Characterization of Browning Formation in Orange Juice during Storage

Chapter 5

Characterization of Browning Formation in Orange Juice during Storage Downloaded by PURDUE UNIV on November 28, 2016 | http://pubs.acs.org Publication Date (Web): November 18, 2016 | doi: 10.1021/bk-2016-1237.ch005

Laurianne Paravisini and Devin G. Peterson* Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, Minnesota 55108 *E-mail: [email protected]; Current address: Department of Food Science and Technology, 317 Parker Food Science & Technology Building, The Ohio State University, 2015 Fyffe Rd., Columbus, Ohio 43210

In citrus juice products, non-enzymatic browning is the main cause of quality loss contributing to both flavor and color changes during prolonged storage. Ascorbic acid degradation and the Maillard reaction have been well established as the main pathways involved in juice browning. However, the contribution of reaction precursors and especially the role of reducing sugars has remained unclear. In order to gain a deeper understanding into browning mechanisms, the α-dicarbonyl compounds, key building blocks of non-enzymatic reactions, were characterized and monitored in orange juice during a ten week storage study. Results indicated that the α-dicarbonyl content was significantly correlated to the development of the brown color. More specifically, threosone and 3-deoxyglucosone, respectively C4 and C6 α-dicarbonyls, showed the highest correlation with color formation. Further investigations using isotope model experiments demonstrated the key roles of fructose and glucose and, thus, the significant implication of the Maillard reaction in juice browning. This work afforded a better understanding of non-enzymatic browning in juice products.

© 2016 American Chemical Society Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by PURDUE UNIV on November 28, 2016 | http://pubs.acs.org Publication Date (Web): November 18, 2016 | doi: 10.1021/bk-2016-1237.ch005

Introduction Orange juice is one of the most consumed juice products in the world due to its valuable nutrional and highly appreaciated sensory properties. During prolonged storage, from production to consumption, juice undergoes a series of transformations, which include the development of a brown color, leading to a significant loss in quality. Along with flavor, color is one of the most important factors determining consumer acceptance as it is the first sensory cue that consumers experience and triggers the purchasing act. Juice browning has been studied for decades but the mechanisms involved are not adequately defined, limiting sustainable solutions to prevent color formation. Non-enzymatic browning in juice involves the Maillard reaction and ascorbic acid degradation, both of which are responsible for flavor and color changes (1). Most previous studies have focused on the evaluation of quantitative changes in ascorbic acid, glucose and fructose and their relationship to browning during storage. Thus, results often report ascorbic acid degradation as the major pathway of non-enzymatic browning in juice due to the high negative correlation between ascorbic acid loss and the formation of markers of browning (brown color, furfural, 5-hydroxymethylfurfural) (2, 3). The Maillard reaction is often considered to be a minor pathway due to reported constant levels of the main reducing sugars present in juice in addition to unfavorable pH conditions. However, focusing solely on the initial reactants (ascorbic acid and reducing sugars) limits the ability to evaluate the extent of the contribution of each pathway to browning. The current study investigated Reactive Carbonyl Species (RCS), especially the α-dicarbonyls, which are key reactive intermediate building blocks for flavor and color formation via non-enzymatic browning pathways (4). The RCS arising from both ascorbic acid degradation and the Maillard reaction polymerize and/or react with nitrogen-containing compounds to generate the brown pigments. The main objectives of this work were to characterize the α-dicarbonyl composition in commercial orange juice samples, examine the correlation to browning during storage, and identify the precursors of key α-dicarbonyls by utilyzing isotope labeling model experiments.

Materials and Methods Materials Glucose, fructose, glyoxal, methylglyoxal, diacetyl, o-ethylhydroxylamine hydrochloride, o-phenylenediamine were purchased from Sigma-Aldrich (St. Louis, MO). 3-Deoxyglucosone was purchased from SantaCruz Biotech (Dallas, TX). 13C6-Glucose and 13C6-fructose were purchased from Cambridge Isotope Laboratories (Andover, MA). 56 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Orange Juice and Shelf-Life Study Single strength orange juice was provided by PepsiCo (Purchase, NY). After processing, 600 mL-bottles were kept for 1, 2, 4, 8, and 10 weeks in refrigerated (4 °C, control) and heated (35 °C) storage conditions in the dark.


