Effect of gamma irradiation on the physicochemical properties and nutrient contents of peanut

LWT - Food Science and Technology 96 (2018) 535–542

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Effect of gamma irradiation on the physicochemical properties and nutrient contents of peanut

T

Kunlun Liu∗, Ying Liu, Fusheng Chen∗∗ College of Food Science and Technology, Henan University of Technology, Zhengzhou 450001, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Peanut Gamma irradiation Physicochemical properties Nutrient

This study aims to assess the effect of gamma irradiation on the physicochemical properties and nutrient contents of two different peanut cultivars. The peanuts were treated with 0, 1, 3, 5, and 10 kGy gamma irradiation doses. The examined physicochemical properties were water activity, fatty acid value (FAV), peroxide value (PV), carbonyl value (CV), malondialdehyde (MDA) content, and lipase activity. The examined nutrient contents were moisture and ash contents, total sugar, protein and fat contents, fatty acid and amino acid composition. Results showed that high irradiation dose (10 kGy) significantly decreased the fat and protein contents and increased the water activity of peanut. However, the moisture and ash contents and total sugar were not affected by irradiation. The FAV, PV, CV, and MDA contents increased depending on the radiation dose. Fatty acid and amino acid composition changed when the irradiation dose increased. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis revealed that irradiation did not modify the protein subunits but disrupted the disulfide bonds. Furthermore, lipase activity decreased with the increased irradiation dose. This phenomenon may positively affect peanut quality during storage.

1. Introduction Peanut is an important source of oil and is consumed by many consumers worldwide because of its rich nutrient substance (fat, protein, unsaturated fatty acid, and amino acid). Peanuts easily oxidize and decompose due to its high fat content (50%) during storage and transportation; this phenomenon affects its nutritional value and agricultural importance (Ren et al., 2017). To maintain good peanut quality during storage, scholars have applied various preservation methods, such as controlling the storage environment, fumigating, irradiating, and surface coating. Irradiation is a non-thermal, environment-friendly and rapid technology that has become a popular pretreatment method in industrial production (El-Rawas et al., 2012). Irradiation can be effective in postharvest pest control because of the gamma rays' ability to kill insects and inhibit mycotoxin biosynthesis. Exposure of peanut seeds to gamma irradiation significantly reduces surface microorganisms and microbial population to below the detection limits of 6 and 9 kGy, respectively (Albachir, 2016). Camargo, Gallo, and Shahidi (2015) reported that gamma irradiation decreased the yeast, mold, total coliform, and Staphylococcus contents of peanut skin. However, the free radical produced by irradiation may produce some molecular substances and influence the food quality. For example,

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Liu), [email protected] (F. Chen).

∗∗

https://doi.org/10.1016/j.lwt.2018.06.009 Received 6 February 2018; Received in revised form 2 June 2018; Accepted 4 June 2018 Available online 06 June 2018 0023-6438/ © 2018 Elsevier Ltd. All rights reserved.

Jittrepotch, Kongbangkerd, and Rojsuntornkitti (2010) reported that the peroxide value (PV), p-anisidine, and free fatty acid (FFA) of extracted peanut oil significantly increased after irradiation. Similar research on other types of nut reported that gamma irradiation did not significantly alter nut oil content but proportionally increased the fatty acid content and PV with the dose (Gecgel, Gumus, Tasan, Daglioglu, & Arici, 2011). Al-Bachir (2015) proved that gamma irradiation decreased the oleic acid content and increased the linoleic acid content of pistachio. On the contrary, some studies reported that irradiation did not influence food quality and nutrient content. Gölge and Ova (2008) reported that irradiation did not influence the nut's physical qualities, such as texture color and fatty acid content. According to Güler, Bostan, and Çon (2017), the FFA value in the untreated hazelnuts were comparable with that of the treated samples. The PV increased proportionally with the dose, but the difference was insignificant. The applied doses did not significantly modify the crude protein content, water activity, crude cellulose content, and moisture content of the hazelnuts. Although substantial studies have reported the effect of irradiation on food, the results are unclear and contradictory. Information about the effect of irradiation on the amino acids, subunits, and disulfide bonds are scarce. These aspects are important to food's nutritional

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and then esterified drastically. The samples were placed in a 1.5 mL bottle and injected in the GC unit. During injection, the injector was operated in the split mode (1:2 split ratio) at a temperature of 330 °C.

quality. Furthermore, irradiation undoubtedly kills most microorganisms in food; however, the enzyme passivation caused by irradiation is rarely mentioned and might play an important role in food preservation. This study aims to explore the effect of increasing gamma irradiation dose on the physicochemical properties and nutrient content of peanuts.

