Ascorbic Acid Degradation in a Model Apple ... - Wiley Online Library

Ascorbic Acid Degradation in a Model Apple Juice System and in Apple Juice during Ultraviolet Processing and Storage Rohan V. Tikekar, Ramaswamy C. Anantheswaran, and Luke F. LaBorde

Abstract: Ultraviolet radiation induced degradation of ascorbic acid in a model apple juice system and in apple juice

was studied using a collimated beam batch UV reactor. In the model system, ascorbic acid degradation was more rapid at higher dose levels and the reaction accelerated with increasing exposure time. Ascorbic acid degradation significantly (P < 0.05) increased as the pH was raised from 2.4 to 5.5, although no difference was observed between 2.4 and 3.3. Increasing malic acid concentration between 0.1 and 1%, increased ascorbic acid degradation (P < 0.05) although there was no difference between 0.5 and 1.0%. Solution absorbance, varied by addition of tannic acid, decreased ascorbic acid degradation with increasing concentration due to absorption of UV radiation. Fructose at levels found in apple juice significantly increased ascorbic acid degradation while glucose and sucrose did not. Factors identified that accelerate ascorbic acid degradation may at least partially explain why ascorbic acid degradation occurred more rapidly in UV-treated apple juice than in the 0.5% malic acid model system. Ascorbic acid degradation continued after UV treatments during dark storage. Storage decreases were faster at higher initial UV dose levels and higher storage temperature. Keywords: fruits and vegetables, nonthermal processing, ultraviolet radiation, vitamin C

Practical Application: The present study shows the effect of UV processing on ascorbic acid, a key vitamin found in many

fruit juices. Process developers and researchers can use this study as a model for designing experiments to identify factors that influence the stability of vitamin C and other bioactive compounds during UV processing.

Introduction

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Because UV penetration is inversely proportional to the absorbance of the medium, the dose required to achieve a given microbial lethality treatment is affected by the presence of UVabsorbing juice constituents. Soluble chemical compounds such as polyphenols, proteins, and vitamins in juice absorb UV radiation while insoluble substances such as protein, carbohydrates, and larger tissue particulates block or scatter radiation (GuerreroBeltran and Barbosa-Canovas 2004; Koutchma 2008). Up to 3 log reductions in total aerobic plate counts have been reported in UV-treated orange juice, guava juice, guava/pineapple mixes, and mango nectars although doses required to achieve a given log reduction vary considerably due to variation in turbidity levels and UV-absorbing constituents (Tran and Farid 2004; Murakami and others 2006; Keyser and others 2008). A commercially available continuous flow UV system (CiderSureTM FPE, Rochester, N.Y., U.S.A.) is capable of reducing Escherichia coli O157:H7 and Cryptosporidium parvum populations in apple cider by 5 log units after treatment with 14.3 mJ/cm2 of UV radiation (Koutchma and others 2004; Quintero-Ramos and others 2004). This technology is therefore suitable for meeting the pathogen reduction standard established in the Juice HACCP regulation (FDA 2001). Ascorbic acid (2-(1,2-dihydroxyethyl)-4,5-dihydroxy-furan-3one) is naturally present in some fruit juices or is added to enhance nutritional appeal. Although apples are not a significant MS 20100882 Submitted 8/3/2010, Accepted 11/26/2010. Authors are with source of vitamin C (USDA 2009), most commercially availDept. of Food Science, the Pennsylvania State Univ., Univ. Park, PA 16802, U.S.A. able apple juices are fortified to contain one or more Reference Direct inquiries to author LaBorde (E-mail: [email protected]). Dietary Intake (RDI) daily values (21CFR101.9) of vitamin C

