Abstract
The effects of two nonchemical methods [controlled atmosphere (CA) storage and postharvest heating, alone or combined] on the quality (firmness, taste, color, and skin wax) and storability (losses resulting from bruising and fungal decay) of apples were investigated in a 3-year study. Fruits of two cultivars (cv. Aroma and cv. Ingrid Marie) were mechanically wounded on two opposing sides, inoculated with conidial suspensions of one of three pathogens [Pezicula malicorticis (bull's eye rot), Penicillium expansum (blue mould), and Colletotrichum gloeosporioides (bitter rot)], exposed to 40 °C for four different exposure periods (24, 48, 72, and 96 h), and stored either in air (21.0 kPa O2 + 0.03 kPa CO2) or in CA storage (2.0 kPa O2 + 2.0 kPa CO2) for 4 months. Effect of postharvest heating on bruise susceptibility of air- or CA-stored apples was also investigated. Cultivar Aroma apples generally showed higher bruise susceptibility than cv. Ingrid Marie. The sun-exposed side of apples was less sensitive to bruising than the shaded side and red phenotypes of these two cultivars also showed increased resistance to bruising as compared with standard phenotypes. Heat treatment and CA storage, either alone or in combination, decreased bruise occurrence in both cultivars. Pz. malicorticis was the more aggressive storage pathogen for both apple cultivars followed by P. expansum and C. gloeosporioides. The highest decay severity occurred in inoculated and nonheat-treated apples stored in air. Heat treatment, especially in combination with CA storage, showed an eradicative effect on the pathogens without any negative effects on apple quality. Heat treatment maintained flesh firmness during storage, reduced ethylene production, and caused clearly visible changes in epicuticular wax structure, resulting in a higher resistance to bruising or to natural and artificial infections with the pathogens. The effective exposure period could be reduced to 24 h, because a combination of heat treatment (at 40 °C for 24 h) and CA storage showed the best protective effect against bruising and fungal decay. This combined treatment decreased bull's eye rot by 86% and 60% and bitter rot by 73% and 65% in cv. Aroma and cv. Ingrid Marie, respectively, in comparison with untreated apples.
Most storage losses in the Swedish apple industry are caused by bruising and fungal decay. Harvesting and postharvest operations (especially grading, packing, and distribution) can cause a high degree of fruit bruising, resulting in a marked deterioration in quality (Ericsson, 1989). Although some bruising can be avoided by various orchard management practices (Tahir et al., 2005, 2007) and some storage systems can reduce fruit sensitivity to bruising, this problem is still difficult to eliminate (Funt et al., 1999; Grajkowski et al., 2004; Zhang, 1994). Clear knowledge on cultivar susceptibility to bruising and on the ability of postharvest nonchemical treatments to reduce this susceptibility is needed by growers and fruit distribution organizations.
Infections by Pezicula malicorticis (bull's eye rot), Penicillium expansum (blue mould), and Colletotrichum gloeosporioides (bitter rot) are the main causes of decay during apple storage in Sweden (Tahir, 2006). Because pre- and postharvest application of fungicides is becoming banned in increasingly more countries, and totally prohibited in organic production (such as in Sweden), more environmental-friendly methods to protect apples against the more common fungal diseases should be developed.
Postharvest heat treatment has been used to reduce fungal rots in apples (Ferguson et al., 2000; Saftner et al., 2003; Sholberg et al., 2000). Decay caused by P. expansum can be avoided by heating fruits at 38 °C for 4 d (Conway et al., 1994; Fallik et al., 1995; Leverentz et al., 2000). Contradictory results have been found for C. gloeosporioides with some studies reporting a positive effect of heat treatment (Conway et al., 2004; Rappel et al., 1991) and others no effect (Janisiewicz et al., 2003). The effects of heat treatment on Pz. malicorticis have not been carefully studied. Regarding apple quality, some authors report that heat treatment leads to faster apple softening and a deterioration in apple acidity and starch content (Conway et al., 2004; Leverentz et al., 2003), whereas others found heated apples sweeter and firmer than nonheated (Abbott et al., 2000; Saftner et al., 2003). Heat treatment is not yet widely used because of variability in effects resulting from infection time (Leverentz et al., 2000), temperature degree and exposure period (Lurie, 1998), and weak residual protection (Leverentz et al., 2003). However, it is important to resolve these practical difficulties with heat treatment and to evaluate the usefulness of heating in combination with other treatments such as biocontrol (Conway et al., 2004).
