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Nonadienal Synthesis Essay

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Watermelon

By Liz Roth-Johnson | August 26, 2014 10:00 am

(Steve Evans/Flickr)

Nothing says “summer” quite like a big, juicy slice of watermelon. Even if you prefer it charred on the grill or blended into an icy agua fresca, watermelon is one of the best ways to beat the late-summer heat.

So what gives watermelon its refreshingly delicate flavor?

Turns out the answer is pretty complicated. Over the last few decades, scientists have identified dozens of flavor and aroma molecules that contribute to watermelon’s unique taste [1].

And here’s an interesting twist: a watermelon’s flavor has a lot to do with its color. Chow down on a yellow ‘Early Moonbeam,’ a pale ‘Cream of Saskatchewan,’ or a deep red ‘Crimson Sweet’ and you’ll likely notice different flavor profiles for each melon.

These watermelons don’t just look different, they taste different, too! (David MacTavish/Hutchinson Farm)

Several of watermelon’s flavor molecules form when colorful chemicals called carotenoids break down into smaller chemical compounds [2,3].

For example, the classic color of red watermelons comes from lycopene, the same molecule responsible for the color of red tomatoes. When lycopene breaks down, it forms key flavor compounds such as lemon-scented citral.

Orange melons don’t have much lycopene, but they make up for it with extra beta-carotene. This chemical – the same one that makes carrots orange – leads to a completely different set of flavor molecules, including floral beta-ionone.

Colorful molecules called carotenoids break down into different flavor compounds. Figure adapted from [2].

The chemistry of watermelon flavor is clearly complex, but scientists are still searching for individual molecules that mimic watermelon’s characteristic taste.

Most recently, a study identified a single molecule – dubbed “watermelon aldehyde” – that has a very distinct watermelon aroma [4]. Unfortunately (or fortunately, depending on your perspective), the molecule is too unstable to be used as a food additive. So for now, artificially flavored “watermelon” products will just have to keep on tasting nothing like watermelon.

Good thing there’s plenty of real, chemically complex watermelon to go around.

References

  1. Yajima I, Sakakibara H, Ide J, Yanai T, Hayashi K (1985) Volatile flavor components of watermelon (Citrullus vulgaris). Agric Biol Chem 49: 3145–3150. doi:10.1271/bbb1961.49.3145.
  2. Lewinsohn E, Sitrit Y, Bar E, Azulay Y, Meir A, et al. (2005) Carotenoid Pigmentation Affects the Volatile Composition of Tomato and Watermelon Fruits, As Revealed by Comparative Genetic Analyses. J Agric Food Chem 53: 3142–3148. doi:10.1021/jf047927t.
  3. Lewinsohn E, Sitrit Y, Bar E, Azulay Y, Ibdah M, et al. (2005) Not just colors—carotenoid degradation as a link between pigmentation and aroma in tomato and watermelon fruit. Trends Food Sci Technol 16: 407–415. doi:10.1016/j.tifs.2005.04.004.
  4. Genthner ER (2010) Identification of key odorants in fresh-cut watermelon aroma and structure-odor relationships of cis, cis-3, 6-nonadienal and ester analogs with cis, cis-3, 6-nonadiene, cis-3-nonene and cis-6-nonene backbone structures University of Illinois at Urbana-Champaign. Available: http://hdl.handle.net/2142/16898.

About the author: Liz Roth-Johnson received her Ph.D. in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.

Read more by Liz Roth-Johnson


CATEGORIZED UNDER: Flavor of the Month

MORE ABOUT: aroma, beta-carotene, carotenoid, chemistry, color, flavor, lycopene, molecules, summer, watermelon

2.1. Formation of Tea Aromatic Volatiles

Volatile compounds in tea products can be formed through many pathways, such as the carotenoid derivatives pathway, the fatty acid derivatives pathway, the terpene derivatives pathway, the phenylpropanoid/benzenoid derivatives pathway, the glycoside hydrolysis pathway and the Maillard reaction pathway.

