Plant material and processing conditions.

Orange-fleshed cantaloupe melon (Cucumis melo L. subsp. melo var. cantalupensis Naudin ‘Olympic Gold’) fruit grown by Del Monte Fresh Produce Co. in Arizona were harvested in Nov. 2007, purchased from a broker in East Lansing, MI, and stored at 3 °C for a maximum of 48 h before use. Fruits (n = 14 to 26 per temperature treatment) with an average weight of 2.5 ± 0.2 kg and 11.7 ± 1.5% (w/w) soluble solids were used in the experiments. Whole fruit were rinsed with tap water, sanitized with 150 μL·L⁻¹ NaClO for 2 min, and air-dried. The rind was removed by hand with a sharp knife and the flesh was cut into trapezoidal sections ≈2 × 2.5 cm wide.

Packaging and storage conditions.

Melon pieces were placed in vented polyethylene terephthalate clamshells (Monte Package Company, Riverside, MI) of 13.0 × 11.1 × 6.7 cm that were inserted into low-density polyethylene (Dow Chemical Company, Midland, MI) pouches (18.5 × 19.0 cm or 18.5 × 19.5 cm), which were hermetically sealed using a heat sealer. Several combinations (n = 12 per temperature treatment) of film surface area and film thickness, and a range of fruit masses varying from 0.04 to 0.36 kg per pouch, were used to assure a wide series of steady-state gas concentrations within the packages. The film thickness used in the experiments ranged from 28.1 × 10⁻⁴ to 77.7 × 10⁻⁴ cm. Film permeabilities to O₂ and CO₂ (Table 1) were calculated with the predicting equations P_O2 = 0.067 × e^(–4423/T) and P_CO2 = 0.118 × e^(–4153/T) derived from the diffusion rate of gases of known concentrations through the film inserted into a permeability cell and the Arrhenius model to account for the effect of temperature on permeability (Gomes et al., 2010). Three replicates of each of the 12 combination of film thickness, film area, and fruit mass were stored at 0, 5, 10, and 15 °C.

Table 1.

Film permeability to O₂ and CO₂ at various temperatures.

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Determination of respiration rate.

The gas composition of the headspace in individual packages was monitored daily until steady-state was reached. Gas (100 μL) was withdrawn from packages through a silicon sampling septum (Gomes et al., 2010). Gases were measured using a paramagnetic O₂ detector (Series 1100; Servomex Co., Sussex, U.K.) and an infrared CO₂ detector (ADC 255-MK3; Analytical Development Co., Hoddesdon, U.K.) connected in series. Rates of O₂ uptake () and CO₂ () production were calculated from Eq. [1] and Eq. [2] using steady-state O₂ and CO₂ partial pressures, package permeability, and fruit weight (Beaudry et al., 1992; Lakakul et al., 1999). Respiratory quotients () for aerobic respiration were computed from the calculated respiration rates.

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where and are respiration rates (mmol·kg⁻¹·h⁻¹) for O₂ and CO₂, and are the O₂ and CO₂ permeabilities (mmol·cm·cm⁻²·h⁻¹·kPa⁻¹), A is the film area (cm²), l is the film thickness (cm), pO_{2 atm}, pO_{2 pkg}, pCO_{2 atm}, and pCO_{2 pkg} are O₂ and CO₂ partial pressure (kPa) outside (_atm) and inside (_pkg) the package, and M is the fruit mass (kg).

Experimental data modeling.

Respiration rate () was described as a function of O₂ partial pressure by a Michaelis-Menten model [Eq. (3)] as described by Lee et al. (1991).

View Expanded

where is the maximal rate of (mmol·kg⁻¹·h⁻¹), pO_{2 pkg} is the oxygen partial pressure inside packages (kPa), and is pO_{2 pkg} at half the value of (kPa).

The Michaelis-Menten model was fitted to the data from temperatures between 0 and 10 °C (n = 66) but not at 15 °C because fruit started to decay before reaching steady-state. The maximal respiration rate was found to be an exponential function of temperature [ = a × e ^{(b × T)}], and the Michaelis-Menten constant was modeled as either an exponential function of temperature [ = q × e ^{(r × T)}] or a constant (Table 2). Parameter estimates (a, b, q, and r) were obtained by non-linear regression using the Levenberg-Marquardt method. The model predictions of respiratory parameters (Table 2) were computed at 0, 5, and 10 °C and were used to calculate activation energies through an Arrhenius plot (Gomes et al., 2010) and the temperature coefficient (Q₁₀).

Table 2.

