Original Paper Effects of Drying Process with Different Temperature on the Nutritional Qualities of Walnut (Juglans Regia L.)

Original paper Effects of Drying Process with Different Temperature on the Nutritional Qualities of Walnut (Juglans regia L.)

1,2 2 2 2 1 2* Yongxiang Han , Yuewen Zheng , Shiliang Li , Runhong Mo , Xiangyu Long and Yihua Liu

1Rubber Research Institute, Chinese Academy of Tropical Agricultural Sciences, Danzhou, Hainan 571737, China, Naobao Village, Nada Town, Danzhou, Hainan Province, China 2Research Institute of Subtropical Forestry, Chinese Academy of Forestry, Fuyang 311400, China, No.73 Daqiao Road, Fuyang, Zhejiang Province, China

Received August 10, 2018 ; Accepted November 12, 2018

The chemical compositions, fatty acids, total flavonoids content (TFC), total polyphenols content (TPC), and minerals of walnuts were investigated after freeze drying (‒55 ℃ ) and heat drying (60 ℃ , 105 ℃ , 140 ℃ ) process. The results showed that the contents of protein, soluble sugar, minerals (except for Mn), and carbohydrate were increased with the rise of temperature from ‒55 ℃ to 105 ℃ . Linoleic acid and oleic acid could be detected as the main fatty acids, with the highest contents were also observed at 105 ℃ (309.0 mg/ g and 79.6 mg/g). However, the TFC and TPC were notably increased by 36.9 % and 33.9 % in kernel when the temperature was increased from ‒55 ℃ to 140 ℃ . In general, the freeze drying and heating drying had an inhibition effect and irritation effect on the nutritional qualities of walnuts, respectively. Therefore, heating treatment could significantly improve the nutritional qualities of walnut, and the recommended temperature for walnut drying was 105 ℃ .

Keywords: walnut, heat drying, freeze drying, temperature, nutritional qualities

Introduction associated with the harvest season (Lynch, et al., 2016) and The industry of nut production has been developing rapidly environmental conditions (temperature) (Amaral et al., 2005). in recent years. Nut is recommended as part of a healthy diet The previous studies indicated that nuts cultivated in region due to their beneficial nutritional profile consisting of with lower mean maximum annual temperature would exhibit unsaturated fatty acids, tocopherols, polyphenols, phytosterols higher contents of unsaturated fatty acids (palmitic, oleic and and flavonoids (Bolling et al., 2011). The health benefits of nut linoleic acids) and phytosterols (β-sitosterol) (Fuentealba et al., consumptions are associated with preventing the cardiovascular 2017). The above-mentioned factors were covered the diseases, colon cancer, coronary heart disease, and type II preharvest period of walnut. However, during the postharvest diabetes (Grace et al., 2014). The health-related properties of period, temperature becomes one of the key factors, which nut are influenced by the cultivar and its growing conditions. could have important effects on the health-related properties of The nutritional values of different cultivars of walnut from nuts. Spain were various (Tapia et al., 2013). The results from three Roasting is one of the main cooking method that applied regions (north, dry coast, south) of Chile were also presented for nuts, which could lead to various physicochemical changes. the effects of growing condition on the contents of minerals in Many previous works investigated the effects of roasting pine nut, in which the contents of Cu, K, Na located in the treatment on the nutritional qualities of hazelnuts (Manzo et north Chile doubled those in the dry coast (Lutz et al., 2017). al., 2017), almonds (Agila and Barringer, 2012), pistachios In addition, the nutrient compositions of nuts are also (Hojjati et al., 2013), Brazilian nuts (Ozcan et al., 2018), and

