Properly cleaning stainless steel in a distillery can be a difficult task. Different substrates and soil loads can often make it hard for distillers to know which method of cleaning is right for them.
The relative ease of cleaning stainless steel is one of its great advantages. Yet knowing how to do it correctly can help ensure that it lasts long and rewards your investment. In previous articles, we’ve looked at how to look after your stainless steel, how to avoid corrosion and monitor for it, and the different types of stainless and what makes them work.
Here, we’ll look in more detail at the most important way that distillers interact with their stainless: everyday cleaning.
The first method—and by far the most common—for cleaning stainless steel in distilleries is chemical cleaning. For this, we use specially formulated chemical cleaning agents to remove biological solids and mineral buildup on stainless steel. There are hundreds if not thousands of different chemical cleaning agents available to distillers. Rather than get into specific products here, we’ll focus on the two distinct categories found in most distilleries.
Alkaline Cleaners
The first category of chemical cleaners is known as alkaline cleaners—so named because they all have a pH above 7.
Alkaline cleaners are most effective at breaking down biological contamination, such as proteins and fats, that can build up on stainless steel. Using them is often the first step in a cleaning regimen because of their ability to remove large soil loads. There are three distinct types of alkaline cleaners: caustic, noncaustic, and oxidizing.
Caustic cleaners are the most-commonly-found alkaline cleaners in distilleries, and they typically contain either sodium hydroxide or potassium hydroxide—both strongly alkaline. These cleaners often require high temperatures and have pH ratings above 12, which makes them dangerous to handle. However, they’re incredibly effective for high-solid loads, and they’re also typically some of the cheapest cleaners available.
Noncaustic cleaners, meanwhile, are formulated with less-alkaline chemicals that allow for easier handling and lower-temperature operations. Noncaustic cleaners also often include chelating agents that can help reduce beer stone and other mineral buildup. The only real downside to noncaustic cleaners is that they can be expensive, and it often requires greater concentrations to be effective. So, distillers tend to use them only when conditions require it.
Finally, oxidizing cleaners use oxidizers such as hydrogen peroxide or sodium percarbonate to break down organic residues and other contaminants. These cleaners are very good at breaking down troublesome organic matter such as biofilms, but they’re not always effective by themselves. Thus, distillers often use them in conjunction with caustic and noncaustic cleaners, to “supercharge” their cleaning power. This can be an effective way to reduce overall chemical usage.
Acid Cleaners
This second category of chemical cleaners have pHs below 7.
Acid cleaners are often made from nitric acid, phosphoric acid, or sometimes a blend of both. These cleaners are effective at removing rust and free iron, something with which alkaline cleaners tend to struggle.
They’re also vital in preventing mineral buildups such as beer stone, and they can help to re-passivate stainless steel between uses. That makes them a central part of many stainless-steel cleaning regimens.
Effective Chemical Cleaning
There are several different ways that distillers can effectively clean stainless steel. We look at three approaches here.
Alkaline, then Acid
While there are different ways that distillers can effectively clean stainless steel with chemicals, most distillers prescribe a two-step approach: first alkaline, then acid. In this method, distillers first use alkaline cleaners to remove biological matter, then they rinse the stainless with water before adding acid cleaners to remove rust or other nonbiological contamination and ensure cleanliness. This is an effective method for keeping stainless pristine, but it does use a considerable amount of chemicals. That’s why many distillers have experimented with reducing the frequency of their acid rinses because mineral buildups and rust are less of a concern than biological contamination.
Regardless which regimen you use, it’s important to consult with your chemical suppliers to determine correct dosing and handling.
Steaming
Another common method that distilleries use to clean and sanitize stainless steel is steaming.
As the name implies, this method involves exposing stainless steel to clean, low-pressure steam for an extended period of time—often 30 minutes or more. This exposure eliminates bacteria on the surface while loosening debris and other contaminants so that they can be easily rinsed off with a hose.
Because of the risk of over-pressurization, you should use steaming only for surfaces and vessels that are open to the atmosphere. For closed vessels, excess steam can be released through a pipe that allows the water vapor to escape—away from any people. For open-top vessels, it’s good practice to stretch heavy-duty cloth sheets over the openings before introducing the steam. That increases residence time while reducing the amount of hot vapor that can escape into the production space.
Although steaming can be effective, it does have some downsides. First and foremost is that steaming, unlike chemical cleaning, isn’t effective at removing corrosion or heavy solid loads. The high temperatures can be dangerous for staff and can also lead to heat stress on the metal. Finally, producing the amount of clean steam to properly clean stainless steel is energy-intensive and expensive. That’s why steam-cleaning is normally found only in larger distilleries with dedicated boilers capable of producing large amounts of clean steam.
