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W.R. Miller, R.E. McDonald, G. Hallman, and J.L. Sharp

Experimental vapor heat (VH) tests [43.5C for 5 hours, 1009” relative humidity (RH)] were conducted to determine treatment effects to freshly harvested Florida grapefruit (Citrus paradisi Macf.). VH treatment reduced peel pitting 5-fold compared to control fruit after 5 weeks of storage (4 weeks at 10C + 1 week at 21C) and did not cause peel discoloration or rind breakdown. There was no difference in volume between treated and nontreated fruit after 1 week of storage or in weight loss after 5 weeks. Also, peel color, total soluble solids concentration, acidity, and pH were not affected by VH treatment. Fruit were slightly less firm after VH treatment and remained less firm throughout storage, compared with control fruit. The VH treatment tested is a potentially viable alternative quarantine treatment for control of the Caribbean fruit fly [Anastrepha suspensa (Loew)] because it is not phytotoxic to grapefruit and has been reported effective for disinfestation of this pest in grapefruit.

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Gilbert F. Simmons, Joseph L. Smilanick, Shama John, and Dennis A. Margosan

Moisture is raised in dehydrated prunes to improve palatability before packaging and potassium sorbate is added to inhibit microbial growth. Vapor phase hydrogen peroxide (VPHP) technology uses hydrogen peroxide pulses to disinfect dried prunes. Dried prunes were obtained from dehydrators. The number of colony-forming units per 10 prunes (cfu/p) was compared between untreated and VPHP treated. Three culture media—dichloran rose bengal chloramphenicol agar base (DRBC, Oxoid), aerobic plate count agar (PCA), and potato dextrose agar (PDA)—were used to evaluate cfu/p. Similar mean microbe populations were observed on DRBC (67) and PDA (70); PCA had higher cfu/p (99). Microbes washed from untreated prunes obtained from dehydrators were 58 to 112 cfu/p, depending on the culture medium used. The number of cfu/p assessed on all media on VPHP-treated prunes was near 0 after 100 min exposure. Unlike potassium sorbate, hydrogen peroxide is a microbiocide rather than a microbiostat.

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M. Brakke and L.H. Allen Jr.

Midday reductions of stomatal conductance and carbon dioxide assimilation rates (Aco2) in Citrus are typically attributed to large leaf-to-air vapor-pressure differences or high atmospheric vapor-pressure deficits (VPD). This study investigated air temperature (Ta) and available soil water (ASW) level as corollary factors of atmospheric VPD that influence midday reduction of net gas exchange in citrus leaves. The influence of elevated atmospheric CO2 under conditions that inhibit net canopy Aco2 was also investigated. Net canopy Aco2 and evapotranspiration rates of Carrizo citrange [Poncirus trifoliata Raf × Citrus sinensis (L.) Osbeck] and Swingle citrumelo (P. trifoliata Raf × C. paradisii Macf.) seedlings grown in outdoor controlled-environment growth chambers were measured under two levels of Ta with concomitant changes in VPD and two levels of atmospheric CO2 concentration, which were changed in steps over time. Cyclical depletion of ASW was allowed to occur at each set of Ta/VPD and CO2 combinations. Highest net canopy Ace, rates at ambient CO2 concentration (330 μmol·mol-1) were obtained at the low Ta/VPD level (29C/2.4 kPa) and ASW >50%. Diurnal canopy CO2 uptake rates decreased at the high Ta/VPD level (37C/3.6 kPa), and midday depression of canopy Aco2 was observed at ASW levels <50%. Net canopy Aco2 decreased at higher levels of ASW under the high Ta/VPD treatment than at the low Ta/VPD treatment. At the elevated CO2 concentration (840 μmol·mol-1) net canopy CO2 uptake rates were double those that occurred at ambient CO2 levels and they did not exhibit midday reduction. Our data indicate that, when soil water is not readily available, citrus seedlings are more sensitive to high levels of Ta and VPD which results in reduction of CO2 uptake. The inhibitory effects of elevated VPD and reduced ASW on citrus net Aco2 were lessened at the elevated atmospheric CO2 level.