Ultraviolet (UV) processing of foods is an emerging nonthermal technology for enhancing the quality and safety of foods. Applications studied include extension of the shelf life of fresh fruits and vegetables (Gonzalez-Aguilar and others 2001; Fonseca and Rushing 2006), increased phytochemical content in fresh fruits (Cantos and others 2000), and sanitization of food contact surfaces (Guerrero-Beltran and Barbosa-Canovas 2004). UV radiation has long been used to decrease microbial levels in drinking water (Legrini and others 1993). However, it has more recently been studied as a food processing technique for juice products (Koutchma 2009). The UV region of the radiation spectrum consists of a range of wavelengths between 200 and 400 nm. This region is divided into 3 types—UV-A (315 to 400 nm), UV-B (280 to 315 nm), and UV-C (200 to 280 nm) (Sastry and others 2000). UV processing methods include pulsed UV systems that utilize very high intensity radiation at wavelengths between 200 and 400 nm and continuous monochromatic UV systems where almost 90% of the energy is from a single wavelength (Tikekar and others 2010). Continuous UV-C radiation at 254 nm, which has germicidal activity, is used in this study.

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R C 2011 Institute of Food Technologists doi: 10.1111/j.1750-3841.2010.02015.x

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Effect of UV processing on ascorbic acid . . . (1 RDI = 60 ppm). The chemistry, physiological role in the body, and the function of ascorbic acid as an antioxidant in foods have been extensively studied (Cameron and others 1979; Englard and Seifter 1986; Buettner and Jurkiewicz 1996). Ascorbic acid degradation in aqueous foods is a complex process that includes multiple free radical reactions (Gregory 2008). When exposed to UV radiation, molecular excitation is induced and photochemical degradation reactions occur (Kagan 1993). UV induced oxidation of ascorbic acid to ascorbic radicals has been demonstrated in animal (Jurkiewicz and Buettner 1994, 1996) and plant (Hideg and others 1997) tissues, and reduced UV skin damage from topical application of ascorbic acid has been attributed to quenching of free radicals (Shindo and others 1993; Fuchs and Kern 1998). There have been few studies on ascorbic acid losses in UVtreated juice products. Tran and Farid (2004) reported a 17% loss in ascorbic acid after exposure of orange juice to 100 mJ/cm2 UV radiation. Ascorbic acid destruction was found to be directly related to the applied dose. In a study by Adzahan (2006), UV treatment of apple cider using the CiderSureTM system at a dose level of 14.3 mJ/cm2 (equivalent to a 5-log reduction of E. coli O157:H7) resulted in a 30% reduction in vitamin C. Koutchma (2009) described experiments in which approximately 50% ascorbic acid degradation occurred in enriched apple juice after 3 passes through R a CiderSure 1500 UV system set at the lowest flow rate possible. The author also reported that ascorbic acid degradation was greater in apple juice compared to orange juice at identical applied UV doses, presumably due to the greater amount of radiation absorbing suspended solids in the orange juice. These studies demonstrated that, similar to the effect of UV treatments on microbial populations, the extent to which ascorbic acid degradation occurs depends on the dose level and the physical and chemical properties of the juice. There have been no studies on the individual effects of juice chemical constituents on UV-induced ascorbic acid degradation in apple juice. The objective of this article is therefore to study the kinetics of ascorbic acid degradation in apple juice and a juice model system during UV-C processing and subsequent storage.

U.S.A). Caramel solution (Product 050) was obtained from D.D. Williamson Inc. (Louisville, Ky., U.S.A.). Acetonitrile and water used to prepare the high-performance liquid chromatography (HPLC) mobile phase were HPLC grade from Fisher (Fair Lawn, N.J., U.S.A.). Formic acid obtained from Fisher was ≥ 95% pure.

UV apparatus A collimated bench-top batch UV reactor was used for all experiments (Figure 1). The apparatus consisted of 3 UV lamps (254 nm, 10 W, Atlantic Ultraviolet Inc., Hauppauge, N.Y., U.S.A.) mounted within a shielded horizontal cylinder fitted over a vertical tube (100 mm dia. × 100 mm length). The inside surface of the vertical tube was painted with nonreflected black paint to increase the perpendicularity of radiation rays reaching the sample surface. Based on the measured distance between the radiation source and the bottom of the tube, the theoretical deviation of radiation rays from vertical was no greater than 20◦ . Incident intensity at the sample surface was measured with a digital radiometer (Black-Ray Model J225, UVP Inc., Upland, Calif., U.S.A.) placed at the bottom of the tube at a length equal to the distance between the radiation source and the sample surface. Variation of incident intensity over the sample surface area was less than 1% and experimental error was further minimized by continuously stirring the reaction solution with a mechanical stir bar (2 mm × 10 mm, 300 rpm). Variation in incident intensity was minimized by allowing the lamps to warm up for at least 30 min prior to treatments. Over the course of all experiments, incident intensities ranged between 1.2 and 1.8 mW/cm2 .

