In our recent review of Charbay’s Double Aged Rum, reader Taylor Fox inquired as to the point of the first round of that double aging. The rum spends five years in stainless steel tanks, then three years in oak barrels.

Well, what’s the point of that? We’ve long been told that stainless steel tanks are “neutral” vessels, and don’t impact the spirit held inside. But is that an oversimplification?

Let’s look first at how stainless steel is typically used in distilling: After primary aging has concluded.

For example, Buffalo Trace’s Sazerac Rye 18 Years Old was long held in a stainless tank after it was aged in barrel, and some of it was released every year until the tank ran out. This was done specifically because the whiskey wouldn’t change (much) over that time. “Stainless tanks have traditionally been a good way to store our whiskeys,” says Buffalo Trace Master Distiller Harlan Wheatley. “The reasons for tanking the product is to stop the aging process in the barrel. The stainless tanks appear to only help the whiskey by settling down and mellowing the flavors, all through the oxidation process of sitting in the tank we presume. But overall the flavors appear to be very stable for years.”

Sure enough, Sazerac 18 was very consistent over those years it was tanked, but it did change a little each year, which made reviewing the spirit annually a lot of fun.

Tanking a finished product makes logical sense, but what about resting in stainless before a product is transferred to a wood barrel? Why would you waste five years with a rum lying a steel tank when it could be gaining complexity in oak? Or getting to market five years earlier? Stainless steel resting is common with white spirits like pisco and tequila (and some spirits that are later aged, like the Charbay rum). So why do they do it?

Let’s try to get to some kind of answer here.

Here’s a simple answer from Laurence Spiewak, co-founder of Suerte Tequila, whose blanco spends a few months in stainless steel before bottling. “We rest our blanco in stainless steel tanks before we bottle it to allow it to breathe, become more full flavored and full bodied. It’s the same philosophy as aerating a wine before it’s served. Oxygen causes changes that we see as beneficial to the end product.”

That’s a pretty common answer I heard in my inquiries, and is essentially the same as what Wheatley said about tanking finished whiskey. Oxygen can be good in moderation, and it helps the spirit to “breathe” before it’s locked up in an airtight bottle.

For a deeper explanation, let’s talk to Charbay’s Marko Karakasevic directly. He says, “Aging in stainless for 5 years let the sugar cane flavors and body develop by themselves without any outside influences. After the stainless aging, we then aged it in oak for 3 years to add to the flavor profile and let the barrel add body to the rum. As a result, the rum was not over-oaked, and all the top notes of the French oak were now joined with all the rum notes. We then bottled it without any filtration, and let it sit for another year before releasing it. This gave it even more time to rest and allowed the entire flavor and body to balance out and line up in its own unique pattern.”

Marko’s wife Jenni (who handles marketing for Charbay) adds, “Spirits (just like wine in the bottle) continue to soften and ‘come together’ when aged in neutral vessels. It’s a way of aging without adding flavor, so the focus can be more on the spirit than the barrel.”

In the world of pisco, the situation has another little twist. Pisco by law has to be rested in a neutral vessel for a minimum of three months before bottling. Stainless steel tanks is common, but so are glass, ceramic, and even plastic vats — all neutral vessels, designed as such because pisco is legally defined as a “pure” grape distillate. The difference between rested and non-rested pisco can be striking. “Pisco coming out directly from the still is called chicharrón, which is a rough product that differs greatly from the rested product,” says Carlos Moreno, overseas manager of Cuatro Gs, a major pisco producer.

“From a chemical standpoint, resting encourages the introduction and integration of new molecules to align and come into orbit with each other, ensuring that everything is in sync,” says Rony Matchus of La Caravedo Pisco. “Resting allows the radicals to calm down and harmoniously integrate with the other components without the influence of wood. It’s a ‘feel’ thing. It’s the same reason you let a steak rest after you cook it and before you cut it.”

“Resting pisco allows the fatty acids and character of pisco to assemble in harmony,” adds Moreno.

Henry Preiss, of the famed Preiss Imports, wraps it up perhaps best, saying, “Much like some recipes taste better the second day, spirits are often rested to let all the various components fully come together. The first spirit off the still is jumpy and often aggressive, and after a period of resting from days to months it rounds out and takes on a different taste and even aroma.”

So there you have it. Stainless steel resting isn’t about the container, and that’s by design. Rather, it’s about the large quantity of liquid inside reacting with oxygen and with itself — and hopefully evolving into something better along the way.