Physiological Physical Chemical Characteristics and Sensory Evaluation of Minimally Processed Grapefruit Segments Packaged under Modified Atmosphere

The objective of this study was to investigate physiological, physical, chemical, and sensory characteristics of minimally processed and modified-atmosphere-packaged grapefruit segments. Grapefruits were sanitized, peeled, segmented and packed under passive (21%O2 +79% N2) and two active modified atmospheres (20% O2+10% CO2 +70% N2 and 70% O2+20% CO2 +10% N2) in polypropylene trays. Physiological, physical, chemical, and sensory properties were monitored at 4 oC for 15 days. In general, there was no significant effect of MAP observed in chemical properties (P > 0.05). There was no significant effect of MAP treatment on a* and b* values (P > 0.05); however, the firmness values increased at all applications (P ≤ 0.05). Grapefruit segments remained viable for 10 days under enriched oxygen (70%) and less than 10 days under active (20% oxygen) and passive MAP applications considering internal atmosphere and quality parameters. These results suggested that grapefruits could be prepared as ready to eat fruits with considerable shelf life, quality and convenience with using enriched oxygen MAP.


Introduction
Ready-to-eat or fresh-cut products are a rapidly growing sector of the horticultural industry (Soliva-Fortuny & Martin-Belloso 2003).Many terms are applied to fruits and vegetables cleaned and prepared in fresh form: lightly processed, minimally processed, prepared, precut, fresh-processed, and partially processed etc.The term fresh-cut is widely used for these products (Cantwell 1997).The major factors responsible for extending the shelf life of fruits and vegetables include careful harvest at optimum maturity and food sanitation.When these are properly practiced, the implementation of optimum storage conditions through modified atmospheres can be quite effective, thus maximizing product shelf life and quality (Farber et al 2003).
Modified atmosphere packaging techniques constitute one of the fresh food preservation methods with no or only minimal use of additives and are accepted now as the technology of the future (Tas & Ayhan 2005).In modified atmosphere (MA) applications, the O 2 and CO 2 concentrations are modified initially and then change dynamically depending on respiration rate of product and the permeability of the film surrounding the produce (Erkan & Wang 2006).Reduced O 2 and/or enriched CO 2 levels reduce respiration and decrease ethylene production, inhibit or delay enzymatic reactions, alleviate physiological disorders, and preserve the product from quality losses (Day 1994;Soliva-Fortuny & Martin-Belloso 2003).However, exposure to O 2 and CO 2 levels outside the limits of tolerance may lead to anaerobic respiration and the production of undesirable metabolites and other physiological disorders (Zagory & Kader 1988).In fresh-cut fruit commodities, operations including peeling, cutting, coring, and/or slicing are critical to delimit the shelf life.These operations increase respiration and mass transfer phenomena, thus microbiological spoilage and quality loss of the processed produce (Martin-Bellaso & Soliva-Fortuny 2006).Agar et al (1999) observed that peeling and slicing of kiwifruit caused 30% mass loss after 3 days of storage in comparison with unpeeled slices.
The benefits of modified atmosphere packaging have been extensively studied in extending shelflife of many fruits and vegetables.However, there are only limited studies regarding ready to eat citrus fruits.The application of MAP limited the microbial growth and enhanced the quality of strawberries (Garcia et al 1998), fresh cut mangoes and pineapples (Martinez-Ferrer et al 2002).It was reported that reduced levels of O 2 combined with appropriate permeability of plastic package extended the microbiological shelf life of fresh cut pears for almost 3 weeks under refrigerated storage (Soliva-Fortuny & Martin-Bellaso 2003).However, packages with low permeability in combination with low O 2 atmospheres can stimulate the growth of anaerobic spoilage or pathogenic microorganisms (Soliva-Fortuny et al 2004).
Some researchers have suggested the use of super atmospheric or high O 2 concentrations ranging from 30 to 100 kpa for fruits and vegetables.Elevated O 2 atmospheres have been suggested to control microbiological growth, inhibit enzymatic discoloration, excessive moisture and flavor loss.Super atmospheric concentrations were reported as more effective against microorganisms when used with high CO 2 levels of 15-20 kPa.High CO 2 atmospheres also mostly inhibit aerobic microorganisms especially gram-negative bacteria and moulds (Kader & Ben-Yehoshua 2000).
Citrus species, especially grapefruits, are important because of their nutritional content and antioxidant properties associated with a low risk of degenerative diseases such as cancer, diabetes, cardiovascular and neurological problems (Del T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 131 Caro et al 2004).That is the reason why citrus species are of interest as "ready-to-eat" packaged products.There was also no study found in the literature on the quality and shelf life of segmented grapefruits.The use of high oxygen for grapefruit segments is especially of interest due to beneficial effects on the control of microbial growth and enzymatic reactions.There is also less risk for anaerobic respiration.
Low pH of the grapefruit, modified atmosphere packaging, and storage at low temperature along with good sanitation practices could prevent significant microbial growth.Pretel et al (1998) observed slight microbial activity in minimally processed ready-to-eat oranges during 11 days of storage at 4°C under passive atmosphere using two PP films with different permeability.There were also no significant change reported regarding to weight loss, acidity, °Brix and carotenoids.However, the orange segments were commercially viable for one week in terms of sensorial quality.PP with high permeability kept sensory quality better than PP with low permeability.They noted problems with loss of aroma, appearance of strange tastes and increase in ethanol level after one week of storage.Rapisarda et al (2006) also stated that there were low microbial counts observed in Tarocco orange slices packaged with 3 films of different permeability during 12 days of storage at 4°C.Minimally processed Tarocco clone slices packaged with the highest oxygen permeable film were the most appreciated regarding sensory evaluation.Terry & Overcast (1976) reported that there was only a slight or no microbial growth in different types of salad plants when the pH was less than 4.5 for mesophilic and psychrophilic bacteria.

