GONG A-na , SHl Ai-min, LlU Hong-zhi YU Hong-wei LlU Li LlN Wei-jing WANG Qiang

1 Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-products Processing, Ministry of Agriculture, Beijing 100193, P.R.China

2 Department of Food Science, Shenyang Agricultural University, Shenyang 110161, P.R.China

1. lntroduction

Peanut (Arachis hypogaeaL.) is an important crop in many parts of the world. China is the world’s largest peanut producer, which has more than 8 000 varieties (Wanget al.2017). In 2017, China produced about 17.40 million tons of peanuts (USDA 2017). A large proportion of peanut production is used for domestic foods, because peanuts contain high levels of oil and protein for producing peanut oil and peanut butter. The dry seed typically contains 40–50%oil by weight, with oil content from conventional peanuts(NP) approximately 41–67% oleic acid (18:1), whereas that of high oleic peanuts (HOP) can be up to 80% oleic acid(Deanet al. 2011). Wilkinet al. (2014) reported that the high monounsaturated and low polyunsaturated fatty acid content can increase the storage time of these cultivars when compared with that of other varieties.

Peanut butter is one of the popular peanut products,which is used for direct consumption or as an ingredient in the preparation of other foods. The unique flavor and nutritional value make peanut butter one of the most favorite American snacks, where peanut butter yield accounts for 40% of the total peanut output. However, preservation and storage stability of these products are an ongoing concern in the peanut industry (Bolton and Sanders 2002).Isleibet al. (2006) and Patteeet al. (2002) found that lipid oxidation is the primary cause of decreased shelf life and the development of off- flavors and aromas in roasted peanuts. Moreover, the factors affecting the lipid oxidation include vitamin E, packaging, hypoxic environment, storage temperature, and time. Previous studies demonstrated that the HOP varieties exhibited the highest oxidative stability compared with conventional peanut, because it contains a higher amount of oleic acid (Jonnalaet al. 2006; Craftet al. 2010). Manufacturers have selected HOP cultivars to prepare peanut butter owing to their low degree of lipid oxidation during storage, which significantly improves the preservation of sensory and chemical quality parameters(Riveroset al. 2010). This effect was first detected in friedsalted peanuts (Valeriaet al. 2006). A study on peanut butter prepared using seven peanut cultivars commonly grown in the major producing states of India showed that peanut variety Somnath is best suited for producing peanut butter (Dhamsaniyaet al. 2012). Other studies have reported differences in the physical and chemical properties of peanuts and of peanut butter from the ?OM and NC-7 cultivars, although little is known of the decreases in antioxidant activity during preliminary storage (?zcanet al. 2003). There is a demonstrated significant correlation between the storage stability of peanut butter and peanut varieties. However, the storage stability of peanut butter prepared with different peanut varieties, particularly those produced with conventional and high oleic acid peanut varieties in China has not been studied. In particular, no study has examined the most suitable Chinese peanut varieties for the best quality and storage of peanut butter.

The objective of this study was to compare the oxidative stability among different varieties of peanut butter. A total of 17 peanut samples, which included 16 conventional and 1 high oleic peanut cultivars were selected for study.First, the quality characteristics of 17 peanut varieties were analyzed; second, across different temperatures and storage durations, the increases in peroxide value (PV), acid value (AV), and centrifugal rate (CR, %) were measured;third, the effects of different peanut varieties on peanut butter quality, as well as the relationship to the storage stability of peanut butter were analyzed.

2. Materials and methods

2.1. Materials

For sample preparation, 17 varieties of peanuts commonly grown in major peanut producing provinces of China were selected. The pods of these selected peanut varieties were provided from the Chinese Academy of Agricultural Sciences. Samples were collected from 6 provinces and autonomous regions in China in 2013: Shandong, Liaoning,Henan, Hebei, Xinjiang, and Guangxi (Table 1). The pods were decorticated manually to obtain medium grade size sound kernels.

2.2. Proximate determination

The physical properties and chemical composition of different varieties of peanut kernels were analyzed according to the AOAC Official Method with minor modifications(Wilkinet al. 2014). The fat content was determined by using Soxhlet extraction. Approximately 1 g of peanuts was ground, placed in a thimble, and refluxed for 2 h using petroleum ether (60–90°C) as the solvent. After 2 h, the solvent was recovered and the residue was weighed. The protein content was determined by the Kjeldahl method using 5.46 as the conversion factor.

