the other a solvent.

One tree, two ways of counting time.