Oxygen and Packaging

Staling of coffee occurs gradually as the result of numerous chemical processes affecting the coffee at different rates. The actual amount of time in which these processes take place will depend upon the state of the coffee (whole bean or ground) and conditions of storage (amount of oxygen contact, heat, moisture, and light).

Two basic processes take place during staling:

In the graph below, downward lines represent the flavors that are being lost, while upward lines represent undesirable stale flavors. The time indicated is under normal ambient conditions:

Any amount of oxygen absorbed by coffee eventually leads to staleness. However, as the result of sugar browning reactions during roasting, freshly roasted coffee exudes carbon dioxide for up to a week in its whole bean form. For this reason, the highest quality coffees are typically packaged as soon as possible after roasting in a moisture-proof laminated bag containing a one-way valve. This valve allows the carbon dioxide to escape without “ballooning” or rupturing the bag, but does not allow oxygen to enter.

Other types of valves allow carbon dioxide to be expelled through a plastic covered pinhole. This prevents carbon dioxide buildup within the package, but once the carbon dioxide is no longer creating a positive pressure on the coffee side, oxygen can leak into the bag. This is a good solution if the product will move through a system quickly.

If the coffee is to be packaged using no valves (including fractionally packaged ground coffee), the coffee must rest (“degas”) before it is packaged. It will pick up some oxygen during the degassing process which will limit shelf life, but otherwise the bag will balloon and possibly rupture, exposing the coffee to ambient levels of oxygen.

Packaging material: Coffee is packaged in materials ranging from paper bags to heavy foil laminates and cans. The packaging type also plays a marketing role. In terms of coffee shelf life, the type should be sealed easily (usually by heat) and moisture-proof.

Oxygen content of no more than 3% in the package is suggested. To ensure freshness at the brew stage, a “use by” date printed on the bags as well as the Julian calendar number printed on the case for the benefit of the retailer might be desirable. If ground coffee is packaged for an individual serving, the weight of coffee per package can be measured (see “Water to Coffee Ratio” in the “Brewing” section following). The actual “use-by” date depends upon the standards of the manufacturer.

Packaging and the maintenance of packaging equipment is one of the biggest challenges in the roasting facility. Coffee Analysts has designed shelf–life tests so that several samples from a batch are tested at once. Especially in the case of packaging, only testing one bag will not give adequate information about the packaging system.

Use of the leak detector: When excessive oxygen is found, it is important to determine the source. If the equipment is not sealing the bag properly, pinhole leaks can develop that allow small amounts of oxygen into the package over time (immediately after manufacture, no oxygen will be detected). By isolating the location of these leaks, the equipment can be adjusted as necessary.

Often, no leaks are detected in a package with excessive oxygen. This can be a sign that the bags are not being fully flushed with nitrogen, possibly as the result of the line moving too fast.

For packages with valves, testing for both carbon dioxide and oxygen is recommended. If the coffee was packaged soon after roasting the carbon dioxide level can be as high as 80% after a week. If no carbon dioxide is found in a plastic-covered pinhole valve, the shelf life of the product may be limited.

For serving size roasted ground coffee, it is suggested that a random case of product be sent for analysis. All bags are quickly checked for leaks and 20% of the total are tested for oxygen and leaks.

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