Browning Measurement by UV-Spectroscopy Juice browning was evaluated by measurement of absorbance at 420 nm. Orange juice was centrifuged for 30 min at 9000 rpm and the supernatant was filtered using a 0.45 µm PTFE filter (MicroSolv, Leland, NJ). Absorbance was measured with a Libra S11 US/VIS spectrophotometer (Biochrom US, Holliston, MA) in 10 mm cells against water at 420 nm. Measurements were performed in duplicate. Sample Preparation Juice samples were spiked with lactose at 5 g/L, used as an internal standard for sugar quantification and centrifuged for 30 min at 9000 rpm. The supernatant was collected and cleaned by Solid Phase Extraction (SPE) prior to derivatization. A 500 mg DSC-18 cartridge (Supelco, Bellefonte, PA) was preconditioned with methanol and nanopure water, 5 mL of sample were loaded onto the cartridge. The permeate was collected and the cartridge was further washed with 1 mL of 0.1% aqueous formic acid solution:methanol (95:5). Both permeate and wash were collected and pooled for analyses. The pooled solution was separated into two aliquotes. One aliquot was used for α-dicarbonyl quantification and was immediately derivatized by adding a methanolic solution of o-phenylenediamine at 0.5 M following incubation at 35 °C for two hours. The other aliquot was diluted 200-fold using nanopure water for quantification of the sugars. Finally, all samples were filtered using a 0.45 µm PTFE filter (MicroSolv) and stored at -20 °C prior to analyses. Identification and Quantification of α-Dicarbonyls in Orange Juice by Liquid Chromatography/Mass Spectrometry (LC/MS) Derivatized samples were analyzed using an Acquity® UPLC system equipped with a Cortecs UPLC C18+ column (50 mm x 2.1 mm i.d., 1.7 µm) (Waters Co., Milford, MA) and coupled with a Xevo G2 Q-ToF Mass Spectrometer (Waters Co.). The mobile phase was composed of solvent A (water + 0.1% formic acid) and solvent B (methanol + 0.1% formic acid). Separation was performed using the following gradient elution: 0−5 min, 80−30% A; 5−5.5 min, 30−0% A; 5.5-6 min 0-0% A; then equilibration in the initial conditions for 1 minute. Two µL were injected at a mobile phase flow rate of 0.4 mL/min, and the column temperature was maintained at 30 °C. MS data collection was performed using electrospray ionization in positive mode (ESI+) and MSE acquisition mode, optimal for structural characterization. Alternating collision energies were set at 10 eV and 30 eV, cone voltage was 40 eV. Source desolvation 57 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

temperatures were set at 120 °C and 500 °C, respectively. Identification was confirmed using MS fragmentation data in comparison with literature. Injection of a pure standard was done when the compound was commercially available. For quantification, MS data were collected using multiple reaction monitoring (MRM) mode. Quantification of seven α-dicarbonyls was performed according to a method previously reported (5). MRM conditions are given in Table 1.


Table 1. Optimized multiple reaction monitoring (MRM) conditions for quantification of seven α-dicarbonyl compounds identified in orange juice and detected as quinoxaline derivatives in positive electrospray ionization (ESI+) α-Dicarbonyl compounds

Parent Ions (m/z)

Cone Voltage (V)

Collision Energy (V)

Sibling Ions (m/z)

