Optimization of an equilibrium modified atmosphere packaging (EMAP) for minimally processed mandarin segments

Journal of Food Engineering 91 (2009) 474–481

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Optimization of an equilibrium modified atmosphere packaging (EMAP) for minimally processed mandarin segments Valeria Del-Valle b, Pilar Hernández-Muñoz a, Ramón Catalá a, Rafael Gavara a,* a b

Packaging Lab, Instituto de Agroquímica y Tecnología de Alimentos, CSIC, Apdo. Correos 73, 46100 Burjassot, Spain Sigdopack S.A, Guacolda 2151, Casilla 4, Quilicura, Santiago, Chile

a r t i c l e

i n f o

Article history: Received 27 February 2008 Received in revised form 3 September 2008 Accepted 26 September 2008 Available online 7 October 2008 Keywords: Mandarin segments MAP modelling EMAP Fermentative volatile compounds Sensory analysis

a b s t r a c t In view of the increasing demand for products with a fresh-like quality that are convenient to consume, the purpose of this research was to develop a ready-to-eat mandarin segments product. Mandarin segments were stored in controlled atmosphere environments with varying compositions at 3 °C. The results observed for different freshness indicators, especially the accumulation of acetaldehyde and ethanol and the sensory test scores, revealed that an atmosphere with low carbon dioxide concentration (3%) is suitable for this product. A mathematical model which considers the respiration rate and the gas mass transfer through plastics was used to make a pre-selection of suitable packages. Due to the high respiration rate of mandarin segments, the model showed the need of using microperforated plastic films to design the modified atmosphere package. The number of micropores was optimized by monitoring the accumulation of fermentative volatile compounds for three weeks at 3 °C. The results confirmed by a sensory test indicated that the optimum equilibrium modified atmosphere packaging for mandarin segments was 19.8/1.2% (O2/CO2%). Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The eating habits of people in the industrialized world have changed over the past decades. Nowadays, consumers are increasingly demanding products with a fresh-like quality that are free from preservatives and artificial colours and convenient to consume. The development of innovative products with these characteristics represents a challenge and a marketing opportunity for the food industry. In view of this, the development of minimally processed fruit could provide the processed fruit industry with an opportunity for diversification and promotion and offer consumers a product in accordance with their expectations, that is to say, ready-to-eat products with an extended shelf-life. As a result of their physiological and morphological characteristics, citrus fruits, including mandarins, are ideal for presentation as a ready-to-eat product. Mandarins are non-climacteric fruits which can be kept for long periods of time without apparently suffering biochemical changes. In addition, due to their anatomy, they can be peeled and segmented easily, practically without affecting their vesicular structure (Pretel et al., 1998; Restuccia et al., 2006). Many studies have been focused on postharvest treatments of whole citrus fruits (Kader, 2002), nonetheless, only a few researchers have addressed the development of minimally processed citrus products (Palma et al., 2003). * Corresponding author. Tel.: +34 963900022; fax: +34 963636301. E-mail address: [email protected] (R. Gavara). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.09.027

Minimal processing has been defined as the handling, preparation, packaging and distribution of agricultural commodities in a fresh-like state, and may include processes such as washing, peeling and slicing (O’Connors-Shaw et al., 1994). The difficulty in developing a new product based on fresh produce, in contrast to other types of foods, is the fact that fruits and vegetables continue their physiological activity, consuming oxygen and releasing carbon dioxide and water vapour into the package headspace. Also, with minimal treatment, microbiological and sensory quality factors such as appearance, texture and flavour are not stabilized and product deterioration may proceed rapidly (Jacxsens et al., 1999). Modified atmosphere packaging (MAP) has been often used to extend the shelf-life of food products. Among the variants of MAP applications, equilibrium modified atmosphere packaging (EMAP) is a technology that is specifically suitable for prolonging the shelf-life of fresh produce. An equilibrium atmosphere is established inside the package when film permeation rates for O2 and CO2 match the respiration rates of the packaged fresh produce (Kader et al., 1989; Jacxsens et al., 2001; Almenar et al. 2007). The concentration of gases at which the headspace atmosphere reaches equilibrium depends on the weight and respiration rate of the plant tissue and on the surface area and gas transmission rate of the packaging material. The equilibrium concentrations are crucial for product quality since exposure of fresh produce to high CO2 levels may cause physiological damage and exposure to too low O2 levels may induce anaerobic respiration and the development of

