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Amots Hetzroni, Denys J. Charles, and James E. Simon

A nondestructive electronic sensory system (electronic sniffer) that responds to volatile gases emitted by fruit during ripening was developed. It is based upon a single semi-conductor gas sensor placed within a rigid plastic cup equipped with a gas inlet to flush the head between samples. This gas sensor reacts with the range of reductive gases such as the aromatic volatiles that are naturally emitted by the ripening melon fruit. The sensor cup is placed on the exterior of the fruit and the change in electrical conductivity is recorded. In 1994, we examined the electronic sniffer as a tool to nondestructively determine ripeness in `Superstar', `Mission', and `Makdimon' melons. Fruits were manually classified into five ripeness stages based on external appearance and slip stage. Melons were first sampled nondestructively for color, weight, size, and slip stage, and then subjected to the electronic sniffer. Then, fruit volatiles, flesh firmness, and total soluble solids were measured. The electronic sniffer was able to accurately classify melons into three ripeness classes: unripe, half-ripe, and ripe for `Superstar' and `Mission'. The sniffer was only able to separate ripe from over-ripe in `Makdimon', which is known to become over-ripe and deteriorate rapidly. Using the sniffer as a tool to nondestructively measure ripeness and its potential application in fruit quality will be discussed.

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Amots Hetzroni, Denys J. Charles, Jules Janick, and James E. Simon

A prototype of a nondestructive electronic sensory system (electronic sniffer) that responds to volatile gases emitted by fruit during ripening was developed. The electronic sniffer is based upon four semiconductor gas sensors designed to react with a range of reductive gases, including aromatic volatiles. In 1994, we examined the potential of using the electronic sniffer as a tool to nondestructively determine ripeness in `Golden Delicious' and `Goldrush' apples. Fruit were harvested weekly from 19 Sept. to 17 Oct. (`Golden Delicious') and 27 Sept. to 18 Nov. (`Goldrush'). Each week, apples of each cultivar were evaluated individually for skin color, weight size, and headspace volatiles. Each fruit was then evaluated by the electronic sniffer, and headspace ethylene was sampled from air within the testing box. Individual fruits were then evaluated for total soluble solids, firmness, pH, total acidity, and starch index value. The electronic sniffer was able to distinguish and accurately classify the apples into three ripeness stages (immature, ripe, and over-ripe). Improved results were obtained when multiple gas sensors were used rather than a single gas sensor.

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Liangli Yu, Denys J. Charles, Amots Hetzroni, and James E. Simon

The volatiles of muskmelon (Cucumis melo L. reticulatis cv. Mission) were sampled by dichloromethane extraction and dynamic headspace methods and analyzed by gas chromatography (GC) and GC–mass spectroscopy (MS). A total of 34 constituents were identified, with esters contributing 8%–92% of the total volatiles. Butyl propionate, ethyl 3-methylpentanoate, hexadecanoic acid, methyl (methylthio)acetate, propyl butyrate, phenylpropyl alcohol, and vanillin, were recovered only by solvent extraction, while hexanal was only detected using dynamic headspace sampling. Methyl butyrate 35.2%, ethyl acetate 17.1%, butyl acetate 11.6%, ethyl propionate 8.3%, and 3-methylbutyl acetate 6.3% were the major constituents by solvent extraction sampling method. Butyl acetate 35.5%, 3-methylbutyl acetate 20.9%, ethyl acetate 7.3%, 2-butyl acetate 5.6%, and hexyl acetate 3.8% were the major constituents recovered by headspace sampling. Fruit tissue was also separated into five layers (exocarp, outer mesocarp, middle mesocarp, inner mesocarp, and seed cavity) and the volatile constituents differed significantly in content and composition by tissue.

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John N. Sacalis

It has previously been demonstrated that exceedingly high concentrations of 2,4-D, when taken up by cut carnations, inhibit petal senescence, while application of low concentrations of this synthetic auxin promote petal senescence. The mode of action of such high concentrations of 2,4-D has not been elucidated.

In previous work, it was observed that significant amounts of volatiles always emanated from those flowers treated with high 2,4-D, and which displayed inhibition of ethylene synthesis as well as petal senescence. In the present work, the headspace of treated flowers was therefore tested by gas chromatography after enclosure for a short period of time. Two of the major constituents of the volatiles produced by the treated flowers were found to be ethanol and acetaldehyde.

Since ethanol has formerly been shown to delay senescence in carnation flowers, and since 2,4-D has been shown to induce alcohol dehydrogenase, it is suggested that the mode of action of 2,4-D in this case is by means of the ethanol produced as a result of the 2,4-D treatment.

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P. Perkins-Veazie

Small fruit share several general characteristics. A significant source of starch is missing in strawberry, blueberry, cranberry, raspberry, blackberry, and grapes; thus, sugars accumulated at time of harvest represent the maximum amount of sweetness available. Total non-volatile acids decrease or stay the same, depending on the fruit. Immature small fruit are astringent, due to the presence of a variety of phenolic compounds that are diluted, metabolized, or immobilized in mature fruit. Ripeness can be determined by obvious changes in color, coinciding with or prior to fruit softening. Berry color is governed by the loss of chlorophyll and the accumulation of water soluble flavanoids and anthocyanins, rather than through accumulation of fat-soluble carotenoids. Environmental changes, especially temperature and rainfall, affect sugars, acidity, and color while storage conditions are more likely to affect color and acidity.

