Evaluation on oxidative stability of walnut beverage emulsions

Food Chemistry 203 (2016) 409–416

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Evaluation on oxidative stability of walnut beverage emulsions Shuang Liu, Fuguo Liu, Yanhui Xue, Yanxiang Gao ⇑ Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Laboratory for Food Quality and Safety, Beijing Key Laboratory of Functional Food from Plant Resources, College of Food Science and Nutritional Engineering, China Agricultural University, Beijing 100083, PR China

a r t i c l e

i n f o

Article history: Received 8 November 2015 Received in revised form 31 January 2016 Accepted 4 February 2016 Available online 4 February 2016 Keywords: Walnut beverage emulsions Lipid oxidation Food antioxidant Light exposure Thermal stability

a b s t r a c t Walnut beverage emulsions were prepared with walnut kernels, mixed nonionic emulsifiers and xanthan gum. The effects of food antioxidants on the physical stability and lipid oxidation of walnut beverage emulsions were investigated. The results showed that tea polyphenols could not only increase the droplet size of the emulsions, but also enhance physical stability during the thermal storage at 62 ± 1 °C. However, water-dispersed oil-soluble vitamin E and enzymatically modified isoquercitrin obviously decreased the physical stability of the emulsion system during the thermal storage. BHT and natural antioxidant extract had scarcely influenced on the physical stability of walnut beverage emulsions. Tea polyphenols and BHT could significantly retard lipid oxidation in walnut beverage emulsions against thermal and UV light exposure during the storage. Vitamin E exhibited the prooxidant effect during the thermal storage and the antioxidant attribute during UV light exposure. Other food antioxidants had no significant effect on retarding lipid oxidation during thermal or light storage. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Walnut kernels contain plentiful fat and protein (Prasad, 1994). The relative ratio of unsaturated fatty acids in walnut oil (relative to total fatty acids) is apparently higher than other edible oils. The diet with polyunsaturated or monounsaturated fatty acids can lower plasma and low-density-lipoprotein (LDL) cholesterol, thus reduce the risk of heart disease (Abbey, Noakes, Belling, & Nestel, 1994). The major fatty acids found in walnut oil are oleic (18:1), linoleic (18:2) and linolenic (18:3) acids (Cunnane et al., 1993). Although unsaturated fatty acids have many benefits, they are extremely sensitive to heat and light during the process and storage, resulting in potential alteration in nutritional composition and food quality (Choe & Min, 2006). These fatty acids are subjected to rapid and extensive oxidation by exposure to light, heat or air during the process and storage (Ng, Lau, Tan, Long, & Nyam, 2013), resulting in the generation of hydroperoxides, and subsequently, off-flavor compounds (Alamed, Chaiyasit, McClements, & Decker, 2009). Thus, the lipid oxidation limits the utilization of walnut oil in processed foods. Formulation and physical stability characterization of walnut beverage emulsions were published for the first time by Gharibzahedi, Mousavi, Khodaiyan, and Hamedi (2012). The appli-

⇑ Corresponding author at: Box 112, No. 17 Qinghua East Road, Haidian District, Beijing 100083, PR China. E-mail address: [email protected] (Y. Gao). http://dx.doi.org/10.1016/j.foodchem.2016.02.037 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.

cation of walnut oil as a functional component in the production of beverage emulsions was investigated using response surface methodology (RSM). Homayoonfal, Khodaiyan, and Mousavi (2014a, 2014b, 2015) investigated the influence of ultrasonic time, walnut oil content and the emulsifier concentration on the physical stability of walnut oil in water nano-emulsion by RSM in conjunction with central composite rotatable design (CCRD). These studies revealed that a linear term of walnut oil concentration was the most significant parameter for all the responses and walnut oil in O/W emulsion could be prepared with polysaccharide, emulsifier and walnut oil. However, all the reports about walnut beverage emulsions only dealt with a buffer system, not a real food system. In the present study, we tried to use walnut kernels as the material and retained all its components in the emulsion and focus on the physicochemical stability during the process and storage. Nowadays, walnut beverage emulsions become more and more popular in plant protein beverage market, which is similar to soybean milk. Therefore, it is necessary to study the physicochemical stability and shelf life of walnut beverage emulsions. We have evaluated the effects of pH, freeze-thaw and thermal sterilization on physicochemical stability of walnut beverage emulsions (Liu, Sun, Xue, & Gao, 2016). It consisted of three parts: an aqueous continuous phase, the droplet’s oil core and the interfacial membrane. The mechanism of lipid oxidation in oil-in-water emulsion differed from bulk lipids, because the emulsions had an aqueous phase containing both antioxidants and prooxidants and the oil-water interface impacted the interaction between oil and water soluble