Reaction media Experiments were conducted using a model system or apple juice. The model system, unless otherwise stated, consisted of a 0.5% (w/v) solution of malic acid at pH 3.3. Values for malic acid concentration and pH were selected based on compositional surveys of apples (Mattick and Moyer 1983; Lee and Wrolstad 1988; Jeuring and others 1979). Solution pH was adjusted using 1 N NaOH or HCl. Unpasteurized apple cider was obtained from a Materials and Methods local cider producer and was clarified by centrifugation (BeckChemicals man Coulter Model Avanti J-26 XPI, Fullerton, Calif., U.S.A.) at Ascorbic acid, malic acid, tannic acid, sucrose, glucose, and 15000 g for 45 min. The maximum juice turbidity was 3 Nephelofructose were obtained from Sigma Aldrich (St. Louis, Mo., metric Turbidity Units as determined with a digital turbidimeter


Figure 1–Collimated beam batch UV reactor used in experiments.

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Effect of UV processing on ascorbic acid . . . R

(Hach ; Model 2100P, Hach Inc., Loveland, Colo., U.S.A.). The where D = UV dose (J/cm2 ), I = incident intensity (W/cm2 ), clarified juice was stored at –15 ◦ C until use. and t = time (s). Ascorbic acid degradation was described using the 2nd-order polynomial function:

Sample preparation and treatment The effects of individual juice constituents were studied by adding appropriate amounts of ascorbic acid, malic acid, tannic acid, sucrose, glucose, or fructose to the 0.5% malic acid model system. Unless otherwise stated, the pH of the solutions was adjusted to pH 3.3. Initial absorbance values for each constituent at experimental concentrations were measured spectrophotometrically at 254 nm (Helios Gamma, Thermo Scientific, Waltham Mass., U.S.A.). When values were outside the range of the spectrophotometer, an appropriate dilution was made, the absorbance measured, and the apparent absorbance was calculated based on the dilution factor. Treatments were carried out by adding 20.0 mL of the reaction solution into an uncovered plastic petri dish (100 mm × 10 mm) and exposing it to UV radiation at appropriate time intervals. Ambient temperature for all experiments was 21 (± 1) ◦ C. At each reaction time interval, 0.6 mL was withdrawn for ascorbic acid analysis. The calculated maximum reduction in sample depth over the course of experiments accounted for incident intensity reductions of no more than 0.03 mW/cm2 . Post-UV treatment storage studies were carried out in 0.5% malic acid or apple juice treated with UV radiation as described above. Following UV treatment, samples were held in a temperature-controlled Thermo Neslab HX 300 water bath (Thermo Scientific) at 4 or 25 ◦ C. Ambient radiation was excluded by covering samples with aluminum foil. After each storage interval, samples were withdrawn for ascorbic acid analysis. Ascorbic acid analysis Ascorbic acid was quantified using a Waters HPLC system (pump model: 600; Autosampler: 71P; photodiode array (PDA) detector: 2998, Waters Inc., Milford, Mass., U.S.A.) and a C18 reverse phase/cation exchange column (Primesep-D, 4.6 mm × 150 mm, particle size 5 μm, SIELC Inc., Prospects Heights, Ill., U.S.A.). The isocratic mobile phase consisted of a water/acetonitrile/formic acid (95%) mixture (95:5:0.095 v/v/v) adjusted to pH 1.80 with HCl. Injection volume was 20 μL. Ascorbic acid identity was confirmed by injecting samples of pure ascorbic acid solution and then comparing retention times (2.2 min) and spectral characteristics between 220 and 300 nm. Standard curves were prepared by plotting the concentration of prepared aqueous solutions of ascorbic acid (0 to 500 mg/L) versus peak area measured at 245 nm. At each reaction time interval, model system or juice samples were injected directly onto the column without prior extraction or clean up.


