In theory, in my world at least, anything that helps soap dissolve a little faster, helps to soften the water/prevent soap scum, or supports bubble structure, should lead to bigger bubbles.
Based on my personal experiences and a lot of experimenting with various recipes and oils, dissolution is helped along by coconut, a recipe with less stearic + palmitic, oleic acid at 40% or more, KOH in a dual lye recipe, or a large amount of sugar added, like >2% of TOW. The chelators probably help with point of use softening and we know they help to prevent soap scum. Sugars, like table sugar, honey and beer definitley increase bubbles in my recipes. They are known to contribute to bubble support and I also think they contribute to dissolution based on a few high sugar soaps I made earlier this year. I haven’t tried sorbitol, so can’t weigh in on how/why it works. Many makers have found that milks, cooking water or purees of oatmeal, rice, potatoes, fruits, aloe and similar help with bubbles. It seem likely that the colloidal starches or sugars in the purees or cooking or rinsing water contribute to bubble support, and possibly also some dissolution. Rice bran oil also increases bubbles in my recipes, which may be because the kinky structure of the polyunsaturated linoleic fatty acid in soap makes it easier to dissolve. On the downside, the linoleic and linolenic fatty acids in RBO have been linked to a higher risk or DOS because the unsaturated bonds are more prone to oxidation. Oils high in oleic acid have a somewhat similar effect in my recipes and are more stable due to oleic acid being monounsaturated. As much as I like a lot of rice bran oil in a soap, I have adjusted my recipes over time to rely more on oleic-rich oils. I now aim to have 40-50% oleic acid in the fatty acid profile of a “balanced” recipe. Due to the very hard water at my house, I keep superfat in the 2-4% range knowing that higher superfatting tends to reduce lather and enhance soap scum.
In the end, any assessment of lather quality is going to be as much influenced by things we mostly can’t control as by variables we control. The unknowns or uncontrollable variables could be undocumented qualities in the fats we use (we use average profiles for fats, but there are variations due to growing conditions, what pigs and cows are fed, processing, etc; unsaponifiables may or may not be present), user perception of what makes great soap, the type of water used for testing and how the soap is manipulated when a person is using it. The role of preference and other uncontrollable variables were apparent in a
lather experiment I did last year. After that experiment, I settled on a few base recipes that I can adjust to have more or less cleansing (coconut oil or sometimes PKO) for gentleness or more bubbles.
