In cold process soap, water has a single job: dissolving NaOH so the alkali can reach the oils. When that water is replaced by coffee, wine, beer or goat milk, the lye pot also receives sugar, alcohol, protein and aroma. Each of them reacts with lye in its own way: heating up, foaming over, changing colour, changing smell. This piece goes through each liquid, looks at how soap makers handle it, and asks what is left in the bar after six weeks.
When water is replaced by another liquid
In the six week piece, NaOH dissolving in distilled water releases strong heat, the solution climbs to about 80°C and is then left to cool to 38°C, the same temperature as the oils warmed to 38 to 40°C. Distilled water is an inert solvent. It brings nothing but itself, so the heat in the mixing jug comes only from the lye dissolving.
Coffee, wine, beer and milk are different. Most of each is still water, but the rest is made of molecules that can react. Sugar is present in wine, beer and milk. Alcohol is present in wine and beer, along with the carbon dioxide dissolved in beer. Protein and fat are present in milk. Coffee carries the aroma compounds of roasted beans and the brown pigments formed during roasting. All of them meet the alkali at its most concentrated, before the oils are even added.
So every liquid used in place of water brings three familiar concerns: the mixture runs hotter than needed, the mixture foams out of its container, and colour or smell shift in an unwanted direction. The sections below follow each group of substances: sugar, alcohol and gas, the protein of milk, then the aroma of coffee.
Sugar meets lye: heat and brown colour
Sugar is an extra source of heat in the reaction. Soap making guides note that a sugary liquid heats up very quickly when it meets NaOH solution, and that a batch rich in sugar overheats easily in the mould. Some beer soap makers advise against adding any further sugar to the recipe, because beer already brings enough heat for the whole batch.
Colour comes with the heat. In a hot, alkaline environment, sugar breaks down and darkens by two routes: the sugar degrading on its own, close to the caramelisation of sugar heated on a stove, and the Maillard reaction between sugar and protein or amino acids. Studies on model systems record that both reactions run faster as the pH rises. A lye solution made with milk or beer often turns yellow orange, sometimes brown, within the first few minutes. If the heat climbs too high, the sugar can scorch and leave an unpleasant burnt smell in the bar.
The general answer is to keep the heat low from the start. The liquid is chilled first, the mixing jug sits in an ice bath, and the lye is sprinkled in a little at a time rather than poured all at once. Once the batter is in the mould, many soap makers also reduce the insulation around it, so the centre of the block does not rise far above the 60 to 70°C of gel phase recorded in the six week piece.
Alcohol and gas: why wine and beer are boiled first
Wine and beer bring two things that distilled water does not: ethanol and dissolved carbon dioxide. When the lye goes in, the heat of dissolution drives both out at the same time. The gas fizzes up. Ethanol boils at about 78°C, below the temperature a lye solution can reach. The result is a solution that boils and spills out of its jug, which soap makers call a lye volcano.
So wine is usually boiled to drive off the alcohol, then cooled and chilled; guides give from a steady boil of five to seven minutes to a simmer of at least ten, and boiling removes only most of the alcohol. Beer is left to go flat, by opening the bottle overnight or by warming it gently so the bubbles escape. Some soap makers hold that letting beer go flat is not enough on its own, because the alcohol is still there, so they boil both beer and wine before use.
Boiling also reduces the liquid. The lost water has to be accounted for, because the amount of water sets the concentration of the lye solution. So guides usually boil more than needed, around one and a half to two times the amount, then weigh out the exact amount the recipe calls for.
Lê Mai's Red Wine Artisanal Soap No.801 uses red wine, Vitis vinifera, in place of part of the water in the recipe, on a base in which almond oil makes up more than 30 percent, with brown rice powder and floral blend No.801. Because the wine replaces only part of the water, less sugar and alcohol reach the lye pot than in a recipe where wine replaces all of it.
Goat milk: protein, lactose and ice
Milk is the most sensitive of the four liquids. Besides water, goat milk carries lactose, fat and protein, and Lê Mai's product description also lists lactic acid. When lye meets milk at room temperature, the heat of dissolution is enough to turn the milk orange and give off an odd smell within minutes, because sugar and protein are being heated together in alkali.
