Impact of temperature and relative humidity on the transpiration rate of pomegranate arils Oluwafemi J. Caleb1*, Pramod V. Mahajan2, Umezuruike Linus Opara1 1
Postharvest Technology Research Laboratory, Faculty of AgricSciences, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa 2 Department of Process and Chemical Engineering, University College Cork, Ireland. *Corresponding author. E-mail:
[email protected]
Abstract A study was conducted to determine the transpiration rate of pomegranate (Punica granatum L.) arils under various combinations of temperature (5, 10 and 15 °C) and RH (76, 86 and 96%) during storage. To evaluate transpiration rate, a weight loss approach was adopted. Transpiration rate ranged from 1.14 g/kg.day to 16.75 g/kg.day across the various combinations of RH and temperature studied. RH had the most significant impact on transpiration rate and the interaction between RH and temperature also had a significant impact on transpiration rate (p < 0.05). After 8 days of cold storage, losses in quality attributes of arils such as colour, firmness and decay incidence were higher with increasing storage temperature and RH. A mathematical model to predict transpiration rate as a function of temperature and relative humidity was developed based on non-linear regression. The model was validated for arils stored at 8 °C and 76, 86 and 96% RH and, a good agreement was found between experimental and predicted data. Keywords: Pomegranate arils; transpiration rate; transpiration model; temperature; relative humidity; storage 1. Introduction Transpiration is one of the critical physiological processes in fresh produce such as fruit and vegetables. Once the produce is detached from the growing plant, it solely depends on its own water content for transpiration (Mahajan et al., 2008). The loss of water from fresh produce results in weight loss and shrivelling, leading to economic loss during retail marketing. Therefore, appropriate packaging and optimal environmental conditions are applied to extend the storage life of both fresh and fresh-cut produce (Opara and Al-Ani, 2010). As fresh-cut produce respire, they produce large amount of water vapour and without appropriate packaging, water vapour could build-up within the package, facilitating the growth of microorganisms (Mahajan et al., 2008). Transpiration rate of produce during postharvest handling and storage is influenced by intrinsic factors such as surface-to-volume ratio, surface injuries, morphological and anatomical characteristics, as well as maturity stage (Sastry and Buffingtin, 1982). As well as, external or environmental factors such as temperature, relative humidity (RH), air movement, and atmospheric pressure (Mahajan et al., 2008). Existing models of water loss or transpiration for fresh produce have been limited to cooling process and bulk storage applications (Sastry and Buffingtin, 1982), and these models may not be suitable for MAP systems (Song et al., 2002). Most models describe moisture loss as a function of the bioand thermo-physical properties such as skin thickness, surface cellular structure and pore fraction in the skin, thermal diffusivity and geometry of produce (Song et al., 2002). Studies on fresh ready-to-eat pomegranate arils using MAP technology and semipermeable or perforated films for their storage have been reported (Porat et al., 2009). However, information on the effects of storage conditions on the transpiration rate of pomegranate arils is lacking. Hence, predicting the rate of water loss is essential for estimating the shelf-life of fresh pomegranate arils and designing optimal storage and packaging conditions. The objective of our study was to quantify the water loss of fresh
pomegranate arils and develop mathematical model to relate transpiration rate to temperature and relative humidity. 2.
Material and Methods
2.1. Plant material and sample processing Commercially ripened pomegranate fruits (cv. Acco) were procured from Robertson valley farm, Western Cape (33°48′0″S, 19°53′0″E), South Africa and air freighted to the Process and Chemical Engineering Laboratory, University College Cork, Ireland. On arrival, the fruit were immediately stored at 5 °C until the next day when fruit samples were processed in a clean cold room at 5 °C, by carefully removing the husks to avoid damaging the arils. Free surface moisture on the arils were gently removed using sterile paper towels after which, the arils were weighed and equilibrated at 5, 10 and 15 °C for 1 h prior to experiment. 2.2. Experimental setup The experimental setup consisted of three test containers placed within refrigerating incubators with temperature maintained with ± 0.5 ° C of the set temperature of 5, 10 and 15 °C. Humidity within the test containers were independently controlled by using saturated salt solutions of sodium chloride (NaCl), potassium chloride (KCl), potassium nitrate (KNO 3) giving 76, 86 and 96 % RH respectively (Patel et al., 1988). The salt solutions were poured into the test containers and supports were mounted above the solution level with large aluminium pans to hold the petri-dishes containing the samples. This setup was found to maintain a constant RH throughout the experimental run, as the temperature and RH within the test containers were monitored continually using battery-powered sensor (HMP50, Campbell Scientific Inc., Utah). To evaluate the transpiration rate, a weight loss approach was adopted (Leonardi et al., 1999). A sample of approximately 10 g of arils was placed in a petri-dish of known weight and, aril weight loss was