Fresh-cut Asian vegetables - pak choi as a model leafy vegetable

Abstract

Pre-prepared fresh-cut salads are becoming increasingly common in the marketplace. Once dominated by lettuce, new vegetables are now being added to increase both flavour and visual appeal. A wide range of Asian brassicas is being sourced as constituents, but short shelf life because of yellowing is a problem to be contended with, and pak choi is a good example of this.
Yellowing in pak choi leaves is associated with a depletion of sugars (the main energy substrate). Increasing the initial leaf sugar level, or slowing the rate of sugar depletion, will directly increase shelf life. Sugars tend to be highest in younger leaves growing close to the tip, and lowest in leaves towards the base of the stem, even though the leaves may look similar in size and appearance. Removal of older leaves will therefore increase the life of a salad. Harvesting later in the day can also increase sugar levels, a result of photosynthesis during the day. Once harvested, leaves require sanitary washing and drying before packaging to avoid postharvest rots. Plastic packaging is vital to prevent wilting of leaves, but may also be used to provide an atmosphere conducive to slowing the rate of sugar depletion. Low oxygen (0.5–2% O2) and enhanced carbon dioxide (2–10% CO2) have been found to almost double shelf life in pak choi. However, for modified-atmosphere packaging to maintain an ideal atmosphere, stable temperature management is required, as high temperatures may lead to anaerobiosis and carbon dioxide toxicity of leaves. Common temperatures used for handling packaged salads range from 4° to 12°C. The above findings are for pak choi, but appear to apply also to many other Asian leafy brassicas used in fresh-cut salads.