Natural rubber (NR) is a vital commodity in world market due to its excellent physical properties that synthetic rubber cannot replicate or prevail. It can be used to manufacture more than 40,000 consumer products. In the past decade, the world's demand for natural rubber has increased dramatically. Alternative sources of natural rubber will not only increase the supply of rubber, but will also provide resource protection for countries engaged in planting. More than 2500 kinds of plants can synthesize natural rubber, most of which produce small quantities of low molecular weight rubber. Only Hevea brasiliensis or para rubber tree, Parathenium argentatum and Taraxacum kok-saghyz can produce rubber with molecular weight over 1000kDa and have in turn applicative values (1). Hevea brasiliensis is currently the only large-scale commercialized rubber-producing plant, mainly distributed in tropical and subtropical regions such as Asia, Africa and South America. Asian rubber production accounts for 93% of the world’s rubber production, while the output from South America as the origin of the Hevea brasiliensis which is easily affected by leaf blight, only accounts for ca. 2%. As a single rubber-producing plant, Hevea brasiliensis may face potential threats. Countries have gradually realized the importance of developing natural rubber resources other than Para rubber, and have successively initiated the research and development of Hevea brasiliensis alternative rubber-producing crops (2).
Taraxacum kok-saghyz (TKS), is a rubber-producing plant native to the mountains of Kazakhstan and the Tianshan Valley-Tex River Basin in Xinjiang, China. TKS plant, inherently rich in natural rubber, inulin and other bioactive ingredients e.g. penta-cyclic triterpenoids, is an important industrial crop. Discovered in the 1930’s by the former U.S.S.R exploration team (3), TKS was further investigated to reduce dependence on foreign rubber sources. The mass fraction of rubber within TKS’s taproot can reach 2.89%-27.89% (3), and TKS rubber is of good quality, with a molecular weight of more than 100,0000 g/mol (4). In addition, gloves and other rubber products made of traditional rubber may cause personal allergic reaction, namely life-threatening Type Ⅰ Hev-b latex allergy, while the rubber of TKS has better biocompatibility and can be used to manufacture hyposensitive medical goods (5). In addition, TKS is easy to grow, which has strong ability to grow in both cold areas and temperate regions, and to resist bacterial infections and pest invasions.
The determination of rubber content is crucial for the growth of alternative rubber plants in the natural rubber industry. The breeding of TKS necessitates the screening of high-yielding varieties, thus rapid determination of rubber content is essential for the work. Over the years, a variety of techniques have been used to determine rubber content, such as gravimetry (6), gel permeation chromatography (GPC) (7), infrared spectroscopy (IRS) (8), mass spectrometry (MS) (9, 10), etc. However, quantification is usually a time-consuming process, which slows down the speed of improving rubber quality and yield through breeding. Several quantitative procedures have been discussed to measure the rubber content in plants (11, 12). In most cases the protocols comprise two successive steps, extraction of rubber, then quantification of the components by gravimetry, chromatography, or a spectral technique (13–19). For example, quantitative Soxhlet extraction method was used as the main technique for rubber quantitation (20). With this method, plant materials were first ground into small particles, and the dry-milled materials were put into cellulose extraction thimbles. Acetone extraction for 4 hours was next used to remove resins, and then samples were extracted with hexane for another 4 hours to obtain the rubber. The collected extractants were evaporated, leaving dry film of rubber. Following evaporation of the solvents, weights were determined for rubber contents using gravimetric analysis (11). This method requires time-consuming purification and extraction steps, and consumes a lot of organic solvents. Moreover, the rubber is easily degraded in boiling acetone, which affects the quality of the rubber consequently.
Pyrolysis gas chromatography (PyGC) has been used to determine the rubber content of plant-derived natural rubber e.g., Hevea brasiliensis, Eucommia ulmoides and synthetic rubber (21). The principle of pyrolysis gas chromatography is easy to understand. Firstly, the sample is heated in a strictly controlled environment. During the heating process, the target polymeric compound is thermally and quantitatively cracked to small volatile molecule compounds, then these mingled small molecules are separated by gas chromatography to obtain a chromatogram. Finally, the polymeric structure and composition of the sample can be inferred based on the qualitative and quantitative data analysis (8). Pyrolysis gas chromatography is a relatively accurate method so far to determine the rubber content indirectly. The method is fast and accurate which does not require complicated sample processing and rubber extraction steps, and eliminates the need of any organic reagents. The rubber content data of a plant sample can be easily acquired in an average of 40 minutes, and both dry and wet samples can be measured.
Near infrared spectroscopy (NIRS) is based on an electromagnetic radiation wave between visible (Vis) and mid-infrared (MIR). The near-infrared region, defined as 780-2526nm by the American Society of Testing Materials (ASTM), is the first invisible region found in the absorption spectrum. The near infrared spectrum originates from the combinatoriallly vibrational/rotational frequency and doubling absorption frequency regions of hydrogen groups (OH, NH, CH) in organic molecules. By scanning with ease, the near infrared spectra of the samples, the characteristic fruitful information of hydrogen groups in organic molecules can be obtained. Sample analysis by near infrared spectroscopy is deemed to be convenient, rapid, efficient, accurate, cost effective, non-destructive and environmentally friend. Therefore, this technology is favored by more and more natural rubber researchers (22).
The main purpose of this study is to develop a facile near-infrared spectroscopic method to determine the content of rubber in the fresh root of Taraxacum kok-saghyz. By collecting different varieties of TKS samples and taking the measured values of pyrolysis gas chromatography as the reference data of the NIR model, the quantitative prediction model of rubber content was established by multivariate regression analysis method. The prediction accuracy and reproducibility of the analysis model were evaluated, and a set of near infrared spectrum model suitable for the fresh root of TKS was explored.