The total sugar content was determined using the vitriolic acid-phenol method (Duboilset al. 1956). Briefly, about 15 mg of ground peanut was placed in an anaerobic jar with 15 mL of deionized water, and was then hydrolyzed using vitriol at the room temperature for 3 h and heated in a vacuum oven at 105°C for 4 h. Finally, sugar content was determined using a UV-3010 spectrophotometer (Hitachi,Tokyo, Japan).

Table 1 Peanut varieties included in a study of peanut butter quality and shelf-stability

2.3. Fatty acid determination

Peanut oil samples were methylated according to standard methods with minor modi fications (Daviset al. 2008; Gajeraet al. 2010). Briefly, 50 mg of oil was hydrolyzed using a 5% sulfuric-methanol solution, and heated in a water bath for 1 h at 90°C, and resulting fatty acids were converted to their methyl esters usingn-hexane (chromatography pure)as a catalyst. The fatty acid methyl esters (FAMEs) were extracted and analyzed by capillary gas chromatography(SHIMADZU Company, Japan) equipped with a flame ionization detector and a capillary column (sp-2560). The carrier gas used was hydrogen and air, and the total gas flow rate was 40 and 400 mL min–1, respectively. The injector and detector temperatures were kept at 260 and 285°C,respectively. The column was initially held at 130°C for 3 min and programmed to increase to 240°C at the rate of 4°C min–1, finally holding at 240°C for 20 min.

The FAMEs were identified according to retention times using reference standards, and their relative contents were calculated as percent distribution of areas under each peak of fatty acids. The fatty acids quantified as percent distribution were myristic (C14:0), palmitic (C16:0), stearic(C18:0), oleic (C18:1), linoleic (C18:2), linolenic (C18:3),arachidic (C20:0) and behenic (C22:0) acids.

2.4. Peanut butter preparation

The peanut butter was prepared by modifying previously described methods (Gajeraet al. 2010; Dhamsaniyaet al.2012). Approximately 150 g of sound peanut kernel for each sample was spread over a Petri dish with an area of 130 cm2, and roasted at 150°C for 30 min using a laboratory digital electrical oven (sensitivity 1°C). The kernels were cooled with forced air and then the skin was removed. The blanched seeds were ground in a domestic grinder, and the additives salt and sugar were added at the rate of 1 and 5%, respectively. The mixture was again ground to create the peanut butter samples.

2.5. Storage conditions and sampling

After preparation, samples were packed in 500 g plastic jars and stored at 23°C (conditioner room) and 37°C(constant temperature and humidity incubator) for the room temperature and accelerated storage condition treatments,respectively. All 17 samples were chemically analyzed after 0, 10, 20, and 30 days storage.

Peroxide value (PV)PV was determined using the AOAC Official Method Cd 8-53 with minor modifications.Briefly, (3.00±0.2) g of peanut oil was weighed and 30 mL chloroform/acetic acid (2:3, v/v) was added. A total of 1.0 mL saturated potassium iodide was added as an indicator and shaken for 30 s with trochanter before 100 mL of deionized water was added. This was titrated against 0.002 mol L–1sodium thiosulphate using a potentiometric titrimeter, while being vigorously shaken until the endpoint was reached.

Acid value (AV)A total of 3.0–5.0 g peanut oil was weighed in a glass beaker, and 50 mL of dimethyl carbinol and phenolphthalein indicator was added. This solution was titrated with 0.5 mol L–1KOH until the color reached pompadour.

Oil separationThe oil separation was analyzed using the centrifugal rate method according to Totlaniet al. (2002)with minor modifications. Briefly, the peanut butter samples were prepared and maintained at room temperature for 24 h.A total of 10 mL peanut butter was centrifuged for 25 min at 4 500 r min–1at 20°C, and then the condensate volume was read. Centrifugal rate (CR, %)=Condensate volume (mL)/Total volume centrifugal peanut butter (mL)×100

2.6. Statistical analysis

Each experiment was performed in triplicate, and results were averaged. Experimental data were analyzed according to variance using SAS Genstat. The significance analysis was measured by analysis of variance (ANOVA) and the statistical differences between the treatment and control were determined by independent-samplest-test, and the significance was determined at levelP＜0.05 and was considered highly significant atP＜0.01.