2.6. Amino acid composition and content analysis Amino acid composition and contents were analyzed according to the method reported by El-Rawas et al. (2012). Samples (50 mg) were subjected to acid hydrolysis with 6 mol/L HCL at 110 °C for 24 h. The hydrolysate was vacuum dried until waterless and then mixed with sodium citrate buffer.

2. Materials and methods 2.1. Preparation of samples Newly harvested peanuts YuHua-9326 (YH-9326) and YuHua-22 (YH-22) were purchased from Henan Academy of Agricultural Sciences, China. The samples were shelled and stored in −20 °C for further treatment.

2.7. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDSPAGE) The peanut protein was diluted with sample treatment buffer and heated in boiling water for 5 min (Maity et al., 2009). Two different experiments, namely, reductive and non-reductive SDS-PAGE, were conducted in this study. The only difference is whether or not reductant was added in the sample treatment buffer.

2.2. Irradiation of samples Irradiation was conducted at the Isotope Institute Co., Ltd., Henan Academy of Sciences. The peanut seeds were treated with Co-60 gamma-ray. The samples were exposed to irradiation with the total amount of radiation absorbed restricted to 1, 3, 5, and 10 kGy at room temperature. The samples were stored at −20 °C until analysis.

2.8. Lipase activity Lipase activity was determined by spectrophotometry, and the experimental process was conducted in four steps, namely, substrate preparation, extraction of crude enzymes, reaction and identification (Cato, Halmos, & Small, 2010). The substrate was the mixture of olive oil and Tween-20 (w: w = 1:1). In extracting the crude lipase, the samples (5 g) were mixed with 3 mL of Tris-HCL (pH 7.5) and grinded with an ice bath until the mixture formed a homogenate. This homogenate was dissolved again (Tris-HCL, pH 7.5), vibrated (4 h), and centrifuged (9000 g, 20 min). Finally, the crude lipase solution was added to the substrate and reacted at 37 °C for 3 h. The reaction mixture was colored by developer, and the absorbance was measured at 715 nm.

2.3. Determination of the proximate composition and water activity Proximate analysis of the peanut moisture, ash, fat and protein contents were conducted using the method described by AOAC (1995). Total sugar was determined by measuring the absorbance at 620 nm with a spectrophotometer. Water activity was estimated by the Labmaster-aw water activity meter. 2.4. Lipid oxidation 2.4.1. Fatty acid value (FAV) FAV of peanut was determined using titrimetric procedure described in AACC method (2000). The FAV was quantified in accordance with mg of NaOH required to neutralize the acid in 100 g of dried peanut sample.

2.9. Statistical analysis Values were expressed as means ± standard deviations, and measurements were obtained in triplicate. Data were analyzed by ANOVA, and significant difference was determined at the P < 0.05 level for Duncan's multiple range test by using SPSS software (version 16.0).

2.4.2. Peroxide value (PV) PV of peanut oil was measured using the method reported by Shantha and Decker (1994). In brief, 5 mL of a mixed solution of chloroform and methanol was used to dissolve the oil extract from peanut. This mixture was adequately blended with ferrous chloride solution and potassium thiocyanate solution and then incubated for 5 min at room temperature. Absorbance of the solution was measured at 500 nm.

3. Results and discussion 3.1. Moisture, ash content and total sugar Peanut seeds of YH-9326 and YH-22 had moisture contents (%) of 3.65 ± 0.05 and 4.05 ± 0.09, ash contents (%) of 2.38 ± 0.01 and 2.54 ± 0.20, and total sugar (%) of 13.27 ± 0.51 and 14.04 ± 0.49, respectively. No significant differences (P > 0.05) were found after irradiation with 0–10 kGy doses.

2.4.3. Carbonyl value (CV) CV was determined according to the method described in AOAC (1995).

3.2. Fat and protein contents 2.4.4. Malondialdehyde (MDA) content MDA content was determined by spectrophotometry (Fan & Thayer, 2002). The samples were mixed with 10% chloroacetic acid and centrifuged for 15 min (4000 r/min), and the supernatant obtained by centrifugation is the MDA extract of the peanut. The extract was mixed with 0.2% thiobarbital acid and reacted in a boiling bath. Absorbance was measured at 450, 500, and 600 nm.

The fat and protein contents in peanut seeds are shown in Fig. 1. No significant differences (P ˃ 0.05) in fat contents were observed during irradiation with 1, 3, and 5 kGy. However, the fat content was significantly reduced when the peanut seeds were irradiated at a high dose (10 kGy). A similar pattern for peanut protein content was noted. Irradiation with doses of 1, 3, and 5 kGy did not significantly (P ˃ 0.05) influence the protein contents, whereas only the highest dose (10 kGy) caused a significant reduction.

