Evaluation of air disinfection equipment during cool storage of strawberries
AgFirst Australia P/L commissioned Agriculture Research Division, DEDJTR to test an air disinfection system for cool storage of strawberries (Fragaria × ananassa). The primary objective of this research was to determine the effect of Meditube disinfection equipment on fungal rot development after cool storage for up to three weeks at 0°C
The results from this trial clearly demonstrated that Meditube treatment effectively reduced Botrytis incidence relative to conventional cool storage under experimental conditions of low ethylene air concentration and no airborne fungal spore load.
Low temperature cold storage of fruit is fundamental in delivering optimal fruit quality in horticultural supply chains. Strawberries are particularly sensitive to fungal diseases such as Botrytis cinerea that greatly reduce their storage potential, shelf life and quality. Often the cool room environment is partially responsible for fruit degradation, either through latent infection due to airborne fungal spores and/or high ethylene concentrations that accelerate ripening.
AgFirst Australia P/L has available an air purifier and disinfection system that may reduce the concentration of air-borne fungi and thus rot incidence in fruit during storage and after ripening. The aim of this trial was to test the effectiveness of the air purifier and disinfection system in reducing rot incidence in strawberry fruit packed in retail punnets in comparison to conventional cool storage. AgFirst Australia P/L provided the commercial Meditube air purification equipment for testing.
Cool room ethylene measurements
Cool rooms used were empty prior to the start of the experiment. A 20 mL air sample was taken from within the centre of each cool room using a syringe prior to placement of fruit within the room, and on the final day of cool storage prior to removal of fruit. Ethylene concentration in air samples was measured by injecting a 1.0 mL sample into a Shimadzu GC-14B packed-gas chromatograph (Column = Packed Alumina SS 80/100 180cm; 140°C; Inj/Det = 180°C, Shimadzu, Kyoto, Japan).
Cool room airborne fungal spore assessment
Initial and final airborne fungal load in each cool room was measured using three unlidded sterile Dichloran Rose Bengal Tetracycline (DRBT) agar plates placed in the same positons within each cool room (i.e., middle shelf; 2 on the left, and one on the right hand side). Unlidded plates were left in each room for 3 to 5 h and then incubated for 5 days at 22°C. After incubation the fungi were identified on each plate and the number of colony forming units (CFU) were counted.
No colony forming units (CFU) were found after incubation on agar plates placed in treated and control cool rooms prior to the start of the experiment indicating that all three rooms were free of airborne pathogens. Treated and control cool room ethylene concentrations were measured prior to storage of fruit (Day 0) and at the end of the cool storage period (Day 19). Initial and final ethylene concentration was zero in all cool rooms used in the trial.
The experimental unit was a cool room. Two cool rooms were randomly assigned to two treatments that consisted of (a) air disinfection using the Meditube unit, or (b) no disinfection (i.e., control treatment). No ethylene was measured in all cool rooms, and no airborne fungal spores prior to application of treatments.
Within each experimental unit (i.e., cool room) 250 g strawberry punnets were randomly allocated to seven shelf positions (Fig.1). Nine punnets were placed at each shelf position, a group of three punnets representing one of three removals (0, 14 and 19 days). Within each cool room and shelf position one punnet per removal was designated for disease assessment, another was inoculated at Day 0 by misting with a Botrytis spore solution and assessed for disease symptoms after storage, and fruit quality was measured on the third punnet at each removal. Another five punnets per cool room were randomly assigned to a shelf position for disease assessment at the final removal. Fruit weight loss was determined as described in the main report.
Fungal disease assessments
At each removal directly out of cool storage fruit was assessed for disease development which consisted of identification of primary fungi, and the incidence of fruit with disease symptoms and disease severity for each fruit within each punnet was determined. Severity was scored on each infected fruit using a five-point rating scale for percent of fruit surface infected, where: 1 = none; 2 = 1 – 10%; 3 = 11 – 25%; 4 = 26 – 50%; and 5 > 50% of surface infected. Mean disease severity per punnet was calculated by averaging severity scores of infected fruit.
