From the Archives of the Rare Fruit Council of Australia, inc.
by S.J.R. Underhill and L.S. Wong


Seasons in Australia are opposite to those in the US.  Summer is Dec. Jan. Feb. Autumn is Mar. Apr. May. Winter is June July Aug. Spring is Sept. Oct. Nov.

A Maturity Standard for Lychee

Litchi chinensis
Sapindaceae


1. Introduction

As fruit mature they undergo a complex series of physiological changes, which dramatically alter their physical and biochemical properties (Nills et al., 1981; Paull et al., 1984). The assessment and subsequent correlation of such changes constitute the principal framework of postharvest maturity standard research. Depending on the end-use of the fruit and its specific quality attributes, some factors will have a greater practical relevance (Wills et al., 1981). Potential indices should be highly correlated to fruit quality either directly or in a secondary manner and display minimal cultivar variability and seasonal consistency.

Lychee maturity is of major concern to the Australian Industry (Menzel et al., 1988). In the absence of maturity regulations, immature fruit are being sold on the commercial markets. Although returns are initially high, poor quality and reduced product reputation dramatically lower subsequent seasonal prices (Sing and Tomes, 1987). When this is combined with the high incidence of 'first buyers' and product competition, the damage caused by retailing immature fruit is severe (Greer, 1989).

With rapidly increasing production (Sing and Tomes, 1987, Menzel et al., 1988) the industry has to expand product demand if price is to be maintained. The assurance of fruit quality through maturity and grade standards would help alleviate this problem to a great extent.

A wide variety of parameters have been previously trialled (Brown et al., unpub; and Batten 1986), with both Brix:Acid ratio and titratable acidity shown to have good potential (Brown unpublished) as maturity standard parameters.


2. Discussion

The Brix:Acid ration level of the pulp is calculated from both Total Soluble Solids (Sugars) and titratable Acidity (Acids). As fruit mature, their sugar content increases and the acidity level is reduced. The resultant effect is a logarithmic increase in the Brix:Acid ratio (Figure 1). The relationship between eating quality and brix: acid ratio depends on the stage of fruit development. Prior to obtaining full maturity, large changes in the Brix:Acid ratio account for relatively small maturity differences. On obtaining full maturity, the reverse situation occurs. If fruit are overmature or are beginning to senesce, the Brix:Acid ratio decreases. This is caused by reduced sugar levels through enzymatic breakdown.

Although eating quality is dramatically affected by both sugar and acid levels, other parameters are also involved. Texture, flavour and the presence of off-tastes are all an integral part of overall eating quality. As a consequence of these factors, the correlation between eating quality and Brix:Acid ratio cannot be absolute, and some error must exist. In addition to this inherent inaccuracy, difference created by both cultivar type, growing region and possibly seasonality will compound this variability. Although the relationship between Brix:Acid ratio is subject to all the above sources of error, compared to other tested parameters, it gave the best and most consistent correlation.

A. Threshold Value
The most important component of a maturity standard is the resultant threshold or minimum value. It is this process to which most of the concern is directed. The decision of where a minimal standard will lie should be the responsibility of the industry. In other crops, a recommendation has been put forward for adoption without active involvement of the Industry. The result is constant debate, requests for further work and general dissatisfaction. The recent situation with the Mango industry provides a good example.
There is no benefit in maintaining this approach as problems are sure to develop. By involving Industry in the developmental stages of maturity standard and allowing them to subsequently select the maturity standard, such problems will be reduced. To the pessimist, this may seem like a relinquishment of one's responsibility. However we feel that if the industry as a whole develops a standard, the interest of all parties will be served, and the potential of the standard enhanced.

3. Summary

The Brix:Acid Ratio of the pulp is highly correlated to eating quality of the fruit. There is little evidence that cultivar variability, growing conditions, seasonality or culture practices have a major impact on the nature of this relationship. Some variability was observed, and should be considered In the ultimate selection of a threshold value.

Figure 1. Brix:Acid ratio versus eating quality
Figure 1. Brix:Acid ratio versus eating quality
of Lychee for all cultivars and regions.

The acidity of the pulp was suggested as an alternative index of maturity. Our work supports the fact that Acidity and mean eating quality are well correlated . The potential of Acidity, however, is diminished by possible seasonal variability.

The Brix:Acid Ratio has some potential for use by the growers directly as an index for harvesting. It is suggested that skin colour, titratable acidity and general appearance should be considered as possible harvest indices. However consideration of the limitation involved should be addressed.

The Brix:Acid Ratio of the pulp is recommended for use as a Maturity Standard for Lychee on the commercial markets.



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Bibliography

Underhill, S.J.R. and Wong, L.S. "A maturity standard for lychee." rfcarchives.org.au. Archives of the Rare Fruit Council of Australia. May 1989. Web. 29 Mar. 2015.

Published 29 Mar. 2015 LR
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