2.5. Fatty acid composition Fatty acid composition was determined by gas chromatography (GC) (Mexis & Kontominas, 2009b). Accurately weighted 0.1 g of peanut oil which was blended with sodium hydroxide and methanol,

3.3. Water activity The water activity of the samples was increased significantly when 536

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activity (Liao, Wang, Yu, & Fu, 2010). 3.4. Lipid oxidation The lipid oxidation of peanut was reflected by the FAV, PV, CV, and MDA contents. The changes in FAV, PV, CV, and MDA contents of two peanut cultivars as a function of irradiation dose are shown in Fig. 3. FAV is an important factor indicating the quality deterioration of peanut seeds. As shown in Fig. 3A, FAV increased significantly (P < 0.05) with increasing irradiation doses, thus indicating that high irradiation dose (˃ 3 kGy) might accelerate the lipid hydrolysis or oxidation of peanut seeds. Hyperoxide can be generated through the oxidation of unsaturated fatty acids and is a good indicator of the degree of fat oxidation. For non-irradiated peanuts, the PVs of these two cultivars were low at 2.453 meq/kg (YH-9326) and 2.896 meq/kg (YH-22). The PVs were not significantly changed when the peanuts were treated with low irradiation doses (< 3 kGy). However, further increase in irradiation dose (5 and 10 kGy) significantly increased the PVs (Fig. 3B). In particular, the PV of YH-9326 increased by 1.6-fold relative to that of the control when the irradiation dose reached up to 10 kGy. The CV corresponds to the fatty acid or glycerol containing an aldehyde ketone group, which is produced during lipid oxidation. This parameter is valuable in monitoring the degree of fat oxidation. In this study, the CVs were only 0.660 and 0.610 meq/kg for the two cultivars of peanut seeds (control) (Fig. 3C). Except for the 1 kGy dose irradiation of YH9326, the CV of peanuts for all other irradiation dose was significantly increased. In particular, the CV of YH-9326 under the 10 kGy dose irradiation dose increased 1.9 times higher than that of the control. MDA is the final product of lipid oxidation and is also an important index of oxidation. The MDA content in peanut seeds increased significantly with the increasing irradiation dose (5 and 10 kGy) (Fig. 3D). The results above were similar to those reported by Mexis and Kontominas (2009a), who found that PV increased with elevated irradiation dose and reaches the maximum value at 7 kGy in hazelnuts. In another study, Ma, Lu, Liu, and Ma (2013) reported that the PV did not significantly different between walnuts treated with 0.5 kGy irradiation and the control, but the value was 36%, 21%, and 114% higher than that of the control at 1.0, and 5.0 kGy doses, respectively. Gecgel et al. (2011) proved that the FFA and PV of four different kinds of nuts (hazelnuts, walnuts, almonds, and pistachios) increased significantly with the increasing irradiation dose (1, 3, 5, and 7 kGy). However, Al-Bachir (2004) reported that immediately after irradiation, the PV of irradiated and non-irradiated walnuts showed no difference. Sánchez-Bel, Egea, Romojaro, and Martínez-Madrid (2008) also reported the lack of PV change after almonds were irradiated at 3, 7, and 10 kGy dose. The findings in this study revealed that irradiation substantially accelerated lipid oxidation, and the degree of lipid oxidation of peanut seeds was strongly correlated with irradiation dose. Irradiation with doses ≤1 kGy was suitable for peanut seeds in relation to its FAV, PV, CV, and MDA contents.

Fig. 1. Effect of irradiation dose on fat and protein contents of peanut seedsYH9326 ( ) and YH-22 ( ). Each data column represents the mean of three replications. Vertical bars represent the standard errors of means. Values of each peanut cultivar followed by the same letter are not significantly different (P ˃ 0.05). Peanut seeds were irradiated at 25 °C with doses of 0, 1, 3, 5, and 10 kGy, respectively.

