Kevin Dunn did a presentation in which he describes his research into what happens when soap is made with milk. The sugar (lactose) in goat milk chemically reacts with NaOH or KOH and changes color in the process -- that's why milk soap tends to be darker than the same soap made without milk. The protein (casein) only slightly reacts -- not enough to worry about. And obviously the milkfat (butterfat) reacts.
Dunn concluded that using reconstituted goat milk as a 100% replacement for water will raise the superfat by 4% to 6% due to the reaction of the lactose as well as milkfat with the NaOH. To compensate for this, since soap recipe calculators don't have a saponification value for actual goat milk, you could reduce the superfat % by that much. I calculated that the fat in full fat milk raises the superfat by about 1% if milk is substituted for the water, so the remaining 3% to 5% increase is coming from the sugar.
As to whether milk sugars or other sugars will cause the soap to volcano, that is not a problem in my experience, so I don't spend any time worrying about it. The volcanoes I've seen and heard about have been triggered by the use of fragrances that contain known accelerants (such as eugenol) or to slowly pouring cool lye solution into unusually hot fat or to heating soap batter on the stove top and not watching the temperature carefully enough.
As far as food and similar ingredients decomposing or otherwise going bad in soap, the high pH of soap will inhibit microbial decomposition if the soap maker is sensible. I've seen evidence of mold or other decomposition on soap in only a couple of instances. One was when a bar of soap at a sink was contaminated with stuff from a person's very dirty hands and the bar was left dirty, damp, and untouched for some time. Another was when the soap maker heavily sprinkled botanical/food ingredients like flower petals or oatmeal flakes on the top of the loaf. In both cases, the contaminated areas were ON the soap, not IN it. Food type additives have been used IN soap for centuries, including potatoes, starches, eggs, and dairy, so the idea is nothing particularly new or unknown. As IL points out, additives like these should be in small particles or liquids, not big chunks. They should be used in low to moderate amounts and mixed well into the soap.
Dunn concluded that using reconstituted goat milk as a 100% replacement for water will raise the superfat by 4% to 6% due to the reaction of the lactose as well as milkfat with the NaOH. To compensate for this, since soap recipe calculators don't have a saponification value for actual goat milk, you could reduce the superfat % by that much. I calculated that the fat in full fat milk raises the superfat by about 1% if milk is substituted for the water, so the remaining 3% to 5% increase is coming from the sugar.
As to whether milk sugars or other sugars will cause the soap to volcano, that is not a problem in my experience, so I don't spend any time worrying about it. The volcanoes I've seen and heard about have been triggered by the use of fragrances that contain known accelerants (such as eugenol) or to slowly pouring cool lye solution into unusually hot fat or to heating soap batter on the stove top and not watching the temperature carefully enough.
As far as food and similar ingredients decomposing or otherwise going bad in soap, the high pH of soap will inhibit microbial decomposition if the soap maker is sensible. I've seen evidence of mold or other decomposition on soap in only a couple of instances. One was when a bar of soap at a sink was contaminated with stuff from a person's very dirty hands and the bar was left dirty, damp, and untouched for some time. Another was when the soap maker heavily sprinkled botanical/food ingredients like flower petals or oatmeal flakes on the top of the loaf. In both cases, the contaminated areas were ON the soap, not IN it. Food type additives have been used IN soap for centuries, including potatoes, starches, eggs, and dairy, so the idea is nothing particularly new or unknown. As IL points out, additives like these should be in small particles or liquids, not big chunks. They should be used in low to moderate amounts and mixed well into the soap.
