Pg. 12-14 Steam and Low pH Dips Improve Litchi (Litchi Chinesis Sonn) Pericarp Colour Retention

The red pericarp (skin) is an important attribute in the appeal of fresh litchi or lychee fruit in the market place. At room temperature, fruit will rapidly discolour due to breakdown of the anthocyanin pigments and are less attractive. In recent years fruit from several countries, including South Africa, have simply received a postharvest gaseous sulphur treatment to overcome browning. However, at best fruit are light green, yellow or pink and they never attain their initial red colour. In addition, several workers have examined the use of low pH solutions in conjunction with sulphur treatments, but the results were not too promising. Kaiser (1994) implemented a novel technique using boiling water to break down the cell membranes, followed by immersion in zero pH, thus eliminating the need for using sulphur. This red colour was retained, but the pulp browned. Steam was then substituted for boiling water and a 2 s treatment of steam (95°C) followed by immersion for 4 min in zero pH and then dipping in 1% Vaporgard^® solution resulted in firm red fruit with no internal browning and acceptable taste (rated on a scale of 0- 5) after 28 days of cold storage at 1°C, compared with sulphur-treated fruit and controls. Consequently, commercial trials of this treatment are recommended.

Summary:

• The red pericarp (skin) color is a key factor in the market appeal of fresh litchi fruit; however, the color rapidly deteriorates at room temperature due to breakdown of anthocyanin pigments.
• Commonly, postharvest treatment involves sulphur dioxide gas fumigation, which prevents browning but results in fruit that is light green, yellow, or pink instead of bright red, and introduces an undesirable aftertaste and health concerns.
• Low pH dips have been tested to maintain red color but results were not promising when combined with sulphur treatments.
• Kaiser (1994) developed a novel technique using boiling water to disrupt cell membranes, followed by immersion in a zero pH solution, which eliminated the need for sulphur and retained the red color but caused pulp browning.
• The study replaced boiling water with a brief (2 seconds) steam treatment at 95°C, followed by immersion in zero pH solution and dipping in 1% Vaporgard^® solution.
• This steam and low pH treatment maintained firm, red-skinned fruit with no internal browning and an acceptable taste after 28 days of cold storage at 1°C, outperforming sulphur-treated and control fruit.
• Ultrastructural examination showed boiling water disrupted cuticles and cell membranes, causing pulp discoloration, while steam treatment was less harsh.
• The steam treatment increased total soluble solids (TSS) due to moisture loss, but adding Vaporgard^® reduced moisture loss and preserved juiciness and texture.
• Sensory evaluation showed improved color, firmness, taste, and juiciness with steam plus Vaporgard^® treatment compared to sulphur-treated fruit.
• The study concludes that the 2-second steam treatment followed by low pH immersion and Vaporgard^® dip is a promising alternative for commercial use to retain litchi skin color and fruit quality without sulphur.
• Commercial trials of this method are recommended for the export and local markets given the increasing consumer resistance to sulphur-treated fruit.