3.5. Fatty acid composition The fatty acid composition of peanut seeds irradiated with various doses is given in Table 1. The major fatty acids identified in peanut oil were 40% oleic acid (18:1), 37% linoleic acid (18:2), 12% palmitic acid (16:0), and 4% stearic acid (18:0). These results were consistent with previous reports. The high unsaturated fatty acid content (80%) suggested the high nutritional value of peanut oil. Gamma irradiation significantly influenced the fatty acid composition of peanut seeds. On the one hand, after irradiation with 5 and/or 10 kGy doses, the saturated fatty acid palmitic acid (16:0) and stearic acid (18:0) were significantly (P < 0.05) increased relative to those of the control. Parallel results were reported by Afify, Rashed, Ebtesam, and Elbeltagi (2013), who noted that the saturated fatty acids contents of C18:0, C24:0, and C27:0 in the irradiated peanut seeds were augmented depending on the

Fig. 2. Effect of irradiation dose on water activity of peanut deeds YH-9326 (■) and YH-22 (×). The results are expressed as the mean of three replications ± SD. Values of each peanut cultivar followed by the same letter are not significantly different (P ˃ 0.05). Peanut seeds were irradiated at 25 °C with doses of 0, 1, 3, 5, and 10 kGy, respectively.

the irradiation dose (5 and 10 kGy) increased in both YH-9326 and YH22 (Fig. 2). The increased water activity may induce some other reactions in peanuts. Under a certain environment, the water activity in food is maintained in a relatively balanced state. Irradiation may change the organizational structure of the material (such as cell wall), thus increasing the internal free moisture of food and altering its water 537

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Fig. 3. Changes in FAV, PV, CV and MDA contents as a function of irradiation dose. Peanut seeds: YH-9326 (■), YH-22 (×). The results are expressed as the mean of three replications ± SD. Values of each peanut cultivar followed by the same letter are not significantly different (P ˃ 0.05). Peanut seeds were irradiated at 25 °C with doses of 0, 1, 3, 5, and 10 kGy, respectively.

However, some research results were contradictory. Afify et al. (2013) stated that the relative percentages of induced unsaturated fatty acid are dose dependent and generally increase with the increasing irradiation dose for different seeds (soybean, peanut, and sesame seed).

irradiation dose. On the other hand, the levels of the unsaturated fatty acids oleic acid (18:1) and linolenic acid (18:3) were significantly decreased in both peanuts. The linolenic acid (18:3) content decreased from 70% to 48% in YH-9326 and from 73% to 63% in YH-22 after irradiation at 10 kGy dose relative to those of the controls. The unsaturated linoleic acid (18:2) content was increased significantly in both varieties of peanuts treated with 10 kGy dose irradiation. It may be due to the transformation degree of linolenic acid (18:3) to linoleic acid (18:2) and linoleic acid (18:2) to oleic acid (18:1). These findings proved that fatty acids were highly sensitive to irradiation, and gamma irradiation can alter the fatty acid composition. These phenomena might be explained by the free radicals produced by irradiation. These free radicals can destroy the double bonds of unsaturated fatty acids instead of transforming them into saturated fatty acids (Mexis & Kontominas, 2009a). Similar results were obtained by Al-Bachir (2015), who reported that gamma irradiation decreased the oleic acid (C18:1) content and increased the linoleic acid (C18:2) content of pistachio oil.

3.6. Amino acid profile analysis Amino acids, especially essential amino acids (EAAs), are important peanut nutrients. Almost all the contents of the tested amino acid, except for alanine and tyrosine, changed significantly with the increasing irradiation dose (Table 2). The contents of glutamic, arginine, leucine, aspartic, valine, isoleucine, and histidine of the two peanut seeds significantly decreased in various levels with the elevated irradiation dose. For example, the valine content of YH-22 decreased by 43%, and the leucine content of YH-9326 diminished by 16% when treated with 10 kGy of irradiation relative to those of the untreated samples. This result indicated that low irradiation doses might cause different amino

Table 1 Fatty acid composition of peanut (%) with various irradiation doses. Peanut variety

YH-9632

YH-22

Fatty acid

Palmitic acid (16:0) Stearic acid (18:0) Arachidic acid (20:0) Behenic acid (22:0) Oleic acid (18:1) Linoleic acid (18:2) Linolenic acid (18:3) Osenic acid (24:1) Saturated fatty acid Unsaturated fatty acid Palmitic acid (16:0) Stearic acid (18:0) Arachidic acid (20:0) Behenic acid (22:0) Oleic acid (18:1) Linoleic acid (18:2) Linolenic acid (18:3) Osenic acid (24:1) Saturated fatty acid Unsaturated fatty acid

Irradiation dose (kGy) 0

1

3

5

10

11.78 ± 0.02c 3.70 ± 0.00c 1.60 ± 0.01a 2.58 ± 0.01b 41.28 ± 0.00a 37.14 ± 0.04b 0.70 ± 0.03a 1.17 ± 0.02a 19.68 ± 0.02c 80.29 ± 0.01a 12.02 ± 0.16c 3.65 ± 0.09c 1.59 ± 0.01a 2.45 ± 0.07c 36.77 ± 0.08a 40.46 ± 0.14b 0.73 ± 0.02a 1.29 ± 0.13a 19.70 ± 0.00e 79.24 ± 0.33a

