J. Cent. South Univ. Technol. (2008) 15(s1): 506-508

DOI: 10.1007/s11771-008-410-3

Rheology study of supercritically extracted tea-oil

ZHANG Dang-quan(张党权)¹, CHEN Sheng-ming(陈胜铭)², PENG Wan-xi(彭万喜)², LIU Qi-mei(刘其梅)²,

GU Zhen-jun(谷振军)¹, FAN Shao-gang(樊绍刚)¹, DENG Shun-yang(邓顺阳)¹

(1. Key Lab of Non-wood Forest Products of State Forestry Administration, Central South University of Forestry and Technology, Changsha 410004, China;

2. Institute of Rheological Mechanics & Material Engineering, Central South University of Forestry and Technology, Changsha 410004, China)

Abstract: The rheological analysis on dynamic shear rate-viscosity relationship of tea-oil extracted from tea-oil tree seeds by supercritical extraction method was carried out at gradient temperatures and constant shear rate, respectively. The results show that at 20, 40, 60 and 80 ℃, once the shear rate increases gradually, the torque enlarges correspondingly, while the viscosity shows little difference. However, at the constant shear rate, the rising temperature results in a steady downtrend on tea-oil viscosity. This results reveal that tea-oil viscosity is not closely correlated with shear rate at constant temperature, yet negatively correlated with temperature at constant shear rate.

Key words: tea-oil; rheology; dynamic shear rate; viscosity; supercritical extraction

1 Introduction

Tea-oil tree (Camellia oleifera) is the key edible oil tree growing specifically in China. As a world-famous edible oil tree, it is regarded as one of the four primary edible oil tree plants together with oil palm, olive, and coconut. C. oleifera originates in China and has developed into the main non-wood forest tree distributed (35 billion m²) mainly south of the Yangtze River valley^[1]. C. oleifera has been the main tree of forestation in the red-soil upland area of south China for its wide adaptability and endurance to the arid and barren environments^[2].

The oil obtained from C. oleifera seeds is named tea-oil, which is the elite edible oil in Asia and profoundly favored by people, especially Chinese and Japanese, and is characterized by its unique flavor, durable storage, and ease of absorption into the body. The kernel of C. oleifera seed has 55% tea-oil, whose oiliness has been proved to be far better than that of palm, rape, bean and peanut oil, and can even exceed that of olive oil, for tea-oil is abundant in unsaturated fatty acids (more than 90%) and abundant vitamins^[3]. It has been found that tea-oil does not contain erucic acid unhealthy to the body^[4]. Besides its high edible merit, tea-oil possesses good medicinal characteristics such as preventing cardiovascular cirrhosis, reducing cholesterol, and decreasing blood pressure^[5-6]. Other uses are advances in cosmetic and raw materials for the perfume industry by deep processing^[7-8]. Therefore, C. oleifera is considered as the Oriental olive.

In recent years, many researches have been focusing on the processing and utilization of tea-oil, especially the deep processing, development of cosmetic and perfume industry, improvement of technological parameters^[9-10], but paying little attention to analyze the rheological characteristic of tea-oil and resultant influence on processing and utilization or elucidative instruction for the technology parameters of industrial development. Therefore, the rheology analysis of dynamic shear rate-viscosity relationship of tea-oil extracted from tea-oil tree seeds by supercritical extraction was carried out at gradient temperatures, to present scientific foundation for the rheological application to tea-oil industry.

2 Materials and Methods

2.1 Materials

Mature C. oleifera fruits of an excellent cultivar Xianglin-1 (XL-1) were collected as material from elite C. oleifera orchard of forestry academy of Hunan Province in the suburb of Changsha, China. All reagents (chromatographic grade) were purchased from the Sigma Chemical Company (USA).

2.2 Supercritical extraction of tea-oil

The kernels of mature C. oleifera fruits were cleaned by deionized water, and then freeze-dried by vacuum freeze drying technology. C. oleifera kernel sample was smashed into powder by FZ102 Disintegrator, which is suitable for plant (Tanjing Taisite Inc. Corp., China), in succession, 75 μm powders were sieved out through AS200 Sieving Instrument (USA). The powders of C. oleifera kernels were treated by supercritical CO₂ extraction under 25 MPa, at 40 ℃ for 2.5 h using SFT-150 supercritical extraction instrument (Supercritical fluids technology Inc, USA). Tea-oil extracted by supercritical CO₂ extraction was used for the analysis on dynamic shear rate-viscosity.

