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  • Author or Editor: Randolph Beaudry x
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The application of low oxygen through modified atmosphere packaging (MAP) is a technique used successfully to preserve the visual quality of lettuce and some other commodities. The expansion of use of low O2 via MAP to preserve quality of most commodities is limited by technical difficulties achieving target O2 concentrations, adverse physiological responses to low O2, and lack of beneficial responses to low O2. Low O2 often is not used simply because the physiological responses governed by the gas are not limiting quality maintenance. For instance, shelf life may be governed by decay susceptibility, which is largely unaffected by low O2 and may actually be exacerbated by the conditions encountered in hermetically sealed packages. Physiological processes influenced by low O2 and limit storability are discussed. The interdependence of O2 concentration, O2 uptake by the product, and temperature are discussed relative to requirements for packaging films.

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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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The steady-state oxygen concentration at which blueberry fruit began to exhibit anaerobic carbon dioxide production. (i.e., the RQ breakpoint) was determined for fruit held at 0, 5, 10, 15, 20 and 25 C using a modified atmosphere packaging (MAP) system. As fruit temperature decreased, the RQ breakpoint occurred at lower oxygen concentrations. The decrease in the RQ breakpoint oxygen is thought to be due to a decreasing oxygen demand of the cooler fruit.

The decrease in oxygen demand and concomitant decrease in oxygen flux would have resulted in a decrease in the difference in the oxygen concentrate on between the inside and outside of the fruit and thus decreased the minimum amount of oxygen tolerated. The implications on MAP strategies will be discussed.

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We tested the sorptive capacity of a number of nontarget materials found in apple storage rooms on their capacity to remove 1-MCP from the storage atmosphere and thereby compete with the fruit for the active compound. Furthermore, we evaluated the impact of temperature and moisture. Nontarget materials included bin construction materials [high density polyethylene (HDPE), polypropylene (PP), weathered oak, nonweathered oak, plywood, and cardboard] and wall construction materials (polyurethane foam and cellulose-based fire retardant). Each piece had an external surface area of 76.9 cm2. We placed our “nontarget” materials in 1-L mason jars and added 1-MCP gas to the headspace at an initial concentration of ≈30 μL·L-1. Gas concentrations were measured after 2, 4, 6, 8, 10, and 24 hours. The concentration of 1-MCP in empty jars was stable for the 24-hour holding period. Little to no sorption was detected in jars containing dry samples of HDPE, PP, cardboard, polyurethane foam, or fire retardant. Inclusion of plywood, nonweathered oak, and weathered oak lead to a loss of 10%, 55%, and 75% of the 1-MCP after 24 hours, respectively. Using dampened materials, no sorption resulted from the inclusion of HDPE, PP, polyurethane foam, or the fire retardant. However, the rate of sorption of 1-MCP by dampened cardboard, plywood, weathered oak, and nonweathered oak increased markedly, resulting in a depletion of ≈98%, 70%, 98%, and 98%, respectively. The data suggest that there are situations where 1-MCP levels can be compromised by wooden and cardboard bin and bin liner materials, but not by plastic bin materials or typical wall construction materials.

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Blueberry fruit were sealed in 0.00254 cm (1 mil) thick, 200 cm2 low density polyethylene pouches, which, in turn, were sealed in containers continually purged with gas mixtures containing 0, 20, 40 or 60 kPa CO2 and held at 15C. Sampling the gas composition of the enclosed package permitted accurate determination of O2 uptake, CO2 production and the respiratory quotient (RQ) despite the high background CO2 levels. O2 uptake was minimally affected by the CO2 treatments. CO2 production, however, increased at CO2 partial pressures over 20 kPa, resulting in an elevated RQ at 40 and 60 kPa CO2. Raising the CO2 partial pressure caused the fruit to become more sensitive to lowered O2, raising the O2 partial pressure associated with the RQ breakpoint.

