That's what caustic is. Sodium hydroxide is the core of Project Mayhem's soapmaking-cum-explosives-making operation, and it's also at the heart of our cleaning processes here at the brewery. Tyler uses powdered lye; we use liquid caustic detergent made of sodium hydroxide (NaOH), potassium hydroxide (KOH), and water (H20).
When water molecules split, they form hydrogen ions (H+) and hydroxide ions (OH-). On the pH scale, a solution with more hydrogen ions than hydroxide ions is an acid, and the converse is a base. Caustic is a strong base. The hydroxide ions that are attached to sodium or potassium atoms in our caustic are essentially carriers for readily available, ready-to-react electrons. This is the opposite of a strong acid, but chemicals in both categories are extremely volatile. The pH scale is logarithmic and ranges from zero to fourteen. That means a solution with pH of ten, like toothpaste, actually has a hundred times fewer hydrogen ions relative to hydroxide ions than a solution with a pH of twelve, like ammonia. Pure distilled water is the benchmark for "neutral" on the scale at seven.
To clean a stainless steel tank, it's first important to have a well-passivated, food-grade stainless steel surface. Not all stainless is equal; there are many different grades that are visually indistinguishable and expensive to test, so it's important to have a trustworthy supplier. The first step in our caustic cycle is to rinse the inside of the dirty tank with a hose, trying to break off as much krausen and hop debris as possible. Fermenters tend to be dirtier than brite tanks because of dry-hopping, the intense foaming that happens during fermentation, and the amount of solids we coax out of suspension through fining and cooling the tank. After rinsing, we run hot water through the spray ball in the tank to heat it within the operational range of our caustic (140-160 degrees). We need to be sure to purge any CO2 from the tank before or during this time, because introducing caustic to a CO2 environment could create a vacuum strong enough to implode a stainless tank.
Our liquid caustic is diluted in one barrel (thirty-one gallons) of hot water, creating a cleaning solution with a pH of about twelve. This solution has plenty of electrons to donate, which bond to particles on the inside of our tanks and break down the matter that's stuck to the stainless. We use the tank as its own cleaning reservoir, attaching a hose from the tank outlet to the pump inlet and from the pump outlet to the cleaning arm, which terminates with a spray ball inside the top of the tank. We run the cleaning cycle for thirty minutes at around 150 degrees. This doesn't sanitize the surface of microbes; that's a different process. The cleaning cycle is only meant to remove soil from the surface of the steel. Once that surface is clean and rinsed, then we can use sanitizer to be sure our beer is going into a microbe-free environment to prevent spoilage and off-flavors.
The sodium and potassium from the caustic form salts that rinse away. Over time, however, some of these salts can cling to the stainless leaving a white film that is difficult to remove. We're able to do so by running an acid cycle on the tank every few months, which also has the benefit of passivating the stainless surface with a layer of passive chromium oxide, which does not bond with other chemicals and inhibits rust. Homebrewers tend to use less abrasive and volatile oxygen-based cleaners. Think Oxi-Clean without the scent and additional chemicals.
This is the fourth post in my “What’s That Thing Over There?” series in which I reveal the extraordinary and mundane of brewery miscellania. In my last three posts, we talked about how beer is like sausage and legislation, got the skinny on the troll that lives under the brewdeck, and proved how reading CO2 levels can make or break you. Next time, who knows? The Thing you’re wondering about may be on deck. Just please, stop asking our poor bartenders what that damn thing over there is.