# Abstract

The degradation kinetics of vitamin C in different citrus juice types (tangerine, grapefruit, orange and lime) were investigated during four hours storage at temperatures of 4 and 25°C under different storage conditions in terms of light and oxygen presence. The loss of vitamin C at each sample followed the first-order kinetic model. During storage, predicted half-life of vitamin C ranged from 137.586 to 467.486 min for tangerine; 105.444 to 203.100 min for grapefruit; 365.702 to 1044.668 min for orange and 540.076 to 722.016 min for lime juices. These results indicated that the highest and lowest retentions of vitamin C were observed in orange and grapefruit juices respectively during storage. Additionally, storage time, light penetration and oxygen presence were the most effective factors on vitamin C degradation while the storage temperature revealed no significant effect on the vitamin C content.

Key words
citrus juices; kinetic modelling; storage; vitamin C

# INTRODUCTION

Vitamin C or L-ascorbic acid is a water-soluble and highly unstable vitamin. The primary function is to prevent scurvy (Karim & Adebowale 2009KARIM OR & ADEBOWALE AA. 2009. A dynamic method for kinetic model of ascorbic acid degradation during air dehydration of pretreated pineapple slices. Int Food Res J 16(4): 555-560.). It is used as an additive in many foods due to its antioxidant potential. Thus, it increases their quality and technological features of foods as well as the nutritional value (Burdurlu et al. 2006BURDURLU HS, KOCA N & KARADENIZ F. 2006. Degradation of vitamin C in citrus juice concentrates during storage. J Food Eng 74(2): 211-216.). Nowadays, there is an increasing demand for nutritious food, and many attempts have been made to maximize the retention of nutrients in the storage as much as the processing. Vitamin C is often considered to be a nutrient quality indicator undergoing the processing and storage of foods, since it is seen that other nutrients are well preserved if vitamin C is well preserved (Sapei & Hwa 2014SAPEI L & HWA L. 2014. Study on the kinetics of vitamin C degradation in fresh strawberry juices. Procedia Chem 9: 62-68.).

The vitamin C content in citrus juices is different from each other depending on processing conditions and raw material, such as 38 mg·100 g-1 for grapefruit juice, 46 mg·100 g-1 for lemon juice, 50 mg·100 g-1 for orange juice and 31 mg·100 g-1 for mandarin juice (Baysal et al. 1991BAYSAL A, KEÇECIOĞLU S, ARSLAN P, YÜCECAN S, PEKCAN G, GÜNEYLI U, BIRER S, SAĞLAM F, YURTTAGÜL M & ÇEHRELI R. 1991. Composition of foods, 3rd ed., Ankara: Turkish Dietitians Association, 53 p.). According to available biochemical, clinical, and epidemiological studies, the current recommended daily acceptance (RDA) for vitamin C is advised to be 100–120 mg·day-1 for obtaining cellular saturation and optimum risk reduction of stroke, heart diseases, and cancer in healthy individuals (Klimczak et al. 2007KLIMCZAK I, MAŁECKA M, SZLACHTA M & GLISZCZYŃSKA-ŚWIGŁO A. 2007. Effect of storage on the content of polyphenols, vitamin C and the antioxidant activity of orange juices. J Food Compost Anal 20(3-4): 313-322.). Citrus juices are rich in vitamin C and seem to be quite effective in meeting the need for daily vitamin C. However, vitamin C of citrus juices is generally readily oxidized and therefore lost storage. There are many factors affecting this oxidation process, such as light exposure, level of dissolve oxygen, storage temperature, sugar and metal ions presence (Sapei & Hwa 2014SAPEI L & HWA L. 2014. Study on the kinetics of vitamin C degradation in fresh strawberry juices. Procedia Chem 9: 62-68.). During storage, L-ascorbic acid oxidizes to dehydroascorbic acid (DHAA) (Cruz et al. 2008CRUZ RMS, VIEIRA MC & SILVA CLM. 2008. Effect of heat and thermosonication treatments on watercress (Nasturtium officinale) vitamin C degradation kinetics. Innov Food Sci Emerg Technol 9(4): 483-488.). This fact does not cause the loss of vitamin C because DHAA can be converted back to ascorbic acid (Kırca & Cemeroğlu 2001KIRCA A & CEMEROĞLU B. 2001. The mechanism of ascorbic acid degradation. The Food 26(4): 233-242.). But, DHAA is easily hydrolyzed to 2,3-diketogulonic acid (DKGA) due to it is highly unstable. DKGA has no biological activity (Wibowo et al. 2015WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151.). It has been found that these oxidation steps are particularly sensitive to availability of oxygen, long-term heat treatment in the presence of oxygen and exposure to light (Cruz et al. 2008CRUZ RMS, VIEIRA MC & SILVA CLM. 2008. Effect of heat and thermosonication treatments on watercress (Nasturtium officinale) vitamin C degradation kinetics. Innov Food Sci Emerg Technol 9(4): 483-488.).