measured daily using a chemical balance (Bosch SAE200, GmbH). Weight loss due to respiration was considered negligible in comparison to that due to transpiration (Shirazi and Cameron, 1993). Transpiration rate (TR) was calculated from the changes in weight over time and expressed as change in weight (g) per kilogram (kg) per day as shown in the following equation: (1) where, TR is the transpiration rate in g/kg day, Mi is the initial mass of arils in g and M is the mass of arils in g at weighed time t in days. Two replicates for each storage condition were used for all analyses and the duration as well as measurement interval was the same for all temperature and RH conditions. Experiments were performed according to a full factorial design with two factors temperature and RH at three levels of 5, 10 and 15 °C, and 76, 86 and 96 % RH respectively. The total number of runs was 18. An additional set of experiments with all the combinations of 76, 86 and 96 RH was performed at 8 °C in order to validate the mathematical model. 2.3. Model building The flow of water vapour through a fruit is proportional to the difference in water activity (aw) (RH/100) between the surface of a commodity and the surrounding air. This can be related to Fick’s law of diffusion (Ben-Yehoshua, 1987). In this model the relative humidity of fruit internal atmosphere was considered as a first approximation to be 1.0 (or 100% RH). This parameter depends on solute content of the fruit and is slightly less than 1.0. The term (aw - awi) is the difference in concentration of the water vapour across the pomegranate aril membrane for the direction of flow from awi to aw. (2)
where, aw is the water activity of the container; awi is the water activity of the arils; Ki is the mass transfer coefficient. At the end of the storage period, the water activity of the arils was measured experimentally (mean awi = 0.984 ± 0.01) and did not differ among the different storage temperature and humidity levels. This showed that aw of the arils was constant throughout the study period and gave a constant humidity gradient across the arils resulting in uniform mass loss. Equation (1) was combined with Equation (2) yielding Equation (3) where TR is transpiration rate (3) Equation (3) was then separated for M which is the mass loss of arils with time as shown in Equation (4). )
(4)
The mass transfer coefficient Ki for each set of experimental conditions was estimated by fitting equation (4) to the experimental data by non-linear regression using Statistica software (Statistica 10.0, Statsoft, USA). Furthermore, temperature term was incorporate in order to estimate the overall effect of temperature on Ki, hence, equation (3) was modified as follows: (5) Equation (5) was then separated for M in order to fit all the experimental data for weight loss of pomegranate aril (M) with time (t) as shown in Equation (6) (6) Experimental data obtained at all combinations of temperature and humidity studied were used to estimate the values of the constants Ki and a. The model equation (6) was fitted by non-linear regression using Statistica software (Statistica 10.0, Statsoft, USA). 2.4. Quality evaluation Visual quality evaluation of each experimental petri-dish on the 8th day of storage was carried out. Colour, firmness and decay of arils were determined subjectively using a 1-5 visual rating scale by adapting visual quality descriptors reported by Nunes et al. (2011). Firmness was determine based on the resistance of the arils to slightly applied finger pressure and recorded using a 1-5 tactile rating. 2.5. Statistical analyses Pareto analysis was carried out at 95 % confidence interval using using Statistica software (Statistica 10.0, Statsoft, USA) to assess the effects of RH and temperature, and the interaction between RH and temperature on the transpiration rate of pomegranate arils. 3. Result and Discussion 3.1 Transpiration rate of pomegranate arils Weight loss of pomegranate arils during storage at 10°C and 86% RH across the experimental combinations, which constantly decreased with time at all combinations of temperature and RH studied. Weight loss was higher at 15°C and 76% RH in comparison to arils stored at 5°C and 96% RH. TR for pomegranate arils as calculated from equation (1) ranged from 1.14 to 16.75 g/kg.day across all the combinations of temperature and RH tested. The TR values obtained in this study were lower than those reported by Mahajan et al. (2008) for mushroom (TR: 6.5 to 92 g/kg.day) at 5, 10 and 15°C with RH 76, 86 and 96%. The high values of TR for mushroom is associated to its lack of a protective skin, therefore, the rate of moisture loss is higher. But, due to a higher surface/mass ratio for smaller fruit this influences transpiration rate compared with large fruit, this explains the higher TR in arils compared to apple. TR was found to be higher at 76% RH and 15°C compared to 96% RH and 5°C as shown in Figure 1. 3.2. Quality of pomegranate arils
