Effect of MAP with argon and nitrous oxide on quality maintenance of minimally processed kiwifruit

Postharvest Biology and Technology 35 (2005) 319–328

Effect of MAP with argon and nitrous oxide on quality maintenance of minimally processed kiwifruit Pietro Rocculi∗ , Santina Romani, Marco Dalla Rosa Universit`a degli Studi di Bologna, Facolt`a di Agraria, Corso di Laurea in Scienze e Tecnologie Alimentari, Via Ravennate 1020, 47023 Cesena, Italy Received 19 May 2004; accepted 4 September 2004

Abstract The shelf-life of minimally processed (MP) kiwifruit is principally limited by softening and colour degradation, caused by increased enzymatic activities as a consequence of wounding. Modified atmosphere packaging (MAP) with non-conventional gas mixtures was tested on the maintenance of some physico-chemical characteristics of MP kiwifruit slices, during refrigerated storage. Kiwifruit slices were sealed in polypropylene boxes that were stored in air (control) and in three different modified atmospheres: N2 (90%), O2 (5%), CO2 (5%); Ar (90%), O2 (5%), CO2 (5%) and N2 O (90%), O2 (5%), CO2 (5%). The packed kiwifruit samples were stored at 4 ◦ C for 12 days and the following quality parameters were monitored during storage: soluble solids content, weight loss, carbon dioxide and oxygen levels in the package headspace, texture changes and surface colour by a reflectance colorimeter (lightness, hue angle and chroma) and by image analysis (percentage of browning area). MA with 90% of N2 O was the best mixture of tested gases in order to maintain the quality of kiwifruit slices. The initial firmness value of kiwifruit slices (about 13 N) decreased only by 10% after 8 days in the sample packed in N2 O, while about 70% firmness loss was detected in the control sample after just 4 days of refrigerated storage. Kiwifruit slices in N2 O also maintained a better initial colour, in particular in terms of L* and hue. Moreover, the use of image analysis showed less browning in both the pericarp and core surfaces of the samples in N2 O, compared to the control. Correlation analysis between texture and all colour results showed that the application of an image analysis technique allowed a good recognition of chromatic changes related to fruit softening. Score plots of principal component analysis (PCA) showed slight modifications in the most important discriminated quality factors for the sample in N2 O, a rapid quality loss for samples in air and in N2 and an acceptable quality maintenance until the 8th day of storage for the sample in Ar. © 2004 Elsevier B.V. All rights reserved. Keywords: Kiwifruit; Minimally processing; MAP; Nitrous oxide; Argon; Image analysis

1. Introduction ∗ Corresponding author. Tel.: +39 0547 636120; fax: +39 0547 382348. E-mail address: [email protected] (P. Rocculi).

0925-5214/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2004.09.003

Minimally processed (MP) fruit are products that maintain their attributes and quality similar to those

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of fresh products (Alzamora et al., 2000). However, the loss of cellular compartmentation, due to peeling and cutting, causes mixing of previously sequestered metabolites of the ethylene generating system stimulating ethylene production (Mazliak, 1983; Watada et al., 1990). Enzymatic activities, softening and ripening of kiwifruit are promoted by ethylene (Arpaia et al., 1985; Wegrzyn and MacRae, 1992). Therefore, it is difficult to maintain the quality of kiwifruit slices once they have been cut (Chien and Buta, 2003). Abe and Watada (1991) found that the use of an ethylene absorbent prevented the accumulation of ethylene and was effective in reducing the rate of softening in packed kiwifruit slices. A sensory evaluation of kiwifruit cubes stored at 4 ◦ C in closed circular polypropylene containers showed a decrease in texture properties after 2 days, and the appearance of a bitter flavour after just 4 days (O’Connor-Shaw et al., 1994). An extension of MP kiwifruit shelf-life from 3 days (control sample) to 10 days using a combination of CaCl2 dips, and storage at 1 ◦ C under an atmosphere of 2% O2 and 5% CO2 with an ethylene scrubber, was obtained by Massantini and Kader (1995). Agar et al. (1999) showed that fresh-cut kiwifruit had a shelf-life of 9–12 days if treated with 1% of CaCl2 or 2% of calcium lactate, and stored at 0–2 ◦ C in an ethylene-free atmosphere of 2–4 kPa of O2 and/or 5–10 kPa of CO2 . The introduction of volatile compounds, in particular methyl jasmonate, inside polystyrene trays was effective in good quality maintenance of kiwifruit slices, up to 3 weeks of storage at 10 ◦ C, compared to the control sample, but did not cause any differences in terms of respiration rate (Chien and Buta, 2003). Most studies have been based on the effects of different CO2 and O2 levels on fruit metabolism to extend the shelf-life of whole and MP vegetables, packed in modified atmospheres (Lee et al., 1991; Mathooko, 1996; Watada et al., 1996; Gil et al., 1998; Beaudry, 2000). Recently, there has been a great interest in the potential benefits of argon (Ar) and other noble gases in MAP applications (Spencer, 1995; Mostardini and Piergiovanni, 2002). In some studies, argon is reported to be biochemically active, probably due to its enhanced solubility in water compared to nitrogen and it seems to interfere with enzymatic oxygen receptor sites (Spencer,