*To whom correspondence should be addressed. E-mail: [email protected] 168 Y. Han et al. walnuts (Schlormann et al., 2015). The roasting treatment purchased from Sigma (St. Louis, MO, USA). Other solvents resulted in the changes of microstructural and chemical were of analytical grade and were purchased from Shanghai compounds, such as moisture content reducing (Aljuhaimi et GuoYao Chemical Reagents (Shanghai, China). al., 2017b), total polyphenols and flavonoids increased (Lerma- Sample preparation and drying About 5 kg fresh walnut Herrera et al., 2017), and fatty acid content changed (Ozcan et (Juglans regia L.) was purchased at a local supermarket in al., 2018). Some studies also showed that roasting was related Hangzhou, Zhejiang province, China. The samples were stored to the formation of special taste and aroma as well as the at 4 ℃ until analysis. brittleness of nuts (Hojjati et al., 2013). The walnut samples were dried as without the shell in a Walnut is one of the important tree nuts in the world. The drying oven at 60 ℃ , 105 ℃ , 140 ℃ ; and in a lyophilizer industry of walnut production has been developing rapidly in (Thermo Biofuge Stratos) at ‒55 ℃ until a constant weight was China in recent years, ranking first in the yield of walnut achieved. Each drying protocol was carried out three times. globally (Han et al., 2018). However, the roasted walnut is not The samples were weighed every 3 min (140 ℃ ), 20 min a typical way to eat in China and some other countries. Walnut (105 ℃ ), 1 h (60 ℃ ) and 4 h (‒55 ℃ ), in order to obtain the is often subjected to thermal treatment to reduce the moisture drying kinetics curves of walnut. According to the previous content, and then packed for sell or further processed into other study (Nawirska, et al., 2009), the variations in the moisture productions. The differences between roasting and thermal ratio MR occurring at drying time τ were established in terms of treatment include the heating temperature, duration and the modified formula: operation. The roasting process is carried out by commercial M(τ) ‒ Me MR = ······Eq. 1 electrical oven and microwave (Manzo et al., 2017; Aljuhaimi M0 ‒ Me and Ozcan, 2018). The typical roasting conditions ranged from where M(τ) is walnut moisture content after drying time τ, minimum 140 ℃ /25 min to 160 ℃ /15 min for almonds, Me denotes equilibrium moisture content, and M0 stands for pistachios and macadamia and up to 176 ℃ /10 min for pecan, initial moisture. which approximately reflect the minimum and maximum range For each drying cycle, temperature were measured every 30 of roasting temperatures and time periods used for industrial s during the drying process using a contact digital electronic roasting of nuts (Stuetz et al., 2017; Magnuson et al., 2016). thermometer HT-9815 (Xintai instrument Co., Ltd., Guangdong Compared with the roasting process, the thermal treatment of province, China). In order to obtain the temperature profile in the walnut in China is often carried out by sunlight or air oven, walnut kernel core during the drying process, three sensors were with a relative lower temperature (< 140 ℃ ) and longer time (> inserted into three samples. The heated and frozen samples were 30 min). Recent studies supported that heating treatment (from separately ground into powder with a grinder before analysis. 70 ℃ to 110 ℃ ) had significant influence on moisture, fat Chemical composition Moisture, fat, protein, soluble content, fatty acid composition, and polyphenols compounds of sugar, and carbohydrate content of walnut samples were pecan kernel (Aljuhaimi et al., 2017b). Except heating determined according to AOAC procedures (AOAC 2000). treatment, freeze drying is now widely used in fruits, Moisture content of kernel was measured after the sample vegetables and nuts, which is a dehydration process typically preparation. Fat content was extracted by Soxhlet apparatus for used to make the material more convenient for transport. 10 h (with petroleum ether), and the solvent was evaporated Freeze drying would keep fruits and vegetables as much fresher using rotary vacuum evaporator at 40 ℃ . Oil was stored at as possible nutritional value practically unchanged (Storey and ‒20 ℃ in brown bottle until GC analysis. The quantity of Anderson, 2018; Eisinaite et al., 2016). However, as far as we protein was calculated by multiplying the nitrogen content, know, there is no research focus on the changes of nutrients in determined using the Kjeldahl method, by the coefficient 6.25 walnut after freeze drying. (AOAC 2000). The aim of this study was to evaluate the influence of For soluble sugar analysis, 0.5 g kernel of homogenate and different heating temperature treatment (60 ℃ , 105 ℃ , 140 ℃ ) 10 mL ethanol/water (8:2, v/v) were placed in a 25 mL on nutrient qualities of walnut. Moreover, we also want to colorimetric tube, heated in water at 80 ℃ for 30 min. know that if freeze drying treatment (‒55 ℃ ) has less effect on Supernatant was transferred to 25 mL tube and added 5 mL nutritional qualities of walnut than the heating treatment. ethanol/water (8:2, v/v). This process was repeated three times. Supernatants were combined and brought to 25 mL with Materials and Methods deionized water. The supernatant was diluted to 25 mL (diluted Chemicals A Milli-Q-Plus water purification system from 25 times) for soluble sugar analysis. The extracted kernel and Millipore (Milford, MA, USA) was used throughout the study 10 mL 3 M HCl were mixed in a 25 mL volumetric flask, and to obtain the ultrapure water for the analyses. GC-grade 2, 2, heated in boiling water for 40 min. After cooling, the solution 4-trimethylpentane and methanol were obtained from Merck was neutralized with 10 mL 3 M NaOH and brought to 25 mL (Darmstadt, Germany). Folin-Ciocalteau and 37-component with deionized water. Finally, the solution was diluted to 25 mL fatty acid methyl esters (FAMEs) mixture (No.47885-U) were (diluted 25 times) for carbohydrate analysis. 2.5 mL diluted Quality Change of Walnut during Drying 169 solutions (soluble sugar and carbohydrate) were carefully in triplicate. The data were showed as milligrams of rutin measured and transferred to 10 mL colorimetric tube, and added equivalents (total flavonoids content) and gallic acid equivalents 6.5 mL anthrone in freezing water. The mixture was heated in (GAE) (total polyphenols content) per kilogram of defatted meal. boiling water for 10 min. After cooling, the soluble sugar and Mineral composition Potassium (K), calcium (Ca), carbohydrate were measured at 620 nm in an Optizen Pop UV/ phosphorus (P), magnesium (Mg), copper (Cu), zinc (Zn), iron Vis spectrophotometer (Perkin Elmer, USA). Blank sample of (Fe), and manganese (Mn), were determined by ICP-OES (iCAP glucose was used as a reference in spectrophotometric analysis. 7000); barium (Ba) was determined by ICP-MS (NexIon 300D, A standard curve was prepared and the results were expressed as Perkin Elmer, Shelton, CT, USA). The walnut samples g glucose equivalent/g sample (%). preparation procedure according to the method developed in our Fatty acid composition 100 µL oil samples mixed with previous research (Han et al., 2018). Briefly, 0.2 g milled 4 mL 2, 2, 4-trimethylpentane in 25 mL colorimetric tubes was samples were accurately weighted and transferred to vortexed for 1 min; then, the samples added to 200 µL 2 M polytetrafluoroethylene (PTFE) tube, then added 6 mL nitric acid

