Using Enzymes to Cure Stuck Fermentation and Low Gravity Problems

by Kevin Hass (, 5/15/95

I wrote this document on enzymes because of the relative lack of information on their use. I had looked thru several books, and for example, Papazian's "Joy" book only made a vague, one sentence comment on the use of enzymes in this manner. The information on using them to fix stuck fermentations, and brewing low FG beers is lacking. Perhaps if more people knew this, we wouldn't be seeing the continual flow of "stuck fermentation" postings - almost everyday! I provide this in the hope that more people will look into this, and maybe have fewer problems with stuck fermentations. I'm not saying that the use of enzymes are the panacea for stuck fermentations, but I do think it would solve a majority of them! Their use has fixed every one that I tried them on - three so far.

Enzymes are very useful in the following circumstances:
  1. To produce a high alcohol, low FG beer.

  2. To correct a "stuck" fermentation, while the SG is still high, due to the large amounts of unfermentable dextrins and complex sugars.

  3. To correct a stuck fermentation when large amounts of glucose (corn sugar, or dextrose) were used to boost OG, only to screw up the yeast's ability to utilize other normally fermentable sugars.

Enzyme action:

The enzymes I am mainly speaking of is a mixture of both alpha and beta amylases - especially the beta version. When boiling wort, even if you are an all grain brewer, the enzymes are deactivated by the boiling. Any conversion of starches and dextrins to fermentable sugars are halted by the boiling. The alpha amylase, if allowed to convert carbohydrates to their lowest form, would result primarily in maltose - a disaccharide. Beta amylase works with alpha amylase, to break down carbohydrates to their simplest form, which is mainly glucose. Although yeast can normally utilize sugars other than glucose, yeast must ultimately convert those sugars within the yeast cell, to glucose to use and ferment those sugars (sugars like maltose, fructose, and sucrose, to name a few).

High alcohol & low FG beers (1):

In most normal brewing situations, there are significant percentages of unfermentable carbohydrates. If the goal of the brewer is to produce a high alcohol beer, and or reduce the carbohydrate content of the finished beer, then enzymes can help attain this goal. If these unfermentables are not converted to fermentable sugars, they remain in the beer, as body and character enhancing components. A significant percentage of these unfermentables can be converted to fermentable sugars by the addition of enzymes, to yield a beer that is higher in alcohol, and lower in both FG and carbohydrate content. As I am diabetic, the reduction of carbohydrate content is important to me.

The enzymes can be added when pitching yeast, or when the higher than desired FG is attained.

NOTE: Both alpha amylase, and the mixture of alpha & beta amylases will work in this case - but beta amylase containing enzymes will be superior in this situation.

Malt's unfermentables (2):

When malt is produced, is is generally a complex mixture of various sugars, and dextrins mainly. Depending upon how the grain was mashed when the malt was produced, it will have some fermentable sugars, and other unfermentable complex sugars and dextrins. I am mainly speaking of malt extracts in this case, but it is also true of all-grain home mashed worts. The percentage of fermentables in the malt is variable, and is dependent upon several factors, including the malted grain used, how much active enzymes are left in it, the temperature(s) it was mashed at, the pH of the mash, and how long it was mashed at certain temperatures. I have read of percentages normally being around 70% fermentable (but that could be wrong - I'm working from memory).

Alpha amylase, which primarily produces poly-saccharide sugars (i.e. NOT glucose), survives higher mashing temperatures than it's beta amylase brother. Beta amylase tends to have about (roughly...) a 10F (or about 5.5C) lower tolerance for heat than does alpha amylase. Beta amylase breaks down significantly at temperatures at or above 149F (65C), while alpha amylase breaks down more rapidly at about 158F (70C). Since most mashes are done at or above 153F (67C), the life expectancy of the beta amylase is short, and is destroyed much quicker than the alpha amylase, resulting in a wort that is richer in poly-saccharides and dextrins. I am not making a suggestion to change the way mashing is done! I am only stating this for informational purposes.

If a malt contains significant unfermentables (and most do - more or less), then the beer will only ferment down to where it has used up all of the fermentable sugars, and stops there. SG values all the way up into the 1.020's can be a result of this problem. Such a problem, in the case of extract brewers, is NOT the fault of the brewer, and the ferment is stuck at that point.

This kind of stuck fermentation can be dealt with by the use of enzymes. They can be added when pitching yeast, or when they notice the fermentation seems to be stuck at a higher than expected SG. The enzymes will slowly break down the unfermentable dextrins and complex sugars, into fermentable sugars. This will result in the fermentation resuming, and help to lower the SG to a more acceptable FG value.

NOTE: Both alpha amylase, and the mixture of alpha & beta amylases will work in this case - but beta amylase containing enzymes will be superior in this situation.

Yeast metabolism (3):

Just like humans, yeast ultimately convert all *fermentable* sugars to glucose before they can be utilized within the yeast cell. This is accomplished via enzymes within the yeast. If there is a major problem within the yeast to manufacture the enzymes needed to convert sugars other than glucose, to glucose, then the yeast cell cannot ferment sugars that it would normally consume rapidly. This is a problem that happens commonly in beer brewing, when large amounts of corn sugar (i.e. glucose or dextrose) are added to boost the OG, in the hope of producing a high alcoholic beer. Under these type of circumstances, the yeast become stressed due to various problems, become weakened, and ultimately lose much of their ability to manufacture the enzymes needed to convert sugars. When this happens, the yeast lose their ability to utilize any other form of sugar except glucose, and leave large amounts of other fermentable sugars unfermented. When this happens, the SG is usually fairly high, and adding yeast nutrients doesn't help. The fermentation is "stuck".

