Any nonvolatile liquid may be used to make soap. There is debate whether or not nutritive components are preserved after being exposed to lye, a highly corrosive, hydroscopic base (ph 13-14). In many instances some of the nutritive ingredients are saved in the final product. To ensure this some people are big on cooking soap and at the end adding nutritive ingredients such as superfat oils, herbs, decoctions and the like so the final product has most of the valuable ingredients preserved.
Beer essentially is a liquid that is comprised of about 90% water with malt, hops, yeast and ethanol held in some kind of carbonated state. The familiar aroma of beer, though varying especially among small breweries comes from the yeast, ethanol, aromatic and bitter hops plant material and hops essential oil. Beer taste is sometimes described as a blend of phenol taste/aroma produced from yeast which produces a medicinal/spicy aroma, esters which produce a fruity aroma and taste and diacetyls which produce a butterscotch or woody aroma and taste.
To create beer soap all components that may be volatile must be made less volatile without distroying the character and value of the brew. To be usable the beer has to be flat - this means it has to be devoid of carbonation but not necessarily of alcohol. While reducing beer to a flat state reduces the carbonation much of the alcohol is left intact. This does not pose a problem as the alcohol will be evaporated during processing. To get beer flat leave out in an open container for several days.
To use beer in soap, substitute the flattened beer for all or part liquid needed. As always, observe hazardous manufacturing precautions, especially since using lye and alcohol.
To prepare the beer refrigerate it before adding lye to the liquid.
Below is a sample recipe for a beer brew soap. Some folks prefer to leave a beer soap unscented as some of the beer scent is evident in the final bar. However, for those who prefer to scent this soap here are a couple pointers - hops essential oil is really good to use in beer soap! It is quite nice blended with orange or mandarin essential oil. Also any citrus and warm earthy essential oils are good such as patchouli, cedarwood, sandalwood, etc.
Beer Brew Soap
24 ounces olive oil
8 ounces coconut oil
4.49 ounces lye
8-12 ounces flattened beer
Total - 32 ounces
Follow directions for regular cold process soap substituting water with the beer. See links below for cold process directions and safety information.