Abstract
We tested the impact of storage atmospheres in which the CO2 and O2 percentages sum to 21% on highbush blueberry (Vaccinium corymbosum L.) fruit condition and quality. The CO2 and O2 combinations, in percent composition, were 19%/2%, 18%/3%, 16.5%/4.5%, 15%/6%, 13.5%/7.5%, 12%/9%, 6%/15%, and 0%/21% for CO2/O2, respectively. Nine blueberry cultivars were evaluated (Duke, Toro, Brigitta, Ozarkblue, Nelson, Liberty, Elliott, Legacy, and Jersey) after 8 weeks of controlled atmosphere (CA) storage at 0 °C. Surface mold, berry decay, skin reddening (associated with fruit pulp browning), fruit firmness, pulp discoloration, and the content of ethanol and acetaldehyde were assessed. Fruit firmness, skin reddening, and decay declined and the proportion of fruit with severe internal discoloration tended to increase as CO2 concentrations increased. Ethanol and acetaldehyde accumulation was minimal, indicating fermentation was not induced by the atmospheric conditions applied. Cultivar effects were far more pronounced than atmosphere effects. Some cultivars such as Duke, Toro, Brigitta, Liberty, and Legacy appear to be well suited to extended CA storage, whereas other cultivars such as Elliott stored moderately well, and Ozarkblue, Nelson, and Jersey stored poorly. The data indicate that responses to high levels of CO2, while O2 is maintained at its maximum level practicable, can, in a cultivar-dependent manner, include significant negative effects on quality while achieving the desired suppression of decay.
Blueberry (Vaccinium corymbosum L.) fruit benefit from elevated CO2 levels, through its impact on suppressing fungal decay, but not from low O2 levels (Blasing, 1993; Ceponis and Cappellini, 1979, 1983, 1985; Schotsmans et al., 2007; Smittle and Miller, 1988). Unlike climacteric fruit, which typically are harvested mature, but not ripe, and require ethylene to drive ripening processes (Asif et al., 2009; Bapat et al., 2010; Pathak et al., 2003), blueberry fruit are ripe at the time of harvest and do not depend on ethylene for ripening (DeLong et al., 2003). Thus, low O2, which is used to suppress ethylene synthesis and response in climacteric fruit storage (Banks, 1985; Elyatem et al., 1994), is not expected to benefit blueberry storability (Beaudry, 1999). The atmosphere for blueberry fruit in CA storage and in modified atmosphere (MA) packaging is usually one in which the CO2 level is of primary importance and O2 is maintained at a level sufficient to prevent fermentation.
Commercially, CA storage of blueberry fruit is commonly used during extended sea shipments and less commonly used in land-based storage facilities. It is recommended that the CO2 partial pressure be maintained between 8 and 15 kPa and O2 levels are maintained above 2 to 4 kPa (Ceponis and Cappellini, 1979, 1983, 1985). However, the concentration of CO2 necessary for decay control is often close to the level of product tolerance (Kader, 1995). Importantly, most, if not all, fruits or vegetables possesses a low O2 threshold, below which fermentation damage is liable to occur, and a threshold to high CO2 levels, above which injury resulting from fermentation and/or acute toxicity is likely (Beaudry, 1999; Thomas, 1925). Sensitivity of plant tissues to both gases is a function of cultivar, temperature, and duration of exposure (Pesis, 2005) and off-flavor development can occur with either low O2 or high CO2 (Couey and Wells, 1970; El-Kazzaz et al., 1983; Harris and Harvey, 1973). An interaction between O2 and CO2 on fermentation has been demonstrated for blueberry; as the partial pressure of CO2 increases, the tolerance of ‘Bluecrop’ fruit to low O2 declines (Beaudry, 1993).
After several iterations of CO2 accumulation and purging, the equation demonstrates that the sum of partial pressures of O2 and CO2 converges on ≈21%, which is ≈21 kPa (1.013 kPa = 0.01 atm) (Fig. 1).
