Illy article on water chem and flow rate

[IanClark]

Seems like this article missed its due attention here on coffeed.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T6R-4W2NDVW-5&_user=8771778&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1128188201&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=8771778&md5=ebc313cd75988ded66e8324dc0b0f9df

It took me three or four reads to finally understand what is being said here. If I've got it right, the abstract is that in untreated water without significant Na+ a key determinant of water flow rate (or brew time if you prefer) is the carbonate/bicarbonate content on the brew water. This is because the bulk of the bicarbonate (HCO3-) breaks down to carbonate (CO3-2). Carbonate then breaks down into dissolved carbonic acid (CO2) during extraction. The author indicates that this release of CO2 as gas contributes to coffee bed compaction and therefor reduced flow rate.

In softened water (where calcium and magnesium ions are replaced with sodium ions), the Na+ content is responsible for the significant impact on flow rate as it reacts with bicarbonate to yield CO2 and NaOH and influences pH in ways that can influence bed properties (somehow).

I find this quite fascinating and it actually has given me direction towards investigating an anomaly in our espresso program. We have 10 locations using largely the same equipment and the same coffee, however one of these locations has always demonstrated higher flow rates than the rest which are pretty much the same. This one location happens to be in a part of town with very old municipal water pipes... hmmm! Come to think of it this shop routinely runs at marginally higher brew temperatures than the rest (0.3F - 0.5F higher than other stores), which would make sense if the water has a slightly lower bicarbonate alkalinity (adjusting brew temp based on the inverse relationship between brew temp and alkalinity when alkalinity is already quite low... < 40 mg/L). Awesome... that would seem to explain this anomaly. Thanks Illy!

Update 09/12/09: This morning I did some preliminary testing of the above theory. At three of our locations pulling the same batch of roasted coffee on the same equipment with similar use & maintenance histories, the one shop that I noted demonstrates higher flow rates required a 0.7F to 1.0F higher brew temp setting over the other two to achieve the same taste balance. This is in accordance with the theory that the flow rate is due to lower bicarbonate content in the brew water.

I still have to get an actual measurement of bicarbonate alkalinity at this location relative to others as there may be variables I'm not considering here.

[Instaurator]

This is interesting. Maybe in the Q-Grader certification for acids they should include: Carbonic acid (H2CO3) which is carbon dioxide+water (CO2+H20) because this acid would be much more highly present in the first few days after roasting due to the fact that much more CO2 is given off in the first few days after roasting. Maybe this is another reason I don't like volatile crema extractions!

[gscace]

Ian:

that link didn't work for me. I must be stupid or something.

question on another topic. How'd it go with the humidity brew temperature thing you were trying to sort out?

-Greg

[IanClark]

Greg,

It's a pay-to-view article being linked to there. Regarding the other issue I was looking at, it wound up primarily being seasonal variation in carbonate hardness (alkalinity) that was effecting the taste balance quite considerably. This was sort of indirectly related to temperature & humidity in the sense that the environmental variables in the Ottawa River Basin influence the water alkalinity (as well as TDS to a lesser extent) and therefor how we pull shots.

The trouble with having a carbonate hardness below 40mg/l turns out to be that any change downward (as we get in winter) will result in both reduced extraction rate and reduced pH, completely messing with the acidity balance. During Spring it's sort of all over the place as run-off and rainfall mess with the water quite a bit. Even a change of 5mg/l will effect the way shots extract dramatically. The ultimate solution is formulation, however using an espresso with seriously low acidity will also help.

Instaurator -- I'm fairly sure all of the carbonic acid dissolved during the extraction precipitates out as soon as it leaves the portafilter and depressurizes. I think most students would agree the Q Grader acid test has quite enough acids in it already

[phaelon56]

Ian:

that link didn't work for me. I must be stupid or something.

question on another topic. How'd it go with the humidity brew temperature thing you were trying to sort out?

-Greg

Try this shorter version of the link:

http://bit.ly/9NqFre

[nick]

Instaurator -- I'm fairly sure all of the carbonic acid dissolved during the extraction precipitates out as soon as it leaves the portafilter and depressurizes. I think most students would agree the Q Grader acid test has quite enough acids in it already

Not to mention, I think that carbonic acid is more of an issue with espresso than with filter (atmospheric pressure) brewed coffee.

Also, I think the organic acids portion of the Q may have changed a bit since you took it, Ian... for better or for worse.