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components (Frankel, Huang, Kanner, & German, 1994). There are many factors that can influence the rate of lipid oxidation in oilin-water emulsions, such as oxygen concentration, particle size, the thickness and interfacial rheology properties (Waraho, McClements, & Decker, 2011). Lipid oxidation in oil-in-water emulsions was extensively investigated. McClements and Decker (2000) reported that the transition metals were the major prooxidants in oil-in-water emulsions. The stabilizer and emulsifier could also have an impact on lipid oxidation of emulsions by changing the charge of the emulsion droplets and the thickness of interfacial film (Hu, McClements, & Decker, 2003). Many studies revealed that the incorporation of antioxidants into foods is one of the most effective approaches in retarding lipid oxidation. Phenolics are from natural plant sources and commonly applied as antioxidants to inhibit lipid oxidation in emulsion systems. The phenolic antioxidants could increase the reactivity of prooxidant metals and the partition into the emulsion droplets where they might scavenge free radicals (Mei, McClements, & Decker, 1999). Nowadays, more and more natural antioxidants have been applied to emulsions system, such as vitamin C, vitamin E, tea polyphenols and natural plant extracts. Grape seed extract was found to be effective in scavenging free radicals and inhibiting lipid oxidation in algae oil-in-water emulsions with certain concentration, and the major antioxidants in the seeds were proanthocyanidins (Hu, McClements, & Decker, 2004). Meanwhile, the natural antioxidants have the characteristics of high efficient and low toxicity, which are better than the synthetic ones, such as BHT, TBHQ and BHA. The antioxidative attribute of anthocyanin-rich purple corn husk extract (PCHE) in mayonnaise was compared with that of BHT and EDTA. During storage, the antioxidative effect of PCHE on the mayonnaise was more significant than that with synthetic antioxidants such as BHT and EDTA (Li, Kim, Li, Lee, & Rhee, 2014). Tea polyphenols (TP) and vitamin E (VE) are extensively utilized in bulk oils and emulsions system as natural antioxidants. However, these antioxidants have never been applied in real walnut beverage emulsions system. Natural antioxidant extract (NAE) and enzymatically modified isoquercitrin (EMIQ) would be applied as novel natural antioxidants in walnut beverage emulsions. The objective of this study focused on the effects of different food antioxidants on the physical and oxidative stability of walnut beverage emulsions.

2. Materials and methods 2.1. Materials Walnut kernels were purchased from Dinghui Food Co., Ltd. (Hebei, China) and stored at 19 °C until used. Xanthan gum was obtained from Xinhe Biochemical Co., Ltd. (Hebei, China). Glycerol monostearate (GMS, HLB: 3.8) was obtained from Danisco Co., Ltd. (Beijing, China). Decaglycerol monolaurate (DML, HLB: 15.5) was obtained from Evonik Food Development Co., Ltd. (Shanghai, China). Sodium azide, BHT and FAME MIX (C8–C22) were purchased from Sigma Aldrich (USA). Methanol and 1-butanol were purchased from Beijing Chemical Work. Ammonium thiocyanate and BaCl2 were purchased from Tianjin Chemical Work. FeSO4
7H2O was obtained from Guanghua Sci-Tech Co., Ltd. (Guangdong, China). 1,1,3,3-Tetraethoxypropane was purchased from Xiya Reagent (China). Water-soluble tea polyphenols (purity >99%) were obtained from Hongyi Biotechnology Co., Ltd. (Henan, China). Water-dispersed vitamin E (purity >50%) and vitamin E (purity >50%) were obtained from DSM (Switzerland) and Lantian Biotechnology Co., Ltd. (Xian, China), respectively. The natural antioxidant extract (NAE, SL25933) and enzymatically modified isoquercitrin

