to wait between harvest and consumption has always been a feature of coffee. In part this is due to on-farm processing requirements of coffee cherry production including pulping, fermentation, washing and drying for premium wet processed Arabica origins and drying and hulling for the dry processed commodity, mostly Robusta and lower grade Arabica in Brazil.
Coffee is produced in hot and wet tropics but predominantly roasted, manufactured, and consumed in countries with temperate climates. This means that production is dependent on longer storage, transit times, and movement across different climatic zones with varying environmental conditions, basically from higher to lower temperature and humidity.
There are additional waiting times for green coffee on arrival at the port, before roasting and grinding, and for manufactured packed products prior to purchase and use by the consumer. Such is the length of the post-harvest process that coffee originating from last year’s harvest may only start to be consumed as the new year’s crop is picked, and considerably longer if demand is for custom-aged coffee with a particular flavor.
Cured coffee beans require strictly controlled conditions in storage and shipment, with quality compromised if consignments experience unfavorable conditions of temperature and humidity. High humidity rather than high temperature is the killer of green coffee quality, but the two are closely linked and must be considered in tandem when formulating the right conditions for safe storage and transit of cured coffee beans.
Storage at Origin
Coffee may be stored in producing countries as dried cherry, dry parchment coffee or cured green coffee. All types of coffee require exacting conditions of storage but less so for dried cherry or parchment coffee since the dry husk and parchment provide a useful barrier against movement of moisture, as well as physical protection against insects. Green coffee is the most demanding commodity having been de-husked, polished, and denuded of any protection against ingress of water to exceed the safe moisture content achieved at the end of the drying period.
For absolute safety the moisture content of green coffee should not exceed 11% w/w (weight to weight—11 g water /100 g coffee beans) a level at which fungal mold growth and enzyme activity are negligible. The safety margin is narrow since a relative humidity exceeding 74% (producing 13% w/w equilibrium moisture content in the beans) is sufficient to trigger the development of various fungal molds.
These will include largely innocuous molds like Rhizopus that are dangerous to human health but still causes taints and off flavors in beans and cupped coffee. Much more sinister is the notorious Aspergillus ochraceous fungus producing ochratoxin A, a carcinogenic and genotoxic mycotoxin regarded as the single biggest threat to the production and manufacture of ‘safe to drink’ quality coffee.
Yeasts and bacteria with the potential to cause more rapid spoilage by wet necrosis and fermentation require a considerably higher humidity of 85%, corresponding to around 18% w/w moisture in the coffee beans, before becoming a problem. That said 85% relative humidity is by no means unusual in the tropics and in lowland areas such as West Africa and South East Asia at or below the usual prevailing level.
Temperature and Humidity
Dried parchment coffee and green coffee are sensitive and vulnerable to high humidity and will rapidly absorb moisture from the air until bean moisture content is in equilibrium with water vapor in the air (% Equilibrium Relative Humidity or ERH). Capacity of air to hold water vapor depends on temperature, the higher the temperature the greater the water holding capacity, thus coffee bean moisture content in relation to ERH must always be considered with respect to temperature.
Coffee bean moisture content at 12% w/w is generally accepted as being close to the upper acceptable limit for stored and shipped green coffee. For coffee kept at 25°C to maintain bean moisture content of 12% w/w the corresponding and required ERH of the storage atmosphere is 60%.
Mold growth on green coffee with accompanying spoilage is likely to begin as soon as bean moisture content exceeds 13% w/w. This level corresponds to an ERH of 67% at 25°C.
Research has shown that wet processed Arabica can maintain original quality for up to 26 weeks storage, provided it is dried to 11.0% w/w beforehand and relative humidity maintained within the 50-70% range and temperature kept at 20°C or less. Controlled ventilation was superior to hermetic sealing of coffee, especially at higher temperatures, and dried parchment coffee shows greater stability and resilience than green (hulled) coffee.
Perhaps the most definitive study into the storage of green coffee was carried out by the U.S. military and documented in 1946 in the U.S. War Department technical manual. It documents approximate keeping times for green coffee of three to five years depending on the temperature, humidity and physical aspects of the storage facility. For coffee kept within a relative humidity range of 55-60% the keeping period falls as the temperature rises, being 5, 4 and 3-4 years at respectively 4°C, 21°C and 32°C.
It goes on to suggest that at 4°C green coffee would probably keep for an indefinite period and identifies 60% relative humidity as the level at which coffee deteriorates. The storage times relate to coffee that was ‘perfectly’ dried prior to storage. The precise conditions provided for the trial are clearly beyond the capability of private organizations in commercial practice but they serve to illustrate the stability and resilience of properly cured green coffee under ideal storage conditions.
There is no general agreement about the optimum storage period for coffee. Parchment coffee is considered to have ‘matured’ after four weeks in store and after six months produces cupped coffee with a distinctly ‘woody’ flavor. Some suggest the flavor of green coffee improves with storage allowing time for development of favorable aroma precursor compounds.