Carotenoids are important precursors of tea volatile compounds, especially the C9- to C13-aromas. Ionone and damascone are important C13-carotenoid-derived compounds (Figure 1) that constitute an essential aroma note in black tea. Testing in a model fermentation system consisting of β-carotene, tea catechins and crude soluble enzymes’ preparation extracted from fresh tea leaves showed that β-ionone was the major volatile product, and β-damascone was the minor product formed during the conversion process by oxidative degradation of β-carotene [2,3]. Concentrations of volatiles, including β-ionone, α-ionone, β-damascone and α-damascone, were significantly increased, and the volatile flavor compound (VFC) index was improved by supplementing carotenoid in tea leaves during black tea fermentation [4].

Lipids, especially fatty acids (FA), contributed greatly to the aroma and flavor volatiles in tea [5]. Glycolipids, which are rich in linolenic acid, are the most abundant lipids in fresh tea leaf. Neutral lipids, which are rich in linoleic, myristic, lauric, stearic, palmitic and oleic acids, are moderately abundant lipids. Phospholipids, which have a high proportion of linoleic, palmitic and oleic acids, are the least abundant lipids in tea [6]. During black tea processing, including withering, rolling and fermentation, the lipids are degraded to produce flavor volatiles by hydrolytic or oxidative action of enzymes on glycolipids and phospholipids [5]. The major fatty acid derivatives include alcohols, aldehydes and lactones. C6 and C9 alcohols and aldehydes are key contributors to the “fresh green” odor of tea. Methyl jasmonate, an important fatty acid derivative, is a major contributor to the jasmine-like aroma of oolong tea [7]. The aldehydes were synthesized via an intermediate, in which the primary reaction is the dioxygenation of an unsaturated fatty acid catalyzed by a lipoxygenase (LOX). The fatty acid-derived aldehydes could be further transformed to their corresponding alcohols by alcohol dehydrogenase [8].

Geraniol, linalool and linalool oxides are important terpene derivatives in tea, especially in black tea. Geranyl pyrophosphate (GPP), a compound produced in the methylerythritol phosphate (MEP) pathway [9], is the precursor of the terpenoid derivatives geraniol and linalool by geraniol synthase and linalool synthase, respectively [10,11] (Figure 2). The linalool oxidation may be catalyzed by a non-specific enzyme [11]. Linalool is usually present in volatile free form in black tea. However, linalool oxides always occur in the bound forms of primeverosides [12].

Phenylalanine, a compound produced in the shikimic acid pathway, is the precursor of volatile phenylpropanoid/benzenoid derivatives [13]. The volatile phenylpropanoid/benzenoid derivatives are important contributors to the fruity and floral smells. Major phenylpropanoids in tea are phenylethanol and phenylacetaldehyde, and major benzenoids are benzyl alcohol and benzaldehyde [13]. The enzymes involving in these biosynthesis pathways in tea remain to be investigated.

Many tea volatile compounds are present in glycosidically-bound forms, which are more water soluble and more easily stored, but less volatile than their free aglycone counterparts. Benzyl β-d-glucopyranoside was the first volatile glycoside isolated from tea leaf [14]. It was later confirmed that the major glycosides in tea leaf were primeverosides and glucosides, among which primeverosides were predominant [15,16]. Biosynthesis of glycosides from free volatile compounds is catalyzed by glycosyltransferases [14]. Glycosidically-bound volatiles are located in the vacuoles, but glycosidases are present in the cell walls and cavity areas among cells [17]. The primeverosides and glucosides were hydrolyzed by the enzymes glucosidase and primeverosidase during tea processing, especially in the stage of rolling. Primeverosides are the major black tea volatile precursors, because they are abundant in fresh tea leaf, but almost disappear in the final tea products, whereas the glucosides are not substantially changed during tea processing [18]. Non-enzymatic hydrolysis of the glycosidically-bound volatiles takes place during tea processing. During the high temperature processing of ready-to-drink green tea and black tea, glycosidic precursors of damascenone are hydrolyzed [19].

