3.1. Physicochemical Characteristics
The results for pH and titratable acidity of the juices are presented in Table 1
. The mixed formulations (BOMJ 1:1 and 1:2) presented titratable acidity (0.41%–0.59% citric acid) with intermediate values between beet juice (BJ or PBJ; 0.08%–0.14% citric acid) and orange juice (POJ; 0.77%–0.79% citric acid) (p
≤ 0.05), as expected. The same trend was observed for pH values, where the mixed formulations (BOMJ 1:1 and 1:2) presented pH (4–4.21) with intermediate values between beet juice (BJ or PBJ; 4.84–5.71) and orange juice (POJ; 3.83–3.88) (p
≤ 0.05). The presence of higher quantities of orange juice (BOMJ 1:2) in the mixed juice resulted in more acidic products, with a decrease in the pH values and an increase in the titratable acidity (p
≤ 0.05) when compared to the product with lower orange content (BOMJ 1:1). The pH and titratable acidities of the beet and orange juices are similar to those reported by other authors [18
Organic acids contribute to the flavor and palatability of fruit juices. The greater acidity of the mixed juices (BOMJ 1:1 or BOMJ 1:2) when compared to the beet juices (BJ or PBJ) could protect them from the development of food spoilage micro-organisms, increasing their shelf life [19
]. However, it can decrease the sensory acceptance of the products, because consumers are not attracted to very acidic products [20
]. It is noteworthy, however, that the mixed juices (BOMJ 1:1 and BOMJ 1:2) still had lower acidity than the conventional orange juice (POJ).
During the storage period, the titratable acidity increased (p
< 0.05) in the BJ juice, decreased in the mixed juices (BOMJ 1:1 and BOMJ 1:2), and was maintained in the PBJ and POJ juices (p
> 0.05). For the pH values, the products with beet (BJ and PBJ) exhibited a decrease (p
≤ 0.05) in pH values during storage, and the products with added orange (POJ, BOMJ 1:1, and BOMJ 1:2) maintained (p
> 0.05) this parameter. Probably, the presence of orange juice (POJ, BOMJ 1:1, and BOMJ 1:2) resulted in an increase in the buffering capacity of the juices, maintaining the pH of the products [20
], which did not occur in the juices with beet only.
The color parameters (L*, a*, b* and Croma) are shown in Table 2
. The total color difference is presented in Figure 1
. The initial values of a* for BJ (0.78) and PBJ (0.95) indicate that the beet-added juices had a slightly red-purple color, probably originating from the presence of betacyanin. The b* values for BJ (1.56) and PBJ (1.67) represent a slight predominance of yellow by the presence of betaxanthins [21
]. The POJ juice presented values of L* (36.61), a* (−2.49), and b* (16.34), demonstrating the yellow color of the orange juice. The mixed juices presented color characteristics of both juices (beet and orange) (L* = 23−24; a* = 4.59−8.76; and b* = 2.19−2.75).