3. Results and discussion

3.1. Constituent analyses of peanut

Table 2 shows the protein, fat, moisture, total sugar, ash,and fiber contents of the 17 studied varieties of peanut. The total fat and protein content are important from nutritional point of view (Wanget al. 2014; Wang 2018). The fat content of selected peanut varieties varied between 42.64 and 57.47%. The lowest fat content was found in Jihuatian 1 and highest in Guihua 17. Protein content varied from 19.19 to 26.58%. The content of total sugar varied between 4.29 and 23.04%, with the average content of total sugar at 12.19%,result shown the significant difference in the total sugar content in different varieties peanut based on coefficient of variation. The Jihuatian 1 variety has the highest sugar content, at about 23.04%, which is five times higher than the 13-2 variety with the lowest sugar content (4.29%). There are significant differences in the compositions of different varieties of peanuts, and between the different constituents of peanuts according to ANOVA (P＜0.05). Other studies have also reported a significant difference among the basic qualities of different peanut varieties (Dhamsaniyaet al.2012; Wanget al. 2014).

3.2. Fatty acid compositions of peanut cultivars

Fatty acid compositions of the oil from 17 peanut cultivars are presented in Table 3. The fatty acids in peanuts are composed of myristic (C14:0), palmitic (C16:0), stearic(C18:0), oleic (C18:1), linoleic (C18:2), linolenic (C18:3),arachidic (C20:0), and behenic (C22:0), with average content of 0.007, 5.00, 1.79, 1.06, 1.22, 21.48, 13.88, and 0.03 g 100 g–1, respectively. Lópezet al. (2001) reported that different varieties of peanut have significant differences in these fatty acids in peanuts. Oleic acid and linoleic acid are the main components of unsaturated fatty acids in peanut oil(Riveroset al. 2010). Wahrburget al. (2004) reported that oleic acid can reduce cholesterol, regulate blood fat, lower blood sugar, and participate in other important physiological functions. Because of these nutritional roles, oleic acid is regarded as the most healthy dietary fatty acid, and is an indicator of product quality. In the present study, we found that the proportions of oleic acid and linoleic acid were similar, showing a roughly 1:1 ratio, except in the Kainong 17-15 peanut varieties with contents of 33.10 and 1.67%,respectively. Thus, the oleic/linoleic ratio (O/L) for Kainong 17-15 was 19.82:1. In addition to nutritional properties,Daviset al. (2016) and Janick (2014) evaluate peanuts and peanut products by the stability index (SI), which is O/L ratio.In addition, other researchers reported that the high stability index implies a good compositional quality of the product and can ensure better shelf life (Dhamsaniyaet al. 2012).Similar results for these fatty acid ratios were also found in other peanut products prepared with high oleic and normal peanut lines from Argentina and other countries (Nepoteet al. 2009), with more favorable ratios indicating longer shelf life (?zcanet al. 2003; Isleibet al. 2006).

3.3. Storage stability analysis of peanut butter

Previous studies found that the PV and CR of peanut butter is altered with an increase in storage time and variability in temperature. The lower the PV and CR is, the lower the oxidation of oil is (Totlaniet al. 2002; Riveroset al. 2010).During the current study, the storage stability of peanut butter varieties was evaluated by lipid oxidation changes which occurred during storage at different temperatures and storage durations. The changes in PV, AV and CR (%)after the 0, 10, 20, and 30 days at 23 and 37°C are shown in Table 4. To clarify the effect of temperature and storage time on the stability of peanut butter, the degree of oil separation was measured to determine the extent of lipid oxidation.

The PV, AV, and CR of the peanut butter samples increased with increasing of temperature and storage time.Samples stored at high temperature have a high degree of oxidation, compared with normal temperature storage.For example, at 23°C, the average PV, AV, and CR of the 17 samples were 0.165 g 100 g–1, 0.429 mg 100 g–1and 19.81%, respectively. However, at 37°C, the average content of three factors of 17 samples were 0.192 g 100 g–1, 0.542 mg 100 g–1, and 23.69%, respectively (Table 4).Abegazet al. (2004) reported that storage time was a significant factor influencing PV in peanut butter confections stored at 21°C. They also reported that PV increased markedly within 4 weeks.

PV is a measure for the initial products of lipid oxidation.Differences in PV across storage temperature and time treatments were significant (P＜0.05; Fig. 1). PV is significantly increased during storage at 23 and 37°C in three different varieties of peanuts (Fig. 1). This trend did not hold true for the HOP peanut butter samples, which hadlower PV. Riveroset al. (2010) revealed that extension of shelf life was the primary rationale for deployment of the high oleic trait in peanut varieties; it has also been observed that peanut butter prepared with HOP peanuts has a longer shelf life than peanut butter prepared with normal peanuts. Okturket al. (2001) and Abegazet al. (2004) reported that storage time was a significant factor that influenced PV in model peanut butter stored at atmospheric temperature. However,in each treatment, the AV value did not show any significant increase during the storage period (Fig. 2). In contrast, HOP samples stored at 23 and 37°C had significantly lower AV than the other treatments.