11.79 ± 0.02c 3.71 ± 0.04bc 1.61 ± 0.02a 2.58 ± 0.04b 40.88 ± 0.03ab 37.21 ± 0.22b 0.69 ± 0.05a 1.17 ± 0.01a 19.69 ± 0.08c 79.95 ± 0.20ab 12.43 ± 0.06b 3.68 ± 0.00c 1.58 ± 0.01a 2.43 ± 0.09c 36.62 ± 0.14a 40.45 ± 0.14b 0.72 ± 0.02a 1.25 ± 0.07a 20.12 ± 0.14d 79.05 ± 0.23ab

11.83 ± 0.19bc 3.91 ± 0.10abc 1.59 ± 0.02 a 2.63 ± 0.00ab 40.54 ± 0.15b 37.20 ± 0.14b 0.57 ± 0.03b 1.15 ± 0.01a 19.95 ± 0.27bc 79.47 ± 0.34bc 12.61 ± 0.10ab 3.79 ± 0.10bc 1.56 ± 0.02a 2.51 ± 0.01bc 35.62 ± 0.13b 40.73 ± 0.14ab 0.70 ± 0.03ab 1.25 ± 0.06a 20.47 ± 0.01c 78.31 ± 0.10bc

12.09 ± 0.14ab 3.92 ± 0.11ab 1.61 ± 0.02a 2.65 ± 0.01a 39.88 ± 0.15c 37.46 ± 0.14b 0.55 ± 0.02bc 1.15 ± 0.01a 20.27 ± 0.00ab 79.03 ± 0.23cd 12.64 ± 0.11ab 3.99 ± 0.15ab 1.55 ± 0.02a 2.63 ± 0.06ab 35.28 ± 0.21bc 40.83 ± 0.14a 0.66 ± 0.02bc 1.20 ± 0.14a 20.82 ± 0.04b 77.97 ± 0.51c

12.25 ± 0.00a 4.05 ± 0.09a 1.59 ± 0.02a 2.66 ± 0.03a 38.76 ± 0.35d 38.21 ± 0.15a 0.48 ± 0.01c 1.05 ± 0.07b 20.54 ± 0.14a 78.50 ± 0.12d 12.86 ± 0.06a 4.16 ± 0.07a 1.56 ± 0.01a 2.68 ± 0.01a 35.02 ± 0.14c 40.97 ± 0.14a 0.63 ± 0.01c 1.15 ± 0.02a 21.26 ± 0.10a 77.77 ± 0.29c

Means in the same row followed by the same letter are not significantly different (P > 0.05). 538

539

5.43 ± 0.00bc 1.86 ± 0.02c 9.89 ± 0.08b 2.83 ± 0.00b 1.73 ± 0.08a 0.05 ± 0.06a 0.46 ± 0.01c 1.57 ± 0.08a 1.60 ± 0.02abc 4.70 ± 0.04abc 0.86 ± 0.02cd 1.68 ± 0.01bc 1.78 ± 0.02a 2.94 ± 0.07b 2.32 ± 0.03a 1.47 ± 0.01cd 41.01 ± 0.20c 11.04 ± 0.09b 29.96 ± 0.11b 26.93 ± 0.00a 36.86 ± 0.00a

5.40 ± 0.06c 1.85 ± 0.01c 9.19 ± 0.13c 2.78 ± 0.08b 1.67 ± 0.25a – 0.46 ± 0.01c 1.57 ± 0.07a 1.53 ± 0.00bc 4.18 ± 0.12c 0.83 ± 0.03d 1.64 ± 0.02c 1.61 ± 0.04b 2.73 ± 0.03c 2.26 ± 0.04a 1.45 ± 0.00d 38.94 ± 0.40d 10.53 ± 0.05b 28.41 ± 0.35b 27.04 ± 0.00a 37.07 ± 0.00a

5.36 ± 0.04c 2.37 ± 0.05a 9.13 ± 0.04c 3.20 ± 0.00a 1.77 ± 0.05a – 0.60 ± 0.01a 1.62 ± 0.08a 1.43 ± 0.11c 4.52 ± 0.23bc 1.37 ± 0.07a 1.44 ± 0.04d 1.54 ± 0.03b 2.67 ± 0.02c 2.36 ± 0.10a 1.67 ± 0.00a 40.82 ± 0.03c 11.04 ± 0.00b 29.78 ± 0.03b 27.03 ± 0.00a 37.05 ± 0.00a

