Aw and Lipid Oxidation in Specialty Green Coffee

   Lipid oxidation is important in green coffee because the oxidation of lipids can produce off flavors.
A common compound in this category is called Trans-2-nonenal (T2N).
T2N is produced by the autoxidation of linoleic acid, the "most important acid in the lipids in coffee" (Flament1.
Trans-2-nonenalis a very potent odorant that is described as smelling like paper, wet cardboard, cedar, or wood.
At high concentrations it can smell like cucumber or green melon.

   We see in the Water Activity Stability Diagram that the rate of lipid oxidation increases in a distorted U shape above and below roughly Aw 0.4.
Keep in mind that this diagram is generically applicable to all products and is not merely specific to green coffee.
In the table below we use our longitudinal data set to investigate the correspondence between water activity and Trans-2-nonenal type flavor descriptors.
We see much higher occurrence of Trans-2-nonenal descriptors in coffees that have water activities above 0.6.
We also see a much higher occurrence of these flavors if we limit ourselves only to the arrivals of coffees beginning with water activities above 0.610.

   It is important to note that we also have the common Trans-2-nonenal flavor descriptors in coffees that do not have high Aw and that never did.
Water activity does not explain everything.
Water Activity is not the only factor at play in green coffee.
Additionally, we should keep in mind that our findings are observational.
We are not measuring, let alone verifying Trans-2-nonenal content.
Trans-2-nonenal is detectable at low concentrations with certain aromatic characteristics.
The identification of these characteristics is not necessarily the identification of Trans-2-nonenal.

   We have observed a higher occurrence of Trans-2-nonenal type flavors in high Aw coffee.
Trans-2-nonenal is formed by the autoxidation of a primary coffee lipid, linoleic acid.
The rate of lipid oxidation increases at Aw above 0.4.
We have observed dramatic increases in these flavors in coffees that are or that have
been measured at Aw higher than 0.610 and moderate increases in occurrence from 0.550 to 0.6.
Coffees below 0.550 Aw have the lowest occurrence of Trans-2-nonenal type flavors.
We will revisit these numbers with reference to moisture content later in the paper.

   While this does not mean that the lipids in coffees with Aw above 0.610 have oxidized, the conceptual path from high Aw to Trans-2-nonenal type aromas in coffee is easily followed.
A more conclusive study would require the direct measurement of Trans-2-nonenal (and related compounds) in relation to water activity and sensory testing.
The cumulative observations of our study seem to support this transformation occurring in specialty coffee and suggest further, direct research into lipid oxidation.

 

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