DRIED LITCHIS
SCIENTIFIC NAME: Litchi chinensis
FAMILY: Sapindaceae
An interesting processed product of the litchi is the fruit in dried condition. The taste of the fruit flesh of the dried litchi is comparable to that of raisins of very high quality, but is sweeter and has an exceptionally characteristic taste.
So far as the appearance of the dried product is concerned, the litchi is exceptional in this respect that the flesh and the seed shrink while the skin hardens and retains its normal size. The colour of the skin also remains fixed, the final product presenting the appearance of well-coloured, fresh litchis. Only ripe fruit which has not been stung by insects should be used for drying. The drying process should commence as soon as possible (within 1 to 2 hours) after picking. If the fruit shows dust or spray deposit it must first be washed in a diluted sodium hydroxide solution (0.25 per cent). The fruit is thereafter washed in clean water.
Before starting with the actual drying of the litchis, the fruit must first be treated with sulphur-dioxide gas to preserve the colour of the skin and the flesh. The sulphur-dioxide also improves the porosity of the skin with the result that moisture can more readily evaporate through the skin from inside. Fruit thus treated before drying has a much better quality, in respect of colour and general appearance, than litchis dried without treatment. The sulphur-dioxide treatment can be applied in different ways. The fruit can be fumigated by burning sulphur in a closed container (¾ lb sulphur/100 cubic ft air in a closed container is recommended). The container must not be made of iron or any other material that can be damaged by H2SO3). Care must be taken that there is not too much moisture in the container which can combine with the SO2 to form H2SO3, which diminishes the effect of the sulphur dioxide. The litchis must be packed on shelves in the container and fumigation lasts about 15 minutes. The sulphur should burn throughout the entire period. The disadvantages of this method are the formation of H2SO3, the fact that sulphur does not burn readily, and the ineffective distribution of SO2 vapours in the container.
Another method is to prepare SO2 from sodium sulphite (Na2SO3) and hydrochloric acid (using 1388g Na2SO3/100 cubic ft of air in the container). The hydrochloric acid (2 litre 0.5N HCI/1388g Na2SO3) is dripped on the sodium sulphite while the air in the container is circulated. This fumigation also lasts about 15 minutes.
An additional method is to dip the litchis into a 2 per cent solution of sodium metabisulphite (Na2S2O5). (150 mg solution/100 g fruit is recommended). Jain (1959) recommends that the fruit be kept in the solution for 48 hours; It was however found that a shorter period is sufficient. Fruit immersed in the solution for more than two hours did not recover the red colour. As a substitute for Na2S2O5, potassium metabisulphite (K2S2O5) can also be used.
It is also maintained that the fruit must first be bleached in boiling water for five minutes, or in steam at atmospheric pressure for 15 minutes before treatment with SO2 and that a further immersion in diluted citric acid improves the taste of dried litchis. It should also be borne in mind that the treatment should be kept as light as possible for good taste and that oven-drying requires less fumigation than sun-drying. If the colour of the fruit is not restored after treatment, the fruit can briefly be immersed in a weak hydrochloric solution to recover the colour. The fruit must then be rinsed in clean water.
The actual drying of the litchis should begin in the sun because sunlight restores the natural colour of the treated fruit. This can be done on wire shelves, but the fruit must be turned from time to time so that the colour can develop on all sides. As soon as the colour of the fruit appears, the fruit can either be further dried in the sun or in an oven. The impression exists overseas that sun-dried fruit has a better quality than oven-dried fruit. It has however been found that oven-drying is a much more convenient and simpler method and that good results can be obtained at low temperatures (below 80°F). This also eliminates the problem of cloudy or rainy days.
The fruit must be dried to a stage where the flesh contains only about 30% of the original moisture. Although litchis dry very easily, there are chiefly two points which must be observed. In the first place, care must be taken that the skin does not shrink together with the flesh, which usually occurs as the result of too rapid drying at the beginning of the drying period. A lower drying temperature for the first six to twelve hours should solve this problem. In the next place it sometimes happens that the skins split because the skins themselves dry and shrink too rapidly. This can be avoided by first drying the fruit in the shade for approximately two hours after SO2 treatment.
The keeping quality of well-dried litchis can, immediately after drying, be kept in plastic bags or can, after husking and stoning, be packed like dates in a plastic material.
DATE: January 1981
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