We get this one at nearly every stop on the Beer Mile: someone spots a hand pump next to the keg taps and asks what the actual difference is. It's a fair question — "cask" and "keg" get used loosely, and plenty of drinkers assume cask just means "warm and flat" and keg means "cold and fizzy". Neither is quite right. The real difference comes down to how the beer finishes its fermentation and how it's served, and once that clicks, a lot of pub and taproom bars suddenly make more sense. Here's the honest, practical breakdown, plus where to taste both side by side in London.
The short answer
Cask ale is unfiltered, unpasteurised beer that finishes a natural secondary fermentation inside the cask it's served from, is stored at cellar temperature and is pulled to the glass by hand pump or gravity, with no added gas. Keg beer is filtered (and often pasteurised), force-carbonated with CO2 or a nitrogen mix at the brewery, then served cold and pushed to the tap under gas pressure from a sealed keg. Both can be ale, and both can be excellent — they're two different serving traditions, not a quality ranking.
How cask ale actually works
Cask ale — often called "real ale" when it meets the strict definition — is delivered to the pub or taproom still lightly fermenting. The cellar team "conditions" it: venting the cask, letting the last of the yeast settle, and tapping it a day or two before it's served. There's no added CO2 or nitrogen; the gentle natural carbonation comes entirely from that final fermentation in the cask. It's then pulled through a beer engine (the hand pump you see on a bar) or served straight by gravity from a stillage, and it's kept at cellar temperature, roughly 11–13°C — cool, but nowhere near fridge-cold.
Because it's alive and unfiltered, cask ale has a shelf life once it's tapped, usually a few days, which is why a well-run cellar and steady turnover matter so much to how it tastes.
How keg beer actually works
Keg beer is finished, filtered and (in most cases) pasteurised at the brewery, then sealed into a pressurised keg with a set level of carbonation already locked in. At the bar, CO2 or a nitrogen-CO2 blend pushes the beer up through the line to the tap — the gas is doing the work of moving the beer and topping up its fizz, not fermenting anything further. Because it's sealed, filtered and chilled, keg beer is far more stable than cask: it travels well, keeps for longer once connected, and pours consistently pint after pint. It's also served noticeably colder, typically 3–8°C depending on the style.
Carbonation and mouthfeel
This is where the two are easiest to tell apart in the glass. Cask ale's natural conditioning gives it soft, fine carbonation — a gentle prickle rather than a fizz — and a rounder, fuller mouthfeel. Keg beer's forced carbonation is typically higher and coarser, giving a crisper, livelier feel on the tongue that suits hop-forward or very cold styles. Nitro-keg beers (Guinness being the best-known example) sit at the extreme end of this — nitrogen produces those famously tiny, creamy bubbles and a thick, cascading head, quite unlike either standard cask or CO2 keg.
Flavour differences
Serving temperature and carbonation both feed directly into flavour. Cellar-temperature cask ale releases more aroma and lets malt sweetness, yeast character and subtler hop notes come through — it's why traditional bitters and milds are almost always served this way. Cold, highly carbonated keg beer suppresses some of that aroma and flavour, which suits crisp lagers and punchy, bitter IPAs that are designed to hit hard and refresh rather than unfold slowly. Neither approach is a fault; a beer brewed for keg and served on cask (or vice versa) would usually taste out of balance, because most breweries design a beer's carbonation and temperature around how it's meant to be served.
Cask vs keg at a glance
- Carbonation: cask — natural, from secondary fermentation in the cask; keg — forced, added CO2 or nitrogen mix.
- Temperature: cask — cellar temperature, roughly 11–13°C; keg — chilled, roughly 3–8°C.
- Dispense: cask — hand pump or gravity, no gas pressure; keg — gas pressure pushes the beer to the tap.
- Filtering: cask — unfiltered and unpasteurised; keg — usually filtered and often pasteurised.
- Shelf life once tapped: cask — a few days; keg — considerably longer, thanks to filtering and sealing.
- Typical styles: cask — bitters, milds, traditional ales; keg — lagers, IPAs, stouts and most modern craft beer.
Is one actually "better"?
Honestly, no — they're suited to different beers and different moods. A well-kept cask bitter, served at the right temperature with proper natural conditioning, is one of the most rewarding pints in British brewing; a hazy keg IPA served ice-cold and full of hop aroma is a completely different, equally valid pleasure. The thing that actually ruins either is poor cellar-craft: a cask that's been left too long once tapped, or a keg line that hasn't been cleaned. Judge the individual pint, not the dispense method.
A quick myth-check
- "Cask ale is warm." No — cellar temperature (11–13°C) is cool. It's simply not fridge-cold like keg lager.
- "Cask ale is flat." A well-kept cask has gentle, natural carbonation. Genuinely flat cask usually means it's past its best.
- "Keg means lager." Not at all — most modern craft beer, including plenty of ales and IPAs, is served on keg. Keg describes the dispense method, not the style.
- "Real ale and cask ale are the same thing." Almost — "real ale" is the strict term for cask-conditioned beer with no added gas and no filtering; it's effectively a synonym for proper cask ale.
Try both side by side on the Beer Mile
The easiest way to actually feel the difference is to taste it back to back, and that's exactly what a run along the Bermondsey Beer Mile lets you do — several of the taprooms pour both hand-pulled cask and cold craft keg lines at the same bar. Our guided Bermondsey Beer Tasting Tour (£49pp, 2–3.5 hours, tastings included) takes you through four independent taprooms with a guide on hand to point out what's on cask versus keg at each stop, so you get a genuine comparison rather than guessing from the pump clip. We meet at Southwark Brewing Co, 46 Druid St, and we've been running the tour since 2014.
If you want the science behind why yeast and temperature matter so much in the first place, our lager vs ale explainer covers the fermentation basics that sit underneath all of this, and our what is craft beer guide unpacks the independence and flavour side of the wider scene.
What to order if you're comparing the two
- Start on cask with a traditional bitter or mild if it's on — you're looking for that rounder, softer mouthfeel and cellar-temperature aroma.
- Move to keg with a pale ale or IPA from the same taproom, if available, and notice the extra fizz and cooler bite.
- Try a nitro pour if you spot one — the tiny bubbles and creamy head are a completely different sensation to both standard cask and CO2 keg.
- Ask your guide (or the bar staff) which is which before you taste — half the fun is confirming what you'd guessed from the glass alone.
What is a taproom pouring, anyway?
If you're new to taprooms generally, it's worth reading our what is a taproom guide first — it covers how a taproom bar differs from a standard pub, and what to expect when you walk up and see a wall of both hand pumps and keg fonts for the first time.
Make a full day of it
Pair a Beer Mile afternoon with our free self-guided heritage walk through Southwark's brewing history, or round the day off with our Saturday cheese, meat & beer tour for a fuller comparison of styles and serving methods alongside good food.
Gift the comparison
If you know someone who'd enjoy working out cask from keg glass in hand, our gift vouchers are delivered instantly by email and never expire, so they can pick their own date to come and taste the difference for themselves.