(EMIQ; purity >30%)were gifted by Ogawa Co., Ltd. (Japan) and San-Ei Gen F.F.I. Co, Ltd. (Japan), respectively. 2.2. Walnut kernel slurry The peeled walnut kernels were mixed with deionized water at the weight ratio of 1:4.5 and processed into the slurry. The crude slurry was treated through a colloid mill and filtered through three layers of gauze to remove the solid residues and obtain the coarse slurry. 2.3. Preparation of walnut beverage emulsions Previously we have optimized the species levels of emulsifiers (GMS, DML, and sucrose ester) employed in the emulsion and polysaccharides (xanthan gum, soybean polysaccharides and arabic gum). In order to obtain the stable emulsion, the formulation of walnut beverage emulsions was designed as follows: coarse slurry (18%), sucrose (6.5%, wt), xanthan gum (0.09%, wt), GMS (0.18%, wt), DML (0.07%, wt), sodium azide (0.02%, wt) and deionized water (75.14%). The coarse walnut beverage emulsions were prepared by mixing the slurry with the aforementioned additives. The water-soluble antioxidants were directly incorporated into coarse walnut beverage emulsions. On the other hand, the oil soluble antioxidants were solubilized in ethanol firstly (1:10, w/w), then, the solutions were added to coarse walnut beverage emulsions. To achieve a fine emulsion with small mean particle size and narrow particle-size distribution, pre-homogenization was performed by using an Ultra-Turrax (T25, IKA, Staufen, Germany) at a speed of 10,000 rpm for 3 min, and then passed through a two-stage valve homogenizer (Niro-Soavi Panda, Parma, Italy) for three cycles at 65 MPa and 65 °C. The final emulsion was immediately cooled down to 25 °C and then transferred into screw-capped brown bottles. 2.4. Light stability of walnut beverage emulsions The emulsions in the presence of different antioxidants were transferred into glass bottles and put into a controlled light cabinet (Q-SUN Xe-1-B, 0.35 W/m2, 45 °C) (Q-LAB, USA). The temperature was regulated on black panel (Ploeger, Scalarone, & Chiantore, 2009). The contents of peroxide and malondialdehyde (MDA) were regularly determined at an interval of 2 h. 2.5. Thermal stability of walnut beverage emulsions Emulsion samples (55 mL) were transferred into brown bottles with metal cap and tighten by the automatic capper. The oxidation reaction was accelerated in a force-draft air oven DHG-9140A (Shengxin Instruments; Shanghai, China) set at 62 ± 1 °C (Ramadan, 2013) and the changes of droplet size, peroxide concentration, MDA and the composition of fatty acids in emulsions were regularly determined at an interval of 2 or 3 days. 2.6. Analytical methods 2.6.1. Mean particle size The mean particle sizes of samples were determined with a Zetasizer Nano-ZS90 (Malvern Instruments, Worcestershire, UK) at a fixed detector angle of 90°. To avoid multiple scattering effects, the emulsions were diluted with deionized water at a ratio of 1:800 (v/v). Droplet size was described as cumulative mean diameter (size, nm). All measurements were performed in triplicate.

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2.6.3. Oxidative stability measurement of walnut beverage emulsions 2.6.3.1. Measurement of peroxide concentration in walnut beverage emulsions. Lipid hydroperoxides were measured using a modified method of Shantha and Decker (1993). The walnut beverage emulsions were stored at 37 °C and 55 °C, respectively. The lipid peroxides were measured at an interval of 2 or 3 days. Three milliliters of methanol/1-butanol (2:1, v/v) were mixed with 20 lL of an emulsion sample and 30 lL of thiocyanate/Fe2+ solution. The thiocyanate/Fe2+ solution was prepared immediately before used by mixing an equal amount of thiocyanate solution (3.94 M ammonium thiocyanate) and Fe2+ solution (obtained from the supernatant of a mixture of 3 mL 0.144 M BaCl2 in 0.4 M HCl and 3 mL of freshly prepared 0.144 M FeSO4). After 20 min, the absorbance was measured at 510 nm with an UV spectrophotometer (UV1800, Japan) and lipid peroxides were quantitated from a standard curve using Fe3+. 2.6.3.2. Measurement of MDA in walnut beverage emulsions. Malondialdehyde (MDA) was determined using the method of McDonald and Hultin (1987). The emulsion sample (0.05 mL) was combined with 0.95 mL of deionized water and 2.0 mL of TBA reagent (15% w/v trichloroacetic acid and 0.375% w/v thiobarbituric acid in 0.25 M HCl) in test tubes and placed in a boiling water bath for 15 min. The tubes were cooled down to room temperature and then centrifuged (1000g) for 15 min. The absorbance was measured at 532 nm with an UV spectrophotometer (UV-1800, Japan) and the concentration of MDA was calculated from a standard curve prepared using 1,1,3,3-tetraethoxypropane.

