Refrigerated mandarin juice was packed in four different containers, three cartons with different composition and one polyethylene terephthalate transparent bottle, and was stored at 4C for 90 days. During the storage of these juices, changes in the headspace gas composition, vitamin C, and CIE L*, a* and b* color coordinates were evaluated. In addition, a consumer panel evaluated the sensory color, fresh mandarin flavor and presence of off-flavors in the juices. Experimental data indicated that the deterioration of mandarin juices (ascorbic acid degradation and darkening of color) was triggered by the rise in oxygen in the headspace of the storage containers. The type of container played a predominant role in determining the juice quality, with carton containers with an inner layer of aluminum foil providing the juices with the best quality throughout their storage.
Results from this study will provide manufacturers of mandarin juice with information dealing with the storage and quality of juices packed in different containers. In this way, if manufacturers want to use transparent polyethylene terephthalate (PET) bottles showing the color and appearance of the juice, they will be aware that the shelf life of the juice will be much shorter than in packed cartons; this reduction will be from more than 90 to 36 days (PET bottle). On the other hand, if manufacturers want to use carton for their packaging, they will be aware that using a container with a thick inner layer of aluminum foil will maintain the quality of the juice for a longer time (over 90 days) compared with a shelf life of about 54 days from cartons with an inner layer of ethylene vinyl alcohol copolymers.
Citrus juices are highly consumed in many countries. For instance, orange juice accounts for 60% of all Western Europe consumption of fruit juices and juice-based drinks. In the U.S.A., orange juice is the most popular juice, with the consumption of the Americans' second choice, apple juice, five times lower (Conrad et al. 2005).
Spain, the second world producer of mandarin (1,779,800 t in 2003), is also the main supplier to the international market, especially to northern European countries and the U.S.A. The most grown and exported mandarin cultivars are Clementine (MAPA 2003). On the other hand, the selling prices of mandarin in eastern Spain sometimes do not even cover the irrigation, pesticides and picking expenses. Thus, there is an imperative need for finding new uses for mandarin in the food industry because otherwise this agricultural and industrial sector is going to collapse, with all the implications that this breakdown can have in the agriculture, industry and culture of eastern Spain. Currently, the main two items elaborated with mandarin fruits are canned slices and juice, and they represent only about 7% of the total trade of this fruit (FAO 2005).
Consumers purchase a product based mainly on their perception of the product quality and the relationship price/quality (Thai and Shewfelt 1991). The importance of color as a quality parameter in citrus juice has been demonstrated in several studies (Meléndez-Martínez et al. 2003). The natural bright colors of citrus juices have been considered traditionally as one of their main advantages over other juices. The main carotenoids responsible for the orange color of orange and mandarin juices are α-carotene and β-carotene, zeta-antheraxanthin (yellow), violaxanthin (yellow), β-citraurine (reddish orange) and β-cryptoxanthin (orange) (Stewart 1980; Lin and Chen 1995). Large quantities of β-cryptoxanthin in orange and mandarin juices produce a highly desirable bright orange color (Meléndez-Martínez et al. 2003).
It is easy to understand why color is an important attribute to the food industry. Consumers frequently judge food quality based on color. In the marketplace, it is rare that consumers are allowed to taste food products prior to purchasing them; however, they can look at the product. Therefore, they finally decide whether to buy a particular product largely based on the overall appearance including color. Frequently, color and flavor are directly related. However, food processors are often limited in their capability to adjust color in the final product. Because of this limitation, manufacturers pay special attention to the color of raw materials/ingredients and to the changes that occur during each step of the production chain (Gunasekaran 1990).
On the other hand, fruit juices and beverages must be handled carefully during processing and storage to control nutrient losses and color changes. Ascorbic acid (AA) is an important nutritional component of many juice products, and the label AA content per serving must be valid throughout the product shelf life (Will et al. 2000). Numerous complex factors, including the protection provided by the container, affect AA loss in foods, and the kinetics of the degradation appears to be dependent on the specific processing system (Soares and Hotchkiss 1999). AA degradation is also associated with nonenzymatic browning (Lee and Nagy 1996).
For these reasons, selection of the packaging materials is one of the basics of the food industry because of their influences on the quality of foods during storage. The permeation through them and the degradation of color and nutrients by oxygen transmission through packages have been an increasing area of research (Soares and Hotchkiss 1999).
