The temporal evolution of the magnetic field associated with electron thermal Weibel instability in optical-field ionized plasmas is measured using ultrashort (1.8 ps), relativistic (45 MeV) electron ...bunches from a linear accelerator. The self-generated magnetic fields are found to self-organize into a quasistatic structure consistent with a helicoid topology within a few picoseconds and such a structure lasts for tens of picoseconds in underdense plasmas. The measured growth rate agrees well with that predicted by the kinetic theory of plasmas taking into account collisions. Magnetic trapping is identified as the dominant saturation mechanism.
•First direct quantification of aroma-related esters in A. chinensis ‘Hort16A’.•Detrimental impact of cold storage (1.5°C) on ester composition in kiwifruit.•Significantly increased ester levels in ...ethylene-treated, stored fruit.•Kiwifruit aroma was strongly but flavour only slightly changed by ethylene.
Fruit esters are regarded as key volatiles for fruit aroma. In this study, the effects of cold storage on volatile ester levels of ‘Hort16A’ (Actinidia chinensis Planch. var chinensis) kiwifruit were examined and the changes in aroma perception investigated. Cold storage (1.5°C) for two or four months of fruit matched for firmness and soluble solids concentration resulted in a significant reduction in aroma-related esters such as methyl/ethyl propanoate, methyl/ethyl butanoate and methyl/ethyl hexanoate. Levels of these esters, however, were restored by ethylene treatment (100ppm, 24h) before ripening. A sensory panel found that “tropical” and “fruit candy” aroma was stronger and “green” odour notes less intensively perceived in kiwifruit which were ethylene-treated after cold storage compared to untreated fruit.
The key findings presented in this study may lead to further work on the ethylene pathway, and innovative storage and marketing solutions for current and novel fruit cultivars.
Volatile esters, typically found in ripe kiwifruit were formed by recombinant alcohol acyltransferases from
Actinidia. However, substrate preferences were observed for two phylogenetically different ...enzymes.
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► Gene transcription of ripening specific
Actinidia ATs is controlled by ethylene. ► Two likely orthologous ATs exhibit benzoyl-CoA:alcohol
O-acyltransferase activity. ► One phylogenetically distinct AT acts as acetyl-CoA:alcohol
O-acyltransferase. ► AAT-substrate specificity and availability modulate kiwifruit ester profiles.
Volatile esters are key compounds of kiwifruit flavour and are formed by alcohol acyltransferases that belong to the BAHD acyltransferase superfamily. Quantitative RT-PCR was used to screen kiwifruit-derived expressed sequence tags with proposed acyltransferase function in order to select ripening-specific sequences and test their involvement in alcohol acylation. The screening criterion was for at least 10-fold increased transcript accumulation in ripe compared with unripe kiwifruit and in response to ethylene. Recombinant expression in yeast revealed alcohol acyltransferase activity for
Actinidia-derived
AT1,
AT16 and the phylogenetically distinct
AT9, using various alcohol and acyl-CoA substrates. Functional characterisation of AT16 and AT9 demonstrated striking differences in their substrate preferences and apparent catalytic efficiencies
(
V
max
′
K
m
-
1
)
. Thus revealing benzoyl-CoA:alcohol
O-acyltransferase activity for AT16 and acetyl-CoA:alcohol
O-acyltransferase activity for AT9. Both kiwifruit-derived enzymes displayed higher reaction rates with butanol compared with ethanol, even though ethanol is the main alcohol in ripe fruit. Since ethyl acetate and ethyl benzoate are major esters in ripe kiwifruit, we suggest that fruit characteristic volatile profiles result from a combination of substrate availability and specificity of individual alcohol acyltransferases.
Kiwifruit (Actinidia spp.) are a relatively new, but economically important crop grown in many different parts of the world. Commercial success is driven by the development of new cultivars with ...novel consumer traits including flavor, appearance, healthful components and convenience. To increase our understanding of the genetic diversity and gene-based control of these key traits in Actinidia, we have produced a collection of 132,577 expressed sequence tags (ESTs).
The ESTs were derived mainly from four Actinidia species (A. chinensis, A. deliciosa, A. arguta and A. eriantha) and fell into 41,858 non redundant clusters (18,070 tentative consensus sequences and 23,788 EST singletons). Analysis of flavor and fragrance-related gene families (acyltransferases and carboxylesterases) and pathways (terpenoid biosynthesis) is presented in comparison with a chemical analysis of the compounds present in Actinidia including esters, acids, alcohols and terpenes. ESTs are identified for most genes in color pathways controlling chlorophyll degradation and carotenoid biosynthesis. In the health area, data are presented on the ESTs involved in ascorbic acid and quinic acid biosynthesis showing not only that genes for many of the steps in these pathways are represented in the database, but that genes encoding some critical steps are absent. In the convenience area, genes related to different stages of fruit softening are identified.
