A: Basically, Greg is right, though I'm not sure I agree with his advice. As a rule, low pitching rates will yield higher levels of esters in finished beers. (For more details on this, see reference 3.) Other factors that affect ester production include pitching and fermentation temperatures (as you said), wort gravity and composition, and, of course, the yeast strain's inherent propensity for ester production.

All other things being equal, higher pitching rates lead to less growth of yeast in the wort -- in other words, higher pitching rates provide a higher percentage of old yeast cells, which tend to be less adventurous about taking minor metabolic pathways like ester synthesis, preferring to stick to the main road, which is alcohol production. Old yeast cells also tend to be less vigorous about fermentation. This is why breweries that repeatedly repitch their yeast generally avoid high pitching rates -- the practice leads to a "graying" of the yeast population. You don't want sensible middle-aged yeast that go to bed at ten o'clock every night (in other words, yeast that get tired and stop fermenting when there is still sugar in the wort); you want a bunch of young, dissolute kids who will party until every last drop of sugar is gone. As Dr. Wen-Ping Hsiu put it, "Yeast are like people -- only the young can be strong!"

For the sake of ensuring a complete fermentation, I think it is essential to aerate your wort fully, even if you choose to use a higher-than-normal pitching rate. If you are a micro- or pub brewer whose production yeast is prone to ester formation, then Greg Noonan's advice is good.

The only thing I would add is, don't reuse the yeast from your batch(es) of Scotch ale. For home brewers, the simplest way to get the clean flavor profile of a typical Scotch ale is to use a yeast that is not prone to ester formation, such as Wyeast 1056 (American Ale). I have made some very clean Scotch ales with this yeast using my normal pitching rate, aeration method, and fermentation temperature.

A: Your concern is reasonable, but in fact chlorine dioxide does not work the way you seem to think. When activated, sodium chlorite will form chlorine dioxide gas, which is dissolved in the sanitizer solution (see the equations on pages 78 and 80 of the article for the details). Chlorine dioxide gas is a compound; it does not have the same properties as the elements from which it is made (oxygen and chlorine) any more than table salt has the same properties as the sodium and chlorine from which it is made.

The way chlorine dioxide kills microbes is that it reacts chemically with certain amino acids that contain sulfur. The amino acids are important building blocks in the proteins that help to form cell walls. When these proteins are destroyed, the cell wall ruptures and the organism dies. In the chemical reaction, the chlorine dioxide gas takes on an electron from the amino acid and reverts back to a chlorite ion. The amino acid gives up an electron, and giving up an electron is what chemists call oxidation. Many common oxidation reactions do in fact involve molecular oxygen (that's how it got the name), but many do not. That is why articles that talk about oxidation reactions can be so confusing to lay persons like us -- especially if we are brewers who get jumpy at the thought of oxygen in our beer.

Just because oxygen is not liberated during the sanitizing process, however, does not mean that chlorine dioxide is innocuous. Other nonoxygen oxidizing sanitizers, including sodium hypochlorite and iodophor, have been demonstrated to produce off-flavors when they come into contact with beer. The issue needs to be settled with a practical test.

One major American brewery undertook such a test some time ago. They dosed beer with 100- and 200-ppm solutions of activated chlorine dioxide, at rates of 0.1% and 1% by volume. Note that these concentrations are far higher than the 40 ppm recommended for general no-rinse sanitizing; they wanted to know what would happen in a worst-case scenario; that is, suppose the doser fed way too much sanitizer into the water and the beer actually got out the door. How bad would the beer be? Note also that the 0.1% addition is a realistic maximum for cans or bottles sprayed with sanitizer and drained before filling. The results were that, at a rate of 0.1%, even 110-day beer dosed with 200-ppm chlorine dioxide could not be detected from control beers that were not dosed. The beers dosed with 1% showed a thinner body (probably due to protein degradation), but no oxidized, phenolic, or other off-flavors. The beers dosed were standard North American-style light lagers, and the evaluations were done by trained taste panels.

This brewery is now using activated chlorine dioxide for container rinsing, filler sanitizing, and a continuous 5-ppm spray biowash on the fillers during operation. Since they adopted this program, baseline bacteria counts on their unpasteurized, aseptically packaged beers have been zero.

Q: I am having a continuing problem getting the attenuation I have come to expect with my lagers. I have had this problem ever since I started brewing five years ago. I have tried adjusting everything I can think of -- the pH of the mash and sparge water, and the mashing and fermentation temperatures. The last liquid yeast I tried should have had an attenuation of 73-77%, but instead came in at 58.3% -- this even though I used a 1-qt starter and fermented for two weeks at 55° F (13° C). This is pretty typical of what happens in my home brewery. I am at a loss as to what might be causing it.

One thing I have noticed is that when I bottle I tend to get a lot of bubbles in the siphon tube, which causes my beer to foam in the bottle. I have determined that the bubbles are coming from the beer and not from a leaky siphon. Is this an indication of something I am overlooking? Is it possible it is still fermenting? It does taste sweet before I bottle it. Is it possible to do some sort of a controlled experiment to find out where the problem lies? I really want to lick this problem if I can.

A: First, a question. I assume you are calculating apparent attenuation? This is what home brewers usually mean by attenuation, and that's what I assume the 73-77% refers to. It means that a 1.048 wort should attenuate to around 1.012 as measured by the hydrometer. If that's the way you're calculating it, then yes, you have a problem.

The usual causes of inadequate attenuation are underpitching (insufficient number of yeast cells), low viability of the pitching yeast, and inadequate yeast growth in the wort. Underpitching with a liquid culture is very common. A 50-mL smack pack of lager yeast requires two stages of propagation at room temperature and frequent aeration of the starter cultures throughout propagation. This method should provide enough healthy yeast to pitch a 5-gallon batch.

To ensure adequate yeast growth in the wort, aerate the holy tar out of it after pitching. Use a stone -- it's the only method that you can count on to achieve saturation. I remain convinced that 90% of all home brewing fermentation problems are caused by inattention to these basic procedures.

Another potential problem with lager beer is thermal shock. Your wort should not be more than 18° F (10° C) colder than your yeast starter at pitching. Greater temperature differences can occur if you propagate in a warm room and pitch into very cold wort.

Your question about gas bubbles is suggestive. How clear is your wort when you pitch it? Lately these pages have seen a lot of discussion about the desirability of clear wort -- especially in lager brewing (8,9). The current consensus among commercial brewers is that the reason clear wort sometimes leads to weak and incomplete fermentations is the under-appreciated phenomenon of CO₂ toxicity. Cloudy wort holds lots of particulate matter in suspension; the particles provide nucleation sites for CO₂ bubble formation and the CO₂ is promptly purged from the fermentor as it is generated. Without nucleation sites, the gas dissolves in the wort and inhibits the yeast. The problem is worst with lager beers because the lower temperatures allow the CO₂ to dissolve more readily. (For those who are interested, a good summary of this research is included in reference 10.) So if your wort is very clear, CO₂ toxicity may be part of your problem.

The best way to track down the cause of the problem is to try changing one thing at a time in your procedures. If you have been racking your wort off the cold trub before pitching, try eliminating this step. Review your brewing notes and see if they suggest a possible cause. Then change that and see if it works. If you propagate your yeast in only one stage, for example, try two stages (one cup, then one quart). Or if you haven't been aerating the starters during propagation, try swirling them around three or four times a day throughout propagation. Both these measures will give you a higher cell count at pitching.

The basic fact is that when you make lagers, you have a lot less room for error than you have with ales. Because of the low temperatures, you never get as much yeast growth in the fermentor as you do with ales. Any mistakes you make will be magnified.

References