V. Del-Valle et al. / Journal of Food Engineering 91 (2009) 474–481

off-flavors (Zagory and Kader, 1988; Exama et al., 1993; Pretel et al, 1998; Pesis, 2005). In studies on the suitability of plastic films for EMAP of different products with medium–high respiration rates (such as mandarins), perforated films appear to have the potential to provide adequate fluxes of O2 and CO2 (Sanz et al., 1999; Bai et al., 2003). Previous studies have presented theoretical predictions for the exchange of gases and vapours through non-continuous films (Del-Valle et al., 2003). The objective of this study was to develop a ready-to-eat mandarin segment product packaged in an equilibrium modified atmosphere. To achieve this objective, the most suitable atmosphere composition for the storage of mandarin segments was determined by means of controlled atmosphere (CA) storage tests. A packaging system that would achieve an appropriate equilibrium modified atmosphere for this product was then developed. The evaluation of product quality was followed by measurement of the different physical and chemical parameters and sensory analysis.

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of nitrogen, oxygen and carbon dioxide. This procedure was used to determine the product respiration and the package headspace composition at equilibrium. The respiration rate of the mandarin segments was experimentally determined by the continuous gas flow method (Lee et al., 1996). A stainless-steel cell of about 1 dm3 was filled with produce and stored at 3 ± 1 °C, while a constant flow of air was maintained throughout the experiment by using precision needle valves and an Aalborg mass flowmeter GFM-1700 (Dakota Instruments, Monsey, NY, USA) (Lee et al., 1996). Before entering the cell, the stream was humidified in a washing bottle to provide ca. 97% RH. Air flowrate was varied between 2 and 25 ml/min to measure changes in the respiration rate with the oxygen concentration. The composition of the exiting stream was periodically analyzed by GC until steady-state. 2.4. Maturity index

Mandarins (cv. Clemenules Clementine) supplied by Cooperativa de Puçol (Valencia, Spain) were washed in a 2% sodium hypochlorite solution, dried, peeled and segmented.

The maturity index is the ratio of soluble solids (°Brix) to total organic acid compounds (represented by mg of citric acid/100 ml of mandarin juice). The soluble solid content was determined by a RX-100 digital refractometer (ATAGO, Japan) and the total organic acid compounds by the AOAC 942.15 method, based on titrating the juice with sodium hydroxide and using a phenolphthalein indicator.

2.1. Controlled atmosphere storage

2.5. Vitamin C

Controlled atmosphere storage was used to determine the suitable range of atmospheric compositions (Akbudak et al., 2007). The study of the best controlled atmosphere composition was performed with an assembly composed of three plastic containers of about 10 l capacity per treatment, each containing 1500 g of mandarin segments stored at 3 ± 1 °C, connected to a flow-through system with a constant flux of a specific gas mixture. The four different gas mixtures were: air, 18% O2/3% CO2 (18/3 mix), 15% O2/6% CO2 (15/6 mix) and 11% O2/10% CO2 (11/10 mix) (Abello-Linde, Paterna, Spain). The flow rate was maintained constant throughout the experiment using precision needle valves and Aalborg mass flowmeter GFM-1700 (Dakota Instruments, Monsey, NY, USA). Before entering the containers, the gas mixtures were humidified by bubbling through water to provide ca. 98% RH. Every week, a container was opened and the quality of the samples was analyzed.

