The lychee fruit, with its striking red skin and translucent, sweet flesh, is highly prized in global markets, but its delicate nature presents a significant post-harvest challenge. This problem, known commercially as “lychee brown,” refers to the rapid discoloration of the fruit’s vibrant outer layer after it is picked. The change from an attractive crimson to an undesirable brown hue severely limits the fruit’s shelf life and marketability, becoming a major constraint for commerce and a disappointment for consumers. Understanding the biological and environmental forces behind this rapid decay is the first step toward preserving the lychee’s unique appeal.
The Anatomy of Lychee Browning
Post-harvest browning is an anatomical event confined almost entirely to the fruit’s skin, or pericarp, while the edible inner portion, called the aril, remains largely unaffected in its appearance. The lychee pericarp naturally contains the red pigments, primarily anthocyanins, that give the fruit its characteristic color. When the fruit browns, these pigments are degraded, and new, darker compounds are formed. The result is a loss of the bright red color, which is replaced by a dull, mottled, or uniform brown surface.
The pericarp is composed of several layers, and the browning reaction is most noticeable in the outer layers, where the color compounds and browning enzymes are concentrated. The delicate pericarp is structurally prone to water loss and physical damage, which triggers the internal chemical process that results in the visible color change.
The Chemical Cause of Color Change
The underlying mechanism for lychee browning is an enzymatic oxidation reaction initiated when the fruit’s cellular compartments break down. The key players in this process are naturally occurring chemical compounds called phenolic compounds, which are abundant in the lychee pericarp, and an enzyme known as Polyphenol Oxidase (PPO). These two components are normally separated within the fruit’s cells: the phenolic compounds are sequestered in the vacuole, and the PPO enzyme is in the cytoplasm.
When the fruit is harvested, stressed, or damaged, the cell membrane integrity is compromised, leading to the mixing of the PPO enzyme and its phenolic substrates. Once mixed, the PPO catalyzes a rapid oxidation reaction, converting the colorless phenolic compounds into highly reactive molecules called quinones.
The quinones are unstable and quickly undergo further reactions, including polymerizing with other compounds, to form large, dark-colored polymers, which are the brown pigments seen on the fruit’s surface. This enzymatic cascade not only produces the brown color but also indirectly causes the degradation of the desirable red anthocyanin pigments, contributing to the overall loss of color.
Environmental Triggers of Spoilage
The enzymatic reaction that causes browning is set off by external, post-harvest stresses. Moisture loss, or desiccation, is considered the single most significant factor triggering pericarp browning because the lychee peel possesses no protective waxy layer. The rapid loss of water causes the tissue to shrink and results in microscopic cracks and increased membrane permeability, which initiates the PPO reaction.
High temperature is another major trigger, as it accelerates the fruit’s metabolic rate and increases the activity of the PPO enzyme. Storing lychees at ambient temperatures can lead to complete browning within 24 to 48 hours. Physical damage, such as abrasion, bruising, or cracking during harvesting and handling, also compromises the pericarp’s cellular integrity, providing a pathway for the enzyme and substrate to mix. Even chilling injury from storage at excessively low temperatures can disrupt cell membranes and accelerate browning once the fruit is returned to room temperature.
Extending Shelf Life
Commercial strategies to combat lychee browning focus on either inhibiting the PPO enzyme’s activity or minimizing the environmental triggers that cause cellular breakdown. Immediate and rapid pre-cooling of the fruit after harvest to temperatures between 2 and 5 degrees Celsius significantly slows down the fruit’s metabolism and enzyme activity. Maintaining a high relative humidity, typically between 90 and 95 percent, is also practiced to reduce the rate of water loss and prevent desiccation-induced cracking.
Sophisticated packaging techniques, such as modified atmosphere packaging (MAP) using polymeric films, help to create a localized environment with high humidity and altered gas levels, which further suppresses respiration and water loss. For long-distance transport, chemical treatments are frequently applied, with sulfur dioxide (\(text{SO}_2\)) fumigation being a common commercial method. The \(text{SO}_2\) acts by acidifying the pericarp, which inactivates the PPO enzyme and stabilizes the red anthocyanin pigments.
Alternative, non-sulfur treatments are also being explored, including dipping the fruit in acidic solutions or using antioxidants like ascorbic acid. These treatments work by lowering the pericarp’s pH to inhibit PPO, or by acting as competitive substrates to intercept the oxidation reaction. These combined post-harvest approaches are necessary to extend the lychee’s market life from a few days to several weeks.