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The generation of dilute vapor phase standards using the static headspace method can be challenging, requiring the construction of specialized chambers or use special methods for adding minute amounts of the compound of interest. The vapor concentration above a dilute water solution can be effective and accurate and has been used to create standards to measure the concentration for a wide range of volatile and semivolatile organic compounds. Such systems are highly temperature-sensitive, however. The goal of this work is to mathematically describe the relationship between vapor concentration above a dilute water mixture for compounds important to postharvest physiology, such as ethanol, acetaldehyde, ethyl acetate, and hexanol. The experiments were carried out in the range of 0 to 40°C and concentration of 0 to 1000 ppm for each compound. Three replications were used for each data point. The concentration was measured after thermal and chemical equilibration by gas chromatography containing a HAYESSEP-N column, by injecting 1 cc of the vapor headspace, using a 8-cm-long needle Hamilton syringe. Relationships for each of the compounds noted were successfully described employing multiple-order equations. For example, the relationship for ethanol vapor concentration was: Y = 12.12356 + 0.9461594*X + 0.5761110e-01*X2 + 0.6565694E-03*X3 + 0.23499598E-04*X4 (R 2 = 1.000), with X being the temperature in °C. The relationships described for those compounds provides an useful tool that allows us to dilute liquid standards across a range of temperatures.
O2 and C2H4 are biologically active molecules of importance in plant metabolism and their availability is manipulated to modify plant behavior during storage and the shelf-life period of harvested plant products. Respiratory curves describing the dependence of O2 uptake on O2 were obtained for slicing and `Roma'-type tomato, and `Jonathan' and `Empire' apple fruit at 20 °C for ripening fruit and for mature, non-ripening fruit. Mature, non-ripening fruit were maintained in that state by the application of 1-methylcyclopropene (1-MCP) or the use of a nonripening mutant in the case of tomato. The range of O2 atmospheres wherein the reduction of O2 relative to ambient yielded a significant (50%) reduction in respiration relative to the maximal rate of respiration, but was above the fermentation threshold, was termed the `safe working atmosphere' (SWA). For apple, there was no SWA for non-ripening apple fruits since a 50% reduction in respiration occurred at the fermentation threshold. During ripening, the respiratory curve shifted, revealing a marked increase in the apparent Km and maximal rate of respiration with no change in the fermentation threshold, resulting in the creation of a SWA of 6.5 kPa O2. A similar, less dramatic, shift in the respiratory curve for tomato fruit also occurred. In a flow-through system, low O2 reduced the rate of respiration of ethylene insensitive tomato fruit by ≈50% and resulted in an approximate doubling of the storability of the fruit. Insensitivity to ethylene yielded fruit with a respiratory rate approximately one-half that of ripening fruit, but storability was improved about 5-fold. The data collectively suggest that inhibition of ripening, rather than global metabolism via reduced respiration is key to preserving fruit quality.
The growth regulator 1-methylcyclopropene (1-MCP) is a vapor under physiological conditions and acts by inhibiting the binding of the hormone ethylene to its binding site and a single exposure can temporarily render plant material insensitive to ethylene when applied at the parts-per-billion level. Apple fruit were harvested 1 week prior to the climacteric (harvest 1), at the onset of the climacteric (harvest 2), and 1 week after the onset of the climacteric (harvest 3). Fruit were stored at 0, 5, 10, 15, and 20 °C and were given treatments with 1 ppm 1-MCP on a once-per-week, once-per-2 weeks, once-per-month, and once-per-year basis or were left untreated. In terms of reduced softening, earlier harvested fruit were more responsive to the 1-MCP treatment and the efficacy of 1-MCP was enhanced by repeated application. At 20 °C, control fruit (all harvests) softened to less than 50 N pressure within 20 days. For fruit treated once with 1-MCP, fruit of harvest 1 reached this threshold by 63 days, those of harvest 2 after 56 days and those of harvest 3 by 40 days. Fruit treated on a once-per-month basis began to soften by 56 days for harvest 3, while those of harvest 1 and 2 did not. Fruit treated once per week or once per 2 weeks did not soften relative to initial firmness (68N) during the first 63 days of the study. 1-MCP effectively prevented softening at all temperatures relative to the controls, however, as temperature decreased, the benefits of 1-MCP application became less pronounced. Decay was a significant problem for fruit stored at 15 and 20 °C storage temperatures. Roughly 30% to 60% of the fruit were lost to decay in the first 60 days of the study. 1-MCP application reduced, but did not prevent decay. Storage of 1-MCP-treated apple fruit at elevated temperatures will likely require some means of controlling decay in storage.
