VOLATILE ESTERS AND SULFUR COMPOUNDS IN DURIANS & A SUGGESTED APPROACH TO ENHANCING ECONOMIC VALUE OF DURIANS

Durio zibethinus, more commonly known as Durian or the ‘king of fruits’ by locals is a Southeast Asian tropical fruit. Smell is a crucial factor in durian acceptance amongst consumers as many are unable to accept the pungent onionlike odour liberated by durians. Due to the controversial aroma of durians, chemical compounds reportedly contributing to the durian smellvolatile esters and sulfur compounds have been widely discussed in the literature. This review article seeks to consolidate the literature which have identified volatile esters and sulfur-containing compounds in durians from Malaysia, Indonesia and Thailand, and studies shedding light on how the economic value of durians can be enhanced. Literature review was conducted using databases Scopus and ScienceDirect and a total of 18 articles were reviewed. In light of the rising demand for durians, factors, namely aroma, flavour and colour, in which consumers consider in the purchase of durians are further looked at, to explore the potential of enhancing favourable traits of durians and increasing its economic value and sales in the global market. By knowing the chemical compounds involved or influencing each factor, further studies can be conducted to explore methods such as breeding of new durian cultivars and metabolic or gene modification for phenotypic manifestation of favourable traits attractive to consumers.


INTRODUCTION
A flight was delayed as a heated argument ensued where passengers complained about an odour likened to rotten onions, which would make flying unbearable.Later a batch of fruits, arguably the world's smelliest fruit-Durian, was removed from the flight [1].There are an estimated 27 species of Durio worldwide but Durio zibethinus remains the most popular.Originating from the Malay Peninsula, Durian or Durio zibethinus belongs to the family of Bombacaceae, is well known by locals as the 'king of fruits'.Recently, it was reported that chinese consumers bought up 80,000 durians within 60 seconds upon its commencement of sales on Alibaba's Tmall platform.It was also announced that a 3 billion yuan deal was closed between Alibaba and the Thai government [2].Growth in durian sales in the global market is remarkable seeing exports from major producers such as Indonesia, Malaysia and Thailand.Despite its controversial aroma, durians have been proven a valuable commodity of trade and governments in South East Asia have been looking to expand their durian industry.
To date, there are many publications discussing the identities and contributory role of volatile esters and sulfur-containing compounds in the aroma, liberated by the durian fruit.Studies have been conducted with durian cultivars from various South-East Asian countries such as Malaysia, Indonesia and Thailand.In order to gather and consolidate data from all available studies that have discussed the chemical names of these esters and sulfur-containing compounds, their respective aromatic contributions, the specific aroma of different durian cultivars, a literature review was conducted.To consider how the economic value of durians can be enhanced, studies discussing aromatic contributions of esters and sulfur-containing compounds in the lai cultivars (used in durian hybridisation), the metabolomics of compounds contributing to aroma, colour and flavour, are also reviewed.

MATERIALS AND METHODS
The literature review was conducted using two databases: Scopus and ScienceDirect.This review focuses on the volatile esters and sulfur compounds.Key terms "Durian", "Durio", "Esters", "Sulfurs", "Volatility", "Volatile", "Flavours", "Fragrance", "Aroma" and "Odour" were included in the search strategy.A filter was applied to search for articles with these key terms appearing in the title, abstract or keywords.A total of 30 and 10 articles were found on Scopus and ScienceDirect respectively.Only English articles (including both abstract and the full text) indexed in the two databases were utilised.There were no changes in the number of articles with the language filter applied.
Articles discussing fermentation, wine or alcoholic fermentation, volatile compounds in other fruits, storage of durians and retention of volatiles, development of tools for quantitation of chemical compounds in durians, were not considered for this literature review.

Chemical contribution of esters and sulfur compounds to durian aroma
The strong smell of durians is favoured only by a limited group of consumers and it has presented as an issue in the transportation of durians when it is marketed in other parts of the world.Many are turned away by the onion-like odour of durians, which have been reported to be due to sulfur-containing compounds present in durians [5].Hence to enhance the favourability of durians, plant breeders have started to look at the cultivation of new breeds of the fruit.To breed new cultivars however, selection has to be based on the characteristics of fruits that breeders would like to see manifesting in the new durian cultivar.One paper, namely the Belgis et al study sheds lights on how characterisation of cultivars can aid selection of candidates for future breeding of new durian cultivars [18].This will be elaborated in the subsequent subsections.
3 papers are consolidated in Table .2 summarising the sulfur compounds found and their contribution to aromatic nuances in durian cultivars [10,11,18].

Esters
Esters are reported to be responsible for the fruity smell of durians [3].
Studies report that different esters contribute different aroma nuances.
Esters and their aromatic nuances from 6 studies done are consolidated in Table .3 and 4 for lai and durian cultivars respectively.

Characteristic Aroma of Durian and Lai Cultivars
Belgis et al reported on the characterisation of lai and durian cultivars in Indonesia by Quantitative Descriptive Analysis (QDA), a method used to characterise aroma through evaluation of sensory properties and subsequent description of these sensory attributes by categories, highlights the type of aroma of each cultivar and identifies chemical compounds possibly influencing these aroma nuances in durian and lai cultivars (as summarised in Table 6 and Table 7) [18].By identifying the characteristic aroma of the different durian and lai cultivar, it will better inform decisions on breeding and the potential traits that might arise in the new breeds of durian fruits.Likely due to the presence of highest ester ethyl-2butenoate

Enhancing Economic Value of Durians: Modifying Aroma, Color, Flavor
The sweet fruity smell of fruits is what attracts consumers but despite having a fruity aroma, the unique sulfur or onion-like odour of the durian fruit turns some consumers away.Smell is a critical factor in influencing consumer decisions.As mentioned in the previous sections, the onion-like odour of durian fruits is unacceptable to many consumers in the market.Apart from sensory attributes, the colour and appearance of durians-often used as a judge of aesthetics and quality-affect consumer decisions [20].
Durians with darker colours are usually less attractive to consumers [21].Taste of the durian fruit is also an important factor of consideration.As such, these criteria in which consumers use to assess the favourability of durians are important to be looked in order to be improved upon, to increase the economic value of the fruit.

