Article pubs.acs.org/JAFC
Vitamin A Is Rapidly Degraded in Retinyl Palmitate-Fortified Soybean Oil Stored under Household Conditions Marc Pignitter,† Bettina Dumhart,† Stephanie Gartner,† Franz Jirsa,‡,§ Georg Steiger,# Klaus Kraemer,⊥,⊗ and Veronika Somoza*,† †
Department of Nutritional and Physiological Chemistry and ‡Department of Inorganic Chemistry, University of Vienna, Vienna, Austria § Department of Zoology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa # NIP Product and Technology Development − HNH, DSM Nutritional Products Ltd., Kaiseraugst, Switzerland ⊥ Sight and Life, Kaiseraugst, Switzerland ⊗ Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205, United States ABSTRACT: Oil fortification with retinyl palmitate is intended to lower the prevalence of vitamin A deficiency in populations at risk. Although the stability of vitamin A in vegetable oil has been shown to depend on environmental factors, very little information is known about the stability of vitamin A in preoxidized vegetable oils. The present study investigated the stability of retinyl palmitate in mildly oxidized (peroxide value < 2 mequiv O2/kg) and highly oxidized (peroxide value > 10 mequiv O2/kg) soybean oil stored under domestic and retail conditions. Soybean oil was filled in transparent bottles, which were exposed to cold fluorescent light at 22 or 32 °C for 56 days. Periodic oil sampling increased the headspace, thereby mimicking consumer handling. Loss of retinyl palmitate in soybean oil by a maximum of 84.8 ± 5.76% was accompanied by a decrease of vitamin E by 53.3 ± 0.87% and by an increase of the peroxide value from 1.20 ± 0.004 to 24.3 ± 0.02 mequiv O2/kg. Fortification of highly oxidized oil with 31.6 IU/g retinyl palmitate led to a doubling of the average decrease of retinol per day compared to fortification of mildly oxidized oil. In conclusion, oil fortification programs need to consider the oxidative status of the oil used for retinyl palmitate fortification. KEYWORDS: retinyl palmitate, vitamin A, vitamin E, soybean oil, cold fluorescent light
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treatment at 180 °C for 25 min, the mean retention of retinyl acetate was only 56%.6 One of the initial factors determining the stability of RP in fortified edible oils and fats is their degree of oxidation prior to RP addition.7 It could be shown that storage of a mixture of fortified sunflower and soybean oil at a ratio of 1:1 on a shelf for 4 weeks led to a higher loss of vitamin A of 31.1% when the oil was characterized by a peroxide value (POV) of 5.8 mequiv O2/kg compared to a loss of vitamin A of only 19.7% in an oil with a POV of 0.4 mequiv O2/kg. Once the fortified oil is transported and stored, the ambient temperature, light, and oxygen exposure also have a major impact on the RP stability. Among these environmental factors, exposure to oxygen plays a crucial role. Whereas vitamin A was stable in fortified soybean oil that was stored in closed metal containers at 23 or 35 °C for up to 98 or 7 months,9 a 30% degradation of vitamin A was found when the fortified soybean oil was stored in open pails at 35 °C for only 1 month.9 These results also demonstrate that ambient temperatures of up to 35 °C do not chiefly contribute to vitamin A degradation in edible oils in the absence of controlled light exposure. The effect of light on the stability of vitamin A in fortified edible oils has also been widely studied, although the source of