# MATERIALS AND METHODS

## Preparation of citrus juice samples

Tangerines (Citrus reticulata), grapefruits (Citrus paradise), oranges (Citrus sinensis) and limes (Citrus aurantifolia) were harvested in 2017 and purchased from a local fruit supplier (Tesco Kipa Mass Marketing Trade Logistics and Food Industry Inc., Izmir, Turkey). Fresh juices were obtained using a household table top citrus juice extractor (Arçelik, Istanbul, Turkey). Then, they were filtered by a double layer cheese cloth to remove their pulps and immediately analyzed.

In this study, there were six samples for each juice and each sample had different storage conditions. Sample 1 and Sample 4 were put in glass cups, while Sample 2 and Sample 5 were placed in capped glass bottles. Sample 3 and Sample 6 were put in capped glass bottles wrapped around with aluminum foil. Sample 1, Sample 2 and Sample 3 were stored at room temperature of 25°C, whereas Sample 4, Sample 5 and Sample 6 were kept at refrigerated temperature of 4°C. All samples were stored for 4 hours and analyzed every 60 minutes. They were prepared in triplicate.

## Determination of vitamin c

Firstly, DHAA was converted to ascorbic acid according to the method of Wibowo et al. (2015)WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151.. Then, the vitamin C contents as the sum of ascorbic acid and DHAA were determined following the spectrophotometric analytical procedure outlined by Hışıl (2013)HIŞIL Y. 2013. The instrumental food analyses-laboratory analyses, 4th ed., Izmir: Ege University Academic Press, 41 p.. The standard curve was formed by ascorbic acid solutions (1-5 mg·100 mL-1). The ascorbic acid solutions were prepared from stock ascorbic acid solution (0.1% (w/v)) containing pure ascorbic acid and oxalic acid solution (%0.4 (w/v)). The blank solution was a mixture of 1 part (in volume) water and 9 parts (in volume) 2,6-dichorophenol indophenol solution while the colored solution was prepared with 1 part (in volume) ascorbic acid solution/sample and 9 parts (in volume) 2,6-dichorophenol indophenol solution. Subsequently, the absorbance was measured at 518 nm using a UV-vis spectrophotometer (Optizen Pop, Mecasys Co., Ltd., Korea). Results were expressed as mg of ascorbic acid·100 mL-1 of juice.

