Conversion and MIlling
Converting starch to sugar is a complicated molecular process but as brewers we witness the simplicity of that process by milling grain, mixing in hot water and waiting for the magic to happen. Depending on your process, sixty to one-hundred-twenty minutes later we have wort (sugar water).
Milling the grain breaks it up so the hot water can kick the enzymes (on the outer layer of the seed) into action and convert the endosperm (starch) into fermentable sugar. This process is known as conversion (literally converting starch to sugar through molecular action) and depending on the mix of grains, mashing pH, water characteristics, and temperature of the water, the available potential differs from the actual results and is not the same as kettle efficiency, which is measured after moving the converted available sugar from the mash tun into the boil kettle, aka - pre boil efficiency.
Conversion of the starch to sugar is directly impacted by higher volumetric water to grain contact. This contact is predicated on the the water volume in the tun and the mill setting gap. The wider the grind setting, the larger the grain particles. Larger grain particles do not expose as much surface area of the grain to water and thus are not as easily converted as smaller particles. This is because the interior portion of the endosperm is not in direct contact with water - it is "trapped" by the outer endosperm. The same logic applies to dough balls - the outside is wet while the inside is dry.
Mill Gap Setting
A low mill gap setting of .015 creates a finer grist which = smaller particles of endosperm = higher water to endosperm contact = faster more complete and thus efficient conversion.
Mill gap settings are/were limited by the mash tun pickup's ability to operate. Meaning, if the grain was milled to fine, the pickup would clog - resulting in a "stuck sparge" (no flow into the kettle) - so home brewers mill their grain based on the type of pickup in the tun or said another way, they mill based on the equipment limitations. So instead of milling based on grain extraction potential, they mill to avoid a stuck sparge.
To avoid the dreaded stuck sparge a filtering medium is added to the grain. Rice hulls "fluff" the grain bed and allow the wort / sparge water to flow through. Some brewers use hulls for every brew session and some use them only with gelatinous grains like wheat or rye.
To find the ideal mill setting for their bottom pickup to work, brewers that sparge experiment and adjust mash water to grain ratios, make note of grains used, focus on sparge water rates and adjust outflow. Too fine a grind and rice hulls are needed, too coarse and efficiency drops. Getting it just right takes some finesse, but once there, they are good to go and most sparge brewers settle for 72 -74% as the norm.
Moving Sugar = Efficiency
Given time, grain converts to nearly the same degree no matter the volume of the mash water, but conversion is not the issue. Getting the converted sugars out of the tun and into the boil kettle is the key to optimizing the entire process.
Kettle efficiency is the measure we all recognize as "it". And this doesn't mean "is the efficiency in the kettle measuring in the 70's or 80's. Those numbers can be reached by adding more grain to the mash. What that means is "to what degree did the available starch in any given grain bill convert to sugar and how much of it was moved into the boil kettle". What most home brewers don't consider is "how much grain did I use to reach X gravity reading" and "if I could get a greater percentage of that sugar into the boil kettle I could use less grain and achieve the same result".
In addition, braided cord and manifold pickups that are generally associated with fly or batch sparging methods rely on a coarser crush. We know that a coarse crush is proven to slow conversion. So, if the conversion potential is limited by the size of the grain particles, the potential available sugars are limited as well. If a finer crush of the grain allows the highest potential conversion - why are home brewers and LHBS's setting their mills wider and adding rice hulls? To avoid a stuck sparge, and that is the tail wagging the dog. Crushing finer to access the complete and full potential of the starch conversion is the dog wagging the tail. A higher conversion ratio means you can use less grain and get the same results and conversion will occur faster and more complete.
But you can't mill finer and eliminate rice hulls without the use of a real filter - not a grain bed sediment filter that changes based on the grains used.
Sparging = Dilution
Does sparging really wash the sugars off the grains? No matter if you're using a fabric filter or using the grain bed as the filter, sparging produces the same results - lower gravity readings as the sparge progresses. I call sparging progressive dilution because as water is added to the mash tun or run through the bag, the resulting wort run off gravity is measurably lower - it is being diluted.
When sparging a mash tun the volume of sparge water and the volume of the kettle must meet at the point of expected gravity. If the gravity reading is too high the tun never empties and wort is wasted. When using a fabric filter and the full volume of mash water, the gravity reading is fixed and the only wort wasted is what is trapped in the grain after squeezing.
All things being equal, the first, second, and third runnings gravity combined from a sparged mash tun will be equivalent to the gravity reading with no sparge after lifting the and squeezing the fabric bag. But they are not equal in all things because full volume mashing with a true fabric filter allows a finer crush, which promotes a higher more complete conversion from less grain in less time.
Conclusion
As provided below, thinner mashes convert faster and more easily. A recently published respected brewer, Denny Conn, concludes that without changing the grind setting, or using fabric, that a 1.65 wtg ratio, (up from the standard 1.25 to 1.50) aides efficiency - and he's right.
The evidence for a thinner mash supported by Kai Troester states that even up to 3 quarts per lb is not too thin. "Thick mashes impede starch gelatinization and inhibit alpha amylase activity"
From a 2010 presentation to NHC - by Kai Troester - braukaiser.com - Brew Free or Die
Sparging and wort quality
● first wort is highest quality wort
● sparging depletes the mash's buffer capacity -> water pH starts to take over -> pH rises -> tannins become more soluble
● Be careful with high lauter efficiencies (90-95+%). They may reduce wort quality.
● Don't fix poor conversion efficiency with excessive lautering
● 75% efficiency for one brewer may not be the same 75% for another brewer:
● 95% conversion eff. x 79% lauter eff. = 75% into kettle
● 79% conversion eff. x 95% lauter eff. = also 75% into kettle
● former is a realistic no-sparge scenario while the latter could be over sparging
Reducing lauter efficiency
● limit the amount of sparge water by using more strike water -> thinner mashes
● German Pilsners have been brewed with thin mashes in order to increase the amount of high quality first wort
● no-sparge. With close to 100% conversion, efficiencies can be in the 70s for average beers
● keep boil-off between 10 and 15% -> limits the amount of sparge water available
● batch spargers may not fully drain the mash before sparge water addition
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