1- and 3-deoxythreosone

175

30

10

157

xylosone

221

50

25

161/173

threosone

191

35

15

173

3-deoxyglucosone

235

35

19

199

glyoxal

131

35

28

77

methylglyoxal

145

35

28

77

diacetyl

159

35

30

77

Quantification of Glucose and Fructose by Liquid Chromatography/Mass Spectrometry (LC/MS) Two uL of sample were injected on an Acquity® UPLC system equipped with a BEH amide column (150 mm x 2.1 mm i.d., 1.7 µm) (Waters Co.) and coupled with a Quattro Premier XE Mass Spectrometer (Waters Co.). MS was operated in MRM mode with the following transitions and conditions for the analytes of interest: glucose (ESI-) 176→89, cone voltage 30 eV, collision energy 8 eV; fructose (ESI-) 176→89, cone voltage 30 eV, collision energy 8 eV and lactose (ESI+) 343→163, cone voltage 33 eV, collision energy 9 eV. The mobile phase was composed of solvent A (30% acetonitrile + 70% water + 0.1% ammonium hydroxide) and solvent B (80% acetonitrile + 20% water + 0.1% ammonium hydroxide). Separation was performed using the following gradient elution: 0−5 min, 0−40% A; 5−5.5 min, 40−40% A; then equilibrated in the initial conditions for 1 minute. Mobile phase flow rate was 0.3 mL/min, and the column temperature was maintained at 30 °C. Isotope Model Experiments Based on sugar quantification results, known amounts of labeled precursors (13C6-glucose and 13C6-fructose) were added separately in orange juice in a ratio of 58 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


unlabeled:labeled of about 2:1. The samples were then aged at 35 °C for 4 weeks. Composition of the models is reported in Table 2. An aseptic process, adapted from Kokkinidou and Peterson (5), was simulated to prevent microbial development and inhibit enzymatic activity. A Combi-Pal auto-sampler (CTC Analytics, Zwingen, Switzerland) equipped with a cooling tray and agitated heating station was used as a bench-top aseptic processing system. Five mL of juice were introduced in a 20 mL headspace vial and placed in a chilled vial holder set at 10 °C. Vials were then transferred to an agitator set at 200 °C and held for 75 seconds to allow the samples to reach a final temperature between 90-95 °C for five seconds. Samples were then returned to the cooler at 10 °C and finally placed in the refrigerator to cool down to 4 °C. α-Dicarbonyl compounds were extracted and analyzed following the protocol described above. Isotopomeric ratios were calculated taking into account the intial amount and the rate of formation based on the shelf-life study data. Experimental ratios were compared to the theoretical ratios to estimate the contribution of glucose and fructose to the formation of α-dicarbonyl compounds.

Table 2. Composition of isotope models, i.e., orange juice (OJ) enriched with 13C6-glucose and 13C6-fructose (concentration is reported as the average of triplicates ± standard deviation). Samples

Native concentrations (g/L)

Concentrations in models (g/L)

glucose

fructose

13C6-glucose

13C6-fructose

control OJ

28.3 ± 4.6

31.8 ± 5.3

-

-

OJ+13C6-fructose

28.3 ± 4.6

31.8 ± 5.3

15.0

-

OJ+13C6-glucose

28.3 ± 4.6

31.8 ± 5.3

-

14.1

Results and Discussion Characterization of the α-Dicarbonyl Composition in Orange Juice During Storage In the first part of this study, the α-dicarbonyl composition in orange juice was characterized by UPLC/MS/MS in juice stored at 35 °C for 4 weeks. α-Dicarbonyl compounds are highly reactive species involved in non-enzymatic browning reactions arising from both ascorbic acid degradation and the Maillard reaction. Seven α-dicarbonyl compounds were identified and reported in Table 1. The characterization of α-dicarbonyls from sugars and ascorbic acid breakdown in various model systems have been described previously (6–8), but the characterization of a single strength orange juice during storage and observed color development is reported here, to the best of our knowledge, for the first time. Methylglyoxal and glyoxal have been previously reported in browned lemon juice, in which ten other carbonyls were also detected (4). In order to gain insight regarding the potential contribution of α-dicarbonyls to juice browning, their quantitative changes were further monitored for 10 weeks of storage under refrigerated (4 °C) and accelerated (35 °C) conditions 59 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


(Figure 1). The total α-dicarbonyl content in the control sample, i.e. immediately after thermal processing, was 23.3±2.3 mg/L. Initial levels of α-dicarbonyls in the control sample were suspected to be primarly generated from the thermal processing pasteurization step. After eight weeks of storage, the maximum concentration of α-dicarbonyls was reported at 62.1±4.5 mg/L and 34.6±1.6 mg/L for samples stored at 35 °C and 4 °C, corresponding to an increase of 3 and 1.5-fold, respectively, when compared to the initial control sample. Threosone and 3-deoxyglucosone (3-DG), respectively C4 and C6 compounds, appeared to be the main α-dicarbonyls in orange juice, constituting approximately 60% of the total content (Table 3).

