The sample used for the study was used to oil extracted from a sancho (Zanthoxylum schinifolium S. et Z.) tree farm located in Hadong from Korea. Sancho seed-pods were collected on 5^th October, 2018 and then the oil was immediately extracted from them.

The extraction of Sancho oil was performed by the compression method. After picking the Sancho seeds, they were extracted with a expella pressing machine (Ubo Sancho, Milyang, Korea). The oil was cooled at room temperature and centrifuged at 2,500 rpm for 5 minutes using a centrifuge. Milked sancho oil was stored at 5℃ until use. The acid value of the sancho oil extracted directly from the compression-type expeller extraction machine was between 1.0 and 2.0.

The acid values of sancho oil and other vegetable oils were compared. The acid value was investigated for commercially available edible oils such as olive oil, grape seed oil, canola oil and sesame oil.

Acid value of sancho oil and blending oil was measured according to American Oil Chemists` Society (AOCS, 1990). 1.0 g of the sample was taken in an Erlenmeyer flask and mixed with 1 : 2 (v/v) ethanol (Sigma-Aldrich Co., St. Louis, MO, USA) 20 ㎖ was added to dissolve the sancho oil. The Phenolphthalein solution (Sigma-Aldrich Co., St. Louis, MO, USA) was used as an indicator and titrated with 0.1 N potassium hydroxide (KOH) (Sigma-Aldrich Co., St. Louis, MO, USA) until the ethanol solution became pale red. The acid value was determined by the following equation, and the experiment was repeated three times. The acidity was measured in an incubator (SH-75B, Seyoung Scientific Co., Bucheon, Korea) adjusted to 25℃.

Acid value = 5.611 × A × F / sample weight (g)

A: Volume of 0.1 N KOH (㎖), F: Tilter of 0.1 N KOH (㎖)

50 ㎖ sancho oil was put in a 100 ㎖ plastic container, put at -17℃, 4℃, and 25℃, respectively and stored in the dark for 6 months. The stored sancho oil was taken out of each incubator and the acid value was measured by the method mentioned above.

50 ㎖ of sancho oil was taken and placed in a erlenmeyer flask and left at 25℃. for 7 days, and the turbidity and acid value were measured.

To measure turbidity, take 0.5 ㎖ of a oil in a 1000 ㎖ erlenmeyer flask, dilute it in a ratio of 1 : 1000 with 0.1% sodium dodecyl sulfate (SDS) solution, and stir it with a homogenizer (Ultra-Turrax T25, IKA Laboretechnik Co., Staufen, Germany) for 3 minutes. After leaving for 2 hours, absorbance was measured at 500 ㎚ with a spectrophotometer (PR-101a, Atago Co., Ltd., Tokyo, Japan) and expressed as turbidity.

The acid value measurement according to sedimentation was performed by the above method.

20 ㎖ of sancho oil was placed in a 50 ㎖ conical tube, and sodium bicarbonate (NaHCO₃) was added at concentrations of 0.5, 1.0, and 2%. The oil was placed in a dark place at 25 ± 2 ℃, and the acid value was measured every 5 days.

All experiments were performed 3 times. Data are presented as means and standard deviation and analyzed by one-way ANOVA using the IBM SPSS statistical package (Ver. 24, IBM Co., Armonk, NY, USA). Means were compared at 5% significance level using Duncan’'s Multiple Range Test (DMRT) comparison (p < 0.05).

The acid values of four commercially available cooking oils and sancho (Zanthoxylum schinifolium S. et Z.) oil were investigated (Fig. 1). The acid values of olive oil and grape seed oil were 0.44 and 0.42, respectively, 0.06 for canola oil and 1.81 for sesame oil, and 2.53 for sancho oil. Unlike other edible oil, sesame oil and sancho oil sold without degumming exhibited relatively high acid values.

Fig. 1.  Acid values of sancho oil and commercial vegetable oils.

1.0 g of sancho oil and vegetable oil were taken and placed in an Erlenmeyer flask and the acid value was measured by the method of AOCS (1990). Data represent the means ± S.E of three replicates. *Different letter for each treatment shows significant difference using Duncan’s Multiple Range Test (DMRT) comparison (p < 0.05).

Sancho oil stored for 6 months had different acid values depending on the temperature (Fig. 2).

Fig. 2.  Changes in acid value of sancho oil by storage temperature.

