Antioxidant properties of ascorbic acid in bulk oils at different relative
humidity
Ji Young Kim, Mi-Ja Kim, Bora Yi, Sumi Oh, JaeHwan Lee ⇑
Department of Food Science and Biotechnology, Sungkyunkwan University, Suwon, Republic of Korea
article info
Article history:
Received 23 December 2013
Received in revised form 19 October 2014
Accepted 18 December 2014
Available online 25 December 2014
Keywords:
Ascorbic acid
Relative humidity
Antioxidant polar paradox
Bulk oil
abstract
The effects of relative humidity (RH) on the antioxidant properties of ascorbic acid (10, 20, 42, and
84 ppm) were investigated in stripped corn oils stored at 60 C. The degree of oxidation in oils was
determined by analysing headspace oxygen content and conjugated dienoic acids. The oxidative stability
of bulk oils without addition of ascorbic acid was significantly different depending on the RH. As the
concentration of ascorbic acid increased from 10 to 84 ppm, oxidative stability increased significantly
irrespective of RH (p < 0.05). Generally, oils containing ascorbic acid at low RH had higher oxidative
stability after storage at 60 C than those at high RH. The antioxidant properties of ascorbic acid were
greatly influenced by both the moisture content in the oil and the ascorbic acid concentration.
2014 Elsevier Ltd. All rights reserved.
1. Introduction
Oxidation products from lipids can cause deterioration in the
sensory attributes and nutritional values of foods, which in turn
decreases consumers’ acceptance of food products. The degree of
lipid oxidation is greatly influenced by many factors including
the content of unsaturated fat, matrix types such as oil-in-water
(O/W) emulsions or bulk oils, and the presence of pro-oxidative
metal ions and antioxidants (Chaiyasit, Elias, McClements, &
Decker, 2007; Choe & Min, 2006; McClements & Decker, 2000).
Ascorbic acid has an enediol-lactone resonant structure, which
provides reducing ability. Due to its hydrophilic characteristics,
the antioxidant capacities of ascorbic acid have been extensively
studied in oil-in-water emulsions (Jayasinghe, Gotoh, & Wada,
2013; Kim, Decker, & Lee, 2012; Kim, Park, Kim, & Lee, 2013),
matrices containing a high proportion of water such as milk (Van
Aardt et al., 2005), or olive oil containing intentionally added
moisture to carry the ascorbic acid (Mosca, Ceglie, & Ambrosone,
2008). Ascorbic acid can effectively retard the formation of lipid
oxidation products in bulk oils (Shahidi & Zhong, 2011) and in
linoleic acid model systems (Watanabe, Ishido, Fang, Adachi, &
Matsuno, 2005).
Generally, ascorbic acid shows antioxidant capacities in bulk
oils and pro-oxidant properties in O/W emulsions (Frankel,
Huang, Kanner, & German, 1994; Kim et al., 2012). This phenomenon
is called the ‘antioxidant polar paradox’, which states that
hydrophilic antioxidants decrease the rates of lipid oxidation most
efficiently in non-polar media such as bulk oil systems, whereas
lipophilic antioxidants show better antioxidant capacities in more
polar media such as O/W emulsions and liposomes. Recently, this
theory has been re-evaluated in both O/W emulsions (Laguerre
et al., 2010; Lee et al., 2013; Lucas et al., 2010) and in bulk oils
(Shahidi & Zhong, 2011). Shahidi and his research group (2011)
proposed that the antioxidant capacities of chemicals in bulk oils
are greatly influenced by the polarity and concentration of the
antioxidants.
The moisture content in bulk oils plays an important role in
lipid oxidation through the association of colloids with amphiphilic
compounds including free fatty acids (FFAs), phospholipids (PLs),
diacylglycerols (DAGs), and monoacylglycerols (MAGs) (Chaiyasit
et al., 2007; McClements, 2004; Schwarz et al., 2000). The surfaces
of association colloids are major sites of lipid oxidation (Frankel
et al., 1994; McClements & Decker, 2000). Park, Kim, Kim, and
Lee (2014) reported that increases in the rates of lipid oxidation
occur concomitantly with increases in moisture content in oils
treated at 100 and 140 C under airtight conditions. Also, moisture
in lipids may participate in the formation of volatiles during lipid
oxidation under airtight conditions (Kim, Kim, & Lee, 2014).
However, the effect of moisture content in bulk oils on the
antioxidant capacities of hydrophilic compounds has not been
reported in the literature. Changes in antioxidant capacities in bulk
oils according to the concentration and polarity of compounds as
suggested by Shahidi and Zhong (2011) could be due to the
http://dx.doi.org/10.1016/j.foodchem.2014.12.079
0308-8146/ 2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Department of Food Science and Biotechnology,
Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do
440-746, Republic of Korea. Tel.: +82 31 290 7809; fax: +82 31 290 7882.
E-mail address: s3hun@skku.edu (J. Lee).