KOH in methanol at room temperature and vortexed for 1 min. (HNO3) and 2 mL hydrogen peroxide (H2O2). The PTFE tube Finally, the mixture added about 1 g sodium bisulfate was heated gradually to 200 ℃ for 20 min with the microwave monohydrate to remove the moisture, and the fatty acid methyl oven (CEM, Matthews, NC, USA). After cooling at the room esters (FAMEs) were analyzed using in an Agilent 7890A gas temperature, the PTFE tube was placed into digestion furnace chromatography (Agilent Technologies, Palo Alto, California, and the solution was heated to near dryness at 200 ℃ . Then the USA) equipped with a flame ionisation detector (GC-FID), a solution diluted to 25 mL polypropylene tubes for ICP-OES (K, PAL3 autosampler (Agilent Technologies, Santa Clara, CA, Ca, P, Mg, Cu, Zn, Fe, Mn) and ICP-MS (Ba) analysis. USA) and an HP-INNOWAX fused silica capillary (30 m × Statistical analysis Analytical determinations were carried 0.25 mm × 0.25 μm) column (Sigma Chemical Co., St. Louis, out in triplicates and the results were expressed as mean value ± MO, USA). Walnut FAMEs was injected at 220 ℃ with an standard deviation. The statistical significance of the measured injector volume of 1 µL. The oven temperature was differences among the means of triplicates was analyzed using programmed from an initial value of 140 ℃ (1 min hold) to the analysis of variance (ANOVA). To evaluate differences, the 250 ℃ at 4 ℃ /min (5 min hold). The detector temperature was Tukey-karmer’s multiple range test was choosing to evaluate the operated at 220 ℃ . Nitrogen was used as a carrier gas at a statistical significance ( p < 0.05) of the data among different constant flow rate of 1.5 mL/min. Fatty acids were quantified temperature by SPSS 18.0 software package (Statistical Package by the external standard calibration method. The results were for Social Scientists, Michigan, USA). performed as mg of fatty acid per g of walnut fresh weight and in relative percentage of each fatty acid. Results and Discussion Total flavonoids and total polyphenols content 1.0 g of oil- Drying kinetics and temperature profile The relationship free and milled samples was added to 10 mL of mixture of ethanol: between the moisture ratio MR of the samples and the time τ of water (70:30, v/v). The mixture was kept in ultrasonic water-bath drying performed (Fig. 1, a, b, c, d) was described by the semi- for 30 h at room temperature, followed by centrifugation at empirical Page’s equation:

_ 8000 rpm for 5 min. The supernatant was diluted 10 times. 0.5 mL κ·τ MR=a·e +y0 ······Eq. 2 of diluent was mixed with 0.5 mL 5 % sodium nitrite (NaNO2, w/ v), kept in the room temperature for 5 min. Then the mixture was where a, κ, y0 were the constant and showed in Table 1. added 5 % 0.5 mL of aluminum nitrate (Al(NO3)3, w/v), kept it in Table 1 shows the equations with the different temperature the room temperature for 5 min. Finally, the mixture was added treatments. The results present high values of the determination 2 mL 2 M sodium hydroxide (NaOH) and the TFC was measured coefficient (R2, 0.993-0.998) and low values of the reduced chi- at 510 nm in an Optizen Pop UV/Vis spectrophotometer (Perkin square (χ2, < 0.001), indicating an acceptable goodness-of-fit. Elmer, USA). Blank sample of rutin was used as a reference in The drying curves with different temperature showed similar spectrophotometric analysis. A standard curve was prepared and variation. The moisture ratio MR decreased rapidly at the former the results were expressed as mg rutin equivalent/g sample. period and decreased steadily at later period. Similar results were 1 mL above mentioned diluent (diluted 10 times) was mixed found for pumpkin (Nawirska et al., 2009) and carrots with 0.5 mL of Folin-Ciocalteu reagent. After 5 min, 2 mL of (Kroehnke, et al., 2018). To reach the equilibrium moisture

75 g/L sodium carbonate (Na2CO3) was added. Finally, the content of samples the drying time required was found to be 24 h, mixture diluted to 10 mL and kept it for 1 h at 40 ℃ without 7 h, 2 h, and 0.25 h at ‒55 ℃ , 60 ℃ , 105 ℃ , 140 ℃ , respectively. light. The absorbance was measured at 750 nm in a Obviously, the drying time was significantly reduced with the spectrophotometer (Perkin Elmer, USA). Blank sample of gallic temperature improved. acid was used as a reference in spectrophotometric analysis. A The temperature profiles of walnuts samples obtained standard curve was prepared and the results were expressed as during drying processing for different temperature are mg gallic acid equivalent/g sample. All samples were carried out presented in Fig. 2. As show in Fig. 2, the temperature varied 170 Y. Han et al.

Fig. 1. Drying curves of walnut samples at different temperature treatments, a--55 ℃ , b-60 ℃ , c-105 ℃ , d-140 ℃ .

Table 1. Drying kinetic equations for different temperature treatments.