1995). However, inconsistent results have been presented on the effect of Ar on inhibition and control of the growth of certain micro-organisms, on the activity of quality-related enzymes and on degradative chemical reactions in selected perishable food products, such as MP fruit (Powrie et al., 1990; Spencer, 1995; Day, 1996, 1998; Kader and Watkins, 2000; Jamie and Saltveit, 2001; Mostardini and Piergiovanni, 2002). Another “new” packaging gas, nitrous oxide (N2 O), has been allowed for food use in the EU; however, little is known about its effects on MAP fruit. It seems that N2 O binds lipids and also proteins, such as cytochrome c oxidase (Gouble et al., 1995; Day, 1996). Sowa and Towill (1991) reported the action of N2 O on partial and reversible inhibition of respiration and cytochrome c activity in mitochondria isolated from seeds, leaves or cellular suspensions. Moreover, Sowa et al. (1993) found that an exposure to 80% N2 O atmosphere reduced seedling respiration and root length of germinating Phaseoulus vulgaris L. seeds. Leshem and Wills (1998) showed that N2 O inhibits ethylene action and synthesis in higher plants. Furthermore, continuous N2 O gas treatment had a significant effect on inhibiting ripening by extending the lag phase which precedes the ethylene rise, delaying colour change in pre-climateric fruit of tomatoes and avocados (Gouble et al., 1995). Benkeblia and Varoquaux (2003) found that an exposure of onion bulbs to N2 O at different concentrations reduced the incidence of rots, especially in bulbs pre-treated with 100% N2 O for 4 days. In previous research, we found that a nonconventional atmosphere (65% N2 O, 25% Ar, 5% CO2 , 5% O2 ), combined with a dipping treatment in an aqueous solution of 0.5% of ascorbic acid, 0.5% of citric acid and 0.5% of calcium chloride for 3 min, maintained the fresh quality of MP apples for 12 days (Rocculi et al., 2004). However, to our knowledge, little data are available on the influence of Ar and N2 O on the shelf-life of MP fruit. The aim of this investigation was to study the effect of the use of non-conventional gas mixtures of Ar and N2 O on the maintenance of high quality characteristics of minimally processed kiwifruit slices during refrigerated storage.

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2. Materials and methods

2.3. Physico-chemical analysis

2.1. Plant material

Total soluble solids (%) content of the juice obtained from kiwifruit slices was determined at 20 ◦ C using a digital refractometer (PR1, Atago, Japan) after filtering through Whatman #1 filter paper. Percentage weight loss was estimated after each period of storage by weighing all sample boxes. Texture analysis was performed at room temperature (20 ± 2 ◦ C), about 1 h after removing samples from 4 ◦ C. Penetration tests were carried out on a 5 mm kiwifruit pericarp tissue sample using a 6-mm diameter stainless steel cylindrical probe. A texture analyser (mod. HD500, Stable Micro Systems, Surrey, UK) equipped with a 50-kg load cell was used. Test speed was 0.5 mm s−1 and data were expressed in N. At each storage time, 30 tests were performed for each sample from the 10 packages, with 3 measurements for each package. Measurements of the CO2 and O2 levels inside all sample boxes during storage were performed with a check point O2 /CO2 instrument (PBI Dansensor, Milano, Italy). The apparatus is based on an electrochemical sensor to record the O2 content and a mini-IR spectrophotometer to record the CO2 content in the package (accuracy: 0.1% O2 ; 2% CO2 ). The instrument was calibrated with O2 and CO2 air percentages.