The vitamin C content was measured by the polarographic method described by Aparicio et al. (1992). The segments were ground and filtered. Filtrate (5 ml) were diluted to 25 ml with an extraction solution (oxalic acid 1%, trichloroacetic acid 2% and sodium sulphate 1%). This solution was filtered and 0.5 ml were placed into the cell of a 747 VA stand polarograph (Metrohm) connected to the 746 VA trace analyzer (Metrohm AG, Herisau, Switzerland), adding 9.5 ml of oxalic acid 1% and 2 ml of buffer solution (acetic acid/sodium acetate 2 M, pH 4.8). The results were expressed as mg of ascorbic acid per 100 ml of juice.

2. Materials and methods

2.2. Modified atmosphere packaging For the EMAP test, 80 g of mandarin segments were packaged in 125 ml PP/EVOH/PP cups (Huhtamaki. Nules, Castellón, Spain), and sealed with 55-lm-thick microperforated films supplied by Amcor Flexible (Bristol, UK). Each pore was ca. 150 lm in diameter. The lids used to close the cups were obtained by precisely cutting circles from those films with 1 (TP1), 2 (TP2) or 3 (TP3) pores per package. Cups were closed in a Mecapack 500 heat-sealer (Elton S.A., Logroño, Spain) without initial atmosphere modification. EMAP mandarins were stored at 3 ± 1 °C. 2.3. Headspace composition The 100 ll gas samples were analyzed in a HP 5890 series II gas chromatograph (Agilent Technologies, Barcelona, Spain) equipped with a thermal conductivity detector (GC–TCD) and a Restek Chromosorb 2/20 1/8”column (Teknokroma, Barcelona, Spain). GC conditions were: oven temperature at 40 °C, injection port at 100 °C, detector at 140 °C and a 20 ml/min He flow as the carrier gas. A previous calibration was carried out by injecting known amount

2.6. Volatile compounds The concentrations of acetaldehyde, methanol and ethanol in the mandarin segments were monitored during the storage period. Mandarin segments (500 g) from the controlled atmosphere storage or modified atmosphere packaging were crushed with a blender and filtered to remove the pulp. Two 5-ml samples were transferred to 8-ml vials and capped with Teflon-lined septa. nPropanol (50 ll) was added as an internal standard and the samples were quickly frozen (35 °C) and stored. For the analysis, each vial was thawed and equilibrated for 35 min at room temperature (20 °C), then heated to 60 °C and maintained at this temperature for 35 min. Gas samples (500 ll) were extracted from the headspace and analyzed by gas chromatography using a HP 5890 Series II Plus equipped with a FID and an Ultra-2 30 m, 0.32 mm, 1 lm column (Restek Corp., Teknokroma, Barcelona, Spain). The GC conditions were as follow: 40 °C/2 min, heating ramp at 15 °C/min and 5 min at 200 °C. The injector and detector temperatures were 220 °C. Prior calibration was carried out by injecting known amounts of the compounds. 2.7. Sensory analysis For the sensory test, the mandarin segments were presented in random order to 30 panellists. Ranking tests were made to determine the consumer opinion of the product. The results were analyzed through Friedman statistical analysis.

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2.8. Statistical analysis The StatGraphics Plus program, version 2.1 (Statistical Graphics Corp., USA), was used for analysis of variance (ANOVA) and to test significant differences between mean at p 6 0.05.

3. Theoretical considerations

NO2 ðtÞ ¼ NO2 ð0Þ þ NO2 ðpermeatedÞ  N O2 ðconsumedÞ ð1Þ

where Ni(t) is the number of moles of substance ‘i’ at any time, is a function of the initial condition and the exchanges caused by permeation and by respiration. The composition of the headspace atmosphere varies with time until a steady-state is achieved, that is when the rate of gas permeation gas equals that of product respiration.

