Headspace Oxygen Content
Packaging material plays an important role in the quality of foods (Plestenjak et al. 2001). The oxygen content in the headspace of the containers was measured throughout the storage of the mandarin juices, and experimental results are summarized in Fig. 1.
Figure 1. CHANGES OF OXYGEN CONCENTRATION WITH TIME IN REFRIGERATED MANDARIN JUICES STORED AT 4C PACKED IN CARTON (A, B AND C) AND IN POLYETHYLENE TEREPHTHALATE (PET) (D) ●, Carton A; ○, carton B; ▾, carton C; Δ, PET bottle.
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The initial oxygen content was zero because during the packaging of the juices, the headspace gasses were displaced using liquid nitrogen (Kimball 2002; FMC 2004). Juices C and D packed in “carton C” and PET, respectively, presented significantly (P < 0.001) higher oxygen concentrations than refrigerated mandarin juices A and B packed in “cartons A and B.” These experimental results provided a real proof that somehow carton C and PET bottle were partially permeable to oxygen; according to our experience, micropores could be present in some of the junctions of the juice C package. According to Berlinet et al. (2003), it is known that PET has oxygen permeability and can absorb some flavor compounds from the food matrix (Ducruet et al. 2001). According to our experimental results, cartons A and B could be considered as high oxygen barriers, while carton C and PET bottle should be considered as intermediate or low oxygen barriers. In our opinion, the main entrance of air into container C must be located in the joints of their upper part. However, further research is needed to find a proper explanation for these experimental findings.
The initial vitamin C concentration for refrigerated mandarin juices (day 0) was 301.2 ± 0.5 mg/L. The type of container significantly affected the changes of vitamin C concentration with time (Fig. 2). At the end of the storage of the refrigerated (4C) mandarin juices, the juice with the highest vitamin C concentration was A (185.3 ± 0.3 mg/L), followed by juice B (159.2 ± 0.2 mg/L), juice C (86.3 ± 0.4 mg/L), and finally, juice D (78.7 ± 0.3 mg/L). These experimental results proved that the container type was important for nutritional values as degradation rate of vitamin C.
Figure 2. CHANGES OF VITAMIN C CONCENTRATION WITH TIME IN REFRIGERATED MANDARIN JUICES STORED AT 4C PACKED IN CARTON (A, B AND C) AND IN POLYETHYLENE TEREPHTHALATE (PET) (D) ●, Carton A; ○, carton B; ▾, carton C; Δ, PET bottle.
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The experts committee of the European Association of Citrus Juice Producers (AIJN) establishes a minimum level of vitamin C concentration (100 mg/L) on orange and mandarin juices that must be maintained throughout the shelf life of these products (AIJN 2005). This minimum content was not reached by juices A and B after a storage period of 90 days, while it was reached on day 70 and 40 by juices C and D, respectively. Therefore, the shelf life of the juices was reduced from more than 90 days on juices A and B to approximately 36 and 54 days for juices D and C, respectively.
The main explanation for the high reductions in the vitamin C content in juices C and D is that higher oxygen contents were present in these juices as compared with juices A and B. Ascorbic acid stability is greatly influenced by temperature, oxygen and metal ion content (Fennema 1996; Polydera et al. 2003). Ascorbic acid in the presence of oxygen and metallic ions will degrade to dehydroascorbic acid (Fennema 1996). Thus, precautions should be used to minimize ascorbic acid degradation, for instance, removal of as much oxygen as possible from equipment and container is needed to improve shelf life of ascorbic acid addition as a nutrient.
The statistical study of instrumental color data showed significant differences among the refrigerated mandarin juices at P < 0.001 (Table 2), for color parameters L*, a*, b*, C* and hab.
Table 2. RESULTS OF THE ANOVA AND TUKEY MULTIPLE RANGE TESTS FOR COLOR COORDINATE CHANGES WITH TIME IN REFRIGERATED MANDARIN JUICES PACKED UNDER DIFFERENT CARTON COMPOSITION AND PET BOTTLES AND STORED AT 4C
|Tukey multiple range test|
The main color characteristics of the initial mandarin juice analyzed (day 0) in this study were L* = 67.29 ± 0.01, a* = 18.90 ± 0.03 and b* = 76.19 ± 0.02. One of the peculiarities of the ultrafrozen orange juices analyzed by Meléndez-Martínez et al. (2003) was their deep orange color (L* = 63.23 ± 1.27, a* = 16.18 ± 0.56 and b* = 64.64 ± 3.78), because the juice was subjected neither to high temperatures nor to concentration process during the production. On the other hand, mandarin juices analyzed by Pérez-López and Carbonell-Barrachina (2006) and subjected to thermal processing (T = 98C for 20 s) showed less vivid orange colors. In this way, the current chroma values, C* = 78.50 ± 0.02, were closer to those reported by Meléndez-Martínez et al. (2003), C* = 66.64 ± 3.66, than to those reported by Pérez-López and Carbonell-Barrachina (2006), C* = 34.27 ± 0.01.
