by Andy Walsh (awalsh@crl.com.au)

The following table shows average carbohydrate levels in sweet wort (from an all malt source), expressed as a % of wort solids (from Hough, Biggs, Stevens and Young, "Malting and Brewing Science", Chapman and Hall, 1981.)

carbohydrate    %of wort fermentable?  Notes

maltose         43.9          yes      disaccharide of 2 glucose units
maltotriose     13.6          yes      trisaccharide
sucrose         4.2           yes      disaccharide
monosaccharides 9.5           yes      mainly glucose and fructose
dextrins        21.2          no       also called amylopectins, alpha glucans
other           5             ?        fermentability dependent on yeast type  

All of the above carbohydrates are normally fermentable by yeast, except for dextrins and "other" (which are up to 40% fermentable by certain yeast strains). Some ale yeasts also do not completely ferment maltotriose. Note that dextrins comprise the bulk of the unfermentable material. Dextrins are said to contribute body and mouthfeel, but no sweetness (M. Meilgaard, chapter VI in H. Broderick's "The Practical Brewer", MBAA, 1977). Sweetness stems from unfermented sugars in the wort.

The sugars must first be taken up inside the yeast cell before they can be fermented. The monosaccharides glucose and fructose are generally first, followed by sucrose. Sucrose must first be split (by the enzyme invertase) outside the cell, into glucose and fructose, before it can be fermented. Maltose is uptaken at a slower rate inside the cell, split into 2 glucose units by the enzyme maltase, and fermented. Maltotriose is transported and converted into glucose by a similar system by the enzyme maltotriase, generally at an even slower rate.

High wort starting gravities are a very common cause of stuck fermentations for the following reasons:

How to avoid a stuck fermentation when making high-gravity beers.

What to do if your fermentation gets stuck

First look at what Al Korzonis says on the subject. Didn't work? Then look at what Kevin Hass writes.


The use of enzymes can be very risky if you are not careful, or are uncertain about the enzymes you are using. After reading Kevin's material, I have on 3 occassions used a commercially available enzyme preparation (exact type of enzyme unknown, but labelled "Modiferm") to "correct" high terminal gravities.

(OK, so I was very stupid doing this to 3 beers, but it was all in the space of 2 weeks and it *looked* like it was working at the time).

Let's see if we can now try and figure out what has happened here. Firstly, notice the extremely low finishing gravities in each. This tends to indicate that all of the dextrins have been broken down by the enzyme to monosaccharides. Why?

Case A looks like a typical case of catabolite repression, where both maltase and maltotriase have been deactivated. This can be determined by the gravity at which it stuck, which is about half way. Note that maltose and maltotriose form about half of the fermentables. The enzyme preparation must have reduced the dextrins, maltose and maltotriose into glucose units for it to have been fermentable.

Cases B and C show slightly highish terminal gravities that are not necessarily associated with a fermentation disorder. The extremely low finishing gravities suggest complete carbohydrate breakdown by the enzyme.

Alpha glucans consist of amylose units (straight chain sugars) connected together by a 1-6 carbon bond. Normal alpha and beta amylase cannot break this bond, although they can break the dextrins into smaller dextrins plus amylose (which can then be further broken down into glucose units). These enzymes would not be able to reduce the gravity to such a low level, as there must always be some residual dextrins.

Dextrinases (debranching enzymes) are capable of breaking the 1-6 link. These are used commercially to make "Dry" beer, through additions of dextrinase into the fermenter. I believe that I must have bought enzymes containing dextrinase. I even think it is likely that most commercially bought enzymes of unknown origin will contain this type since they are used in huge commercial volumes. Dextrinase is completely unsuitable for correcting a stuck fermentation, unless you like drinking rocket fuel.

This would also explain the differences obtained between Kevin and myself. His technique of using malt-derived alpha and beta amylase may well be preferable to using dextrinase (it couldn't be any worse!), but there are certain thermophilic strains of lactobacillus that can survive mash temperatures, making the risk of infection very high.

Be wary, too, of asking the retailer what type of enzymes you are buying. Mine said, "Papain", with great authority, which is clearly drivel, as papain is a proteolytic enzyme used for eliminating protein hazes, and would do nothing to dextrins.

Remember, prevention is better than cure, and try and make it right the first time.

Andy Walsh from Sydney
Ph. (02) 212 6333
E-mail: awalsh@crl.com.au