Percent of atmosphere for O2, CO2, and the sum of O2 and CO2 in controlled atmosphere (CA) chambers for which the atmosphere control strategy is to maintain elevated CO2 and using air as a purge gas. In this example, the target concentration for CO2 is 12%, the actionable threshold to instigate a purge event is 13.5% CO2, and O2 is not actively controlled. Initial conditions are depicted as resulting from an initial addition of CO2 to obtain the target CO2 concentration. Dashed line represents the sum of O2 and CO2 at equilibrium.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Percent of atmosphere for O2, CO2, and the sum of O2 and CO2 in controlled atmosphere (CA) chambers for which the atmosphere control strategy is to maintain elevated CO2 and using air as a purge gas. In this example, the target concentration for CO2 is 12%, the actionable threshold to instigate a purge event is 13.5% CO2, and O2 is not actively controlled. Initial conditions are depicted as resulting from an initial addition of CO2 to obtain the target CO2 concentration. Dashed line represents the sum of O2 and CO2 at equilibrium.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Percent of atmosphere for O2, CO2, and the sum of O2 and CO2 in controlled atmosphere (CA) chambers for which the atmosphere control strategy is to maintain elevated CO2 and using air as a purge gas. In this example, the target concentration for CO2 is 12%, the actionable threshold to instigate a purge event is 13.5% CO2, and O2 is not actively controlled. Initial conditions are depicted as resulting from an initial addition of CO2 to obtain the target CO2 concentration. Dashed line represents the sum of O2 and CO2 at equilibrium.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
For blueberry fruit, effective CO2 levels have generally been reported to be in the range of 8 to 20 kPa. Ceponis and Cappellini (1979, 1983, 1985) and Schotsmans et al. (2007) reported that a CO2 partial pressure in the range of 8 to 15 kPa is effective in suppressing decay and preserving fresh blueberries stored for several weeks at low temperature.
The aim of this study was to investigate the sensitivity of nine blueberry cultivars (Duke, Toro, Brigitta, Ozarkblue, Nelson, Liberty, Elliott, Legacy, and Jersey) to combinations of O2 and CO2 that are commercially attainable in CA storage and perforated MA packages when fruit are to be stored for relatively extreme durations. The 8-week storage duration simulated the time needed for transoceanic container shipments plus distribution and retail sales periods.
Materials and Methods
Full-blue fruit from nine blueberry (Vaccinium corymbosum L.) cultivars were hand-harvested from trial plots at the Michigan Blueberry Growers Association near Grand Haven, MI on 9 July (‘Duke’), 15 July (‘Toro’), 22 July (‘Brigitta’), 29 July (‘Ozarkblue’ and ‘Nelson’), 3 Aug. (‘Liberty’), and 5 Aug. (‘Elliott’, ‘Legacy’, and ‘Jersey’) of 2009. Except for ‘Liberty’, fruit from each cultivar were from the first harvest, which took place after 30% to 50% of the fruit on the bush were full-blue. ‘Liberty’ fruit were from the second harvest, which followed the removal of the first harvest fruit by 10 d. After each harvest, fruit were transported to Michigan State University in insulated containers and transferred that afternoon to a cold storeroom at 0 °C. The next morning, fruit of each cultivar was segregated into 32 lots, discarding fruit with obvious defects. The weight of each lot was adjusted to ≈200 g and the fruit placed into 12 × 12 × 5-cm (length, width, and height, respectively) ventilated plastic clamshell containers. The number of fruit in each clamshell ranged from 70 to 120, depending on fruit size. For each cultivar, the clamshells were randomly assigned to eight treatments with four replicate clamshells per treatment in a randomized complete block design.
Initial fruit firmness was determined on 10 fruit randomly selected from each of the four replicates. Fruit firmness was determined as resistance to deformation using a durometer (Type 00; Shore Instruments, Jamaica, NY) fitted with a 2.4-mm diameter hemispherical probe and imparting a force of 0.49 N (50 g-force) per millimeter deformation. The maximum displacement of the probe was 2 mm. Durometer readings of 25, 50, 75, and 100 are equivalent to a force of 0.25, 0.49, 0.75, and 0.98 N and 1.5, 1.0, 0.5, and 0 mm of deformation, respectively. At harvest, the durometer was applied for ≈6 s to take a firmness measurement. After storage, the durometer was applied for 3 s to minimize the potential for puncturing the fruit skin.