The mode of action of heat treatment is still unclear. Previous works reported a direct interaction with fungal germination and growth (Schirra et al., 2000) and/or physiological responses by apple tissue (Ben-Shalom et al., 1996). Postharvest heating is reported to affect fruit ripening by enhancing peel degreening (Klein and Lurie, 1992) and starch conversion (Conway et al., 2004) and by delaying fruit softening and volatile production (Fallik et al., 1995). Apple epicuticular wax is relatively smooth with numerous surface cracks arranged in interconnected networks. These networks disappear after heating as a result of recrystallization or melting of the wax platelets into the cracks (Lurie et al., 1996; Roy et al., 1994; Schirra et al., 2000). However, the relationship between structural changes in epicuticular wax and the resistance of heat-treated fruits to rots is still unclear.
This study investigated relationships between bruise occurrence and fungal decay in two apple cultivars with postharvest heating alone or in combination with controlled atmosphere (CA) storage. The aim was to determine the shortest effective exposure period required to significantly decrease bruising, and fungal rots resulting from Pezicula malicorticis, and Colletotrichum gloeosporioides, and maintain fruit quality and thereby develop a commercially attractive method to decrease storage losses.
Materials and Methods
Apples.
Two of the most commonly grown Swedish apple cultivars, cv. Aroma and cv. Ingrid Marie, were harvested from nonfungicide-treated trees at the experimental station orchard in Kivik, the Swedish University of Agricultural Sciences, during 2003–2006. Apples were picked at preclimacteric stage, when ethylene production was 4.0 pmol·kg−1·s−1, firmness was 79 to 82 N, and starch conversion was 4/10 for cv. Aroma and 3.0 pmol·kg−1·s−1, 86 to 88.0 N, and 3/10, respectively, for cv. Ingrid Marie (Tahir, 2006). After harvesting, apples were randomly divided into 65 lots per cultivar, 40 of which were used for pathogen experiments, 10 for quality control experiments, 10 for bruising experiments, and five for investigation of ethylene production and respiration rate. The first 60 lots comprised three replicate batches of 60 apples each, whereas the last five lots comprised three replicate batches of 150 apples each.
Pathogens.
Of the 40 lots per cultivar, half were stored in air and the other half in CA storage after treatment. From each half:
Five lots were inoculated with conidial suspensions of Pz. malicorticis. One was then left unheated (control), whereas the other four lots were heat-treated at 40 ± 2 °C for 24, 48, 72, or 96 h.
Five lots were inoculated with conidial suspensions of P. expansum and then treated as in (1).
Five lots were inoculated with conidial suspensions of C. gloeosporioides, and then treated as in (1).
Five lots were used to investigate the natural infection (without inoculation). One was left unheated (control), whereas the other four lots were heat-treated at 40 ± 2 °C for 24, 48, 72, or 96 h.
The three pathogens used were isolated from naturally infected apples showing typical symptoms of the diseases, maintained on petri dishes with potato dextrose agar, and stored separately as pure cultures at 4 °C. Pathogen virulence over time was confirmed by periodic transfers through apples. For the present experiments, spores of the three pathogens were removed from the surface of 10-d-old cultures and suspended in 5 mL sterile distilled water containing 0.05% (v/v) Tween 80. The suspensions were filtered through four layers of sterile cheesecloth to remove any adhering mycelia, and spore concentrations were adjusted to 1 × 105 conidia/mL (by hemacytometer). The apples used were washed for 2 min with water to remove natural occurring fungi and wounded twice on both sides to a depth of 2 mm with a nail (4 mm diameter); one wound was placed 10 mm above the equator and the other 10 mm below and inoculated by pipetting 20 μL of freshly prepared conidial pathogen suspension into each of the wound sites.
Bruising.
Of the 10 lots per cultivar used for the bruising experiments, half were stored in air and the other in CA storage. From each half, one lot was stored within 6 h of harvesting (control), and the other four lots were heat-treated at 40 ± 2 °C for 24, 48, 72, or 96 h before storage. After 4 months of storage, apples were tested for bruise susceptibility by dropping them from 30 cm through vertical hollow polyvinyl chloride pipe onto a wooden table. Bruised fruits were watched and examined after 12 h (when bruised area totally browned at room temperature). They were cut longitudinally; bruise depth (h), diameter (d), apple diameter (D) and altitude (x) were measured; and brown tissue was excised and weighed. Bruise susceptibility was estimated as: bruise volume (BV) = 6−1 Π h d2 (Chen and Sun, 1981). To evaluate the relationship between coloration and bruise susceptibility, an additional lot of 150 apples per cultivar (cvs. Aroma and Ingrid Marie and their red phenotypes cv. Amorosa and cv. Karin Schneider) were picked at commercial harvesting dates, divided equally in three replicates each, and bruised within 6 h after picking by dropping them on their exposed or shaded side as described previously. Apple coloration of each side was measured as described subsequently.
