Effect of Drying Methods on Peanut Quality during Storage

To maintain peanut quality during storage and reduce eco-nomic losses, many researchers have studied the quality changes of peanut under different storage conditions 15 − 17 ） . Mutegi 18 ） found that the moisture content, rancidity, physical damage, and aflatoxin contents of peanuts notably changed at different humidity. Martín 19, 20 ） demonstrated that peanut stored at 10 ℃ was associated with less quality deterioration than that stored at 25 ℃ , and peanut qualities （ peroxide value, conjugated dienes, free fatty acids, saturated to unsaturated fatty acid ratio, oleic to linoleic ratio, iodine value, roasted peanutty flavor etc. ） were better when packed in ethylene vinyl alcohol bags than that in polypropylene ventilated bags except for free fatty acids. Fu 21 ） investigated that the influences of different packing materials on inhibiting aflatoxin production and maintain-ing the quality of peanuts, and found that peanuts stored in the polyester/aluminum/polyamide/polyethylene and polyamide/polyethylene bags were both better at inhibiting and controlling aflatoxin production. Liu ’ s study 22 ） exhibited that storage temperature （ 15 ℃ , 25 ℃ , and 35 ℃） led to different degrees of lipid oxidation of peanuts. Tempera-Abstract: Storage is an important step after peanut harvest and drying. Many factors could affect the peanut quality during storage. The quality change differences of peanut after being dried by solar radiation and at 35℃, 40℃, 45℃, 50℃ during later storage were investigated, including moisture content (MC) and germination percentage (GP) of peanut kernels, acid value (AV), peroxide value (PV), iodine value (IV), vitamin E (VE) content and fatty acid composition (FAC) of extracted peanut oil. And the impact of four storage conditions, air-room temperature (A-RT), air-low temperature (A-LT), vacuum-room temperature (V-RT) and nitrogen-room temperature (N-RT) on peanut quality after 10 months’ storage were also studied in this paper. The results revealed that drying conditions had only a little influence on peanut quality during later storage. Peanut dried by solar radiation was more easily oxidized than that dried under other drying conditions. The effects of storage time were much greater. The GP, AV, PV, VE content and FAC, showed significantly changes along with storage. GP and VE content decreased, AV and PV increased, and some linoleic acid was oxidized to oleic acid after 10 months’ storage. In addition, A-LT exhibited best performance in keeping peanut quality than A-RT, V-RT and N-RT, which demonstrated that low temperature was more advantageous for peanut storage than controlled atmosphere. These results above would provide useful information and reference for the peanut storage to apply in food industry.

To maintain peanut quality during storage and reduce economic losses, many researchers have studied the quality changes of peanut under different storage conditions 15 17 .
Mutegi 18 found that the moisture content, rancidity, physical damage, and aflatoxin contents of peanuts notably changed at different humidity. Martín 19,20 demonstrated that peanut stored at 10 was associated with less quality deterioration than that stored at 25 , and peanut qualities peroxide value, conjugated dienes, free fatty acids, saturated to unsaturated fatty acid ratio, oleic to linoleic ratio, iodine value, roasted peanutty flavor etc. were better when packed in ethylene vinyl alcohol bags than that in polypropylene ventilated bags except for free fatty acids. Fu 21 investigated that the influences of different packing materials on inhibiting aflatoxin production and maintaining the quality of peanuts, and found that peanuts stored in the polyester/aluminum/polyamide/polyethylene and polyamide/polyethylene bags were both better at inhibiting and controlling aflatoxin production. Liu s study 22 exhibited that storage temperature 15 , 25 , and 35 led to different degrees of lipid oxidation of peanuts. Tempera-tures could affect peroxide value and malondialdehyde MDA content of oil extracted from peanuts significantly, which were the indicators of lipid oxidation degree 23 .
Besides the quality deterioration of lipid in peanut along with storage, the quality of peanut protein also changed. Sun 24 investigated the molecular functional properties of peanut protein isolate during storage. Extending storage time resulted in a decrease in free sulfhydryl content, fluorescence intensity, surface hydrophobicity and emulsifying properties and an increase in protein particle size. The results also showed the content decline of α-helix and β-sheet, and the content rise of β-turn and random coil.
In summary, substantial studies have reported that the effects of different storage conditions on the quality of peanuts. However, few works have investigated the effect of different drying conditions on the later storage of peanut. The present study aimed to investigate the quality changes of peanuts during storage after being dried by solar radiation and at 35 , 40 , 45 , 50 . The effect of low temperature and controlled atmosphere on peanut storage were also studied.