## Kinetic modelling

To determine the influence of storage conditions on the vitamin C content of citrus juices, vitamin C degradation was kinetically modeled as a function of the storage conditions such as time, temperature and packaging type. In this study, the zero- (Eq. (1)) and first-order (Eq. (2)) kinetic models were respectively applied, assuming previous related studies (Burdurlu et al. 2006BURDURLU HS, KOCA N & KARADENIZ F. 2006. Degradation of vitamin C in citrus juice concentrates during storage. J Food Eng 74(2): 211-216., Al-Zubaidy & Khalil 2007AL-ZUBAIDY MMI & KHALIL RA. 2007. Kinetic and prediction studies of ascorbic acid degradation in normal and concentrate local lemon juice during storage. Food Chem 101(1): 254-259., Abbasi & Niakousari 2008ABBASI A & NIAKOUSARI M. 2008. Kinetics of ascorbic acid degradation in un-pasteurized iranian lemon juice during regular storage conditions. Pak J Biol Sci 11(10): 1365-1369., Valdramidis et al. 2010VALDRAMIDIS VP, CULLEN PJ, TIWARI BK, & O’DONNELL CP. 2010. Quantitative modelling approaches for ascorbic acid degradation and non-enzymatic browning of orange juice during ultrasound processing. J Food Eng 96(3): 449-454., Torres et al. 2011TORRES B, TIWARI BK, PATRAS A, CULLEN PJ, BRUNTON N & O’DONNELL CP. 2011. Stability of anthocyanins and ascorbic acid of high pressure processed blood orange juice during storage. Innov Food Sci Emerg Technol 12(2): 93-97., Remini et al. 2015REMINI H, MERTZ C, BELBAHI A, ACHIR N, DORNIER M & MADANI K. 2015. Degradation kinetic modelling of ascorbic acid and colour intensity in pasteurised blood orange juice during storage. Food Chem 173: 665-673.).

$C t = C 0 − k 0 ⋅ t$ (1)
$C t = C 0 exp ( − k 1 ⋅ t )$ (2)

where Ct is the concentration of vitamin C (mg ascorbic acid·100 mL-1 of juice) at any given time (t), C0 is the initial vitamin C concentration, k0 and k1 are the (apparent) reaction rate constants (min-1), and t is the reaction time (min).

The reaction order and the relevant coefficients were determined through non-linear regression analyses by comparing the coefficient of determination (R2) and root mean square error (RMSE) (Eq. (3)) amongst zero- and first-order kinetic models, respectively. The model with both higher value for R2 and the lower value for RMSE shows a better fit with the results (İçier et al. 2014İÇIER F, BAYSAL T, TAŞTAN Ö & ÖZKAN G. 2014. Microwave drying of black olive slices: effects on total phenolic contents and colour. The Food 39(6): 323-330., Süfer et al. 2017SÜFER Ö, SEZER S & DEMIR H. 2017. Thin layer mathematical modeling of convective, vacuum and microwave drying of intact and brined onion slices. J Food Process Preserv 41: e13239.).

$RMSE = ∑ i = 1 N ( C exp , i − C p r e , i ) 2 N$ (3)

While the half-life (t1/2) of vitamin C is the estimated time where the concentration of vitamin C is decreased by 50% from its initial value, the decimal reduction time (D-value) is the heating time where causes 90% degradation of vitamin C in a heated medium at constant temperature (Özkan et al. 2010ÖZKAN M, CEMEROĞLU B & KIRCA-TOKLUCU A. 2010. Reaction kinetic in food engineering, Ankara: Food Technology Association Publications No: 42, 174 p., Sapei & Hwa 2014SAPEI L & HWA L. 2014. Study on the kinetics of vitamin C degradation in fresh strawberry juices. Procedia Chem 9: 62-68.). Half-life and decimal reduction time of Vitamin C in each citrus juice were calculated according to the kinetic model (Eq. (1) or Eq. (2)) that has the best fit between model and experimental data.

## Statistical analysis

All experiments were performed in duplicate. Mean values with standard deviation were calculated using standard statistical procedures. The data obtained were subjected to statistical evaluation, and non-linear regression analysis for kinetic data analysis performed with SPSS v.20.0 statistical package. To understand the mutual importance of the storage conditions for vitamin C degradation among each other, a multivariate data analysis (MANOVA) and Duncan’s multiple range test were applied. In kinetic data analysis, among the relevant coefficients related to reaction order, RMSE was calculated by Microsoft Excel 2010 program, while R2 was determined by the SPSS v.20.0 program. Estimated kinetic parameters were compared using one-way analysis of variance (ANOVA) and Duncan’s multiple range test to determine differences among storage conditions. Evaluations were based on the p<0.05 significance level.

