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

1. Introduction

Dry-cured ham is generally classified based on the origin. In particular, the three main forms from southern China, southern or central Europe and the southeastern United States, have many differences in their sensory properties [1]. Jinhua ham, Parma ham, Iberian ham and the American ham are their best known representatives [2,3,4,5]. In China, Jinhua ham, along with “Xuanwei ham” and “Rugao ham”, are well known as the “three hams”. The traditional processing technology for making Jinhua ham is composed of multiple steps, including raw material selection, salting, soaking and washing, sun drying and shaping, fermentation, ripening, post-ripening, grading and storage [6]. The unique flavor of Jinhua ham is appreciated by consumers all over the world. Nowadays, ham quality is graded by its aroma intensity and persistence on the bamboo stick, but different processing technologies can make a great difference in the flavor quality of ham. Therefore, the control of ham flavor formation during processing is very important for ham grading, so comprehensive research of Jinhua ham flavor is crucial for better ham quality and the establishment of a national traditional meat products standard.

In the fermentation process, relevant chemical and biological reactions take place in the muscle of Jinhua ham, such as lipid degradation and oxidation, Maillard reactions, Strecker degradation, etc., resulting in the special ham flavor [7]. Many exploration methods and technologies have been used to analyze the odorants in Jinhua ham, including dynamic headspace sampling (DHS) [8], purge-and-trap (P&T) [9], solid phase microextraction (SPME) [10], but to our knowledge, there are few studies on the identification of variations of key odorants of Jinhua ham at different fermentation stages, compared to that of Western dry-cured hams such as Parma and Iberian ham [11,12,13,14,15,16].

The objective of this study was to identify and characterize the aroma-active compounds of Jinhua ham under different processing times and operation conditions by gas chromatography-olfactometry-mass spectrometry (GC-O-MS), aided by both dynamic headspace dilution analysis (DHDA) and aroma extract dilution analysis (AEDA) techniques.

2. Results and Discussion

2.1. Aroma-Active Compounds

A total of 81 aroma-active compounds of Jinhua ham in different processing time were identified by DHS-GC-O-MS and SAFE-GC-O-MS. The compounds included 15 saturated and unsaturated aldehydes (Figure 1), 11 ketones (Figure 2), 12 alcohols (Figure 2), 11 acids (Figure 3), 11 esters and lactones (Figure 3), five sulfides (Figure 4), seven benzene derivatives compounds (Figure 4), three pyrazines and six others (Figure 4).

2.2. Key Aroma-Active Compounds by Dynamic Headspace Dilution Analysis (DHDA)

Sixty-seven compounds were identified as odorants by DHS-GC-O-MS (Table 1) and eight compounds remained unknown. Among the identified compounds, 3-methylbutanoic acid (odor: sour and sweaty), 2-acetyl-1-pyrroline (odor: popcorn), trimethylamine (odor: fishy) and γ-nonalactone (odor: peachy and sweet) had average FD factors over one hundred (where Average FD factor = Sum of the FD factors in one compound of 18, 15, 12 and 9 months ham/4). Seven other identified compounds: acetic acid (odor: sour), hexanal (odor: cut-grass), 2,6-dimethylpyrazine (odor: toast and nutty), butanoic acid (odor: cheesy), methional (odor: cooked potato), γ-decalactone (odor: peachy and burnt sugar) and 1-nonen-3-one (odor: mushroom) had average FD factors ≥ 50.

Figure 1. Structures of the saturated and unsaturated aldehydes in Jinhua ham.

Figure 1. Structures of the saturated and unsaturated aldehydes in Jinhua ham.

Figure 2. Structures of the ketones and alcohols in Jinhua ham.

Figure 2. Structures of the ketones and alcohols in Jinhua ham.

Figure 3. Structures of the acids, esters and lactones in Jinhua ham.

Figure 3. Structures of the acids, esters and lactones in Jinhua ham.

Figure 4. Structures of the sulfides, benzene series compounds, pyrazines and other compounds in Jinhua ham.

Figure 4. Structures of the sulfides, benzene series compounds, pyrazines and other compounds in Jinhua ham.

Table 1. Aroma-active compounds in Jinhua ham by DHS.

Nr aCompound Name bR.I. cIdentification dOdor Property eFD f
R.I.(DB-5ms)R.I.(DB-WAX)1815129 c
38methanthiol-627RI,Orotten egg251251-
76trimethylamine503848RI,O,MSfishy12512512525
50ethyl acetate610877RI,O,MS,STDfruity/sweet25251-
23-methylbutanal638927RI,O,MSchocolate/malty2512551
27ethanol-941RI,O,MS,STDalcohol11--
77triethylamine677970RI,O,MSfishy2525--
172-pentanone-971RI,O,MSfruity525--
3pentanal700984RI,O,MSfermented/yoghourt2555-
542-methyl-butanoic acid ethyl ester8501051RI,O,MSfruity/sweet25551
51acetic acid butyl ester8121059RI,Ogreen/fruity55--
182-methyl-3-pentanone-1068RI,O,MSmint255--
61disulfide, dimethyl7501079RI,O,MScooked cabbage/onion252511
4hexanal8031094RI,O,MS,STDcut-grass125125255
321-methoxy-2-propanol-1137RI,O,MSplastic1111
331-penten-3-ol6881164RI,O,MS,STDbuttery/grassy/green51--
unknown-1179RI,O,MSpopcorn525--
5heptanal9051183RI,O,MSoily/green2551-
78D-limonene10281191RI,O,MS,STDsweet /orange5255-
79pyridine7571193RI,O,MSspicy1111
unknown-1199Ocooked potato251251-
303-methyl-1-butanol-1201RI,O,MSfermented/oily/fruity5---
802-pentylfuran9951231RI,O,MS,STDfruity/green55--
52hexanoic acid ethyl ester10001232RI,O,MSfruity/apple--51
281-pentanol7601250RI,O,MSgreen15-1
10(E)-2-hexenal855-RI,O,MSgreen/fatty55--

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