‘Seasoning’ of a group head refers to the practice of making one or more ‘sink shots’ after cleaning an espresso machine – that is, shots that are thrown away with no attempt to taste them. Advocates of seasoning claim that the first shots have an unpleasant metallic or chemical taste, either due to some residual cleaning chemical, or because of the coffee coming into contact with bare metal. However, many people, including us at Barista Hustle, no longer recommend seasoning.

First, let’s deal with the possibility of chemical residue. If you’re backflushing and rinsing correctly, and using a chemical designed for that purpose, there should be no detectable trace of chemical left. I recently blind-tested this with a few baristas: we ran a chemical backflush on one group, and on the other group pulled a few ‘seasoning’ shots then backflushed with just water to remove coffee taint. When we tasted the water from the groups side by side, none of us could detect any difference.

However, the possibility of metal contamination is not so straightforward. Groupheads are made of a number of different materials, often including brass, chrome, plastics, stainless steel, and copper. Espresso, being acidic, will react with these materials differently to just water, making it harder to run this kind of side by side tasting in a controlled way.

Many metals are known to impart an off taste to water – for example copper ions can be tasted at concentrations as low as 2.6mg/L (I Zacarías et al., 2001). However, according to the US Department of Agriculture National Nutrient Database, a typical espresso contains only 0.5mg/L of copper, so the seasoning shots would have to have nearly five times as much copper as this for it to be detectable. If the acids in espresso were dissolving this much copper into the first shot, it seems unlikely that subsequent shots would have so much less, even if coffee oils deposited on the brass might protect it to some extent.

Iron is also associated with a distinctive smell or taste. The characteristic smell that you get from touching coins for example is actually a human body odor, formed by the iron reacting with compounds found on the skin surface (D Glindemann et al., 2006). The chemicals responsible for this distinctive smell are 1-octen-3-one, and to a lesser extent nonanal and decanal – incidentally, all of which have been detected in coffee (Coffee Flavour Chemistry by von Flament, 2002). Again, it’s unlikely that these odours would be created on contact with the first shot out of an espresso machine, but not subsequent ones.

So if it’s not metals causing this flavour, what could it be? In a 2009 blog post, James Hoffmann theorised that the off taste is just poor extraction of the first shot thanks to the groups not being up to temperature. Alternatively, it could be the case that the water in the machine that has been sitting overnight has acquired some taint. If either of these is the case, then simply flushing the groupheads with an appropriate amount of water will solve the problem. The amount of water required will vary from machine to machine.

Ultimately, we advise letting your own taste buds be your guide. In our experience, we at Barista Hustle don’t notice any off flavours in the first shot from a properly cleaned and flushed machine. However, every machine is different, so if you suspect your machine is giving a taint in the first few shots even after proper rinsing and flushing, then try a blind tasting or two, and let that be your guide. Even if you decide you don’t want to drink the first shot, try not to treat it as ‘garbage’. Use it to help dial in to a recipe, or teach someone some latte art perhaps, rather than just tossing all that hard work down the sink.

References

I Zacarías, CG Yáñez, M Araya, C Oraka, M Olivares, R Uauy, 2001. Determination of the Taste Threshold of Copper in Water. doi: 10.1093/chemse/26.1.85
(https://academic.oup.com/chemse/article/26/1/85/465395)

D Glindemann, A Dietrich, H-J Staerk, P Kuschk, 2006. The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines. doi: 10.1002/anie.200602100
(https://onlinelibrary.wiley.com/doi/full/10.1002/anie.200602100)