2.7. Statistical analysis All experiments were performed in triplicate and the results were expressed as mean value ± standard deviation (SD) in this study. Data were subjected to analysis of variance (ANOVA) on SPSS 18.0 for Windows (SPSS Inc., Chicago, USA), and the difference was considered to be significant with p < 0.05.

3. Results and discussion 3.1. Effect of different antioxidants on physical stability of walnut beverage emulsions The influence of tea polyphenols, water-dispersed vitamin E, vitamin E, BHT, NAE and EMIQ on physical stability of walnut beverage emulsions was shown in Figs. 1 and 2. In order to evaluate the effect of different antioxidants on the physical stability, the size change was monitored during the accelerated oxidation period. Fig. 1 displayed the change of integral transmission vs centrifugation time. The slope of this graph was equal to velocity of creaming of the primary emulsions. It could be found that the slopes of all curves were almost the same, except for the emulsion with 0.02% tea polyphenols, as shown in Fig. 1(a). Therefore, walnut

a

50

control TP-0.04% water-dispersed VE-0.04%

45

Integral transmission (%)

2.6.2. Physical stability measurement of walnut beverage emulsions The physical stability of walnut beverage emulsions was measured by the LUMiSizer (L.U.M. 290 GmbH, Germany) according to the method of Xu, Wang, Jiang, Yuan, and Gao (2012) with some modifications. LUMiSizer instrument is a multisample analytical instrument employing centrifugal sedimentation to accelerate the occurrence of instability phenomena such as sedimentation, flocculation or creaming (Sobisch & Lerche, 2008). The integration graph shows the percentage of light absorbance as a function of time and position over the entire sample, the ‘‘creaming rate”. The rate is correlated to the physical stability of the emulsion: the lower the creaming rate, the higher the stability. The parameters used for the measurement were set as follows: rotational speed, 4000 rpm; time 7650 s; temperature, 25 °C; time interval, 30 s (Lei, Liu, Yuan, & Gao, 2014).

40

VE-0.04%

35

BHT-0.04%

30 25 20 15 10 5 0

2000

3000

4000

5000

6000

7000

8000

6000

7000

8000

Time (s)

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50

control NAE-0.02% NAE-0.04% EMIQ-0.04%

45

Integral transmission (%)

2.6.3.3. Measurement of the composition of fatty acids in walnut beverage emulsions. Walnut oil was extracted from the emulsion using the method of CN102288714A (Shi et al., 2011). Briefly, 15 mL of walnut beverage emulsions was combined with 0.1 mL of HCl water solution (1:1, v/v) and 20 mL of n-hexane (chromatographic grade) in a test tube with stopper. The tube was centrifuged (2500g) for 10 min. Then, 2 mL of supernatant and 0.8 mL of KOH-methanol (5.0g of KOH dissolved into 1000 mL of methanol) were mixed by vortex mixer thoroughly. The fatty acids of walnut oil in emulsions were initially identified using an Agilent 6890N GC equipped with an Innowax 19091N-113 capillary column (30 m length; 0.32 mm i.d. and 0.25 lm film thickness). The injection was performed in the splitless mode. Oven temperature was initially held at 45 °C for 1 min, followed by an increase of 4 °C/min to 180 °C, and finally by 2 °C/ min to 210 °C for 10 min. Nitrogen was used as carrier gas with a flow rate of 1 mL/min (Lu, Yang, Bi, Liang, & Mei, 2010). The fatty acids of walnut oil in emulsions were analyzed by an Agilent 6890N GC equipped with a flame ionization detector (FID) and the same column and operating condition were used.

1000

40 35 30 25 20 15 10 0

1000

2000

3000

4000

5000

Time (s) Fig. 1. Recorded evolutions of time dependent integral transmission profiles of walnut beverage emulsions with different antioxidants.

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600 550

control TP-0.04% water-dispersed VE-0.04%

500

Droplet size (nm)

medium, such as pectin; (3) steric effects, limiting the number of available binding sites of proteins to readily fit polyphenols, such as dextran (Liu et al., 2016; Yang et al., 2015). As shown in Fig. 1, vitamin E, BHT, NAE and EMIQ did not alter the creaming rate of walnut beverage emulsions. However, during the storage, the droplet sizes of the emulsions in the presence of water-dispersed vitamin E, vitamin E and EMIQ were significantly. This indicated that the emulsion system with vitamin E or EMIQ prone to be damaged during thermal storage. In summary, although tea polyphenols could increase the droplet size of the emulsion, they could maintain better physical stability during the storage. However, vitamin E and EMIQ could damage the emulsion system during thermal storage. Meanwhile, BHT and NAE had little influence on the physical stability of walnut beverage emulsions.

