Low-Carb Beer Methodology
According to research published by Ipsos Public Affairs, over one-half of American adults believe that the carbohydrate count in vodka is high or medium and 55% believe this about tequila, even though neither vodka nor tequila contain any carbohydrates, nor does gin, rum or whiskey. The survey also found over two-thirds of adults falsely believed that fruit juice and regular soda are lower in carbohydrates than distilled spirits.
Carb dieters must understand that carbs are not the same as calories. Starches and sugars are carbohydrates, alcohol is not, but all three contain calories. Therefore, making a low gravity beer does not necessarily give you a low-carb beer. The carb count in beer is more determined by process variables than gravity.
Let's first take a look at carbs: Carbs are essentially grain starches. In the mash tun, any starch absorbs increasing amounts of water (hydrates) and gradually expands as the ambient temperature increases from cold to warm to hot. Between 158 and 176 degrees F barley starch forms a viscous paste as it gelatinizes. This is important because starches are more susceptible to enzymatic conversion after they have gelatinized. Starches thin out above 176 degrees F. At this point they can leach unconverted into the wort causing a chill haze in the finished beer and contributing to the beer's carb count. That's why you should never let the mash temperature rise above 176 degrees F.
Barley is about 60-65% starch by weight. Before these complex carbohydrates can be metabolized by yeast, they need to be broken down by enzymes in the mash into chunks of two molecules (maltose) or just one molecule (glucose). These sugars are then extracted into the wort. Maltose makes up about half of all wort sugars while glucose makes up no more than one-tenth of all sugar. The remaining 40% of sugars are too complex to be fermented by most yeast and tend to remain in the beer as residual sweetness and thus carbs. The objective for the low-carb brewer is to make a non-standard wort. To do this requires conversion in the mash of as many starches as possible into fermentable sugars.
To accomplish this, first note the connection between enzyme activity and temperature ranges: Starch converting enzymes are also called diastatic enzymes. There are two types of diastatic enzymes, alpha-amylase and beta-amylase. Alpha-amylase breaks starch molecules into complex, mostly unfermentable sugars. Beta-amylase breaks both starches and complex sugars into simple, fermentable sugars. Alpha-amylase, the complex sugar maker, starts to show activity around 140 degrees F, reaches its peak performance around 162 degrees F and virtually ceases all activity around 176 degrees F. Beta-amylase, the simple sugar maker, starts to show activity around 104 degrees F, reaches its peak performance around 149 degrees F and virtually ceases activity around 158 degrees F.
From these temperature values follow three logical requirements for low-carb beer:
1. Keep the mash as close to the optimum beta-amylase temperature of 149 degrees F as possible. This will produce almost exclusively simple, fermentable sugars. As saccharification rest at this temperature of at least 45 minutes is recommended.
2. Because at a mash temperature of 149 degrees F, alpha-amylase is still 13 degrees F below its peak performance, but within its active range the amount of starch converted into simple sugars can be increased by adding alpha-amylase enzymes to the mash. Thus, decreasing the amount of carbs in the finished beer.
3. After the saccharification rest, move the mash as quickly as possible past 158 degrees F where beta-amylase enzymes stop working, to a mash out temperature of roughly 172 degrees F where alpha-amylase enzymes slow down.
The eventual amount of carbs in finished beer is determined by yeast metabolism. Here's how that logic works for the carb-conscious:
Ale yeast ferments monosaccharides (glucose) and disaccharides (mostly maltose and some melibiose) up to roughly 50% of the starches that are naturally in the grain.
Lager yeast ferments monosaccharides and disaccharides as well as some trisaccharides (maltotriose and raffinose) for an additional roughly 5% of the original carbs in the grain.
Conclusion: If you want to make a really low-carb beer, mash primarily for beta-amylase activity, rest thoroughly, raise the temperature to mash-out quickly past the peak alpha-amylase temperature, sparge fast, make sure you leave the unconverted starches in the spent grain and use alpha-amylase in the mash.