Green coffee beans aged in storage: How does lipid oxidation affect flavor?

Green coffee beans are not a stable material during storage. Even before roasting, the beans continue to be affected by oxygen, temperature, humidity, and storage environment. One of the notable changes is lipid oxidation - a process that can alter the fatty acid composition and contribute to the formation of unwanted flavors.

This explains why two batches of coffee with the same origin and processing method can have different qualities after a few months in storage, if the storage conditions are different.

How do lipids in coffee beans change?

Coffee beans contain many lipid components, including triglycerides and fatty acids. When exposed to oxygen, some unsaturated fatty acids can undergo oxidation reactions. This process forms primary oxidation products such as hydroperoxides; they then continue to break down into secondary compounds, including aldehydes and ketones.

Some secondary compounds have low odor thresholds. When accumulated, they can contribute to the perception of old, rancid, or stale flavors or reduce the freshness of the roasted aroma.

Do not assume that all signs of old coffee are due to lipid oxidation. Flavor during storage is also affected by the loss of volatile compounds, changes in other organic compounds, humidity, temperature, and the structural state of the beans.

A study comparing burlap, LDPE bags, and GrainPro airtight bags

A 2022 study published in the Foods journal investigated the changes in lipid oxidation of green Arabica coffee beans under accelerated storage conditions.

The research group compared three types of packaging: burlap, LDPE bags, and commercial GrainPro airtight bags. The samples were stored at 30°C, 40°C, and 50°C, with a relative humidity of 50%, and then sampled at short intervals for analysis.

The results showed that high storage temperatures accelerated the quality decline of the beans. In the experimental conditions, the GrainPro group maintained better quality than the other two groups according to the monitored oxidation indices.

This is evidence that packaging can affect the rate of change of coffee beans. However, the study used accelerated storage conditions; it is not possible to directly convert 20 days at 50°C to a number of months in normal storage temperature.

What indicators help recognize the oxidation process?

In food chemistry research, lipid oxidation is often monitored by multiple indicators rather than a single measurement.

• Peroxide Value (PV): Reflects the amount of primary oxidation products, particularly peroxides and hydroperoxides.
• p-Anisidine Value (p-AV): Supports the evaluation of some secondary oxidation products, especially aldehydes.
• Acid Value (AV): Reflects the amount of free fatty acids, which can increase due to lipid hydrolysis and related changes.
• TBARS: A quantitative test for the group of compounds reacting with thiobarbituric acid, often used as an indicator of secondary oxidation products.
• Water Activity (Aw): Does not directly measure lipid oxidation, but helps evaluate the water state and the risk of changes during storage.

These indices need to be read together. For example, PV may increase initially and then decrease as peroxides continue to break down. Therefore, low PV does not automatically prove that the sample has never been oxidized.

Why is storage temperature important?

High temperatures often accelerate the rate of many chemical reactions. In coffee bean storage, temperature can also alter the moisture balance between the beans and the environment, thereby affecting the stability of the material.

A 2023 study on green coffee and roasted coffee storage conditions published in Scientific Reports also found that the sample stored at 20°C had a faster aging rate than the sample stored at 10°C or -10°C in the experimental design of the authors. This result does not mean that every roaster needs to freeze green coffee beans; cold storage also requires control of condensation when bringing the beans to normal environment.

Establishing a green coffee bean storage facility

For roasting and grinding facilities and green coffee bean trading companies, quality control in the storage facility should start with measurable and recordable factors such as

• Avoid placing bags directly on the floor or against walls that are prone to moisture
• Limit temperature and humidity fluctuations in the storage facility
• Use packaging that can effectively limit water vapor and oxygen exchange suitable for the expected storage period
• Do not open bags frequently if there is no plan for use; after opening, close tightly and record the date
• Store by lot according to the date of entry, processing, moisture, Aw, and storage history
• Regularly roast and cupping samples to monitor sensory changes

For specialty coffee or fermented coffee beans, the choice of packaging should also consider the stability of flavor compounds and the product's sensitivity to oxygen. A good packaging system cannot compensate for initial moisture content that is not suitable or a hot and humid storage facility

The value of green coffee beans is not only determined at the time of harvest and processing. The period from bagging to roasting is also part of the quality management process

References

1. Lipid Oxidation Changes of Arabica Green Coffee Beans during Accelerated Storage with Different Packaging Types. Foods, 2022. DOI: 10.3390/foods11193040.
2. Effect of green and roasted coffee storage conditions on selected characteristic quality parameters. Scientific Reports, 2023.
3. Influence of Packaging on the Storage Stability of Fermented Green Coffee Beans. Applied Sciences, 2026. DOI: 10.3390/app16125925.