Samples
Tianfu No.3 peanut was planted in April in Weihui County 35.4 N, 114.1 E , Henan Province, China, and harvested in October. After preliminary drying in the field by the sun, the peanut was picked. And the initial moisture contents of the peanut were tested between 17.7 and 22.9 . Some of the wet peanuts were spread on the ground and dried by the sun as called solar radiation for 3 days, and some of them were dried by hot air at 35, 40, 45, 50 and 55 for 18 h, 14 h, 10 h, 8 h and 6 h, respectively until the moisture contents were all below 10 as mentioned in our previous study 25 . Dried peanuts obtained under different drying conditions were then shelled by a dehusking machine Yuexin Machinery Factory, Gongyi, China , respectively. And the shelled peanuts were used as samples for storage experiments.

Storage conditions
The shelled peanuts were stored under four conditions as below.
a Air-room temperature A-RT The shelled peanuts were put into polyamide and polyethylene PA/PE bags, which was 0.16 mm in thickness, 40 cm in width and 60 cm in length. One bag can hold 1500 g samples. Then the bags with peanut samples were stored at room temperature Fig. 1 . b Air-low temperature A-LT The shelled peanuts were placed in PA/PE bags as described in a and stored at quasi-low temperature less than 20 , Fig. 1 .
c Vacuum-room temperature V-RT The shelled peanuts were first placed in PA/PE bags. Then the air in the bags was drawn using a vacuum packing machine DZ400, Shanghai Afanlao Machinery Co, Ltd, China to vacuum degree 90 kPa. The vacuum bags were sealed by a plastic film sealing machine FS-300, Qingye Company, China . And the samples were then stored at room temperature Fig. 1 . d Nitrogen-room temperature N-RT The vacuum bags with shelled peanuts as mentioned in c were connected to steel cylinder by syringe needle, rubber tube and pressure reducing valve. And the high-purity nitrogen ≥ 9.999 slowly filled the vacuum bag. Then the pinhole was sealed by the sealing machine. The peanut samples were then stored at room temperature Fig. 1 .
The qualities of the shelled peanuts stored under these different conditions were analyzed after 0, 2, 4, 6, 8 and 10 months of storage, respectively.

Moisture content determination
The moisture content of shelled peanuts was measured according to the American Society of Agricultural and Biological Engineers ASABE method S410.2 at 130 for 6 h 26, 27 .

Germination tests
Based on Chinese National Standard GB/T 5520-2011, samples of 50 shelled peanuts were sowed in wet sand and cultivated at 25 in a climate incubator. The germination percentage was defined as the number of successfully spouted peanuts out of the total number of peanuts after 10 days cultivation.   25 , aqueous enzymatic extraction 28 30 and thermosonication assisted extraction 3 were the common extraction method of peanut oil. In this paper, peanut oil was extracted from the sliced peanut kernels by petroleum ether, and then the petroleum ether was removed with a rotary vacuum evaporator according to Qu et al. 25 The extracted oil was then used for further chemical analysis.

Acid value
The acid values of peanut oil were determined according to the Chinese National Standard GB 5009.229-2016. The extracted oil 20 g was placed into conical flask for dissolving the sample in ether-isopropanol solution. Then the solution was titrated with standard NaOH solution 0.1 mol/ L with phenolphthalein as the indicator.

Peroxide value
The peroxide values of peanut oil were measured by the Chinese National Standard GB 5009.227-2016, using 3 g oil from each sample and expressed as millimoles of active oxygen in 1 kg oil 25 .

Iodine value
Method for iodine values determination of peanut oil followed Chinese National Standard GB/T 5532-2008. Iodine value of peanut oil was expressed in grams of iodine absorbed by 100 g oil 25 .

