Influence of Oxygen and Temperature on the Respiration Rate of Fresh-cut Cantaloupe and Implications for Modified Atmosphere Packaging

The respiratory behavior of fresh-cut melon under modified atmosphere packaging at various temperatures was characterized to assess the potential for shelf life extension through low-oxygen and to generate information for the development of appropriate packaging conditions. Cantaloupe melon (Cucumis melo var. cantalupensis ‘Olympic Gold’) cubes were packaged and stored at 0, 5, 10, and 15 8C. Packages attained gas equilibrium after 5 days at 10 8C, 6 days at 5 8C, and 10 days at 0 8C. In cubes stored at 15 8C, decay started before steady-state gas levels were reached. Respiration rates were measured and respiratory quotient calculated once steady-state O2 and CO2 partial pressures were achieved inside the packages. O2 uptake increased with temperature and O2 partial pressure (pO2 pkg), according to a Michaelis-Menten kinetics described by RO2 = [(R max;T O2 · pO2 pkg)/(K T m;O2 + pO2 pkg)]. Respiratory parameters were modeled as an exponential function of temperature: RO2= {[1.34 · 10 · e · T) · pO2 pkg]/[1.15· 10 · e · T) + pO2 pkg]} (R 2 = 0.95), Q10 = 3.7, and EaR max;T O2 = 84 kJ mol. A good fit to the experimental data was also obtained considering K T m;O2 as constant: RO2 = {[4.36 · 10 L14 · e · T) · pO2 pkg]/[0.358 + pO2 pkg]} (R = 0.93), Q10 = 2.8, and EaR max;T O2 = 66 kJ mol. These results provide fundamental information to predict package permeability and steady-state pO2 pkg required to prevent anaerobic conditions and maximize shelf life of fresh-cut cantaloupe. The kinetics of respiration as a function of pO2 suggests that no significant reductions in respiration rate of fresh-cut cantaloupe can be achieved by lowering O2 levels. Melons are large fruit whose preparation requires slicing and disposal of the rind and seeds. Therefore, convenience of consumption is valued in this fruit and, not surprisingly, fresh-cut melons account for a major part of the growing fresh-cut fruit market (Offner, 2011). Fresh-cut processing invariably involves tissue wounding with the concomitant healing response. Wound response in plant tissues is mediated by ethylene and often involves increased respiration. Enhancement of respiration rate after cutting of cantaloupe mesocarp has been documented (McGlasson and Pratt, 1964), although the steady-state respiration rates of cut melon pieces can be similar to those of whole fruit under refrigeration (Aguayo et al., 2004; Watada et al., 1996). Respiration rates have been reported for fresh-cut melons (cantaloupe and other cultivar groups) at various temperatures (Aguayo et al., 2004; Gorny, 1998; Watada et al., 1996) and were predicted under non-equilibrium gas concentrations for an inodorus-type melon (‘Piel de Sapo’) stored at 4 C with initial oxygen partial pressures of 2.5, 21, and 70 kPa (Oms-Oliu et al., 2008). During the first 10 d of storage at 4 C, the respiration rates of cut ‘Piel de Sapo’ ranged from 0.13 mmol CO2/kg/h to 0.83 mmol CO2/kg/h (Oms-Oliu et al., 2008), values similar to those reported by Gorny (1998). For the same storage period at 5 C, Aguayo et al. (2004) measured respiration rates of 0.16 to 0.25 mmol CO2/kg/h for cantaloupe and inodorus melons, values 1.5 to two times higher than those obtained at 0 C. Modified atmosphere packaging (MAP) often complements refrigeration as an additional hurdle to help maintain the quality and food safety of fresh-cut fruit. The practical benefits of MAP are considered relevant for fresh-cut cantaloupe with favorable gas partial pressures ranging from 3 to 5 kPa O2 and 6 to 15 kPa CO2 (Gorny, 1998). Recommended gas compositions for MAP of fresh-cut produce in general and fresh-cut melon in particular have been established based on few published references (Gorny, 1998) and on experiments with a limited number of combinations of O2 and CO2 concentrations (Bai et al., 2001; Oms-Oliu et al., 2007). Optimal packaging geometry and film permeability to achieve the target gas levels can be deducted from the respiration rates (Jacxsens et al., 2000; Lakakul et al., 1999). However, despite the benefits observed in the few reports on MAP of fresh-cut melon (Bai et al., 2001, 2003; Oms-Oliu et al., 2007, 2008), an observation of the European and American markets for fresh-cut melon reveals that most operators do not aim at optimizing gas compositions inside the packages given that anaerobiosis is prevented. The discrepancy between the potential benefits of optimal MAP reported in the literature and the apparent lack of adoption of this knowledge by the industry may be the result of deficient knowledge transfer. Alternatively, the putative benefits of optimal MAP in fresh-cut melon fail to materialize in actual supply chains, and the efforts to achieve optimal MAP conditions have little or no economic benefit. Physiological limitations to the reduction of respiration rate by MAP must also be considered. The reduction in respiration by MAP depends on the kinetics of the respiration rate as a function of oxygen partial pressure at given temperatures. In whole fruit, significant differences in the kinetic parameters are found among species (Beaudry, 2000; Hertog et al., 1999). Postharvest treatment of apple with the ethylene action inhibitor 1-methylcyclopropene, inducing changes in the ripening stage, significantly alters the kinetic parameters of respiration as a function of O2 substrate (Beaudry, 2000). The kinetics of respiration vs. oxygen concentration in fresh-cut ‘Rocha’ pear suggests that no significant reduction in respiration rate can be achieved through MAP (Gomes et al., 2010); consistent with this physiological limitation, no significant improvement of metabolism-dependent quality attributes was observed in fresh-cut pear under various MAP conditions (Gomes et al., unpublished data). Therefore, the fundamental knowledge of respiratory parameters as affected by O2 partial pressure is essential to establish the Received for publication 9 Jan. 2012. Accepted for publication 11 Apr. 2012. This work was funded by Fundac xão para a Ciência e a Tecnologia, Portugal, and co-funded by POPH, European Social Fund, through project grant PTDC/ AGR-ALI/66144/2006 and the PhD grant SFRH/ BD/22628/2005 to M.H. Gomes. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 47(8) AUGUST 2012 1113 physiological limits of the tissue, to predict the benefits of low oxygen, and, eventually, to design packages aimed at targeted atmosphere conditions. The objective of this study was to determine the effect of steady-state oxygen concentration on the respiration kinetics of fresh-cut cantaloupe at various temperatures, to provide detailed information to predict the benefits of MAP, and to assist in the design of adequate packages for this convenient product. Materials and Methods 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 mL L 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 to 77.7 · 10 cm. Film permeabilities to O2 and CO2 (Table 1) were calculated with the predicting equations PO2 = 0.067 · e and PCO2 = 0.118 · e 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. Determination of respiration rate. The gas composition of the headspace in individual packages was monitored daily until steadystate was reached. Gas (100mL) 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 (RO2 ) and CO2 (RCO2 ) 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 (RQ = RCO2=RO2 ) for aerobic respiration were computed from the calculated respiration rates. RO2 = PO2 3 A l 3 pO2atm pO2pkg M [1] RCO2 = PCO2 3 A l 3 pCO2pkg pCO2atm M