Potatoes are a common, inexpensive vegetable. And those starchy tubers can be put to use in brewing. Potatoes can be used in any recipe that calls for a relatively flavorless starchy adjunct, such as flaked barley, flaked maize, or flaked rice, or when sugar additions to the kettle are called for. They are easy to use in the brewhouse, and are a fun experience for home gardeners or adventurous brewers to try.
Potatoes (Solanum tuberosum) are members of the nightshade family (Solanaceae). The vegetable part of the potato is a stem tuber, an enlarged portion of stem — around ground level — that stores starch. They are the fourth largest food crop after corn (maize), wheat, and rice. Most modern potato cultivars are descended from plants domesticated 7,00-10,000 years ago in Peru. I have brewed successfully with potatoes three times (and once unsuccessfully).
I have also brewed with sweet potatoes (Ipomoea batatas) a few times. (See, for example, my sweet potato ESB recipe). Sweet potatoes, a root tuber, are in the same order (Solanales) as potatoes, but in the different family — the morning glory family (Convolvulaceae). In addition to the carbohydrates they contribute to a beer, they also add a little orange color. Unless they are roasted, they do not contribute an appreciable amount of sweet potato flavor.
Although I have no personal experience with them, you could probably also brew with yams. Yams (Dioscorea sp.) are a member of the family Dioscoreaceae.
Both potatoes and sweet potatoes contain approximately 75–80% water. They are ow in protein — 2% for potato and 1.6% for sweet potato, by wet weight. Multiplying these figures by 5 gives the dry weight of protein (assuming 80% water), which is slightly lower than typical barley malt. Most of the rest of their dry weight is starch. In contrast, corn and rice — two other low-protein starchy adjuncts — contain between 10–12% water and 7% and 9% protein, with most of the rest being starch.
The amylose to amylopectin ratio varies among cultivars. In mealy potatoes (Russets, Yukon Gold), it varies from being somewhat similar to cereal grains (including barley and wheat) to higher in amylopectin. Waxy potatoes (red and new potatoes) have less starch (more water) and at least some of them some have more amylopectin than mealy potatoes. (My potato-brewing experiences have been with Russet potatoes.)
Use in Brewing
You can use flaked potatoes (the kind designed to make easy mashed potatoes) or raw potatoes for brewing. If you choose flaked potato, read the ingredients and choose one without added salt or flavorings. Flaked potatoes may contain an emulsifier and a preservative, but this doesn’t preclude them from being useful brewing. At the rate flaked potatoes are used, the proportion of emulsifier and preservative is minimal in the mash.
Potato starch has a low gelatinization temperature, around 130 °F (54 °C) — well below the single infusion mash temperature range. As such, you can simply stir flaked potatoes or raw potato slices into a mash. You can also make whipped potatoes from raw potatoes and stir that into the mash. (Peel the potatoes first.) You can use the whipped potato addition to raise your mash temperature, if you are doing a step mash. Or, if you are employing a single infusion mash, simply add the whipped potatoes to your strike water while it’s heating. Once you have stirred the potatoes into your mash, your brew day should proceed as it would for any other beer.
You can substitute flaked potatoes for any flaked brewing adjunct on a one to one basis. To calculate the dry weight of raw potatoes, simply divide the weight by five. (This assumes the potatoes contain 80% water.) In other words, five pounds of raw potatoes equals one pound of potatoes by dry weight. [The remaining 4 pounds (1.8 kg) is water, a little less than half a gallon (~2 L).]
You could use potatoes as the starchy adjunct in a cream ale or American-style Pilsner. I have brewed a couple examples of this. When doing so, the faint taste of corn is subtracted from the beer and the very faint taste of potatoes is added. For a 5.0-gallon (19-L) batch, swap up to 2.0 pounds (910 g) of the flaked maize in the recipe for the same (dry) weight of potatoes.
I have also used sweet potatoes in brewing an ESB. The sweet potatoes added a bit of an orange color, but in this maltier, hoppier beer, I could not detect and flavors or aromas from the sweet potatoes — the ESB tasted like a regular ESB. In my 5.0-gallon (19-L) ESB recipe, I used 5.0 pounds (2.3 kg) of peeled sweet potatoes (wet weight) in place of 1.0 lb. (450 g) of barley malt.
Swapping flaked potatoes for the flaked barley in a dry stout recipe would probably work well. For standard Guinness-type recipe, just substitute the 10% of flaked barley usually called for with the same amount of flaked potatoes, or 5 times as much raw potato (wet weight). Make whipped potatoes, then stir them into your strike water while you are heating it.
I haven’t tried this, but you could also substitute potatoes for the sugar portion of a tripel or double IPA recipe.
To keep your brew day easy, hold the percentage of potatoes to under 20% of the grist (by dry weight). If you exceed 30%, you will likely encounter problems with lautering. However, if you are using the brew in a bag (BIAB) method, you could use much higher percentages of potato.