Kiwifruit (Actinidia deliciosa (A. Chev) C.F. Liang and A.R. Ferguson var. deliciosa ‘Hayward’) were harvested in March in the south of Italy and stored for 1 month at 0 ◦ C in a controlled atmosphere (1% O2 and 2% CO2 ). When the fruit was transferred to the University of Cesena, they had about 12.5% total soluble solids content and an initial firmness of 13.2 N. Kiwifruit were selected for regular shape and uniform size (about 45 mm diameter). 2.2. Sample preparation After washing in tap water and soap, 750 kiwifruit were hand-peeled and cut in 1-cm thick slices using a sharp knife. Knife and cutting board were washed with water and soap and rinsed with 1000 mg l−1 sodium hypochlorite solution prior to use. Two slices from the central part of each fruit were cut into four pieces and polypropylene boxes were filled with 200 g (about 40 pieces) per box and wrapped in polypropylene film (200 ␮m thick). Boxes and film were purchased from Hot Mould System srl (Torino, Italy). The permeability data of the plastic film at 25 ◦ C were the following: water vapour transmission rate (WVTR): 1.9E − 18 to 3.8E − 18 mol s−1 m m−2 Pa−1 ; O2 TR: 2.5E − 19 to 3.6E − 19 mol s−1 m m−2 Pa−1 ; CO2 TR: 7.6E − 20 to 1.3E − 19 mol s−1 m m−2 Pa−1 . The samples were packed using a quaternary gas mixer (CVC, Milano, Italy) and a compensated vacuum-packing welding machine (Food Basic, Vigevano, Italy). The following atmospheres were created: [A] air; [B] 90% nitrogen (N2 ), 5% carbon dioxide (CO2 ), 5% oxygen (O2 ); [C] 90% nitrous oxide (N2 O), 5% CO2 , 5% O2 and [D] 90% argon (Ar), 5% CO2 , 5% O2 . Samples were prepared in a cold room at 12 ◦ C. All the samples were analysed just before packaging (zero time) and after storage in a refrigerated cell at 4 (±1) ◦ C for 4, 8 and 12 days. Fifty kiwifruit slices were used for zero time evaluations and 30 packages for treatment were prepared. At each storage time, 10 packages per sample were randomly taken and analysed.

2.4. Colour measurements 2.4.1. Tristimulus reflectance colorimeter Surface colour of pericarp tissue was measured using a reflectance colorimeter (Chromameter-2 Reflectance, Minolta, Japan), equipped with a CR-300 measuring head. Colour was measured using the CIE L* , a* , b* scale. Illuminant “C” (6774 K) was used. The instrument was calibrated with a white tile (L* = 98.03, a* = −0.23, b* = 2.05) before the measurements. Numerical values of L* , a* and b* were converted into hue angle: h◦ = tan−1 (b* /a* ) and chroma: C* = (a*2 + b*2 )1/2 (McGuire, 1992). Surface colour of kiwifruit core portions was not measured because of the small dimensions of the slices. At each storage time, 30 measurements were performed for each sample from the 10 packages, with three measurements for each package. The measurements were done on the centre of pericarp portion of three different slices.

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2.4.2. Image analysis Images of MP kiwifruit were obtained by digitalisation, using a colour plane scanner (Scan Jet 6300 C) (Fig. 1a). All images obtained under the same conditions (true colour – 24 bit, resolution of 300 bit × pixel (BPP)), were taken by positioning the kiwifruit slices on a scanner held on a black box, in order to exclude the surrounding light. After acquisition, digitalised kiwifruit images were isolated from the background according to Roudot (1989) and the pericarp and core areas were digitally separated using the software Photoshop® v. 5.0 (Adobe Systems Incorporated, USA), having different chromatic characteristics (Fig. 1b). After conversion in grey scale (8BPP), as detailed by Roudot (1989), and elimination of seed area, pericarp and core images were evaluated in two steps with advanced Image Analysis Software (Image Pro-Plus® v. 4.1, Media Cybernetics, USA), selecting total sample area and areas with different levels of browning. On the basis of chromatic characteristics of all samples, two colour models (pericarp and core) were built up (Reale et al., 2003; Rocculi et al., 2004), relating two virtual colours to pixel ranges with different grey levels. The same colour models were applied to