The changes in volatile-aroma of Penicillium expansium and Botrytis cinerea fungi and apple fruit inoculated with these fungi were studied using GC-MS. A specially designed chamber with raised end glass tubes with access ports fitted with Teflon-lined septa was used to determine the volatile profile for fungi on agar. Inoculated fruit were placed in glass flow-through chambers similarly fitted with sampling ports. Volatile collection from fruits or fungi was accomplished using solid phase micro-extraction (SPME) device (Supelco, Inc.). In fungi-inoculated fruits, volatiles not produced by uninfected fruit included formic acid, 2-cyano acetamide; 1-hydroxy-2-propanone, and 1-1-diethoxy-2-propanone, which were initially detected 6 hr after inoculation. These new volatiles are suggested to be synthesized specifically by the action of fungi on fruits as they were not detected from fungi that were grown on agar or bruised fruits. In general, esters, alcohols, aldehydes, ketones, acids, and hydrocarbons other than α-farnesene declined in fungi infected fruits.
Autoxidation products alpha-farnesene of have been implicated in superficial scald induction for apple (Malus domestica cv. Cortland Apple) fruit. We suspect the apple cuticle acts as a sink where α-farnesene can accumulate and eventually autoxidize into hydroperoxides, conjugated trienes, 6-methyl-5-hepten-2-one (ketone), and other compounds. These oxidized byproducts may diffuse back into the peel, thereby initiating the scald process. Cortland apples were stored at 0.8°C. Volatile cuticular components were analyzed at 2-week intervals by gas chromatography–mass spectroscopy. Only two scald-related volatiles were found, 6-methyl-5-hepten-2-one and α-farnesene. The identification of these compounds may allow the determination of cuticular involvement in superficial scald, as well as a possible correlation between the volatiles and apple scald development. α-farnesene concentrations initially increased and was followed by a decline, possibly due to its autoxidation.
Superficial scald is still one of the most important postharvest physiological disorders in apples. Commercial control of this disorder has been accomplished by selecting resistant cultivars, treating fruit with DPA and ethoxyquin, using oil-soaked fruit wraps and storing the fruit under low O2. However, the causal reason for scald development is still a mystery. Research has indicated that the scald-promoting factor or inducing compound may be formed or accumulated in apple cuticle then rediffused back into the hypodermis, thereby causing damage. Hydroperoxides, auto-oxidative product from α-farnesene, have been thought to be the toxic compounds, inducing scald; however, it is not explained how the hydroperoxides move from the cuticle to the hypodermis. The identification and dynamic changes of 6-methyl-5-heptene-2-one as a natural volatile in apple fruit during ripening were made, which accumulated in higher quantitaty in cuticular wax than in headspace. The close relationship between the chloroplast breakdown and amount of α-farnesene changes, the induction of scald-like symptom on the surface of apple fruit by 6-methyl-5-heptene-2-one and the sensitivity of fruit to this ketone damage were investigated. Our results suggest that the accumulation of 6-methyl-5-heptene-2-one in the cuticular wax of apple fruit might be the causal reason for scald development in apples.
Aroma analysis of horticultural produce is an emerging field in which both flavor producing and malodorous compounds are detected from within a complex sample matrix. Qualitative and quantitative information is desired to monitor produce ripeness and provide quality control over processed products such as juices, preserves and canned products. Conventional analysis methods such as purge and trap and gas chromatography–mass spectrometry provide much of this information but are laborious and time consuming. Faster techniques are required when large numbers of samples are being analyzed and when rapid feedback to the produce harvester is required. Solid-phase microextraction (SPME) has recently been shown to significantly reduce the sampling times required by more conventional methods. The use of fast gas chromatographic techniques along with the recently commercialized time-of-flight mass spectrometer has also significantly reduced the separation and analysis times. We have combined SPME with gas chromatography–time-of-flight mass spectrometry as a rapid and quantitative tool for the analysis of flavor volatiles in apples and tomatoes. The sampling and analysis processes provide significant improvements to sample throughput, with analysis times taking only 2–6 minutes. The linear response of this system to butylacetate, ethyl-2-methylbutanoate and hexylacetate ranges from ppb to ppm levels, and the identification of unknown flavor compounds is possible even in the presence of other co-eluting compounds. The SPME technique is able to investigate volatiles changes in apple cuticle and tissues, which open the new possibility for flavor biochemistry research.