Temperature Equation Determination coefficient (R2) Reduced chi-square (χ2) ‒55 ℃ (τ/h) y=1.0151·e-0.1511τ‒0.0186 0.9984 0.0002 60 ℃ (τ/h) y=0.9882·e-0.5228τ‒0.0084 0.9926 0.0008 105 ℃ (τ/min) y=0.9854·e-0.0437τ+0.0078 0.9966 0.0004 140 ℃ (τ/min) y=0.9931·e-0.3521τ+0.0041 0.9983 0.0002

at different rates with the different drying conditions. The temperature inside dried walnuts reached a plateau around ‒55 ℃ and 60 ℃ with 20 min and 55 min for treatment, respectively, whereas the temperature increased rapidly and reached the 105 ℃ and 140 ℃ with about 15 min and 20 min, respectively. Effects of different temperature on the chemical composition Moisture, total fat, crude protein, soluble sugar as well as carbohydrate are the basic components of walnuts. The effects of different temperature on these components are presented in Fig. 3. Fat was the predominant nutrient in walnut, the concentration of which was ranged from 65.8 % to 67.3 %. Obviously, the moisture content in kernel was decreased from 5.0 % to 3.7 % with the increasing temperature from ‒55 ℃ to 140 ℃ . The result showed that the high temperature was more conducive to the evaporation of moisture. There was no significant difference Fig. 2. Temperature profiles of walnut samples dried at ‒55 ℃ , ( p > 0.05) in the carbohydrate contents among the different 60 ℃ , 105 ℃ , and 140 ℃ . Temperature data were measured every treatments, with the levels ranged from 1.3 % to 1.6 %. Except 30 s during drying process. for carbohydrates, all the other analyzed chemical compositions in walnuts exhibited significant variations ( p < 0.05) among the Quality Change of Walnut during Drying 171

Fig. 3. Chemical composition of walnut (Juglans regia L.) in different temperature. (a)-(d) Means within each column with different letters are significantly ( p < 0.05) different. Data are presented as mean ±standard deviation (n = 3). four treatments. When the temperature was increased from 110 ℃ , and reduced to 72.6 % as the temperature increased to ‒55 ℃ to 105 ℃ , the values of protein, soluble sugar, and 130 ℃ (Aljuhaimi et al., 2017b). In addition, the contents of carbohydrate were enhanced by 1.6 %, 0.7 %, and 0.1 %, protein in Canarium indicum (Bai et al., 2017) and carbohydrate respectively. However, as the temperature was increased to in walnut (Arinola and Adesina, 2014) also decreased when the 140 ℃ , a descending performance occurred. Therefore, the heating temperature above 110 ℃ . contents of protein, soluble sugar and carbohydrate in walnuts Effects of different temperature on the fatty acids exhibited the highest values at 105 ℃ , with 19.6 %, 3.0 % and composition Fatty acids are both important dietary sources of 1.6 %, respectively. During the heating process, nut cells and energy for human and important structural components for tissue structures were physically damaged, which led to more body cells. The fatty acids profiles of walnuts are shown in chemical components were released; some enzymes (esterase) Table 2. The unsaturated fatty acids accounted for more than maintain active at the high temperature, which might further 90 % of total fatty acids. The polyunsaturated fatty acid help the release of chemical contents (Bai et al., 2017). However, (PUFA) represented more than 72 % of total fatty acids. when the temperature was up to 140 ℃ , the above-mentioned Linoleic acid (n-6) and α-Linolenic acid (n-3) could be detected effects induced by heating might be weakened or stopped. In as the main PUFA, with the concentrations ranged from 245.4 addition, the high temperature might destroy the construction of ± 4.40 mg/g to 309.0 ± 41.61 mg/g and 34.3 ± 0.06 mg/g to nuts and reduce enzymes active thus result in denaturation of 43.1 ± 5.82 mg/g, respectively. The monounsaturated fatty chemical compounds during drying (Arinola and Adesina, 2014). acids (MUFA) were ranged from 16.8 % to 17.3 %. The oleic According to the results from this study, heating with an acid was the most abundant MUFA, constituting more than appropriate temperature could increase the chemical composition 98 % of MUFA. Saturated fatty acid (SFA) was less than 10 % of walnut, whereas those contents could decrease with the of total fatty acids in all samples. keeping elevated temperature. The similar performance was also The present work revealed that the different drying observed for the fat of pecan walnut, which was increased from conditions have not significantly effect on each individual fatty 69.1 % to 73.6 % with the heating temperature from 70 ℃ to acids fatty acid. Interestingly, the highest contents were 172 Y. Han et al.