10

0

5.60 ± 0.01a 1.79 ± 0.04b 9.40 ± 0.20a 2.92 ± 0.25a 1.43 ± 0.24a 0.02 ± 0.00b 0.60 ± 0.02a 1.60 ± 0.27a 1.74 ± 0.05a 4.75 ± 0.10a 1.43 ± 0.05b 1.84 ± 0.09a 2.00 ± 0.02a 2.81 ± 0.10a 2.13 ± 0.02c 1.51 ± 0.10b 44.57 ± 0.70a 11.72 ± 0.03a 29.85 ± 0.73a 28.19 ± 0.01b 39.26 ± 0.01b

5.54 ± 0.02ab 1.96 ± 0.02bc 10.37 ± 0.07b 2.84 ± 0.01b 1.87 ± 0.01a 0.02 ± 0.00a 0.49 ± 0.01bc 1.65 ± 0.05a 1.70 ± 0.02a 5.06 ± 0.20ab 0.94 ± 0.01bc 1.72 ± 0.01ab 1.86 ± 0.02a 3.18 ± 0.04a 2.43 ± 0.05a 1.49 ± 0.01bc 43.10 ± 0.31b 11.62 ± 0.11a 31.47 ± 0.19a 26.97 ± 0.00a 36.93 ± 0.00a

5

5.64 ± 0.06a 2.10 ± 0.14b 10.67 ± 0.35a 2.88 ± 0.05b 1.89 ± 0.06a 0.03 ± 0.00a 0.52 ± 0.03b 1.66 ± 0.05a 1.66 ± 0.09ab 5.16 ± 0.34a 1.00 ± 0.00b 1.76 ± 0.03a 1.84 ± 0.10a 3.19 ± 0.05a 2.45 ± 0.20a 1.52 ± 0.03b 43.91 ± 1.38a 11.75 ± 0.40a 32.16 ± 0.98a 26.76 ± 0.00a 36.53 ± 0.00a

3 YH-22

1

YH-9632

0

Irradiation dose (kGy)

Means for the same amino acid and the same variety followed by the same letter are not significantly different (P > 0.05). TAA: total amino acids, EAA: essential amino acids, NEAA: non-essential amino acids. E/T (%) = EAA/TAA; E/N (%) = EAA/NEAA .

Asp Ser Glu Gly Ala Cys Met Tyr His Arg Thr Val Ile Leu Phe Lys TAA EAA NEAA E/T (%) E/N (%)

Amino Acid

Table 2 Amino acid composition of peanut (%) with various irradiation doses.

5.57 ± 0.05a 1.82 ± 0.09b 9.33 ± 0.13a 2.99 ± 0.16a 1.57 ± 0.02a 0.02 ± 0.00b 0.59 ± 0.05a 1.49 ± 0.06a 1.72 ± 0.04a 4.73 ± 0.08a 1.34 ± 0.03bc 1.86 ± 0.05a 1.93 ± 0.05a 2.79 ± 0.05a 2.15 ± 0.04c 1.45 ± 0.01b 41.30 ± 0.25ab 11.51 ± 0.05a 29.79 ± 0.20a 27.86 ± 0.00b 38.62 ± 0.00b

1

5.35 ± 0.08ab 1.71 ± 0.02b 8.76 ± 0.13a 2.87 ± 0.07a 1.30 ± 0.10a 0.03 ± 0.01b 0.56 ± 0.04a 1.54 ± 0.21a 1.51 ± 0.08b 4.70 ± 0.04a 1.30 ± 0.01cd 1.68 ± 0.02a 1.73 ± 0.06b 2.69 ± 0.04a 2.30 ± 0.01b 1.50 ± 0.01b 39.50 ± 0.35ab 11.19 ± 0.04b 28.31 ± 0.31ab 28.32 ± 0.00b 39.51 ± 0.00b

3

4.97 ± 0.44b 1.70 ± 0.02b 7.64 ± 0.62b 3.16 ± 0.49a 1.28 ± 0.20a 0.02 ± 0.00b 0.51 ± 0.06a 1.59 ± 0.34a 1.41 ± 0.04b 4.77 ± 0.05a 1.24 ± 0.03d 1.32 ± 0.17b 1.60 ± 0.09b 2.67 ± 0.01a 2.36 ± 0.00b 1.49 ± 0.01b 37.73 ± 2.49bc 10.68 ± 0.23c 27.05 ± 0.27ab 28.36 ± 0.01b 39.60 ± 0.02b

5

4.34 ± 0.12c 2.21 ± 0.02a 6.39 ± 0.32c 2.84 ± 0.71a 1.52 ± 0.59a 0.02 ± 0.00a 0.62 ± 0.04a 1.52 ± 0.44a 1.13 ± 0.02c 4.14 ± 0.13b 1.61 ± 0.03a 1.05 ± 0.04c 1.37 ± 0.04c 2.41 ± 0.06b 2.53 ± 0.08a 1.88 ± 0.08a 35.77 ± 1.69c 10.86 ± 0.13c 24.92 ± 1.57c 30.38 ± 0.01a 43.65 ± 0.02a

