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Triglycerides form the core structure of every vegetable oil, consisting of glycerol bound to three fatty acids. The character of an oil comes mainly from the ratio of these fatty acids. Saturated fatty acids such as palmitic and stearic give solid or thick texture at room temperature. Unsaturated fatty acids such as oleic, linoleic, and linolenic give liquid texture.

Oleic acid (omega-9, one double bond) is the most common fatty acid. It gives oils a balanced feel, moderate absorption, and good oxidative stability. Olive, almond, camellia, and macadamia carry over 60 percent oleic.

Linoleic acid (omega-6, two double bonds) gives oils a dry feel, fast absorption, and suits oily and acne-prone skin. Grape seed, hemp seed, rosehip, and safflower are rich in linoleic. It oxidises faster, so shelf life is shorter.

Linolenic acid (omega-3, three double bonds) is rare in vegetable oils. Flaxseed and hemp seed carry significant amounts but the oils are very prone to rancidity and need refrigeration.

Lauric acid (saturated, 12 carbons) characterises coconut and babassu and produces strong lather when saponified. Ricinoleic acid is unique to castor oil, with a double bond and a hydroxyl group, which makes castor thick, sticky, and a strong humectant.

Skin feel relates directly to the linoleic to oleic ratio and to molecular polarity. Dry oils absorb fast and leave no greasy film, suiting oily or acne-prone skin and facial formulations. Grape seed, rosehip, hemp seed, safflower, hazelnut, and camellia fall in this category.

Balanced oils absorb moderately and leave a light emollient layer. Sweet almond, apricot kernel, jojoba, argan, and macadamia are typical. This group serves as body oils and massage oils.

Greasy oils absorb slowly and leave a heavy film. Olive extra virgin, avocado, castor, sesame, and softening shea butter belong here. They suit dry skin, balms, foot care, and occlusive applications for dry seasons.

The comedogenic rating measures an oil's tendency to clog pores and trigger acne, on a scale of 0 to 5. The scale originated from rabbit-ear testing, is not absolute, but remains a practical guide for facial formulation.

Oils rated 0 to 1 are safe for acne-prone skin: argan, hemp seed, safflower, sunflower, camellia, shea butter. Rating 2 is tolerated by most skin: sweet almond, jojoba, rosehip, sesame, olive. Rating 3 to 5 needs caution on acne-prone skin: avocado, coconut, cocoa butter, wheat germ.

The comedogenic rating applies only to leave-on formulations. In cold process soap the oils are saponified into fatty acid salts and no longer carry the comedogenic profile of the free oil, so coconut (rating 4) remains acceptable in soap bars.

Saponification value (SAP) is the amount of alkali needed to fully saponify one unit weight of oil. The usual unit is mg KOH or mg NaOH per gram of oil. Cold process formulation uses NaOH to produce solid soap bars.

Each oil has its own SAP because different fatty acid structures need different alkali amounts. Coconut has a high SAP (NaOH ~0.190) due to short lauric acid. Olive has a lower SAP (NaOH ~0.135) because of longer oleic. A soap recipe calculates total alkali from the proportion of each oil, with a 5 to 8 percent superfat to leave unreacted oil that softens the bar.

Each oil also contributes a different property to the finished bar. Coconut and palm give hard bars with bubbly lather. Olive gives a soft, conditioning bar that traces slowly. Castor adds creamy lather. Shea and cocoa butter give firm, conditioning bars. Hemp, rosehip, and evening primrose are used as superfat after saponification to preserve their actives.

Vegetable oils go rancid through oxidation of unsaturated fatty acid double bonds. Linolenic with three double bonds oxidises fastest, linoleic with two double bonds moderate, oleic with one double bond slow. Saturated-rich oils like coconut, cocoa, and shea stay stable longest.

Shelf life drops sharply under heat, light, and oxygen exposure. Store in tightly sealed amber bottles in a cool place away from sun. Refrigeration extends life significantly for linolenic-rich oils such as flaxseed, rosehip, and hemp.

Vitamin E (tocopherol) is a natural antioxidant present in many oils, and can be added at 0.5 percent to extend shelf life. Rosemary oleoresin extract (ROE) at 0.1 percent is another common antioxidant for carrier oils.

Signs of rancidity: stale wax smell, paint-oil smell, fishy notes; colour shift to deep yellow or brown; precipitate or separation. Rancid oil should not be used on skin and will not saponify cleanly.

Sweet almond (Prunus dulcis) is the most common carrier. Balanced, moderate absorption, high oleic, vitamins E and A. Suits most skin types except nut allergies. Used neat or 10 to 100 percent in body and facial formulations.

Apricot kernel (Prunus armeniaca) is close to sweet almond but lighter and faster absorbing. High vitamin A, suits dry, mature, and sensitive skin. Used 20 to 50 percent.