After sorting and segregation into lots, the fruit were placed into aluminum-walled CA chambers (Storage Control Systems, Sparta, MI) measuring 143 × 71 × 96 cm (length, width, and height, respectively) and held at 0 °C for 8 weeks. Chamber atmospheres were regulated with an automated atmosphere control system (ICA 61 Laboratory System; International Controlled Atmosphere Ltd., Paddock Wood, UK) and managed so that flow rates for CO2, N2, and air were similar for all chambers. CO2 and O2 proportions were constrained to sum to 21% using the following combinations: 19%/2%, 18%/3%, 16.5%/4.5%, 15%/6%, 13.5%/7.5%, 12%/9%, 6%/15%, and 0%/21% for CO2/O2. At the altitude of the research site, roughly 270 m above sea level, 1% of atmospheric pressure equals 0.98 kPa. Target atmospheres were established within 24 h of fruit placement into the CA chambers. The relative humidity of the chambers was not measured.
Post-storage quality evaluations were carried out 4 to 6 h after removal of the fruit from the storage environment to allow the fruit to warm to room temperature. Weight loss was calculated for each clamshell by subtracting weight on removal from storage from initial weight. Fruit were removed from the clamshells and five fruit were removed for determination of ethanol and acetaldehyde content (see subsequently). Ten additional fruit, free from obvious defects, were selected for firmness measurements, and 50 of the remaining fruit were used to assess external and internal quality. Firmness was determined as described previously. External quality assessments included the incidence of skin reddening and visible surface mold. Skin reddening refers to the discoloration of the fruit skin from blue to a brownish red color indicative of dead and discolored epidermal and hypodermal cells. Determination of surface mold was based on the presence of obvious fungal mycelia on the surface of the fruit. No attempt was made to determine the identity of the fungal organism(s).
Internal quality was assessed by measuring the incidence and intensity of pulp (mesocarp and carpel) discoloration. Berries were cut in half using a stainless steel knife and fruit were assigned to quality categories based on the severity of pulp darkening resulting from water soaking and/or pigment bleeding (Beaudry et al., 1998). Fruit quality classes were 1 (less than 25% internal discoloration, white–green pulp; acceptable), 2 (between 25% and 50% internal discoloration, white–green to light pink pulp; edible but not acceptable), 3 (50% to 75% discoloration with some portion of white–green to light pink flesh; inedible), and 4 (75% to 100% discoloration with little to no white–green to light pink flesh; inedible). The number and percentage of fruit in each category for each replicate was determined. Determination of the incidence of berry decay was based on the presence of surface mold and/or liquefaction of the interior of the fruit with or without obvious shrivel.
Ethanol and acetaldehyde levels were determined according to the method of Pesis and Avissar (1990), based on the partition ratio between air and water (Harger et al., 1950), with minor modifications. For each replicate, five fruit were halved and macerated to a puree in a 22-mL clear glass vial using a Teflon-coated pestle. The vial was sealed with a valved septum (Mininert valve; Supelco, Bellefonte, PA) and placed within a water bath maintained at 25 °C. After a 10-min equilibration, a 1-mL headspace sample was removed from the vials using a plastic syringe and injected in the inlet septum of a gas chromatograph (Carle AGC Series 100; Hach Co., Loveland, CO) fitted with a 2-m long, 2-mm i.d. stainless steel column packed with Chromosorb OV-103, 60/80 mesh (Altech Associates Inc., Deerfield, IL) and equipped with a flame ionization detector. The gas chromatograph was run isothermally at 140 °C and the flow rate of the carrier gas (helium) and combustion gases (H2 and O2) were ≈50, 50, and 200 mL·min−1, respectively. Ethanol and acetaldehyde concentrations in the sample headspace were calculated using standards of 0.1% aqueous solutions of ethanol and acetaldehyde in identical vials to those used for samples and held in the same water bath. At 25 °C, a 0.1% solution of ethanol and acetaldehyde yield headspace concentrations of 120 and 203 μL·L−1, respectively.
Analysis of variance was performed using commercially available software (SAS 9.1, Cary, NC) using the PROC MIXED procedure. Atmosphere and cultivar were considered fixed effects and Tukey's test was used for multiple comparison analysis. An arcsine square root transformation was performed on all percentage data before statistical analysis. Non-transformed data are presented.
Results and Discussion
The rate of weight loss differed almost threefold between cultivars and ranged from 0.6% to 2.3% over the 8 weeks of the trial (Table 1), but no shrivel was observed on any fruit. The cultivar Jersey experienced 50% more weight loss than the next two nearest cultivars Toro and Legacy. This may reflect differences in cuticle permeance to water vapor, differences in the stem scar morphology, and/or the surface-to-volume ratios. In fact, ‘Jersey’ has the largest stem scar of all of the varieties evaluated, supporting the role of the stem scar in moisture loss during storage (Hancock et al., 2001; Hancock, personal observation).
