Soybean Oil

Moisturising is one of the cosmetic functions the European Commission's ingredient database assigns to soybean oil, and it is the plainest thing this oil does. It leaves skin feeling softer, more comfortable and less tight after use — a description of how skin looks and feels rather than of anything happening beneath the surface, which is the line a cosmetic is allowed to stand on.

Soybean oil is an emollient, the plain cosmetic word for something that works on the surface rather than underneath. It settles into the gaps between the flakes of partly shed skin that make a dry patch feel rough, so heels, elbows and hands feel smoother under your fingers within a minute of putting it on.

This is one of the lighter carrier oils, and that is its signature. It goes on thin, carries almost no colour or smell, and does not leave the heavy film a richer oil can — so a lotion or a body oil built on it feels clean rather than greasy, and people who dislike a heavy oil often get on with this one.

Work published in the Journal of Cosmetic Science measured oil-treated hair rather than describing it, recording lower combing force both wet and dry and higher gloss. Soybean oil is light enough to do that on fine hair without weighing it down, so it leaves hair smoother and glossier — a fixed-oil property, not a claim about growth or strength.

Soybean oil goes into the oil phase of lotions, creams and body oils, and it is light enough to work in leave-on products that a heavier oil would make greasy. It is high in linoleic acid, which suits normal and oily skin, and it carries almost no colour or smell into a formula. Because it is high in polyunsaturated fats, pair it with an antioxidant.

Soybean oil suits the lighter hair formats: pre-wash oils, conditioners, masks and leave-in serums, where its low weight means it rinses cleaner than a heavy oil and does not flatten fine hair. It brings slip and a smoother feel rather than any strengthening claim, and its neutral smell leaves the fragrance of the product in charge.

Soybean oil is a classic soap oil, contributing a mild, conditioning lather; soap makers usually balance it with a harder oil for a firmer bar, since on its own it makes a soft one. It also works in bath oils, melt-and-pour bases and body-oil blends, and its neutral colour keeps a pale base pale.

Treat soybean oil as a light fixed oil. It joins the oil phase with your other oils before emulsification, needs no solubiliser in anhydrous work and a proper emulsifier for anything water-based, and it shifts viscosity very little. Being high in polyunsaturated fats it oxidises faster than a saturated oil, so add a tocopherol antioxidant and keep it cool. One name on the INCI list.