all kiwifruit slice images. Grey ranges were the following: in the pericarp, 0–177 BPP for the browning area (blue colour in Fig. 1c) and 178–253 BPP for the normal area (red colour in Fig. 1c); in the core, 0–215 BPP for the browning area (blue colour in Fig. 1c) and 178–253 BPP for the normal area (red colour in Fig. 1c). The software, examining all pixels in the image, calculated the percentage of browning (Fig. 1c). The measurements were obtained from the same slices used for colour determinations made with the tristimulus colorimeter. 2.4.3. Statistical analyses Analysis of variance (ANOVA) and the test of mean comparisons according to Fisher’s least significant difference (LSD) were applied, with a level of significance of 0.05. Data were also evaluated using Pearson’s correlation analysis between tristimulus colour evaluations, image analysis and firmness. Principal component analysis (PCA) was used to reduce the number of variables in the data matrix and to select the most discriminating parameters. The statistical package STSG Statistica for Windows, version 6.0 (Statsoft Inc., Tulsa, UK) was used.

Fig. 1. Kiwifruit slice images obtained by digitalisation using a colour plane scanner (a), after core and pericarp separation and isolation from the background (b) and virtually coloured on the basis of different levels of browning areas calculated with advanced Image Analysis Software (c).

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3. Results In Table 1 data of some physico-chemical characteristics of kiwifruit packed with different methods during refrigerated storage are shown. Samples packed in air (A) and with 90% N2 (B) showed the highest values of soluble solid contents (SSC), reaching values around 13.5% after 12 days of refrigerated storage. SSC of sample C (packed with 90% N2 O) did not show any significant changes over the whole storage period. This was associated with slow ripening of the fruit. Sample D (packed with 90% Ar) showed a significant decrease in SSC, becoming lower by 2% than the fresh sample after 12 days of storage. A, B and D samples showed a similar tendency for weight loss (WL) during storage reaching the same values on the 12th day (1.3–1.4%), almost double that of sample C (about 0.6%). Sample C had the lowest CO2 production during storage. As expected, the MP kiwifruit packed in air (A) had the highest CO2 production during storage (Park and Jung, 2002), while the CO2 levels inside B and D sample packages reached about the same values at the end of the storage period. The O2 in B and D samples Table 1 Change in physico-chemical characteristics of kiwifruit slices packed with different modified atmospheres (A: air; B: 90% N2 , 5% CO2 , 5% O2 ; C: 90% N2 O, 5% CO2 , 5% O2 ; D: 90% Ar, 5% CO2 , 5% O2 ) during storage at 4 ◦ C Sample

Time (day)

Physico-chemical characteristicsa SSC (%)

WL (%)

CO2 (%)

O2 (%)

A

0 4 8 12

12.40 a 13.30 b 13.33 b 13.40 b

– 0.83 a 0.99 a 1.40 b

0.03 a 34.00 b 46.75 c 54.60 d

20.80 a 11.70 b 6.30 c 0.25 d

B

0 4 8 12

12.40 a 12.66 b 13.40 c 13.60 d

– 1.07 a 1.20 ab 1.33 b

5.00 a 28.40 b 40.80 c 49.60 d

5.00 a 0.70 b 0.05 c 0.03 c

C

0 4 8 12

12.40 a 12.00 b 12.30 a 12.25 ab

– 0.42 a 0.50 a 0.66 b

5.00 a 16.05 b 24.40 c 26.25 d

5.00 a 9.15 b 5.40 c 1.45 d

D

0 4 8 12

12.40 a 11.00 b 10.60 c 10.45 c

– 0.60 a 1.02 b 1.43 c

5.00 a 22.25 b 31.70 c 49.85 d

5.00 a 0.03 b 0.05 b 0.03 b

a Means within the same column and the same sample with different letters are significantly different (p < 0.05).

Fig. 2. Firmness (F) values of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D.

was almost depleted after just 4 days of storage, while it remained higher in the C sample for all the storage period. The O2 level of sample A, 20.8% at the beginning of storage, rapidly decreased, reaching values close to zero by the 12th day. As shown in Fig. 2, the firmness of the pericarp tissue of all kiwifruit samples decreased, with samples A and B undergoing a greater loss of firmness, more than 50% of the initial value, during the first 4 days of storage. Sample D and, in particular, sample C, underwent a slighter firmness decrease, about 10% after 8 days of storage, maintaining the highest firmness level until the 12th day. All kiwifruit samples showed a decrease in luminosity (L* ) (Fig. 3) and chroma (C* ) (Fig. 4) during storage. MP kiwifruit packed in atmosphere C maintained the initial value of L* better than the other samples. In terms of L* , sample A and, in particular D, were more brown, showing the lowest values during storage. C* values of all samples (Fig. 4) suddenly decreased after 4 days of cold storage, reaching similar values after 12 days. In terms of hue angle values (Fig. 5), sample C did not change during storage, while a reduction in values for samples A and B suggested a shift to a more yellow colour during storage. Minor changes were observed in sample D (Ar), with a slight increase in hue angle. The tristimulus approach to colour measurements was compared with image analysis to evaluate chromatic changes of slice surfaces. The use of image analysis permits colour evaluation of all product areas at

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Fig. 3. Lightness (L* ) values of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D.