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A.L. Moyls, P.L. Sholberg, and A.P. Gaunce

A fumigation technique using brief exposure of fruit to a low concentration of acetic acid vapor was combined with modified-atmosphere packaging to reduce storage rots and increase shelf life of grapes (Vitis vinifera L.) and strawberries (Fragaria ×ananassa Duch.) by two or three times normal values. Both commodities were inoculated with spores of Botrytis cinerea Pers. before fumigation with acetic acid, packaging, and storage at lowered O2 levels. Fumigation with acetic acid at 8.0 mg·L–1 followed by modified-atmosphere packaging for 74 days at 0 °C reduced the percentage of rotted grapes from 94% to 2%. Strawberries fumigated with acetic acid at 5.4 mg·L–1 were free of decay compared to 89% rotted for the control fruit stored for 14 days at 5 °C.

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Harold E. Nordby and Roy E. McDonald

It has been reported that temperature conditioning (TC), intermittent warming (IW), and film wrapping (FW) reduce chilling injury (CI) on grapefruit. Our objective was to determine if IW, FW, and vapor heat (VH) affected the composition of the epicuticular wax of grapefruit similar to the effects we previously reported with TC. Waxes were analyzed by gas chromatography. C25 to C34 aldehydes and alkanes decreased in all treatments in 5C storage for 21 days. Squalene increased in both the TC (7 days at 21C) and VH (43.5C for 4 hr) treatments. Terpenoids increased in both the TC and IW (4 cycles of 5 days at 5C and 2 days at 21C) treatments, and the greatest increase in C24 aldehyde occurred in the TC treatment. A VH-TC sequential treatment kept C27 to C34 aldehydes at fresh fruit levels following 5C storage. FW did not cause any wax increase. It is possible that these wax changes may have a role in reducing CI.

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W.R. Miller and R.E. McDonald

`Marsh' grapefruit (Citrus paradisi Macf.) produced in Florida must be certified for security against unwanted pests before entry into some domestic and export markets. Application of heat by hot water (HW) has been shown to cause severe injury to grapefruit; however, direct comparisons between forced vapor heat (VH) and HW have been lacking. Grapefruit preharvest-treated with gibberellic acid (GA) or not treated, were postharvest-treated with VH or HW such that the surfaces of fruit were exposed to the same rate of temperature increases and treatment durations. Condition and quality attributes were then compared with ambient air (AA) and ambient water (AW) controls after storage. After 4 weeks' storage at 10 °C plus 1 week at 20 °C, scald affected 5% of HW and 20% of VH-treated fruit. No scald developed on control fruit. At the end of storage, mass loss for HW and VH fruit was ≈5%. HW-treated fruit had a 5-fold higher incidence of aging than VH fruit; however, control fruit showed significantly more aging than all heat-treated fruit. Gibberellic acid (GA) and the heat treatments reduced decay relative to the control. GA-treated fruit remained greener during storage than control fruit. These findings indicate that VH and HW treatments at the temperatures and durations to control the Caribbean fruit fly (Anastrepha suspensa, Loew) will likely cause peel injury to `Marsh' grapefruit produced in Florida, regardless of treatment with GA.

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Guy J. Hallman

`Arkin' carambolas (Averrhoa carambola L.) were subjected to the fruit fly quarantine treatments of hot water immersion at 43.3 to 43.6C for 55 or 70 rein, 46.0 to 46.3C for 35 or 45 rein, or 49.0 to 49.3C for 25 or 35 rein, or vapor heat at 43.3 to 43.6C for 90 to 120 rein, 46.0 to 46.3C for 60 or 90 rein, or 49.0 to 49.3C for 45 or 60 min. Marketability, color, weight loss, internal appearance, flavor, total acids, and soluble solids content were determined. The 49.0 to 49.3C treatments resulted in excessive damage to the carambolas 2 to 4 days after treatment. There were no statistically significant differences in the variables measured among the other treatments and control; however, heat-treated carambolas appeared duller in color than control fruits. Overall, fruit treated at 46.0 to 46.3C lost significantly more weight than that treated at 43.3 to 43.6C.