ð4Þ

where npores is the number of pores of diameter d in a package and DO2;Air is the diffusivity of oxygen in air as calculated through the Fuller, Schettler and Giddings relation

DO2 ;Air ¼ 3:2  104

A previous selection of materials applicable for the packaging of mandarin segments was made by mathematical prediction of the equilibrium atmosphere composition. The evolution of the packaging atmosphere during commercialization can be predicted by a mass balance which considers the gas exchange through the package walls and the respiration of the product

NCO2 ðtÞ ¼ NCO2 ð0Þ þ NCO2 ðpermeatedÞ þ NCO2 ðgeneratedÞ NN2 ðtÞ ¼ NN2 ð0Þ þ N N2 ðpermeatedÞ

2 dðNO2 Þ npores  pd DO2 ;Air  ðpO2 ;0  pO2 ;HS Þ ¼ RT ð4L þ 2dÞ dt

 1=2 M O2 þ MAir M O2  M Air

h

T 7=4

1=3 pT ðRmÞ1=3 O2 þ ðRmÞAir

ð5Þ

i2

in this equation, the constant has been recalculated in order to enter all values in the International Unit System. Rm is the diffusion volume which can be found in tables Perry and Green (1984) and pT is the total pressure. At 3 °C and atmospheric pressure, DO2;Air is: 1.73  105 m2/s, 1.43  105 m2/s and 1.78  105 m2/s. At equilibrium, the rate of oxygen permeation through the pores should be equal to that of oxygen consumption by fruits, and therefore, substituting Eq. (4) in Eq. (3)

dðNO2 Þ ¼ RRðO2 Þ; dt

2 npores  pd DO2 ;Air  ðpO2 ;0  pO2 ;HS Þ ¼ RRðO2 Þ: RT ð4L þ 2dÞ

ð6Þ This expression can be used to predict the number and size of the pores needed to obtain a particular equilibrium atmosphere or, as in this case, the equilibrium headspace composition can be predicted as a function of the number of pores of a given size.

3.1. Continuous films The gas exchange rate at the steady-state through a continuous membrane can be expressed through the definition of permeability (Lee et al., 1996; Techavises and Hikida, 2008)

dðN O2 Þ P  A  ðpO2 ;0  pO2 ;HS Þ ¼ L dt

ð2Þ

accordingly, the oxygen permeation rate ðdðN O2 Þ=dtÞ is a function of the material permeability (P), the package surface area (A), the film thickness (L) and the difference in partial pressure between the external and internal atmosphere at equilibrium. Similar expression can be written for CO2 and N2. Once the packaged fruit reach the equilibrium, the rate of gas permeated and the respiration rate of the gas should be equal. Therefore

dðN O2 Þ ¼ RRðO2 Þ; dt

P  A  ðpO2 ;0  pO2 ;HS Þ ¼ RRðO2 Þ: L

ð3Þ

A similar expression can be derived for carbon dioxide, in which the permeation rate of this gas out of the package equals the CO2 generation through fruit respiration. Substituting the diverse parameters of package and product in this expression, the mass exchange properties of the package can be predicted, and from the values of P and L, a previous selection of materials can be carried out. 3.2. Porous films Mass exchange through porous films cannot be explained by applying Henry’s and Fick’s laws, and therefore, Eq. (3) is not applicable. Instead, diffusion mechanisms in gas media are more appropriate. In some studies (Fonseca et al., 2000), an effective permeation coefficient was considered although it was found to be dependent on the pore size. In a previous paper (Del-Valle et al., 2003), the applicability of the diverse models of mass transport through porous films to the specific conditions of a package for EMAP was discussed. For the case of a package with a fixed volume, in which total pressure at the headspace is equal to the external pressure, the permeation rate at the steady-state can be expressed as