# RESULTS AND DISCUSSION

The vitamin C content determined at the start of storage in fresh home-made citrus juices were 31.35 ± 0.72 mg·100 g-1 for tangerine juice, 45.75 ± 0.65 mg·100 g-1 for grapefruit juice, 31.37 ± 0.97 mg·100 g-1 for orange juice and 34.34 ± 1.21 mg·100 g-1 for lime juice. These values were close to those reported by other researchers (Klimczak et al. 2007KLIMCZAK I, MAŁECKA M, SZLACHTA M & GLISZCZYŃSKA-ŚWIGŁO A. 2007. Effect of storage on the content of polyphenols, vitamin C and the antioxidant activity of orange juices. J Food Compost Anal 20(3-4): 313-322., USDA 2018USDA. 2018. Food composition databases show nutrients list. Retrieved in January, 16, 2018 from https://ndb.nal.usda.gov/ndb/nutrients/report/nutrientsfrm?max=25&offset=0&tot Count=0&nutrient1=401&nutrient2=&nutrient3=&subset=0&fg=9&sort=f&measureby=g.
https://ndb.nal.usda.gov/ndb/nutrients/r...
). Figure 1 demonstrates the percentages of vitamin C degradation calculated for fresh home-made citrus juices stored under different conditions assayed. During storage, vitamin C contents of all samples were decreased significantly with time at a rate depending on the storage temperature, light and oxygen presence (p<0.05). It was determined that the loss of vitamin C varied between 14.73% (Sample 6 for orange juice) and 79.36% (Sample 1 for grapefruit juice) depending on the type of sample and storage conditions (p<0.05). As storage time increases, vitamin C loss occurs by the more or less cumulative effects of storage conditions such as temperature, light, and oxygen presence. This is in agreement with the findings of Wibowo et al. (2015)WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151., Remini et al. (2015)REMINI H, MERTZ C, BELBAHI A, ACHIR N, DORNIER M & MADANI K. 2015. Degradation kinetic modelling of ascorbic acid and colour intensity in pasteurised blood orange juice during storage. Food Chem 173: 665-673. and Abbasi & Niakousari (2008)ABBASI A & NIAKOUSARI M. 2008. Kinetics of ascorbic acid degradation in un-pasteurized iranian lemon juice during regular storage conditions. Pak J Biol Sci 11(10): 1365-1369..

Figure 1
Comparison of vitamin C loss in fresh home-made citrus juices ((a) tangerine juice, (b) grapefruit juice, (c) orange juice, (d) lime juice) during storage [Sample 1 (), Sample 2 (), Sample 3 (), Sample 4 (), Sample 5 (), Sample 6 ()].

Temperature is one of the parameters that influence the content of vitamin C during storage. During the storage of fresh home-made citrus juices for 240 min, the change of temperature from 4°C to 25°C was not determined to have a significant effect on vitamin C loss of all samples depending on the time (p>0.05). This is thought to be due to storage at temperatures close to each other. This result in the study correlates with the finding of Kavousi (1997)KAVOUSI CK. 1997. Accelerated shelf-life testing in lime juice, M.Sc. Thesis, Shiraz University, Shiraz, Iran. (Unpublished). who reported that the change of vitamin C in commercially pasteurized lime juice showed a low dependence on the temperature change between 4ºC and 25ºC. However, Burdurlu et al. (2006)BURDURLU HS, KOCA N & KARADENIZ F. 2006. Degradation of vitamin C in citrus juice concentrates during storage. J Food Eng 74(2): 211-216., Wibowo et al. (2015)WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151., and Remini et al. (2015)REMINI H, MERTZ C, BELBAHI A, ACHIR N, DORNIER M & MADANI K. 2015. Degradation kinetic modelling of ascorbic acid and colour intensity in pasteurised blood orange juice during storage. Food Chem 173: 665-673. expressed that loss of vitamin C increased with the increasing storage temperature, while Zerdin et al. (2003)ZERDIN K, ROONEY ML & VERMUË J. 2003. The vitamin C content of orange juice packed in an oxygen scavenger material. Food Chem 82(3): 387-395. determined that the level of ascorbic acid in pasteurized orange juices stored at 4°C was higher than those stored at 25°C.