Table 2. Fatty acid composition of walnut (Juglans regia L.) in different temperature

Temperature/℃ ‒55 60 105 140 Group 1 Palmitic acid 25.27±0.46a 25.44±0.05a 30.98±4.24a 26.72±3.96a (mg/g) Stearic acid 12.28±0.22a 11.73±0.02a 14.49±1.95a 12.99±1.89a Arachidic acid 0.36±0.05a 0.34±0.04a 0.3±0.01a 0.34±0.01a Oleic acid 65.85±1.21a 64.76±0.12a 79.56±10.71a 68.93±10.09a Palmitoleic acid 0.32±0.07a 0.37±0.03a 0.31±0.08a 0.35±0.09a Gadoleic acid 0.82±0.11a 0.73±0.09a 0.65±0.02a 0.67±0.02a α-Linolenic acid (n-3) 36.93±0.66a 34.26±0.06a 43.09±5.82a 38.7±5.52a Linoleic acid (n-6) 245.44±4.40a 251.36±0.47a 309.04±41.61a 267.35±38.76a

Group 2 Palmitic acid 6.52±0.00a 6.54±0.00a 6.47±0.02b 6.42±0.02c % Stearic acid 3.17±0.00a 3.01±0.00d 3.03±0.00c 3.12±0.00b Arachidic acid 0.09±0.01a 0.09±0.01a 0.06±0.01b 0.08±0.01ab Oleic acid 17.00±0.00a 16.65±0.00b 16.63±0.00b 16.56±0.02c Palmitoleic acid 0.08±0.02a 0.1±0.01a 0.07±0.01a 0.08±0.02a Gadoleic acid 0.21±0.03a 0.19±0.02ab 0.14±0.02b 0.16±0.02ab α-Linolenic acid (n-3) 9.54±0.01a 8.81±0.00d 9.01±0.00c 9.3±0.02b Linoleic acid (n-6) 63.38±0.04c 64.62±0.02a 64.59±0.01a 64.26±0.01b n-6/n-3 6.65±0.01d 7.34±0.00a 7.17±0.01b 6.91±0.02c SAF 9.79±0.01a 9.64±0.01b 9.57±0.01c 9.63±0.01b MUFA 17.30±0.03a 16.93±0.01b 16.83±0.02c 16.81±0.02c PUFA 72.91±0.05c 73.43±0.02b 73.60±0.01a 73.56±0.02a

SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids. (a)-(d) Means within each column with different letters are significantly ( p < 0.05) different. Value are expressed as mean ± standard deviation (n = 3). Group 1: Fatty acid composition content (mg/g) of walnut. Group 2: Fatty acid composition (%) of walnut exhibited at 105 ℃ (exclude gadoleic acid). The performance optimal ratio of n-6/n-3 was 6.65 (from the freeze drying was likely due to the hydrolytic enzymes in damaged cells, treatment), which indicated that the drying process (heating) which could lead to increased fatty acids after dry heating (Bai could have a negative effect on the ratio of n-6/n-3. et al., 2017). Effects of different temperature on flavonoids and Besides the contents of fatty acids, the proportion of fatty polyphenols Flavonoids and polyphenols are important acids composition is also an important indicator to characterize antioxidant substances, which show much stronger antioxidant the nutritional qualities of walnuts. There were significant activities than other nutrient compounds (Tas and Gokmen, differences ( p < 0.05) among the different drying temperature 2017). The total flavonoids contents (TFC) and total in fatty acids compositions (except palmitoleic acid). The polyphenols contents (TPC) of walnut methanol extracts highest percentage of stearic acid, arachidic acid, oleic acid, obtained with different treatments are shown in Fig. 4. The gadoleic acid, and α-Linolenic acid could be detected at ‒55 ℃ . contents of flavonoids were notably raised from 8.4 mg/g to Walnut exhibited the highest percentage of palmitic acid, 11.5 mg/g, along with the temperature from ‒55 ℃ to 140 ℃ . palmitoleic acid, and linoleic acid at 60 ℃ . Compared with After drying at 60 ℃ , the TFC was increased by 12.7 % samples dried at lower temperature (‒55 ℃ or 60 ℃ ), the compared with the values from ‒55 ℃ . However, with the sample exhibited lower percentage of specific fatty acids dried temperature from 60 ℃ to 105 ℃ , the TFC of walnut increased at higher temperature (105 ℃ or 140 ℃ ). Our results is in line by 6.6 %. Interestingly, the highest growth rate for TFC came with one reported literature (Aljuhaimi et al., 2017b), in which from the temperature interval of 105 ℃ to 140 ℃ . Some studies the portions of fatty acids reduced with the heating temperature indicated that TFC were notably increased by 33.5 % in cashew improved from 70 ℃ to 130 ℃ . In addition, the n-6/n-3 ratio of and 131.4 % in almond when the temperature was increased walnut has great significance for human beings, and the from 110 ℃ to 175 ℃ (Ghazzawi and Al-Ismail, 2017). The recommended dietary n-6/n-3 intake ratio for human beings increased TFC in nuts could be contributing to the increased ranges from 1:1 to 5:1 (Simopoulos 2008). From our work, the anthocyanin content during heating, as the anthocyanin was Quality Change of Walnut during Drying 173

