Search Results

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 O<sub>2</sub> atmospheres wherein the reduction of O<sub>2</sub> 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.

Peel discs (0.2 mm in diameter) of refrigerated, air-stored apple that were immersed in a medium isotonic with that of fruit sap were allowed to photosynthesize either in a closed or flow-through system. The photosynthetic net evolution of O₂ in the light or consumption in the dark was used to predict the duration of the experiment that would be within the critical limits of aerobiosis. Using GC-MS system, volatile emissions from these tissues were determined, in the head space and liquid medium. The volatile profile generated from head space analysis was essentially similar to that of whole fruit. Light, compared to the dark treatment, stimulated the synthesis of nearly all volatiles, especially α-farnesene. The synthesis of 6-methyl-5-hepten-2-one, an oxidative product of α-farnesene that causes scald like symptoms in whole fruit, was observed only under lighted conditions. While O₂ content in the medium seemed to have no effect on the volatile build-up in the head space, a C15 sesquiterpene with a mass spectrum similar to hydroperoxide breakdown products of α-farnesene was synthesized only in presence of O₂ in the reaction medium. Inhibition of light reactions of photosynthesis with DCMU, suppressed the synthesis of various volatiles in the head space suggesting the role of chloroplast activity in aroma development. We suggest that peel discs work well as a model system to study flavor chemistry in apple.

Chlorophyll fluorescence of three cultivars of apple, representing fruits that are considered highly susceptible (`Cortland'), moderate- to highly susceptible (`Red Delicious'), and resistant (`Empire') to superficial scald were studied in relation to scald development during storage. The preclimacteric harvested fruits from each variety were divided into two equal lots, lot one was treated with DPA (1000 ppm) and all the fruits (treated and untreated) were air-stored in separate bins at 0°C for 4 months. Chlorophyll fluorescence parameters, minimal fluorescence (F<sub>o</sub>), maximal fluorescence (F<sub>m</sub>) and the ratio of (F<sub>m</sub> – F<sub>o</sub>)/F<sub>m</sub>, and various quenching components of variable fluorescence were measured at regular intervals during storage. The maximal level of fluorescence (F<sub>m</sub>) at harvest varied between varieties; it was highest in `Empire', followed by `Red Delicious' and `Cortland', respectively. DPA dip treatment seemed to have no influence on chlorophyll fluorescence at harvest. Decline in F_m was found to be related to scald development during storage. The data on fluorescence quenching pattern and kinetics in relation to development of storage scald will be discussed. Changes in ${\text{O}}_2^{-}$ radical (a possible cause of apple scald) scavenging system during storage will also be presented.