During the storage period, all formulations had the same behavior for the L* and a* parameters, as the products stored for 30 days had higher L* and a* values than the newer ones (day 0) (p
≤ 0.05). These results indicate that the juices became redder and with a lighter color. The juices (BJ, PBJ, POJ, BOMJ 1:1, and BOMJ 1:2) also had a decrease in the parameter b*, demonstrating a discoloration of the yellow color. In the POJ juice, a more intense yellow color was observed (i.e., an increase in b* values), and the total difference in color (∆E*ab) was the greatest (6.25). Therefore, the beet juice made the color of the juices more stable to the effects of refrigerated storage. The alterations in the color parameters were probably caused by the oxidative and non-oxidative reactions of polyphenols, resulting in colored condensation products. Furthermore, the Maillard Reaction or melanoidins’ formation could cause the alteration [20
The physicochemical results demonstrated that the presence of orange juice in the mixed juices improved the pH stability of the products to the effects of refrigerated storage, as demonstrated by the maintenance of the pH of the orange and mixed juices. The alterations in the titratable acidity were slight (0.2% citric acid for BOMJ 1:1 and 0.05% citric acid for BOMJ 1:2). On the other hand, the presence of beet juice improved the color stability of the products, as the mixed juices showed a reduction of only 7%–10% in yellow color intensity and an increase in color purity (Chroma) by 22%–27%, probably due to the betalains’ stability. These results reinforced the viability of the production of mixed juices with orange and beet. The physicochemical stability is desirable, because it confirms that the products remain similar to those that are newly manufactured, even after several weeks of storage [19
3.2. Ascorbic Acid Content
The ascorbic acid content is shown in Figure 2
. The initial values were 37.19, 25.93, and 39.84 mg/100 mL for POJ, BOMJ 1:1, and BOMJ 1:2, respectively, with the mixed juice with a higher quantity of orange juice (BOMJ 1:2) presenting higher ascorbic acid content than the other mixed juice (BOMJ 1:1), as expected. The pasteurized orange juice (POJ) showed an ascorbic acid loss of 25% after 30 days of storage (p
≤ 0.05). The ascorbic acid stability is influenced by many factors, such as storage temperature, concentration of salt, sugars, and minerals, pH, oxygen levels, and the presence of enzymes or light [22
]. However, the final values were in compliance with the legislation that established a minimum limit of ascorbic acid of 25 mg/100 mL for juice [23
In the mixed juices (BOMJ 1:1 and BOMJ 1:2), a rapid loss of the ascorbic acid content occurred, with reduction right after 5 days of storage (p
≤ 0.05). In BOMJ 1:1, the ascorbic acid content was completely lost at 15 days of storage, whereas in the BOMJ 1:2 juice a very low content was observed at day 30. The ascorbic acid can act as an antioxidant, removing the oxygen and controlling the activity of polyphenol oxidases enzymes, increasing the stability of the pigments [24
]. In this study, it is possible that the decrease in the ascorbic acid content in the mixed juices is associated with a greater stability of the betalains pigments.
3.3. Betalain Content
shows the results for the betacyanin and betaxanthin content during the refrigerated storage of the juices. The initial betacyanin content (Day 0) in pasteurized pure beet juice (PBJ) (53.39 mg/100 mL) was lower than the content in the raw beet juice (BJ) (65.32 mg/100 mg), showing that the pasteurization caused a reduction in betacyanins by 18% (p
≤ 0.05). For the betaxanthins, the thermal treatment had no significant effect (p
The betacyanin and betaxanthins content of the mixed juices BOMJ 1:1 (31.12 mg/100 mL and 11.96 mg/100 mL) and BOMJ 1:2 (22.31 mg/100 mL e 8.80 mg/100 mL) were lower (p ≤ 0.05) than those of the juices only with beet (BJ with 65.32 mg/100 mL and 29.99 mg/100 mL and PBJ with 53.39 mg/100 mL e 31.76 mg/100 mL). These results were expected, due to the dilution of the beet juice with the orange juice, proportional to each mixed juice formulation.
At the end of the storage time, the beet juices (BJ and PBJ) showed a significant reduction of 37–41% in betacyanin content, while for the betaxanthins, a reduction of 19–24% was observed. In the mixed juices (BOMJ 1:1 and BOMJ 1:2), the betacyanin (22.31–31.63 mg/100 mL) and betaxanthin (7.82–12.08 mg/100 mL) content was maintained, when comparing the products at 0 and 30 days of storage (p > 0.05). As discussed before, the results demonstrate that the orange juice contributed to the stability of both classes of betalains in the mixed juices, probably due to the action of the citric acid and the ascorbic acid. The citric acid is an antioxidant and chelating agent, and acts synergistically with the ascorbic acid. The maintenance of these beneficial compounds in the mixed juices is of primary importance in the development of functional foods.