Apple quality.
Of the 10 lots per cultivar used for fruit quality experiments, half were stored in air and the other half in CA storage. One lot from each half was stored within 6 h of harvesting as control and the other four lots were heat-treated at 40 ± 2 °C for 24, 48, 72, or 96 h. After storage, apple quality was monitored using 45 apples. Apple coloration was measured as hue angle (h°) using a Chromameter CR200 with a 8-mm diameter window (Minolta Ltd., Osaka, Japan). Measurements were conducted on three widely spaced spots at the apple equator and expressed as h°, in which 0° = red–purple, 90° = yellow, 180° = bluish green, and 270° = blue (McGuire, 1992). Apple firmness was determined by penetrometer (Model FT-327; Effigy, Alfonsine, Italy; plunger diameter 11.1 mm, depth 7.9 mm) on opposite sides of each apple and the results expressed as force (N) required for penetration. Soluble solids content (SSC) in apple juice was measured by a refractrometer (Atago, Tokyo, Japan) and the results expressed as a percentage. Titratable acidity was analyzed by titration of apple juice as malic acid with 0.05N NaOH to pH 8.1 (Radiometer PHM64, Copenhagen, Denmark). Storage rot (natural infection) was determined by visual examination according to Rein (1996). Decay severity resulting from artificial inoculation was measured as surface decay lesion area. Skin of heated and unheated apples was examined under a scanning electronic microscope (LEO 435VP, Cambridge, UK) with secondary electron detector at 2 kV low voltage and on magnification 1000×. Two apples per replicate batch and lot were assessed. Equatorial peel (4 to 5 mm2) from either the sun-exposed or shaded side of apples was freshly mounted on double-sided tape and on specimen stubs. Areas of wax at least 5 μm thick were chosen.
Respiration activity and ethylene production.
Of the five lots per cultivar used for respiration rate and ethylene production determination, one was left untreated (control), whereas the other four lots were heat-treated on 40 ± 2 °C for 24, 48, 72, or 96 h. All lots were stored in air (21.0 kPa O2 + 0.03 kPa CO2, 2 ± 0.5 °C and 90% relative humidity). Once a week, six apples from each lot were transferred to 0.5-L sealed glass jars (three replicate jars per treatment) for measurement of apple respiration rate and ethylene production. The jars were hermetically sealed with rubber stoppers for 2 h at 20 °C before measurements. One milliliter of holder atmosphere was withdrawn by gas syringe and the ethylene quantified by gas chromatography, Agilent 6850 (column Agilent 19095P-Q04E Hp-Plot Q 30 m × 530 μm × 40 μm). For respiration rate measurements (CO2), one additional milliliter was analyzed by gas chromatography, Varian Inc. 3700 (two-column Hayesep Q1/4″ × 8″ and Molsict 5, 3/8″ × 3″).
Heat treatment.
At harvest, the field temperature of cv. Aroma and cv. Ingrid Marie was 16 and 14 °C, respectively. Within 6 h of inoculation, apples were placed in separate new plastic boxes, covered with polyethylene to provide water-saturated conditions and to protect the apple from heat damage (Miller et al., 1990), and kept in thermostatically controlled (± 2 °C) small chambers (4.5 m3) at 40 °C for 24, 48, 72, or 96 h. Relative humidity (RH) was always more than 80%. The heated apples were allowed to equilibrate to room temperature for 12 h before being transferred to air or CA storage.
Storage and shelf life.
Apples were stored either in air (21.0 kPa O2 + 0.03 kPa CO2, 2 ± 0.5 °C and 90% RH) or in CA (2.0 kPa O2 + 2.0 kPa CO2, 2 °C and 90% RH) (Haffner, 1993). CA storage conditions with computer-controlled gas composition were established in 350-L chambers (Nino-laboratory, Onsala, Sweden) 3 d before placing the apples in the chambers. At the end of storage, all apples were transferred to a plastic chamber (18 ± 2 °C and 80% RH) for 1 week before quality and natural and artificial infections with Pz. malicorticis, P. expansum, and C. gloeosporioides were evaluated. Changes in quality parameters were also estimated.
Statistical analyses.
