Dear Mr. Wizard,

I have started to drain my mash tun completely before sparging to my desired volume. So far, I have not experienced a stuck mash and have increased my efficiency from 70 to 75 percent. Besides the increased risk of lautering problems, are there any other problems that I am likely to encounter, such as increased tannin being leached from the grain?

Andrew Stewart
Moreno Valley, California

Mr. Wizard replies:

There are really two common methods of sparging: continuous sparging and batch sparging. The method you have switched to is batch sparging. The most significant problem that I am aware of with this method is the potential for slower wort collection rates after the first sparge. The reason for a decline in wort flow rate is the presence of oxygen — when the bed is exposed to air, proteins begin to crosslink and effectively increase in size. Much of the tannish or gray “teig” or “dough” seen in a lauter bed is crosslinked proteins.

The other problem that can accompany this method is cloudy wort following each drain cycle of the bed. Cloudy wort is primarily known for high lipid levels and can contain higher levels of tannins, although high tannin levels are typically associated with low-gravity, high-pH last runnings (“glattwasser” in German). Most commercial brewers who batch sparge add the next batch of water, underlet their bed and recirculate the wort for a few minutes (“vorlauf”) after each drain cycle. The main goals of this method are to improve wort clarity and to lift the grain bed off of the false bottom to improve run-off rates between cycles.

One of the reasons that brewers use batch sparging is for an increase in yield and it looks like this method has significantly increased the yield of your system. There is no magic behind the increase in yield if you think about what happens when you drain the grain bed.

Gravity drains much of the free liquid contained in the little grain bits when the bed runs dry. This extract-rich liquid collects on the bottom of your lauter tun and runs into your kettle. And the second addition of sparge water then leaches even more extract from the grain.

This is different than continuous sparging because, with continuous sparging, the only physical phenomenon driving the extract out of the grain bits is the difference in concentration between the liquid around the grain and the liquid in the grain.

Within the grain bits are concentration gradients, where the highest concentration of extract is found in the center of the grain bit and the lowest concentration is found on the outside of the grain bit. Diffusion is the term used to describe the movement of molecules from an area of high concentration to an area of low concentration. This concept is ubiquitous in biology, chemistry and physics. The rate of diffusion attaches a time factor to this migration of molecules.

The concentration gradient of extract — coupled with the rate of diffusion — has real practical implications on both continuous and batch sparging. Diffusion rates slow down when resistance is introduced. Big pieces of grain will release their extract slower than small pieces of grain and this explains why finely-milled malt typically has better extract efficiencies than coarsely milled malt.

This concept also explains why very fast wort collection rates can reduce extract yield — the sparge water zips through the bed faster than the extract can completely diffuse into the liquid. And it explains why batch sparging produces a better yield — the liquid around the grain is drained and replaced with water and the goodies inside the grain are driven by the increase in concentration gradient.

If your beer tastes fine, looks fine and smells fine coupled with an increase in extract yield, I say go for it, Andrew! By the way, most commercial breweries are equipped with lauter tuns running a continuous sparge.

Dear Mr. Wizard,

How long would I have to aerate with oxygen in order to incur negative effects on my yeast? I have read several articles that skirt the issue, but most cover commercial brewing and don’t give homebrewers an idea of how much is too much. I have read the recommended length of time to aerate, but not the maximum times.

Perry Launius
Jackson, Mississippi

Mr. Wizard replies:

It sometimes seems like homebrewing has advanced from “Relax, don’t worry, have a homebrew” to “Stress out so much that only a homebrew can calm you down.” (Nothing personal! Just an observation based on seven years of Wizard questions.)

Unfortunately, there is no exact answer to this question. To leap-frog to my recommendation, I encourage homebrewers to worry more about under-aeration and not to spend too much time on concerns with over-aeration. The caveat is with propagation. Yeast can be stressed when oxygen is continuously or intermittently bubbled into a propagation container. I will give some insight that may help understand why there is no exact answer to your question and will present a list of facts about oxygen and yeast that may help.

