Superfat and lye discount

The number every recipe carries and that half the craft describes using the wrong term.

Two names for two different things

Superfat means using more oil than the lye can saponify, leaving free oil in the bar.

Lye discount means using less lye than the oils require, which produces the same result from the other direction.

Expressed as a percentage, and the two are arithmetically equivalent, which is why calculators use them interchangeably.

What they are not is water discount, which is a different decision entirely and is frequently confused with this one.

What superfat does

Guarantees no free lye, which is the safety function and the main reason it exists.

Leaves unsaponified oil in the bar, which contributes to feel.

Softens the bar and shortens its life, because free oil is not soap.

And gives the rancidity clock something to work on, since free unsaturated oil is what goes off.

Choosing a level

Five percent is the conventional default and suits most bars.

One to three percent for a harder, longer-lasting bar and for laundry or utility soap.

Eight percent or more for a bar positioned as rich, at the cost of a softer bar and a shorter life.

Zero is not advisable, because it leaves no margin for the several places small errors enter.

The safety argument

This is why superfat is not optional.

Scales have tolerance. Oils vary in SAP value between lots. Lye absorbs moisture and a container open for months is not the purity on the bag. Measurement has error.

All of those push in both directions, and a recipe calculated at exactly zero superfat can come out lye heavy on a bad day.

Five percent absorbs every one of those and costs almost nothing. It is the margin that makes an ordinary process forgiving.

References