Open a bottle of neem seed oil and the scent arrives before you have looked at the colour: sharp, heavy, like crushed garlic mixed with damp earth. It is a plant oil, not an essential oil, yet its smell overwhelms familiar carriers such as olive or almond. In cold process soap, neem is usually a small line in the ingredient list. This piece follows the neem seed, the volatile part that makes the scent, the fatty acid table, and the reasons soap makers use it only in small amounts.
A tree from the Indian subcontinent
Neem, Azadirachta indica, belongs to the mahogany family, Meliaceae, together with the chinaberry tree and big leaf mahogany. It is considered native to the Indian subcontinent and parts of Indochina. It is an evergreen tree of 15 to 20 metres that sheds part of its foliage in the dry months. It tolerates drought well, so in dry regions it is planted widely as a shade tree and along roads.
The names carry part of its history. The word neem comes from the Sanskrit nimba. The genus name Azadirachta comes from the Persian āzād dirakht, meaning free or noble tree. In India the tree has been part of daily life for a very long time: its wood, shade, leaves, twigs and seeds each have their own place in the household and the village.
Neem oil is pressed from the seed. Inside the ripe fruit a hard white shell encloses one elongated seed, sometimes two or three. The pressed oil is yellow, bitter and smells of garlic. Those three traits come from three different groups of compounds in the same bottle, and each group passes through the soap pot in its own way.
The garlic note comes from sulfur
Most plant oils are almost odourless, because triglycerides are large molecules that do not evaporate. The scent of a carrier oil usually comes from a small amount of accompanying compounds. In neem, that small amount includes organic compounds containing sulfur. Studies of neem seeds record this volatile group, and di-n-propyl disulfide is the most prominent component in the headspace above freshly crushed neem seeds.
Di-n-propyl disulfide is a disulfide: two sulfur atoms joined together, each end carrying a short carbon group. The main constituent of garlic oil, diallyl disulfide, has the same frame and differs only in the groups at the two ends. The library describes diallyl disulfide as pungent raw garlic, sharp and persistent. Di-n-propyl disulfide itself is described in flavour ingredient sheets as sulfurous and alliaceous, so these two close relatives sit side by side for the nose.
The human nose is very sensitive to sulfur compounds, so a small amount is enough to be noticed. That is why an oil with so little volatile matter still carries such a large scent. The earthy part is harder to assign to a single molecule; it sits under the garlic like a slightly musty ground.
The bitterness comes from elsewhere. Neem seeds contain limonoids, a group of large triterpenoid molecules, of which azadirachtin and nimbin are the names met most often. The pressed seed oil carries part of these limonoids, with an azadirachtin content that varies widely between seed lots. These molecules are heavy and hardly evaporate, so they contribute to taste more than to scent.
The fatty acid table of neem oil
As far as the fat goes, neem is a fairly ordinary oil. A survey of 60 neem seed samples from Rajasthan, India, analysed by gas chromatography, recorded oleic acid at 25 to 58 percent, palmitic at 16 to 34, stearic at 6 to 24 and linoleic at 6 to 17. The ranges are very wide, because the trees grow on different soils, in different climates and from different lines.
Placed beside the fatty acid table in the library, neem sits between olive oil and the butters. Extra virgin olive oil has 65 to 80 percent oleic and 14 to 18 percent saturated. Shea butter has 42 to 50 percent saturated, cocoa butter 57 to 64. Neem has less oleic than olive, while its saturated part, palmitic plus stearic, is usually much higher.
That saturated part explains a familiar physical trait: neem oil tends to thicken or gel when the weather is cool, and returns to an even liquid when gently warmed. The library notes that saturated fatty acids such as palmitic and stearic give oils a solid or thick texture at room temperature; neem carries enough of them to stand on the border.
Linoleic acid, with two double bonds, is a small share. According to the library, linoleic oxidises faster than oleic, and oils rich in saturated fatty acids last longest. With proportions like these, neem is not among the oils that turn rancid easily, even though its scent sometimes makes a first time user think the bottle has gone off.