Cultivar (C) and atmospheric treatment (T) effects on quality characteristics of highbush blueberry fruit held in cold storage at 0 °C for 8 weeks.z
Atmosphere affected moisture loss more than cultivar, yielding a 13-fold difference between the 0%/21% treatment (0.25% weight loss) and the 19%/2% treatment (3.3% weight loss). The greater weight loss for the highest CO2/lowO2 treatment may stem largely from physical causes. An elevated CO2 and low O2 environment requires more frequent and aggressive atmosphere modification than other treatments, so the greater flux of dry gas through the CA chambers likely contributed to the extreme in water loss. Although a physiological impact on moisture loss may have occurred because high levels of CO2 can stress blueberry fruit (Beaudry, 1993), a linkage is not obvious. Moisture loss in some cultivars resulting from the atmosphere treatment was more marked than in others. ‘Jersey’, for instance, lost over 6% of its weight in the high CO2 19%/2% treatment, whereas in this atmosphere, ‘Elliott’, ‘Toro’, and ‘Legacy’ each lost ≈4% of their weight; the other cultivars lost ≈2.5% of their original weight.
Initially, the firmest fruit was ‘Duke’ and the softest cultivar was Jersey (Table 1; Fig. 2). After storage, ‘Jersey’ fruit were markedly softer than all others, although ‘Nelson’, ‘Ozarkblue’, and ‘Elliott’ were quite soft as well. The relatively low initial firmness of the ‘Jersey’ fruit and the excessive firmness loss during storage might be partially indicative of a somewhat more advanced stage of maturity. It seems probable that the extreme loss in firmness of ‘Jersey’ also was at least partly a result of the high moisture loss (Table 1). The cultivar effect on firmness is consistent with previous publications. ‘Jersey’ is known to store poorly and several of those cultivars with the highest firmness after storage, Brigitta, Toro, Liberty, and Legacy, have been demonstrated to store well (Hancock et al., 2008).
Effects of cultivar and storage atmosphere on the firmness of highbush blueberry fruit as determined using a Type 00 durometer after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the firmness of highbush blueberry fruit as determined using a Type 00 durometer after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the firmness of highbush blueberry fruit as determined using a Type 00 durometer after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
The effect of atmosphere on fruit firmness was not as pronounced as the effect of cultivar, but fruit firmness was negatively affected by an increasing ratio of CO2/O2 with the treatment of 19%/2% having the greatest negative impact on firmness (Table 1). Schotsmans et al. (2007) found a similar result for rabbiteye blueberries (Vaccinium ashei Reade) with an atmosphere of 15 kPa CO2 and 2.5 kPa O2 leading to a loss in firmness relative to air storage in as little as 4 weeks’ storage for Centurion and Maru cultivars. Cranberry (Vaccinium macrocarpon Aiton) fruit undergo firmness loss at CO2 partial pressures of 15 kPa or more at O2 levels ranging from 2 to 70 kPa or under a partial pressure of 0 kPa O2 without added CO2, but firmness is not diminished by low O2 (2 kPa) relative to air storage (Gunes et al., 2002). Interestingly, high CO2 is known to have a positive influence on the firmness of strawberry (Fragaria ×ananassa Duch.) fruit (Harker et al., 2000; Pérez and Sanz, 2001). The mechanism of the impact of CO2 on fruit firmness is hypothesized to be related to a reduction in the apoplastic pH and its impact on the interaction of cell wall constituents (Harker et al., 2000). It is not clear why this mechanism would behave one way in strawberry but the opposite in Vaccinium species.
The firmness response to atmosphere differed between cultivars with seven of the nine cultivars exhibiting lower firmness for 19% CO2 treatment than for the air treatment (Fig. 2). Conversely, the firmness of ‘Duke’ and ‘Ozarkblue’ fruit was lower for air-stored fruit than for fruit exposed to any of the other atmospheres. The low firmness readings were the result of the durometer probe occasionally penetrating the skin of the berry fruit for this treatment. Approximately 10% to 15% of readings for air-stored ‘Duke’ and ‘Ozarkblue’ fruit were below 15, symptomatic of puncturing during the firmness measurement (data not shown), despite the average firmness for the air treatment being 45 and 40, respectively. This apparent weakening of the skin and/or cuticle of these two cultivars during air storage was not associated with higher rates of moisture loss. The relationship between storage atmosphere and skin and cuticle properties is, to our knowledge, unexplored in blueberry.
