For the past five years, canned cocktails have been one of the strongest forces shaping the alcohol industry.
Ready-to-drink (RTD) spirit-based products have posted double-digit dollar sales growth the past two years, up 58 percent in 2022 and 47 percent in 2023, according to data from Bump Williams Consulting. As the year comes to a close, spirit-based canned cocktails have grown to almost 9 percent of overall spirits sales in the United States. Even while these white-hot growth rates are cooling slightly as the market matures, new brands continue to enter this space.
Yet the barriers to entry—particularly when it comes to packaging—can’t be discounted. Most beverages will erode bare aluminum; it’s why can liners exist. However, canned cocktails—thanks to their delicious alchemy of distilled spirits, acid, and sometimes salt—can pose a more significant risk of corrosion. Not all cans are appropriate to hold them, and beverage companies must vet their can-liner technology carefully. This is particularly true for high-strength products: anything approaching, at, or above 14 percent ABV.
“Our suspicion is that higher ethanol concentrations could induce liner swelling and/or degradation over time,” says Matthew Sheehan, a graduate student conducting research in Cornell University’s College of Agriculture and Life Science. “This leads to a greater degree of aluminum-beverage contact and therefore corrosion and other undesirable reactions.”
Sheehan, like any good scientist, is careful to point out that his lab’s can-corrosion research is ongoing. It’s only within the past few years that products such as kombucha, sour beer, wine, and cocktails have been canned in any significant volume.
“Because of the increasing popularity of the can as a high-quality and sustainable beverage package, there are more types of beverages going into cans than ever before,” says David Racino, cofounder and CEO of American Canning. “We’re testing these [packages] in ways they’ve never been tested before.”
Examining how cans and liners perform with novel beverages—particularly over months, not days—takes time. Yet as a result of research by can manufacturers and labs like the one at Cornell, some best practices have already emerged.
Think Beyond pH
First, some myth busting: pH is not public enemy number one when it comes to can corrosion. (If pH were the only issue, soda’s 2.5 to 3.5 pH would regularly dissolve its cans.) Yes, pH does correlate with acidity, but dissolved acids—their overall levels as well as their particular types—are a much more important factor when canning hard-to-hold beverages such as cocktails.
Understanding the raw level of titratable acidity is step one; step two is drilling down to what types of acids are present in a particular cocktail.
Determining titratable acidity requires some math, but it should be simple enough for most distilleries to calculate in-house; otherwise, labs will analyze it for $10 to $15, says Cornell professor and associate chair of food science Gavin Sacks. Research at the Cornell lab shows that corrosion issues—measured as dissolved aluminum—arise after 24 weeks, when titratable acidity is greater than 6 grams per liter.
Lactic, acetic, citric, and phosphoric acids all have applications in cocktails. So-called volatile acids—lactic and acetic—are more corrosive than other typical beverage acids, such as citric and phosphoric. The precise reason isn’t completely understood, but researchers suspect that it’s connected with fact that lactic and acetic acids exist in their natural state (neither positively nor negatively charged). A charged form of an acid likely has a hard time permeating a can’s nonpolar liner.
So, Cornell researchers have found that the best predictor of whether a beverage will do serious damage to its can is not the total amount of lactic or acetic acid, but the amount that exists in neutral form. Levels of neutral acetic and lactic acids are thus critical data points when canning cocktails. If nothing else, understanding a beverage’s neutral lactic and acetic as well as total acid levels will get a beverage maker far in determining their risk for can corrosion.
But those are not the only factors in can corrosion: Beverage makers must also watch for corrosion caused by chloride and other halides (present in teas and kombuchas), copper ions, ethanol, and sulfur dioxide (present in some wines, ciders, fruit juices, and RTD cocktails). In short, makers of higher-alcohol canned cocktails should be especially careful to test their particular combinations of ingredients.
“It’s the acid, not the pH,” says Kevin McCabe, founder of Double Strand Consulting and a former brewery quality-assurance manager. “But it’s also a mix of other things: It’s the acid plus the alcohol, the mix of different acids in combination, and the presence of certain flavoring agents that make things troublesome. That’s where it gets tricky.”
Look Closely at Liners
Can liners exist to keep a beverage from coming into contact with an aluminum can. Cornell’s Sheehan says most of the issues with can corrosion in hard-to-hold beverages are suspected to be due to channels or pores in the liner that allow the beverage to interact with the aluminum. Once a liquid breaches the liner and touches that aluminum, beverages with standard pH (less than 4) will begin to corrode the can.
Corrosion is responsible for multiple types of problems. One is staling, or accelerated oxidation, because of a loss of the hermetic seal. Another is tainting, or the formation of undesirable flavors resulting from aluminum or other can components after corrosion occurs.
While “non-aggressive” beverages can be packaged in cans with liners as thin as 1 to 2 µm (micrometers), the Cornell lab recommends hard-to-hold beverages be packaged in cans with thicker liners. Researchers also suggest makers of RTD cocktails shy away from liners made of acrylic in favor of those made with epoxy. That’s because acrylic liners are no match for the corrosive power of sulfur dioxide, particularly at higher alcohol concentrations. (Another note on sulfur dioxide: Molecular sulfur dioxide above 0.5 mg/L is highly corrosive and will generate hydrogen sulfide, responsible for a rotten egg smell.)
“We’ve generally seen worse performance for acrylic liners than epoxy-based liners—BPA and BPA-NI-Epoxy—with 10 percent ethanol solutions,” Sheehan says.
He notes that the lab has typically seen comparable performance between BPA and BPA-NI-Epoxy liners, but there is considerable variation in performance among different brands or producers of those liners. Much of that is attributable to what’s going on in the headspace of a can, which is the most difficult part to evenly cover with a polymeric liner. How well-lined a can’s headspace is depends on the can’s manufacturer and choice of liner, something canned cocktail makers should pay particular attention to.
What’s Ahead?
As noted earlier, research into can-liner technology is ongoing. In particular, both the Cornell lab and American Canning are in the midst of testing a new type of liner called aTULC. The technology is notable because aTULC cans are made from a pre-coated aluminum sheet, rather than the liner being sprayed onto the can after formation. The laminated aTULC liner coats both the interior and exterior of the can, reducing the risk of secondary corrosion (i.e., when liquid from a busted can drips onto the exterior of another and eats away at it from the outside). So far, aTULC liners appear to be effective for use with hard-to-hold beverages, but research is not yet conclusive.
Work is also being done to find out whether can liners could potentially stop corrosion before it starts via “anticorrosive beverage additives.” Corrosion inhibitors such as modified starches, organic amines, and phosphates are used in other applications; however not all are food safe—some, in fact, are toxic. The Cornell lab has received a grant from the U.S. Department of Agriculture and the National Institute of Food and Agriculture to study the use of anticorrosive additives in hard to hold beverages and the efficacy of liner materials made from anticorrosive ingredients.