Argan (Argania spinosa) from Morocco, strong anti-aging, high vitamin E and squalene. Suits hair and mature skin. Acne-prone skin should use with care.

Avocado (Persea americana) is thick, the richest fruit in vitamin E, stimulates collagen. Suits dry, dehydrated, mature skin. Used 10 to 20 percent in lighter carriers.

Camellia seed (Camellia oleifera), also called tea seed oil. Oleic 78 to 85 percent, fast absorbing, non-greasy. The Japanese tradition uses it for hair and skin. Used 2 to 12 percent or neat.

Coconut (Cocos nucifera) comes in two forms: virgin (solid below 24°C, high lauric, coconut scent) and fractionated (permanent liquid, MCT-separated, odourless). Virgin goes into cold process soap for lather. Fractionated serves as a light carrier for essential oils.

Grape seed (Vitis vinifera) is thin, high linoleic, dry absorbing. Suits oily skin. High antioxidant content. Used 1 to 100 percent for non-greasy facial serums.

Hemp seed (Cannabis sativa) is dry, with balanced omega-3 and omega-6 resembling skin lipids. Dry and mature skin. Refrigerate. Used 2.5 to 10 percent.

Jojoba (Simmondsia chinensis) is actually a liquid wax ester, not a triglyceride. Its structure mimics human sebum. Resists rancidity through high tocopherol. Suits all skin types including acne-prone. Used neat or 10 to 100 percent.

Macadamia (Macadamia integrifolia) is high in palmitoleic, close to skin lipids, fast absorbing, stable. Suits dry, mature, all skin.

Olive (Olea europaea) in two grades: extra virgin (green, high polyphenol, for skin and castile soap) and mild or pomace (general soap use). High oleic, suits dry skin. Castile soap uses 100 percent olive.

Rice bran (Oryza sativa) is emollient with vitamin E and gamma-oryzanol, with mild UV absorption. Used 5 to 100 percent. Replaces nut oils when allergies are a concern.

Rosehip seed (Rosa mosqueta) is strongly regenerative through essential fatty acids and lycopene. Scarred and mature skin around eyes and mouth. Used at 10 percent or neat by drops. Refrigerate.

Safflower (Carthamus tinctorius, high linoleic) is a dry barrier-supporting oil. Oily and combination skin.

Sesame (Sesamum indicum) carries lipids, vitamin E, sesamol antioxidant, mild UV absorption. Ayurvedic tradition. Used 10 to 15 percent.

Shea butter (Vitellaria paradoxa) is a hard butter with vitamins A and E, phenolic compounds, non-comedogenic. Body cream, balm, lip products. Used 5 to 30 percent.

Cocoa butter (Theobroma cacao) is very firm with chocolate scent unless deodorised. Stretch mark cream, lip balm, hard soap bar. Used 3 to 10 percent.

Mango butter (Mangifera indica) is hard, mildly scented, with vitamin A and phenolics. Lipstick, balm. Used 5 to 15 percent.

Castor (Ricinus communis) is thick, sticky, a strong humectant from ricinoleic. Lip gloss, mascara, soap (boosts creamy lather at 5 to 8 percent), hair, lashes. Not for ingestion (strong laxative). Avoid during pregnancy.

Essential oils need a carrier oil for dilution before skin application. Typical ratios: 1 to 2 percent for facial, 2 to 5 percent for body, 5 to 10 percent for spot treatment. Match the carrier to the skin profile and the essential oil profile.

Anti-aging: Rosehip seed + Frankincense or Myrrh. Argan + Helichrysum for heavily mature skin.

Facial blend with citrus essentials: Jojoba is the preferred carrier as it is stable and non-comedogenic. Bergamot, Lemon, Lime in jojoba for night serums (avoid sun due to phototoxicity).

Acne-prone skin: Hemp seed or grape seed + Tea tree, Lavender, or Geranium. Avoid coconut and cocoa butter in leave-on facial.

Calming and sleep: Sweet almond + Lavender, Roman chamomile, or Neroli.

Massage and body: Sweet almond or apricot kernel + Sweet marjoram, Ylang ylang, or Sandalwood.

Incoming oils need verification of batch number, production date, and expiry. On opening, scent and colour must match spec for that oil. Oils with off-notes or colour shift should not be used leave-on; lightly off oils may still be usable for soap if not severely rancid.

Store oils in amber glass bottles or stainless drums. Warehouse temperature 18 to 22°C, humidity below 60 percent. Linolenic-rich oils (flaxseed, rosehip, hemp) go in cold storage at 4 to 8°C.

Before formulation, solid oils (shea, cocoa, mango butter, virgin coconut) need melting at 40 to 50°C in a bain-marie. Do not exceed 60°C to preserve vitamin E and phenolics.

After opening, oils contact oxygen, so shelf life drops to 50 to 70 percent of sealed shelf life. Label the opened date. Adjust purchase quantity so the oil is used within shelf life, avoid stocking beyond demand.