Best estimates of and for fresh-cut cantaloupe melon as a function of temperature (K) and corresponding model accuracies.

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Accuracy of the parameter estimates was calculated as the ratio between se and estimated value. Differences between the simulated and the experimental results were ascertained by the root mean square error (RMSE) (Yang and Chinnan, 1988). All statistical analyses were performed using the software package SPSS for Windows Version 16.0 (SPSS, Chicago, IL).

Effect of oxygen concentration and temperature on respiration rate.

Packages with fresh-cut melon attained gas equilibrium after 5 d at 10 °C, 6 d at 5 °C, and 10 d at 0 °C, but steady-state was not reached at 15 °C because the tissue started to decay. Experimentally determined respiration rates ( and ) between 0.1 and 16.2 kPa O₂ (Table 3) are within the range reported in the literature for fresh-cut cantaloupe (Aguayo et al., 2004; Gorny, 1998). The reported respiration rates of fresh-cut cantaloupe melon are 0.09 to 0.36, 0.13 to 0.71, 0.27, and 1.34 mmol CO₂/kg/h at 0, 5, 10, and 20 °C, respectively (Gorny, 1998).

Table 3.

Experimental rates of O₂ uptake and CO₂ production, respiratory quotient and fermentation threshold for packaged fresh-cut cantaloupe melon stored at various temperatures.

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RQ of fresh-cut cantaloupe melon stored at various temperatures ranged from 1.6 at 0 °C to 3.1 at 15 °C (Table 3; Fig. 1). The RQ values observed are higher than the theoretical value of 1.3 for the oxidation of organic acids (Kader et al., 1989) that would be expected in melon fruit, in which malic and citric acids are dominant (Lamikanra et al., 2000). The higher RQ values obtained in our study at suboptimal temperatures are likely a sign of slight fermentation at steady-state oxygen concentrations. This is not uncommon in fresh-cut melons, in which the loss of malic acid at 20 °C can be related to malo-lactic fermentation by the lactic acid bacteria (Lamikanra et al., 2000). Also, in fresh-cut ‘Piel de Sapo’ melon stored at 4 °C under 21 kPa O₂ and 2.5 kPa O₂ + 7 kPa CO₂, the RQ increases during storage in association with ethanol production (Oms-Oliu et al., 2008). In addition, the rise in CO₂ production observed in fresh-cut honeydew after 3 d at 10 °C or 6 d at 5 °C was attributed to tissue deterioration (Qi et al., 1999).

Effect of steady-state O₂ partial pressure and storage temperature on the rate of O₂ uptake (left) by cantaloupe pieces in sealed packages and corresponding respiratory quotient (right) and headspace CO₂ (open triangles). On the left, solid lines represent the MM model fit, whereas dashed lines represent the MM_k model fit. The oxygen partial pressures at which respiration reaches half its maximum (apparent K_m) and the fermentation threshold (FT) are indicated with a vertical bar.

Citation: HortScience horts 47, 8; 10.21273/HORTSCI.47.8.1113

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Effect of steady-state O₂ partial pressure and storage temperature on the rate of O₂ uptake (left) by cantaloupe pieces in sealed packages and corresponding respiratory quotient (right) and headspace CO₂ (open triangles). On the left, solid lines represent the MM model fit, whereas dashed lines represent the MM_k model fit. The oxygen partial pressures at which respiration reaches half its maximum (apparent K_m) and the fermentation threshold (FT) are indicated with a vertical bar.

Citation: HortScience horts 47, 8; 10.21273/HORTSCI.47.8.1113

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Effect of steady-state O₂ partial pressure and storage temperature on the rate of O₂ uptake (left) by cantaloupe pieces in sealed packages and corresponding respiratory quotient (right) and headspace CO₂ (open triangles). On the left, solid lines represent the MM model fit, whereas dashed lines represent the MM_k model fit. The oxygen partial pressures at which respiration reaches half its maximum (apparent K_m) and the fermentation threshold (FT) are indicated with a vertical bar.