Fig. 4. Total flavonoids contents (TFC) and total polyphenols contents (TPC) in walnut Juglans( regia L.) in different temperature. (a)-(d) Means within each column with different letters are significantly ( p < 0.05) different. Value are expressed as mean ± standard deviation (n = 3) heat-resistant (Aljuhaimi et al., 2017a). mineral elements were ranged from 7.3 % to 23.3 %. However, The variation of TPC was in agreement with TFC. The when the sample was submitted to the heating process at TPC were ranged from 16.8 mg/g at ‒55 ℃ to 22.5 mg/g at 140 ℃ , all the mineral elements were decreased, with the 140 ℃ . At the different temperature intervals, the TPC was decrease percentages of 4.7 %‒9.8 %. This similar performance increased by 4.5 % (‒55 ℃ to 60 ℃ ), 17.0 % (60 ℃ to 105 ℃ ), was reported in roasted walnuts, in which the decreased and 14.0 % (105 ℃ to 140 ℃ ), respectively. The increase of percentages of Fe and Mg were 169.2 % and 9.8 % with the TPC with the temperature was also reported from hazelnut roasting temperature, improved from 180 ℃ to 220 ℃ (Djikeng (Marzocchi et al., 2017), Sacha inchi (Sterbova et al., 2017), et al., 2018). Previous works showed that some antinutritional and cashew (Chandrasekara and Shahidi, 2011). The substances were present in nuts and could form tight complexes performance could be due to the Maillard reaction, in which with the mineral elements, which could be significantly the formation of Maillard derivatives (such as pyrroles and destroyed by heat (Djikeng et al., 2018; Makinde and Akinoso, furans) reacts with Folin-Ciocalteu reagent (Ghazzawi and Al- 2013). We conclude that the heating process resulted in the Ismail, 2017). Additionally, it is supposed that the increase of dissociation of the complexes and the improvement of free TPC by the heating process is probably an easier extraction of mineral elements. The dissociation effect might reach a top at polyphenols substances linked to the matrix, due to a greater 105 ℃ , or the correspondingly increased temperature (140 ℃ ) matrix destructuration when the heating temperature is even had a negative effect on the dissociation process. increased (Marzocchi et al., 2017). However, with the treatment temperature form 120 ℃ to Effects of different temperature on the mineral composition 180 ℃ , the mineral contents in sesame increased percentages Mineral contents of kernels with freezing and heating treatment from 12.3 % (Cu) to 91.7 % (Mg) (Makinde and Akinoso, are given in Fig. 5. Walnuts are rich in K, Ca, P, and Mg, in 2013). The results in sesame are significantly different from which the concentrations were 3719.4‒4372.0 mg/kg, ours, which probably caused by the different nut and treatment 1717.6‒1890.9 mg/kg, 3936.8‒4511.4 mg/kg, and temperature. 1801.2‒2100.6 mg/kg, respectively. Walnuts also contain other Effects of different temperature on the nutritional qualities elements such as Fe, Mn, Zn, Cu, and Ba (data of Ba not The comparisons of nutritional qualities of walnuts dried by shown), with the detected concentrations were 25.7‒31.7 mg/ heating and freezing treatments are presented in Fig. 6 (a, b, c). kg, 45.2‒54.6 mg/kg, 31.6‒34.8 mg/kg, 17.1‒20.3 mg/kg, and Notably, the heating treatment had an irritation effect for most 1.5‒1.6 mg/kg, respectively. Interestingly, the highest Mn of nutritional qualities. Dried walnut by heating treatment content was recorded at 60 ℃ , whereas the lowest content was showed higher values for the chemical compositions, minerals, found at 105 ℃ . This result was different with other elements. antioxidant substances (polyphenols and flavonoids), and part Except for Mn, the variations of the analyzed mineral elements of fatty acids than dried samples by freezing treatment. showed a similar performance under the four drying process. Compared with freeze drying treatment, the improved When the temperature was raised from ‒55 ℃ to 105 ℃ , the percentages of nutritional qualities were from 1.0 % to 34.0 % contents of mineral elements were improved and reached to a during heat drying treatment. Among these changed qualities, top at 105 ℃ . The increase percentages for the analyzed some chain reactions occurred. During heating treatment, 174 Y. Han et al.

