Elsevier

Ultrasonics Sonochemistry

Volume 58, November 2019, 104614
Ultrasonics Sonochemistry

Changes of amygdalin and volatile components of apricot kernels during the ultrasonically-accelerated debitterizing

https://doi.org/10.1016/j.ultsonch.2019.104614Get rights and content

Highlights

  • Ultrasonically-debitterizing did not cause the epimerization of d-amygdalin.

  • Ultrasound rapidly debitterized the apricot kernels by degrading the amygdalin.

  • Ultrasound can make the debitterizing water more safe and flavors.

  • Ultrasound can be used to accelerate the removal of bitterness from apricot kernels.

Abstract

Ultrasound has been regarded as an efficient novel technique for debitterizing of the apricot kernels, but its influence is severely concerned on the possible epimerization of d-amygdalin to the l-amygdalin, a more potentially toxigenic compound. Considering this, the experiments were conducted to investigate the epimerization of d-amygdalin and the volatile components in the debitterizing water, which were separated and identified by the high performance liquid chromatography (HPLC) and gas chromatography with a mass spectrometer (GC-MS), respectively. The results indicate that the ultrasonically-debitterizing did not cause the epimerization of d-amygdalin to the l-amygdalin, while the procedure can be greatly accelerated due to the rapid mass transfer and degradation of d-amygdalin induced by ultrasound irradiation. In addition, the water from the ultrasonically-debitterizing of apricot kernels exerted more aromas compared with that of the conventional debitterizing, which might have more applications about this kind of water. In a word, ultrasound can be safely applied in the debitterizing industry of apricot kernels.

Introduction

Apricot, Prunus armeniaca L., the genus prunus of the subfamily Prunoideae in the family Rosaceae [1], mainly distributes throughout the Central Asia, West Asia, Mediterranean region and Western China, which is an internationally popular crop, with the global production reaching 4.1 million tons [2]. As the nut seed, apricot kernels contain abundant compounds like carbohydrates, protein, polyphenols, flavonoids and amygdalin [3], [4]. Among them, the amygdalin is very distinctive and determines the healthy function of the apricot kernel to some extent, which has a content of 2–5 g/100 g apricot kernels [4]. According to the chemical structure, the amygdalin, ever called vitamin B17 or Nitrilosides, is composed of two molecules of glucose, one benzaldehyde and one hydrogen cyanide [5]. Generally, the amygdalin itself is considered to be nontoxic, while its metabolic product like HCN has the potential toxicity, and its lethal dose for human is 0.5–3.5 mg/kg body weight [6]. Therefore, the amygdalin is widely regarded as a potentially toxigenic compound [7], [8], and its occurring has been as an obstacle to the utilization and commercialization of the apricot kernels for human or animal nutrition. Considering this situation, the detoxification has become an indispensable operation unit in the processing of apricot kernels, by which the amygdalin can be lowered to the safe content, simultaneously removing the bitterness and potential toxicity of the apricot kernels.

The commonly employed methods of detoxifying the apricot kernels include two categories: conventional and novel methods, the former comprises the cold water debitterizing [9], hot water debitterizing [10], [11], acid solution debitterizing [12] and acid-base alternate debitterizing, while the latter contains the vacuum debitterizing [13], ultrasound debitterizing and microwave debitterizing [7], etc. To the conventional methods, the main disadvantages are high energy consumption (water and electricity), time waste (from 6–7 h to 6–7 days), labour-intensive, high pollution and high loss of compounds in apricot kernels. The contents of protein, carbohydrates, phenols, flavonoids and amygdalin have been detected in the debitterizing water concentrate (DWC) from the conventionally industrial debitterizing. Among them, the amygdalin has the highest content of 23.89 ± 0.03 (g/100 g dry base weight) in the DWC, and the total loss of weight is about 17% of the apricot kernels after the debitterizing [14]. In comparison, the novel techniques are relatively low-cost, eco-friendly, rapid and easy-industrialization. As a non-thermal processing technology, ultrasound is regarded as the most promising technique for accelerating the debitterizing of apricot kernels. With this method, the debitterizing time can be reduced to about two hours and the water consumption can also be greatly decreased by ultrasound irradiation [11], while the safety is severely considered about the transformation of configuration (epimer) and the products available generated from the amygdalin during exposure to ultrasound irradiation.