The experiment was carried out over three seasons. Three variables were controlled: two storage methods, five exposure periods, and three seasons. Decay incidence was defined as the percentage of natural occurring fungal infection and decay severity was measured as surface decay lesion area caused by artificial infection. Data were subjected to arcsine square root transformation before analysis. Data were subjected to analyses of variance using the general linear model to determine main effects and interactions (SAS Inc., Cary, NC). Least significant difference values (0.05) were calculated for comparison of means. Regression analyses and Pearson correlation were carried out (using SAS) to quantify the relationships among bruising occurrence, heat exposure time, and skin color.
Results
Apple quality.
Fruit firmness at the end of shelf life was affected by cultivar, storage method, season, and treatment (Table 1; P < 0.05). The highest loss in firmness was found for control apples of either cultivar, whereas the loss was significantly lower for apples heated at 40 °C for a minimum of 48 h (or 24 h in 2003 for cv. Aroma and in 2005 for cv. Ingrid Marie) and for apples stored in CA (Table 1). Combined heat treatment (for 24 h for cv. Aroma and 48h for cv. Ingrid Marie) and CA storage maintained better firmness, showing 26%, 12%, and 10% higher firmness in comparison with control, CA storage, and heat treatment (at 40 °C for 24 or 48 h), respectively (Table 1). Extending the heat exposure period more than 24 h for cv. Aroma and 48 h for cv. Ingrid Marie did not improve the effect of heat treatment on apple firmness, and instead had a negative influence for cv. Aroma in 2003 and 2005 (Table 1). No relationships were found between SSC or acidity and the heat treatments or CA storage in either cultivar, whereas heating apples at 40 °C for 24 h before CA storage improved the maintenance of the taste by 15% (higher acidity:SSC ratio) in comparison with other treatments (Table 2; P < 0.05). Heat-treated apples had better coloration (lower h° value) after shelf life compared with untreated fruit (Table 2). No interactions between treatment effect and season or between exposure period and coloration were found. Nonheated apples (control) showed higher ethylene production than those heated at 40 °C. This difference was first detectable in cv. Aroma after 5 weeks and in cv. Ingrid Marie after 6 weeks of air storage (Table 3; P < 0.05). Ethylene production was inversely related to exposure period. This effect became gradually visible until the last week in which the lowest ethylene production was found in apples heated at 40 °C for 96 h followed by heating for 72, 48, and 24 h (Table 3). Heat-treated apples showed higher respiration rate than nonheated during the first week in air storage, but this changed later (Weeks 5 to 7 in cv. Aroma and Weeks 3 to 7 in cv. Ingrid Marie) when nonheated apples showed a higher respiration rate than heat-treated apples. No visible influence of exposure time was observed in the beginning, whereas after 2 months of storage, heat-treated apples for 24 to 48 h showed higher respiration rate than heat-treated apples for 96 h (Table 4). The wax layer on the sun-exposed side of cv. Aroma apples usually had clumps (Fig. 1A1), whereas the wax layer on the shaded side often had nail-like crystals (Fig. 1A2). Cracks and fungal hyphae occurred on both sides, but particularly on the shaded side (Fig. 1A3). Wax layers in cv. Ingrid Marie were heavier, thicker, and less homogenous than those in cv. Aroma. Wax with nail-like crystals, microcracks, and fungal hyphae were found on both sun-exposed (Fig. 2A1) and shaded sides (Fig. 2A2).
Reduction of flesh firmness of heated and non heated apple fruits during storage and shelf life.
Changes in skin color and fruit taste of heated and nonheated apple fruits during storage and shelf life.
Ethylene production in heated and non heated apple fruits during storage in air.
Respiration rate (CO2 production) in heated and nonheated apple fruits during storage in air.
Epicuticular wax in cv. Aroma apples (A) in untreated and (B) in heat-treated apples at 40 °C for 24 h. Scale bars =5 μm, magnification 1000×; Leo. 435 up. (A1) Wax clumps on sun-exposed side. (A2) Nail-like wax crystals on shaded side. (A3) Crack in wax layer, shaded side. (B1) Wax clumps fusing as a result of heat treatment. (B2) Fungal hyphae captured in melted wax. (B3) Wax layer on shaded side less influenced by heat treatment.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1648
Epicuticular wax in cv. Ingrid Marie apples (A) in untreated and (B) in heat-treated apples at 40 °C for 24 h. Scale bars = 5 μm, magnification 1000×; Leo. 435 up. (A1) Layers of nail-like wax crystals on shaded side. (A2) Wax clumps and nail-like crystals on exposed side. (A3) Crack and hyphae in the wax layer. (B1) Wax clumps fusing very slowly resulting from heat treatment. (B2) Melted wax filling cracks. (B3) Fungal hyphae captured in melted wax.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1648
Bruising.