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Laura Dellevigne, Peter Vergano, Lee Wiles, Andy Hale, and Jeff Adelberg

Commercially produced membrane rafts containing Celgard D-304 microporous, polypropylene film are used to support the growth of micropropagated plants in liquid media. This method is used because growth experiments using membranes resulted in plant growth rates equal to or greater than those grown on agar. Inconsistent results of plant growth on these rafts led to an interest in measuring the water vapor transmission rates (WVTR) of the membranes. A modified WVTR test (ASTM E96-80) was used on 15 raft samples. Results showed that the supplier's value of 49.2 g/m2 ·hr falls within the range of measured values of 39.2 to 54.8 g/m2·hr. Inconsistencies in growth of micropropagated plants may not be due to variability in the WVTR of the Celgard film. It is possible that the WVTR of the film is not the most important factor in facilitating liquid nutrient transport across the membrane. Other properties of the film need to be measured to determine factors affecting growth rates observed.

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W.R. Miller and R.E. McDonald

`Marsh' and `Ruby Red' grapefruit (Citrus paradisi Macf.) were harvested in Florida during Oct. and Nov. 1990, degreened in an ethylene chamber, exposed to vapor heat (VH) treatment (43.5 ± 0.1C for ≈240 min), and evaluated for deterioration in quality and development of injury after various storage regimes. Symptoms of aging averaged 6% and 8% of the surface on `Ruby Red' and `Marsh' fruit, respectively, and the VH treatment had reduced the incidence of aging by 45% after 5 weeks of storage (4 weeks at 16C plus 1 week at 21C). Total decay, mostly stem-end rots (Diplodia spp. and Phomopsis spp.), remained relatively low (≈5%) in both treated and nontreated fruit after 5 weeks of storage. The VH treatment had little effect on change in peel color during treatment or subsequent storage. After the final inspection, `Marsh' fruit was higher in total soluble solids and titratable acidity than `Ruby Red' fruit, but these quality indicators and pH were not affected by the VH treatment. VH treatment did not adversely affect the quality of `Marsh' or `Ruby Red' grapefruit harvested early in the season; hence, VH should be considered as a viable quarantine treatment for Florida grapefruit.

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Gilbert F. Simmons, Joseph L. Smilanick, Nuria Denis-Arrue, Dennis A Margosan, and Shama John

A new vapor phase hydrogen peroxide (VPHP) technology that uses relatively dry hydrogen peroxide pulses is a promising method for the disinfection of surface-borne bacteria, yeasts, and molds on walnut nutmeats. The number of colony forming units per gram (cfu/g) on untreated nutmeats was compared to those VPHP treated. Three culture media; dichloran rose bengal chloramphenicol agar base (DRBC, Oxoid), aerobic plate count agar (APC, Oxoid), and potato dextrose agar (PDA, Sigma), were utilized to evaluate cfu/g. Similar numbers of cfu/g of product were observed on APC and PDA. The more selective DRBC had lower cfu/g. Microorganisms washed from untreated walnut nutmeats purchased at retail outlets ranged between 17,000-29,000 cfu/g depending upon the culture medium used. The number of cfu/lg on nutmeats after VPHP treatments was reduced to 500-1400, a 95% reduction. VPHP may offer an alternative to propylene oxide fumigation. The moisture content of nutmeats was not significantly altered by VPHP. The Food and Drug Administration lists hydrogen peroxide as a “generally recognized as safe substance” (GRAS). Hydrogen peroxide is already produced in a food grade for aseptic packaging.