3.4. Total Phenolic Compounds Content and Antioxidant Activity
Total phenolic content and the antioxidant activity (DPPH Trolox or ABTS trolox) are presented in Table 4
. All juices were significant sources of polyphenols. The raw beet juice (BJ) and the pasteurized beet juice (PBJ) presented 521 and 497 µg EAG/mL, respectively; the mixed juices had 484–485 µg EAG/mL; and the orange juice (POJ) had 448 µg EAG/mL at day 0. In fact, orange juices are good sources of polyphenolic compounds, including hydroxycinnamic acids and flavonoids (mainly flavonas) [25
], while beet juice has a high antioxidant action and is a rich source of polyphenols. Among the beet extract phenolic acids, the 4-hydroxybenzoic acid is the main constituent, followed by the cynamic, vanillic, and chlorogenic acids, the trans ferulic acid, and caffeic acid [2
During storage time, the beet juices (BJ and PBJ) showed a significant loss (p ≤ 0.05) of phenolic compounds by 14% and 13%, respectively, while the orange and mixed juices did not show any significant variation (p > 0.05) when comparing the products at day 0 and 30 of storage. This was probably due to the joint action of the citric and ascorbic acids, which inhibited the oxidative reactions of the phenolic compounds presented in the juices, demonstrating the beneficial effect of the addition of orange juice to the beet juice.
In both methodologies of antioxidant activity evaluation (DPPH and ABTS methods), the samples with greater antioxidant activity were the raw (2733 and 2179 µg Trolox/mL) and pasteurized beet (2813 and 2081 µg Trolox/mL) juices, and the sample with the lowest antioxidant activity was the orange juice (1408 and 1872 µg Trolox/mL). In fact, betalains’ antioxidant activity is greater than that of ascorbic acid, and beet is one of the most powerful green vegetables with regard to antioxidant activity [26
], when beet juice is compared to other fruits that are well-accepted as having high antioxidant contents, such as pomegranate and cranberry [27
]. The mixed juices had intermediated antioxidant activity, with 1930–2083 µg Trolox/mL in the DPPH method and 1840–1854 µg Trolox/mL in the ABTS method.
During storage, the antioxidant activity decreased in all of the juices evaluated in one or both methodologies (DPPH or ABTS) (p ≤ 0.05). Pasteurized beet juice (PBJ) showed the greatest loss (37%) in the DPPH method, followed by POJ (32%), BJ (21%), BOMJ 1:2 (19%), and BOMJ 1:1 (no significant loss). In the ABTS methodology, the decrease was: POJ (21%), PBJ (14%), BOMJ 1:1 (13%), and BOMJ 1:2 (12%). The decrease in ascorbic acid and total phenolic compounds is the probable cause for the reduction in antioxidant capacity in the juice samples analyzed in this study. Therefore, this study demonstrated that the orange juice maintained the stability of the phenolic compounds, and reduced the loss of the antioxidant activity of the juices.
3.5. Sensory Acceptance
Consumer acceptance is presented in Table 5
. Pasteurized beet juice (PBJ) acceptance was lower (p
≤ 0.05) than that of the mixed juices (BOMJ 1:1 and BOMJ 1:2) with regard to the attributes aroma, flavor, and overall acceptance. For the color attribute, it was not different from the BOMJ 1:2 (p
> 0.05). The PBJ juice had satisfactory acceptance in the color parameter (a score of 7.5 in a 9-point hedonic scale), but, in aroma, flavor, and overall acceptance, the scores were low (<5.9).
The addition of orange juice to the beet juice increased the acceptance of the products in all evaluated parameters (color, aroma, flavor, and overall acceptance). The mixed juices received scores between 6.3 and 8 in a 9-point hedonic scale, indicating that the consumers liked the products from slightly to much. The color was the attribute with the highest score (7.8–8), probably because of the intensity and attractiveness that the beet gives to the product due to the presence of betalains.
The results of the present study demonstrate the applicability of the mixed juices, as the products were well accepted. The pasteurized beet juice (PBJ) had a higher content of betacyanin, betaxanthin, total phenolic content, and antioxidant activity; however, it was not well accepted by consumers, and the functional components were less stable to the effects of refrigerated storage in this juice.
Currently, several studies have focused on the antioxidant capacity and consumption benefits of the fruit and green vegetables juices, including beet juice [2
]; however, there are no data related to the sensory acceptance of these types of beverages by population.