Bruising after storage was affected by cultivar, season, and fruit side. Cultivar Aroma and cv. Ingrid Marie apples were significantly (P < 0.05) more sensitive to bruising than the respective red phenotype cv. Amorosa (by 15% on the shaded side and 12% on the sun-exposed side) and cv. Karin Schneider (by 14% on the shaded side and 10% on the sun-exposed side during only two seasons) (Table 5). The shaded side of fruit was more sensitive (larger BV) to bruising than the sun-exposed side (Table 5). Generally, a positive Pearson correlation was found between h° value and bruise occurrence with fruit with high h° value (poor red color) showing larger BV than fruit with low h° value (Table 5). Heating apples decreased bruise susceptibility (lower BV) in both cultivars (Table 6). A positive correlation was found between heat exposure period and BV. The shortest effective exposure period (at 40 °C) was 24 h in cv. Aroma, decreasing BV by 58%, and 48 h in cv. Ingrid Marie, decreasing BV by 43%, compared with unheated fruits (Table 6). A combination of postharvest heat treatment (at 40 °C for 24 h) and CA storage increased the positive effect of each separate treatment on bruise susceptibility of the two cultivars. This combined treatment reduced BV by 54% and 40% compared with only CA storage and by 23% and 35% compared with only heat treatment (at 40 °C for 24 h as shortest effective period) in cv. Aroma and cv. Ingrid Marie apples, respectively (Table 6). No significant interactions with season were noted.
Relationships between apple color and bruise occurrence.
Postharvest heating at 40 °C increases apple resistance to bruising (average of the means 2003–2005).
Fungal decay resulting from natural infection.
Natural decay incidence was affected by cultivar, decay–causing agent, season, storage method, and treatment. Cultivar Ingrid Marie apples showed more resistance to Pz. malicorticis than cv. Aroma apples (by 30%), whereas cv. Aroma was always more sensitive to P. expansum than cv. Ingrid Marie (by 40%). Resistance to C. gloeosporioides varied between the seasons (data not shown). CA storage decreased decay resulting from the three pathogens by 50% compared with the air stored cv. Aroma apples (Table 7) and by 50%, 38%, and 27% resulting from Pz. malicorticis, C. gloeosporioides, and P. expansum, respectively, compared with the air stored cv. Ingrid Marie apples (Table 7). Heat treatment at 40 °C for 24 h reduced storage decay in cv. Aroma as a result of each of the three pathogens by 28% in air and 62% in CA (Table 7), whereas decay resulting from Pz. malicorticis and C. gloeosporioides in cv. Ingrid Marie apples was also decreased by 25% and 33% in fruits stored in air and by 50% and 38% in fruits stored in CA, respectively, in comparison with control. This positive influence on decay caused by P. expansum was shown only when heat treatment was combined with CA storage (Table 7).
Natural fungal decay in heated and nonheated apples after 4 months of storage and 1-week shelf life.
Fungal decay resulting from artificial inoculation.
Decay resulting from artificial inoculation was affected by cultivar, decay-causing agent, storage method, and treatment. In both cultivars, Pz. malicorticis caused the largest lesion area (13.0 to 17.0 cm2) followed by P. expansum (8.2 to 14.0 cm2), whereas C. gloeosporioides gave the smallest lesions (4.0 to 4.8 cm2). Heating apples at 40 °C for 24 h in air gave 50%, 63%, and 56% smaller lesion area in cv. Aroma and 48%, 56%, and 32% smaller lesion area in cv. Ingrid Marie resulting from inoculation with Pz. malicorticis, C. gloeosporioides, and P. expansum, respectively, compared with the nonheat-treated apples (Table 8). The shortest protective exposure period was 24 h, because no further beneficial effect with longer exposure periods was observed (Table 8). CA storage decreased decay severity resulting from Pz. malicorticis, C. gloeosporioides, and P. expansum by 53%, 52%, and 50% in cv. Aroma and by 40%, 16%, and 50% in cv. Ingrid Marie, respectively, in comparison with the air-stored apples (Table 8). Combined heat treatment (40 °C for 24 h) and CA storage improved resistance to Pz. malicorticis, C. gloeosporioides, and P. expansum further and resulted in lower decay severity compared with only CA-stored apples (by 60%, 70%, and 50%, respectively, in cv. Aroma apples and by 60%, 57%, and 40% in cv. Ingrid Marie) (Table 8).