Dose measurement and analysis Reaction rates were compared by plotting ascorbic acid concentrations versus time or UV dose. Linear portions of the data were characterized using the zero-order kinetic equation: Ct = C0 − k0 (t )

(1)

y = a x2 + b x + c

(3)

where y = percent ascorbic acid remaining, x = UV dose (J/cm2 ), and a, b, c are derived coefficients. For each analysis, r 2 values for the polynomial fits versus actual data were ≥ 0.98, with no systematic error observed in the plots for residuals versus fits. To compare reaction rates for different treatments, the UV dose required to achieve a 50% reduction in ascorbic acid was calculated from the polynomial equation by setting y = 50 and solving for x using the quadratic formula. This calculated dose is termed the D50 value.

Statistical analysis Statistical significance between treatments was carried out either by single-factor analysis of variance or Student’s t-test using Mi R crosoft Excel 2007 (Redmond, Wash., U.S.A.). All experiments were performed in triplicate by exposing samples to UV radiation at 3 separate times. For each run, a single ascorbic acid concentration determination was made at each sampling time interval.

Results and Discussion The reverse-phase analytical column used in this study contained embedded basic ion-pairing groups that do not retain highmolecular weight components. This permitted direct injection of model system and juice samples without observable peak interference from other compounds. Based on the linearity of the standard curve, the lower limit for quantification was 5 mg/L. Dehydroascorbic acid (DHAA), which also has vitamin C activity, was not detected or quantified. In this study, we assume that ascorbic acid concentration alone provides a good estimate for vitamin C activity since DHAA in apple juice is low (5% to 10% of ascorbic acid) (Behrens and Medere 1987) and other ascorbic acid degradation studies have made the same assumption for apple and citrus juices (Tran and Farid 2004; Adzahan 2006; Burdurlu and others 2006).

Comparison of ascorbic acid degradation in juice and the model system Ascorbic acid degraded more rapidly in apple juice (C o = 170 mg/L, pH 3.5) than in the malic acid model system (C 0 = 190 mg/L, pH 3.3) (Figure 2). Previous UV microbial inactivation studies have demonstrated that UV-absorbing compounds decrease radiation penetration and therefore increase the dose required to achieve a target microbial reduction (Koutchma and others 2004; Murakami and others 2006). Given that the A254 value for apple juice was 250 times greater than 0.5% malic acid (Table 1), these results were unexpected. Apple juice is a complex mixture of phenolic compounds, organic acids, and sugars that absorb radiation in the UV region to varying degrees. It appears that some of these compounds enhance ascorbic acid degradation despite their UVabsorbing properties. To further explore this phenomenon, the effects of individual apple juice constituents on the kinetics of UV degradation of ascorbic acid were studied using a 0.5% malic acid model system.

where Ct = the concentration of ascorbic acid (mg/L) at time t (s), C 0 = initial ascorbic acid concentration (mg/L), and t = time (s). Effect of initial ascorbic acid concentration Dose values were obtained using the equation: The effect of initial ascorbic acid concentration (C o ) at 25, 50, D= I ×t (2) 100, 150, and 200 mg/L on UV-induced ascorbic acid degradation H64 Journal of Food Science r Vol. 76, Nr. 2, 2011