Figure 1. Total α-dicarbonyl contents (seven compounds) in orange juice during storage for 10 weeks at 4 °C and 35 °C (concentration is reported as the average of triplicates ± standard deviation).

Browning in juice was evaluated by measuring the absorbance at 420 nm. No significant changes in absorbance were observed in the juice sample stored at 4 °C with values ranging from 0.291±0.007 for the control and 0.310±0.021 after 10 weeks of storage (Figure 2). These results indicated that the observed chemical changes in the system, i.e. slight increase in α-dicarbonyl contents (23.3±2.3 vs 34.6±1.6 mg/L; significance t-test p=0.0022), were not enough to render significant changes in color, remaining stable under refrigerated conditions. However, after 10 weeks of storage at 35 °C, development of brown color was evident along with a significant increase in absorbance, from 0.291±0.007 to 0.490±0.001 (Figure 2).

60 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Table 3. Quantified levels of α-dicarbonyl compounds in orange juice during a 10 weeks storage period at 35 °C (concentration is reported as the average of triplicates ± standard deviation).


α-Dicarbonyl compounds

a

Storage time at 35 °C (weeks) Control

1

2

4

8

10

1- and 3-deoxythreosonea

2.2 ± 0.1

2.5±0.1

2.3 ± 0.2

2.3 ± 0.2

2.8± 0.1

2.5 ± 0.2

xylosonea

1.1 ± 0.1

1.4± 0.2

1.3± 0.1

1.3 ± 0.1

1.6 ± 0.1

1.5 ± 0.1

threosonea

8.2± 0.9

19.8± 5.5

16.8± 2.1

18.5± 0.5

29.6± 3.8

29.7± 4.3

3-DG

4.8± 0.3

6.3± 0.1

6.5± 0.6

8.7± 0.1

16.2± 1.0

18.3± 2.1

glyoxal

2.5± 0.3

4.0± 0.6

3.1± 0.2

3.3± 0.1

5.2± 0.4

3.5± 0.5

methylglyoxal

0.9± 0.1

1.1± 0.1

1.0± 0.1

1.0± 0.1

1.4± 0.1

1.1± 0.1

diacetyl

3.7± 0.6

3.3± 0.4

2.5± 0.2

2.6± 0.3

5.3± 0.5

3.5± 0.6

TOTAL

23.3± 2.3

38.3± 7.7

33.5± 2.9

37.8±0.6

62.1± 4.5

60.2± 6.6

quantified using 3-deoxyglucosone response factor

Figure 2. Absorbance at 420 nm in orange juice during storage for 10 weeks at 4 °C and 35 °C (concentration is reported as the average of duplicates ± standard deviation).



Pearson correlation coefficients were computed to assess the relationship between the α-dicarbonyls and the absorbance during storage of the orange juice. Significant positive correlation was obtained between the total α-dicarbonyl content and the absorbance (r=0.888; p=0.018). This observed relationship was driven by high correlation between the levels of the two main carbonyls, threosone and 3-DG, and the absorbance values (threosone r=0.840; p=0.034; 3-DG r=0.960; p=0.002). Similar findings have been previously reported in honey monitored in samples from different botanical origins. A correlation between the color of honey and the α-dicarbonyl content was demonstrated; dark honeys had higher α-dicarbonyl contents than lighter colored honeys (9). It can be hypothesized that along their formation, reactive carbonyls species are involved in the formation of brown polymers. A positive correlation likely corresponds to different kinetics of formation between the carbonyls and the brown polymers, i.e. carbonyls are likely formed at higher rate than the polymers. In juice products, the presence of α-dicarbonyls in the early stages of storage of lemon juice has been demonstrated and indicated their important association with the initiation of non-enzymatic browning (4). In Maillard model systems, the implication of α-dicarbonyls in color formation has been widely demonstrated (10, 11). In sucrose models, investigations clearly indicate that individual α-dicarbonyls are involved in formation of specific molecular weight colored fractions. For example, 3-DG is mainly associated with the formation of low molecular weight fractions whereas methylglyoxal promotes the formation of higher molecular weight fractions (12). As a conclusion of the first part of this study, in accordance with literature, the correlation between α-dicarbonyls and color formation was clearly demonstrated. For the first time, threosone and 3-DG are reported as key α-dicarbonyls in orange juice browning, with the highest concentrations among the RCS and a positive correlation with color formation. Investigation of the Role of Reducing Sugars as Precursors of Key Carbonyls During storage of orange juice, α-dicarbonyls could arise from both ascorbic acid degradation and the reducing sugars involved in Maillard reaction pathways. Previously, diacetyl, methylglyoxal, glyoxal, threosone and 3-deoxythreosone have been identified in model systems as products from ascorbic acid degradation induced by heat treatment via an oxidative pathway (6). These same α-dicarbonyls are also known to be major intermediates arising from the breakdown of glucose and fructose (13). In order to identify the main precursors of the identifed α-dicarbonyls in this study, and bring new insights regarding the key precursors of browning in orange juice, isotope model experiments were performed utilizing single strength orange juice. Reaction pathways in foods are commonly investigated in simplified model systems to provide a basis to control cofounding effects, such as the reaction composition. Even though model systems can provide relevant insight on chemical pathways, they override the effect of the food matrix and can lead to erroneous conclusions. In this study, stable isotope labeled precursors were added to a commercial single strength orange juice to monitor the chemistry and 62 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