The Maillard reaction is defined as a chemical reaction between reducing sugars and amino acids at high temperature, which produces brown pigments with caramel flavor. Roasted and pan-fired green teas contain high levels of Maillard reaction products, such as 1-ethyl-3,4-dehydropyrrolidone, pyrazines, pyrroles, pyrans and furans. Pyrazines and 1-ethyl-3,4-dehydropyrrolidone play an important role in developing a roasted flavor in both roasted and pan-fired green teas [20,21].

2.2. Variation of Aromatic Volatiles between Various Kinds of Tea

White tea, green tea, oolong tea, black tea and Pu-erh tea are major teas consumed in China. White tea is a minimally-processed tea. Fresh tea shoots with abundant leaf trichomes are withered under natural ambient conditions for a few days and then solar dried. The final product looks whitish owing to the dense grey white leaf trichomes covering the tea shoots. Green tea is an unfermented tea. During green tea processing, fresh leaves are heated in a steamer or a hot pan to inactivate the polyphenol oxidase so as to prevent tea polyphenols from oxidation. The fixed leaves are then rolled and dried. Green tea contains a high level of tea polyphenols. Black tea is a fully-fermented tea prepared by a processing procedure “withering—rolling—fermentation—drying”. Fresh leaves are withered in hot air and rolled in a cylindrical rolling machine or a machine called a CTC (crush, tearing and curling). The rolled leaves are fermented under controlled temperature and humidity, during which the tea polyphenols are oxidized by the polyphenol oxidase enzyme, resulting in the formation of red and orange tea pigments. Oolong tea is a semi-fermented tea whose processing procedure includes withering, shaking, rolling and drying. The aim of shaking processing is to partially bruise the leaf edge by hand touching or by a rotating drum. The polyphenols in the bruised parts of the leaves are oxidized. Pu-erh tea is a post-fermented tea, and its manufacture includes raw tea processing, fermentation and compressing. Fresh leaves are fixed and rolled as green tea. The rolled leaves are sun-dried and used as raw tea for post-fermentation. The raw tea is made damp and piled up for a few months, during which post-fermentation takes place by the action of mold. The fermented leaves are finally compressed into cakes or bricks.

Many aromatic compounds in fresh tea leaves are in the forms of non-volatile precursors or glycosides, which are liberated by glycosidase during tea processing [22]. The aromatic precursors and glycosidase enzymes vary with tea cultivars and processing methods. There are great variations of aromatic volatiles between various kinds of tea owing to the differences in tea cultivar and processing method.

There were 133 aromatic volatiles formed during black tea processing, among which there were 48 carbonyls, 30 esters, 25 alcohols, 11 hydrocarbons, 3 phenols, 3 lactones and 13 volatiles of miscellaneous structures [23]. Volatile carbonyl compounds can be formed from the degradation of membrane lipids by enzymatic catalyzation. During black tea fermentation, catechins are oxidized. The oxidized catechins are strong oxidizing agents, which may oxidize other compounds, such as amino acids, carotenoids and unsaturated fatty acids, resulting in the formation of volatile compounds contributing to black tea aroma [24,25]. Linalool, methyl salicylate and C6-aldehydes increased appreciably during the rolling and fermentation of black tea, accompanying a decrease in unsaturated fatty acids. Trans-2-hexenal is increased with the loss of cis-3-hexenal during black tea withering and fermentation [6,25]. The aromatic volatiles are changed with the black tea fermentation, and so, the optimum fermentation time of black tea can be monitored using an electronic nose fitted with quartz crystal microbalance sensors [26,27].

