Lactic acid is found in yogurt, buttermilk, kefir, and other fermented dairy products. It is created when lactobacillus bacteria convert sugars into lactic acid.
Pure lactic acid powder can be purchased from companies that supply beer and wine brewers.
What does lactic acid do in soap?
Lactic acid and sodium hydroxide (NaOH) make sodium lactate. Sodium lactate will increase the hardness of bar (NaOH) soap, make bar soap easier to remove from a silicone mold, make hot process NaOH soap more fluid and stirrable, and make liquid (KOH) soap paste easier to dilute.
How much pure lactic acid powder can be used?
Lactic acid powder is not often added directly to soap. If you decide to try this, I suggest 5 to 10 g lactic acid per 1000 g fats (0.5% to 1% of the total fat weight).
How to calculate the weight of pure lactic acid powder to add to soap?
1. Decide the percentage of lactic acid you want to use in your recipe (see above).
2. Calculate the weight of lactic acid powder:
Lactic acid powder, grams = (Total weight of fats, grams) X (Percent lactic acid) / 100
How much fermented dairy can be used?
Fermented dairy product can be used instead of pure lactic acid powder. Fermented dairy can be used up to 100% of the water in a soap recipe, although I get the impression most soap makers do not use that much.
How much fermented dairy is typically used?
For hot process soap, stir in 1-2 tablespoons per pound (or 500 grams) of the total fat weight (about 7-14 grams based on total fat weight) after the soap is saponified.
For cold process soap, the amount of fermented dairy typically ranges from a low of 1-2 tablespoons per pound (or 500 grams) of fats and as much as half of the total water weight for the batch.
Fermented dairy is seldom used in liquid (KOH) soap making; sodium lactate is preferred.
How much lactic acid is in fermented dairy products?
Here are estimates of the lactic acid content in fermented dairy products --
Yogurt: Up to 0.9% by weight (0.9 gram acid per 100 grams yogurt) (1, 2)
Buttermilk: Up to 4% by weight (4 grams acid per 100 grams buttermilk) (2)
Kefir: Up to 1% by weight (1 gram acid per 100 grams kefir) (3)
Acid whey drained from yogurt: 0.6% to 1.40% by weight (0.6 to 1.4 grams acid per 100 grams of whey) (4)
How to calculate the weight of lactic acid in fermented dairy?
1. Estimate the percentage of lactic acid in the dairy product (see above).
2. Decide the weight of dairy product to use in your recipe.
3. Calculate the weight of lactic acid in this amount of dairy:
Lactic acid, grams = (Weight of dairy, grams) X (Percent lactic acid in dairy product) / 100
Example: I want to add 250 grams of yogurt to my soap recipe. I estimate the lactic acid is about 1% by weight. About how much lactic acid is in this yogurt? Calculate the answer this way:
Lactic acid, grams = 250 X 1 / 100 = 2.5 grams
How much lye does lactic acid neutralize?
10 g lactic acid neutralizes 4.44 g NaOH. 10 g lactic acid neutralizes 6.23 g KOH.
How to calculate the extra lye to react with lactic acid?
When using a fermented dairy product or pure lactic acid powder 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.
Sodium hydroxide, NaOH
1. Calculate the weight of lactic acid to be added (see above).
2. NaOH for lactic acid, grams = Lactic acid, grams X 4.44 / 10
3. Total NaOH, grams = NaOH for lactic acid, grams + NaOH for saponification, grams
Potassium hydroxide, KOH
1. Calculate the weight of lactic acid to be added (see above).
2. KOH for lactic acid, grams = Lactic acid, grams X 6.23 / 10
3. 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 to add lactic acid powder or fermented dairy to my soap?
If a fermented dairy product is used as a partial substitute for some of the water in the recipe, blend it 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.
References
(1) N. Niamsiri, C.A. Batt. Dairy Products in Encyclopedia of Microbiology, 3rd ed, pages 34-44. Academic Press, 2009.
"...total lactic acid content to be around 0.7 to 0.9% w/w..."
Source: https://www.sciencedirect.com/science/article/pii/B9780123739445001206
(2) Shannon Romanowski. The Natural Exfoliator Hiding in Your Fridge on the Self website. Conde Nast, publisher.
"...yogurt only contains about 0.9% available acid after fermentation.... Buttermilk contains between 3-4% lactic acid which is much higher than levels found in yogurt...."
Source: https://www.self.com/story/can-you-exfoliate-with-yogurt
(3) M. Nuñez. Existing Technologies in Non-cow Milk Processing and Traditional Non-cow Milk Products in Non-Bovine Milk and Milk Products, pages 161-185. Academic Press. 2016.
"...Kefir undergoes a mixed lactic-alcoholic fermentation that yields a foamy fermented milk of slight acidic taste and aromatic odor, with up to 1.0% lactic acid and 1.2% ethanol...."
Source: https://www.sciencedirect.com/science/article/pii/B9780128033616000077
(4) Karastamatis, S.; Zoidou, E.; Moatsou, G.; Moschopoulou, E. Effect of Modified Manufacturing Conditions on the Composition of Greek Strained Yogurt and the Quantity and Composition of Generated Acid Whey. Foods 2022, 11, 3953. https://doi.org/10.3390/foods11243953
Data in the literature reviewed by the authors:
"...Acid whey, the by-product of Greek strained yogurt manufacture, has ... 0.6–1.40% lactic acid..."
See also Table 3 in this paper for lactic acid test data collected by the authors.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9778196
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