Pg. 32-35 Litchi (Litchi Chinensis Sonn) Pericarp Colour Retention

The South African litchi industry is predominantly export orientated and pericarp (skin) colour retention is of primary importance, where export markets demand red fruit. Until now, gaseous sulphur treatments were thought to play a major role in the maintenance of this colour by firstly, fixing the anthocyanins, and secondly acting as a fungicide thus preventing browning reactions. Sulphuring of fruit however, results in undesirable aftertastes, constitutes a health problem and is ineffective against some fungi. In addition, since sulphur-fixed anthocyanins are colourless, the sulphuring process is then reversed by dipping the fruit in low pH solutions. Depending on the pH of the solution, the anthocyanins reflect light at different wavelengths, ranging from red to blue. At a pH around 5.0, they may even be colourless due to the excessive loss of protons from their structure. The pH of the solution is thus critical and investigations are implicated.

Summary:

• Pericarp (skin) color retention is critically important for the South African litchi export market, which demands red-colored fruit.
• Historically, sulphur dioxide (SO₂) treatments were used to maintain red color by fixing anthocyanins (red pigments) and acting as a fungicide to prevent browning.
• SO₂ treatment has drawbacks including undesirable aftertaste, health risks, ineffectiveness against some fungi, and colour loss issues since sulphur-fixed anthocyanins are initially colourless.
• Post-treatment dipping of fruit in low pH solutions (e.g., dilute hydrochloric acid) helps restore red color by altering anthocyanin light reflection, but pH must be carefully controlled.
• Anthocyanins are water-soluble pigments belonging to flavonoids and are responsible for the red color; their structure changes with pH and temperature, affecting color stability.
• Anthocyanins degrade via chemical and enzymatic reactions during storage, influenced by pH, temperature, oxygen, and enzymatic activities such as polyphenol oxidase and peroxidase.
• pH plays a major role in color changes; increasing pH leads to proton loss in anthocyanin structures, causing a shift in color or loss of color.
• Colour loss is also accelerated by desiccation and fungal attack; micro-cracks on the fruit expose pigments to degradation.
• Attempts to retain color include various chemical dips (ascorbic acid, citric acid), wax coatings, blanching, plastic packaging, and controlled atmosphere storage but with limited success.
• Aluminium and tin complexes with anthocyanins show promise for stabilizing red coloration through pigment fixation.
• SO₂ residues in fruit and tighter export regulations are encouraging the search for sulphur-free alternatives.
• Understanding the physiological and chemical mechanisms controlling pericarp color is essential for developing effective color retention strategies.
• The article concludes that pH stabilization, fungal control, and pigment fixation methods should be further explored to maintain red color while minimizing chemical treatments.