Moisture Content Revisited

  If we cut off coffee purchases at 12% moisture, we will cut off many Aw-related problems.
The model gives an error of about 1 percent.
If we pull the criterion back to 11% the problem region for Aw in specialty green coffee is reduced.

 

  This does not mean that all of these coffees arrive as they pre-shipped and then last forever.
We have not addressed any problems arising from change in water activity post sampling.
It just means that the majority of known Aw-caused problems are increasingly unlikely to apply because in green coffee processed for specialty export it is uncommon to find 11 percent moisture coupled with Aw greater than 0.610 (below which very little can be attributed to Aw).
Remember, in water activity coffee is just coffee and moisture is an imperfect-not-terrible predictor of water activity.
The general uniformity of the coffee substrate, at least as far as moisture is concerned, means that moisture interacts with different coffees in much the same way.

  While people stress that it is water activity and not moisture content that determines the rate, direction and likelihood of a large number of transformations in a substrate, this does not mean that there is no relation between moisture and water activity.In general, in a given system, more moisture will mean higher water activity.

 

  The likelihood of a higher than 0.610 Aw when using 11%-11.1% MC (moisture content) as a criterion is small.
While this is still around 7 percent, the main point here is that we can limit our exposure to water activity related problems by anticipating the relationship between Aw and MC. Fifty-one of the 703 coffees shown below with 11%-11.1% MC had Aw higher than 0.6.
Looking more closely at 11% moisture content samples we see the following:

  This suggests that even if we miss our criterion and accept a coffee slightly above 11% moisture, that coffee will have to be more than a full standard deviation above the mean Aw for that specialty-coffee moisture level in order to be greater than 0.610 Aw.

 

  Early on we hoped that water activity would allow us to learn to safely buy coffees with higher moisture content.
Technically, this is the case.
A coffee with 12.5% moisture and 0.550 Aw can be maintained as such with environmental controls.
Such a coffee would not be subject to Aw-controlled degradations that occur above 0.5.
Of course, a coffee with that level of moisture is very unlikely to have such a low Aw.

  It is important to remember that, like moisture content, an Aw reading is just a single Aw reading.
It does not tell you if the Aw is stable or moving.
If moving, it does not tell you the direction.
It does not tell you about either the past or the future.
It does not tell you about the entire bag, let alone lot of coffee.
A water activity measurement tells you the water activity of your sample at the time of measurement.

  In our observations, we've found water activity to be more changeable than moisture.
In drying trials that we performed in Costa Rica and Colombia, we found that moisture content declines relatively steadily from day to day, while water activity fluctuates as it declines.
In the charts below the wildly fluctuating blue and gray lines are water activity measurements taken of different coffees.
The steadier orange, yellow and green lines toward the bottom of the charts are the corresponding moisture contents.

  In addition to these, we were able to run a corresponding trial with coffee from Finca Hernandez in a mechanical dryer.
We unfortunately missed a series of measurements due to events beyond our control (and in the process learned a little about arranging trials), but nevertheless did get enough readings at least to get a glimpse of Aw and moisture behavior as a coffee progressed through a mechanical drying process.

  Even hourly measurements pulled from a mechanical dryer show fluctuations within the larger downward trend in water activity.
We also measured relative humidity (RH) and temperature in the above drying trials.
In general, the RH would increase overnight as the temperature dropped, and then would decrease as the temperature rose during the day.
While the water activity of drying coffees did
decrease as the coffees dried, it tended to follow the same daily pattern as the relative humidity.

  Today it is clear that what water activity has really done for us is reinforce and push back the criterion that needs to be drawn for moisture content.
While the standard cutoff of 12% moisture is reasonable (actually, in terms of Aw it is a bit high around 0.6, it is water activity and not moisture content that we are often concerned with.
In some ways the assumed use of moisture content is already as an estimate for water activity.
That we may not have known that prior to measuring water activity does not negate the fact.

We can remember as well that in some situations, moisture content is just exactly what we want-as when we want to know how much of the mass, and therefore price, of a given shipment is water.

  We have seen that water activity is more reactive than moisture content to environmental cues.
We will see below that this volatility carries through to the shipment period as deltas can be significant.
Despite being more sensitive and capable of making close calls on upper end or very specific Aw levels (Aw measures Aw directly, thus a given Aw target can be aimed at and reached within a very close tolerance when compared to estimating that target with moisture content), and in particular given "black box" shipping periods of one to four months, water activity is probably less well-suited to making those close calls.

  These conclusions about the relationship between moisture content and water activity in green coffee are further supported by analysis of the longitudinal data.
We will dig more deeply into this data below.
While there is an error in estimating water activity from moisture content, we will show that this error becomes much less significant when taken in the context of and compared to the
changeability of water activity itself observed over the shipment

  As an estimate of Aw in specialty green coffee, moisture content is not perfect.
This means that if we determined that 0.610 was our Aw criterion, and that we wanted to use moisture to estimate it rather than Aw to measure it, we would need to do more than just find the correlation moisture level (~11.7%) for that water activity.
Because the correlation is not perfect, a coffee measuring 11.7% moisture may have anywhere along a range of Aw readings (~0.6 +/-0.0.
Our job would be to determine the MC at which we are satisfactorily unlikely to go above 0.610 Aw.

  There are two further topics that we should discuss before moving on to examine our longitudinal data for specialty coffee: browning reactions and lipid oxidation.
Looking back to the Water Activity Stability Diagram, we see that the rates of both of these reactions increase as we leave the normal water activity range of specialty green coffee on the high end.
Lipid oxidation also increases at extremely low Aw.

 

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