a

VE-0.04%

450

BHT-0.04%

400 350 300 250 200 0

5

10

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25

Time (day) 800

Walnut kernel oil is chemically unstable and prone to oxidative deterioration, especially when exposed to oxygen, light, moisture and heat (Ng et al., 2013). Temperature is an important factor affecting lipid oxidation and the purpose of these experiments was to explore the change of oxidative products in the presence of different antioxidants during accelerated oxidation period. The changes of the primary product (peroxide) and secondary product (MDA) concentrations were shown in Fig. 3.

b

Droplet size (nm)

700

control NAE-0.02% NAE-0.04% EMIQ-0.04%

600

500

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200 0

5

10

3.2. Effect of different antioxidants on peroxide and MDA concentrations during the thermal storage

15

20

25

Time (day) Fig. 2. Size change of the droplets in walnut beverage emulsions with different antioxidants at 62 ± 1 °C as a function of storage time. Error bars represent standard deviations (n = 3).

beverage emulsions in the presence of tea polyphenols exhibited poorer physical stability than those with other antioxidants. On the other hand, the droplet size of the emulsion with tea polyphenols was larger than others at the first day. This was because polyphenols could interact with proteins through hydrophobic or hydrophilic effects (Ozdal, Capanoglu, & Altay, 2013), leading to the formation of soluble or insoluble complexes (Bandyopadhyay, Ghosh, & Ghosh, 2012), which destroyed the physical stability of the emulsion. However, during accelerated storage test, the droplet size of the emulsion in the presence of tea polyphenols was scarcely changed, as shown in Fig. 2(a). This result indicated that the emulsion in the presence of tea polyphenols remained better thermal stability during the storage, which could be attributed to xanthan gum and it was an important factor that influenced the interaction between proteins and polyphenols. Xanthan gum, as an anionic polysaccharide, could prevent the protein–polyphenol aggregation, reported by Carvalho, Póvoas, Mateus, and De Freitas (2006). There are three mechanisms responsible for the inhibition of protein–polyphenol aggregation by some polysaccharides: (1) molecular association between polysaccharides, such as arabic gum, b-cyclodextrin, and polyphenols, competing with protein aggregation; (2) the formation of a protein–polyphenol–poly saccharide ternary complex, enhancing its solubility in aqueous

3.2.1. Effect of different antioxidants on peroxide concentration during the thermal storage As shown in Fig. 3(1a), in the absence of antioxidants (control sample), the peroxide concentration began to rapidly increase after 14 days of the storage, which indicated that the lipid entered a period of oxidation and oil began to deteriorate. However, the emulsion containing vitamin E had the highest peroxide concentration during the storage, which indicated that vitamin E exhibited the prooxidant effect. For oil phase, the concentration (0.04%) of vitamin E was too high and it was prone to be oxidized into hydrogen peroxide radicals (Cillard & Cillard, 1980). For water-dispersed vitamin E, it was easy to be solubilized in water phase and it could inhibit the formation of hydroperoxides. Meanwhile, the emulsions with BHT and tea polyphenols had significantly peroxide values than that in control sample during thermal storage. They are phenolic antioxidants and have the capacity to donate one hydrogen atom to a free radical and reduce propagation of the radical chain reaction (Gallego, Gordon, Segovia, Skowyra, & Almajano, 2013). On the other hand, peroxide values showed that the oil in the emulsions containing NAE and EMIQ was oxidized at the faster rate over 17 days compared with others, as shown in Fig. 3(1b). This result indicated that these antioxidants disabled to inhibit the formation of hydroperoxides for a long time during thermal storage. As shown in Fig. 3(1), the emulsions in the presence of 0.02% NAE and 0.04% EMIQ showed that peroxide values were significantly (p < 0.05) reduced for 25 days of the thermal storage. These results might be attributed to that the hydroperoxides were decomposed into volatile substances and similar results were also reported in the literature (Cardenia, Waraho, Rodriguez-Estrada, McClements, & Decker, 2011). 3.2.2. Effect of different antioxidants on MDA concentration during the thermal storage Hydroperoxides usually suffered further oxidation and turned into secondary oxidation products, such as hexane, MDA (Barriuso, Astiasarán, & Ansorena, 2013). As shown in Fig. 3(2a), the change of MDA values showed that the emulsions containing tea polyphenols, BHT, vitamin E, and water-dispersed vitamin E