Regarding the amygdalin, it has two epimers of l-amygdalin and d-amygdalin (Fig. 1), the former does not exist in nature, but it can be epimerized from the d-amygdalin during the decoction, particularly under basic conditions, because of the weakly acidic character of the benzylic proton [15], [16]. That is to say, the amygdalin coexisted in d- and l- amygdalin forms at neutral or basic conditions, while the D-isomer only exists under acid conditions. This phenomenon suggests that the dominant amygdalin epimer might be easily changed during the debitterizing or sample pretreatment. Furthermore, the d-amygdalin tastes bitter, while the l-amygdalin has no bitterness and different physiological properties, so special concerns should be focused on this case during the debitterizing of apricot kernels, since both d- and l- amygdalin can be degraded into the HCN causing toxicity available by the gastric acid or enzymes in the digestive tract of human body. In other words, if the bitter d-amygdalin only converted into the non-bitter l-amygdalin during the debitterizing of ultrasound irradiation, its danger will be greater for the consumers eating this kind of debitterized apricot kernels.

In a word, ultrasound irradiation does greatly shorten the debitterizing time of apricot kernels and has the promising application in the apricot kernels processing industry, while its influencing mechanism on the amygdalin should not be neglected. In the meantime, the debitterizing water, containing abundant components especially volatile compounds, can also be as the main ingredients of some oral liquid (cough syrup). In pharmaceutical industry, the water soaked with the apricot kernels (namely aqua armeniacae) is the dominant constitute for cough syrup to treat the symptom of asthma, bronchitis, emphysema and cough in China. So special interests were centered on the available utilization of the debitterizing water in pharmaceutical industry by our group. Before considering the re-utilization of the debitterizing water, some investigations should be conducted about the effect of ultrasound irradiation on the compositions of the debitterizing water, especially the typical flavor compounds. However, to the best of our knowledge, little information is available in this field.

The main purpose of this paper was to investigate the effects of ultrasound irradiation on the amygdalin of apricot kernels and the volatile components in debitterizing water during the accelerated debitterizing process, so as to evaluate the safety of the novel debitterizing technique and the re-utilization of the debitterizing water, finally reducing the pollution of environment and promoting the added-value in the processing industry of apricot kernels.

Section snippets

Materials and reagents

The apricot kernels (harvested in June 2017) were purchased from the Northwest Herb Market of Xi’an, Shaanxi province, China. The water content of the apricot kernels is about 4.84 g water/100 g seeds, and the single weight is about 0.53 ± 0.05 g for each seed.

Standard of d-amygdalin was obtained from Chengdu Manst Biotechnology Co., Ltd. Amygdalin was purchased from the Chengdu Preferred Bio-Technology Co. Ltd (Sichuan Province, China). HPLC-grade methanol and acetonitrile were bought from the

Simultaneous determination of the d- and l- amygdalin by HPLC

Regarding the determination of d- and l- amygdalin, although some methods such as high-performance liquid chromatography (HPLC) [25], carbon-13 nuclear magnetic resonance (13CNMR) spectroscopy [26], chemical ionization mass spectrometry [27] and gas chromatography with flame ionization detection (GC-FID) [28] have been employed for the identification and quantitative determination of the amygdalin epimers, yet these methods required some time-consuming complicated procedures to prepare the

Conclusions

In summary, ultrasound irradiation can accelerate the debitterizing of apricot kernels by promoting the mass transfer and degradation of the d-amygdalin. Moreover, it can not cause the epimerization of the d-amygdalin to the l-amygdalin, which means that this novel ultrasonically-debitterizing technique has the same mechanism as the conventional debitterizing of hot water, i.e. there is no novel safety issue occurred in the ultrasonically-debitterizing based on the changes of d-amygdalin, while

Acknowledgments

This study was funded by National Natural Science Foundation of China [No. 31101324], Natural Science Foundation of Shaanxi Province, China [No. 2015JM3097], Technology Transfer Promotion Project of Xi’an, Shaanxi Province, China [No. 2017071CG/RC034(SXSF003)], Key Research Development Program of Shaanxi Province, China [No. 2017NY-167, 2018ZDXM-NY-086] and the Fundamental Research Funds for the Central Universities of China [No. GK201602005, GK201802012].

Declaration of Competing Interest

No potential conflict of interest was reported by the authors.

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