Aroma
Metabolomics of durians reported that amino acids leucine and cysteine are important precursors that contribute to the aroma of fruits, where leucine is responsible for volatile esters while cysteine is responsible for sulfur-containing aroma production [23].Difference in amounts of the 2 amino acids present in Chanee and Mon Thong durian cultivars reported in a study by Pinsorn et al could possibly account for the difference in aroma volatiles [22].

Colour
Lai cultivars, which have a orange and red colour, are more visually attractive than durian cultivars, which have pale yellowish colour.Association between the yellow and orange colours of durians and lai, and -Carotene has been established in earlier studies [20,24].

Flavour
2 cultivars Chanee and Mon Thong durian cultivars from Thailand by Pinsorn et al.The study revealed that metabolites glutamate, citrate, glutamylcysteine, and glutathione, present in durian pulps from both cultivars are taste-related compounds.Amino acids highly abundant in both cultivars, such as alanine, aspartate and glutamate are likely responsible for the flavour of the durian pulps [22].Another study found that amino acids alanine, proline, phenylalanine and isoleucine are likely to have contributed to the bitter taste in durians sampled [20,25].The sweet taste is one of the most important traits in fruits, which is influenced by the sugar content (i.e.sucrose, glucose, fructose, maltose) present.By QDA, lai cultivars from Indonesia were found to have higher sweetness than durian cultivars [20].

DISCUSSION
To select candidates for breeding, the analysis of the characteristics of different cultivars could be studied and based on these characteristics' breeders would like to see manifesting in new breeds of durian, they can select accordingly the pair of cultivars.Belgis et al suggested that "Batuah" lai, which has a fruity and sweet aroma, and "Hejo" durian, with the mildest sulfur aroma of all 4 durian cultivars, can be used to produce new cultivars, likely with a milder aroma [18].Cross breeding has been conducted in previous experiments and their genetic diversity has been studied at the molecular level.By using plant breeding techniques, it is believed that durian cultivars of better quality can be produced [26].Metabolomics is also important in highlighting important contributors to aroma and flavour of durian fruits.In combination with studies such as those of Belgis et al where they had identified volatile esters and sulfur compounds and their aromatic contribution, further studies can be conducted to investigate how certain metabolites or precursors and their eventual product formation can be manipulated to change the aromatic nuances in durians [20].Metabolites resulting in the formation of sulfur compounds such as 3,5-dimethyl 1,2,4-trithiolane, diethyl disulfide, 3-mercapto-2methyl propanol, bis(ethylthio)methane, 1,1-bis(methylthio)-ethane, and 1,1-bis(ethylthio)-ethane, which contributes to the pungent durian odour, may be further studied to look at possible inhibition of pathways leading to the formation of such sulfur compounds.Since research has been conducted on the durian genome, the identification of important metabolites and precursors can potentially enable genetic modification of the durian genome to enhance favourable traits in durians [23].The genes coding for metabolites and precursors can be manipulated, perhaps modifying amounts of esters formed to enhance the fruity aroma, amounts of -Carotene produced to enhance the colour of durian cultivars, amounts of sugars and amino acids produced to change the flavour nuances of durians, and remove unfavourable traits such as the onion-like odour disliked by many consumers.Since dark colours are visually less attractive to consumers, molecules resulting in the dark colour of the durian pulp and methods to lighten the colour by modifying such molecules merits further studies.Price differentiation amongst different durian cultivars exist and are influenced by demand as some cultivars are preferred by consumers over others.With the improvement of durian cultivars to be acceptable by more consumers, a general increase in demand will follow and as such, the economic value of the durians can be increased.

CONCLUSION
In conclusion, considering that the aroma of durians is unbearable to many, and its smell being an issue in air travel, further studies can analyses consumer preferences in relation to the aromatic contributions of specific esters and sulfur-containing compounds in durians.This would allow understanding of the science behind the appeals or rejection of the fruit.Future material science research, in relation to esters and sulfurcontaining compounds identified to contribute to smell, can be conducted to mitigate issues regarding diffusion of the durian aroma.With this, the probability of durian batches removed from air travel will be minimised, reducing losses and thereby also expanding the market of durians amongst individual consumers who adopt air travel transport.
Considering that studies report lai cultivars as good candidates for durian hybridisation, it highlights the possibility of other Durio cultivars as potential choices for durian hybrid production.Further research is recommended to explore new combinations for breeding and the aromatic profiles of new hybrids produced.Players in the durian industry should look at enhancing factors taste, appearance and aroma according to consumer preferences to leverage on the rising demands for durians.This is the recommended approach to increasing the economic value of durians.

Figure 1 :
Figure 1: Flowchart of included literature for review

Table 1 :
Esters and Sulfur Compounds Identified in Studies

Table 5 :
Characteristic Aroma in Durian Cultivars

Table 6 :
Characteristic Aroma in Lai Cultivars