Vitamin E determination
The contents of vitamin E for peanut oil were determined using VE test kits Nanjing Jiancheng Bioengineering Institute, China according to the manufacturer s protocol, which was on the basis of the reaction of VE and phenanthroline to form a pink complex with 533 nm absorbance.

Fatty acid composition determination
The composition of fatty acid of peanut oil was determined by gas chromatography 7890 B, Agilent Technologies, USA with flame ionization detector GC-FID according to the Chinese National Standard GB 5009.168-2016 external standard method 7.1. The HP-88 capillary column 100 m 0.25 mm 0.20 μm was employed to separate different fatty acids with 1.00 mL/min flow rate of carrier gas nitrogen . The initial temperature was 140 for 5 min, temperature programmed to 240 at 4 /min and holding at 240 for 20 min. Split ratio was 1:50 and FID temperature was 260 .

Data analysis
Origin version 9.0 and SPSS version 21.0 were used to data processing and statistical analysis. The significant difference was determined at the p 0.05 level for One way analysis of variance ANOVA and Duncan s test.

Results and Discussion
3.1 The effect of different drying conditions on the later storage of peanut A-RT was chosen as storage condition to compare the quality change differences of peanut after being dried by solar radiation and at 35 , 40 , 45 , 50 during later storage.

Moisture content
Moisture content is one of the most important key factors for peanut storage. The moisture content of peanut at harvest is usually more than 35 -45 6 . If it is not dried in time, the mold would reproduce and grow fast at appropriate temperature. In case the dried peanut is stored in the humid environment, it is easy to absorb moisture and become moldy. And in severe cases, toxins can also be produced 31,32 . According to the DB 21/T 2496-2015 peanut storage technical regulations, the safe moisture content of peanut kernel is less than 8 . Salamatullah et al. 33 reported that the moisture content of peanut kernels of different harvesting time was determined between 4.47 and 7.93 . In our experiment, the moisture contents of peanut kernels dried by solar radiation and at 35 , 40 , 45 , 50 during later storage were displayed in Fig. 2 a . It can be seen that the moisture contents were all between 4.03 0.06 and 5.94 0.12 . Therefore, the moisture contents of peanut kernels were all within the safe range.

Germination
Germination percentage GP is an important indicator to reflect the vitality of peanuts. The environmental conditions and moisture content could affect peanut germination 34 . The columns in Fig. 2 b displayed the effects of different drying conditions on the GP of peanut stored for same period of time in A-RT condition, which showed no significant difference. Therefore, drying conditions solar radiation and at 35 , 40 , 45 , 50 had no influence on the GP of peanut during later storage. The point-line diagram in Fig. 2 b exhibited the effect of storage time on GP of peanut. The GP showed significant difference after 10 months storage, from 95 3 at the beginning, to 91 4 after 8 months and 86 3 after 10 months. Hence, storage time had an impact on the germination of peanut.

Acid value
Acid value AV is a measure of free fatty acid content in oil, and can be used as an indicator of rancidity. It can be seen from Fig. 3 a that the effect of different drying conditions on AV during storage. On the whole, the AVs of oil that obtained from peanut dried by solar radiation, at 35 and 50 were a little higher than that obtained at other drying temperatures. This was due to that the moisture contents of peanut dried by solar radiation, at 35 were higher than others during storage Fig. 2 a , making the oil easier to hydrolyze. And from our previous study, the AV of oil extracted from peanuts dried at 50 0.124 mg/g was significantly higher than that from peanut dried by solar radiation 0.087 mg/g , at 35 0.090 mg/g , 40 0.085 mg/g and 45 0.093 mg/g , showing that higher drying temperature was unfavourable for peanut quality. Figure 3 a displayed that the storage time had an obvious influence on AV. The AV increased along with storage. For instance, the AVs of peanut oil obtained from peanut dried at 35 were 0.09 0.01 mg/g before storage, which increased to 0.62 0.01mg/g after 10 months. This was due to the hydrolysis of lipid in peanut during storage, which was in accordance with Martín s research 19 .

