Maximisation and Retention of Litchi Fruit Colour

Critical Guidelines: #

Sunlight and humidity:

  • Light is needed for the development of the red pigment, anthocyanin, which colours the fruit
    skin, but excessive sunlight and low humidity can break down this pigment.
  •  On farm trials indicate that litchis grown under shade netting develop better colour and more
    uniform colouration between individual fruit. In addition to improving fruit colour, shade
    netting produces a more humid environment. Shade netting may either be permanent or
    white drape nets placed over the trees as soon as the flower panicles start bending
    downwards.
  • It is important that there is sufficient light penetration in the orchard; inter-tree shading may
    result in poorer fruit colour development.
  • Research in India has shown that the use of overhead micro-jet sprinklers effectively increased
    relative humidity in the orchard aiding in fruit colouration.

Tree nutrition

  • Trees with high N levels (>1.6% leaf N) tend to have more green fruit, especially in young trees.
  • Apply the majority of N fertiliser after harvest to build the tree’s reserves to support flowering
    and fruit set. Limit N application after fruit set.
  • Foliar application of Magnesium chloride may aid in biosynthesis of anthocyanins and thus
    fruit colour development.
  • A 2% mono potassium phosphate (MKP) application on the developing fruit at the 10 g stage
    (2 cm x 3 cm) aids colour development in ‘Fay Zee Siu’ but not ‘Mauritius’.

Degree of fruit development

  • Fruit maturity varies between fruit on a tree and between trees in an orchard. Some fruits
    develop more quickly and colour up sooner than others. Strip picking results in a range of fruit
    maturities, different sizes and different degrees of colouration. To optimise fruit size and
    colour, a few rounds of selective picking are advised. Selective picking will result in a higher
    proportion of well-coloured fruit and fewer smaller fruit on the packline, as fruit left on the
    tree to colour up will, in most cases, also grow bigger.

Packhouse acid bath treatment

  • Litchi fruit colouration does not develop further after harvest.
  •  Post-harvest treatments cannot enhance the degree of colouration present at harvest.
  • SO2 fumigation bleaches the fruit, but some red colouration returns as the SO2 levels in the
    fruit skin dissipate.
  • Acid dips in the packhouse post SO2 fumigation speed up restoration of some of the original
    red colour.

Unsulphured litchis

  • In unsulphured litchis (exported by airfreight) modified atmosphere / modified humidity bags
    play a very important role in reducing fruit moisture loss, hence maintaining red colouration
    for longer.
  • There are different types of bags for different fruit types. All of these bags are perforated or
    micro-perforated to be semi permeable to water vapour, O2 and CO2.
  • StePac XTEND bags are commonly used (Stone fruit combination)- see the Services tab on
    stepac.com (http://www.stepac.com/post-harvest-services).
  • Litchis destined to be exported unsulphured should be picked early in the morning to reach
    the packhouse before 9 am to be packed and cooled to 10 °C as soon as possible before
    shipping to the airport in refrigerated transport. The fruit is then held at 10 °C in the export
    agent’s cold stores until loading into the aircraft. Approximately 80% of litchis exported by air
    freight are shipped in the lower deck (cargo hold) of passenger flights which is maintained at
    cabin temperature (18- 22 °C). Pallet caps help reduce the degree to which the fruit warms up
    during the flight.
  • In wet, humid seasons, free water can form in the bottom of modified atmosphere bags.
    Solutions to this problem e.g. absorbent pads in the bags or bags with higher degree of water
    vapour permeability or a mechanism to elevate fruit above where water would develop,
    should be considered and incorporated into packaging where possible.

Litchis on the branch / ‘bouquets’

  • Litchis exported unsulphured on the branch or in ‘bouquets’ fetch good prices. These fruits
    are also exported in modified atmosphere bags and may be more susceptible to water forming
    in the bag as they cannot currently be dried on traditional packlines.
  • Litchis exported in this format should not be picked when it is raining, and wet litchis should
    not be sent to the packhouse. However, damp litchis (no free water on surface) can be sent
    to the packhouse.
  • Packhouses should investigate modifying drying equipment to dry litchis on the branch.

Controlled atmosphere

  • Litchis exported by sea freight should be shipped in controlled atmosphere (CA) containers.
  • CA settings: 4% O2 and 8-10% CO2.
  • CA does not influence litchi colouration per se but reduces mould development.

Supportive Documents: #