INTRODUCTION Vitamin A deficiency is a major public health problem, especially in Africa and Southeast Asia. It is the leading cause of preventable blindness in children and increases the risk of disease and death from severe infections. In pregnant women, vitamin A deficiency causes night blindness. About 190 million pre-school-age children and 19.1 million pregnant women were affected by vitamin A deficiency during the past decade.1 To mitigate vitamin A deficiency, various fortification programs have been implemented. Fortification of staple foods such as refined sugar, flours, condiments, and edible oils and fats with vitamin A has been demonstrated to be a cost-effective intervention for reducing vitamin A deficiency, especially in settings where the variety of vitamin A rich foods is limited.2−4 As for edible oils and fats, for example, a dosage of at least 30 IU retinyl palmitate (RP) vitamin A per gram of oil has been recommended to cover 50% of the daily vitamin A requirement for a male adult.2 For pre-school-age children, a daily intake of 1333 IU/day is recommended,1 although this is rarely met in developing countries. Targeted high-dose vitamin A supplementation was reported to reduce childhood mortality rate by 24%.5 Oil matrices have been shown to have protective effects on vitamin A stability by delaying its oxidative degradation.4 Vitamin A has been suggested to be used as retinyl acetate or RP, of which RP showed superior stability during heat treatment. Whereas about 80% of RP was retained after heat © 2014 American Chemical Society
Received: Revised: Accepted: Published: 7559
May 7, 2014 July 4, 2014 July 8, 2014 July 8, 2014 dx.doi.org/10.1021/jf502109j | J. Agric. Food Chem. 2014, 62, 7559−7566
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light has rarely been specified. Most precisely, “exposure to sunlight” was applied, although these experimental conditions might not reflect retail or consumer handling. In retail stores and households, cold fluorescent lights are most abundant, whereas exposure to sunlight might be more prevalent in developing countries where the oil is scooped into polyethylene terephthalate (PET) bottles from large open containers stored outside. Sunlight-exposed storage of RP-fortified soybean oil in closed PET bottles for 4 weeks at room temperature has been demonstrated to reduce the initial vitamin A concentration by 27.1 ± 12.1%, whereas additional oxygen exposure, induced by intermittent opening of the PET bottles, augmented the vitamin A loss to 88.6 ± 1.97%.10 When the same oil samples were stored at room temperature in opened PET bottles in the dark, the mean retention of vitamin A was 97.4 ± 1.28%, clearly demonstrating that, at room temperature, protection from sunlight is more critical for preserving the vitamin A stability in edible oils than exclusion of oxygen. Whether the impact of cold fluorescent light, commonly installed in retail stores and households, is comparable to that of sunlight has not yet been studied. Besides the need to study the vitamin A stability, the stability of vitamin E (total tocopherols) in soybean oil stored under household conditions needs also to be investigated as tocopherols were shown to affect the stability of vitamin A.11 The presence of α-tocopherol, at a concentration of 156 μM, led to retention of 91 ± 4% of RP after air oxidation of the ethanolic RP solution for 4 h, compared to only 69 ± 4% retention of RP in the absence of α-tocopherol. However, the literature studying the stability of vitamins E and A in vegetable oils stored under domestic handling is scarce. Thus, the current study investigated, for the first time, the stability of RP and vitamin E in vitamin A fortified soybean oil stored under household conditions in the presence of cold fluorescent light at 22 and 32 °C. Soybean oil was filled in transparent PET bottles, which were reopened periodically for sampling to mimic consumer handling. Mildly and highly oxidized soybean oil was used for fortification with RP to elucidate the effect of oxidative oil quality on RP stability.
