Most commercially produced beer sold in cans and bottles is pasteurized. Draft beer, on the other hand, is typically not pasteurized in the United States, which is why kegs need to be kept refrigerated at all times. Craft beer falls somewhere in between: many small breweries skip pasteurization entirely to preserve flavor, relying instead on refrigeration or sterile filtration to keep their beer stable.
The answer depends on how the beer is packaged, where it’s brewed, and how far it needs to travel before someone drinks it. Here’s how it all breaks down.
Which Beers Are Pasteurized and Which Aren’t
In the U.S., the general rule has held since at least the 1930s: bottled and canned beer gets pasteurized, while domestic draft beer does not. This distinction exists because kegs are already kept in a cold chain from brewery to tap. Cans and bottles, by contrast, may sit on a warm warehouse shelf or in a delivery truck for weeks before reaching your fridge.
Imported draft beers are usually pasteurized before shipping, since maintaining a cold chain across international supply lines is difficult. Even so, those kegs still need to be stored and served at around 38°F for proper taste and dispensing.
Craft breweries are the big exception. Most microbreweries choose not to pasteurize at all, preserving the raw character of the beer. That’s why craft beer often comes with a “keep refrigerated” notice and has a shorter window before it starts to decline.
How Pasteurization Works in Beer
Breweries use two main methods, and they work quite differently.
Tunnel pasteurization is the older approach, based on a principle that predates Louis Pasteur himself. After beer is already sealed in bottles or cans, the containers ride through a long tunnel and are showered with hot water at about 140°F for roughly 30 minutes, then gradually cooled. Because the beer is pasteurized inside its sealed container, the process is considered essentially foolproof for killing microorganisms. The equipment is large, typically around 6 feet wide by 20 feet long.
Flash pasteurization (also called HTST, for high temperature, short time) heats the beer to about 162°F, but only for 15 to 20 seconds, before it’s rapidly cooled and then packaged. This method requires a much smaller footprint and uses significantly less energy, recovering up to 95% of its heat compared to 40 to 50% for a tunnel system. The tradeoff is that the beer must be filled into containers under sterile conditions after treatment, adding a step where contamination could theoretically occur.
Pasteurization Units
Brewers measure heat exposure in pasteurization units (PUs). One PU equals one minute of exposure at 140°F. A typical lager receives anywhere from 5 to 100 PUs, though as few as 5 PUs is sufficient in most cases. Higher PU counts provide a wider safety margin but increase the risk of flavor changes.
What Pasteurization Protects Against
Beer is already a hostile environment for most bacteria. The low pH (high acidity) and the natural antiseptic compounds from hops suppress a wide range of microorganisms. But a handful of bacteria have evolved to thrive in beer anyway. Two groups, Lactobacillus and Pediococcus, are responsible for nearly 70% of microbial beer spoilage. Certain strains like Lactobacillus brevis and Pediococcus damnosus have developed resistance to hop compounds and can continue fermenting inside the bottle, producing a buttery off-flavor, sourness, haziness, and a ropy, stringy texture.
Wild yeasts can also cause problems, continuing to ferment residual sugars and producing off-flavors or excess carbonation. Pasteurization kills all of these organisms, giving packaged beer a stable shelf life of 120 days or more at room temperature. Compare that to unpasteurized draft beer, which lasts 45 to 60 days even when properly refrigerated.
How Pasteurization Affects Flavor
This is the reason many craft brewers avoid it. Heating beer promotes chemical reactions that alter its sensory profile. The most significant are Maillard reactions (the same browning chemistry that gives toasted bread its flavor) and lipid oxidation, both of which create new flavor compounds that weren’t in the original beer. Heat also accelerates colloidal haze formation, breaking the balance between proteins and polyphenols, which can make a previously clear beer turn cloudy faster over its shelf life.
Research published in Foods found that pasteurization, while excellent for microbiological stability, is “one of the main causes of changes in the sensory properties that affect the qualitative characteristics of beer.” The effects are subtle in a standard lager, but in hop-forward or delicate styles where fresh aroma is central to the experience, even minor heat exposure can dull the flavor profile that the brewer worked to create.
Sterile Filtration as an Alternative
Many breweries that want shelf stability without heat turn to sterile filtration. This process pushes beer through specialized filters with pores small enough to physically trap bacteria and yeast cells, removing them without any heat treatment. The result is a microbiologically stable beer that retains more of its original flavor and aroma.
Sterile-filtered beer can achieve at least six months of shelf life, according to research in Foods, while avoiding the costs and flavor changes associated with thermal pasteurization. In the U.S., beer that has been sterile-filtered and aseptically packaged (but not heat-treated) can legally be labeled “draft beer,” even when sold in bottles or cans. That’s the origin of the “draft style” labels you see in stores.
The limitation is that filtration can strip out some body and flavor compounds along with the microorganisms, and it’s less of a guarantee than pasteurization. Some spoilage organisms may survive filtration, giving sterile-filtered beer a shorter shelf life than its heat-treated counterpart. It also requires extremely clean packaging conditions, since the beer is no longer being treated inside its sealed container.
How to Tell if Your Beer Is Pasteurized
Labels rarely say “pasteurized” outright. Instead, look for indirect clues. If a beer says “draft,” “draught,” or “cold-filtered” on a bottle or can, it likely was not heat-pasteurized. If it requires refrigeration (sometimes printed near the date code or on the neck label), it’s almost certainly unpasteurized. Large domestic brands sold in room-temperature displays are nearly always pasteurized.
For kegs, domestic draft beer from major American breweries is typically unpasteurized, while imported kegs are typically pasteurized. Either way, kegs should be kept at 38°F. The difference is that an unpasteurized keg left warm will start developing off-flavors from secondary fermentation within days, while a pasteurized import keg is more forgiving of brief temperature swings during shipping.
Shelf Life at a Glance
- Pasteurized cans and bottles: 120 days or more at room temperature
- Unpasteurized draft (kegs): 45 to 60 days refrigerated at 38°F
- Pasteurized draft (kegs): 6 to 9 months, still best served at 38°F
- Sterile-filtered, unpasteurized: at least 6 months with proper storage
If you’re buying craft beer that hasn’t been pasteurized or sterile-filtered, keeping it cold from store to glass is the single most important thing you can do for flavor and freshness. Warm storage accelerates every spoilage process that pasteurization would have prevented.