Sustainable strategies to obtain bioactive compounds from citrus peels by supercritical fluid extraction, ultrasound-assisted extraction, and natural deep eutectic solvents

Citrus peels, often discarded as waste, harbor a treasure trove of bioactive compounds—and a new green strategy unlocks them without harmful solvents.

Every year, the world produces over 152 million tons of citrus fruit, and nearly half of that weight ends up as waste—peels, seeds, and pomace. These leftovers are more than just garbage; they're packed with valuable bioactive compounds. But extracting them has often relied on toxic solvents and energy-hungry methods. Now, researchers propose a cleaner, sequential approach using supercritical CO2 and ultrasound-assisted extraction with natural deep eutectic solvents (NaDES).

The team tested grapefruit, lime, and lemon peels. They found that a specific NaDES—Choline Chloride:Tartaric acid (1:2) for grapefruit and lemon, and Choline Chloride:Glycerol (1:2) for lime—with 50% water, pulled out the highest phenolic content. Surprisingly, pre-treatments like cryogenic grinding or supercritical CO2 didn't boost phenolic recovery; they actually reduced efficiency by shrinking particle size and increasing surface tension. Direct UAE-NaDES extracts showed the greatest phenolic diversity, with naringin dominating grapefruit and hesperidin in lime and lemon.

But the supercritical CO2 step wasn't a dead end. It delivered extracts with higher anticholinergic capacity and a rich terpenoid profile. This sequential strategy—first SC-CO2, then UAE-NaDES—offers a more holistic use of citrus peels, turning waste into a goldmine of health-promoting compounds.

Key Points

• Direct UAE-NaDES yields highest phenolic diversity in citrus peels.
• SC-CO2 pretreatment doesn't boost phenolics but enriches terpenoids.
• NaDES with 50% water optimizes extraction from grapefruit, lime, lemon.