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Figure 1. Study design. Fortified soybean oil was stored under different household conditions at 22 and 32 °C. Samples were withdrawn on the indicated days. time of fortification on the stability of RP, a nonfortified soybean oil, which was exposed to cold fluorescent light at 22 ± 1 or 32 ± 1 °C, was stored until a POV of >10 mequiv O2/kg was reached on day 28. Then, this oil was fortified with 31.6 IU/g RP and stored until the end of the study (day 56). To mimic consumer handling, the bottles were reopened on days 1, 7, 14, 21, 28, and 56, and sample volumes of 51.4 mL, 42.4 mL, 42.4 mL, 37.0 mL, 42.4 mL, and 51.4 mL, respectively, were withdrawn. On day 56, a headspace volume of 334 mL was determined. On days 1, 7, 14, 28, and 56, samples were withdrawn for analyses after gently agitating the bottles. Thus, the samples, which were withdrawn on day 21 to mimic consumer handling, were not subjected to further analyses. Quantification of Copper in Soybean Oil. Copper concentrations in oil samples were determined by means of atomic absorption spectrometry using a PinAAcle 900Z graphite furnace atomic absorption spectrometer (PerkinElmer, Vienna, Austria) as described previously.12 A volume of 8 mL of 34% HNO3 (TraceSELECT Fluka) was added to 0.2 g of oil sample and heated to 180 °C in a microwave MARS XPRESS system (CEM Corp., Kamp-Lintfort, Germany). After treatment in the microwave oven, double-distilled water was added, leading to a final volume of 20 mL, which was passed through a 0.2 μm PTFE filter prior to measurement. The limit of detection for copper was determined to be 0.02 μg/g. Quantification of Retinol in Soybean Oil. Retinyl palmitate was analyzed as retinol after saponification. The quantification of retinol in soybean oil was performed according to the method of Bognar.13 Sample preparation was conducted under nitrogen atmosphere in the dark. For saponification, 1 g of oil sample was mixed with 30 mL of double-distilled water, 20 mL of KOH (10.7 M), 100 mL of ethanol, 1 g of sodium ascorbate, and 2 mL of sodium sulfide (0.51 M) and heated to 80 °C under reflux for 30 min. Afterward, the reflux apparatus was rinsed with 20 mL of water. The mixture was cooled to 15 °C before 28 mL of cold, double-distilled water and 100 mL of nhexane were added. The extraction was repeated twice with 100 mL of n-hexane and twice with 50 mL of n-hexane. The pooled hexane extracts were washed four times with 100 mL of cold, double-distilled water and dried with sodium sulfate. The solvent was removed from the dried extract under nitrogen by means of a rotating evaporator. The residue was dissolved in 5 mL of methanol before HPLC analysis. HPLC analysis was carried out on a Dionex Ultimate 3000 LC system (Thermo Fisher Scientific, Vienna, Austria), coupled to a diode array detector set to 325 nm. A volume of 20 μL of the samples was separated on a C18 column (Luna, 250 × 3.00 mm, 5 μm, Phenomenex, Aschaffenburg, Germany) with a flow rate of 0.5 mL/ min at 25 °C. A mixture of methanol and double-distilled water (98:2, v/v) was used to elute retinol after 5.05 min. Quantification of retinol was performed by applying external calibration. Calibration curves for retinol (y = 4.64x + 0.06) were prepared in the beginning, middle, and end of the study. To determine the limit of detection the signal-tonoise method was applied by measuring the peak height of the analyte and the noise in proximity of the analyte. A signal-to-noise ratio of 3 was considered as the lowest amount for detection. The limit of detection for retinol was 3.27 ± 0.25 ng/mL. The corresponding limit of quantification for retinol was calculated to be 10.9 ± 0.84 ng/mL, corresponding to a signal-to-noise ratio of 10. The recovery of retinol was determined by spiking oil samples with 30.5 μM retinyl palmitate
MATERIALS AND METHODS