There are two kinds of green tea, i.e., steamed green tea made in Japan and pan-fired green tea made in China. All green teas contain high levels of indole pyridine, linalool, geraniol, benzyl alcohol, 2-phenyl-ethanol, 2-ethylhexanoic acid and maltol. Sulfinylbismethane and sulfonylbismethane are considered to be important volatiles contributing to the typical aroma of green tea. (Z)-1,5-octadien-3-one (metallic), 4-methoxy-2-methyl-2-butanethiol (meaty), 4-mercapto-4-methyl-2-pentanone (meaty), (E,E)-2,4-decadienal (fatty), β-damascenone (honey-like), β-damascone (honey-like), indole (animal-like) and (Z)-methyl jasmonate (floral) are the most important odorants of Japanese steamed green tea [28,29]. Geraniol (rose floral), linalool (sweet floral), linalool oxides (sweet floral), indole (animal-like), dihydroactinidiolide (sweet,), cis-jasmone (jasmine floral), 6-chloroindole (indole like), coumarin (sweet), methyl jasmonate (floral), trans-geranylacetone (fruity), phytol (sweet), 5,6-epoxy-β-ionone (rose floral) and phenylethyl alcohol (floral) are identified as various aroma-active compounds of green tea beverages prepared using steamed green tea [30,31]. However, Chinese roasted green teas contain high amounts of pyrazine (nutty), pyrrole (nutty), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (sweet strawberry), 3-hydroxy-4,5-dimethyl-2(5H)-furanone (sweet strawberry), coumarin (sweet), vanillin (vanilla-like), geraniol (rose floral), (E)-isoeugenol (clove like) and 2-methoxyphenol (smoky). They have a high flavor dilution (FD) factor, which is defined as the highest dilution in which the tested odorant is perceived by trained panelists. (E)-isoeugenol (clove like) content is closely related to the green tea manufacturing process [32]. Moreover, 4-mercapto-4-methyl-2-pentanone, a roasted odorant contributing to the aroma of roasted green tea, increased with the increasing roasting temperature [33]. During the storage of green tea, 2-butyl-2-octenal (fruity) increased with time [34].

Oolong tea is a semi-fermented tea with a floral aroma. The major volatiles isolated from oolong tea were cis-jasmone (jasmine floral), β-ionone (rose floral), nerolidol (woody), jasmine-lactone (jasmine floral), methyl jasmonate (jasmine floral), indole (animal like), linalool and its derives (sweet floral) and geraniol (rose floral) [35,36]. These odorants are formed from precursors, such as (S)-linalyl β-primeveroside, cis- and trans-linalool 3,7-oxides 6-O-β-d-apiofuranosyl-β-glucopyranosides, (Z)-3-hexenyl β-d-glucopyranoside, methyl salicylate 6-O-β-d-xylopyranosyl-β-d-glucopyranoside and 8-hydroxygeranyl-β-primeveroside [12,37].

Microorganisms, such as fungus, are involved in the post-fermentation of Pu-erh tea processing, resulting in a moldy or musty smell. The major volatiles in Pu-erh tea included methoxyphenolic compounds, hydrocarbons and alcohol compounds, such as 1,2,3-trimethoxybenzene (storage musty), 1,2,4-trimethoxybenzene (storage musty), 2,6,10,14-tetramethyl-pentadecane (unknown), linalool and its oxides (sweet floral), α-terpineol (lilac), and phytol (sweet) [38]. The volatile composition of Pu-erh tea depends on the extraction method. Head space solid-phase microextraction (HS-SPME) was suitable for testing Pu-erh tea flavor [39,40].

White tea is a special tea processed using shoots with abundant leaf trichomes. There is abundant essential oil in the trichome joint by which the trichome is attached to the leaf lower epidermis [41], and so, white tea has high levels of volatiles, especially hexenal (green grassy odor) and (E)-hexenol (green grassy odor) [27].

The volatile composition is differentiated between various kinds of tea owing to the differences in cultivar and processing method, resulting in variation of aromatic characteristics. Table 1 lists the top ten volatiles in five kinds of tea. The different kinds of tea can be partially classified by cluster analysis using the index of individual catechins and volatile components [27].

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