Effect of UV processing on ascorbic acid . . . in 0.5% malic acid (pH 3.3) is shown in Figure 3. No reduction in ascorbic acid was observed in untreated samples held in the dark at pH 3.3 for the same amount of time as treated samples (data not shown). Ascorbic acid degradation did occur in UV-treated

samples at all concentrations. In samples initially containing 25 and 50 mg/L ascorbic acid, the entire degradation reaction could be described by zero-order kinetics. However, at 100, 150, and 200 mg/L, degradation rates followed zero-order kinetics for up to 40, 60, and 75 min, respectively, after which the reaction rate accelerated. Zero-order rate constants obtained from the linear Table 1–Absorbance (254 nm) of solutions used in experiments. portion of each curve were not significantly (P > 0.05) different at Chemical Absorbance each C 0 concentration (k0avg = 0.55 ± 0.037/min) suggesting that degradation followed a similar mechanism during the initial part constituent Concentration 254 nm of the reaction. Koutchma and others (2009) reported that UVMalic acid 0.10% 0.07 induced ascorbic acid degradation followed zero-order kinetics 0.50% 0.30 at C 0 concentrations between 341 and 660 mg/L. However, the 1.0% 0.50 Caramel 60 mg/L 2.98 experiments in that study were conducted using apple juice, and Tannic acid 25 mg/L 0.55 only 20% to 40% decreases in ascorbic acid were achieved. In 100 mg/L 2.24 the present study, 0.5% malic acid was the reaction medium and 200 mg/L 4.58 ascorbic acid reductions of up to 90% were achieved. The observed Fructose 10.0% 0.16 Glucose 10.0% 0.007 acceleration of ascorbic acid degradation with time may be the Sucrose 10.0% 0.065 combined result of greater penetration of UV radiation into the Apple juice Single strength 17.5 reaction solution as ascorbic acid levels decrease and as well as Figure 2–UV degradation of ascorbic acid in apple juice (C 0 = 170 mg/L, pH 3.5) and in 0.5% malic acid (C 0 = 190 mg/L, pH 3.3). Each data point represents an average of 3 measurements ± standard deviation (SD).


Figure 3–UV degradation of ascorbic acid in 0.5% malic acid at varying initial C 0 concentrations. Each data point represents an average of 3 measurements ± SD.


Effect of UV processing on ascorbic acid . . . generation of side products that accelerate the degradation rate. UV enhanced oxidation of ascorbic acid to form ascorbic radicals has been demonstrated in animal (Jurkiewicz and Buettner 1994, 1996) and plant (Hideg and others 1997) tissues. It is possible that free radical reactions, enhanced by exposure to UV radiation, are important factors responsible for ascorbic acid degradation in UV-processed juices. Because reaction rates tended to increase at longer treatment times, the entire data could not be fitted to zero-, 1st-, or 2ndorder kinetic model. Therefore, the data were fitted to a quadratic function and the dose required to achieve a 50% reduction in ascorbic acid (D50 ) were calculated for each experiment. Although the use of a quadratic function did not help to determine reaction mechanisms, it served to quantitatively compare the effect of individual chemical constituents on UV-induced ascorbic acid degradation. An advantage to using dose as the independent variable was that day-to-day and longer term variation in output intensity of the UV lamps can be corrected in reaction rate calculations.

The effect of tannic acid concentration on ascorbic acid degradation rate is demonstrated in Figure 4. When tannic acid was added to the model system (C 0 = 100 mg/L, pH 3.3) at levels between 0 and 200 mg/L, solution absorbance increased by a factor of 15 (Table 1). Corresponding D50 values for ascorbic acid degradation significantly (P < 0.05) increased from 7.16 ± 0.13 to 14.97 ± 0.84 J/cm2 . It should be noted that, similar to other polyphenols, tannic acid has some antioxidant properties (Scalbert and others 2005) that may influence ascorbic acid degradation. Nevertheless, these results are analogous to UV microbial inactivation studies demonstrating a positive relationship between solution absorbance and dose levels required to inactivate bacterial cells (Guerrero-Beltran and Barbosa-Canovas 2004; Koutchma 2008).

