Ask Mr. Wizard

Starch Conversion


Bill Broderick • Newnan, Georgia asks,

As I understand it, 100% mash efficiency on a brewers level is actually 80% due to grain hull mass and insoluble starches. So a mash that contains Pilsner malt having a maximum 37 possible gravity points per gallon, at 80% efficiency, would yield approximately 30 gravity points per gallon; a very efficient mash by most standards. If you achieve less than 100% of a brewer’s possible yield, are there not unconverted starches present in the beer that will cause a haze? Or are all the remaining starches insoluble? If you perform an iodine test for complete conversion, and the results show no remaining starches, and you have a mash efficiency of 70%, does that mean 30% of those starches are insoluble no matter what, or does it mean your specific mash regimen could not convert the remaining starches and they remain solid and not in suspension? Is the final product missing something without insoluble starches present to the point I should intentionally decrease my efficiency?  How will the wort from a mash tun that produces very high efficiencies with accepted normal water to grain volumes, grain crush, mash and sparge times, temperatures, pH, etc. compare to wort produced under the same parameters by a mash tun that is less efficient?


Mash efficiency is normally expressed by comparing what is extracted from malt during brewing to the hypothetical or so-called “laboratory” yield. You are correct by stating that when the yield in the brewery equals the laboratory yield that 100% of the hypothetical has been extracted. That is rarely the case because the lab method used to determine the maximum yield uses malt that has been very finely milled and the mash is “sparged” with excess water so that anything soluble in the malt is extracted. Rarely do brewers use this type of mashing and sparging method and the result is that most brewers get somewhere between 85% and 95% of the hypothetical yield.

The stuff that is not extracted into wort during mashing is mainly comprised of husk material and protein. There is often some starch contained in spent grain, but not much. There also may be some large molecular weight carbohydrates that are associated with cell walls in spent grain, such as beta glucans, pentosans and arabinoxylans, but again these compounds don’t make up the majority of the solids contained in spent grains. If the lab yield (hypothetical yield) is 80% that means that for every pound of malt used in mashing that 0.80 pounds of extract can be dissolved into wort. The 20% that cannot be dissolved into wort are the compounds I have just named, again mainly husk and protein.

When brewers get yields that are less than the lab yield, they are leaving behind wort in the spent grains, so the loss is primarily in the form of fermentable and unfermentable carbohydrates originating from starch and possibly some unconverted starch. Yielding less than the lab yield is caused by a combination of factors. The primary things that effect yield are milling (fine grist yields more extract), mashing technique (thin stirred mashes tend to yield better than thick infusion mashes) and lauter tun or mash tun design and mode
of operation.

You ask about possible problems when the efficiency is less than 100% of lab yield. The problems actually arise when yield is too high. If you measure the concentration of various compounds extracted from the mash during sparging you will find that the wort composition changes. Sugars are continually being leached from the mash bed during sparging and the concentration of sugars (approximated by wort gravity measures) steadily declines as sparge water dilutes the wort. Other compounds begin showing up in wort as the mash/wort pH begins to increase towards the end of wort collection. The concentration of polyphenols or tannins from the malt husk is the main compound of interest that begins to pop up in higher concentration as pH increases. Unconverted starch can also move from the grain bed into the wort as sparging continues. Therefore, many brewers intentionally stop sparging before extracting everything possible from the grain.

Some brewhouses routinely produce less than stellar yields. This is usually due to deficits in equipment design or because of certain techniques. While low yields can be expensive to commercial brewers, they do not have a negative impact on the finished beer. Brewhouses with high yields are normally preferred by commercial brewers because brewing raw material loss in the form of low yield becomes expensive, but it is possible that too much efficiency can be detrimental to finished beer quality.

Response by Ashton Lewis.