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John K. Fellman

Volatile ester molecules are important contributors to the perception of fruit taste. Biosynthesis of volatile compounds occurs via several biochemical pathways. Ongoing studies have concentrated on alcohol acetyl transferase, the terminal step in the acetate ester synthesis pathway. Our studies on volatile biosynthesis in apples have revealed several interesting phenomena. First, the nature and amount of volatile compounds are cultivarand strain-dependent. Studies with `Delicious' show a relationship between amount of peel coloration and flavor volatile content of tissue. Second, it is possible to manipulate the preharvest growing environment to influence the content of some volatiles in the fruit. Third, generation of volatiles is closely linked to the onset of climacteric ripening. Other experiments show the response of apples to different storage conditions with regard to volatile ester synthesis. In some cultivars softening apparently provides ester precursor molecules, leading us to speculate that there are glycosidically bound intermediates that are liberated by the action of cell-wall degradation.

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Randolph M. Beaudry

A theoretical model was developed that predicts how volatiles synthesized by fruit accumulate in the fruit interior and the fruit cuticle. Model inputs include temperature, rates of volatile synthesis, solubility of the volatile in the cuticular material, and the permeability of the volatile through the cuticle. The model indicated that the accumulation of volatiles was highly temperature-dependent and dependent upon the nature of the interaction between the volatile and the cuticle. For volatiles whose cuticular permeability declined rapidly with temperature, the concentration in the fruit and fruit cuticle tended to increase with decreasing temperature. This accumulation of volatiles in the fruit and fruit cuticle with decreasing temperature was enhanced by a decrease in the heat of solution (i.e., temperature sensitivity of solubility) and diminished by an increase in the Q10 Of the rate of volatile synthesis (i.e., the temperature sensitivity of the rate of synthesis). The model suggests that storage temperature can influence volatile retention and, hence, the volatile profile.

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J.P. Mattheis, D.A. Buchanan, and J.K. Fellman

Fruit quality and volatile compounds produced by apple fruit (Malus ×domestica Borkh. `Gala') were characterized following regular atmosphere (RA) or controlled atmosphere (CA) storage at 1°C. Static CA conditions were 1, 1.9, 2.8, or 3.7 kPa O2. Fruit stored under dynamic CA conditions were exposed to ambient air 1, 2, or 3 days per week for 8 hours then returned to 1 kPa O2. All CA treatments included 2 kPa CO2. Ethylene production was reduced following CA storage plus 1 day at 20°C compared with apples stored in RA. Apples stored in static 1 kPa O2 and the dynamic treatments had lower ethylene production compared with apples stored in 1.9 to 3.7 kPa O2 after 90 and 120 days. Ethylene production by apples from all CA treatments recovered during a 7-day poststorage ripening period at 20°C. Ester production was reduced following CA at 1 kPa O2 after 60 days compared with RA-stored fruit. Production of butyl acetate by apples stored in 1 kPa O2 static CA was 29%, 30%, and 7% of that produced by RA-stored fruit after 60, 90, and 120 days storage plus 7 days at 20°C. Amounts of 2-methylbutyl acetate were not affected by CA storage, however, production of other 2-methylbutyrate esters was reduced following 1 kPa O2 storage. Ester production increased with O2 concentration after 90 days in storage. The dynamic treatments resulted in greater ester emission after 120 days storage plus 7 days at 20°C compared with apples stored in static 1 kPa O2. Production of 1-methoxy-(2-propenyl) benzene by apples subjected to dynamic treatments was also higher after 120 days storage plus 7 days at 20°C compared with apples stored in RA or static CA. No differences in firmness, titratable acidity or soluble solids content were observed between apples stored in 1 kPa O2 and the dynamic treatments. Firmness and titratable acidity were maintained better by dynamic treatments compared with static atmospheres containing > 1 kPa O2.

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Wayne T. Iwaoka, Xiaorong Zhang, Richard A. Hamilton, C.L. Chia, and C.S. Tang

The volatile compounds in soursop (Annona muricata L.) were obtained by a liquid-liquid continuous extraction procedure from the aqueous solution of blended soursop pulp and analyzed by gas chromatography (GC) and GC-mass spectrometry (MS). Twelve volatiles were identified by comparing their mass spectra and Kovats retention indexes with those of standard compounds: five were identified tentatively from MS data only, eight are being reported for the first time. (Z) -3-hexen-l-ol was the main volatile present in mature-green fruit, while methyl (E) -2-hexenoate, methyl (E) -2-butenoate, methyl butanoate, and methyl hexanoate were the four main volatiles present in ripe fruit. Concentrations of these five volatiles decreased and several other unidentified volatiles appeared when the fruit became overripe.

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Weimin Deng and Randolph M. Beaudry

A simple packaging system was developed to simultaneously measure volatile production by plant organs and the permeability of the packaging film to those volatiles. In this system, apple (Malus domestica Borkk cv Golden Delicious) was packaged in low-density polyethylene (LDPE) bag and placed into a glass jar with a low air flow. The package and jar head spaces were sampled for aroma volatile analysis by gas chromatograph. Analysis was by gas chromatography/mass spectrometry. This system allowed at least 10 volatile compounds and their permeabilities to be measured. This system permits volatile production to be measured for products in the package so the product need not be removed from its storage environment. This may be a useful method for determining the dynamic relationship between flavor volatile synthesis and package atmosphere for packaged produce.