Effect of pH UV degradation of ascorbic acid (C 0 = 50 mg/L) in 0.5% malic acid at pH 2.4, 3.3, and 5.5 is shown in Figure 5. D50 values at pH 2.4 (4.77 ± 0.07 J/cm2 ) and 3.3 (4.23 ± 0.38 J/cm2 ) were not significantly different (P > 0.05). However, at pH 5.5, ascorbic acid degradation was significantly more rapid as evidenced by a lower D50 value of 2.72 ± 0.03 J/cm2 . No significant decreases in ascorbic acid were observed in control samples simultaneously held in the dark at pH 2.4 or 3.3 (data not shown). However, in pH 5.5 samples, a 23% reduction in ascorbic acid occurred in samples stored for the same amount of time as UV-treated samples (5.04 J/cm2 , t = 60 min). The observed reductions in pH 2.4 and 3.3 UV-treated samples can therefore be entirely attributed to the effects of UV radiation. However, ascorbic acid reductions at pH 5.5 were likely the result of the combined effects of UV radiation and the inherent instability of the ascorbate ion species that is predominant at pH values above the pKa value for ascorbic acid (pK1 = 4.2). Most fruit juices have pH values lower than 4.0. However, some low-acid fruits and vegetables juices have pH greater than 4.2. Therefore, it may be necessary to take pH into consideration when predicting ascorbic acid losses during UV processing of higher pH juices.

Effect of solution absorbance Earlier UV microbial inactivation studies utilized varying concentrations of UV-absorbing caramel solutions as chemical surrogates to demonstrate the protective effects of UV-absorbing polyphenols, organic acids, and insoluble haze particulates that scatter radiation (Koutchma and others 2004; Murakami and others 2006). We conducted preliminary experiments to determine if caramel solution had a similar protective effect on ascorbic acid. Although the addition of caramel to the model system increased solution absorbance at 254 nm (Table 1), unexpected increases in the rate of ascorbic acid reduction occurred (data not shown). Caramel is a complex mixture of polymeric compounds formed from unsaturated 5- and 6-membered cyclic compounds (Schwartz and others 2008). In this study, we did not further explore caramel effects on ascorbic acid degradation since it is not naturally present in juice products. However, it is in agreement with our hypothesis that UV active chemical compounds exist in juice that enhance UV-induced ascorbic acid degradation. To avoid the confounding effect of caramel, tannic acid was used to study solution absorbance effects on UV degradation of ascorbic acid. Levels for tannic acid, Effect of malic acid concentration a mixture of polyphenolic glucose esters of gallic acid, were choFigure 6 shows UV degradation of ascorbic acid (C 0 = sen to approximate known levels of polyphenols present in apple 100 mg/L) at malic acid concentrations of 0.1, 0.5, and 1.0% juice (Picinelli and others 1996). each held at a constant pH of 3.3. D50 values in 0.5% (7.14 ± Figure 4–Effect of tannic acid concentration on UV degradation of ascorbic acid (C 0 = 100 mg/L) in 0.5% malic acid. Each data point represents an average of 3 measurements ± SD.

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Effect of UV processing on ascorbic acid . . .

Effect of sugars Unsweetened commercial apple juice typically contains 9.6% (w/w) total sugars with fructose (5.7%), glucose (2.6%), and sucrose (1.3%) being the most abundant types (USDA 2009). UV degradation of ascorbic acid (C 0 = 100 mg/L) in 0.5% malic acid (pH 3.3) and identical amounts (10% w/v) of glucose, fructose, or sucrose are shown in Figure 8. Compared to the control (no added sugar) (D50 = 7.14 ± 0.13), sucrose (D50 = 6.97 ± 0.56 J/cm2 ) did not have a significant (P > 0.05) effect on ascorbic acid degradation. Glucose (D50 = 8.86 ± 0.37 J/cm2 ) had a small,