Manual Cleaning
The last type of cleaning involves physically scrubbing and polishing stainless steel to remove dirt and grime.
Although manual cleaning may sound basic and somewhat obvious, it’s important to mention because it plays a role in many stainless-steel cleaning regimens: Manual methods can quickly and efficiently remove heavy solid loads that would be difficult and expensive to remove via chemical or steam cleaning alone. Furthermore, because manual cleaning relies mostly on physical exertion rather than chemicals and utilities, it’s inherently more eco-friendly—something that’s increasingly of interest to distilleries of all sizes.
How you go about manually cleaning your stainless depends greatly on your equipment and setting. Yet there are some core principles that all distillers should consider.
First, as explained previously, you should never use hard or abrasive instruments on stainless steel. Nor should you enter stainless-steel tanks with standard boots—these risk damaging the passivated layer that keeps stainless steel effective.
Second, if you’re using chemicals in conjunction with a manual cleaning regimen—perhaps in a dip bucket for brushes—it’s important to carefully choose the strength and type of chemical both for effectiveness and safety. (For example, cleaner that isn’t effective below 140°F/60°C cannot be safely handled.)
Finally, and most importantly: You should always evaluate manual cleaning methods for both effectiveness and safety. Quickly hand-scrubbing a troublesome patch of dirt might be the fastest way to clean the inside of a tank, but that doesn’t mean you should risk entering a confined space without proper safety precautions. Safety is always a priority.
While manual cleaning might seem mundane and ineffective compared to chemical and steam cleaning, don’t forget that it’s a valid option that can make your distillery more eco-friendly and save money. Indeed, in some parts of the world—such as Japan—there are distillers who keep their equipment clean and their facility infection-free using only manual cleaning and, occasionally, the judicious use of dish soap.
Measuring the Effectiveness of Your Cleaning
Regardless which method—or, more likely, which combo of methods—you choose to clean your stainless, it’s critical to be able to measure a cleaning regimen’s efficacy.
There are many ways to do so. For most distillers, a simple visual inspection paired with a pH test of the rinse water is enough to ensure cleanliness and a lack of residual, harmful cleaning chemicals. But if you’re worried about lingering infections, a visual inspection is not enough. Instead, you should conduct something known as an ATP test.
ATP, or adenosine triphosphate, is a chemical produced by all living organisms, and it’s essential for energy transfer. ATP can exist both within and outside of cells, and the quantity of ATP in a given sample is directly related to the amount of biological activity within that sample. When exposed to enzymes known as luciferases, ATP undergoes a bioluminescent reaction, causing it to give off electromagnetic radiation—in other words, light. You can measure that light by using a specialized device known as a luminometer.
The brightness and intensity of the light that this device detects effectively measures how much ATP is present on the surface—and, thus, whether it’s free of infection or needs further cleaning.
Passivation
As we discussed previously, the passivated layer of stainless is a thin layer of chromium oxide that allows stainless steel to resist corrosion, and it’s crucial in maintaining cleanliness.
Although stainless steel naturally creates a passivated layer over time, natural passivation can be time-consuming and uneven. Therefore, it’s sensible to use chemicals to ensure complete and even passivation.
Like all other types of cleaning and chemical applications, there are a number of different ways to passivate stainless steel. The most common method is to use a nitric-phosphoric acid blend to thoroughly clean the surface before introducing an oxidizing agent to encourage the creation of chromium oxide.
That method is highly effective, but it does require the handling of strong chemicals. For distilleries with restrictions on the amount and types of chemicals that can be used, there’s a second method that uses citric acid—much less dangerous—to achieve similar results. The only downside is that the chromium oxide layer created by this method isn’t as thick, and it takes more time and effort.
Regardless which method you use, passivation is an important part of caring for stainless steel and ensuring its longevity. If your stainless steel isn’t passivated, you’ll never be able to fully clean it—no matter which cleaning method you use. You should passivate any brand-new stainless equipment before using it, and you should occasionally passivate older equipment, too. Otherwise, you risk contamination as well as damage to the stainless steel’s structural integrity.
There aren’t many things worse than discovering that an otherwise effective cleaning regimen was rendered useless by a piece of equipment that isn’t passivated.
As a distiller, one of your chief responsibilities is to choose the right cleaning regimen for your stainless steel. A good cleaning regimen is one that safeguards against contamination while ensuring that you realize all the benefits that stainless steel has to offer.
Cleaning methods don’t have to be complicated, but they must be effective. It’s important to routinely evaluate your cleaning regimens to determine whether they’re achieving your goals. If not, it may be time to re-evaluate.