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irradiation at doses of 1–25 kGy (Maity et al., 2009). Cho, Yang, and Song (1999) reported that protein aggregates with high molecular weight can be obtained after gamma irradiation by generating interprotein crosslinking reactions and electrostatic and hydrophobic interactions. Malik, Sharma, and Saini (2017) reported that gamma irradiation also significantly affected the molecular weight profile of sunflower protein isolates, and the bands completely disappeared after gamma irradiation at all tested doses (10–50 kGy). Non-reductive SDS-PAGE can reveal the effect of gamma irradiation on the disulfide bonds of peanut protein. As shown in the non-reductive SDS-PAGE analysis results (Fig. 4 C and D), most of the protein bands (below 43 kDa) were previously not visible relative to the reduced conditions, and all of the bonds reached up to 60 kDa. This finding suggested the possible presence of numerous disulfide bonds in the peanut protein structures. The molecular weight distribution was mainly exhibited in two obvious regions (31–35 and 50–66 kDa). No significant changes were noted under low-dose irradiation treatment (1 and 3 kGy); however, the 50–66 kDa band decreased in intensity, and the 25–30 kDa band increased under 5 and/or10 kGy radiation treatment relative to that of the control. This result showed that gamma irradiation of high dose might break the disulfide bond between peptides and disintegrate the protein. A parallel report proposed that the 52 and 55 kDa bands of cowpea flour protein under SDS-PAGE decreased in intensity under both reducing and non-reducing conditions at 10 and 50 kGy, and the observed intensity decreases were evident at 50 kGy and indicated the association of the 52 and 55 kDa polypeptides during irradiation (Abu, Muller, & Duodu, 2006). Therefore, irradiation may affect the secondary structure, such as the disulfide bonds of peanut protein, but does not change the basic subunit composition of the peanut protein.

acids to join together through dehydration synthesis and thus lead to decreased amino acid content (El-Rawas et al., 2012). On the contrary, some other amino acids (threonine and lysine) were significantly increased at high irradiation dose (10 kGy) but not under low irradiation doses (≤5 kGy). This finding manifested that high irradiation doses might disintegrate some macromolecular substances into small molecules. Lysine, which is generally deficient in peanut, has been recognized as the first limiting amino acid of peanut (Hamaker et al., 1992). However, cystine and methionine levels are lower than lysine levels because the former two were partially destroyed by hydrochloric acid during hydrolysis. Irradiation with doses from 0 kGy to 10 kGy did not significantly affect (P > 0.05) the alanine and tyrosine contents of the two peanut seeds. However, El-Rawas et al. (2012) reported that the tyrosine content decreased significantly (P < 0.05) at the irradiation dose of 7.0 kGy. This effect might be due to the e-beam-induced freeradical formation that resulted in the deamination of certain amino acids. The total amino acid (TAA), EAA and non-EAA contents were also significantly reduced after irradiation (10 kGy) relative to those in the control. EAA/TAA, an important nutritional factor, was not changed in YH-9326 but significantly increased in YH-22 when treated with 10 kGy irradiation dose. Maity et al. (2009) reported that the TAA contents of C. arietinum L. seeds increased significantly and by nearly three folds compared with that of the control at 6 kGy irradiation. Khattak and Klopfenstein (1989, pp. 2257–2261) reported no significant changes in the threonine, valine, arginine, and histidine contents after irradiating four different cultivars (wheat, maize, chickpea, and mung-bean seeds) and found significant decrease in phenylalanine only for maize. Tyrosine is more susceptible to irradiation than phenylalanine and was significantly decreased in all cultivars at 0.5 kGy. On the contrary, Abu, Muller, Duodu, and Minnaar (2005) found that except for the tyrosine content, that increased significantly, all the amino acid contents in the cowpea flour decreased significantly at 50 kGy. The data above showed that different amino acids exhibited different sensitive degrees to irradiation. These differences may be due to the varying side-chain groups in the amino acids, and the changes in amino acid are largely attributed to the cultivar differences. During irradiation, the reaction groups between free amino acids and protein molecules might be oxygenized, hydrolyzed, and deaminized, thus resulting in different changes in different kinds of amino acids (El-Rawas et al., 2012).