Table 2 Composition of the nutrition of 17 peanut varieties (%)

Table 3 Composition and content of fatty acids of peanut varieties

Table 4 Storage stability analysis of peanut butter

The CR is the primary method to evaluate the degree of oil separation, with higher rates indicating greater oil separation. Oil separation rates of varieties of peanut butter were significantly different (Fig. 3), with oil separation of HOP significantly different than that of Jihuatian 1.According to Totlaniet al. (2002), CR would be able to predict the shelf life of the product in the shortest time interval. Gillset al. (2000) reported that USDA product regulations for peanut butter specify a maximum of 0.5 mL free oil of freshly manufactured product after 24 h storage at 30°C. There were no differences found in peanut butter stored at 30 and 45°C.

3.4. Correlation analyses of peanut quality

Fig. 1 Changes in peroxide value (PV) in three peanut butter during different storage temperatures and days. HOP, high oleic peanuts.

Table 5 shows the fiber, ash, carbohydrate, fat, moisture,protein, and oleic and linoleic acid contents of the 17 peanut varieties. The protein content was negatively correlated with fat content (r=–0.623＊＊,P＜0.01). The negative correlation between the oleic acid and linoleic (r=–0.817＊＊,P＜0.01) was in accordance with the results reported by Singkhamet al.(2010) and Shinet al. (2010).

Fig. 2 Changes in acid value (AV) in three peanut butter during different storage temperatures and days. HOP, high oleic peanuts.

3.5. Correlation analyses of peanut quality and peanut butter storage stability

Across time (0, 10, 20, and 30 days) and temperature (23 or 37°C) treatments, the variables (PV, AV, and CR) that changed during storage were correlated using Pearson coefficients (Tables 6 and 7). The PV of peanut butter has a significant negative correlation with oleic acid, a significant positive correlation with linoleic acid and a significant negative correlation with the O/L value (P＜0.05), while the rest of the stability index and each quality component had no significant correlations. During storage for 0, 10, 20,and 30 days at 23 and 37°C, negative correlations between PV and oleic acid content of peanuts were found. Positive correlations between PV of peanut butter and the content of linoleic acid of peanuts in each treatment were found.The oxidative stability and shelf life of oil is in fluenced by the concentration of oleic acid and linoleic acids (Sanders 1980a, b; Lópezet al. 2001). Savage and Keenan (1994)and Lópezet al. (2001) showed that high oleic acid peanut and its products had high storage stability, when measured with PV as the standard for liquid oxidation. Similar results were also found in other peanut products (Chunet al. 2005)prepared with high oleic peanut and normal peanut lines from Argentina.

Fig. 3 Changes in CR (%) in three peanut butter during different storage temperatures and days. HOP, high oleic peanuts.

4. Conclusion

Total of 17 varieties of peanuts from different regions were used for peanut butter preparation, and to study thedifference in lipid oxidation stability across time intervals at different temperatures. Fat content, protein content,and sugar content varied significantly among different peanut varieties. In particular, the oleic and linoleic acid ratio for most of the varieties was 1:1, except for that of HOP (Kainong 17-15) varieties, where it was 19.82:1. The markers used to study the oxidative stability of different peanut varieties were PV, AV, and CR. The HOP variety,with its high oleic acid content, showed the highest stability against lipid oxidation. The PV value for all varieties (except HOP) increased significantly with increasing time and temperature. Likewise the AV increased in all varieties with time, but HOP had a relatively lower AV value. Furthermore,the oleic acid content is negatively correlated with the PV value, and linoleic acid is positively correlated with PV in peanut butter products. This study found that the HOP is the most suitable variety for making peanut butter, which can allow farmers and processors to choose the specific variety for better product and shelf life.

Table 5 Correlation analyses of peanut quality1)

Table 6 Correlation analyses of peanut quality and peanut butter storage stability at 23°C

Table 7 Correlation analyses of peanut quality and peanut butter storage stability at 37°C

Acknowledgements

We acknowledge the financial support from the National Key Research and Development Program of China(2016YFD0400200), the Agricultural Science and Technology Innovation Program of the Chinese Academy of Agricultural Sciences (CAAS-ASTIP-201X-IAPPST) and the Central Public-interest Scientific Institution Basal Research Fund,China (Y2017CG10). The authors also wish to thank the Key Laboratory of Agro-products Processing, the Ministry of Agriculture of China for allowing us to use its laboratory facility and providing us with some technical assistance.

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