Abstract
Several environmental and physical factors affect the kinetics of ethylene release from (2-chloroethyDphosphonic acid and (2-chloroethyl)methylbis(phenylmethoxy)silane. Target surface chemistry exerted a strong influence on the evolution of ethylene from both compounds. Ethylene release from (2-chloroethyl)methylbis(phenylmethoxy) silane was slowed by glass and hydrophobic substances such as wax, surfactants in the spray solution, and high concentrations of the parent molecule, but not by epicuticular waxes on leaves. Ethylene evolution from (2-chloroethyl)phosphonic acid was inhibited by glass and high levels of epicuticular waxes. The rate of ethylene release from both compounds was positively correlated with temperature; however, ethylene released from (2-chloroethyl)methylbis(phenylmethoxy)silane was much less affected by temperature increases. Increases in light intensity promoted the initial release of ethylene from (2-chloroethyl)methylbis(phenylmethoxy)silane, but decreased long-term yield. Light intensity had no effect on the breakdown of (2-chloroethyl)phosphonic acid.
Volatile compounds produced by apple (Malus domestica Borkh) fruit partition into the cuticle and epicuticular waxes and may play an important role in superficial apple scald. Of these volatiles, α-farnesene, conjugated trienes, hydroperoxides, and 6-methyl-5-hepten-2-one have been identified as playing a crucial role in scald production. Volatiles from the epicuticular wax of four different apple cultivars have been analyzed by gas chromatography/mass spectroscopy. A correlation was found between scald incidence and 6-methyl-5-hepten-2-one content and the 6-methyl-5-hepten-2-one:α-farnesene ratio. α-Farnesene is the most-abundant volatile at the beginning of storage, whereas 6-methyl-5-hepten-2-one is present in minute quantities. These two volatile compounds appear to have an inverse relationship with respect to one another since the levels of 6-methyl-5-hepten-2-one increased and α-farnesene decreased prior to the onset of apple scald. This changing ratio may have been due to an autoxidative process resulting in the breakdown of α-farnesene to 6-methyl-5-hepten-2-one. Analysis of the volatiles emanating from the apple wax revealed a number of compounds associated with aroma that also partition readily into the fruit surface.
The purpose of this work was to investigate the influence of O2 and CO2 partial pressures on glycolytic carbon flux, phosphorylated intermediates, phosphate, pyrophosphate, and phosporylated nucleotides in asparagus spears tips stores at 1 °C. The effects of CO2 (0, 5, 10, and 20 kPa) combined with O2 pressures ranging from 0.1 to 16 kPa (1% O2 = 1.013 kPa O2 at 1 atm) were investigated. Spears were enclosed within a low-density polyethylene (LDPE) package (for the 5-, 10-, and 20-kPa CO2 treatments) having a surface area of 462 cm2 and enclosed in 1.95-L glass jars. Low O2 enhanced the interconversion of phosphoenolpyruvate (PEP) to pyruvate (PYR) and F6P to F1,6P2 relative to high O2. When spears tips at 16 kPa O2 were compared to those at harvest, little change occurred in the adenylate or phosphate pools. PPi and ATP contents decreased as the O2 partial pressure declined below 16 kPa O2. In general, as CO2 increased, PPi and ATP decreased, while Pi, ADP, and AMP increased. The adenylate energy charge (AEC) declined with a decline in the O2 partial pressure, declining most rapidly below 2 kPa O2. Low O2 reduced AEC relative to high O2. Increasing CO2 partial pressure reduced AEC, an effect not evident at lower O2. The data suggest low O2 and elevated CO2 impair oxidative phosphorylation and induce nonsustaining carbon metabolism, which may limit asparagus spear survival under O2-deficient conditions.