3.8. Lipase activity Lipase is an enzyme that can gradually break down triglycerides into glycerol and fatty acids. The active site of lipase belong to the serine protease and is composed of serine, aspartic acid, and histidine (Kaar, Jesionowski, Berberich, Moulton, & Russell, 2003). The effects of increased irradiation doses on peanut lipase activity are shown in Fig. 5. The lipase activities of YH-9326 and YH-22 were 1078.92 and 811.64 U before irradiation and were significantly decreased after gamma irradiation. Even low irradiation dose (3 kGy) can diminish the lipase activity. For example, the lipase activity of YH-9326 was decreased by 5.2% under 1 kGy dose. On the contrary, high irradiation dose (5 and 10 kGy) did not further reduce the lipase activity. This result indicated that peanut lipase might be sensitive to irradiation and can be inactivated by weak irradiation. Similarly, Wada et al. reported that (1998) irradiation had a certain effect on lipase structure and decreased the lipase activity. Numerous studies revealed that different enzymes in seeds exhibit various sensitivity degrees to irradiation doses. For example, the amylase activity of the seeds increased under radiation stress at doses of up to 3 and 4 kGy and decreased at high doses (5–6 kGy). This result might be due to the metabolic boosting of the seeds to counter radiation stress. The drastic depletion of the enzyme activity during irradiation at high doses manifested system breakdown (Maity et al., 2009). Substantial literature has proven that gamma irradiation can maintain the good quality of food by killing most of the bacteria during preservation. However, the decline in enzyme activity may also play an important role in food preservation.

3.7. SDS-PAGE Reductive and non-reductive SDS-PAGE analyses, the type of which differs on the presence or absence of β-mercaptoethanol in the sample buffer that breaks the disulfide bond, were conducted to investigate the effects of irradiation on the protein changes in peanut seeds. The protein bands of the peanut seeds under different irradiation doses are shown in Fig. 4. Twelve obvious protein bands were attained in the reductive SDS-PAGE (Fig. 4 A and B). Three molecular weight distribution regions consisting of the con-arachin subunit (60–66 kDa), acidic arachin subunit (37–43 kDa) and basic arachin subunit (below 25 kDa) were found. These findings were similar to the results previously reported by Lin et al. (2015). Under reductive conditions, the band patterns of controls and samples were not significantly different after irradiation with doses of 1–10 kGy. This result indicated that irradiation (1–10 kGy doses) did not significantly affect the protein subunit of peanut. El-Rawas et al. (2012) reported that 3.2 and 7.0 kGy irradiation did not significantly (P > 0.05) influence peanut protein levels, but high irradiation levels (27.7 kGy dose) substantially reduced the protein contents of peanuts with high molecular weights. This observation suggested that high irradiation dose caused protein degradation. Likewise, Huang, Herald, and Mueller (1997) found no alteration in the band pattern of egg yolk protein after exposure to 2.5 kGy of irradiation relative to that in an untreated sample. However, the myosin heavy chain gradually disappeared in surimi treated with

4. Conclusions This study showed that irradiation has no significant effect on the moisture and ash contents and total sugar of peanuts. Low-dose irradiation did not significantly alter the water activity and protein and fat contents, but high irradiation levels significantly decreased the fat and 540

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Fig. 4. Reductive and non-reductive SDS-PAGE analysis of peanut protein. A and B represent the reductive SDS-PAGE of YH-9326 and YH-22, respectively; C and D represent the non-reductive SDSPAGE of YH-9326 and YH-22, respectively. Lane M: protein molecular weight markers (kDa). Lanes I to V represent the irradiation doses of 0, 1, 3, 5, and 10 kGy, respectively.

might break the disulfide bonds of peanut protein. In addition, the lipase activity decreased with the increasing irradiation dose. Furthermore, irradiation of 1 kGy is suitable for peanut seed according to this study. Acknowledgements This study was supported by the Natural Science Foundation of Henan Province (162300410046), the Foundation for University Young Key Teachers of Henan Province (2016GGJS-071), and the Talent Projects from Henan University of Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.lwt.2018.06.009. References Fig. 5. Effect of irradiation dose on lipase activity of peanut seeds YH-9326 ( ) and YH-22 ( ). Each data column represents the mean of three replications. Vertical bars represent the standard errors of means. Values of each peanut cultivar followed by the same letter are not significantly different (P ˃ 0.05). Peanut seeds were irradiated at 25 °C with doses of 0, 1, 3, 5, and 10 kGy, respectively.

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protein contents (10 kGy) and increased the water activity (5 and 10 kGy). Gamma irradiation accelerated the degree of lipid oxidation and consequently increased PV, CV, FAV, and MDA contents. The fatty acid and amino acid composition were changed after irradiation treatment. Irradiation might not affect the peanut protein subunits but 541

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