Procedure for processing and packaging
PP trays were sanitized with 3% hydrogen peroxide solution.During the entire production processes, laboratory coats, sterile gloves, and caps were used to comply with industrial hygienic standards.All equipment was sanitized using 200 ppm chlorine solution prior to processing.
Fruits were selected for uniformity, washed with tap water and immersed in 200 ppm NaOCl for 3 min, then dried and peeled manually.After the peeling, fruits were carefully segmented and undamaged segments were placed inside the PP trays (400 g per tray) which were sealed with CPP/OPP (Cast Polypropylene/Oriented Polypropylene) film at 170°C.In this study 3 different gas compositions such as normal atmosphere (passive), active MAP1 (20% O 2 + 10% CO 2 + 70% N 2 ), and active MAP2 (70% O 2 + 20% CO 2 + 10% N 2 ) were applied with the modified atmosphere packaging machine (MECA 501, Mecaplastic, France) combined with a triple gas mixer (KM60-3; Witt, Germany).The PP trays were stored at 4ºC for 15 days, and the physical and chemical quality parameters were measured at 0, 2 nd , 5 th , 10 th and 15 th days.Sensory analysis (Taş 2007) was performed at 0, 5 th , 10 th and 15 th days.

Physiological analysis (headspace oxygen and carbon dioxide content)
The headspace oxygen (%) and carbon dioxide (%) content of PP trays sealed with CPP/OPP film was T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 132 determined using a gas analyzer (PBI Dansensor, Ringsted, Denmark) by a needle through an adhesive septum fixed on the lidding material.The gas measurements were taken from two parallel packages for each treatment on each analysis day.The measurements were taken at 2 different sides of each package and the average of 4 measurements was calculated.

Physical analysis (colour and firmness)
The colour measurements were performed with a colorimeter (Model CR 400, Minolta, Osaka, Japan).The colorimeter was calibrated to a standard white tile.The CIE scale L*, a* and b* values were recorded in the middle of the one side of each segment.Twenty segments from two parallel packages (10 segments per package) were measured for each treatment on each sampling day (Taş 2007).
The firmness was measured with a texture analyzer (TA-XT Plus; Stable Micro Systems, Surrey, England) using a blade set (HDP/BS).Speed of 12 mm s -1 and penetration distance of 35 mm were used to cut the segments at the center, and the firmness was expressed as maximum cutting force (N).The segments similar in thickness were selected for firmness measurement.Twenty segments from two parallel packages (10 segments per package) were measured for each treatment on each sampling day (Taş 2007).

Chemical analysis
Each sample (about 50g from) was blended for 2 min and filtered by using cheesecloth.The prepared juice of the sample was used for chemical analysis.Acidity was determined by potentiometric titration with 0.1 N NaOH up to pH 8.1 using 10 ml of macerate diluted with 50 ml water.Results were expressed as citric acid %.Soluble solids were measured by hand refractometer (Model N-50E; Atago, Tokyo, Japan) and expressed as ºBrix at 20 ºC.The pH was determined by using a pH meter (Model pH-315i; WTW, Weilheim, Germany) (AOAC 1990).The juice of the sample was centrifuged at 3000 rpm for 10 min and the supernatant was filtered by using a 0.45-µm-poresize filter to determine sugar content (fructose, glucose, sucrose, and total sugar) of grapefruit segments by using a HPLC (Shimadzu, Japan) equipped with refractive index detector (Model RID-10A, Shimadzu, Japan) (Escalona et al 2005).Chemical analysis was repeated twice for each package and the average of four measurements of two parallel packages were calculated (Taş 2007).