Improvement of apple resistance to fungal decay [as lesion area (cm2)] by postharvest heating and controlled atmosphere (CA) storage (means 2003–2005).
Changes in skin wax.
Heat treatment caused important changes in epicuticular wax. After heating of cv. Aroma apples at 40 °C for 24 h, wax clumps started to fuse together to discernibly continuous parallel layers (Figs. 1B1 and 2B1) with occasional remains of fungal hyphae captured in the wax (Figs. 1B3 and 2B3). However, the degree of wax fusion varied even within the same apple. The nail-like wax crystals held their form longer than the wax clumps (Fig. 2B2). Heating had less effect on the wax structure of cv. Ingrid Marie apples. However, microcrack structure was altered by heating and filled with melted wax (Figs. 1B1 and 2B1). Fungal hyphae were also captured in the wax (Figs. 1B3 and 2B3).
Discussion
Bruising was not totally prevented by CA storage, but bruise volume was decreased by combining CA storage with heat treatment at 40 °C for 24 h, to commercially acceptable levels in both cultivars, because apples with bruised area smaller than 1 cm3 in volume were accepted by consumers (Ericsson and Tahir, 1996). The inhibitory effect of heat treatment on ethylene production and uronic acid degradation (Klein et al., 1990) and synthesis of cell wall hydrolytic enzymes such as polygalacturonase (Lurie et al., 1996), resulting in better firmness might cause higher ability to withstand impact injury. A clear correlation was found between apple red color and resistance to bruising, because shaded sides were more sensitive to impacts than sun-exposed sides. Also, cvs. Aroma and Ingrid Marie had higher bruise susceptibility than their red phenotypes cvs. Amorosa and Karin Schneider. The sun-exposed side of apples contained thicker and more numerous wax clumps than the shaded side. Heat treatment is known to melt skin wax and change its structure (Roy et al., 1994), perhaps providing some cushioning and decreasing the impact pressure. Cultivar Ingrid Marie apples, which had thicker, nail-like, and heavier wax, also had higher resistance to bruising than cv. Aroma.
C. gloeosporioides did not cause severe decay in either cultivar, whereas the dominant storage pathogen was Pz. maliorticis followed by P. expansum. Both of these fungal species were the more aggressive pathogens and not only P. expansum as previously suggested (Conway et al., 2004; Janisiewicz et al., 2003). All three pathogens were sensitive to heat treatment (at 40 °C) reducing natural fungal infection or decay resulting from artificial inoculation of apples. The effectiveness of heat treatment depended on cultivar and decay-causing agent, confirming previous studies (Barkai-Golan and Philips, 1991; Spadaro et al., 2004), although the present study did not indicate any positive effect of extending heat exposure period on the reduction of the decay.
Shortening the exposure period of heat treatment from 96 to 24 h did not cause any negative change in the method effectiveness. Heating apples for longer than 48 h increased skin damage, particularly in stored cv. Ingrid Marie, possibly because of stomata destroyed at higher heat becoming sites of subsequent decay (data not shown). The positive effects of combined heat treatment and CA storage on bruising, bull's eye rots (Pz. malicorticis), and bitter rot (C. gloeosporioides) reported here are novel information. The method improved the eradicative effect and increased residual activity of heating and CA storage, because it caused significantly lower decay than heated apples stored in air or than nonheated apple stored in CA.
Apple defense systems against fungi, which comprise either complex constitutive or nonconstitutive interactions, can be improved by heating (Ben-Yehoshua et al., 1998). Heat treatment was previously found to inhibit respiration rate and ethylene production (Lurie et al., 1995). Our study showed that heat treatment caused an acceleration of CO2 production in the first week of storage, which recovered during the subsequent storage period, but had no such effect on ethylene production. From the fifth week until the end of the storage, both parameters remained lower in the heat-treated fruits than in the nonheated. Thus, heat treatment delayed ripening and softening and thereby improved apple resistance to fungal infection during storage (Janisiewicz et al., 2003; Saftner et al., 2003). Heat treatment may also enhance host resistance to fungal attack by physiological or structural changes in the fruit surface. Fungal spores and latent infections occur either in the fruit surface or in the few cell layers under the apple peel. Cracks, wounds, and other breaks in the fruit surface are therefore the most important vents for hyphal penetration (Porat et al., 2000; Schirra et al., 2000). Nonheated fruit have an interconnected network of numerous deep surface cracks in their epicuticular wax, whereas the epicuticular wax of heat-treated fruit has no such network (Lurie et al., 1996; Roy et al., 1994). In this study, we observed significant changes in epicuticular wax resulting from heat treatment with melted wax filling microcracks; covering germinated spores, conidia, and hyphae; and preventing inoculation. Differing cultivar responses to heating may be the result of variations in skin wax amount and structure. Apples with heavier and hardened epicuticular wax layers, containing few or no cracks, showed better resistance to pathogen attack, whereas apples with a clumpy wax layer showed better response to heat treatment. Nail-like wax crystals needed higher temperatures to melt and protect against decay. Heat treatment melted wax platelets into cracks and did not recrystallize them, as reported previously (Roy et al., 1994).