Data on Table 2 showed that the refrigerated mandarin juice with the lowest value of the L* coordinate, lightest color, was the fresh one; storage in any container at any storage time and stored at refrigeration temperature (4C) made the juice darker. Therefore, the darkest samples were found after 90 days of storage: 63.12 ± 0.02, 60.14 ± 0.04, 56.98 ± 0.03 and 54.22 ± 0.02 for juices A, B, C and D, respectively. As can be seen, more than 13 units (13.06) of difference were detected in lightness between the lightest (initial mandarin juice) and the darkest mandarin juice (D).
For the green-red coordinate, a*, again, the highest value (a more reddish color) was found in the initial mandarin juice, and the lowest one in juice D stored for 90 days (11.95 ± 0.02). There were no significant differences in the values of a* during the storage of juice A; however, there was a sharp decrease in this coordinate for juice D, even after only 18 days of storage, 17.72 ± 0.01.
Similar trends to those previously described for a* were also found for the blue-yellow coordinate, b*, with the initial juice having the highest value and juice D presenting the lowest value of all juices after 90 days of storage.
Because both chroma and hue angle parameters are calculated from a* and b* values, their patterns were absolutely identical to those previously described for a* and b*.
In summary, it can be concluded that refrigerated mandarin juice A packed in carton provided the juice with the highest values of L*, a*, b* and C*, thus providing a high intensity of the orange color. Finally, the experimental results proved that carton composition was an important quality control parameter in determining the degradation of the initial vivid orange color of the refrigerated mandarin juices. It seems evident that “carton A” provided better experimental results than “cartons B and C” and PET bottle.
Another instrumental measurement, reflection spectra, was carried out to prove that there were significant differences on the color characteristics of the studied mandarin juices. Figure 3 shows that the sample with more intense yellow, orange and red colors was the initial refrigerated mandarin juice; regarding the packed and stored juices, juices A, B and C, packed in carton, showed a better behavior than juice D packed in PET bottle.
Figure 3. ABSORPTION SPECTRA OF THE INITIAL REFRIGERATED MANDARIN JUICE (I) AND THE FOLLOWING REFRIGERATED MANDARIN JUICE SAMPLES AFTER 90 DAYS OF STORAGE PACKED IN CARTON (A, B AND C) AND IN POLYETHYLENE TEREPHTHALATE (D) AND STORED AT 4C
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Once this point is reached, the next question is clear: Is this instrumentally detected change of mandarin juice color due to container type and composition, detected by the regular consumer of citrus juices?
In each session (there was one session for each sampling day [0, 18, 36, 54, 72 and 90 days]), consumers were asked to arrange the four mandarin juices (from four different containers) according to their liking of the sample color, fresh mandarin flavor and intensity of off-flavors. Experimental results are summarized in Table 3.
Table 3. STATISTICAL ANALYSIS OF THE RANKING DATA OF COLOR, FRESH MANDARIN FLAVOR AND OFF-FLAVORS AFTER 0, 18, 36, 54, 72 AND 90 DAYS OF STORAGE AT 4C (20 ASSESSORS; 4 SAMPLES; T(5%; 3df) = 7.81; LSDRANK = 16.0)
|Time (day)||Container||Rank sum|
|Statistics||0.66‡ N.S.||0.54 N.S.||0.42 N.S|
|Statistics||15.2*||0.42 N.S.||1.26 N.S.|
The first sensory attribute to present significant differences among the studied juices was color (P < 0.05); after only 18 days, juice D (container IV, PET bottle) presented a less intense color than the rest of the juices. After 36 days, both juices A and B presented a more intense orange color than C and D juices; no significant differences were found between these last two samples.
Fresh mandarin flavor started to disappear after 36 days of refrigerated storage in juice D. After 54 days, both juices A and B presented significantly higher intensities of fresh mandarin flavor than juices C and D (P < 0.05); this statement was true until the end of this study.
Finally, 54 days of refrigerated storage was needed in order to find off-flavors (negative aromas) in mandarin juices C and D. After 90 days, juice D presented a significantly higher intensity of off-flavors than any other juice.
Once again, these affective data proved that the container nature played an important role in determining the quality of the refrigerated mandarin juices, and that transparent PET bottle provided the worst experimental results in this sensory study.