This large EST resource will allow researchers to undertake the tremendous challenge of understanding the molecular basis of genetic diversity in the Actinidia genus as well as provide an EST resource for comparative fruit genomics. The various bioinformatics analyses we have undertaken demonstrates the extent of coverage of ESTs for genes encoding different biochemical pathways in Actinidia.
Kiwifruit are novel in that they contain high levels of quinic acid (1-2% w/w), which contributes to the flavour, sugar/acid balance and health-giving properties of the fruit. In a study of quinic ...acid storage and metabolism in three kiwifruit species (Actinidia chinensis Planch. var. chinensis, Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa and Actinidia arguta (Sieb. et Zucc.) Planch. ex Miq. var. arguta) quinic acid accumulation occurred principally in the early stages (<60 days after anthesis; (DAA)) of fruit development. The present study established that there are separate quinate dehydrogenase (QDH) and shikimate dehydrogenase (SDH) activities in kiwifruit, probably representing different proteins. Quinate dehydrogenase activity was at a maximum around the time of greatest quinic acid accumulation and declined markedly in late fruit development, and was also higher in the species that accumulated the largest amounts of quinic acid (A. chinensis and A. deliciosa). In contrast, SDH activity was highest in the early stages of fruit development and only declined to 30-50% at later stages of fruit development in all three species. Dehydroquinate synthase gene expression levels measured by quantitative real-time PCR showed a high level in the early season, which was sustained through the mid-season. The quantitative real-time PCR results for a kiwifruit EST that had homology to chloroplastic isoforms of SDH showed an induction in the middle to late season; therefore, the high level of SDH activity in the early season (<30 DAA) may have resulted from the expression of a cytosolic isoform of the enzyme. The results are also consistent with the relative levels of the bifunctional dehydroquinate dehydratase/SDH enzyme and QDH enzyme controlling the accumulation and utilisation of quinic acid in kiwifruit.
We suggest that (methylsulfanyl)alkanoate-ester production in ‘Hort 16A’ kiwifruit is likely to depend on gene expression of ripening-specific AATs, which is regulated by ethylene and inhibited by ...prolonged cold storage.
Four 3-(methylsulfanyl)propionate esters, ethyl 3-(methylsulfanyl)prop-2-enoate, two 2-(methylsulfanyl)acetate esters and their possible precursors 2-(methylsulfanyl)ethanol, 3-(methylsulfanyl)propanol and 3-(methylsulfanyl)propanal were quantified from the headspace of
Actinidia chinensis ‘Hort 16A’ kiwifruit pulp by GC–MS–TOF analysis. The majority of these compounds were specific for eating-ripe fruit and their levels increased in parallel with the climacteric rise in ethylene, accumulating towards the very soft end of the eating firmness. No ethylene production could be observed after long-term storage (4–6
months) at 1.5
°C and the levels of all methylsulfanyl-volatiles, except methional, declined by 98–100% during that period. This depletion of (methylsulfanyl)alkanoate-esters after prolonged cold storage points towards little flavour impact of these compounds on commercial ‘Hort 16A’ kiwifruits. However, ethyl 3-(methylsulfanyl)propionate is suggested to be odour active in ripe ‘Hort 16A’ fruit that has not been stored. Gene expression measured by q-RT PCR of six ripening-specific alcohol acyltransferase (AAT) expressed sequence tags and (methylsulfanyl)alkanoate-ester production of cell-free extracts were also significantly decreased after prolonged cold storage. However, (methylsulfanyl)alkanoate-ester synthesis of cell-free extracts and AAT gene transcript levels could be recovered by ethylene treatment after five months at 1.5
°C indicating that the biosynthesis of (methylsulfanyl)alkanoate-esters in ‘Hort 16A’ kiwifruit is likely to depend on ethylene-regulated AAT-gene expression. That the composition but not the concentration of (methylsulfanyl)alkanoate-esters in fresh fruit could be restored after ethylene treatment suggests that substrate availability might also have an impact on the final levels of these volatiles.
Minor fruit volatiles are likely to be missed using sampling techniques optimized for the extraction of major compounds. This can be a disadvantage if these minor compounds contribute to ...characteristic fruit flavors. In this comparative study, headspace solid-phase microextraction (HS-SPME) and dynamic headspace sampling (DHS) parameters were systematically optimized to ensure highest extraction yields of methylsulfanyl-volatiles from kiwifruit tissue samples. A significant “salting-out” effect from the fruit matrix was observed using both sampling techniques after (NH4)2SO4 saturation. HS-SPME at optimized conditions (polydimethylsiloxane–divinylbenzene-coated fiber, (NH4)2SO4 saturation, 5min equilibration and 20min sampling at 40°C) was faster and more convenient to use than DHS for qualitative purposes. Despite this, the qualitative and quantitative methylsulfanyl-volatile profile was improved using optimized DHS ((NH4)2SO4 saturation; sampling time 20h; flow rate 30mL min−1) compared with HS-SPME, making this the more sensitive and preferred method for quantitative studies. The optimization strategies for increasing headspace extraction yields of trace compounds presented in this study can easily be applied to tissue samples from other fruit.
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