Fig. 5. Hue angle (h◦ ) values of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D.

the same time (Russ, 1995) and has already been used in a previous experiment to evaluate appearance of kiwifruit slices (Roudot, 1989). As shown in Fig. 6a (BAP) and in Fig. 6b (BAC), kiwifruit slices in sample C maintained the best colour. The control sample (A) had more than 90% browning area after just 4 days of storage, in both pericap and core surfaces. Treatment B had the highest BAP and BAC values, especially at the end of refrigerated storage. In terms of BAP, treatment D showed increasing values from 50% on the 4th day to 80% at the end of storage and a sudden increase in the percentage of core browning area after 4 days of storage.

Fig. 4. Chroma (C* ) values of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D.

Fig. 6. (a) Percentages of brown area of pericarp (BAP) of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D. (b) Percentages of brown area of core (BAC) of MP kiwifruit samples during refrigerated storage. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C (䊉): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . Data represent the mean ± S.D.

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It is important to ensure that the changes in the physico-chemical characteristics of MP kiwifruit slices over time were not influenced by microbial contamination. Preliminary studies carried out on kiwifruit slices packed in the same MAP conditions and prepared in the same way, showed that the total and yeast load values remained under 103 CFU, for up to 12 days of refrigerated storage (data not shown). For a better understanding of the relation between colorimetric results obtained with different methods (tristimulus colorimeter and image analysis), and firmness data, Pearson correlation analysis was carried out (Table 2). With regard to the tristimulus colorimetric parameter results, C* data showed a positive correlation only with L* results, and not with those of image analysis and firmness. Only L* values were correlated with image analysis results (BAP and BAC). As expected, from the highly significant correlation between the browning area of pericarp and core, colour changes occurred in both zones of kiwifruit slices in a proportional manner. Firmness data were positively correlated with L* and h◦ values, but more strongly and negatively correlated with both browning area percentages (BAP and BAC). To assess which factors were the most important to describe the quality of different samples and to get a better idea of the differences between the tested packaging methods, a principal component analysis (PCA) was performed. The two PCs explained 87.2% of the x-variables selecting 6 parameters, including C* , h◦ , BAC, BAP, WL and firmness. BAC, BAP and firmness were mainly accounted for with PC1 and C* mainly with PC2. As shown in Fig. 7, both PCs separated the treatments well.

325

Fig. 7. Principal component comparison of weight loss (WL), colorimetric (h◦ , C* , BAC, BAP) and firmness (F) results of MP kiwifruit samples. Samples: A (
): air; B (): 90% N2 , 5% CO2 , 5% O2 ; C ( ): 90% N2 O, 5% CO2 , 5% O2 ; D (): 90% Ar, 5% CO2 , 5% O2 . (+): fresh; (
), (), ( ), (): 4 days of storage; (
), (), ( ), (): 8 days of storage; (
), (), ( ), (): 12 days of storage.

Evaluating geometric distance, sample C (N2 O) was the closest to the fresh product and the most grouped, suggesting that it underwent the least changes during storage. Moreover, considering the positions of the other samples in relation to the time of storage, A and B were grouped by time, showing a high and rapid quality deterioration, in particular in terms of firmness decrease and BAP and BAC increase. Sample D exhibited a more gradual decay in quality showing a shift from C to A and B sample groups, during storage.

4. Discussion Table 2 Correlation coefficients between tristimulus and image analysis colorimetric and firmness results N = 16

L*

h◦

C*

BAP (%)

BAC (%)

L*

– NS 0.685** −0.772*** −0.766*** 0.603*

– NS NS −0.600* 0.636**

– NS NS NS

– 0.904*** −0.918***

– −0.849***

h◦ C* BAP (%) BAC (%) F (N)

NS, not significant. * , p-level, respectively.

