DOS spots: dreaded orange spots and how to prevent them

TL;DR: DOS (orange spots) = oxidation of unsaturated oils. Risk oils: linseed, hemp, sunflower. Prevention: antioxidants (vitamin E, ROE), quality oils, dark storage. Small DOS are not a health risk, but large ones are not marketable.

DOS - Dreaded Orange Spots - are orange, brown or yellow spots that begin to appear on the surface or in the cross section of the soap typically 4-8 weeks after manufacture, sometimes sooner. They are one of the most frustrating problems because they affect soap that was seemingly fine at the time of manufacture.

DOS are a flag Oxidation unsaturated fatty acids - the same process that causes the oil to go rancid. The result is not only an aesthetic blemish, but also an unpleasant rancid smell.

What happens: chemistry DOS

The unsaturated fatty acids (oleic, linoleic, linolenic) in the unsaponified superfat portion of soap are susceptible to oxidation. Oxidation occurs by a chain reaction - one oxidized lipid triggers an adjacent one to spread.

Catalysts of oxidation:

• Oxygen from the air
• Light (especially UV radiation)
• Heat
• Presence of metal ions (Cu²⁺, Fe³⁺) - trace amounts from water or raw materials

The presence of antioxidants (vitamin E, ROE) slows down this reaction, not stops it.

Which oil causes DOS most often

The risk of DOS depends on the degree of unsaturation of the oils in the recipe:

High risk of DOS:

• Flaxseed oil - highest content of linolenic acid (omega-3), oxidizes very quickly
• Hemp oil - high linoleic + linolenic acid
• Macadamia oil - palmitoleic acid, relatively unstable
• Sunflower (standard) - high linoleic acid
• Grapeseed oil - high linoleic acid

Medium risk:

• Olive oil - predominantly oleic (mono) but depends on quality (extra virgin has antioxidants, refined less so)
• Almond oil - mono + linoleic

Low risk:

• Coconut oil - saturated fatty acids, very stable
• Palm oil - saturated + palmitic
• Tallow (tallow) - saturated + oily
• Shea butter - oil + stearic

When DOS comes first

• Soap with linseed oil: DOS can be visible within 3-4 weeks
• Soap with hemp oil: 4-8 weeks
• Olive oil soap: 3-12 months (depends on the quality of the oil)
• Standard coconut/palm recipe: 12-24+ months (unless other causes arise)

Why DOS comes early: accelerating factors

Rancid or old oils

Oil that has been poorly stored or is close to its expiry date already contains primary oxidation products (hydroperoxides). These "inoculate" the soap - the process is then significantly faster.

Rule: Always sniff the oils before use. Put rancid oil (cardboard, greasy, pungent smelling) in the garbage.

Metal contamination

Trace amounts of copper or iron in raw materials or contamination from tools (stainless steel is safe, brass or copper is not) accelerates oxidation.

Rule: Use only stainless steel, plastic or silicon tools and molds.

Too high superfat with unstable oils

SF 10% from linseed or hemp oil = high proportion of unstable, easily oxidized free fatty acids in soap.

Rule: If using flax or hemp oil, reduce the total SF to 5% or less and add antioxidants.

Improper storage of finished soap

Soap in direct sunlight, in a warm and humid environment (bathroom) or in contact with metal.

How to prevent DOS

Antioxidants

Vitamin E (alpha-tocopherol): Add 0.5-1% of the total weight of oils to the trace. Binds free radicals and slows the oxidation chain reaction.

ROE (Rosemary Oleoresin Extract): Stronger than vitamin E. Add 0.1-0.3% of the total weight of oils. Has a stronger herbal scent - almost imperceptible in the finished soap.

Combination: ROE + vitamin E = best protection for formulations with unstable oils.

Distilled water instead of tap water

Tap water may contain trace amounts of metal ions (Cu, Fe) that catalyze oxidation. Distilled water or filtered water (reverse osmosis) eliminates this factor.

Restriction of unstable oils

• Flaxseed oil: maximum 5-10% in the recipe, ideally with ROE
• Hemp oil: maximum 10-15 %
• Sunflower: prefer the high-oleic variant (72% oil instead of 65% linoleic)

High-oleic sunflower oil is significantly more stable than standard (conventional) sunflower. It has similar conditioning properties but a much lower linoleic acid content.

Correct storage

• Dark, dry, cool place (cellar, pantry - not bathroom)
• Paper or fabric packaging (soap must breathe - it won't last in a fully sealed plastic bag)
• Separate from metal surfaces

What to do with soap that has DOS

Small spots (1-3 dots) in otherwise healthy soap: The soap is safe to use. The orange color and slight rancid smell are aesthetic defects, not a health hazard. Please inform the customer or use for your own use.

Stains spread or the soap smells strongly of rancidity: Soap is not for sale. Stop aging and eliminate from sale.

Entire batch hit by DOS within 4 weeks: Look for the cause - probably rancid oil in the raw material or contamination of the tools.

DOS vs. other colour spots

Not every orange or brown spot is a DOS:

Vanillin browning: Uniform browning of the entire cross-section, not spots. Vanilla aroma, not rancidity. Causes FO with vanillin.

Stearic spots: White or light grey spots/grains (not orange). They are caused by unstabilised palm oil or high stearic acid.

Soda ash: White dust coating on the surface (not spots), not in the cross-section.

The natural colour of the oil: Turmeric, spirulina, spinach - color spots when mixed unevenly.

Quick diagnostic key

Frequently asked questions

Is DOS a health risk? Small VOCs are not a health threat - they are just oxidised fatty acids with an unpleasant smell. They are not suitable for sale.

How long can it take for DOS to appear? Depends on the oil: linseed (3-4 weeks), sunflower (4-8 weeks), olive (3-12 months), coconut (12+ months).

Can an antioxidant prevent DOS altogether? Antioxidants slow down the process, but do not stop it. With risky oils (flaxseed, hemp) the DOS comes later, not never.

Can I combine high superfat with oils at risk of DOS? You shouldn't - a combination of SF 10% + flaxseed oil = DOS in 3-4 weeks. Reduce SF to 5%.

What is the best combination of antioxidants? ROE (0.2%) + vitamin E (0.5%) is very effective. Only vitamin E is cheaper.

How do I store the soap so the DOS doesn't show up? A dark, dry, cool place. Paper wrapper. No metal surfaces. Temperatures below 20 °C are ideal.

See also:

• Troubleshooting encyclopedia - overview of all problems
• Soda ash - white surface coating
• Soap additives - vitamin E and ROE as antioxidants
• Oil encyclopedia - stability of oils and fatty acids