Statistical analysis
Two parallel packages were used for each analysis.Data were subjected to analysis of variance (ANOVA) and a Duncan multiple comparison test to determine significant differences between treatments using the MSTAT statistical package (Freed 1991).Significance of differences was represented at 5% (P ≤ 0.05).

Development of headspace atmosphere in the packages
The modified atmosphere inside the bags as oxygen and carbon dioxide and the time necessary to reach 16 (2010) 129-138 133 equilibrium depended on the respiratory intensity of the fruits and the permeability of the film (Pretel et al 2000).The concentrations of O 2 and CO 2 (%) in the package headspace during storage are presented in Figures 1 and 2, respectively.For all treatments, decrease in oxygen and increase in carbon dioxide levels were observed during storage.Oxygen levels decreased continuously in all treatments until the concentrations reached 0% for passive and active MAP1 at day 10, and 2.4% for high oxygen treatment (active MAP2) at day 15, whereas CO 2 levels increased in passive and active MAP1 for 10 days and tended to stabilize after 10 days.The CO 2 level continued to increase at MAP2 during storage as seen in Figure 2. The atmospheres underwent anaerobic condition at passive and active MAP1 before 10 days.Anaerobic respiration could stimulate the growth of anaerobic spoilage and pathogenic microorganisms (Garcia et al 1998;Kader et al 1989).Thus, the storage time of grapefruit segments was limited to less than 10 days for passive and active MAP1 by considering the internal atmospheres.Although the atmosphere was aerobic at the 15 th day of storage for high oxygen MAP (MAP2), the segments were not acceptable by the sensory panel which could be attributed to very high CO 2 (54.98%) level in the headspace.Thus, the storage time is suggested as 10 days under high oxygen application.

Influence of modified atmosphere on physical quality of grapefruit segments
Fruit colour is an important attribute in purchase decisions, especially when a product is packaged and can not be touched or smelled (Gonzalez-Aguilar et al 2000).Effects of modified atmosphere packaging on physical parameters (colour and firmness) in grapefruit segments stored at 4ºC are shown in Table 1.No differences were observed in the colour (a* and b* values) of grapefruit segments among all treatments (passive MAP, active MAP1 and MAP2) (P > 0.05).However, L* value was significantly affected by MAP treatment (P < 0.01).Grapefruit segments under passive MAP had brighter color than that of the active MAP treatments.Storage time significantly affected a* (P < 0.05) and b* values (P < 0.0001) but no significant effect on L* value (P = 0.0917).
In general there was no significant difference in segment firmness among 3 MAP conditions (P = 0.0842).However, storage time affected segment firmness significantly (P < 0.0001).Increases in firmness values were observed in the first 5 days of storage and tended to reverse for the rest of the storage at all applications.Increase in firmness may be attributed to surface drying of the segments due to water loss by respiration and transpiration T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 134 (Comparisons could be made between treatments for a given storage day or between storage days for a given treatment) ** Active MAP1 with low oxygen (20% O2+ 10% CO2+ 70% N2) *** Active MAP2 with high oxygen (70% O2+ 20% CO2+ 10% N2) as stated by Martin-Belloso & Soliva-Fortuny (2006) and Erkan & Wang (2006).However, our sensory panel did not find significant changes in firmness for all treatments during storage.Water loss of minimally processed fruits was one of the important problems and this might be overcome using packaging materials with low water vapor transmission rate and low storage temperature (Martin-Bellaso & Soliva-Fortuny 2006).