Peroxide value
Peroxide value PV is usually used as an indicator of lipid oxidation 22,24 . This parameter increases while the lipid deterioration advances during grain and food storage 25 27 . At the same storage period, peanut dried by solar radiation revealed higher PV Fig. 3 b . This was due to that in addition to autoxidation, lipoxygenase produced by microorganisms can also catalyze lipid oxidation. Firstly, the drying conditions with higher temperature played the role of sterilization. The longer the storage time, the more obvious the sterilization impact. Secondly, the moisture contents of peanut dried by solar radiation were higher than that of other drying conditions during storage, which were shown in Fig. 3 b . In addition, the PV increased more rapidly after 6 months. For instance, the PVs of the obtained peanut dried at 35 oil were 0.24 0.02 mmol/kg before storage, which increased to 0.38 0.03 mmol/kg after 6 months, 0.93 0.05 mmol/kg after 8 months and 1.15 0.03 mmol/kg after 10 months. This phenomenon may be owing to the increase of room temperature Fig. 1 and the acceleration of oxidation rate.

Iodine value
Iodine value IV is important in evaluating the quality and shelf life of oils and fats, as it gives a good estimation of their susceptibility to oxidative degradation 35 . Generally, the higher the IV, the greater the unsaturation degree of the oil, and the higher the content of unsaturated fatty acids in the oil. The IV results were presented in Fig. 3 c , which showed that IVs of peanut oil obtained from peanut dried in different conditions did not significantly changed during storage. And storage time also showed no significant effect on IV.

Vitamin E
Vitamin E VE content could also be used as an indicator of oxidative state for peanut 36 . It stabilizes polyunsaturated fatty acids in membrane lipid bilayers, thus protecting them from lipoxygenase attack and scavenging radicals 37 . Lipid oxidation process results in loss of VE. It can be seen from Fig. 3 d that VE content of oil extracted from peanut dried by solar radiation was much lower than that dried by hot air at the same storage time. This result was consistent with the PV results. The PV of oil extracted from peanut dried by solar radiation was higher than that of other drying conditions. The PV and the VE content showed negative correlation. Moreover, the VE content reduced more rapidly after 4 months. The VE content of the obtained peanut dried at 35 oil were 110 0.58 μg/g before storage, which decreased to 104 0.85 μg/g after 4 months and 70 1.11 μg/g after 10 months Fig. 3 d . Table 1 showed that there were more than 80 unsaturated fatty acids in peanut oil, which mainly included oleic acid and linoleic acid. After 10 months storage, the contents of these fatty acids, C16:0, C18:0, C18:1-9, C18:2-6, C20:0, C20:1-9, C22:0 and C24:0, had no significant difference under different drying conditions Table 1 .

Fatty acid composition FAC
However, linoleic acid C18:2-6 decreased from 34.35 1.21 to 32.48 0.20 , and oleic acid C18:1-9 increased from 43.96 0.06 to 47.02 0.50 after 10 months storage as suggested by Table 2. This was due to that the  The moisture content of peanut kernels before storage was 4.80 0.12 Fig. 4 a . After 10 months storage in A-RT, A-LT, V-RT and N-RT conditions, the moisture contents were 4.88 0.06 , 5.24 0.14 , 4.70 0.05 and 4.94 0.07 , respectively Fig. 4 a . They were all in the safe moisture range below 8 .

Germination
The changes in the GP of peanut kernels under different storage conditions A-RT, A-LT, V-RT and N-RT were investigated. The results were presented in Fig. 4 b . It can be found that the GP decreased from 95 3 to 86 3 in A-RT storage condition after 10 months storage, which was lower than that under other storage conditions. The GP of peanut stored in A-LT, V-RT, N-RT conditions showed no significant difference. Therefore, low oxygen concentration and low temperature were the favorable factors to maintain the GP of peanut.

