The Physics of Filter Coffee - Completed Reading

 

Quick Summary

I finally finish this book.  It was a very good read.  It is insightful for those who want to delve deeper into filter coffee brewing.  Perhaps, you don't need another book to guide you on pour over.

It covers extraction, water, grinding,  percolation, filters, kettles and agitation, drippers, etc.  All elements related to pour over are comprehensively covered and supported by scientific findings. 

Extraction

This is the fundamental that every serious coffee lover should understand about coffee.  Coffee brewing is the process which extracts the chemical solubles contained in coffee grounds to the coffee beverage that we love.  Poor extraction simply ruins any coffee beans of whatever quality.  A general guide to judge whether proper extraction has been conducted is as follow: underextracted coffee tastes sour, overextracted coffee tastes astringent.  The ideal range for extraction is recommended as 19%-24%, i.e. 19%-24% of mass of the coffee grounds is dissolved in the coffee beverage.  Lesser than 19% and higher than 24% may be deemed as underextraction and overextraction respectively.  The measurement of extraction % can be done by drying the coffee grounds or measuring TDS% with refractometer.  While the former is tedious, the latter is expensive.  Normally, one can tell intuitively with the taste of the coffee beverage.  

This chapter discusses the two mechanisms how the coffee solubles are extracted by water, i.e. diffusion and advection.  It discusses the factors that affect the extraction, concentration, distance (ground size), temperature and time and also how they are related to one another in mathematical formulae.

While the slightly uneven extraction, i.e. some coffee grounds are extracted more than the others, may enhance the complexity to the taste of coffee beverage, too broad the range of extraction may be a sign of the poor coffee brewing technique.  The coffee grounds are  overextracted and underextracted at the same time, i.e. astringent and sour.  Even without measuring the extraction, this chapter provides the readers a map on which factor to be adjusted such that a more even and proper extraction can be conducted.  

Water

This chapter discusses how the hardness and alkalinity of water affect the coffee extraction.  Hard water is preferred as this tends to facilitate the extraction of coffee solubles and the coffee beverage may taste sweeter.  However, the hard water may damage the coffee brewing equipements.  On the other hand, the alkalinity of water suppresses the extraction of the coffee's acidic solubles.  One may find water with lower alkalinity may extract the charateristics of light roast coffee better.  Yet, one may use water with higher alkalinity if he/she does not prefer acidic coffee.  

For those who are living in a place without water of proper quality or interested in tailor-made water for coffee brewing, this chapter advises how to prepare the water and the reference hardness and alkalinity to be achieved.

Grinding

This chapter discusses the ground size distribution and the governing parameter which controls the coffee bed's hydraulic permeability.  The difference in coffee ground sizes yields the different durations required for the solubles to reach the coffee ground surface for extraction, hence the difficulty to extract completely the boulders.  The fines are prone to clog the brewers.

The readers learn the effects of ground size distribution on coffee brewing and the importance of grinders' quality. 

Percolation

Filtered coffee are commonly brewed by percolation.  This chapter discusses the percolation mechanism, the importance of preinfusion and how the geometric properties of the dripper affect the coffee extraction.

Filters

Coffee fines are the controlling factor of clogging.  Clogging is the controlling factor of randomness of coffee brewing.  Randomness of coffee extraction means coffee brewing is not repeatible.  This chapter discusses the law of physics behind clogging of filters. 

Kettles and Agitation

Kettles control the flow rate and the temperature of the water.  This chapter discusses the most important factors in choosing and using the kettles.  The readers may intuitively find the steady water stream important for pour over.  This chapter suggests the pouring height should be carefully monitored in order to achieve the thorough agitation and teaches how to reach the optimum pouring height. 

Drippers 

While the shape of drippers is the obvious factor affecting coffee extraction, this chapter also discusses the science behind the common materials used in manufacturing drippers.  The thermal properties of dripper materials may not be intuitively understood by normal people.  The readers who did not receive any training in science should find this very useful in understanding the thermal properties of drippers and hence how to choose drippers.

My Thoughts

I think this book is the holy grail in filtered coffee.  Filtered coffee brewing is commonly known as an art.  Not many can tell precisely the laws behind filtered coffee brewing and hence many have contradicting experience in brewing filtered coffee.  It is like a maze.  

Jonathan Gagne has accomplished a great job and shared the most crucial (also comprehensively) the laws of nature behind filtered coffee brewing.  Gagne is a researcher and this book may seem to be more like an integrated science text book to the common readers.  Mathematical formulae with differentiations, integrations and exponential functions are alien to normal people.  Tons of charts are provided in scientific ways.  One may find this book a little bit intimidating.  However, I suggest the readers with difficulty to digest this book on first sight not to give up immediately.  Gagne provides sufficient physical explanations to help normal readers to understand.  One needs not to really understand the formulae completely.  The formulae show the weightings between the factors that controlling the coffee extraction.  The controlling factors may have already known by the readers.  The formulae tell further which ones are more controlling than the others. 

The final chapter discusses how Gagne carried out the measurements.  This seems to me that it is nearly impossible for normal people to conduct similar studies and measurements without research background.  The instruments are expensive and the implementation of the instruments may be just incomprehensible for normal people.  Furthermore, for people outside the science research world, we just do not know what natural phenomena have completely been understood by scientists.  And thus we can apply the existing science knowledge to understand filtered coffee brewing.

So, there may be hardly another book which may reach the same height and depth of this coffee book.

However, I have my own thoughts on some of the aspects discussed in the book.  I prefer ignoring water retained ratio in the evaluation of extraction.  Yes, I agree the solubles leave the coffee grounds and reach the water retained in the coffee bed.  But these solubles are never tasted.  Therefore, for my own measurements of extraction, I only take the solubles extracted in the final beverage into account.

Gagne suggests setting refractometer zero with distilled water every time before measurements.  I don't adopt this approach.  The solubles contained originally in the brewing water should not be taken into account as the extracted solubles.  Thus, my approach is setting zero with the brewing water instead of distilled water.

  

On lilydrips, while Gagne suggests that lilydrips have no discernible effects on the taste of coffee.  I often find lilydrips facilitating extraction.  The flow is faster.  Finer grinding can be applied and hence a higher extraction %.  The coffee beverage also tastes more evenly extracted.

I think this may be due to the difference normal coffee dose used by Gagne and me.  Gagne brews 22g for V60 coffee and I do 15g.  Obviously, mine is more prone to the depth variations caused by putting lilydrips into the dripper. 

 

Despite a number of disagreements, I highly recommend this book to anyone who are interested in coffee.