From the provided data, it could be concluded that heat treatment at 40 °C and exposure period for 24 h in combination with CA storage is a promising strategy for increasing resistance to bruising and fungal decay in both cv. Aroma and cv. Ingrid Marie. This method provided acceptable protection against bull's eye rot (Pz. malicorticis) and bitter rot (C. gloeosporioides) and shorted the exposure period.
Literature Cited
Abbott, J.A., Klein, J.D., Campbell, T.A., Conway, W.S. & Sams, C.E. 2000 Sensory and firmness measurements of calcium- and heat treated apples J. Texture Stud. 31 109 121
Barkai-Golan, R. & Philips, D.J. 1991 Postharvest heat treatment of fresh fruits and vegetables for decay control Plant Dis. 75 1085 1089
Ben-Shalom, N., Hanzon, J., Pinto, R. & Lurie, S. 1996 Cell wall changes and partial prevention of fruit softening in prestorage heat treated ‘Anna’ apples J. Sci. Food Agr. 72 231 234
Ben-Yehoshua, S., Rodov, V. & Peretz, J. 1998 Constitutive and induced resistance of citrus fruit against pathogens 78 92 Johnson G.I., Highly E. & Joyce D.C. Disease resistance in fruit ACIAR Proc. No. 80 Canberra, Australia
Chen, P. & Sun, Z. 1981 Impact parameters in relation to bruise injury in apples Amer. Soc. Agr. Eng. Pap. 81 3041
Conway, W.S., Leverentz, B., Janisiewicz, W.J., Blodgett, A.B., Saftner, R.A. & Camp, M.J. 2004 Integrating heat treatment, biocontrol and sodium bicarbonate to reduce postharvest decay of apple caused by Colletotrichum acutatum and Penecillium expansum Postharvest Biol. Technol. 34 11 20
Conway, W.S., Sams, C.E., Wang, C.Y. & Abbott, J.A. 1994 Additive effects of postharvest calcium and heat treatment on reducing decay and maintaining quality in apples J. Amer. Soc. Hort. Sci. 119 49 53
Ericsson, N.A. 1989 Possibilities for reduction of mechanical damages during distribution of apples Obst und Gemüse DGQ XXIV 126 136
Ericsson, N.A. & Tahir, I.I. 1996 Studies on apple bruising. I. Estimation of incidence and susceptibility differences in the bruising of three apple cultivars Acta Agr. Scand. Sect. B. Soil and Plant Sci. 46 209 213
Fallik, E., Grinberg, S., Gambourg, M., Klein, J.D. & Lurie, S. 1995 Prestorage heat treatment reduces pathogenicity of Penicillium expansum in apple fruit Plant Pathol. 45 92 97
Ferguson, B., Ben-Yehoshua, S., Mitcham, E.J., McDonald, R.E. & Lurie, S. 2000 Postharvest heat treatments: Introduction and workshop summary Postharvest Biol. Technol. 21 1 6
Funt, R.C., Cameron, E.A. & Banks, N.H. 1999 The effect of apple fruit bruising on total returns Research Circular—Ohio Agricultural Research and Development Center 299 55 61
Grajkowski, J., Ochmian, I., Chelpinski, P. & Mikiciuk, G. 2004 Evaluation of susceptibility to mechanical damage in several cultivars of apple trees Folia Universitatis Agriculturae Stetinensis. Agricultura 93 113 117
Haffner, K.E. 1993 Storage trial of ‘Aroma’ apples at the Agricultural University of Norway Acta Hort. 326 305 313
Janisiewicz, J.W., Leverentz, B., Conway, S.W., Saftner, R.A., Reed, A.N. & Camp, M.J. 2003 Control of bitter rot and blue mold of apples by integrating heat and antagonist treatments on 1-MCP treated fruit stored under controlled atmosphere conditions Postharvest Biol. Technol. 29 129 143
Klein, J.D. & Lurie, S. 1992 Heat treatments for improved postharvest quality of horticultural crops HortTechnology 2 316 320
Klein, J.D., Lurie, S. & Ben Arie, R. 1990 Quality and cell wall components of ‘Anna’ and ‘Granny Smith’ apples treated with heat, calcium, and ethylene J. Amer. Soc. Hort. Sci. 115 954 958
