Most keg beer from large commercial breweries is either pasteurized or sterile filtered before kegging, but not all of it. Craft brewery kegs are typically unpasteurized, and even some major brands skip the process. Whether your keg beer has been heat-treated depends on who brewed it and how they chose to stabilize it.
How Large Breweries Handle Kegs
Major industrial breweries generally pasteurize or sterile filter their keg beer to extend shelf life and ensure consistency across a massive distribution network. The most common method is flash pasteurization, where beer is heated to roughly 71°C (160°F) for about 20 seconds, then rapidly cooled before being sent to the filling line. This kills the yeast and bacteria that could cause spoilage or unwanted fermentation inside the keg.
Some large breweries use sterile filtration instead. This achieves the same goal without heat: specialized filters with pores small enough to physically trap yeast and bacteria strip them from the beer before it’s sealed in the keg. Because no heat is involved, sterile-filtered beer avoids the flavor changes that can come with pasteurization while still gaining a longer shelf life. Under U.S. regulations, beer that has been sterile filtered and aseptically filled can even be labeled “draft beer” alongside traditionally unpasteurized products.
Craft Kegs Are Usually Unpasteurized
Craft breweries and microbreweries typically skip pasteurization entirely. The goal is to preserve the beer’s original character, keeping the full range of flavors and aromas intact from brewery to glass. Some craft brewers do lightly filter their kegs for clarity, but many don’t even do that, preferring to let natural haze and residual yeast remain in the product.
This means if you’re drinking a keg from a local or regional craft brewery, it’s almost certainly unpasteurized. The tradeoff is a shorter window of freshness, which is why craft kegs tend to move through shorter, more local distribution chains and need careful temperature control from brewery to bar.
Cask Beer Is Always Unpasteurized
If you’re comparing keg beer to cask-conditioned ale (sometimes called “real ale”), the difference is clear-cut. Cask beer is unfiltered and unpasteurized by definition. It undergoes a secondary fermentation inside the cask itself, with live yeast continuing to develop flavors right up until it’s poured. Keg beer, by contrast, is filtered in most cases and often pasteurized to lock in stability. Cask beer is served at cellar temperature with natural or gentle carbonation, while keg beer is served cold under pressure with higher carbonation.
How Pasteurization Affects Shelf Life
The practical difference between pasteurized and unpasteurized kegs comes down to how long they last. A pasteurized keg stays fresh for roughly 90 to 120 days when stored at the correct temperature. An unpasteurized keg has a window of about 45 to 60 days.
Unpasteurized beer retains active biochemical components, and those components cause the beer to change over time. The rate of change is directly tied to temperature: colder storage slows aging, while warmer conditions accelerate it. This is why unpasteurized kegs need strict refrigeration throughout the entire supply chain, ideally between 33°F and 38°F (0.6°C to 3.3°C) from the moment they leave the brewery. Even during transport, the temperature should stay below 60°F. Once a keg reaches a retailer, it should be held below 45°F.
Pasteurized kegs are more forgiving. They still benefit from cold storage, but brief temperature fluctuations are less likely to cause rapid deterioration.
What Pasteurization Does to Flavor
Heat treatment isn’t invisible in the glass. Pasteurization reduces the beer’s content of phenolic acids, including ferulic acid, a compound that contributes to certain spicy and grainy flavor notes. These acids break down readily when exposed to heat. The process also causes a measurable drop in B vitamins (B2, B3, B5, and B6), though this matters more nutritionally than in terms of taste.
More noticeably, pasteurization increases aldehyde levels in the finished beer. Aldehydes are closely associated with stale or “aged” flavors, and their relative content can rise by a percentage point or more after heat treatment. Ester levels, which contribute fruity and floral aromas, tend to decrease. The net effect is subtle in a well-made lager but can flatten the complexity of more aromatic styles like ales. This is one reason many brewers, especially those making hop-forward or yeast-driven styles, avoid pasteurization even when it would simplify distribution.
Sterile filtration sidesteps these flavor issues entirely since it never heats the beer. However, it can strip out some body and haze-forming proteins, which is why certain brewers prefer to skip both methods and simply keep their kegs cold.
How to Tell if Your Keg Is Pasteurized
There’s no universal label requirement that tells you whether a specific keg was pasteurized. A few clues can help. If the beer is from a major domestic brand and widely distributed, it’s likely been pasteurized or sterile filtered. If it’s from a small or mid-sized craft brewery, it’s probably unpasteurized. Beers labeled “real ale” or “cask-conditioned” are always unpasteurized. Some craft breweries note “unpasteurized” on their packaging as a selling point, but many simply don’t mention it either way.
If freshness matters to you, the best indicator is the date on the keg. Regardless of pasteurization status, beer closest to its packaging date will taste the most like the brewer intended. For unpasteurized kegs, that freshness window is about six to eight weeks, so checking dates and asking your local bar about their keg turnover can make a real difference in what ends up in your glass.