When the effect of light presence on the loss of vitamin C was investigated depending on the time, it was determined that the presence of light caused a significant decrease in vitamin C content of all samples except Sample 3, Sample 5 and Sample 6 for grape juice and Sample 1, Sample 2 and Sample 4 for lime juice (p<0.05). In Sample 3 and Sample 6 for each type of citrus juice, vitamin C was generally better preserved. The lower vitamin C losses in these samples are associated with lower light penetration from capped glass bottles wrapped with aluminum foil (Abbasi & Niakousari 2008ABBASI A & NIAKOUSARI M. 2008. Kinetics of ascorbic acid degradation in un-pasteurized iranian lemon juice during regular storage conditions. Pak J Biol Sci 11(10): 1365-1369.). This finding correlates with the result of Abbasi & Niakousari (2008)ABBASI A & NIAKOUSARI M. 2008. Kinetics of ascorbic acid degradation in un-pasteurized iranian lemon juice during regular storage conditions. Pak J Biol Sci 11(10): 1365-1369..

While Sample 1 and Sample 5 were in constant contact with oxygen due to storage in open glass cups, the other samples were stored in capped glass bottles. The highest time-dependent losses of vitamin C in orange and lime juices were observed in Sample 1 and Sample 5 (p<0.05). In Sample 1 and Sample 2 for tangerine and grapefruit juices, it was determined that the time-dependent vitamin C losses were higher than other samples (p<0.05). In the packed juice, oxygen can be present as dissolved oxygen, which is included during processing and preparation, as well as headspace oxygen. Addition, it can enter to the package from the environment by diffusion (Wibowo et al. 2015WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151.). Oxygen accelerates vitamin C oxidation, thereby increasing the tendency to further reaction of the carbonyl compounds formed via aldol condensation or reaction with amino groups to yield brown pigments that have no vitamin C activity (Rassis & Saguy 1995RASSIS D & SAGUY IS. 1995. Oxygen effect on nonenzymatic browning and vitamin C in commercial citrus juices and concentrate. LWT - Food Sci Technol 28(3): 285-290.). Therefore, it is thought that one of the reasons of the decrease in vitamin C contents of the citrus juices is oxygen presence. Many researchers reported that oxygen was really important for the loss of vitamin C in the citrus juice (Rassis & Saguy 1995RASSIS D & SAGUY IS. 1995. Oxygen effect on nonenzymatic browning and vitamin C in commercial citrus juices and concentrate. LWT - Food Sci Technol 28(3): 285-290., Zerdin et al. 2003ZERDIN K, ROONEY ML & VERMUË J. 2003. The vitamin C content of orange juice packed in an oxygen scavenger material. Food Chem 82(3): 387-395., Wibowo et al. 2015WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151.).

Multivariate data analysis was performed to determine the mutual importance of the investigated storage conditions for vitamin C loss. According to the result of analysis, statistically significant correlations were found between the type of fresh home-made citrus juice and the loss of vitamin C during storage (p<0.05). The vitamin C losses in fresh home-made citrus juices increased with the increasing storage time (p<0.05). Other storage conditions were evaluated statistically by taking into account the storage period as a whole. The type of fresh home-made citrus juice which lost most of its vitamin C throughout storage was grapefruit juice and followed by tangerine juice, orange juice and lime juice, respectively (p<0.05). It was also determined that when the effect of the packaging type on the loss of vitamin C was examined among the fresh home-made citrus juices stored with different packaging types, it was observed that the highest loss of vitamin C belonged to Sample 1 and followed by Sample 2, Sample 4, Sample 3, Sample 5 and Sample 6, respectively (p<0.05). When the effect of increase in temperature on the amount of vitamin C during storage was examined, statistically insignificant correlations were observed between temperature change and vitamin C loss of fresh home-made citrus juices (p>0.05). Ascorbic acid degrades by two ways as oxidative and non-oxidative mechanism. Temperature is one of the factors related to the non-oxidative degradation mechanism. The non-oxidative degradation rate of ascorbic acid is lower than the oxidative rate between 15ºC and 20ºC (Wallington et al. 2013WALLINGTON N, CLARK AC, PRENZLER PD, BARRIL C & SCOLLARY GR. 2013. The decay of ascorbic acid in a model wine system at low oxygen concentration. Food Chem 141(3): 3139-3146.). Therefore, it is an expected result that there is no significant effect of moderate temperatures ranging from 4ºC to 25ºC on vitamin C loss during storage. Furthermore, it was determined that packaging types (namely, the presence of oxygen and light), citrus juice types and storage time had mutual and concurrent significant effects on the loss of Vitamin C (p<0.05). Temperature was not observed as a significant function when the mutual and concurrent effects of all parameters were evaluated (p>0.05).

