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If you're looking to buy barley for an all-grain batch of beer, you'll probably come across the phrases "six-row" and "two-row." What do they mean?
These words refer to the way the barley kernels grow on the shaft of the plant. In two-row, they're... lined up straight on either side of the shaft, just like in the first picture. In six-row, they're in six much more closely spaced lines, like in the second picture.
So What's the Difference?
Pretty much everywhere but North America, it's the difference between beer and animal feed. Two-row is used for brewing, while six-row is used to feed livestock.
In North America, it's a different story. Barley is native to Europe, and when European settlers brought it to America, they found six-row to be much more suited to the growing climate. Six-row production took off, and North American beer styles adapted to it. A lot of North American beers today use both two- and -six-row barley, but it's really the only place you'll see six at all.
So why is two-row preferred by the rest of the world? There are a few main reasons. For one, the kernels in two-row barley tend to be bigger and more uniform. Since the kernels grow on opposite sides of the shaft, they have plenty of room to get big and tend to be the same size. Six-row barley follows the same pattern, with two big opposite rows, but space between is filled by four more rows of smaller kernels.
This uneven size in six-row kernels could lead to problems in milling, an essential process that grinds up the grains and exposes the starches inside. If you tried to mill small and large kernels together, most of the small ones would slip through unmilled. Because of this, most six-grain kernels are separated by size at harvest. The bigger ones are made into beer and the smaller ones into animal feed.
Why don't we brew with the smaller kernels? As a rule of thumb, the bigger a kernel is, the more starch it contains. So bigger kernels mean more sugar and more efficient brewing.
This is another reason many brewers prefer two-row: Even the biggest six-row kernels are a little smaller than their two-row counterparts, and using two-row grain usually results in a 1-2% higher extraction rate. Basically, two-row gives you more bang for your buck, and if you're a big commercial brewery, every buck is important.
Tied to that lower starch content is a high protein content, which can sometimes lead to problems like haziness and unintended flavors.
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So why use six-row malt at all? One thing six-row really has going for it is greater diastatic power. This means it has a higher concentration of malt enzymes that convert the grain's starches to sugar during the mashing process. If you're brewing with all base malts, you pretty much automatically have enough diastatic power to convert your sugars.
But start adding specialty grains and unmalted materials like corn and oats (called adjuncts), and you may run into a problem. These... ingredients have little or no diastatic power of their own, so they rely on the base malts to convert their starches into sugar. If you use six-row malt, you have more diastatic power at your disposal and can add more specialty grains and unmalted adjuncts to your brew.
So Which Is Better?
It really depends upon what you're brewing. Keep in mind that the international divide has led to some traditional differences. North American beers, and particularly those with specialty malts and adjuncts, are typically brewed with six-row. If that's the style you're after, you'd do best to follow in their footsteps.
By the same token traditional European beers are almost always going to be brewed with two-row grain. If you want the flavor of a German or Belgian ale, you should try two-grain.
Are the two mutually exclusive? Of course not. If you want to brew a German beer but all you can find is six-row malt, your beer is not ruined. Experiment, mix things up and see if you can taste the difference.