2.3 Determination of dynamic shear rate-viscosity

At the constant temperature, the variety of torque and viscosity derived from the dynamic shear rate was measured by rheometer (TA, USA). The temperature gradients 20, 40, 60 and 80 ℃ were checked respectively to analyze the corresponding varieties of torque and viscosity of supercritically extracted tea-oil. The scale of dynamic shear rate increases from 0 gradually by exponential form. It was measured for twenty times for each 10 exponential span. The largest scale of dynamic shear rate at 20 ℃ and 40 ℃ is no more than 100 s^-1, and that of shear rate at 60 ℃ and 80 ℃ is no more than 1 000 s^-1.

3 Results and discussion

According to the designed experimental program, the fresh kernels of C. oleifera seeds treated by freeze-drying were drastically smashed and sieved uniformly into 75 μm powders. Then the pretreated sample was treated by supercritical CO₂ extraction under given conditions. The extraction efficiency of tea-oil by supercritical CO₂ is up to 96.8%, which suggests that the supercritically extracted tea-oil is suitable for the posterior analysis of dynamic shear rate-viscosity.

The results of dynamic shear rate-viscosity of supercritically extracted tea-oil at gradient temperatures 20, 40, 60 and 80℃ show that the gradually increasing shear rate leads to the corresponding augments of torque, but the viscosity of tea-oil does not vary distinctly. The dynamic viscosities of tea-oil at gradient temperatures 20, 40, 60 and 80 ℃ are almost constant at about 0.8 Pa·s (Fig.1), 0.36 Pa·s (Fig.2), 0.22 Pa·s (Fig.3) and 0.16 Pa·s (Fig.4) respectively, especially when the shear rate is more than 10 s^-1. Nevertheless, when the shear rate is relatively small (less than 10 s^-1), the viscosity of tea-oil fluctuates obviously around the constant. Especially, the viscosity of tea-oil at 80 ℃ and small shear rate is drastically fluctuant, for the relatively small shear rate only brings small torque, which cannot counteract the thermal motion of liquid^[11-16].

Fig.1 Relationship between dynamic shear rate and viscosity of tea-oil at constant 20 ℃

Fig.2 Relationship between dynamic shear rate and viscosity of tea-oil at constant 40 ℃

Fig.3 Relationship between dynamic shear rate and tea-oil viscosity at constant 60 ℃

Therefore, relatively large shear rates can be used  to analyze the influence of different temperatures at constant shear rate. The analytical results show that

Fig.4 Relationship between dynamic shear rate and tea-oil viscosity at constant 80 ℃

temperature rise results in corresponding gradual fall of tea-oil viscosity at each constant shear rate (Fig.5). The results reveal that tea-oil viscosity is not correlated with shear rate at constant temperature, but is negatively correlated with temperature at constant shear rate. Therefore, when applied to cosmetic industry, the continual decrease of tea-oil viscosity should be considered for the technological parameters of industry.

Fig.5 Relationship between temperature rate and tea-oil viscosity at constant shear rate

4 Conclusions

1) Tea-oil is successfully extracted by supercritical CO₂ extraction from kernels of mature tea-oil seeds.

2) The dynamic viscosities of tea-oil at gradient temperatures 20, 40, 60 and 80 ℃ are almost constant when the shear rate is more than 10 s^-1. However, temperature rise leads to corresponding gradual fall of tea-oil viscosity at each constant shear rate.

3) Tea-oil viscosity is not correlated with shear rate at constant temperature, but negatively correlated with temperature at constant shear rate.

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(Edited by ZHAO Jun)

Foundation item: Projects(2007CB210201) supported by the Major State Basic Research Development Program of China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: ZHANG Dang-quan, Associate professor; Tel: +86-731-562346; E-mail: zhangdangquan@163.com