The interiors of preformed board cartons are composed of different barrier materials depending on the nature of the food to be packed. For example, aluminum cartons are used for fruit juices while those coated with polyethylene are primarily for products such as milk (Lee and Nagy 1996). Increasingly, ready-to-serve beverages are packaged in different kinds of packing materials such as polyethylene terephthalate (PET), PET/polyethylene napthalate (PEN) and cardboards. Exposure to light and/or oxygen content (headspace and/or dissolved) may be a degradative factor for some beverages packaged in transparent cartons (Sahbaz and Somer 1993; Main et al. 2001; Flouros et al. 2003), but no information is available on the effects of carton materials on mandarin juice.
The main objective of this study was to compare the color properties, AA degradation and changes of gas composition in the headspace of refrigerated mandarin juice stored in four different containers. Results from this study will allow to make recommendations on which container is the best for refrigerating mandarin juice.
MATERIALS AND METHODS
Mandarins (Citrus reticulata, cultivar Clemenules) were all grown under identical conditions of soil, irrigation and illumination in eastern Spain (Librilla, Murcia). The fruits were collected in winter (December 2004). The fruits from these cultivars were selected based on their diameter, 82.4 ± 0.2 mm, and maturity index, 11.7 ± 0.2.
Mandarin juice was processed in a commercial plant (Murcia, Spain). The juice was obtained using a Premium Juice Extractor (FMC Corporation, Vero Beach, FL) (FMC 2004). This type of machinery leads to a juice with a low content of essential oils (Kimball 2002). A total of 60,000 L of juice was prepared in 12 different batches of 5,000 L each (4 treatments × 3 replicates); thus, three different batches of 5,000 L were prepared for each treatment (packaging materials). All analyses were run, at least, in triplicate, meaning that at least one analysis was carried out in each one of the three different batches belonging to each treatment (packaging material).
The pulp content has a direct effect on the measurement of reflectance, and to avoid this effect, all samples must have similar pulp content (FMC 2004).The fiber contents of all mandarin juice samples were equilibrated by using an FMC FoodTech Quick Fiber device (FMC Corporation); in this way, the juice samples presented a mean value for the centrifugable pulp content of 15.0 ± 0.4 g/L (no significant differences were found among the juice samples). Besides, the mean value for the soluble solid contents (SSCs) of the mandarin juices was 11.7 ± 0.2°Brix, and no adjustment was needed because of the close values of all studied samples.
The juices were processed under aseptic packaging and thermal treatment of pasteurization (86C/20 s). These juices were packaged in three different nontransparent plastic containers (I, II or III) and one transparent plastic container (IV), and were stored under refrigeration (4C) conditions. The juice samples were analyzed on days 0, 18, 36, 54, 72 and 90 (end of storage time). The main materials used in the manufacture of the containers used in this study are described in Table 1. Mandarin juices A and B were stored in containers made of polyethylene and aluminum foil, respectively; juice C was stored in containers made of polyethylene and ethylene vinyl alcohol copolymers; and juice D was stored in bottles made of PET. Besides, all mandarin juices were stored under refrigeration.
Table 1. MAIN MATERIALS USED IN THE MANUFACTURE OF REFRIGERATED MANDARIN JUICE USED
SSC, expressed as °Brix, was determined using a portable refractometer (model C3, Comecta S.A., Barcelona, Spain). Vitamin C (reduced ascorbic acid) was measured following the Association of Official Analytical Chemists Official Method 985.33. Ascorbic acid was estimated by titration with colored oxidation-reduction indicator, 2,6-dichloroindophenol. Ethylenediaminetetraacetic acid (EDTA) was added as a chelating agent to remove Fe and Cu interferences (Horwitz 2000). The physicochemical analyses were run, at least, in six replicates.
Headspace Oxygen Content
Oxygen production was measured in the juice containers by extracting 1 mL of the headspace using a gas syringe, and oxygen was quantified using a Shimadzu model 14A gas chromatograph (Kyoto, Japan) equipped with a thermal conductivity detector and a 3-m stainless steel column with an inner diameter of 3.3 mm containing Chromosorb 102. The column temperature was 55C, and the injector and detector temperatures were 110C. Results were the mean ± standard error of five determinations for each of the replicates used, and were expressed as percentage of oxygen in the headspace atmosphere.