For starters, not all brewing yeast strains have the same oxygen requirements for satisfactory fermentation. This observation is documented in Malting and Brewing Science, Volume II (Hough, Briggs, Stevens and Young), although many practical brewers know this to be true from anecdotal evidence. This book has a very nice graph showing peak yeast density as a function of wort oxygen content at the beginning of fermentation. The graph shows a dramatic increase in yeast density as oxygen levels increase from 0 mg/L (which equals 0 ppm) to 2 ppm and very little change from 2–8 ppm. Another graph shows the relationship between the duration of fermentation and wort oxygen content at the beginning of fermentation. This relationship is a bit more interesting since fermentation time decreases as oxygen content increases.
In the book Brewing (Lewis and Young), the point is made that alcohol content in beer declines as wort oxygen levels increase. This reduction can be greatly exaggerated in fermentations that are continuously aerated, such as yeast propagation. Cell density in a commercial propagator with aeration and stirring provisions can reach as high as 200 million cells/mL; this is about five times higher than the peak density seen in a typical beer fermentation where the fermenter is neither aerated nor stirred.

The explanation for this phenomenom is relatively simple — alcohol is not produced from glucose when yeast are consuming glucose to synthesize the building blocks for new yeast cells. Wort aeration also has a dramatic influence on beer flavor formation during fermentation because it affects yeast metabolism. For example, if wort oxygen is limited then ester production increases and, in turn, the production of fatty acids within the yeast cell is limited. Likewise, fatty acid production increases with wort oxygen level and ester production decreases.

Yeast propagation is really the place in commercial breweries where over-aeration has been examined. Why? Because yeast propagators are equipped with sparging devices designed to deliver a lot of air to the propagation and increase cell growth. After all, the goal of propagation is growing yeast and not making beer. Both practical brewers and brewing scientists have observed that yeast can be damaged when excessive amounts of oxygen are delivered during propagation. The term used to describe this stress is “oxidative damage.” While oxygen is required for a wide array of biochemical functions, it is also related to cellular aging. The free radical theory suggests that cellular aging results from damage caused by reactive oxygen species known as “free radicals” — sounds like a punk rock band!

Veronique Martin of Oxford Brookes University presented a poster at the 1999 European Brewing Congress (EBC) in Cannes entitled “The Oxidative Stress Response of Ale and Lager Yeast Strains.” This poster showed stationary phase yeast (the phase after the increase in yeast density) to be less sensitive to oxidative stress than cells growing during the exponential growth phase. Furthermore, the negative affects of oxidative stress show up in subsequent fermentations that use yeast cropped from a stressed environment.

At the same EBC meeting, Chris Boulton from Bass gave a talk called “A Novel System for Propagation of Brewing Yeast.” This method uses oxygen injection into the propagator, but the oxygen flow is regulated using a mass flow meter and oxygen measurement within the propagator to maintain a level of oxygen not exceeding 0.5 ppm. The propagator is also slightly pressurized with nitrogen to minimize foaming. The purported advantage of this method is that yeast is not exposed to oxidative stress during the sensitive growth phase of their life cycle.

This is clearly a topic without an exact answer as research is ongoing. In fact, much of the research is believed to relate to aging in humans and other animals. I will close with a list of facts and my own opinion.

Fact: Wort oxygen levels very quickly drop after the lag phase of fermentation ends when aeration or oxygenation is performed only once. This is the typical method of aerating wort.

Fact: Wort has an oxygen content of about 8.5 ppm when saturated with air (79% nitrogen and 21% oxygen) and an oxygen content of about 43 ppm when saturated with oxygen.

Fact: 0.57 liters of oxygen at standard temperature and pressure weighs 813 mg. When dissolved in 5 gallons or 18.8 liters of wort, this results in a concentration of 43 ppm. After the saturation point is reached, no more oxygen can be dissolved into wort. In other words, it doesn’t take long to saturate wort with oxygen (or air when aeration is being performed).

Fact: Oxygen content in wort cannot be known without measuring it since wort temperature, gas bubble size and the contact time between the bubble and wort all have a profound effect on gas diffusion. Small bubbles diffuse much, much more quickly than big bubbles. Small bubbles also are less buoyant, rise slower through the wort and as such have a longer contact time. That’s why aeration stones are designed to produce very fine bubbles.

Fact: The major concerns with commercial brewers and over-aeration are primarily focused on propagation where aggressive aeration/oxygenation can cause problems due to oxidative stress.

Opinion: This topic has incredible depth and becomes extremely confusing if one attempts to create a Unified Theory of Aeration. There is no exact answer to your exact question. Homebrewing is a hobby of exploration. I think the idea is to learn from what others have done and explore the art of brewing in a fun and creative manner. Along the way, the experienced brewer will come up with their own special techniques and interpretations to the tremendous number of ideas floating around the brewing (and homebrewing) world.

I personally use pure oxygen for a one-time saturation shot for yeast propagation. I have never had any problems with this method. When it comes to wort aeration for making beer, I use air and saturate with air. Again, this works well for me and, most importantly, my yeast!

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