4. Results and discussion 4.1. Controlled atmosphere storage In order to determine the most suitable atmosphere composition for extending the segments’ shelf-life, the mandarin segments were stored in containers with different controlled atmosphere compositions. Every week, one container of each of the gas mixture treatments was opened and analyzed to determine the effect of the atmosphere compositions on product quality. The storage time results for the maturity index are given in Table 1. The initial value of 22 is in agreement with values reported at harvest for this mandarin variety (Carbonell et al., 2007). The results show similar behaviour in all the samples: a general increases during the first 14 days and a slight drop at the end of the storage period. With regards to the effect of the atmosphere composition, the maturity index decreased with less oxygen and more carbon dioxide, significantly so for the samples stored in the 11/10 mix. This trend agrees with the expected reduction in the respiration rate caused by the atmosphere composition. Although a relationship between MI and consumer acceptance has not been published, Pérez et al. (2005) reported that a MI increase improves palatability of mandarins. Regarding the vitamin C content (see Fig. 1), the general trend was a decrease at higher storage times but without significant difTable 1 Effect of different treatments on the maturity index (mean and standard deviation, x ± r) of mandarin segments stored for different times Time (days)

Air (x ± r)

0 7 14 21

22.0 ± 0.9 26.9 ± 0.8 30.5 ± 0.4 28.6 ± 0.6

15/6 mix (x ± r)

18/3 mix (x ± r) z y x a,x,y

22.0 ± 0.9 24.8 ± 0.3 25.3 ± 0.5 27.8 ± 0.5

z y b,x,y a,x

22.0 ± 0.9 26.3 ± 1.5 28.6 ± 1.3 27.4 ± 0.7

11/10 mix (x ± r) z y b,y a,y

22.0 ± 0.9 25.8 ± 0.7 26.9 ± 0.9 24.1 ± 0.8

z x,y b,x b,y,z

a and b indicate significant differences (p < 0.05) caused by atmosphere composition. x, y and z indicate significant differences (p < 0.05) caused by the storage period.

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Fig. 1. Evolution of vitamin C content during controlled atmosphere storage.

ferences among the controlled atmosphere compositions (p > 0.05). Nevertheless, the values at the end of the test followed the order of the atmosphere compositions. The higher oxygen concentration of the storage atmosphere and the higher vitamin C loss. Both the concentration of several volatile compounds and sensory analysis were used to monitor product freshness. The occurrence and perception of off-flavors in citrus fruit is closely associated with the accumulation of acetaldehyde and ethanol. Ethanol fermentation is a two-step process in which pyruvate is first decarboxylated to acetaldehyde by pyruvate decarboxylase (PDC) and then acetaldehyde is converted to ethanol by alcohol dehydrogenase (ADH). Acetaldehyde and ethanol normally accumulate at low levels during fruit maturation and ripening and are used in the synthesis of citric aromas. However, their accumulation results in off-flavors which often deteriorate the sensorial value of the product. Fig. 2 shows the ethanol evolution during storage time for the four gas mixtures tested. As can be seen, the ethanol concentration increased during the first day of storage in all the samples, showing values ranging from 150 ll/l for the 11– 10 samples to 60 ll/l for the other gas mixtures. After the first week, the concentration remained constant or decreased slightly for atmospheres with the lower CO2 concentrations. The initial increase could be the result of the stress suffered by the fruit during the peeling and segment separation processes. For the highest CO2 content, however, the concentration was significantly different to

Fig. 2. Evolution of ethanol concentration in mandarin segments during controlled atmosphere storage.