Citation: HortScience horts 47, 8; 10.21273/HORTSCI.47.8.1113

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Fermentation occurred at higher oxygen partial pressure as the storage temperature increased from 0 to 10 °C (Table 3; Fig. 1), consistent with previous observations in other fruit (Beaudry et al., 1992; Lakakul et al., 1999). To avoid anaerobic respiration in honeydew melon, oxygen partial pressure must be increased from 2 kPa O₂ at 5 °C to 4 kPa O₂ at 10 °C (Qi et al., 1999). Our results indicate that fresh-cut cantaloupe melon should be packaged with oxygen partial pressure higher than 0.7 kPa at 0 °C and 1.3 kPa at 5 and 10 °C to prevent fermentation. At these oxygen concentrations, carbon dioxide levels were 7.6, 11.3, and 14.0 kPa at 0, 5, and 10 °C (Fig. 1), respectively. Package atmospheres commonly considered potentially beneficial to the quality of cantaloupe cubes require higher levels of oxygen, usually ≈3 to 5 kPa O₂ combined with 6 to 15 kPa CO₂ (Gorny, 1998). Similar gas mixtures have been tested by other authors with fresh-cut cantaloupe or honeydew melon, e.g., 4 kPa O₂ + 10 kPa CO₂ (Bai et al., 2001), 5 kPa O₂ + 10 kPa CO₂ (Amaro et al., 2012), 5 kPa O₂ + 5 kPa CO₂ (Bai et al., 2003), 2.5 kPa O₂ + 7 kPa CO₂ (Oms-Oliu et al., 2007), and 2 to 4 kPa O₂ + 10 kPa CO₂ (Qi et al., 1999).

Modeling the effect of oxygen concentration and temperature on respiration rate.

Oxygen uptake rate increased with temperature and pO_2pkg in a way consistent with Michaelis-Menten kinetics (Fig. 1). Respiratory parameters were modeled as exponential functions of temperature (Cameron et al., 1994). Alternatively, was treated as constant coupled with as an exponential function of temperature (Hertog et al., 1998; Jacxsens et al., 2000). These models provided a good fit to the experimental data (Fig. 1) as assessed by the RMSE (Table 2). The fit was slightly poorer (RMSE = 0.013) when was taken as constant (MM_k), and a deviation from experimental data was observed at high O₂ partial pressures at 10 °C (Fig. 1).

Maximal O₂ uptake rate is highly dependent on the temperature. The activation energy was estimated at 66 to 84 kJ·mol⁻¹ (Table 2) and the Q₁₀ was from 2.8 to 3.7 (Table 2). Q₁₀ values of 3.3 to 3.6 for cubes of muskmelon and honeydew melons have been reported in the interval 0 to 10 °C (Watada et al., 1996) and a Q₁₀ of 3.1 can be computed for whole cantaloupe melon from the data compiled by Exama et al. (1993). Activation energy of intact produce generally range from 50 to 89 kJ·mol⁻¹ (Exama et al., 1993; Fonseca et al., 2002) and is often higher for fresh-cut produce (Gomes et al., 2010; Jacxsens et al., 2000).

Respiratory responses to low oxygen and implications for modified atmosphere packaging.

A positive difference between for O₂ uptake and the fermentation threshold—named “safe working atmosphere” by Beaudry (2000)—can be taken as an indicator that significant decreases in metabolic activity can be obtained through the reduction in oxygen partial pressure. To reduce the respiration rate of fresh-cut cantaloupe by 50%, oxygen partial pressure must be less than 0.6 kPa, values lower than the fermentation threshold of 0.7 kPa at 0 °C (Table 4; Fig. 1). Therefore, a safe working atmosphere does not exist for fresh-cut cantaloupe, and limited extension of shelf life is anticipated through the reduction of oxygen levels. A similar response was found in packed strawberry fruits (Hertog et al., 1999) and fresh-cut ‘Rocha’ pear (Gomes et al., 2010) suggesting that low-oxygen MAP provides little or no benefit to extend quality attributes that depend on the metabolic rate of these commodities. Consistently, color, firmness, and soluble solids content of fresh-cut cantaloupe were unaffected by oxygen levels (Amaro et al., 2012). However, the production of flavor-important esters is favored under high oxygen levels (16 to 18 kPa) and suppressed at 5 kPa O₂, suggesting that high oxygen is required for desirable aroma revelation by fresh-cut cantaloupe (Amaro et al., 2012).

Table 4.

Parameter best estimates for fresh-cut cantaloupe melon stored at various temperatures.

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If this assumption proves correct, the potential benefits of MAP reported for fresh-cut cantaloupe (Gorny, 1998) are not attributable to the low oxygen levels; instead, they are likely explained by the high CO₂ partial pressure, reduced water loss, and lower microbial contamination resulting from the package barrier.

In conclusion, the respiratory parameters reported here suggest that there is a physiological limitation to the extension of shelf life of fresh-cut cantaloupe through the reduction of oxygen partial pressure, justifying the limited attention of the industry operators to the optimization of MAP for oxygen levels as long as the tissue is under aerobic conditions.