** , ***

Significant to 0.05, 0.01 and 0.001

In general, sample B and control (sample A) showed analogous changes of physico-chemical characteristics during storage, as the modified atmosphere with 90% of N2 was not effective in inhibiting the phenomena responsible of the loss of quality of MP kiwifruit. The observed O2 and CO2 results of sample C confirm the N2 O inhibition effect on vegetable tissue respiration, reported in previous investigations for seed germination (Sowa and Towill, 1991; Sowa et al., 1993) and onion bulbs (Benkeblia and Varoquaux, 2003). As well, the minor weight loss and firmness

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decrease of sample C might be a consequence of a lower activity of the enzymes responsible for pericarp softening (pectinesterase, polygalacturonase and betagalactosidase) and cell juice loss (Agar et al., 1999), imposed by the high level of N2 O in the package head space. The best colour maintenance of sample C, in terms of L* , h◦ , BAC and BAP, could be a consequence of an indirect enzyme inhibitory effect of N2 O. Ethylene is responsible for increased chlorophyllase activity, that causes chlorophyll destruction and its conversion to the olive brown pheophorbide, with changes in chromatic characteristics of the vegetable tissue (Shimokawa et al., 1978; Amir-Shapira et al., 1987). The detected decrease of L* and C* in all kiwifruit samples suggests chlorophyll degradation. These results are in agreement with those of previous research on minimally processed apples, where the same modified atmospheric composition showed the best effect of colour preservation (Rocculi et al., 2004). Kiwifruit slices packed in modified atmospheres with 90% of Ar (sample D) showed a better firmness maintenance compared with samples packed in air and in N2 (A and B) at the beginning of storage and a slightly lower CO2 production, but not better colour characteristics. This finding confirms the results obtained by Zhang et al. (2001) that showed a lower activity of malic dehydrogenase, one of the key enzymes in the Krebs cycle of respiration metabolism, when treated with Ar, compared with N2 treatment. The solubility in water of N2 O, Ar and N2 is respectively 0.665, 0.034 and 0.016 g L−1 (Atkins, 1994). The stronger ability of N2 O and Ar to retard kiwifruit physiology compared with N2 , might be due to the higher capacity of these gases to dissolve in the aqueous layer of the cut fruit and consecutively through the cells of the flesh. Therefore, they can inactivate some chemically-active sites on the enzymes more effectively than N2 and/or reduce the level of dissolved oxygen, whose presence is necessary for oxidative enzymes to catalyse metabolic reactions. Image analysis results were able to better differentiate samples in terms of colour rather than tristimulus colorimeter, also because this technique permitted core colour evaluation, difficult to detect with tristimulus colorimeter because of the small area of the core slices.

As shown in a previous investigation (Talens et al., 2001), changes in texture of kiwifruit can influence colour. Physiological events, associated with the loss of firmness in kiwifruit include degradation of hemicellulose (Soda et al., 1987), solubilization of polyuronide and release of galactose from pectic polymers (Redgwell et al., 1990, 1991, 1992), cell wall swelling (Hallett et al., 1992) and a decrease in water and osmotic potential (Redgwell and Fry, 1993), and can therefore increase the translucency of the flesh. The strong correlation between firmness and the image analysis results suggests that chromatic models allow for good recognition of such chromatic changes associated with kiwifruit softening. In conclusion, under the examined packaging conditions, the modified atmosphere containing 90% of N2 O, 5% of O2 and 5% of CO2 , was the best mixture of gases regarding colour retention, firmness and soluble solid content of sliced kiwifruit, lowering respiratory activity. Our results extend the knowledge on the preservative effects of N2 O on quality of whole climateric fruit, shown in previous studies (Gouble et al., 1995; Leshem and Wills, 1998), to wounded cut kiwifruit. The use of a calcium salt treatment (Massantini and Kader, 1995; Muntada et al., 1998; Agar et al., 1999) together with a N2 O modified atmosphere would have contributed to prolonging high quality characteristics of MP kiwifruit. The effects of argon were positive only for firmness preservation and slightly for slowing down respiration but not for colour preservation, confirming inconsistent results shown in previous research (Jamie and Saltveit, 2001; Mostardini and Piergiovanni, 2002; Day, 1996, 1998; Zhang et al., 2001).

Acknowledgements The authors express their gratitude to Dr. F. Gomez (Lund University, Lund, Sweden) for the critical reading and reviewing of the manuscript.

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