## Kinetics of vitamin c degradation

During storage, the changes in vitamin C content of all fresh home-made citrus juices were fitted to zero- and first-order kinetic models given in Eq. (1) and Eq. (2), respectively. The best fit between model and experimental data was obtained from first-order kinetic models with higher R2 values (0.750-0.995) and lower RMSE (0.409-4.667) (Table I), conversely, these coefficients were much worse with zero-order kinetic models (with R2 range values of 0.655–0.891 and RMSE range values of 0.530–5.138) (not shown). Although the light presence and citrus juice type affect the loss of vitamin C, oxygen is mainly responsible for the loss of vitamin C. The vitamin C content of citrus juices decrease more rapidly at the beginning of storage due to the reaction of vitamin C with dissolved oxygen and then, the vitamin C degrades gradually (Sapei & Hwa 2014SAPEI L & HWA L. 2014. Study on the kinetics of vitamin C degradation in fresh strawberry juices. Procedia Chem 9: 62-68.). The first-order kinetic model was used in most studies to describe the loss of vitamin C in citrus juices during storage, just as it is in this study (Burdurlu et al. 2006BURDURLU HS, KOCA N & KARADENIZ F. 2006. Degradation of vitamin C in citrus juice concentrates during storage. J Food Eng 74(2): 211-216., Al-Zubaidy & Khalil 2007AL-ZUBAIDY MMI & KHALIL RA. 2007. Kinetic and prediction studies of ascorbic acid degradation in normal and concentrate local lemon juice during storage. Food Chem 101(1): 254-259., Torres et al. 2011TORRES B, TIWARI BK, PATRAS A, CULLEN PJ, BRUNTON N & O’DONNELL CP. 2011. Stability of anthocyanins and ascorbic acid of high pressure processed blood orange juice during storage. Innov Food Sci Emerg Technol 12(2): 93-97., Wibowo et al. 2015WIBOWO S, GRAUWET T, SANTIAGO JS, TOMIC J, VERVOORT L, HENDRICKX M & VAN LOEY A. 2015. Quality changes of pasteurised orange juice during storage: a kinetic study of specific parameters and their relation to colour instability. Food Chem 187: 140-151., Remini et al. 2015REMINI H, MERTZ C, BELBAHI A, ACHIR N, DORNIER M & MADANI K. 2015. Degradation kinetic modelling of ascorbic acid and colour intensity in pasteurised blood orange juice during storage. Food Chem 173: 665-673.).

Table I
Kinetic parameters of the first-order kinetic model for vitamin C degradation in fresh home-made citrus juices during storage.