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a

b 2500

control TP-0.04% water-dispersed VE-0.04%

2000

Peroxide concentration ( mol/g oil)

Peroxide concentration ( mol/g oil)

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VE-0.04% BHT-0.04% 1500

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control NAE-0.02% NAE-0.04% EMIQ-0.04%

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MDA ( g/g oil)

MDA ( g/g oil)

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Time (day)

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control NAE-0.02% NAE-0.04% EMIQ-0.04%

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Fig. 3. Peroxide concentration (1) and MDA (2) change of the oil in walnut beverage emulsions with different antioxidants at 62 ± 1 °C as a function of storage time. Error bars represent standard deviations (n = 3).

successfully retarded the formation of MDA during the thermal storage, respectively. Therefore, associated with the result of 3.2.1, tea polyphenols, BHT and water-dispersed vitamin E could inhibit the formation of both primary and secondary products. The emulsions with NAE and EMIQ had slightly lower MDA concentration than that in control sample, as shown in Fig. 3(1b). This result implied that the addition of NAE and EMIQ did not have a major impact on inhibiting the formation of the secondary product (MDA) during the thermal storage. In summary, tea polyphenols, BHT and water-dispersed vitamin E showed a significant (p < 0.05) effect on retarding lipid oxidation during thermal storage. 3.2.3. Effect of different antioxidants on the composition of fatty acids during the thermal storage Lipid oxidation is a major problem leading to the deterioration of polyunsaturated fatty acids, which needs to be prevented because it would cause undesirable changes in flavor, texture, appearance, and nutritional quality of food products (Waraho et al., 2011). Palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2 n-6), and linolenic acid (18:3 n-6) are the major fatty acids in walnut oil, which were analyzed by GC. As shown in Table 1, linoleic acid (18:2 n-6) was the most abundant fatty acid in walnut oil, contributing to about 60% of total fatty acids.

The composition of fatty acids in walnut oil with and without different antioxidants during the thermal storage was presented in Table 1. In the presence of different antioxidants, the degradation of unsaturated fatty acids was increased with the extension of storage time. The unsaturated fatty acids percentage of the oil in control sample was decreased from 89.15% to 84.27% throughout 25 days of the storage. Moreover, in the presence of tea polyphenols, water-dispersed vitamin E or BHT, there existed slight degradation of unsaturated fatty acids of the oil in walnut beverage emulsions, which indicated that these antioxidants could significantly inhibit the lipid oxidation during the thermal storage. Nevertheless, the effect of vitamin E, NAE and EMIQ was not obvious on inhibiting the degradation of unsaturated fatty acids. This result was in accordance with the change of peroxide and MDA concentrations. 3.3. Effect of different antioxidants on peroxide and MDA concentrations under light exposure The UV light exposure has been known to play an important role in lipid oxidation. Oils, such as walnut oil, orange oil and lemon oil, underwent the accelerated deterioration in their fatty acids composition when they were stored in the presence of light (Zhao, Xiang, Wei, Yuan, & Gao, 2014). The deterioration of oils led to the formation of off-flavor compounds, which might affect

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Table 1 Fatty acid composition change of the oil in walnut beverage emulsion with different antioxidants as a function of storage time at 62 ± 1 °C. Storage time (day)

