You may have accidentally left a bar of soap sitting around for a few months or maybe even a year or more and noticed it doesn’t quite smell as good as it used to. This might be especially true for organic or handmade soap. You might be wondering if soap can go bad like food in the refrigerator or if it can expire like old medication or other personal care products.
The short answer is no: if left undisturbed, bar soap remains soap for years. It should still be just as effective at cleaning no matter how old it is. However, certain changes in a bar of soap make it smell less good and seem less pleasant to wash with. As soap gets older, there are two main ways it deteriorates. The first way is that the fragrance evaporates out of it, and the second way is that some of the oils may go rancid.
Why doesn’t old soap smell as good as it used to?
It seems obvious that the scent evaporates out of a bar of soap as it ages, but have you ever wondered why that is? The most common natural scent for bar soap is essential oil. Another name for essential oil is “volatile oil.” “Volatile” means that it has the tendency to vaporize. Essential oils are distilled in a similar way to alcohol because they vaporize at a lower temperature than water. The particles of vaporized oil are what you smell when you detect the pleasant aroma of a bar of soap. But of course, there is only so much essential oil in each bar, so if you wait long enough, all the essential oil will eventually evaporate out.
Can I still use soap with no essential oil?
There is one piece of good news about soap and essential oils. As soapmakers, we often take old inventory for our own personal use. On many occasions, I’ve noticed the old soap bars don’t smell like much, but as I use them, and as the bar shrinks, they begin to smell more strongly again. I am guessing that in these cases, the essential oil has only evaporated from the outer layers of the soap. So as you use the soap, and these outer layers are washed away, you expose the inner core that still has plenty of scent left. So if your soap doesn’t smell quite as strongly as it used to, you might as well take a chance and use it anyway. You might be pleasantly surprised after a few showers.
Does organic soap go rancid?
Another factor in soap going bad is rancidity. Rancidity is a process when, over time, oils break down or oxidize. Rancidity isn’t the same as food spoiling, such as chicken salad left in a hot car for a week. When food spoils in this way, it’s because of bacterial growth, and if you eat it, it will make you sick immediately! If you eat rancid oils, they won’t make you sick in the same way, but also, they are not terribly good for you. For one, as the oils break down, so do most of the vitamins. For another, rancid oils can contribute to oxidative stress, which accelerates the aging process. And finally, when a compound slightly damages cells like this , it increases the risk of cancer. Fortunately, rancid oils smell terrible, so you wouldn’t want to eat them in the first place.
I think I accidentally used rancid soap. Am I going to get cancer?
This is sort of like winning the lottery. If you just buy one ticket , your chances of winning the lottery are smaller than getting hit by lightning. Each time you’re exposed to a harmful compound, it’s like buying a lottery ticket (for the f-ed up lottery.) So you need to buy a lot of them, for example, smoking several times a day for many years. If you use rancid soap, it’s unlikely the rancid oil will soak through your skin and make it to the inside of your cells where it can damage your DNA. And even after that, the soap lather only sits on your skin for a minute or two before it’s washed off. This is the same reason why we don’t make cannabis soap. Just make sure you don’t eat rancid soap regularly.
Is organic or handmade soap more likely to go bad?
It’s not that big of a stretch to assume that soap made out of natural or organic ingredients might not last as long because natural or organic ingredients don’t last as long as synthetic, or because we don’t include any preservatives, etc. This is sort of true, for a couple of reasons I have already talked about. For one, essential oils only last for so long. Another reason has to do with a common practice in natural soapmaking called “superfatting.” This is when we use a recipe that leaves a little bit of oil in the soap bar that doesn’t react with the lye. Superfatting tends to make a very gentle soap bar that is pleasant to wash with. But it is also an added safety measure that ensures all of the lye is used up in the soapmaking process and none remains in the soap bar. So it is this superfatted bit of leftover oil that goes rancid first.
Are certain oils more prone to going rancid than others?
Yes. Polyunsaturated oils such as corn, canola and soybean oil go rancid more quickly than others. Fortunately, monounsaturated oils such as olive and avocado oil do not go rancid as quickly and behave pretty much the same way in a soap formula. We use olive oil for this portion of our soap formula, although we weren’t thinking about rancidity when we decided on a recipe! If you want to read more about why we use certain oils in a soap recipe, you can read our post on base oils.
How can you tell if the oil in soap has gone rancid?
You can tell if soap has gone rancid because it has a stale or acrid smell. It also develops spots.
What can you do with an old bar of soap?
A lot of soapmakers might tell you to throw out your stinky, spotted soap and buy new because they want you to buy more soap. But I say use it! Now, I’m guessing if you’re shopping for organic soap you’re probably very conscious of what you put in and on your body. I’m not saying to use it to wash yourself! Use it to wash your car. Or your bath tub. But probably not the dog. Or use it to pre-treat stains in your clothes. Maybe to wash the dishes, that is, if…you’re feeling lucky.