‘Ozarkblue’ had a markedly greater incidence of reddening than all other cultivars, followed, in order of declining severity of response, by ‘Nelson’, ‘Duke’, and ‘Toro’ (Table 1). Reddening was almost negligible in the remaining cultivars. Where found, reddening was most severe among those fruit held in air storage, averaging over 11% of the fruit affected. A 6%/15% atmosphere resulted in ≈3.5% reddened fruit, with 1%, or less, in higher CO2/O2 ratio treatments. The decrease in reddening with increasing CO2 and decreasing O2 can probably be ascribed to the impact of the CO2 rather than O2 given that the most marked decline in the disorder took place as O2 dropped from 21 to 15 kPa. O2 levels in this range are not considered to confer marked physiological responses (Beaudry, 1999). The interaction between cultivar and atmosphere results from the high incidence of reddening in the air treatments of ‘Duke’, ‘Toro’, ‘Brigitta’, ‘Ozarkblue’, and ‘Nelson’, averaging 7%, 16%, 10%, 46%, and 11%, respectively, with close to 0% for the other cultivars (data not shown).
Cultivar and atmosphere impacted the internal quality of the fruit as measured by the distribution of fruit having internal discoloration classes of 1, 2, 3, or 4. Cultivar had a statistically significant effect on each quality class. ‘Jersey’ and ‘Nelson’ had a greater fraction of fruit in the more severe categories 3 and 4 (summing to 85% and 93%, respectively), whereas ‘Duke’, ‘Toro’, ‘Brigitta’, and ‘Liberty’ had lower fractions in the more severe categories (47%, 36%, 49%, and 54%, respectively), and ‘Ozarkblue’, ‘Elliott’, and ‘Legacy’ had intermediate fractions in the more severe categories (72%, 64%, and 66%, respectively) (Fig. 3). The relatively poor condition of ‘Jersey’ and ‘Nelson’ fruit at the end of the storage period is consistent with previous evaluations of the storability of these two cultivars (Hancock et al., 2008). Similarly, the better maintenance of internal coloration by ‘Brigitta’ and ‘Toro’ is also consistent with previous findings (Hancock et al., 2008). The storability of the remaining cultivars appears to be somewhat more variable from year to year, but the current results are generally in agreement with those published by Hancock et al. (2008) with ‘Legacy’, ‘Liberty’, and ‘Elliott’ storing moderately well to very well depending on the season. There was a significant negative correlation between the percentage of fruit experiencing severe internal discoloration and fruit firmness (P < 0.001, R2 = 0.42).
Effects of cultivar and storage atmosphere on the incidence of severe (greater than 50%) internal discoloration of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the incidence of severe (greater than 50%) internal discoloration of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the incidence of severe (greater than 50%) internal discoloration of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
The impact of atmosphere on internal quality was not as marked as cultivar, affecting only the fractions of fruit classed as 2 and 4 (Table 1). Relatively, a greater portion of the variability of the response is explained by significant interactions for classes 2, 3, and 4. ‘Brigitta’, ‘Ozarkblue’, ‘Nelson’, and ‘Liberty’ exhibited an increase in the fraction of fruit in the more severe classes 3 and 4 as CO2 levels increased and O2 levels declined, but the internal condition of the remaining cultivars was not influenced by atmosphere. The data suggest that some cultivars may respond to more extreme CO2 atmospheres by loss of cellular integrity leading to darkening and discoloration of the pulp, whereas others are more resistant to the influence of atmosphere. The response of ‘Liberty’ and ‘Brigitta’ to the range of atmospheres is instructive; both cultivars had among the lowest levels of severe internal discoloration for the 6%/15% and 12%/9% treatments and nearly twofold higher severely discolored fruit in the atmospheres having the highest CO2 partial pressures. ‘Toro’, on the other hand, exhibited low levels of severe discoloration for all atmospheres.
