Preparation for Market

THE lemons should be brought to the packing area soon after harvest to begin the steps of preparing the fruit for market. These steps involve cleaning, grading, and packing. In addition, fruit destined for export may need to be treated with ethylene in order to improve the external peel colour. The ethylene treatment should be done prior to cleaning, grading, and waxing.

De-Greening
Lemon fruits may have sufficient juice content for harvest when the peel is still green. However, some export markets prefer yellow-coloured lemon fruit. In order to change the external colour and de-green the peel, lemons can be exposed to either ethylene gas or liquid ethylene. These treatments break down the green chlorophyll pigment in the peel surface and allow the yellow carotenoid pigments to be expressed. Ethylene treatment is solely cosmetic in effect and does not alter the flavour of the fruit or the juice content.

The de-greening protocol involves exposing green-skinned lemons to low concentrations of ethylene (usually between 1 to 10 ppm) at 20-25’C (68-78’F), 907o humidity for several days. The optimal ethylene concentration and treatment duration vary by the cultivar and growing conditions. Excessive ethylene (above 10 ppm) can cause stem end rot and increase decay. In order to achieve good de-greening results, adequate internal air movement is necessary. Also, regular ventilation with fresh outside air is needed to keep the COz levels inside the treatment chamber low enough (below 2000 ppm) to avoid the inhibition that high COz levels have on the effectiveness of ethylene.

A liquid ethylene-releasing compound, called ethephon [(2-chloroethyl) phosphonic acid, is another effective de-greening agent. It is applied by dipping the lemons for 1 minute in a tank of clean water with 500 ppm ethephon. It is important that the water is properly sanitised with chlorine (150 ppm hypochlorous acid at a pH of 6.5). A fungicide should also be added to prevent postharvest decay. Recommended fungicides are benomyl (500 ppm active ingredient) or thiabendazole or imazalil (1000 ppm active ingredient).
A negative consequence of de-greening lemons is the increase in senescence of the stem end and abscission of the button. De-greening should always be done prior to waxing. The wax coating will partially restrict gas exchange through the peel and inhibit the action of ethylene.

CLEANING
Washing of the lemons after harvest is necessary to improve the appearance of the fruit by removing dirt, sooty mould, scale insects, and spray residues. Lemons can be cleaned manually by hand rubbing or brushing individual fruit dumped in a tank of sanitised water with detergent. Either sodium hypochlorite or sodium o-phenylphenate (SOPP) can be used as sanitising agents for the wash water. Sodium hypochlorite is readily available in the form of household bleach. It is typically sold in a 5.25Yo solution.

The wash water should be sanitised with a 150 ppm sodium hypochlorite concentration and maintained at a pH of 6.5. Lemons can also be cleaned mechanically by passing the fruit over a series of soft-bristled roller brushes. The lemons are thoroughly wetted as they pass under a series of spray nozzles. Rotating brushes will remove most of the debris. Soap or detergent may be added to the fruit to improve the effectiveness of the cleaning operation as the fruit continues across the brushes.

Adequate cleaning usually requires about 30 seconds on the brushes, rotating at about 100 rpm. The fruit is then thoroughly rinsed as it passes over the last of the brushes. Excess water on the fruit can be eliminated with sponge rollers. The lemons may then continue down a slow-moving conveyor belt for sorting and grading or be put on a mesh or screen table for drying and eventual fading.

Thiabendazole (TBZ), imazalil, and benomyl are the most effective postharvest fungicides for lemons and can be applied as high-pressure sprays after washing. They are typically applied at a dose of 500 to 1000 ppm active ingredient in water. These fungicides can also be applied in water-emulsion waxes.