Leverentz, B., Conway, W.S., Janisiewicz, W.J., Saftner, R.A. & Camp, M.J. 2003 Effect of combining MCP treatment, heat treatment, and biocontrol on the reduction of postharvest decay of ‘Golden Delicious’ apples Postharvest Biol. Technol. 27 221 233
Leverentz, B., Janisiewicz, W.J., Conway, W.S., Saftner, R.A., Fuchs, Y. & Camp, M.J. 2000 Combining yeasts or a bacterial biocontrol agent and heat treatment to reduce postharvest decay of ‘Gala’ apples Postharvest Biol. Technol. 12 87 94
Lurie, S. 1998 Postharvest heat treatments Postharvest Biol. Technol. 14 257 269
Lurie, S., Handros, A., Fallik, E. & Shapira, R. 1996 Reversible inhibition of tomato fruit gene expression at high temperature Plant Physiol. 110 1207 1214
Lurie, S., Othman, S. & Borochov, A. 1995 Effects of heat treatment on plasma membrane of apple fruit Postharv. Biol. Physiol. 5 29 38
McGuire, R. 1992 Reporting of objective colour measurements HortScience 27 1254 1255
Miller, W.R., Chun, D., Risse, L.A., Hatton, T.T. & Hinch, R.T. 1990 Conditioning of Florida grapefruit to reduce peel stress during low temperature storage HortScience 25 209 211
Porat, R., Pavoncello, D., Peretz, J., Weiss, B., Daus, A., Cohen, L., Ben-Yehoshua, S., Fallik, E., Droby, S. & Lurie, S. 2000 Induction of resistance to Penicillium digitatum and chilling injury in ‘Star Ruby’ grapefruit by a short hot water rinse and brushing treatment J. Hort. Sci. Biotechnol. 75 428 432
Rappel, L.M., Cooke, A.W., Jacobi, K.K. & Wells, I.A. 1991 Heat treatment for postharvest disease control in mangoes Acta Hort. 291 362 371
Rein, A. 1996 Apple storage disorders (original title in Swedish: Skador på äpplen vid Inlagring) Swedish Board of Agr. Agro. Info 3810 Gvarv, www.sjv.se.1-16.
Roy, S., Conway, W.S., Watada, A.E., Sams, C.E., Erbe, E.F. & Wergin, W.P. 1994 Heat treatment affects epicuticular wax structure and postharvest calcium uptake in ‘Golden Delicious’ apples HortScience 29 1056 1058
Saftner, R.A., Abbott, J.A., Conway, W.S. & Barden, C.L. 2003 Effects of 1-Methylcyclopropene and heat treatments on ripening and postharvest decay in ‘Golden Delicious’ apples J. Amer. Soc. Hort. Sci. 128 120 127
Schirra, M., D'Hallewin, G., Ben-Yehoshua, S. & Fallik, E. 2000 Host–pathogen interactions modulated by heat treatment Postharvest Biol. Technol. 21 71 85
Sholberg, P., Haag, P., Hocking, R. & Bedford, K. 2000 The use of vinegar vapor to reduce postharvest decay of harvested fruit HortScience 35 898 903
Spadaro, D., Garibaldi, A. & Gullino, M.L. 2004 Control of Penecillium expansum and Botrytis cinerea on apple combining a biocontrol agent with hot water dipping and acibenzolar-S-methyl, baking soda, or ethanol application Postharvest Biol. Technol. 33 141 151
Tahir, I., Johansson, E. & Olsson, M.E. 2005 Groundcover materials improve quality and storability of ‘Aroma’ apples HortScience 40 1416 1420
Tahir, I., Johansson, E. & Olsson, M.E. 2007 Improvement of quality and storability of apple cv. ‘Aroma’ by adjustment of some pre-harvest conditions Scientia Hort. 112 164 171
Tahir, I.I. 2006 Control of pre- and postharvest factors to improve apple quality and storability PhD Diss. Plant Breeding and Biotechnology Dept., Swedish University of Agricultural Sciences Alnarp, Sweden
Zhang, W. 1994 Apple impact bruise analysis PhD Diss. Biological Systems Engineering Dept., College of Engineering and Architecture, Washington State University Pullman, WA