The vitamin C content decreased depending on time and packaging types for all juice type stored (Figure 2). The kinetic parameters k1, C0, D-value, t1/2 together with their R2 and RMSE values of the first-order kinetic models for vitamin C losses are given in Table I. All kinetic parameters were evaluated separately for each juice type and statistically important difference was determined among the samples evaluated (p<0.05). The k values that support the degradation ratios of vitamin C shown in Figure 1 ranged from 0.0015 to 0.0050 min-1 for tangerine juice, from 0.0034 to 0.0066 min-1 for grapefruit juice, from 0.0007 to 0.0019 min-1 for orange juice and from 0.0010 to 0.0013 min-1 for lime juice. The samples with the highest k values were Sample 1 and Sample 2 for tangerine juice, Sample 1 for grapefruit juice, Sample 1 for orange juice and Sample 1, Sample 2, Sample 4 for lime juice (p<0.05). The losses of vitamin C in these samples were faster and higher than in other samples. Samples with the highest t1/2 according to the degradation rate of vitamin C were found as Sample 6 for tangerine juice, Sample 3, Sample 4, Sample 5, Sample 6 for grapefruit juice, Sample 6 for orange juice and Sample 3, Sample 5, Sample 6 for lime juice (p<0.05). It was observed that orange juice was the citrus juice type with the highest vitamin C retention (1044.668 min for t1/2) and grapefruit juice was the lowest one (189.230 - 203.100 min for t1/2). D-values of citrus juices ​​varied from 458.621 to 1558.286 min for tangerine juice, from 351.481 to 676.999 min for grapefruit juice, from 1219.007 to 3482.228 min for orange juice and from 1800.254 to 2406.719 min for lime juice. The common point in all samples is that the degradation rates of citrus juices stored in glass cups are high and their vitamin C preservation are low whereas the degradation rates of citrus juices stored in capped glass bottles wrapped around with aluminum foil, are low and their vitamin C preservation are high.

Figure 2
Degradation kinetics of vitamin C in the fresh home-made citrus juices ((a) tangerine juice, (b) grapefruit juice, (c) orange juice, (d) lime juice) during storage [Sample 1 (), Sample 2 (), Sample 3 (), Sample 4 (), Sample 5 (), Sample 6 ()].

When all the samples stored for each fresh home-made citrus juice type were evaluated according to their kinetic parameters t1/2 and D-values, the following information was obtained. The storage conditions did not have a significant effect on shelf life of vitamin C in tangerine juice stored at 25°C (p>0.05). But, a longer shelf life in 4ºC storage could be obtained by removing oxygen and light from the product, respectively.

Storage of grapefruit juice at both 4°C and 25°C with capped glass bottles wrapped around aluminum foil did not have a statistically significant effect on the shelf life of vitamin C (p>0.05) and provided the best protection. The worst protection of vitamin C was in the grapefruit juice stored at 4°C in capped glass bottles (p<0.05).

The best retention of vitamin C in orange juice was observed in the product with capped glass bottle wrapped around with aluminum foil at 4°C (p<0.05). The worst retention of that was obtained from the products stored in glass cups at 4°C and 25ºC. No significant difference was determined between them (p>0.05).

The products with the highest shelf life of vitamin C for lime juice were the juices in the capped glass bottles wrapped around with aluminum foil at both 4°C and 25°C and the juice in the capped glass bottle at 4°C. No statistically significant difference was found between these three products (p>0.05). There was also no significant difference among the other products (p>0.05) and shelf life of vitamin C was shorter than these.

# CONCLUSIONS

This present study evaluated the effect of different storage conditions (time, temperature, light and oxygen presence) on the loss and kinetic behavior of vitamin C in the fresh home-made citrus juices. Vitamin C losses of citrus juices during storage were best explained by first-order kinetic model. Kinetic parameters such as a rate constant of degradation reaction, a half-life and decimal reduction time of vitamin C were determined for each sample. The length of storage period, light and oxygen presence were the most efficient factors in the degradation of vitamin C, while storage temperatures between 4°C and 25°C had no significant effect on vitamin C loss in the citrus juices during storage (p>0.05). Kinetic studies have shown that oxygen is the most important factor in vitamin C degradation. It should be preferred that fresh home-made citrus juices are stored primarily in an environment free from oxygen and then away from light. The shelf life of Vitamin C in the citrus juices can be extended by reducing the storage time and avoiding oxygen and light permeation through the packaging.

# ACKNOWLEGMENTS

The authors thank Furkan Ediz Hacihasanoğlu for his technical support.

# REFERENCES

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# Publication Dates

• Publication in this collection
12 June 2020
• Date of issue
2020