In the above situation, the "stuck" fermentation - IF it is the same situation as described above, can be corrected by adding enzymes to the fermenting beer. This will allow beta amylase to breakdown, and convert the various sugars to glucose, which the yeast still can utilize. This will again help start up an otherwise stuck fermentation. Before adding enzymes, this condition can be tested by withdrawing two samples of fermenting wort, and adding corn sugar to the one, and adding sucrose to the other. If the corn sugar added sample ferments, while the sucrose sample either slowly ferments, or doesn't ferment at all - then you have this situation, and can be corrected by adding enzymes.

These "stuck" situations that were discussed ASSUMED that there had been a healthy fermentation prior to when the fermentation got "stuck". If the beer won't even begin to ferment at all, even after making sure the wort had been oxygenated well, then I doubt that enzymes will help either.

NOTE: Enzymes containing beta amylase are needed to fix this situation, because only glucose is being used by yeast in this circumstance, and only beta amylase will produce glucose.

My personal experience:

My first brew kit I had brewed, contained a package of "pilsner enzymes". I couldn't find information on these under that name. The beer made from that kit did ferment down to the target FG of 1.002. You added these enzymes when pitching the yeast, so they did their action in the fermenting wort. These enzymes mystified me, until I looked further into the amylase enzymes.

Recently, I had two batches of beer that had stuck at higher than expected SGs. The main one was stuck at 1.043. I tried adding yeast nutrient, adjusting the pH, adding minerals, and even putting a heating pad on the carboy. All attempts did very little to improve the stuck fermentation. It sat for three weeks like this, and then the idea of adding amylase enzymes to the stuck fermentation came to mind. However, my local brew store did not carry enzymes, so I came up with the next best thing under the circumstances. I considered the beer a loss, so what I did was risky, and only had a batch of beer to gain by it (it was lost otherwise...). I extracted the active enzymes from ground malted barley, and added them to the ferment. I extracted the enzymes at 140F (60C) for 15 minutes, well below the temperature where beta amylase significantly breaks down. I started with 1 lb (454gm) of malted barley, and yielded 20 oz (600ml) of extracted liquid. I added 15 oz (450 ml) to this "main" batch of beer.

The beer began to ferment within 2 hours, and returned to almost a normal rapid fermentation rate within 24 hours. At it's peak, it was bubbling once every 1-2 seconds, so it was definitely becoming very active again! It took a few days, and then it slowed down again. I bottled it when it reached an FG of 1.014, which is well below the OG of 1.090, and the stuck SG of 1.043! A success story! It did suffer from absorbing lots of yeast taste, due to the long period of sitting in the primary fermenter, but it is drinkable. I won't repeat that a second time!

I also tried the enzymes on two other batches of beer, one of which was a loss anyway due to unrelated problems, but both resumed normal fermentation after adding enzymes.

I have suggested this course of action to several people who have posted either to rec.crafts.brewing or the HDB, in regards to stuck fermentations. I have heard back from several that have tried this, and it fixed their problem. While I have not heard back from them all, I have NOT heard back from any that had TRIED it, and it didn't work - all responses that have tried it, have affirmed that adding enzymes helped their stuck fermentations. Take that for what it's worth.

Enzymes & their sources:

If you do try to add enzymes to your beer, you should use purified enzymes that can be obtained from some brewing stores. It is advisable to seek out enzymes that have beta amylase in them also -so it will work in all three situations that I have listed in this document. This may be a difficult bit of information to get from some brew store owners, because they may be several steps removed from the original manufacturer, and may have no way of KNOWING for sure what form of amylase enzyme they are selling. While the enzymes may be in a purified form, they probably are not sterile, and you may risk infection, just as you do when you add dried sugar to prime your beer at bottling.

I had previously used enzymes to help mash corn, when I *legally* distilled alcohol during the fuel alcohol craze of the early 80's (I did have a permit from the BATF, BTW). I KNEW that they worked well back then, and I have since located the manufacturer of them - however they have minimum sales of 50kg - far out of the reach of homebrewers! I do now own a more reasonable quantity of them myself, made by that same company. I am now using them regularly.

I would be interested to know if anyone out there knows brand names, and mail order sources for these enzymes - especially beta amylase containing enzymes. Perhaps I could assemble a list of companies that cater to the homebrewer, that supply enzymes that will work, and post it at a later date.


There are numerous causes for stuck fermentations. Yeast will not ferment if they are not being supplied with the various foods they need, which include (not a thorough list) amino acids, minerals, fermentable sugars, and an environment that they can live in. Enzymes will not fix every stuck fermentation, but I do feel that they would correct a MAJORITY of the stuck fermentation posts we see weekly! This is only my opinion. I also DO NOT suggest the method of extracting enzymes from malted barley (as I did), due to the potential risk of infection in your beer. I only mentioned it in the description of what I did for informational purposes.

Kevin Hass,