More clear cut are the benefits of special storage conditions for several now well recognized ‘aged’ coffees although most all appear to have originated a long time ago and more by accident than design. This has been proven in practice for the so-called harsh ‘Rio-y’ coffees from Brazil. ‘Rio-y’ (spoken with an extended ‘i’ even though it originates from the state of Rio) is a cup quality classification term describing a distinctive flavor in which the coffee tastes harsh, medicinal, inky or iodine-y.
Most consuming countries shun and dismiss the taste as serious bean defect but it is avidly sought after in Middle Eastern and Eastern Mediterranean countries like Turkey, Greece and Cyprus and is the factor giving Middle Eastern coffees their distinctive flavor. The ‘Rio-y’ taste can be traced back to the activity of specific molds becoming active on ripe and over-ripe cherry during wet season harvests in the central region of Brazil. With heavy and slow-to-clear morning mists molds proliferate and the cherry acquires the basis for the ‘Rio-y’ flavor before it is even picked. The flavor thought to result from butyric fermentation is enhanced during drying, especially if the cherries are not turned frequently, and with extended storage.
Monsooned coffee from India is another example of aged coffee benefiting from extended storage under special conditions and with an even more bizarre beginning. In name and taste this unwashed Arabica is called Monsooned Malabar. The Malabar Coast is the western shore of the state of Kerala in southern India from where much coffee traditionally left India. In the early 18th century days of the sailing ship, coffee took months to reach Europe. During the voyage, the beans developed a characteristic and unique taste caused by prolonged exposure to sea-air and high humidity. And the color changed from green to a strange shade of yellow.
Many European consumers became fond of and attached to the enhanced body and enriched if slightly strange flavor of this well traveled coffee. When steamers replaced sailing ships and drastically cut journey time the Indians started to copy the conditions experienced on long sea voyages. They artificially induced the flavor by exposing coffee for six weeks to the humid southwest monsoon winds that begin in the month of May or June.
More widely known and appreciated are the aged Indonesian coffees traditionally coming from Java but also Sumatra and Sulawesi. The ageing process is not simply an extension of normal storage conditions but is carried out in the hot steamy naked climate of South East Asia, where the earth smells rich and heavy of leaf mold and mushrooms and produces coffee to match.
Just like the Indian Monsooned Malabar, Old Java, Old Brown, and Old Government have their roots in the days of colonial governors and sailing ships. Once coffee production was well underway in the Dutch East Indies (Indonesia) surplus beans piled high, sweating profusely in the heavy humid heat and turning yellow after several years and eventually into a deep shade of brown.
Under contemporary thinking these origins would have been disregarded as lacking sparkle (acidity), bland and well past their ‘sell by date.’ Enterprising coffee connoisseurs harnessed the ageing process to produce a very special low acidity origin with mellow, earthy notes that drinkers could relate to the heavily spiced atmosphere of Indonesia.
With an inherent low acidity reduced to rock bottom and sweetness enhanced the brew gives an even smoother dense cup with body full to the brim. The fame of these increasingly rare aged Indonesian coffees has led connoisseurs to suggest replacing after dinner liqueurs with a small cup of Old Java for a completely new experience — mellow, sweet, and syrupy and imparting a warm glow, all without alcohol.
In the wider market there are specific distinctions relating to age of coffee beans such as new crop coffee (freshly harvested and processed), second crops and old crops. The latter will have been stored for longer periods in the origin country by necessity or design, but find equally ready acceptance on the market for distinctive separate qualities.
Given a wide-range of variables relating to type and origin of coffee as well as storage conditions, it is difficult to define the shelf life for green coffee. In practice temperature and humidity are likely to fluctuate widely over the time coffee is actually in store. Any prediction or appraisal of quality and flavor change needs to be considered for the regimes of temperature and humidity prevailing over the entire period of storage.
Color change with storage is an important consideration especially in producing countries where color of cured green coffee is used to differentiate quality. Beans stored for a brief time at a high temperature with high humidity can lose their color as well as their flavor. The color of green coffee beans is distinctively varied according to type and source and can therefore be used to as a preliminary measure of quality.
Wet processed Arabica premium coffee beans are uniformly blue-green to grey-green. Dry-processed Robusta beans are generally brown but vary markedly in shade and intensity of color. Color and appearance is used in Kenya and Brazil as a contributing factor in bean classification systems related to quality with ‘green’ coloration placing beans firmly at the high end of the quality scale. Inappropriate storage of green coffee invariably causes a bleaching of color from beans and therefore reduced visual appearance and quality rating.
About the Author: Dr. Terry Mabbett is a technical writer with a Ph.D. degree in Tropical Agriculture. He has worked in crop production and processing throughout the tropics – India, South East Asia, West Africa, and the Caribbean – and in his home country of the U.K. Dr Mabbett has been writing professionally for over 20 years.