Fruit quality assessments
At each removal five punnets from each cool room were assessed for both visual quality and maturity. Fruit was allowed to warm to 18°C, each punnet weighed, and from 8 – 12 fruit selected from each punnet for quality measurements. Each fruit was scored for visual quality based on degree of flesh deterioration, drying and softening using the rating scale in Figure 2.
Surface colour was measured at the equator of each fruit with a hand-held tristimulus reflectance colorimeter (model CM-2600d, Minolta Corp.) Colour was recorded using the CIE L*a*b* uniform colour space (CIE Laboratories), where L* indicates lightness, a* indicates chromaticity on a green (-) Commercial-in- Confidence to red (+) axis, and b* chromaticity on a blue (-) to yellow (+) axis. Numerical values of a* and b* were converted to hue angle [H° = arctan (b*/a*)].
Flesh firmness of each fruit was measured at the equator with an Effegi small fruit penetrometer attached to a 3 mm Magness-Taylor type probe. The penetrometer and probe were mounted on a Dremel Model 220 manual drill press. Firmness was measured in kg/cm2 and converted to newtons (N) using the following formula: N = kg/cm2 x 9.81. After completion of flesh firmness measurements 8 – 12 fruit per punnet were crushed inside a plastic bag and soluble solids concentration (SSC, °Brix) of the extracted juice was measured with a temperature-compensated PAL-1 Atago digital refractometer.
Disease incidence (i.e., % of berries with disease symptoms) within a punnet was calculated. Average fruit quality for each punnet (i.e., berry size, skin colour, visual quality, flesh firmness, SSC) was determined from measurements on 8 – 12 berries per punnet. These data were analysed using restricted maximum likelihood (ReML) in GenStat 14 (VSN International Ltd., Oxford, UK) with fixed effects consisting of cool room treatment x storage removal and the random effects model consisting of Cool room/Mid shelf position/Punnet representing the physical structure of the experiment. Comparison of treatment means was conducted using Fisher’s least significant difference (LSD) test at a 5% significance level (α = 0.05) with the standard error of the difference between means (SE diff) calculated from the number of replicate punnets assessed at each removal.
Strawberry punnets on middle shelf of cool room.
Strawberry visual quality rating scale used in air disinfection experiment based on percent of total fruit surface with flesh breakdown and/or discolouration. Note that only one side of strawberry surface shown in figure above.
Strawberry skin colour range encountered during air disinfection experiment as measured by hue angle (°).
Results and Discussion
Initial latent infection and fruit weight loss during storage
Seven strawberry punnets per experimental cool room were incubated at 18°C for 3 days at the beginning of the trial to determine the level of field (latent) infection in fruit. After incubation there was no statistically significant difference in rot incidence among fruit allocated to each experimental cool room (Table A1), indicating that latent infection was similar among fruit assigned to each cool room at the beginning of the trial.
Berry weight loss after cool storage for 19 days at 0°C and 90% ±5% RH was approximately 20% in all cool rooms (Table A1).
Table A1. Strawberry botrytis incidence at Day 0 after incubation at 18°C for 3 days and effect of cool room treatment on berry weight loss after cool storage at 0°C for 19 days; n=7 punnets per treatment. Means for each quality parameter with the same letters are not significantly different.
Fungal rot development
As Botrytis symptoms were observed in fruit directly after removal from cool storage at 0°C incubation of fruit at 18°C was not required. Thus disease incidence and severity results are likely to better correspond to commercial situation. The Meditube unit significantly reduced botrytis incidence and severity in strawberries compared to conventional cool storage after 19 days cool storage at 0°C.
The Meditube unit significantly reduced botrytis incidence in inoculated fruit compared to conventional cool storage after 19 days
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