50 ㎖ of sancho oil extracted by expella method were taken, placed in an erlenmeyer flask, placed at -17℃, 4℃, and 25℃ for 6 months, and the acid value was measured by the method of AOCS (1990). Data represent the means ± S.E of three replicates. *Different letter for each treatment shows significant difference using Duncan’s Multiple Range Test (DMRT) comparison (p < 0.05).

The acid value of sancho oil was lowest at -17℃ among the three storage temperatures, and the acid value increased as the temperature increased. Sancho oil stored at room temperature has a very high acid value. According to the Food Code (KFDA 2007), when fresh soybean oil and canola oil are generally released, the acid value is 0.6 or less, shortening is 0.8 or less, extra virgin (oil squeezed from fruit) and pure olive oil is 2 or less, and the standard for fried fat is 3. Therefore, it seemed that sancho oil could not be used if stored without treatment such as antioxidants and degumming.

The acid value of sancho oil tended to increase as the number of days left to stand at room temperature to settle the sediment increased (Fig. 3). The acid value of sancho oil gradually increased until the 4th day and then increased rapidly after that. On the other hand, turbidity decreased sharply after starting sedimentation, and decreased relatively slowly thereafter.

Fig. 3.  Changes in acid value and turbidity according to the sedimentation period of sancho oil.

50 ㎖ of sancho oil was taken and placed in a Erlenmeyer flask and left at 25℃. for 7 days, and the turbidity and acid value were measured. *Different letter for each treatment shows significant difference using Duncan’s Multiple Range Test (DMRT) comparison (p < 0.05).

As for vegetable oils including sancho oil, the acid value changes as the sedimentation days pass. In this study, the change in acid value of sancho oil occurred much faster than that of perilla and soybean oil. The acid values of perilla and soybean oil increased after 4 days of standing and increased relatively rapidly after 8 days of culture (Lee et al., 2014). It was reported that the difference in rancidity of the vegetable oil depends on the fatty acid composition. In particular, the content of linolenic acid among unsaturated fatty acids is known to play a role of very lowering oxidation stability.

The major constituent fatty acids of sancho oil were eicosenoic acid 30.88%, linoleic acid 23.4%, oleic acid 19.94%, palmitic acid 10.52%, and n-9 monounsaturated fatty acids accounted for more than 60% (Cha et al., 2000). Kim et al. (2012) reported that oleic acid accounted for about 30% of the total fatty acids, followed by linoleic acid and linolenic acid, accounting for 27% and 20%, respectively.

In this study, it was found that rancidity and turbidity increased as the sedimentation period passed. This implies that effective anti-runtime methods and purification techniques are needed.

These impurities can act as a factor of rancidity during the storage of pepper oil. In general, purification of oils and fats goes through a degumming and neutralization process (Kang et al., 2017). As a prior study, our research team has de-gathered and refined sancho oil. The degumming process is a method of removing phospholipids (lecithin) dissolved in fats and oils, and the deoxidation process is a process of removing coexisting phospholipids (lecithin) by neutralizing the free fatty acids contained in crude oil with an aqueous alkali solution (NaOH).

The acid value of the sancho oil started to increase from the 5th day after being placed at 25℃, and the acid value started to increase rapidly from the 20th day. However, no increase in acid value was observed with sodium bicarbonate treatment. There was no increase in acid value regardless of the sodium bicarbonate treatment concentration (Fig. 4). Therefore, it can be seen that sodium bicarbonate is a material that delays rancidity of sancho oil.

Fig. 4.  Changes in acid value depending on the concentration of sodium bicarbonate treatment.

Sancho oil to which sodium bicarbonate was added was placed in an incubator at 25℃ and the acid value and turbidity were measured.

Changes in acid value and turbidity were observed by mixing sodium bicarbonate and ascorbic acid (Table 1, Fig. 5.). The acid value of the ascorbic acid treatment decreased as the concentration increased compared to the untreated group. Also, when sodium bicarbonate and ascorbic acid were mixed, the acid value was low. However, the mixed treatment of sodium bicarbonate and ascorbic acid did not show a significant difference according to the concentration. It was recognized that mixing sodium bicarbonate and ascorbic acid could prevent rancidity.

Fig. 5.  Changes in turbidity of sancho oil by sodium bicarbonate treatment.

A; change in turbidity according to the standing time of sodium bicarbonate (4 g NaHCO₃ treatment per 250 ㎖ of sancho oil) from the left on the day of treatment, 2, 3, 4, 5, 7 days). B; changes in turbidity depending on the concentration of sodium bicarbonate treatment (from left 0, 0.4, 0.8, 1.2, 1.6, 2.0%, respectively).

