About Vacuum Packaging
Vacuum packaging does not make coffee last forever, in fact there have been some coffees that we've pulled from shrub because of them showing a little wear on the cupping table. Tremendous efforts have been put into better understanding the various elements that contribute to the production of a truly remarkable coffee. It's now commonplace to see not only the roast date and geographic specifics on a bag of roasted coffee, but also statistical information that reveals cultivation and processing specific details. This information is perhaps more tangible and objective than a standard flavor description in terms of telling the story of any particular coffee, but all of this information means absolutely nothing if a coffee in its green state has not been stored and handled properly.
The shortcomings of packaging coffee in traditional natural fiber bags are now becoming more widely understood; the main shortcoming being the material's inability to stabilize the water activity of the coffee. This isn't just a concern during coffees' shipment from origin to port in the U.S., but in many ways it is even more of a concern during transport across the U.S. and storage at a roasting facility. Water activity is defined as the ratio of the water vapor pressure in a material to the water vapor pressure of pure water at the same temperature, and it is responsible for exchanging materials in and out of a coffee when the moisture equilibrium is thrown out of balance. It is commonly understood that the moisture content of green coffee must be below 13% percent and is optimal at about 11%. As a coffee is moved through different environments with dramatic ranges of temperature and humidity, or even stored in a warehouse without a controlled climate of both temperature and humidity, the fluctuating equilibrium of the coffees' water activity can cause the coffee to lose moisture and allow important volatiles that distinguish the coffee to oxidize, or allow the coffee to take on more moisture and along with it take on undesirable flavor characteristics or even molding.
Research has shown that if the ideal moisture content of a coffee is 11%, then the ideal level of relative humidity needed to stabilize the water activity is about 60% with a temperature of about 75 degrees. Taking regular moisture readings of your green coffee and monitoring the temperature and humidity of your storage facility are important quality control measures, but you can protect your green coffee from water activity fluctuations by storing it in an hermetically sealed container or in a vacuum packed and nitrogen flushed "brick". Storage in a Grain-Pro lined bag or box also helps stabilize the coffee and reduce water activity.
When vac-packing a coffee, it is crucial to maintaining the integrity of a coffee that it has been well rested and has a stabilized water activity and low enough moisture content in order to prevent the development of off flavors and molds within the "protected" packaging. If you are simply exchanging the packaging of a green coffee and not roasting it, even if you are moving it from one hermetically sealed environment to another, you must be sure that the water activity is stabilized. Also, it is important to note whether or not the environment that it is being opened into differs greatly from the ideal 60% humidity and 75 degree temp. The degree to which you may be off in these factors could greatly determine what kind of a window of time a coffee might need to either rest or be used or repackaged in relation to realizing and maintaining a coffee's quality.
Despite the research that has gone into the packaging of food products in general, there is virtually nothing to be found on usage of a product once it has been removed from its protective packaging. One coffee professional who uses vac-packing told me that it is best to use a coffee right away once you have opened the packaging while some coffee roasters familiar with vac-packing pierce the bags upon arrival in an effort to normalize them within the climate of their roasting shops (the nitrogen flushing process that Coffee Shrub uses is an effort to help protect a pierced package from taking in unwanted oxygen). What we can draw from this is that no matter whether you rest or instantly use your coffee after opening it that it is best to roast it as soon as possible, and the vac-packing volume that is used at the shrub and widely throughout the specialty coffee industry is close to a single batch size or two depending on the size of roasting equipment used.
This whole vac-packing coffee endeavor is rather new and still largely experimental. Most of the in depth research on it is twenty or more years old and was done to determine shrinkage and loss, and not on a possible way of maintaining a coffee's quality. We are still learning about how to best preserve a coffee in its raw state. Vac-packing is used to try to keep a green coffee as fresh as you can for as long as you can and it does succeed in extending its life in its best condition, but even this can't keep it forever. Coffee ages, which has always been one of the things that makes working with exceptional coffees both frustrating and exciting. In my opinion, I don't feel that vac-packing has added any unnatural length to the lives of these coffees, only given them a fighting chance to last as long as they could. This really creates a whole different insight into the use of the buzz word "seasonality" for selling coffee. We need to embrace the things like this that make coffee such a unique product and work to better understand them, rather than use language that simplifies all of these complexities without really explaining the processes.
For more information on Coffee Packaging and Water Activity look at these sources (which were used extensively for this posting):
Roast Magazine May/June Issue, For the Keeping, By Dr. Luke W. Harris and Andrew Miller
Roast Magazine July/August Issue, Keeping it Real, By Dr. Luke W. Harris and Andrew Miller