“It was like gun shots ringing out from the warehouse,” said an anonymous employee of a distributor as an entire pallet of beer popped can by can. The brewery didn’t assume responsibility but instead blamed the distributor for excessive temperatures in storage.
At a retail location, distended and ruptured cans from another brewery were strewn across the floor of a window display, having fallen from their shelves. Sunlight exposure and excessive temperatures were again blamed for the misfortune. What causes exploding beer cans? Will a rise in temperature alone create enough pressure for a can to burst?
Can Integrity and Fill Volume
Carbon dioxide (CO2) gas is very soluble in beer, but as the temperature increases, gas solubility decreases. In other words, when a beer warms, it loses carbonation because the gas comes out of the beer. In a closed container, the CO2 has nowhere to go but the headspace of the can; thus the pressure increases.
But these internal pressure changes are expected and accounted for in can design. All can manufacturers supply quality standards that define buckle resistance, the ability to withstand elevated pressures and temperatures, commonly specified for beer cans at 90 psig (6.2 bar).
In addition to can integrity, accurate fill volumes are essential, not only for legal reporting, but for accommodating the increase in gas volume as it comes out of solution. The typical headspace for a standard 12 oz can is 0.47 inch (12 mm).
American brewers describe the amount of CO2 in beer as volumes. Volumes (vol) of CO2 can be thought of as the ratio of the physical volume of the gas at atmospheric pressure to the pressurized volume in beer. Levels of carbonation are very important for consumer perception because most people detect a difference as little as 0.2 vol.
As a general standard, open to stylistic interpretation of the brewer, normal carbonation values are 1.5–2.6 vol while highly carbonated beers may approach 4.0 vol, requiring specialized bottles with caged corks to accommodate the elevated pressure. With can integrity and appropriate fill volumes, the pressure of CO2 in beer at 2.6 vol and at 140°F (60°C) is about 70 psig (4.8 bar).
This is within the buckle-resistance specifications of the can. (In tunnel pasteurization systems, canned beer temperature is briefly held at 140°F (60°C) to kill any contaminating microorganisms, so the ability of a filled beer can to withstand this temperature is critical.)
The key factor here is that under normal conditions, a canned beer should withstand higher temperatures. But in the example above, if the carbonation level increases to 4.0 vol at 140°F (60°C), the pressure increases to 126 psig (8.7 bar)! Pop! The root cause of an exploding can is not the elevated temperature alone but also excessive carbonation.
Refermentation
Excessive carbonation is typically caused by refermentation by yeast or spoilage microorganisms after packaging. The beer may be packaged at 2.5 vol, but it ends up at 4.0 vol over time. Small breweries are most at risk due to the use of mobile canning units, the packaging of unfiltered, unpasteurized beer (i.e., active yeast), and/or the presence of contaminating microorganisms. There are several factors that must be controlled in the brewery to prevent refermentation.
First, and most obvious, is that all beer should be fully fermented at terminal gravity before packaging. During fermentation, a 1° Plato drop yields 2 vol of CO2! In addition, the recent trend of adding unfermented fruits and extracts to unfiltered beer just before packaging represents an extreme risk for refermentation.
Second, there is evidence that dry hopping causes refermentation. With excessive dry hopping comes the potential for “hops creep,” also called the “freshening power of hops,” where an additional drop in terminal gravity is observed after the hops addition.
This refermentation is caused by glycolytic enzymes from the hops that convert remaining, unfermented polysaccharides (dextrins) into simple, fermentable sugars. After dry hopping, it is important to ensure that the beer has reached terminal gravity before packaging.
And finally, excessive carbonation may be caused by contamination with wild yeast or bacteria. A thorough micro-program is highly recommended before venturing into canning operations.
Simple Test
Regardless of laboratory equipment, all brewers can do a simple test to check for refermentation. Simply store a selection of cans from each lot in an incubator or warm location. Over time, check for pressure and distension by simply squeezing the can by hand.
Cans are rated to withstand common beer-carbonation volumes at elevated temperatures. Exploding cans are likely due to refermentation after packaging, which creates an initial increase in carbonation made worse with elevated temperatures. Simply insisting that consumers refrigerate their product is not adequate prevention because the average consumer doesn’t understand explosion risk. The most important step to keeping cans intact and on the shelf is for brewers to prevent refermentation.