Fatty acid composition (%) C16:0

C18:0

USFB

10.85 10.98a 11.00a 11.34a 11.63b 13.71c 14.80d 15.74e

89.15a 89.02a 89.00a 88.66a 88.36a 86.29ab 85.19ab 84.27b

7.00a 7.15ab 7.50b 7.50b 7.98c 8.11c 8.21c 8.65d

3.41a 3.91b 4.24c 3.69d 3.29a 3.82bd 3.43a 3.71bd

17.17a 17.25a 17.20a 17.29a 18.20b 17.48ab 17.85ab 17.50ab

59.83a 59.20a 58.58a 59.09a 58.01a 58.57a 58.54a 58.37a

12.58a 12.50a 12.4a 12.43a 12.53a 12.03ab 11.97ab 11.76b

10.41a 11.06b 11.74cd 11.19bc 11.27bc 11.93de 11.64bcd 12.36e

89.58a 88.95a 88.25a 88.81a 88.74a 88.08a 88.36a 87.63a

1 4 7 11 14 18 21 25

7.25a 7.44ab 7.67b 7.57ab 7.77bc 8.08c 8.50d 8.73d

2.76a 2.72a 2.91b 3.61c 3.48cd 3.42d 3.43d 3.90e

17.51ab 18.04ab 17.45ab 17.25a 17.73ab 18.12b 18.18b 17.73ab

59.98a 59.50a 59.69a 59.07a 58.88a 58.28a 58.16a 58.03a

12.49a 12.30ab 12.28ab 12.50a 12.14ab 12.00ab 11.74b 11.62b

10.01a 10.16a 10.58b 11.18c 11.25c 11.50c 11.93d 12.63e

89.98a 89.84a 89.42a 88.82a 88.75a 88.40a 88.08a 87.38a

VE-0.04%

1 4 7 11 14 18 21 25

6.70a 6.92a 7.70b 7.90b 8.25c 8.95d 8.26c 8.84d

3.59a 3.56a 3.90b 3.96b 4.08b 4.47c 5.24d 5.62e

17.58ab 17.84a 17.46ab 17.59ab 17.48ab 16.92bc 16.51c 16.24c

59.81a 59.05a 58.74a 58.55a 58.35a 58.4a 58.33a 58.35a

12.32ab 12.60a 12.20ab 12.00ab 11.84bd 11.19ce 11.56cd 10.96e

10.29a 10.48a 11.60b 11.86bc 12.33c 13.42d 13.50d 14.46e

89.71a 89.52a 88.40a 88.14a 87.67a 86.58a 86.40a 85.55a

BHT-0.04%

1 4 7 11 14 18 21 25

6.67a 6.77a 6.79a 7.79b 7.96b 8.36c 8.38c 8.50c

3.65a 4.08bd 4.12d 3.30c 3.18c 3.83a 3.89ab 4.02b

17.71b 17.42ab 17.60ab 17.36ab 17.90b 17.72ab 16.93ab 16.84a

59.13a 59.25a 59.05a 59.27a 58.80a 58.48a 58.92a 58.83a

12.85a 12.47ab 12.45ab 12.28ab 12.17bc 11.62c 11.87c 11.81c

10.32a 10.85b 10.91b 11.09c 11.14c 12.19d 12.27d 12.52d

89.69a 89.14a 89.10a 88.91a 88.87a 87.82a 87.72a 87.48a

NAE-0.02%

1 4 7 11 14 18 21 25

6.52a 6.77a 6.97b 7.85c 8.58d 8.59d 8.84d 8.94d

3.65a 4.09b 4.10b 3.09c 2.83d 3.40e 3.82a 4.25b

17.65a 17.59a 17.26a 17.44a 17.99ab 17.63a 18.87b 18.37b

58.67a 59.11a 59.05a 59.54a 58.67a 58.76a 57.36a 57.26a

13.51a 12.44bc 12.61b 12.08bcd 11.93cd 11.62de 11.11e 11.18e

10.17a 10.86bc 11.07bc 10.94bc 11.41cd 11.99d 12.66e 13.19e

89.83a 89.14a 88.92a 89.06a 88.59a 88.01a 87.34a 86.81a

NAE-0.04%

1 4 7 11 14 18 21 25

6.45a 6.33a 6.97c 7.62d 8.56e 8.72e 9.40f 9.55f

4.23a 4.76c 4.48b 4.67bc 3.65d 4.22a 3.96e 4.17af

17.66a 17.98a 17.35a 17.23a 17.77a 17.82a 17.29a 17.78a

58.85a 58.40a 58.73a 58.44a 58.11a 58.06a 57.83a 57.34a

12.81a 12.54ab 12.47ab 12.04bc 11.73ce 11.17d 11.53de 11.16d

10.68a 11.09a 11.45b 12.29c 12.21c 12.94d 13.36de 13.72e

89.32a 88.92a 88.55a 87.71a 87.61a 87.05a 86.65a 86.28a

EMIQ-0.04%

1 4 7 11 14 18 21 25

7.65a 7.74a 7.86ac 7.87ac 8.36d 8.20cd 9.93e 10.07e

3.28ab 3.33ab 3.23a 3.41b 3.93c 3.92c 3.97c 4.01c

17.29a 17.47a 17.51a 17.57a 17.18a 17.86a 17.19a 17.97a

59.57a 59.41a 59.32a 59.08a 58.65a 58.29a 57.39a 56.98a

12.21a 12.05ab 12.08ab 12.06ab 11.89ab 11.72ab 11.53b 10.96c

10.93a 11.07a 11.09a 11.28a 12.29b 12.12b 13.90c 14.08c

89.07a 88.93a 88.91a 88.71a 87.72a 87.87a 86.11a 85.91a

1 4 7 11 14 18 21 25

Water-dispersed VE-0.04%

a

SFA

12.44 12.53a 12.60a 12.15ab 12.54a 11.76b 10.34c 9.95c

TP-0.04%

a

C18:3

59.25 59.14a 59.09a 58.75a 58.54a 57.23a 56.93a 56.41a

6.19 6.65b 7.08c 7.92d 8.37e 10.34f 10.74f 10.77f

a

C18:2

17.46 17.35a 17.31a 17.76a 17.28a 17.30a 17.92a 17.91a

1 4 7 11 14 18 21 25

a

C18:1

4.66 4.33a 3.92b 3.42c 3.26c 3.37c 4.06b 4.97d

Control

a

a

