We've all experienced this problem from time to time (or perhaps annually) when a fantastic coffee is sourced and delivered only to have the quality diminish very quickly. In the past we used Water Acitivity (PAw) to assess the quality and potential of green coffee; high water acitivity indicating potential of microbial activity (fungus), mold ...ochratoxin! etc. and low water activity indicating improper drying, storage, transit conditions or perhaps past crop coffee (It is an unfortunate part of the business, but there are times when old and new crop coffees are intermixed and sold off as current crop).
At the recent ASIC meeting I was introduced to an interesting test that, based on the presentation and research, seems to have great potential for the coffee industry in assessing green coffee quality. The seed industry uses a variety of tests to ensure that seeds are viable prior to sale or use including X-Ray and TZ testing. First a bit of background on coffee that I'm sure everyone already knows...green coffee is a seed and it's goal in life is to grow up and reproduce. In it's native conditions it doesn't leave the branch until ripe or somewhere near ripe (eh hum...strip picking and mechanical picking) and doesn't lay itself out on cement patios to be exposed to solar/ultraviolet radiation or pack itself into a mechanical dryer. Green coffee drying can be performed in a manner that preserves the viability of the green coffee but often it is done in an agressive manner by subjecting the coffee to high temperatures and/or prolonged temperatures which compromises the viability of the seed.
Why does viability of the seed matter to us? There is a very strong (VERY VERY STRONG) coorelation between viability of the seed and cup quality as well as longevity of the green coffee in storage. If the coffee was processed and transported in a manner that maintained the viability of the seed then the seed is still alive and will remain stabile for a longer period of time. When a seed dies, it changes chemcially and, specific to roasting and cup quality, the acids and sugars breakdown and no longer behave in the manner you would expect during roasting and perform poorly in the cup.
Description of the TZ test from Tasmanian Botonical:
"The TZ test
Seed viability can also be determined over three consecutive days with the tetrazolium chloride (TZ) staining technique2. Hydrated seeds are perforated and placed overnight into a solution of tetrazolium chloride. This compound is colourless until in the presence of living tissue when it is converted to a red dye. This causes living tissue to stain red. Seed staining patterns are examined the following day to determine percentage seed viability.
Usually only uniformly stained red / dark pink embryos are considered 'viable'. However, staining patterns must be interpreted carefully and, depending on the species and the level of viability, the TZ test can be highly subjective."
The presentation explained the approach to the study in great detail. To summarize several samples were soaked in TZ (the TZ test does not always require perforation of the seeds as it suggests), digital images was taken of all samples. The digital images were then interpreted in order to quantify the red area of the image as a ratio to the total area of each image. Parallel samples were roasted, cupped and assesed for several attributes (sweetness, acidity, freshnes, etc) and overall quality. This was repeated over the course of a year with dying and cupping repeated to understand trends in the beans longevity. The results were very promising to say the least, since this is a new application in terms of coffee quality/longevity and cup quality, it will need to be developed, confirmed and subjectivity removed as much as possible.