Surface mold incidence was extremely low, averaging less than 1%. However, cultivar and atmosphere both had effects and the interaction was significant. ‘Duke’, ‘Toro’, and ‘Ozarkblue’ had a markedly higher incidence of surface mold than the other six cultivars. For the atmosphere treatments, the incidence of surface mold in the air-storage treatment (0%/21%) was ≈4%, roughly 10 times higher than for the various MAs. All the CO2-enriched atmospheres suppressed surface mold similarly such that the atmosphere combination containing 6 kPa CO2 was as effective as the more extreme CO2 treatments. The interaction was the result of uneven responses of the cultivars to the atmosphere treatments because ‘Toro’, ‘Ozarkblue’, and ‘Jersey’ were essentially decay-free except for the air-stored fruit, which averaged 16%, 19%, and 6%, respectively. ‘Duke’ had between 0.5% and 2% surface mold and there was no relationship to atmosphere. The other cultivars had little surface mold even in the air storage treatment (data not shown).
The extent of berry decay was relatively low compared with previously published reports (Ceponis and Cappellini, 1979, 1983, 1985; Hancock et al., 2008) given the long storage duration of the current study (Table 1). The average decay incidence of the nine cultivars ranged from ≈3% to 16% with ‘Elliott’ and ‘Jersey’ exhibiting two - to threefold higher decay rates than the other cultivars and ‘Liberty’ having the lowest rate of decay. The degree of decay declined as the ratio of CO2 to O2 increased (Fig. 4). CO2, but not O2, applied at similar partial pressures to those used in this study have been demonstrated to suppress decay (Ceponis and Cappellini, 1979, 1983, 1985; Schotsmans et al., 2007; Smittle and Miller, 1988). The effect of atmosphere was therefore the result of the CO2 partial pressure rather than changes in the ratio of the two gases per se.
Effects of cultivar and storage atmosphere on the incidence of berry decay of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the incidence of berry decay of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Effects of cultivar and storage atmosphere on the incidence of berry decay of highbush blueberry fruit after 8 weeks storage at 0 °C. Each data point represents the average of 40 fruit; vertical bars represent 1 sd.
Citation: HortScience horts 46, 1; 10.21273/HORTSCI.46.1.74
Ethanol and acetaldehyde concentrations were extremely low throughout the study (Table 1). The highest individual reading for both compounds was less than 10 μL·L−1, which means the maximum concentrations of ethanol and acetaldehyde in the fruit were less than 0.01% and 0.005%, respectively. If fermentation is actively engaged, then it would be expected that concentrations of these fermentation products in the berry pulp would be in the tenths-of-a-percent range (Beaudry, 1993; Beaudry et al., 1993; Toivonen and DeEll, 2001). Inasmuch as this was not the case, the data are likely not indicative of fermentation as a result of the atmospheric treatments, suggesting that none of the treatments were sufficiently stressful to modify respiratory metabolism. This is consistent with the data from Beaudry (1993) on ‘Bluecrop’ fruit, which fermented at CO2 partial pressures above 40 kPa when the O2 partial pressure was below 4 kPa. Similarly, Cameron et al. (1995) found no evidence of CO2 levels as high as 16 kPa altering either the apparent Km for oxygen or the maximal rate of respiration at temperatures between 0 and 20 °C.
The data indicate that important differences exist between these blueberry cultivars in their capacity to store well in air or in MAs. Responses to high levels of CO2, while O2 is maintained at its maximum level practicable, can include significant moisture loss, softening, and external and internal discoloration in addition to the well-documented and desirable suppression of decay. Some cultivars such as Duke, Toro, Brigitta, Liberty, and Legacy appear to be well suited to extended storage. ‘Toro’ seemed to respond well to the range of atmosphere combinations used. This variety underwent little softening even at relatively high CO2 partial pressures and had the lowest overall level of internal discoloration. On the other hand, ‘Liberty’ had extremely low levels of berry decay and skin reddening, especially under the influence of atmospheres with elevated CO2, and, when the CO2 levels were between 6 and 12 kPa, had among the lowest incidence of severe internal discoloration and the highest firmness levels. Collectively, the data suggest that CA storage can improve storability of highbush blueberry fruit relative to air storage, but very high levels of CO2 are to be avoided to prevent softening or internal discoloration in susceptible cultivars. When the intent is to establish an atmosphere enriched in CO2 to suppress decay while maximizing the O2 level, the data suggest a CO2 partial pressure near 12 kPa is broadly useful for blueberry and near optimal for specific cultivars.
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