Influence of modified atmosphere on chemical quality of grapefruit segments
Important chemical properties such as titratable acidity, pH, soluble solids, and sugar content of the grapefruit segments are shown in Table 2.For citrus fruits (grapefruit, orange, mandarin), citric acid is a major organic acid followed by malic acid and quinic acid (Cemeroglu et al 2001).No significant changes were observed in titratable acidity in terms of citric acid, pH and soluble solids among passive T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 135 followed by the different letter for a given parameter are significantly different (P ≤ 0.05) (Comparisons could be made between treatments for a given storage day or between storage days for a given treatment) ** Active MAP1 with low oxygen ( 20% O2+ 10% CO2+ 70% N2) *** Active MAP2 with high oxygen ( 70% O2+ 20% CO2+ 10% N2) and active MAP applications (P > 0.05).The storage time also did not have significant effect on titratable acidity (P = 0.7979) and pH (P = 0.052).However, there was significant effect of storage time on total soluble solids (P < 0.001).The effects of storage became significant for active MAP1 application after day 5 and for high oxygen application after day 10.MAP application with different gas compositions did not have any significant effect on the sugar content of grapefruit segments as shown in Table 2 (P > 0.05).The sugar content (fructose, glucose and total sugar) of grapefruit segments did also not change significantly during storage (P > 0.05).Studies with fresh-cut pears (Senesi et al 1999) and kiwifruit (Agar et al 1999) showed that sugar content did not significantly change under refrigerated storage, as expected because sugar content is not much influenced by the atmospheric conditions (Soliva-Fortuny & Martin-Belloso 2003).Our results are in agreement with the results by Pretel et al (1998) who reported no significant changes in chemical

Influence of modified atmosphere on sensory quality of grapefruit segments
The results of the sensory evaluation are presented in Table 3.In general, there was no significant difference in sensory quality of grapefruit segments between passive (air) and active MAP1 (P > 0.05) for 15 days.There were no significant effect of MAP treatment*storage time interaction on any of the attributes tested (P > 0.05).However, MAP treatment had significant effect on visual appearance (P < 0.001), sweetness (P < 0.05) and product acceptability (P < 0.0001).Storage time significantly affected all sensory attributes tested and product acceptability (P < 0.05).Although grapefruit segments packaged under air or active MAP with 20% oxygen were found acceptable for 15 days in terms of sensory quality, the atmosphere was anaerobic after the 10 th day which could be risky for consumption.The scores for grapefruit segments treated with high oxygen were below acceptability limit (score 3) at the 15 days of storage for most of the sensory attributes and the product acceptability.That's why the viability of grapefruit segments packaged with high oxygen was limited to 10 days.This might be due to negative effects of CO 2 (about 55%) on the sensory attributes at the 15 th day of storage under MAP2.(Comparisons could be made between treatments for a given storage day or between storage days for a given treatment) ** Active MAP1 with low oxygen ( 20% O2+ 10% CO2+ 70% N2) *** Active MAP2 with high oxygen ( 70% O2+ 20% CO2+ 10% N2) T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 137

Conclusion
The grapefruit segments were viable for 10 days under high oxygen (70% oxygen) due to sensory attributes and acceptability scores, and less than 10 days for passive and active MAP with 20% oxygen due the internal atmosphere at 4 ºC.Although the package atmosphere was aerobic during 15 days at high oxygen application, the segments were found not acceptable by the sensory panel after 10 days possibly due to negative effects of high amount of carbon dioxide on the sensory attributes.High oxygen application provided further advantages over passive MAP or active MAP1 considering the internal oxygen level at 10 days of storage.Grapefruit segments can be prepared as ready-to-eat fruits with considerable shelf life of 10 days, good quality and convenience to the consumer using enriched oxygen MAP using PP trays sealed with CPP/OPP film.This study could be repeated using packaging film with higher permeability to increase the shelf life more than 10 days.

Figure 2 -
Figure 1-Headspace oxygen concentration (%) during cold storage (Points without error bars are smaller than symbols) Şekil 1-Soğuk depolama sürecinde tepe boşluğu oksijen konsantrasyonu (%) (Hata çizgisi görünmeyen noktalar sembollerden daha küçüktür) T a r ı m B i l i m l e r i D e r g i s i -J o u r n a l o f A g r i c u l t u r a l Sc i e n c e s 16 (2010) 129-138 136 attributes (pH, titratable acidity, and soluble solids) during 11 days of storage of minimally processed oranges(Pretel et al 1998).

Table 1 -Effect of modified atmosphere packaging on physical parameters (colour and firmness) of grapefruit segments stored at 4ºC
* Mean values followed by the different letter for a given parameter are significantly different (P ≤ 0.05)

Table 2 -Effect of modified atmospheres on chemical parameters (titratable acidity, pH, soluble solids, sugars) of grapefruit segments stored at 4ºC
* Mean values

Table 3 -Effect of modified atmosphere packaging on the sensory attributes of grapefruit segments stored at 4ºC
Mean values followed by the different letter for a given parameter are significantly different (P ≤ 0.05). *