Color determinations were made, at 25 ± 1C, using a Hunterlab Colorflex (Hunterlab, Reston, VA). This spectrophotometer uses an illuminant D65 and a 10° observer as references. A sample cup for reflectance measurements was used (5.9 cm internal diameter × 3.8 cm height) with a path length of light of 10 mm. Blank measurements were made with the cup filled with distilled water against a reference white background.
Color data were provided as CIE L, a, b coordinates (Bower and Baxter 2000), which define the color in a three-dimensional space. L* indicates lightness, and a* and b* are the chromaticity coordinates, green-red and blue-yellow coordinates, respectively (Minolta, 1994). Chroma (C*) and hue angle (hab), defined as and , were also studied (Minolta, 1994); Pérez-López and Carbonell-Barrachina 2006). The color analyses were run, at least, in six replicates.
Reflection spectra of the juice samples were measured, using the color equipment described previously, to determine color intensities in the spectral wavelength range from 595 to 700 nm (580–595, yellow; 595–605, orange; and 605–750, red) (Genard and Bruchou 1992). Measurements were carried out every 10 nm.
Sensory evaluation (hedonic tests) was used to determine the color and flavor acceptability of the mandarin juices. Twenty consumers were recruited with a small advertisement in Alicante and Murcia (35.1% men and 64.9% women, all of them between 18 and 40 years of age). The principal selection criterion was that subjects had to be regular consumers of juice at least twice a week. Details on selection and training of this panel could be found in Pérez-López and Carbonell-Barrachina (2006).
Mandarin juice samples stored under refrigeration in different containers were given to consumers for sensory evaluation at 0, 18, 36, 54, 72 and 90 days of the intensities of different organoleptic attributes: color, fresh mandarin flavor (impressions perceived via the chemical senses from a juice in the mouth) and off-flavors (impressions of nontypical or expected aromatics from a juice in the mouth). The samples (30 mL) were served in 50-mL transparent cups, at a temperature of about 10 ± 2C (Anzaldúa-Morales 1994), and were marked with randomized three digital codes. A complete block design was made, and the juice was presented one by one following a Williams Latin squared design balanced for order and first-order carryover effects to avoid the position error (MacFie et al. 1989).
Measurements were performed in individual booths with controlled illumination (70–80 fc; incandescent lighting) and temperature (23 ± 2C) (Meilgaard et al. 1999). Assessors were trained to rinse their mouths with water, and wait at least for 2 min between samples.
Sensory analysis was conducted in triplicate, and one session was carried out for each sampling day. However, one session consisted of three different subsessions with 1 h of rest between each subsession.
The consumers participated in a ranking test in which the samples of mandarin juice should be sorted out according to their preference for their color and flavor (the subjects were instructed to assign rank 1 to the sample with a more intense orange color, fresh mandarin flavor and off-flavors, and 4 to the sample with a less intense orange color, fresh mandarin flavor and off-flavors. This test was run in triplicate (Meilgaard et al. 1999).
All data were subjected to analysis of variance and the Tukey least significant difference multicomparison test to determine significant differences among mandarin juice samples as affected by packaging materials (Genard and Bruchou 1992). Significance of differences was represented as P ≤ 0.001. Finally, ranking data were analyzed by a Friedman test (ISO 8587:1988). The statistical analyses were performed using SPSS 12.0 (SPSS Science, Chicago, IL), and the figures using Sigma Plot 8.0 (SPSS Science).
RESULTS AND DISCUSSION
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.
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.
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
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.
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)
Significant differences at P < 0.05.
Experimental “t” value.
Values followed by the same letter, within the same column, are not significantly different (P < 0.05), LSD test.
N.S., not significantly different; LSD, least significant difference.
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.
Mandarin juice was packed in four different packaging materials, and its quality was studied during 90 days of storage at 4C. Experimental results proved that cartons consisting of polyethylene, cardboard and an inner layer of aluminum foil provided better results than cartons with an inner layer of ethylene vinyl alcohol copolymers and transparent PET bottles. This high quality of mandarin juice was based on a high vitamin C content (related with lower oxygen content in the headspace of the containers), intense orange color, fresh mandarin flavor and absence of negative off-flavors.