Classic Bells > Soapy stuff > Lactic acid

Lactic acid in soap

What does it do in soap? Lactic acid is the acid in yogurt, buttermilk, kefir, and other fermented dairy products. Lactic acid is also produced by bacterial fermentation of sugars. Pure lactic acid can be purchased from companies that supply beer and wine brewers. Lactic acid and Sodium hydroxide (NaOH) make Sodium lactate.

How much should I use? Lactic acid is usually added indirectly to soap by using yogurt, kefir, or buttermilk in the recipe. Here are my estimates of how much lactic acid would be added by using fermented dairy as a full replacement for water in a soap recipe --

Yogurt: 3-4 grams per 1000 grams of oils (0.3% to 0.4% ppo)

Buttermilk, traditional (the liquid from making butter with sour cream, not fresh cream): 2-3 g per 1000 g of oils (0.2% to 0.3% ppo)

Buttermilk, commercially cultured: No information found. See buttermilk, traditional.

Kefir: 3-4 g per 1000 g of oils (0.3% to 0.4% ppo)

If using pure lactic acid in soap, try 5 to 10 g lactic acid per 1000 g oils (0.5% to 1% ppo).

How much lye does it neutralize? 10 g lactic acid neutralizes 4.44 g NaOH. 10 g lactic acid neutralizes 6.23 g KOH. When using lactic acid in your recipe, add the appropriate extra weight of lye needed to react with the acid. If you do not add extra lye, the acid will increase the superfat in your soap.

NaOH for lactic acid, grams = Lactic acid, grams X 4.44 / 10
Total NaOH, grams = NaOH for lactic acid, grams + NaOH for saponification, grams

KOH for lactic acid, grams = Lactic acid, grams X 6.23 / 10
Total KOH, grams = KOH for lactic acid, grams + KOH for saponification, grams

Making a dual-lye recipe? For recipes that use both NaOH and KOH as well as lactic acid, please see my tips here....

How should I add it to my soap? If used as a partial substitute for some of the water in the recipe, fermented dairy is often blended into the oils before adding the lye solution. If using pure lactic acid, dissolve the lactic acid in about 2 times its weight of water. Stick blend that mixture into your oils.