Texture in terms of firmness and pigment stability (colour) are very important attributes associated with high quality ready-to-eat pomegranate arils and, are directly related to consumer acceptance and commercial value (Artés & Tomás-Barberán, 2000). After 8 day storage, firmness and colour of arils decreased with increase in storage temperature. The least firm, with the highest percentage of decay being arils stored at 15 °C and 96% RH (Figure 2). The effect of increase temperature was more pronounced on visual quality, and this deleterious effect was enhanced at high RH (96%). However, the arils were best kept at storage temperature of 5 °C and optimally with 96% RH. Arils stored at 5 °C: 96% RH had the overall best keeping quality on the 8th day in comparison to those at 76 and 86% RH. An optimal storage RH is essential for successful extension of shelf-life of MAP-packed pomegranate arils to minimise losses during processing. 3.3. Model development and validation The coefficient constant Ki as determined by fitting equation (4) for each set of experimental conditions. Ki was found to increase with temperature as shown in Figure 6, with R2 values > 99.5%. The increase in temperature created a higher shear stress or turbulence along the membrane surface and, this consequently, results in the observed correlation between our experimental temperatures and the coefficient constant Ki. This is phenomenon is similar to that reported by Mahajan et al. (2008) for mushrooms. Fitting equation (6) with the experimental data at all combinations, the model described the change in mass adequately as shown with a R2 value of 94.3%, Ki and a with standard error value of 89.96 (± 6.87) and 0.09 (± 0.01), respectively. Figure 3 shows a good agreement between the observed and predicted mass of pomegranate arils. Both the experimental data and the model prediction showed a decrease in mass with decrease in RH from 96 to 76% as well as with the decrease in temperature from 15 to 5°C. In order to validate the mathematical model, its predictions of transpiration rate at 8 °C with 76, 86 and 96 % RH were compared with experimental data. A good agreement was observed between experimental and predicted TR, at the set temperature of 8 °C. The experimental TR at RH of 76, 86 and 96% were 9.93 (± 0.83), 5.50 (± 0.11) and 1.5 (± 0.59) g/kg.day respectively, while the model predicted TR were 10.5, 5.8 and 1.1 g/kg.day at the respective RH. 4. Conclusion Weight loss was highest at experimental combinations of 15°C with 76% RH. Additionally, RH was the variable with the greatest influence on TR, and arils were best kept at 5°C and 96% RH. This highlights the significance of maintaining an optimal produce storage condition. The applicability of the transpiration model developed was verified based on adequate prediction of TR of pomegranate arils during storage at different combinations of temperature and RH. The model would be useful towards understanding the rate of water loss as affected by temperature and humidity over time, and thus provides a valuable guide for the storage and designing MAP-system for pomegranate arils. Experimental and model prediction results showed that both RH and temperature had significant effects on transpiration rate and quality of stored arils, highlighting the need to maintain optimal storage condition to assure high quality ready-to-eat pomegranate arils with maximum shelf-life. 5. Acknowledgement This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation. The authors are grateful to Citrogold Ltd and Perishable Products Export Control Board (PPECB) for financial support. 6. References Artés, F., & Tomás-Barberán, F. A. (2000). Postharvest technological treatments of pomegranate and preparation of derived products. In Melgarejo-Moreno et al (Eds.), Production, processing and marketing of pomegranate in the Mediterranean Region, CIHEAM-Options Mediterraneennes: Série A. Séminaires Méditerranéens (no. 42) (pp. 199204).
Ben-Yehoshua S. (1987). Transpiration, water stress, and gas exchange. In Weichmann J (Ed.) Postharvest physiology of vegetables (113-170). New York: Marcel Dekker Inc. Leonardi C, Baille A, & Guichard S. (1999). Effects of fruit characteristics and climatic conditions on tomato transpiration in greenhouse. Journal of Horticultural Science and Biotechnology, 74(6), 748-756. Mahajan, P. V., Oliveira, F. A. R., & Macedo I. (2008). Effect of temperature and humidity on the transpiration rate of the whole mushrooms. Journal of Food Engineering, 84, 281-288. Nunes, M. C. N., Emond, J-P., Dea, S., & Yagiz Y. (2011). Distribution centre and retail conditions affect the sensory and compositional quality of bulk and packaged slicing cucumbers. Postharvest Biology and Technology, 59, 280-288. Opara, U. L. & Al-Ani, M. R. (2010). Antioxidant components in fresh-cut and whole fruit and vegetables. British Food Journal, 112(8), 797-810. Patel, P. N., Pai, T. K., & Sastry, S. K. (1988). Effects of temperature, relative humidity and storage time in the transpiration coefficients of selected perishables. DA-88-20-3 (PR-442); 1563-1587. Sastry, S. K., & Buffington, D. E. (1982). Transpiration rates of stored perishable commodities: A mathematical model and experiments on tomatoes. Transactions of the ASHRAE, 88, 159-184. Song, Y., Vorsa, N., & Yam K. L. (2002). Modelling respiration-transpiration in modified atmosphere packaging system containing blueberry. Journal of Food Engineering, 53, 103109. Shirazi, S.K., & Cameron, A.C. (1993). Measuring transpiration of tomato and other detached fruit. HortScience, 28(10), 1035-1038.
FIGURE 1. Fitted square surface showing the effect of temperature and RH on transpiration
rate (g/kg day).
5°C 76% 5
Colour
Firmness 15°C 96%
5°C 86%
4
Decay 3 2 1
15°C 86%
5°C 96%
0
15°C 76%
10°C 76%
10°C 96%
10°C 86%
FIGURE 2. A cluster graph of quality attributes (firmness, colour and decay incidence) of pomegranate arils after 8 days under different storage conditions.
10.8
Experimental values of M, g
10.6 10.4 10.2 10
9.8 9.6 9.4 9.2 9 9
9.2
9.4
9.6
9.8
10
10.2
10.4
10.6
10.8
Predicted values of M, g
FIGURE 3. Relationship between experimental and predicted values of pomegranate aril mass.