Neem in the saponification pot
Each oil needs its own amount of alkali to saponify completely. The library records a SAP of about 0.190 grams of NaOH per gram for coconut oil and about 0.135 for olive oil. Supplier SAP tables list neem at about 0.136 to 0.141, close to olive, because both oils are built mainly from fatty acids with 16 and 18 carbons.
In the pot, the triglycerides of neem go through exactly the reaction described in the six week piece: NaOH cleaves the ester bonds and releases glycerin and fatty acid salts. The salts of palmitic and stearic add to the hardness of the bar, as the library notes for shea butter and cocoa butter. The salts of oleic give the bar suppleness, like the olive part.
Limonoids and the sulfur compounds are not fatty acids, so they do not become soap salts in that way. Azadirachtin in particular is unstable in alkaline conditions, so it is hard to say how much of the limonoids survives intact in the bar. Even so, soapmakers report that the colour and scent of neem carry through into the finished bar; they are only diluted by the rest of the formula.
When a batch is prepared, neem is warmed together with the firm fats such as shea butter and cocoa butter, then stirred so that any gelled part melts completely before it meets the lye solution. A bottle of neem kept in a cool place can be thicker at the bottom than at the top, so it is usually shaken or stirred before weighing.
Why it is only a small part
The first reason is scent. A little neem in a batch is already noticeable; more than that and the garlic and earth cover everything else. One soap supply vendor lists a cold process usage rate of 3 to 6 percent, while some other soapmaking guides give 5 to 10 percent of total oils. The same vendor notes that the neem scent is less noticeable at a low rate and beside a strong scent.
The second reason is colour, and the third is variability. The oil is deep yellow, and soapmakers report that the colour carries into the bar; in a bar with herbal powders the shift blends into the overall colour, in a white bar it shows more. And when oleic can run from 25 to 58 percent depending on the seed lot, a small proportion keeps the bar less dependent on each batch of oil.
Lê Mai's Olive Artisanal Soap No.104 shows that position clearly. Olive oil makes up more than 70 percent of the formula; neem seed oil, shea butter and cocoa butter sit in the rest of its 12 ingredients. As the Castile piece explains, this is not a pure Castile: neem is one of the oils standing beside olive, not the main oil.
Soapmakers report that the neem scent is still there on a cured bar, only fading over time like every other scent in handmade soap. At a small proportion, it becomes a slightly earthy ground under the essential oils. Scent fading on the curing rack is also what was described for essential oils in the piece on essential oils through saponification.
Neem beside essential oils
A base that smells of garlic and earth does not suit every essential oil. Light florals and citrus are easily overpowered, since their top notes already thin out over six weeks of curing. Oils with a strong character, spicy, pungent or cool and camphoraceous, stand beside neem better: they are bold enough not to sink, and their spicy or green side covers part of the sulfur edge.
Lê Mai's Piper Betle Leaf Artisanal Soap No.105 uses fresh betel leaf juice in place of distilled water, with betel leaf essential oil, matcha and turmeric powder, in a 15 ingredient formula that includes neem seed oil and cocoa butter. According to the betel page in the library, eugenol makes up 20.5 to 33.2 percent, giving the warm spice of the leaf. The lightly pungent betel and the green of matcha stand in front; neem lies beneath, in the same herbal key.
Cinnamon Artisanal Soap No.058 goes another way: Cinnamomum zeylanicum bark oil and cinnamon powder from Sri Lanka, with cocoa butter, shea butter and neem seed oil among its 13 ingredients. The brown of the cinnamon powder covers the yellow of neem, and the cinnamaldehyde of cinnamon is bold enough to lead. In the olive bar, the scent comes from essential oil blend #104 of eucalyptus, cajuput, lavender, tea tree and wintergreen; according to the library, 1,8-cineole makes up 65.4 to 83.9 percent of eucalyptus oil, laying a cool camphoraceous layer over the neem base.
The three bars take three different routes, but follow the same principle: neem is part of the base, and the essential oils are chosen to stand on that base.
The garlic of neem comes from sulfur, its bitterness from limonoids.
Its fat is closer to olive and the butters than to anything strange.
Used sparingly, under an essential oil bold enough, neem becomes a base.