The peel, husk, and coffee silver skin can potentially become a source of antioxidants.

During the coffee processing stage, most attention is typically focused on the green coffee bean. However, to obtain coffee beans, producers must remove various layers of material, including the peel, pulp, husk, and silver skin.

These parts are often considered by-products, although they still contain phenolic compounds with antioxidant activity in experimental systems. A study published on November 9, 2026, in the Discover Food journal investigated the possibility of recovering these compounds using the glycerol-water solvent, followed by the application of the extract in mayonnaise to evaluate its ability to slow down oxidation.

The study did not demonstrate that all coffee by-products can be used directly as food. The value of the research lies in the investigation of a specific extraction process and the application of the obtained compounds.

Four types of by-products were included in the study

The authors investigated four lines of by-products from Coffea arabica:

• Skin & pulp: the peel and pulp separated during the initial processing stage.
• Husk: the by-product of the peel obtained depending on the processing method and material classification.
• Silverskin: the thin layer of coffee silver skin surrounding the bean, which often peels off during roasting.
• Parchment: the husk surrounding the green coffee bean, removed during the hulling process.

These by-products differ in structure, chemical composition, and origin, so they cannot be grouped into a single raw material and assumed to have the same extraction efficiency.

Why was glycerol-water used as the solvent?

To recover phenolic compounds, the research team used a microwave-assisted extraction method combined with an aqueous deep eutectic solvent.

In the system investigated, glycerol combined with water formed a solvent capable of dissolving some polar compounds. Glycerol is a polyol used in various food and industrial applications. However, the use of a familiar solvent component does not automatically mean that the entire extract after the process meets food standards.

The authors compared three solvent systems. The glycerol-water system with a 61.3% volume ratio of glycerol showed the highest total phenolic content in the tested conditions.

Which by-product has a higher phenolic content?

In the optimal extraction conditions of the study, two prominent raw materials were:

• Skin & pulp: total phenolic content of 16.5 mg GAE/g.
• Husk: total phenolic content of 15.2 mg GAE/g.

GAE is the abbreviation for gallic acid equivalent, which is a way to express the result of the total phenolic content test, not a claim that each gram of by-product contains the corresponding amount of gallic acid.

The compounds analyzed showed chlorogenic acids, particularly 5-CQA, as a notable group of phenolics. The reported amounts in the by-product lines ranged from 0.07 to 2.3 mg/g. Caffeine was also detected, with amounts ranging from 5.3 to 13.6 mg/g depending on the by-product.

Thus, the by-product not only contains compounds with potential antioxidant activity but also contains caffeine, so the composition of the extract should be determined before considering its application in food.

From extraction to application in mayonnaise

To check the applicability, the research group added the extracts to mayonnaise – an oil-in-water emulsion containing a phase oil prone to oxidation.

The authors monitored two important indices:

• Peroxide value (PV): an index reflecting primary oxidation products, particularly hydroperoxides formed in the early stage of lipid oxidation.
• p-Anisidine value (p-AV): an index commonly used to evaluate some secondary oxidation products, mainly aldehydes.

The results showed that adding the extracts significantly reduced the peroxide and p-anisidine indices under experimental conditions. Notably, the most prominent effect was observed in the formula using extracts from skin & pulp at a ratio of 1.5%.

The research group also noted that the color of the mayonnaise changed due to natural pigments in the extracts. Meanwhile, the pH ranged from 4.3 to 4.4, and the textural characteristics such as hardness, stickiness, adhesion, and elasticity generally did not change significantly.

This result is specific to a particular formula and storage conditions. It should not be inferred that all coffee by-products can replace commercial antioxidants or extend the shelf life of all types of food.

Not all coffee by-products can be used in food

To transform by-products into food-grade materials, manufacturers must control many issues beyond phenolic content:

• Raw material quality: by-products must be collected, sorted, and stored properly to avoid mold and contamination.
• Residual chemicals: check for pesticides, heavy metals, and unwanted substances depending on the raw material source.
• Extraction process: determine the solvent composition, treatment conditions, and residual solvent levels in the final product.
• Extract composition: quantify caffeine, polyphenols, and other compounds that may affect flavor, taste, or safety.
• Stability: evaluate the antioxidant activity over actual storage time, not just at a single test point.

Especially, a laboratory test of antioxidant activity does not equate to the health benefits demonstrated when humans consume food containing the extracts. This study assesses the ability of mayonnaise to limit oxidation, not a clinical trial.

The value of by-products lies in controlled recovery

In coffee production, by-products are often viewed through the lens of processing costs or value as fertilizer, animal feed, and biomass fuel. The new study shows a different direction: recovering phenolic compounds to create functional ingredients in food systems.

However, economic efficiency still depends on the amount of by-products that can be collected, drying and transportation costs, extraction efficiency, solvent reuse potential, safety standards, and market demand. A good result in the laboratory is not enough to confirm a profitable production model at industrial scale.

What is noteworthy is that coffee by-products can be evaluated as a line of raw materials with their own unique chemical composition, rather than considering all as uniform waste. When properly classified and selected, a portion of the remaining value after processing can be recovered in a circular economy direction.

References

• Adoyo, G. O., Owino, W. O., Wafula, E. N., Engelsen, S. B. & Abong, G. O. (2026). Green extraction of bioactive compounds from Coffea arabica sidestreams with aqueous deep eutectic solvent and their application in mayonnaise as antioxidant agents. Discover Food. https://doi.org/10.1007/s44187-026-01300-5