Chemicals. Transparent PET bottles (0.5 L; 19.4 ± 0.05 g) and corresponding screw caps were kindly provided by Rauch Fruchtsäfte GmbH, Rankweil, Austria. Refined soybean oil was purchased from Henry Lamotte Oils GmbH, Bremen, Germany. For illumination of the oil with visible daylight, a cold cathode fluorescent lamp (Philips Tornado 1450 lm, 103 W) was obtained from Philips, Eindhoven, The Netherlands. d12-Hexanal was ordered from C/D/N Isotopes, Quebec, Canada. The internal standard all-rac-tocol was obtained from Matreya, Pleasant Gap, PA, USA. RP, which was stabilized with tocopherols, was kindly provided by DSM Nutritional Products, Kaiseraugst, Switzerland. All other chemicals were ordered from Sigma-Aldrich, Vienna, Austria, and Carl Roth, Karlsruhe, Germany. Study Design. Soybean oil was homogenized and partly fortified with 31.6 IU/g (17.4 mg/kg) RP. To ensure a homogeneous distribution of RP in the oil, RP was premixed in 50 g of oil before it was slowly added under agitation to 20 kg of soybean oil. A total of 460 ± 1 g fortified soybean oil was filled in each of the 0.5 L transparent PET bottles, which were closed with a screw cap afterward. The soybean oil was stored at 22 ± 1 or 32 ± 1 °C following a 12 h dark/light cycle in the presence of cold fluorescent light for 56 days (Figure 1). To expose soybean oil to visible light, a 103 W (1450 lm) lamp was installed 34 cm above the bottles. A distance of 15 cm was kept between the bottles to ensure access to illumination. To investigate the influence of the oxidative status of soybean oil at the 7560
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adding 0.5 μg/mL tocol as internal standard to the samples. By comparing the area under the curve of tocol in 2-propanol with the area under the curve of tocol in the samples, a recovery of 94.4 ± 12.1% could be determined. The limits of detection for α-, β-, γ-, and δ-tocopherols were calculated to be 0.24 ± 0.02, 0.20 ± 0.02, 0.04 ± 0.01, and 0.02 ± 0.01 μg/mL, respectively, according to the signal-tonoise method described above. The corresponding limits of quantification for α-, β-, γ-, and δ-tocopherols were calculated to be 0.80 ± 0.07, 0.67 ± 0.07, 0.13 ± 0.03, and 0.07 ± 0.03 μg/mL, corresponding to a signal-to-noise ratio of 10. Statistical Analysis. All experiments were performed in quadruplets with two technical replicates. The results are shown as the mean value ± standard deviation after exclusion of outliers by applying the Nalimov outlier test. To analyze time effects and differences between storage conditions, one-way ANOVA followed by the Holm−Sidak post hoc test was applied using SigmaPlot 11 (Systat Software Inc., Chicago, IL, USA). To study the relationship between retinol or tocopherols and hexanal or POV, the Pearson productmoment correlation coefficient was calculated. Differences were considered statistically significant when the p value was 0.05). When highly oxidized soybean oil samples with POVs of 12.1 ± Table 1. Percent Decrease of Retinol Concentration in Mildly and Highly Oxidized Soybean Oil, Which Was Fortified with RP on Days 1 and 28, Respectively, after Storage under Light Exposure at 22 and 32°C for 56 Daysa POV at RP fortification (mequiv O2/kg)
total decrease of retinol concentration at day 56 (%)
average decrease of retinol concentration per day (%)
mildly oxidized oil fortified with RP at day 1 (POV < 2 mequiv O2/kg) 22 °C 1.43 ± 0.24 84.8 ± 5.76*a 1.52 ± 0.10a 32 °C 1.43 ± 0.24 81.6 ± 11.1*a 1.46 ± 0.19a highly oxidized oil fortified with RP at day 28 (POV > 10 mequiv O2/kg) 22 °C 12.1 ± 0.23 72.0 ± 1.72*a 2.57 ± 0.06b 32 °C 17.3 ± 0.18 80.3 ± 13.6*a 2.87 ± 0.49b
Data are expressed as the mean ± SD (n = 4). Distinct letters within a column indicate statistically significant difference (p < 0.05). Asterisks indicate statistically significant difference with regard to the day of fortification with RP (p < 0.05).
a
Figure 6. Correlation analyses between retinol and lipid oxidation markers, such as POV (A) and hexanal (B), as detected in soybean oil exposed to cold fluorescent light in the presence of RP at 32 °C for 56 days. 7563
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0.23 mequiv O2/kg (stored at 22 °C) and 17.3 ± 0.18 mequiv O2/kg (stored at 32 °C) were fortified with RP on day 28 and stored for another 28 days, average decreases of retinol concentration of 2.57 ± 0.06 and 2.87 ± 0.49% per day were determined. Thus, the daily decrease of retinol concentration was accelerated when highly oxidized soybean oil with a POV of >10 mequiv O2/kg was used for fortification with RP, compared to mildly oxidized soybean oil with a POV of 1.43 ± 0.24 mequiv O2/kg (p < 0.05). No statistically significant difference was reached with regard to the overall decrease of retinol concentration on day 56 between the highly oxidized oil fortified with RP on day 28 of the study compared to the mildly oxidized oil, which was fortified already on day 1 of the study (p > 0.05). However, the rate of RP degradation was approximately 2-fold higher (p < 0.05) in the highly oxidized oil compared to the mildly oxidized oil.