The data presented in this table consist of average value ± SD of three independent analyses. Different letters in the superscript represent statistically significant difference (p < 0.05). A Saturated fatty acids. B Unsaturated fatty acids.

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the quality of final products. Photo-oxidation is triggered by highly reactive singlet oxygen species, which were formed by excitation of triplet molecular oxygen, under UV light exposure (Barriuso et al., 2013). Walnut oil in the emulsion is rich in unsaturated fatty acids (Table 1) and they are very prone to be oxidized under UV light exposure. Fig. 4 showed the changes of the oxidized primary product (peroxide) and secondary product (MDA) in the presence or absence of antioxidants with the extension of UV light exposure time.

3.3.1. Effect of different antioxidants on peroxide concentration under light exposure As shown in Fig. 4(1), over 10 h of light exposure, the peroxide concentration of all the emulsions was increased at different rates. From the change of peroxide concentration, light exposure induced the formation of lipid peroxide, and the oils in the emulsions containing tea polyphenols, BHT, vitamin E and EMIQ were oxidized at lower rate over 10 h, which interpreted that these antioxidants could effectively (p < 0.05) inhibit the formation of hydroperoxides under UV light exposure. For vitamin E, it showed a prooxidant effect during the thermal storage and exhibited the antioxidant attribute during UV light exposure. This phenomenon revealed that the antioxidant mechanisms of these antioxidants were different

1800

under different environment conditions. In a word, tea polyphenols, vitamin E and synthetic BHT could retard the formation of hydroperoxides under UV light exposure.

3.3.2. Effect of different antioxidants on MDA concentration under light exposure As shown in Fig. 4(2a), in the presence of tea polyphenols, vitamin E and BHT, MDA concentration was not obviously changed after 10 h of UV light exposure, which indicated that these antioxidants could significantly (p < 0.05) inhibit the formation of MDA. For control sample, MDA concentration was changed obviously after 6 h of UV light exposure. The change of MDA in the emulsions with EMIQ and NAE exhibited similar trends with control sample, and its concentration was significantly (p < 0.05) increased after 6 h of UV light exposure, as shown in Fig. 4(2b). The rate of MDA formation in the emulsions was in the order: NAE (0.02%) > NAE (0.04%) > EMIQ (0.04%). In summary, the oxidative stability of walnut beverage emulsions was listed as follows: BHT (0.04%)  tea polyphenols (0.04%) > vitamin E (0.04%) > EMIQ (0.04%) > NAE (0.04%) > NAE (0.02%)  water-dispersed vitamin E (0.04%), based on the determination of both lipid hydroperoxides and MDA under UV light exposure.

1800

a

1600

Peroxide concentration ( mol/g oil)

1600

Peroxide concentration ( mol/g oil)

b

1400

control TP-0.04% water-dispersed VE-0.04%

1200 1000

VE-0.04%

800

BHT-0.04%

600 400 200

1400

control NAE-0.02% NAE-0.04% EMIQ-0.04%

1200 1000 800 600 400 200 0

0 0

2

4

6

8

0

10

2

800

800

a

600

control TP-0.04% water-dispersed VE-0.04%

500

VE-0.04%

6

8

10

8

10

b

700

control NAE-0.02% NAE-0.04% EMIQ-0.04%

600

MDA ( g/g oil)

MDA ( g/g oil)

700

4

Time (hour)

Time (hour)

BHT-0.04% 400 300

500 400 300

200

200

100

100 0

0 0

2

4

6

Time (hour)

8

10

0

2

4

6

Time (hour)

Fig. 4. Peroxide concentration (1) and MDA (2) change of the oil in walnut beverage emulsion exposed to UV light for 10 h. Error bars represent standard deviations (n = 3).

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