On the other hand, there was no significant difference in turbidity of sancho oil when treated with ascorbic acid. And there was no change in turbidity even in mixed treatment with sodium bicarbonate.

Antioxidants are compounds that inhibit or interfere with the oxidation process and are widely used to delay lipid oxidation. Today, questions have been raised about the safety of synthetic antioxidants, suggesting the addition of natural antioxidants to edible fats and oils.

Antioxidants are used as primary or chain breaking and secondary or prophylactic. Primary antioxidants can react with peroxyl radicals before reacting with additional unsaturated lipid molecules to convert them into more stable products. Tocopherols fall into this category. Secondary antioxidants act by other processes such as metal ionic bonding, oxygen scavenging, decomposition of hydroperoxides into non-radical products, absorption of UV radiation, and single antioxygen deactivation (Jadhav et al., 1996). Ascorbic acid is a common secondary antioxidant. Primary or a combination of the two antioxidants, primary and secondary, can have a synergistic effect.

Ascorbic acid is a natural antioxidant. Ascorbic acid can regenerate tocopherols and therefore extend their antioxidant activity (Cillard and Cillard, 1987), which is indicated by significantly lower conjugated diene values in the ascorbic acid emulsion systems after 4 days of storage (p < 0.05).

Ascorbic acid increased the antioxidant power of vegetable oil. Kartika et al. (2005) reported that the antioxidant power was increased as a result of adding ascorbic acid to sunflower oil. Ascorbic acid appears to be the most effective antioxidant according to the Rancimat oxidative stability index.

Sodium bicarbonate showed a change in acid value according to the storage temperature (Table 2). As the storage temperature increased, the acid value slightly increased. However, the acid value was significantly lower than that of sancho oil without sodium bicarbonate. However, turbidity was not significantly changed regardless of the addition of sodium bicarbonate.

Sodium bicarbonate is an abbreviation for sodium bicarbonate and is a colorless crystal. The solubility in 100 g of water is 8.8 g (15℃), and the aqueous solution becomes weakly alkaline because it hydrolyzes, and when heated to 65℃ or higher, CO₂ is released to become sodium carbonate. It is most widely used as an antacid and baking powder. It is a white powder and has a characteristic taste. It is weakly alkaline, neutralizes stomach acid in the stomach and generates carbon dioxide gas.

The mixed treatment of sodium bicarbonate and vitamins prevented rancidity of sancho oil. vitamin C is a natural antioxidant that helps get rid of free radicals (Khassaf et al., 2003; Imik et al., 2012). Vitamin C is widely used for antioxidant, anti-aging, antioxidant stress toxicity, and is widely used by athletes to eliminate free radicals (Khassaf et al., 2003; Morrison et al., 2015; Jaiswal et al., 2017). The treatment of vitamin C in sancho oil may also be related to the tocopherol contained in sanchoi oil. Cuomo et al. (2020) reported that vitamin C added to olive oil regenerates vitamin E by vitamin C, and as a result, it was very effective that the peroxide value did not change for about 40 days after storage.

Sodium bicarbonate also maintains an acid-base and electrolyte balance (Mujahid, 2011). There are no reports of extension of the shelf life of plant oils with sodium bicarbonate. However, it has been said that sodium bicarbonate maintains the acid-base and electrolyte balance (Mujahid, 2011). Qin et al. (2017) reported that when sodium bicarbonate was treated with tartary buckwheat sprouts, the α-diphenyl-β-picrylhydrazyl (DPPH) scavenging ability increased along with the increase in bioactive substances such as flavonoids. However, the mechanism of preventing rancidity of plant oils is unknown, and further research is needed.

What is clear from our study is that it prevented rancidity of the sancho oil, thereby increasing its storage capacity. It seems that these two have created synergistic effects, such as vitamin's ability to remove excess reactive oxygen species (ROS) and sodium bicarbonate's ability to maintain acid-base and electrolyte balance.

As a result of this study, sodium bicarbonate is considered to play a very good preservative for long-term storage of sancho oil. However, in this study, the treatment period for sodium bicarbonate was only 6 months, and the change of properties due to treatment other than turbidity analysis was not investigated. Therefore, it is necessary to investigate changes in the properties of sancho oil following sodium bicarbonate treatment and long-term storage. Sodium bicarbonate is expected to be of great help in the storage and distribution of sancho oil as it has the advantage of not having any restrictions on the food code and is not harmful.