477

the other samples, as it increased throughout the three weeks of storage. The acetaldehyde content presented a very similar evolution to that observed for ethanol although absolute concentration values were 20–40 times lower, in agreement with most reports on the development of off-flavors in citrus (Shi et al., 2007; Luengwilai et al., 2007). As Fig. 3 shows, acetaldehyde concentration increased during the first week of storage and then decreased to values close to the initial concentrations with the exception of the product stored in the highest CO2 gas mixture, where the concentration remained high for the three weeks of storage. All these results are in accordance with those of other authors who attribute this observation to the fermentative processes induced by high CO2 or low O2 atmospheres (Baldwin et al., 1995). No relevant effect on the ethanol/acetaldehyde ratio was observed, being the increase of both compounds rather proportional. Similar results have been reported by Shi et al. (2007) when they found that the concentration of pyruvate and the activities of both PDC and ADH enzymes increased as a consequence of a reduction on the concentration of oxygen in the surrounding atmosphere. They conclude that the production of both compounds increases proportionally and their concentrations increased as a result of a reduction in the permeation capacity of the cuticle. Methanol evolution was also monitored. Fig. 4 shows that the concentration increased linearly with the storage time, reaching concentrations of 100 ll/l at the end of the storage period, irrespective of the gas mixture. Some authors have attributed this increase to a pectin demethylation catalyzed by pectin methylesterase (Lund et al., 1981; Nisperos-Carriedo and Shaw, 1990). Thus, juice cell breakage due to the peeling and segmenting processes allows contact between enzymes and substrate, inducing the generation of methanol. Although the increase of methanol during storage appears to be independent of the headspace composition, as mentioned above, the concentration of methanol in the segments stored in air increased to higher levels in the last week. This difference in behaviour could be attributed to the growth of microorganisms. The inhibiting effect of CO2 on microorganism growth is widely known. The lack of CO2 in the sample stored in air could allow moulds to grow. These microorganisms produce enzymes with the purpose of breaking down the cell wall. Such cell wall hydrolysis could cause further methanol generation in addition to the previouslymentioned mechanism. Besides the parameters already described, texture and colour were also monitored during storage (data not shown). However,

Fig. 3. Evolution of acetaldehyde concentration in mandarin segments during controlled atmosphere storage.

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4.2. Modified atmosphere packaging

Fig. 4. Evolution of methanol concentration in mandarin segments during controlled atmosphere storage.

they did not show any effect of storage time or atmosphere composition and were therefore discarded as freshness indicators (DelValle, 2003). In order to corroborate the results obtained by the analysis of the volatile compounds and their influence on product quality, a sensory analysis was carried out. Ranking tests were carried out to determine the consumers’ acceptance and preferences among the samples. As can be observed in Fig. 5, the sample with the worst score was that stored in the 11/10 mixture, the one that showed the highest ethanol concentration. This result indicates that in the mandarin segments stored in the 11/10 atmosphere, the high concentrations of the volatile compounds tested may lead to consumer rejection of the product. It was also observed that samples stored under 18/3 mixture obtained the best scores during the two weeks of storage, the segments with the lowest concentrations of ethanol, methanol and acetaldehyde. These results confirm that ethanol concentration in the product is a very good quality indicator, as Davis (1971) proposed, because it is an important detector of storage abuse. In addition, these results indicate that the maximum headspace concentration of CO2 for mandarin segments is 3%, in agreement with the low CO2 tolerance reported for citrus fruits (Arpaia and Kader, 2002). Nevertheless, it is important to remember that a minimum CO2 concentration is required in order to inhibit microbial growth.

Once the most suitable atmosphere composition had been determined, the design of a modified atmosphere packaging system was attempted. For this purpose, the mass of mandarin segments (wp = 80 g) and the cup (described in the experimental Section 2) were fixed, and the lid was the variable to be optimized. In order to avoid time-consuming trial and error tests with diverse film lids, a previous selection was made by mathematical simulation. As mentioned in the Theoretical considerations (Section 3), any attempt to predict the equilibrium atmosphere inside a package containing fresh produce requires the previous measurement of the respiration rate. In this study, the respiration rate was determined by the continuous flow method. As an example, Fig. 6A shows the evolution of the CO2 concentration inside a cell containing 712 g of mandarin segments and with an air stream of 11.3 ml/ min. From the value at steady-state, the respiration rate was determined. By changing the air flowrate, the respiration rate values were determined at several steady-state oxygen concentrations. Fig. 6B shows the values obtained. As can be seen, the respiration rate of mandarin segments decreased as the oxygen partial pressure diminished. Within the range of oxygen concentrations tested, the respiration rate (RR(O2)) can be described by a linear function of the oxygen partial pressure (Garfinkel and Fegley, 1994; Fonseca et al., 2002)

