PTD - Potato taste defect.

It only takes one bad egg according to the old saying and spoilage is wasteful, not to mention disappointing and uneconomic.

We have spoken about PTD Potato Taste Defect previously (Mothers Day collaboration 2020) and how it is understood to originate and the effect of it upon coffee beans.

Defects (including PTD) in coffee samples lower cup scores and subsequently the prices paid to producers, but despite rigorous quality controls occasionally defects get through and it only takes that one bad apple or bean to spoil your cup.

If you’re like us and enjoy East African coffees such as Rwanda and Burundi, its is likely that you have experienced this when you’ve ground your coffee, grumbled at the waste and inconvenience of having to dose and grind again because of that one pesky bean.

But what if that bean has spoiled an entire bag? How you ask?


A subject that we haven’t yet covered, despite it having a large influence on your coffee.

(If you’re interested in digging further into this we recommend this article by Perfect Daily Grind)

In the days after roasting, the gases such as carbon dioxide are released, generally speaking the darker the roast the shorter the “degassing” period (subsequently the faster the bean will stale also).

If you use super fresh coffee may have noticed that when you pour your bloom onto fresh roasted coffee, the coffee bubbles up like you’ve made some kind of school science experiment volcano, this is generally speaking an indication of coffee that is a bit fresh and the abundance of gas will negatively impact your brew as the gas keeps water away from the bean resulting in lower extraction. Our experience has been of flat or even harsh brews, now we generally wait a minimum of 7 days after roast and continue to enjoy many light roasted coffees up to 6 weeks or more after roast, noticing that flavours continue to develop over this time.

It is also why when you walk down the coffee aisle in your local supermarket you can smell freshly roasted coffee, thanks to the nifty one way degassing valve in the bags.

Our experiment:

Inadvertently we have conducted a few experiments, this translates to “we are mildly upset by losing some of our favourite coffees we were trying to save, but we are looking at the education aspect of it and turning it into a blog”

First lesson, about 12 months ago happened with a Rwandan coffee, stored air tight in tins.

Opened up the tin and the aroma immediately it hit, that unmistakable smell of PTD, something that I hadn’t experienced from whole bean coffee (until now). I proceeded to weigh out a dose and grind it, no distinct PTD aroma that is usually encountered with grinding the effected bean.

Thinking all was well, continued on to brew the coffee only to discover it had a taste defect. Thinking I must have been missed it, I repeated the process, weigh, dose, brew. Same result.

The only conclusion I can draw (no science involved, sorry) is that when degassing occurs in a sealed environment over a long enough period of time, it results in spoilage of the entire batch of beans, almost like a marination.

Fast forward a year (ish).

Having started a coffee library in my freezer to savour some of the favourites (and subsequently running out of space) I had 2 bags of Burundi - Businde, one a fully washed from our June 2020 collaboration with Merito and another, a Natural from another roaster who we also love and have collaborated with more than once.

The plan was to share these coffees as a 2 part Black Friday at The Village Snob.

First up, The Merito, still tasting amazing (like a cherry cola on V60), within an hour of removing the Natural to brew as Part 2, the PTD smell was prevalent through the degassing valve in the still sealed bag, despite 6 months or more at -20degrees.

So much disappointment, but interesting to again be able to brew and taste the effects of PTD. What we’ve come to notice is that as the coffee cools the taste defect becomes more noticeable and it tends to dry the palette, the cooler it gets the more unpleasant it becomes.

The aroma of the bean is not totally lost or completely over powered by PTD, to the degree that at at first sniff it smell so inciting, its the second sniff that has an air of disappointment.

Undecided upon how to savour our favourites from East Africa in the future, but individual dosing and freezing is looking like it might be the safest bet currently.

Posted: Tuesday 26 January 2021

Insight. Understanding. Change.

A Bunch of Snobs

© Copyright A Bunch of Snobs
New Zealand