but significant (P < 0.05) protective effect. In contrast, fructose (D50 = 1.46 ± 0.09 J/cm2 ) resulted in a dramatic and significant increase in ascorbic acid degradation and the effect was significantly dependent (P < 0.05) on fructose concentration (Figure 9). D50 values at 0, 2, 5, and 10% fructose were 7.14 ± 0.13, 4.77 ± 0.24, 2.91 ± 0.08, and 1.46 ± 0.09 J/cm2 , respectively. Because the A254 of 10% fructose was greater than that for the same concentration of glucose or sucrose (Table 1), the observed ascorbic acid degradation rate differences between sugars cannot be explained by differences in absorbance values for each solution. Triantaphylides and others (1981) reported that the carbonyl group in the open chain configuration of fructose is highly susceptible to 254-nm photolytic UV degradation, while the ring structure is unaffected by UV radiation. Upon exposure to UV radiation, fructose undergoes Norrish type-1 reactions that lead to the formation of hydroxyalkyl and acyl radicals (Binkley and Binkley 1998). Glucose is comparatively unaffected by UV radiation because the proportion of glucose in the chain configuration (0.024%) is lower than that for fructose (0.8%) (Triantaphylides and others 1981). Fan and Geveke (2007) confirmed this effect when they reported that among the 3 primary hexose sugars present in

Figure 5–Effect of pH on UV degradation of ascorbic acid (C 0 = 50 mg/L) in 0.5% malic acid buffer. Each data point represents an average of 3 measurements ± SD.

Figure 6–Effect of malic acid concentration (pH 3.3) on UV degradation of ascorbic acid (C 0 = 100 mg/L) degradation. Each data point represents an average of 3 measurements ± SD.


0.13 J/cm2 ) and 1% (7.89 ± 0.43 J/cm2 ) malic acid were significantly (P ≤ 0.05) lower compared to the reaction in 0.1% malic acid (10.13 ± 1.01 J/cm2 ). Thus, malic acid at levels greater than 0.5% appears to have stronger effect on the ascorbic acid degradation reaction. This was an interesting result given that solution UV absorbance values increased with increasing malic acid concentration (Table 1). It is possible that higher levels of malic acid induced side reactions that accelerated the destruction of ascorbic acid. It should be noted that malic acid was not essential for UVinduced ascorbic acid degradation since the reaction also occurred in unbuffered distilled water (Figure 7).


Effect of UV processing on ascorbic acid . . . apple juice, only fructose reacted to form d4 -furan in solutions exposed to 6 to 8 J/cm2 UV of UV-C radiation. Sucrose is not subject to photolytic degradation because it can only exist in the more stable ring configuration. The reason for the slightly protective effect of glucose is not known. The observed differences in UV degradation of ascorbic acid when each of the sugars is present (Figure 8) suggest that free radicals formed by UV degradation of fructose reacted with ascorbic acid to produce ascorbic radicals that continued to degrade into DHAA and further end products (Gregory 2008).

Ascorbic acid degradation during storage Ascorbic acid degradation in untreated and UV-treated samples was observed for up to 30 h after treatments ended (Figure 10). After initially exposing samples (C 0 = 87 mg/L) to UV doses of 0.96, 1.92, and 5.76 J/cm2 , ascorbic acid levels decreased to 85, 72, and 52 mg/L, respectively. During subsequent storage in the dark at 25 ◦ C, ascorbic acid continued to decrease. Zero-order rate constants increased slightly, but significantly (P ≤ 0.05), at higher UV dose values (Table 2). Post-UV degradation was significantly (P < 0.05) affected by storage temperature (Figure 11). In model system samples initially

Table 2– Post-UV treatment degradation rates for samples (C0 = 100 mg/L) stored at 25 ◦ C in the dark. Each number is the average of 3 determinations. Numbers in columns with different letters indicate statistically significant difference (P < 0.05). Initial UV dose (J/cm2 ) 0 0.96 1.92 5.76

Post-UV ascorbic acid degradation rate constant (h−1 ) 1.62 ± 0.12 a 1.62 ± 0.03 a 2.19 ± 0.22 b 2.28 ± 0.11 b

treated with 1.2 J/cm2 UV and then stored in the dark at 4 ◦ C, ascorbic acid levels decreased by 40% after 50 h. When identically treated samples were held at 25 ◦ C, representing a temperature abuse condition, over 75% of the ascorbic acid was lost after only 17 h. Accelerated degradation of ascorbic acid after UV treatment is confirmed in apple juice as shown in Figure 12. No significant decrease (P > 0.05) in ascorbic acid occurred in untreated apple juice for up to 35 h. However, over 60% of the ascorbic acid in UVtreated (1.2 J/cm2 ) juice was lost during the same storage interval. Figure 7–UV-induced ascorbic acid degradation (C 0 = 100 mg/L) in distilled water (pH 6.0). Each data point represents an average of 2 measurements ± SD.