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The concentrations of free amino acids in the peel and pulp of banana (Musa sp., AAA group, Cavendish subgroup, cv. Valery) fruit during ripening at 22 °C were measured. All 20 amino acids were quantified at seven distinct ripening stages as defined by measures of internal ethylene, O2, and CO2 concentrations, aroma volatile emissions, and peel color. Volatile production commenced 2 days after the peak in ethylene production and 1 day following the climacteric peak in internal CO2. The maximum rate of branched-chain ester synthesis occurred 2 to 3 days after its onset. Production of 2-methylpropyl and 3-methylbutyl esters was much higher in the pulp compared with the peel, confirming that the pulp, rather than the peel, is the primary site of banana aroma synthesis. Of the amino acids measured, only leucine, valine, and cysteine increased concomitantly with ester formation. This was observed in the pulp, but not in the peel. The data suggest the metabolic pathways for valine and leucine formation also support, respectively, the synthesis of 2-methylpropyl and 3-methylbutyl esters. It is not clear how leucine and valine can accumulate despite the fact that they act as feedback inhibitors of their respective synthetic pathways. There was a slight peak in the formation of several other amino acids in the pulp (e.g., alanine, arginine, asparagine, glutamine, and methionine) coinciding with the climacteric respiratory peak in CO2, but a similar pattern was not seen for the peel. These data are the first to demonstrate distinct differences in amino acid metabolism in the peel and pulp of banana related to their role in ripening and aroma biosynthesis.

Open Access

We hypothesized that the blocking of O2 influx and CO2 efflux in banana (Musa acuminata) by sealing nearly 100% of the pores over a fraction of the surface would generate a modified internal atmosphere in a manner similar to fruit coatings that cover 100% of the banana surface but only block a fraction of the pores. This hypothesis was based on the observation made by previous workers that the flesh of mature green bananas has insignificant resistance to O2 diffusion relative to the resistance imposed by the skin of the fruit. We modified the O2 diffusion pathway in bananas by covering, beginning at one end, ¼, ½, ¾, and ⅞ of the fruit surface with paraffin, which sealed essentially 100% of the surface where it was applied. Large end-to-end O2 and CO2 gradients developed within coated fruit, relative to the uncoated control, suggesting that the diffusive resistance in the pulp was not insignificant. Since the large gradients of O2 generated caused uneven ripening, using fractional coatings may help analyze gas exchange properties, but it is not suitable for commercially controlling ripening of bananas.

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Abstract

Flux of ethylene from adaxial applications of Ethrel and Silaid to amphi- and hemistomatous leaves was examined. Following application of Ethrel to amphistomatous leaves in the dark (i.e., closed stomata) or hemistomatous leaves in the light, steady-state ethylene evolution was almost entirely adaxial. When stomata of amphistomatous leaves were fully open, abaxial ethylene flux for Ethrel was about 45% of the total ethylene evolved. Abaxial ethylene flux could then be dramatically reduced by stomatal closure induced by low light levels. Steady-state abaxial flux of ethylene from Silaid on amphistomatous leaves in the dark or hemistomatous leaves in the light was usually equal to or greater than adaxial ethylene flux. When stomata of amphistomatous leaves were fully open, flux of ethylene from Silaid was invariably equal from both leaf surfaces. Flux of Silaid- or Ethrel-derived ethylene from one leaf surface was reduced by increasing air velocity on the opposite side of the leaf, but only on amphistomatous leaves following light-induced stomatal opening. For Ethrel, the effect of air velocity was greater when the side of the leaf to which Ethrel had been applied was exposed to the increased air flow. No similar effect was found for Silaid. Closure of stomata on amphistomatous leaves and use of hemistomatous leaf material prevented any air velocity effect. Data indicate little to no entry of Ethrel or Ethrel-derived ethylene into the side of a leaf that lacks stomata or whose stomata are tightly closed. Significant movement of Silaid into leaf tissues probably occurs regardless of stomatal status, resulting in considerable release of ethylene within the leaf. Chemical names used: (2-chloroethyl)phosphonic acid (Ethrel); (2-chloroethyl) methylbis(phenylmethoxy)silane (Silaid).

Open Access

Abstract

Ethylene-induced abscission of pepper (Capsicum frutescens L. cv. Hungarian Hot Yellow Wax) flower buds, leaves, and fruit depended on ethylene source (i.e. ethylene gas from a compressed gas source vs. ethylene released from Silaid) and concentration. In response to ethylene from either source, flower buds and small fruit (< 10 mm long) abscised most readily and fully expanded leaves least readily. Concentrations of Silaid that induced fruit abscission comparable to a given concentration of ethylene gas induced significantly greater leaf abscission than ethylene gas. Application of Silaid at dusk resulted in a small, but significant, increase in abscission relative to early morning application. Progressive increases in temperature between 18° and 32°C enhanced fruit and leaf abscission in response to ethylene gas. Abscission mediated by ethylene gas was not affected by light intensities between 120 and 300 µmol·m–2·s–1 PAR. Chemical name used: (2-chloroethyl)methylbis(phenyImethoxy)silane (Silaid, CGA-15281).

Open Access