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POINT OF BREW -- Michael J. Lewis

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A Brewery Fermentation and Herd Immunity

Porton Down!

When I read those words my chest tightened, my heart beat a little faster, breath came in a rush and I broke a sweat.

I guess that is the fight or flight reaction but in this case the emotions were alternately fearsome dread and fascinated curiosity.

So why the emotional response to Porton Down?

Well. Porton Down is the British government’s Ministry of Defence Dstl, or Defence Science and Technology Laboratory. It is a large hush-hush military research laboratory for weapons of chemical and biological warfare and, of course, for defense against them. I was both excited and repulsed by this and, in 1960 as a newly graduated biochemist, considered employment there. I was interested that my academic preparation could lead in two such disparate directions from the benign (beer) to the brutal. Fortunately the opportunity to explore the USA came along instead.

Why this re-enactment of ancient things? I ran across a research paper on the Internet published in 2013 about home made facemasks that is a topic of interest now. The authors’ scientific approach seemed to be impeccable. Although their data suggested that such masks were better than nothing, which makes sense to me, the authors could not recommend them. 

The source of this research? Porton Down Defence Science and Technology Laboratory.

Watching the novel corona virus spread exponentially in this country (and many others), although that is disturbing even frightening, it is fascinating to see biology behaving in real life exactly the way one might find described in any biology text.

Thomas Friedman in a recent NYT column wrote that economists in the future will think of the economy in terms of BC meaning Before Coronavirus and AC for After Coronavirus. Unfortunately AC is almost certainly wishful thinking. After all, do we have AF (After ‘Flu) or AM (After Measles) or really A-anything else? More likely we shall have DC, During Coronavirus, for the rest of our days with regular seasonal recurrence against which we shall line up every fall for vaccination. 

With all the dystopian evidence available I am astonished there are still Governors in some states who don’t act urgently in face of the imminent danger of this virus. I think some of this blindness arises from basic science denial arising from devoted evangelical faith plus a feast of opinion from Fox & Friends. But the peculiarity of exponential growth may also play its part. Exponential growth begins slowly, HOHUM, and then explosively accelerates to the OMG stage. A brewer’s fermentation might serve as an illustration of this effect.

There are three general phases to a brewer’s fermentation that might act as a metaphor or illustration for a Governor’s view of the pandemic:

1: The lag phase. Here, nothing much seems to happen after yeast is added. This lag might last about 12 hours and can occasionally make brewers nervous. But it is typical of the deceitful way exponential growth starts out: slowly. A Governor might not wish to impose draconian counter measures on his citizens to counteract such a meager threat. In more rural states with modest populations where the rate of doubling may be low, a Governor could be lulled into the assumption that the virus will pass over and do nothing but wash his hands of it. Whew!

2: The growth phase. This phase concerns the onset of vigorous fermentation when the rate of changes appears to go from nothing to full speed within a few hours. A Governor observing this sudden change in virus infections might well dither wondering if it will be sustained. He might find his previous rosy (and incorrect) comments to the public a political embarrassment. But the fermentation and the viral pandemic are now proceeding apace and it’s panic time. Find a scapegoat! Shelter in place! Close everything! Lockdown! Yikes!

3: The stationary phase. No exponential growth can continue forever, and stationary state will eventually arrive for both the brewer’s fermentation and for the viral infection too. It arrives in a brewer’s fermentation because all the fermentable sugar has been used up and there is nothing left to feed the yeast. The virus, of course, feeds on susceptible people and so spreads; “social distancing” works because it makes it harder for the virus to find new hosts. When enough people have become immune through infection (or have died) and so are removed from the equation the pandemic dies out. That’s why it’s absolutely necessary to develop an effective, safe, cheap, and widely available vaccine    to create a massive population of immune people.

When a pandemic dies of natural causes that is called herd immunity. Without intense mitigation efforts or, better yet, severe efforts at suppression that we can only engage in when we have effective widespread testing, herd immunity is the ultimate line of defense. The number of potential deaths that come along with that can be calculated. The number depends on how many susceptible people each infected person infects (R0) and the mortality rate of those infected.

Unfortunately, none of the mortality numbers are small.


Michael J Lewis MUG Picmonkey

Michael J. Lewis, Ph.D., is professor emeritus of brewing science at the University of California, Davis, and the academic director and lead instructor of UC Davis Extension’s Professional Brewing Programs. Lewis has been honored with the Master Brewers Association of the Americas’ Award of Merit and the Brewers Association’s Recognition Award. He is an elected fellow of the Institute of Brewing & Distilling. He is also a recipient of the UC Davis Distinguished Teaching Award.  He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.

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