Figure 8–Effect of fructose (10%), glucose (10%), and sucrose (10%) on UV degradation of ascorbic acid (C 0 = 100 mg/L) in 0.5% malic acid. Each data point represents an average of 3 measurements ± SD.

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Effect of UV processing on ascorbic acid . . . Figure 9–Dependence of UV degradation of ascorbic acid (C 0 = 100 mg/L) on fructose concentration in 0.5% malic acid. Each data point represents an average of 3 measurements ± SD.

Figure 10–Effect of initial UV dose on postprocessing degradation of ascorbic acid (C 0 = 100 mg/L) in 0.5% malic acid at 25 ◦ C. Each data point represents an average of 3 measurements ± SD.


Figure 11–Effect of storage temperature on ascorbic acid (C 0 = 100 mg/L) in UV-treated (5.76 J/cm2 ) samples held in 0.5% malic acid (pH 3.3). Each data point represents an average of 3 measurements ± SD.


Effect of UV processing on ascorbic acid . . . Figure 12–Postprocessing degradation of ascorbic acid (C 0 = 200 mg/L) in UV-treated (1.2 J/cm2 ) apple juice (pH 3.5) during storage at 4 ◦ C. Each data point represents an average of 3 measurements ± SD.

The more rapid degradation of ascorbic acid in the model system and in apple juice after exposure to UV radiation may be caused by ascorbic acid radicals formed during the initial treatment. Increased formation of ascorbic radicals as result of UV radiation exposure in animal and plant tissues has been demonstrated in animal (Jurkiewicz and Buettner 1994, 1996) and plant (Hideg and others 1997) tissue although there have been no studies that show this effect in UV-treated food products. An analogous effect was reported in a study by Kabasakalis and others (2000) where it was observed that ascorbic acid degradation in thermally processed orange juice occurred more rapidly than in fresh (unheated) juice. Since the half life of ascorbic radicals is only 50 s (Buettner and Jurkiewicz 1993), it is likely that a cascade of temperature dependent radical reactions occur which cause ascorbic acid degradation to continue for extended storage times.

Conclusions


Interactions between UV radiation and juice chemical and physical properties affect ascorbic acid degradation during and after UV treatments. Ascorbic acid degradation occurred more rapidly at higher UV dose values and the reaction accelerated with increasing exposure time. Rates were also greater at higher pH values, malic acid concentrations, and fructose concentrations. Ascorbic acid degradation was slower at higher solution absorbance values and in the presence of glucose. Ascorbic acid continued to degrade during postprocessing storage; the rate was more rapid at higher initial UV dose levels and storage temperature. The data suggest that UV-induced formation of free radicals may be responsible for accelerated loss of ascorbic acid during and after UV treatments. The factors identified in this study that accelerate ascorbic acid degradation provides insight on why ascorbic acid reductions occur more rapidly in UV-treated apple juice compared to a model system. Further studies are needed to confirm the observed effects in commercial UV systems and to identify ascorbic acid degradation pathways and end products that occur during UV processing and storage of juice systems. Processors can minimize losses in ascorbic acid fortified juices by adding the vitamin after UV treatments have occurred and by storing the product at low temperatures. Researchers who are studying ascorbic acid loses in UV-treated juices should also take care to conduct chemical analyses within a consistent posttreatment time interval to avoid misinterpretation of the data. Because of differences in UV dose H70 Journal of Food Science r Vol. 76, Nr. 2, 2011

measurement techniques between different research studies, it is difficult to compare the ascorbic acid degradation kinetics with microbial death kinetics. Therefore, confirmation studies using commercial equipments such as the CiderSure are needed to compare the ascorbic acid degradation kinetics with microbial death kinetics.

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