RRðO2 Þ ¼ a  wp  pO2 ;HS

at 3 °C, the value of the proportionality constant was found to be 12.9 ± 0.8 lmol/(min  kg) (r2 = 0.978). First of all, the potential use of a continuous flexible film was considered. By substitution of Eq. (7) into Eq. (3) and rearranging the terms, the permeance of a suitable film can be predicted as

a  wp :pO2 ;HS P ¼ L A  ðpO2 ;0  pO2 ;HS Þ

ð8Þ

by substituting the diverse parameters of package and product in Eq. (8), and considering that 0.18 is the minimum desired oxygen partial pressure at equilibrium, the permeance of the film should be higher than 0.13 lmol/(cm2  min  atm). Unfortunately, this high permeance cannot be achieved with any polymeric material. For instance, using low density polyethylene (LDPE), one of the polymers with the highest permeation to oxygen, the thickness of the films should be below 4 lm, value very difficult to achieve and which would not provide other necessary properties such as mechanical strength. Applying the same model to CO2, considering that 0.03 is the maximum CO2 partial pressure at equilibrium, the same permeance value is obtained, which in this case would drive to a 12lm film of LDPE. Therefore, the use of porous materials should be considered. As it is described in the Theoretical considerations (Section 3), we have made use of the gas diffusivity through a pore of known dimensions (PET/PP film 55 lm thick and with perforations of 150 lm) to determine the headspace atmosphere composition at equilibrium. The headspace volume and the total pressure were considered constant during the storage. In these conditions, Eq. (6) can be used to determine the oxygen concentration at equilibrium as a function of the number of pores. Introducing Eq. (7) into Eq. (6), and rearranging, the partial pressure at equilibrium can be predicted as a function of the number of pores npores pd2 DO

pO2 ;HS ¼

Fig. 5. Sensory analysis. Ranking test results for mandarin segments stored in a controlled atmosphere.

ð7Þ

2 ;Air

pO

2 ;0

ð4Lþ2dÞRT

a  wp þ

npores pd2 DO

:

ð9Þ

2 ;Air

ð4Lþ2dÞRT

A similar equation can be used to predict the concentration of CO2. The theoretical results obtained with this model with lids contain-

V. Del-Valle et al. / Journal of Food Engineering 91 (2009) 474–481

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Fig. 6. (A) Evolution of the CO2 concentration during a respiration rate test for a sample of 712 g of mandarin segments with an air flowrate of 11.3 ml/min. (B) Respiration rate of mandarin segments at 3 °C as a function of the oxygen partial pressure and representation of the best fit of Eq. (7).

ing 1, 2 or 3 pores, were 18.0/3.0, 19.3/1.7 and 19.8/1.2 O2/CO2%), respectively, values within the desired range of concentrations as determined in the controlled atmosphere storage test. Mandarin segments (80 g) were packaged in cups sealed with films containing 1 (TP1), 2 (TP2) or 3 (TP3) pores per package. The experimental O2/CO2 concentrations inside the package was measured daily by GC. According to the results, the equilibrium atmosphere composition was reached during the first day and the concentrations at equilibrium were 18.0 ± 0.5/3.0 ± 0.5, 19.8 ± 0.3/1.2 ± 0.4 and 20.5 ± 0.2/0.5 ± 0.3 O2/CO2%, respectively, very close to those theoretically calculated. An independent experiment was carried out to determine experimentally the evolution of the atmosphere during the first hour after packaging with the TP1 film. The results are collected in Fig. 7. As can be seen, the atmosphere composition reached equilibrium during the first 6 h. The finite difference method was used to predict the evolution of the atmospheric composition (Almenar et al., 2007). According to this method, differentials can be substituted by finite increments and Eq. (1) can be transformed into

Fig. 7. Experimental values of the CO2 (d) and O2 (4) concentrations in the headspace atmosphere for mandarin segments packaged with a TP1 lid and theoretical evolutions predicted by Eq. (10) (lines).

