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 pils 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
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.
I brewed a weizenbock with an OG of 1.070 and transferred it into secondary conditioning where it has been ever since at room temperature. I plan to keg it soon, but I was wondering if I should cold condition it first, as to my knowledge, in hefeweizens and weizenbocks, a certain amount of yeast left in suspension is desirable for the style? Finally, is there anything else I need to consider before kegging it, as I know hefeweizens and weizenbocks are generally bottle conditioned.
I think the key with kegging unfiltered beers where you really do want haze and some yeast in the keg is in haze control. If you simply rack cloudy beer from the primary into a keg, force-carbonate using a proven method, and tap the beer, say, three days after racking you are likely to be pretty unhappy with the results. My guess is that the first several pours from the keg will be in the form of yeast slurry, which is probably not what you want in your pretty weizen glass. You will eventually blow the yeast from the bottom of the keg and net what you seek, but then the next time you pour from this keg you may have a few more pours of yeast slurry.
Haze control is attempting to balance the impossible. You want a beer that is cloudy and has some yeast in suspension, but you don’t want yeast slurry pouring from your keg. Haze control at Springfield Brewing begins for our wheat beers with wheat malt selection. I have empirically found a type of malted wheat that works quite well for our brews and it is important to use malt that is able to result in cloudy beer because not all haze is from yeast. We also use a little raw wheat for our American-style unfiltered wheat (and of course we skip this addition when brewing weizens.)
The next major step in haze control is what to do after fermentation. This really depends on the yeast strain. Most weizen beer yeasts are true top cropping yeasts and if you have the ability to skim a fermenter or use a vacuum skimmer (check out “Brewing with a Vacuum Cleaner” in the June 1996 issue of BYO) you will be able to remove much of the dense yeast from the top of the fermenter after fermentation is complete. But since most weizen yeast have fairly low flocculation characteristics there will still be a fair amount of yeast in the beer giving a nice haze.
Our American-style wheat is fermented using WLP001 from White Labs. This yeast is fairly flocculent so it is important to allow flocculation to occur before racking. We cool our unfiltered wheat down to 32 °F (0 °C) and hold it for two days before racking, which knocks the bulk of the yeast from solution. It helps if you have some way of dealing with yeast that settles in your keg. We put a ring around the outlet of our serving tanks to prevent yeast that settles from being pulled out into our draft lines since most people prefer drinking beer to yeast sediment. If you use Cornelius kegs you can trim the outlet tube by about a half-inch (1.2 cm) to limit yeast carry-over.
Another approach is to forget about trying to remove yeast before racking and to rack the beer after fermentation is complete and the beer has had time for diacetyl and acetaldehyde reduction. This is where your brews are right now. You could simply rack the beer warm, cool it down in a refrigerator and then carbonate. The trick to prevent half-liter yeast slurry pours is to give your keg a careful “rock” before each tapping session. This moves the yeast off of the bottom of the keg and makes for a nice cloudy pour minus the little yeast clumps that may appear otherwise. There are several commercial brewers of unfiltered wheat beers who suggest keg rocking to draft accounts in order to maintain consistent haze.