Sterols and squalene in apricot (Prunus armeniaca L.) kernel oils: the variety as a key factor

Abstract The profile of sterols and squalene content in oils recovered from the kernels of 15 apricot (Prunus armeniaca L.) varieties were investigated. Nine sterols (campesterol, β-sitosterol, Δ5-avenasterol, 24-methylene-cycloartanol, cholesterol, gramisterol, Δ7-stigmasterol, Δ7-avenasterol and citrostadienol) were identified in apricot kernel oils. The β-sitosterol was the predominant sterol in each cultivar and consisted of 76–86% of the total detected sterols. The content of total sterols and squalene were significantly affected by the variety and ranged between 215.7–973.6 and 12.6–43.9 mg/100 g of oil, respectively.

tucumanensis Lillo, can be considered as source of bio-compounds with anti-inflammatory and antioxidant properties (Bach et al. 2013).
Plant stanols and sterols are steroid compounds that belong to the cholesterol analogues group with carbon side chains with/without a double bond. The most common forms of sterols in the plant world are β-sitosterol, campesterol and stigmasterol. Plant sterols, despite the similarity to cholesterol, have reported ability for reducing the levels of cholesterol in blood serum (Chen et al. 2008).
Squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,20-tetracosahexane) is a triterpenic hydrocarbon, precursor of steroid hormones, vitamin D and cholesterol, present at high concentrations in shark liver and in some oils, e.g., virgin olive oil. Squalene reduces the levels of reactive oxygen species in vitro and protects against oxidative DNA damage in human mammary epithelial cells. Hence, virgin olive oil rich in squalene may be an important foodstuff, which has been stated to play a significant role in the prevention of human breast cancer (Warleta et al. 2010).
Agro-industrial by-products are promising alternative sources of natural bioactive compounds, however the impact of the variety/cultivar has not been investigated in sufficient detail, especially in the case of industrial by-products. The impact of cultivar on the profile of tocochromanols in fruit seeds and kernels has been reported to be significant (Górnaś, Mišina, Grāvīte, Lācis, et al. 2015;Górnaś, Mišina, Grāvīte, Soliven, et al. 2015;Górnaś, Mišina, Ruisa, et al. 2015;; however, much less is known about the impact of variety on the content of sterols and squalene. Therefore, in the present study, the concentration of sterols and squalene in 15 apricot kernels were investigated.

Results and discussion
Nine sterols (campesterol, β-sitosterol, Δ5-avenasterol, 24-methylene-cycloartanol, cholesterol, gramisterol, Δ7-stigmasterol, Δ7-avenasterol and citrostadienol) were identified in apricot kernel oils. The β-sitosterol was the predominant sterol in each variety and consisted of 76-86% of the total detected sterols. The same observation was reported in previous studies (Lazos 1991;Hassanein 1999;Turan et al. 2007;Hassanien et al. 2014). The predominance of β-sitosterol was also significant in apple seed oils recovered from different cultivars, however, varied considerably and ranged from 51 to 94% (Górnaś, Rudzińska, et al. 2014). Despite the very similar percentages of the β-sitosterol in all study samples, its content varied in different cultivars (184.6-821.5 mg/100 g oil in cvs. 'edelveis' and 'Veselka' , respectively). Nevertheless, in the most varieties the range of β-sitosterol was between 184.6 and 427.8 mg/100 g oil (Table 1). Low concentrations were recorded for campesterol, Δ5-avenasterol and cholesterol (11.2-48.7, 9.5-31.4 and 0.0-52.6 mg/100 g oil, respectively). For 24-methylene-cycloartanol, gramisterol, Δ7-stigmasterol, Δ7-avenasterol and citrostadienol were noted values below 10.2 mg/100 g oil. The total amount of sterols ranged from 215.7 to 973.6 mg/100 g oil in cvs. 'edelveis' and 'Veselka' , respectively. Recorded range of total sterols covering the range reported in apricot kernels of nine cultivars originated from Turkey where the concentration of sterols was more comparable (300.6-376.1 mg/100 g oil) (Turan et al. 2007) compared with the present study. Similar range of total sterols, as in present study, was noted in seed oils recovered from five dessert apple cultivars (182-780 mg/100 g oil) (Górnaś, Rudzińska, et al. 2014). The content of squalene in various apricot kernel oils ranged from 12.6 to 43.9 mg/100 g oil in varieties HL PSŠ 5 and Apguldes1, respectively, with the average concentration 23.0 mg/100 g oil. Squalene concentrations in apricot kernel oils are similar to those reported in dessert apple seed oils (9.0-34.0 mg/100 g oil) (Górnaś, Rudzińska, et al. 2014). To the best of our knowledge this is the first report of the squalene in apricot kernel oils.

Conclusion
Our research has shown that the content of sterols and squalene in apricot kernel oils is significantly affected by the variety, since the abiotic factors (which have a proven impact on the chemical composition of plant material) were the same for all studied samples. The oil recovered from apricot kernels can be considered as a valuable source of sterols, mainly β-sitosterol.

Disclosure statement
No potential conflict of interest was reported by the authors.