oil stored for 56 days in this study. The reference values reported by the European Food Safety Agency (EFSA) are 7.5, 1.5, 79.7, and 26.6 mg/100 g for α-, β-, γ-, and δ-tocopherol, respectively.20 Another reason for the instability of vitamin A might be the exposure of the oil to cold fluorescent light and/or the storage-induced progress of lipid oxidation. Photooxidative deterioration of vegetable oils was demonstrated to be induced by natural pigments in vegetable oils.21 In the current study, storage of the oil, which was exposed to cold fluorescent light, for 14 days did not change the RP concentration independent of the conditions applied, likely due to sufficient availability of not yet degraded tocopherols or the less pronounced progress of lipid oxidation at the beginning of the study. The content of total tocopherols significantly decreased to almost 50% of its initial value after storage of the soybean oil for 56 days. These results are in agreements with findings reported by Player et al.,22 demonstrating a remarkable loss of individual tocopherols when the soybean oil was stored in a forced-air oven at 50 °C for 24 days: whereas α-tocopherol was completely degraded after 16 days of storage in the oven, γ- and δ-tocopherols decreased by 28 and 17% after 24 days of storage at 50 °C. In the current study, α-tocopherol was rapidly degraded to 64.1 ± 10.5% of its initial value in soybean oil after the first 28 days of storage under domestic conditions at 22 °C, whereas γtocopherol was degraded to 88.9 ± 5.65% of its initial concentration (p < 0.05). The slight increase of δ-tocopherol in soybean oil after 28 days of storage might indicate antioxidant activity of δ-tocopherol on day 1 because its original concentration might be regenerated between days 1 and 28. δ-Tocopherol might be regenerated by phytosterols in soybean oil, such as brassicasterin. The certificate of analysis of the study oil confirmed the presence of brassicasterins ( 10 mequiv O2/ kg) oxidative status at RP fortification. It could be demonstrated that the RP concentration in soybean oil depends not only on the storage duration under domestic conditions and handling but also on the oxidative quality of the oil at the time of fortification with RP. Whereas fortification of the mildly oxidized soybean oil, characterized by a POV of 1.43 ± 0.24 mequiv O2/kg at day 1, led to average decreases of retinol concentration of 1.52 ± 0.10 and 1.46 ± 0.19% per day at 22 and 32 °C, respectively, fortification of oxidized oil, characterized by a POV of 12.1 ± 0.23 or 17.3 ± 0.18 mequiv O2/kg, induced more pronounced decreases of retinol concentrations of 2.57 ± 0.06 and 2.87 ± 0.49% per day at 22 and 32 °C, respectively. Thus, the decay of RP was approximately twice as high when oxidized soybean oil was used for fortification. Comparable results were obtained by Laillou et al.,7 who demonstrated a higher loss of vitamin A of 23.1% in an oxidized (POV = 5.8 mequiv O2/kg) oil mixture of soybean oil and sunflower oil (1:1), which was exposed to uncontrolled light during domestic storing on a shelf at 30 ± 5 °C for 4 weeks compared to a mildly oxidized oil mixture (POV = 0.4 mequiv O2/kg) with a loss of vitamin A of only 7.3%. There, the degradation of vitamin A was about 3 times higher in the highly oxidized than in the mildly oxidized oil mixture. The relative loss of vitamin A in highly oxidized compared to mildly
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AUTHOR INFORMATION
Corresponding Author
*(V.S.) Mail: Althanstraße 14, 1090 Vienna, Austria. Phone: +43 1 4227 70601. Fax: +43 1 4227 9706. E-mail: veronika.
[email protected]. Funding
This study was financially supported by Sight and Life, a humanitarian nutrition think tank of DSM. Notes
The authors declare the following competing financial interest(s): This study was financially supported by Sight and Life, a humanitarian nutrition think tank of DSM. Dr. Klaus Kraemer, one of the authors, is heading Sight and Life. The author Georg Steiger is an employee of DSM.
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ABBREVIATIONS USED
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REFERENCES
PET, polyethylene terephthalate; POV, peroxide value; RP, retinyl palmitate
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