Food Chemistry 176 (2015) 302–307
Contents lists available at ScienceDirect
Food C
available moisture content in bulk oils because antioxidants should
be located near the surface of association colloids. In this study,
ascorbic acid was selected as a representative hydrophilic antioxidant
and conditions of different moisture content were generated
using saturated salt solutions.
The objective of this study was to determine the effects of
relative humidity (RH) and ascorbic acid concentration on the
oxidative stability of stripped bulk oils. Double vials containing
saturated salt solutions were designed to generate the desired
relative humidity conditions.
2. Materials and methods
2.1. Materials
Phosphorus pentoxide, lithium chloride, magnesium chloride,
magnesium nitrate, sodium chloride, potassium nitrate, and activated
charcoal powder were purchased from Daejung Chemical
Co. (Seoul, Korea). Ascorbic acid and silicic acid were purchased
from Sigma–Aldrich (St. Louis, MO, USA). Corn oil was purchased
from a local grocery market (Suwon, Korea). Other reagent grade
chemicals were obtained from Daejung Chemical Co.
2.2. Sample preparation
To remove impurities, corn oil was stripped according to the
previous report of Kim, Yi, Kim, and Lee (2015). Saturated salt solutions
were prepared using phosphorus pentoxide (RH approximately
0%), lithium chloride (RH 11%), magnesium chloride (RH
32%), magnesium nitrate (RH 52%), sodium chloride (RH 75%),
and potassium nitrate (RH 93%). Ascorbic acid was dissolved in
methanol and added to the stripped oil at final concentrations of
10, 20, 42, and 84 ppm, respectively. Solvent in oils was removed
under nitrogen gas flow. One gram of saturated salt solution was
placed in 10-mL bottles and 2-mL bottles filled with 0.5 g of
stripped corn oil containing various concentrations of ascorbic acid
were placed inside the 10-mL bottles. The double vials were sealed
airtight with rubber septa and aluminium caps (Fig. 1). All of the
sample bottles were incubated at 60 C in a drying oven (Hysc
CO. Ltd, Seoul, Korea) for 62 h. Samples were prepared in triplicate.
Samples with phosphorus pentoxide (RH 0%), lithium chloride
(RH 11%), magnesium chloride (RH 32%), magnesium nitrate (RH
52%), sodium chloride (RH 75%), and potassium nitrate (RH 93%)
were abbreviated PP, LC, MC, MN, SC, and PN, respectively. Samples
with addition of ascorbic acid and without salt solution were designated
CON and samples without addition of ascorbic acid or
deionised water/salt solution were designated CON W/O (Kim
et al., 2015).
2.3. Headspace oxygen analysis
Content of headspace oxygen was analysed to determine the
degree of lipid oxidation in corn oil under airtight conditions.
The headspace oxygen in airtight sample bottles was analysed
according to methods of Kim, Yi, Kim, and Lee (2014) using a Hewlett–Packard
7890 gas chromatograph (GC) (Agilent Technologies,
Inc., Santa Clara, CA, USA) equipped with a 60/80 packed column
(3.0 m 2 mm ID, Restek Ltd., USA) and a thermal conductivity
detector (TCD) from Agilent Technologies (Palo Alto, CA, USA).
The flow rate of helium gas was 20 mL/min. Temperatures of oven,
injector, and detector were 60, 180, and 180 C, respectively.
2.4. Conjugated dienoic acid (CDA) analysis
The CDA content of samples was measured according to AOCS
method Ti la-64 (2006).
2.5. Moisture content analysis
The moisture content in oils was determined using a coulometric
KF titrator (C20, Mettler-Toledo Intl., Columbus, OH, USA)
according to the manufacturer’s instructions.
2.6. Statistical analysis
Data for headspace oxygen content, CDA, and moisture content
were analysed statistically by ANOVA and Duncan’s multiple
range test using SPSS software program (SPSS Inc., Chicago, IL, USA).
A p value <0.05 was considered significant.
3. Results and discussion
3.1. Oxidation of bulk oils at different relative humidity
Effects of relative humidity on the headspace oxygen content,
CDA, and moisture content in stripped corn oils at 60 C are shown
in Fig. 2. Headspace oxygen content in samples stored at 60 C was
significantly influenced by RH (p < 0.05) although the oxidative
stability in stripped bulk oils was not proportionally dependent
on relative humidity. Samples with SC (the 75% RH environment)
had the lowest headspace oxygen content whereas those with LC
and MC environment (11% and 32% RH, respectively) had the highest
headspace oxygen content (p < 0.05). Samples with very low RH
such as PP had lower headspace oxygen content than those with LC
and MC. Also, samples at very high RH such as PN (93%) had
significantly higher headspace oxygen content than those at SC
(p < 0.05), resulting in a trend of a ‘‘N’’-shape from PP to PN
samples (Fig. 2a).
Samples in the SC environment had significantly higher CDA
values and those with LC and MC showed significantly lower
CDA values compared with other environments (p < 0.05). The plot
of CDA values against RH gave a reverse ‘N’ curve or ‘B’ shape.
However, the order of oxidative stability based on CDA values
was not as clear as that based on h
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