NO2 ;tþDt ¼ NO2 ;t þ

2 npores  pd DO2 ;Air  ðpO2 ;0  pO2 ;HS;t Þ RT ð4L þ 2dÞ

ð10Þ

Dt  apO2 ;HS;t  wp  Dt where the number of moles of O2 at a time t + Dt is calculated from the previous value at time t considering the oxygen which enters into the package by permeation and the oxygen consumed by the respiration of the mandarin segments (mass, wp) during a Dt step. Similar expressions can be obtained for nitrogen and carbon dioxide. Every time step the partial pressure of each gas in recalculated by a mass balance. Fig. 7 also shows the theoretical results. As can be observed, the predicted evolution advances towards equilibrium slightly faster than that experimentally measured, although the model describes acceptably the evolution of oxygen and carbon dioxide within the package headspace. Figs. 8–10 show the evolution of selected quality parameters measured for the mandarin segments stored in the three MAP systems. The evolution of the ethanol concentration is presented in Fig. 8. As can be seen, the evolution of this parameter is similar to that observed for the 18/3 samples of the controlled atmosphere experiment. The results showed that the concentration increased

Fig. 8. Ethanol concentration in mandarin segments stored in packages with different numbers of pores.

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Fig. 9. Methanol evolution for mandarin segments stored in packages with different numbers of pores.

this metabolite in the headspace, increasing the concentration gradient along the segment cuticle and therefore, the ethanol release. The evolution of the methanol concentration in mandarin segments packed in porous materials is presented in Fig. 9. It can be seen that its behaviour is the same as in controlled atmosphere storage: an increase in all the samples regardless of the atmosphere composition. As mentioned in the Controlled atmosphere storage (Section 4.1), the increment of methanol concentration should be related to pectin demethylation. Although no significant differences were observed between the samples, the plots show that the higher the permeability of the package the higher the methanol concentration, in agreement with the results previously obtained in the controlled atmosphere storage test. Finally, packaged samples were also subjected to a sensory test. Mandarin segments from the diverse EMAP samples were presented in random order to 30 panellists. They were asked to indicate their preference between the diverse mandarin samples. Fig. 10 shows the panellists’ opinion of the samples. The results indicate that the segments packaged with two pores per lid (TP2) obtained the highest scores. The equilibrium atmosphere composition for this sample corresponded to 19.8/1.2% O2/CO2. Nevertheless, the general comment of the panellists was that all samples presented a good visual aspect and a fresh-like flavour. 5. Conclusions

Fig. 10. Sensory analysis. Ranking test results for mandarin segments packaged with porous films.

during the first week and subsequently remained constant or decreased slightly. However, it was observed that the ethanol concentrations of all the samples were higher than the values observed in the controlled atmosphere test. This variation can be explained by the difference between the experimental conditions of the tests. In controlled atmosphere storage, the atmosphere is constantly renovated by a gas flow, reducing the concentration of ethanol and other volatile substances in the atmosphere surrounding the mandarin segments to values near zero. As permeation is a mass transport process being the driving force the difference in concentration of the permeant along the separating membrane, the release from the segments of ethanol and other volatiles largely increases. On the contrary, the headspace atmosphere in MAP is static and, with the exception of mass transport through the package, the volatile compounds released from the product accumulate in the package headspace during storage, reducing the concentration gradient and therefore the exchange of ethanol through the segment skin. At all events, due to the low CO2 concentrations (<3%) reached in the three porous packages no differences were obtained among the samples, although the ethanol concentration appears to decrease as the permeability of the package increases. Again, an increase in the number of pores results in an increase of the rate at which ethanol leaves the package, reducing the concentration of

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