fate during browning. To define the contribution of Maillard-type precursors toward the generation of RCS, glucose and fructose were chosen because they are the main reducing sugars present in orange juice (14). Model experiments were prepared by supplementing 13C-glucose and 13C-fructose at half their native levels in orange juice. Figure 3 illustrates the mass spectra obtained by LC/MS of threosone (m/z 191) in orange juice spiked with 13C6-fructose, and its +4 isotopomer (m/z 195). In this example, the ratio 13C4-threosone/threosone is equal to the theorical ratio expected if fructose contribute to 100% of the formation, meaning a ratio of 1:2 due to the fact that half of the amount of labeled fructose was added compared to natural occurring unlabeled fructose. Consequently, fructose was identified as the main precursor of threosone in the juice system whereas glucose does not appear to contribute significantly. On the other hand, 3-deoxyglucosone showed an equivalent contribution of glucose and fructose with an incorporation of the isotope of 80 and 70%, respectively. It was noted that the sum of the contribution from glucose and fructose in the different models can exceed 100% considering that the initial quantities of sugar in those models were higher than the native amount present in the juice. However, this still provides an improved understanding of the formation pathways and allows for the identification of the main precursors involved in the generation of targeted carbonyls.

Figure 3. LC/MS (ESI+) spectra of threosone (m/z 191) and its 13C6-isotopomer (m/z 195) detected in control orange juice (i) and orange juice model enriched in 13C6-fructose (ii) 63 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


These results suggest the Maillard reaction as the main formation pathway for these two key α-dicarbonyls, which are positively correlated with browning. 3-DG is usually formed during the second step of the Maillard reaction, from the degradation of the Amadori and Heyns rearrangement products. The shorter sugar fragments, on another hand, can be formed from either the Maillard reaction or the sugar breakdown directly. A suggested pathway for threosone is from 1-deoxyglucosone (1-DG) via α-dicarbonyl cleavage with the C4-enediol as the reactive intermediate (15). Other authors suggested the formation of threosone from erythrose though the breakdown of D-glucosone (16). The utilization of isotope enriched models allowed for the identification of glucose and fructose as main precursors of threosone and 3-DG, potential key reactive intermediates in non-enzymatic browning of orange juice. Further work is ongoing to quantitatively define the significance of the selected carbonyls on color development.

Conclusion The α-dicarbonyl composition in aseptically processed orange juice was characterized. Seven main carbonyl intermediates were identified and monitored during storage at 4 °C and 35 °C during a 10 week shelf-life study. Results revealed that α-dicarbonyl content was positively correlated to the development of the brown color in orange juice stored at 35 °C. More specifically, threosone and 3-deoxyglucosone showed the highest significant correlation with color formation. Isotope model experiments performed in orange juice identified fructose and glucose as the key precursors of those α-dicarbonyl compounds and, thus, were proposed as key contributors of browning. Moreover, these results implied that the Maillard reaction may be a more important contributor to juice browning than previously suggested. In future steps, the use of natural chemistry and carbonyl trapping agents will be investigated in order to develop novel technologies to control color formation in juice.

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Characterization of Browning Formation in Orange Juice during Storage

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