Acid value
The AVs of oil extracted from peanut stored under A-RT, A-LT, V-RT and N-RT conditions were displayed in Fig. 5 a . The initial AV of peanut oil obtained from peanut dried at 35 was 0.09 0.00 mg/100 g, which indicated good quality in terms of the degree of lipid hydrolysis. The AV increased significantly after 10 months of storage under A-RT, A-LT, V-RT and N-RT storage conditions. Compared with other storage conditions, peanut kernels stored at A-LT had the lowest acid value, 0.45 0.03 mg/100 g. The acid values reached 0.66 0.02 mg/100 g and 0.71 0.01 mg/100 g in V-RT and N-RT storage conditions, which was a little higher than that in A-RT condition 0.62 0.01 mg/100 g . These results illustrated that low temperature could inhibit the hydrolysis of triglycerides in peanuts to produce free fatty acids, which controlled atmosphere could not.

Peroxide value
Considerable variability in PV of oil extracted from peanut dried at 35 was detected under different storage conditions, which was shown in Fig. 5 b . At the beginning of storage, the PV was 0.24 0.02 mmol/kg. After 10 months storage, there were significant differences among different storage conditions, varied between 0.64 0.06 mmol/kg L-RT and 1.15 0.03 mmol/kg A-RT . The peanut stored under V-RT and N-RT conditions showed lower PVs than that of A-RT storage, which were 0.88 0.05 mmol/kg and 0.77 0.04 mmol/kg, respectively. From these data, it can be concluded that temperature and oxygen concentration influenced the PV. And low temperature and controlled atmosphere could retard the oxidation of peanut.

Iodine value
There were no significant difference in IV of peanut oil after 10 months storage under different storage conditions, which was displayed in Fig. 5 c . This indicated that unsaturation degree of peanut oil under different storage conditions showed no significant difference.

Vitamin E
Peanut is a rich source of polyphenolic antioxidant compounds, including resveratrol, tocopherol, phytosterol, catechin, epicatechin, and quercetin 33 . Salamatullah et al. 33 reported that gallic acid, 3,4-dihydroxy-benzoic acid, catechin, and 1,2-dihydroxybenzene were the key phenolic compounds of peanut kernels. The gallic acid content of peanut kernels ranged from 1.36 mg/L to 2.85 mg/L. The 3,4-dihydroxybenzoic acid content of peanut samples were 1.73 mg/L-3.56 mg/L. Additionally, the -catechin content of the peanut kernel samples changed between 2.17 mg/L and 5.15 mg/L, and the 1,2-dihydroxybenzene content of peanut kernels were 2.67 mg/L-5.85 mg/L. In this paper, the results of the VE content of peanut oil during storage were shown in Fig. 5 d . The initial VE contents of peanut oil was 110 0.58 μg/g. After 10 months of storage, VE contents of peanut oil extracted from peanut stored at A-RT, A-LT, V-RT and N-RT were 70 1.11 μg/g, 84 1.53 μg/g, 72 1.03 μg/g and 81 1.16 μg/g, respectively. Therefore, low temperature showed good performance on protecting the antioxidant system of peanut.

Fatty acid composition
The dominant fatty acids in peanut oil were oleic acid 43.96 -47. 34 , linoleic acid 32.39 -34.35 and palmitic acid 10.77 -11. 27 , followed by behenic acid 2.55 -2.97 , stearic acid 2.77 -3.18 , lignoceric acid 1.37 -1.71 , arachidic acid 1.43 -1.51 and gadoleic acid 0.73 -1.12 , which can be seen in Table 2. It can be seen that there was no significant difference in the fatty acid composition after 10 months storage under ART, ALT, VRT and NRT storage conditions.

Conclusion
This study demonstrated that drying conditions solar radiation, 35 , 40 , 45 , 50 had only a little impact on peanut quality during later storage. Peanut dried by solar radiation was more easily oxidized than that dried under other drying conditions. The PV and VE content of oil extracted from peanut being dried by solar radiation was significantly different with that of other drying conditions. The effect of storage time on peanut quality was more obvious than that of drying condition. Most indicators, GP, AV, PV, VE content and FAC, showed significantly changes along with storage. For storage conditions, low storage temperature performed best to keep peanut quality than controlled atmosphere storage and A-RT storage. CC BY 4.0 Attribution 4.0 International . This license allows users to share and adapt an article, even commercially, as long as appropriate credit is given. That is, this license lets others copy, distribute, remix, and build upon the Article, even commercially, provided the original source and Authors are credited.