Standard: Specialty grade green coffee beans shall have a water activity measurement lower than 0.70a_w.

Aim:

To control wholesomeness and sensory quality of coffee raw material by minimizing the possibility of microorganism infection.

Background:

Compared to moisture content, water activity (a_w) is considered a better parameter for quantifying the degree of binding of all water present in food, both free and bound and, therefore, its availability to act as a solvent and participate in chemical, biochemical and microbiological transformations (Fellows, 2006), becoming the main indicator of food deterioration.

Presence of water in coffee may be seen as undesirable for several reasons:

• As a foreign material present in raw coffee, water content is part of the weight at time of purchase and therefore affects coffee price;
• As a spoilage factor of raw coffee at field and transport, water enhances mold attack opportunities, eventually leading to lot rejection;
• As a spoilage factor of raw coffee during final storage, water leads to enzymatic reactions that in the long term produce off-tastes;
• As a roasting/quenching/packaging technological aid, water is part of the weight of the coffee in the finished product and therefore affects final wholesale and retail price.

Water activity (a_w) is a measurement of the availability of water for survival and growth of microorganisms on nutritious substrates, including green coffee.

It differs from moisture content inasmuch as water can be bound in different ways to food substrate, making it unavailable for microorganisms' growth and production of metabolites, e.g. undesirable contaminants like flavor taints or toxins. Only the fraction of water that can go in equilibrium with vapor phase (i.e. evaporate) can be profited by microorganisms like bacteria, yeasts and – more importantly – molds for germination, survival and growth. Such fraction can be accurately inferred by measurement of a_w.

Mycotoxins are toxic secondary metabolites produced by fungi. Toxigenic fungi can grow both in the field during harvesting and storage, due to several factors inherent in the substrate and factors inherent in conditions involving the substrate (Scussel, 1998).

The most common mycotoxin related to coffee is ochratoxin A: there are three major ochratoxin A-producing fungi that are associated with coffee: Aspergillus carbonarius, Aspergillus ochraceus and, rarely, Aspergillus niger (Silva et al., 2008).

The fungus Aspergillus ochraceus is the leading producer of ochratoxin A in coffee. It develops in environments with temperatures between 8°C and 37°C, with the optimum temperature between 24 and 31°C. The minimum water activity for its development is 0.76 at 25°C, with the optimum a_w ranging from 0.95 to 0.99 and pH between 3 and 10 (Hocking and Pitt, 1997). Although Aspergillus ochraceus develops from a water activity of 0.76, the toxin is produced in coffee beans from 0.85, and 0.97 is the optimal a_w (Moss, 1996).

Implementation:

Water activity is defined as the ratio of the vapor pressure of water in a given material to the vapor pressure of pure water at the same temperature. When vapor and temperature equilibrium are obtained in a sealed measurement chamber, the water activity of the sample is equal to the relative humidity of air surrounding it.

Relative humidity measurement is a straightforward albeit delicate analysis, best performed in dedicated apparatus commercially available.

Guidelines:

Follow the calibration and operations instructions given by the water activity testing equipment manufacture, placing the appropriate quantity of coffee beans into the device.

Caveat 1: Any analytical determination is at most as good as the sampling step that precedes it.

Caveat 2: a_w determinations cannot assure that previous infection and spoilage did not occur on the green coffee in the past. Monitoring plans for extant presence of mycotoxins should always be in place throughout the farm-to-roaster supply chain.