Fig. 5. Minerals composition of walnut (Juglans regia L.) in different temperature (µg/kg). (a)-(d) Means within each column with different letters are significantly ( p < 0.05) different. Value are expressed as mean ± standard deviation (n = 3)

Maillard reaction could occur between soluble sugar and indicated that heating treatment had an irritation effect for most protein in nuts, thus resulting in the formation of variety of of nutritional qualities, with the highest irritation effect of byproducts, which might increase the TPC and TFC polyphenols. Although a high temperature could improve the (Chandrasekara and Shahidi, 2011). On the other hand, the drying efficiency, some nutritional qualities could be destroyed TPC in sesame increased significantly as the heating or decreased by the high temperature, especially when the temperature increased due to the liberation of polyphenols temperature up to 140 ℃ . However, freezing treatment had an compounds (Jeong et al., 2004). On the contrary, heat drying inhibition effect for most of nutritional qualities of walnuts. treatment had an inhibition effect for some fatty acids such as Although samples by freeze drying were found to be beneficial gadoleic acid and arachidic acid, in which the inhibition effect for PUFA and ratio of n-6/n-3, the chemical compositions, for these mentioned qualities were not more than 21 %. minerals, antioxidant substances, and some of the fatty acids The comparisons of nutritional qualities of walnuts dried exhibited a notable decrease performance, with the highest by 105 ℃ and other temperatures are shown in Fig. 6 (d, e, f). inhibition effect of gadoleic acid. Therefore, taking into Obviously, dried walnut by other temperatures (‒55 ℃ , 60 ℃ , account the variations of the comprehensive nutritional 140 ℃ ) had a notable inhibition effect for most of nutritional qualities, the heat drying of 105 ℃ was recommended as an qualities. The inhibition effects for these qualities were ranged optimal process for walnut drying, in which the protein, soluble from 0.3 % to 25.1 %. However, 105 ℃ is an optimal drying sugar, carbohydrate, palmitic acid, stearic acid, oleic acid, temperature to improve these qualities. In addition, dried at α-Linolenic acid, and minerals showed the highest values. 105 ℃ had an irritation effect for the gadoleic acid, arachidic acid, palmitoleic acid, SFA, and Mn. The irritation effects on Acknowledgments This study was supported by the Opening these nutritional qualities were ranged from 0.6 % to 20.7 %. Project Fund of Key Laboratory of Rubber Biology and Genetic Resource Utilization, Ministry of Agriculture (RRI- Conclusion KLOF201703); the Fundamental Research Funds of CAF In this work, the effects of heat and freeze drying on (CAFYBB2017QC002). nutritional qualities of walnuts were investigated. The results Quality Change of Walnut during Drying 175

Fig. 6. Effects of temperature treatments on the health-related properties

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