The most common method is to freeze the milk first. The milk is frozen into cubes or a slush, then the lye is sprinkled in a little at a time and stirred slowly, with the jug in an ice bath. Soap makers note that frozen milk still turns orange where it first touches the lye, but when the heat is kept low, the final colour of the bar leans towards cream or pale beige.
Another method is not to dissolve the lye in the milk at all. The lye solution is made with a small portion of water and left to cool, while the milk is blended into the oils before the two sides meet. This way the milk never meets the alkali at its strongest. In exchange, the lye holds less water, so the solution is more concentrated and needs more careful handling.
The fat in milk takes part in saponification too. It is made of triglycerides, the same kind of structure as the plant oils described on the vegetable oils page of the library, so the lye also splits it into soap and glycerin. A milk bar is therefore usually cream to beige, darker if the block goes through a hot gel phase, because the extra heat darkens the milk sugar further; soap makers who want a pale milk bar often put the mould in the fridge to avoid gel phase.
Coffee: the colour stays, the roast is harder to keep
The smell of roasted coffee is a complex mixture. The Coffee CO2 extract page in the library records that it is made of hundreds of compounds, of which only a few dozen play a major role: 2-furfurylthiol is considered the key odorant, guaiacol and 4-vinylguaiacol give smoky and spicy notes, and pyrazines give roasted, nutty notes. These substances are present in trace amounts, and many of them are highly volatile.
In the lye pot, that aroma passes through the three stages described in the piece on essential oils through saponification: alkali, heat and time. Thiols and phenols such as guaiacol turn into their salt forms in an alkaline environment, while the light fraction leaves during gel phase and the curing weeks. Soap makers report almost unanimously that strong brewed coffee used in place of water gives a dark brown bar and slightly brown lather, but that the coffee smell after curing is very faint or gone.
The colour is more durable. The brown of coffee comes mostly from melanoidins, large pigments formed during roasting that do not evaporate, so they pass through the lye pot and stay. The stronger the coffee, the darker the bar. Coffee grounds stirred in at trace add another layer: small brown particles scattered through the bar, with a medium abrasiveness, 3 out of 5 in the table on the clays and exfoliants page of the library.
Because the roasted smell has a hard time surviving saponification, a coffee soap is usually scented with a separate blend. Lê Mai's Coffee Bean Artisanal Soap No.234 works that way: concentrated coffee and Arabica coffee powder give the colour and the particles, blend No.234 Coffee Milk gives the scent, on a base that includes cocoa butter and almond oil.
After six weeks, what the bar keeps
Six weeks of curing take away most of what is volatile. The alcohol left during boiling, or escaped as the lye solution heated up. The carbon dioxide is gone. The water in the bar decreases: the six week piece records a loss of about 10 to 15 percent of the weight after four weeks, and a pH that falls from 9 to 10 at cutting to 8 to 9.
What stays is what does not evaporate. The brown of sugar darkened in alkali, the colour of roasted coffee: these remain in the bar and go nowhere. Wine holds a surprise. The red pigments of grape wine are anthocyanins, a group of pigments that change colour with pH: as the pH rises, the colour moves from red towards purple and blue, and these molecules are only stable in fairly strong acid; outside it they break down readily. Soap makers note that red wine usually gives the batter a dark tan colour.
The solids of each liquid stay as well: sugar not fully broken down, the protein of beer and milk, minerals. They do not make the bar smell stronger, but they contribute to its colour and feel. With beer there is no separate measurement, but by the same mechanism the smell of malt and hops most likely fades over the curing weeks as the coffee smell does, so the most visible thing beer leaves behind is usually colour.
Lê Mai's Turmeric and Beer Artisanal Soap No.093 uses beer in place of distilled water, on a base that includes tamanu oil, neem seed oil and turmeric powder, with essential oil blend No.093. In this bar, the pale yellow comes from the turmeric powder and the warm, rustic scent from the essential oil blend. The beer is the liquid of the lye pot, not the main source of scent.
A liquid in place of water brings sugar, alcohol, protein and aroma.
Alkali turns sugar into heat and colour, drives off alcohol and takes most of the smell.
After six weeks, what stays in the bar is mostly colour; scent is the work of essential oils.