J. Cent. South Univ. Technol. (2008) 15(s1): 525-528

DOI: 10.1007/s11771-008-414-z

Liquid rheology study on refined rapeseed oil

LIU Qi-mei(刘其梅)¹, LUO Ying-she(罗迎社)¹, YIN Shui-ping(殷水平)¹, CHEN Sheng-ming(陈胜铭)¹,

ZHANG Dang-quan(张党权)², PENG Wan-xi(彭万喜)¹

(1. Institute of Rheological Mechanics and Material Engineering, Central South University of Forestry and Technology, Changsha 410004, China;

2. Key Laboratory of Non-wood Forest Products of State Forestry Administration, Central South University of

Forestry and Technology, Changsha 410004, China)

Abstract: The rapeseed oil extracted from the mature seeds was purified by refining processing, and the rheological characteristic analysis of the viscosity and dynamic shear rate at gradient temperatures was made. The result shows that at 20, 40, 60 and 80 ℃ respectively, when the shear rate gradually rises, the torque increases accordingly but its viscosity does not vary distinctly. The result suggests that when rapeseed oil is used as the raw of edible oils and industries, the working procedures at high temperature will not influence its rheological characteristic distinctly.

Key words: rapeseed oil; liquid rheology; dynamic shear rate; viscosity; torque

1 Introduction

As the uppermost herbaceous edible oil crop in China, rape (Brassica Napus) has been planted for thousands of years. There are many fine rape varieties bred in recent years and widely cultivated in most areas of China. The rapeseed oil extracted from the rape seed is top-quality edible oil, which contains rich fact acids, fat-soluble vitamins, phospholipids and pigments but lacks cholesterol^[1]. The color of rapeseed is deep yellow. The digestive utilization ratio of rapeseed oil is up to 99%. Therefore, rapeseed has become the main edible vegetable oil in China^[2]. Rapeseed oil is also the third vegetable oil in yield, and is considered four main worldwide vegetable oils together with bean oil, palm oil and sunflower seed oil^[3]. The characteristic of large viscosity, least saponification value among semi-drying oils and high-content erucic acid can be used to distinguish rapeseed oil from other vegetable oils^[4].

Besides the direct application of edibility, rapeseed oil is also the important raw industrial material^[5] and widely applied to industries of machinery, rubber, chemical, plastic, painting, textile, soap, pharmaceutical and bio-diesel^[6-8]. Especially, rapeseed oil can be used as the good lubricating oil in cast steel industry, margarine, fire extinguishing oil and mental preservative^[9-11]. Moreover, the arisen biodiesel engineering in recent years around the whole world results in the increase of the proportion of rapeseed transformed into biodiesel year by year ^[12-13].

The crude rapeseed oil has the unpleasant odor and pungency, and is unsuitable direct edibility. Therefore, crude rapeseed oil needs refining process of degelatinizing, deodorization, deacidification and decoloration to obtain edible high-quality rapeseed oil ^[14]. The refined rapeseed oil becomes the neutral, colorless, odorless and flavorless liquid without volatility, and can also dissolve some organic compounds and fat-soluble vitamins. Meanwhile, the refined rapeseed oil is widely applied to food industry, especially to produce salad oil, butterine and shortening oil^[15]. The common flow of producing refined oil form rapeseed is: first, crude rapeseed oil by mechanical pressing is obtained, and then secondary crude oil from the oil cakes is obtained, finally, the total crude rapeseed oil is treated by refining procedure to form the high-quality edible oil.

Lots of researches had focused on the processing and utilization of rapeseed oil, especially on the deep refining and industrial application in recent years. However, they didn’t analyze the liquid rheological characteristic of rapeseed oil to elucidate the instruction for the technology parameters of industrial operation^[16-17]. In the paper, the liquid rheological analysis of dynamic shear rate-viscosity relationship of rapeseed obtained from good rapeseeds by mechanical pressing and refining was carried out at gradient temperatures, providing scientific foundation for the rheological application of rapeseed oil industry.

2 Materials and methods

2.1 Materials

The rapeseeds were purchased from the wholesale market of Hongxing district of Changsha, Hunan province, China. All reagents (chromatographic grade) were purchased from the Sigma Chemical Company (USA) unless noted otherwise.

2.2 Preparation of refined rapeseed oil

After thorough drying, the rapeseeds were treated by cleaning, crushing, softening, flaking and cooking in succession before oil extracting. The rapeseeds were firstly treated with mechanical pressing to obtain crude rapeseed oil. The extracting byproducts and oil cakes were treated by organic solvent extraction to obtain the residual crude rapeseed oil. The two groups of crude rapeseed oils were mixed, and then followed by the refining procedures including degelatinizing, deodorization, deacidification and decoloration to finally obtain the refined high-quality edible rapeseed oil.

2.3 Determination of shear rate-viscosity of refined rapeseed oil

After quality check, the refined rapeseed oil was analyzed by liquid rheological characteristic. At the constant temperature, the variety of torque and viscosity derived from the dynamic shear rate was measured by rheometer (TA, USA). The gradient temperatures including 20, 40, 60 and 80 ℃were analyzed respectively to analyze the corresponding variety of torque and viscosity of refined rapeseed oil. The scale of dynamic shear rate began from 0 s^-1 and then increased gradually by exponential form. It was measured for twenty times for each 10 exponential span. The biggest scale of dynamic shear rate was no more than 100 s^-1.

After treated by successive drying, cleaning, crushing, softening, flaking and cooking, the rapeseeds were treated by machinery pressing to obtain the crude rapeseed oil. The residual oil in cakes was also extracted by organic solvents extracting. After mixing, the two crude rapeseed oils were treated by deep refining procedure including degelatinizing, deodorization, deacidification and decoloration. The extraction efficiency of rapeseed oil is up to 94.3%, and the refining procedures are proved to be high-efficient.

The analytic result of extracting and refining shows that the quality of rapeseed oil obtained by this method is suitable for the posterior analysis of dynamic shear rate—viscosity. The result of dynamic shear rate—viscosity of refined rapeseed oil at gradient temperatures (20, 40, 60 and 80 ℃, respectively) shows that the gradually increasing shear rate results in the corresponding augment 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 0.41 Pa?s (Fig.1), 0.23 Pa?s (Fig.2), 0.22 Pa?s (Fig.3) and 0.13 Pa?s (Fig.4), respectively, especially when the shear rate is bigger than about 8 s^-1.

Fig.1 Relationship between dynamic shear rate and viscosity of rapeseed oil at 20 ℃

Fig.2 Relationship between dynamic shear rate and viscosity of rapeseed oil at 40 ℃

However, when the shear rate is relatively small (especially less than about 8 s^-1), the viscosity of refined rapeseed oil fluctuates distinctly around the average. Especially, the viscosity of refined rapeseed oil at 80 ℃ and small shear rate are drastically fluctuant. Moreover, when the dynamic shear rate is increased from 0 s^-1 to near 8 s^-1, the viscosity of rapeseed oil decreases from a relative value to the average. The reason is that the relatively small shear rate only brings small torque, which cannot counteract the thermal motion of liquid^[18-23].

Fig.3 Relationship between dynamic shear rate and viscosity of rapeseed oil at 60 ℃

Fig.4 Relationship between dynamic shear rate and viscosity of rapeseed oil at 80 ℃

Therefore, the relatively big shear rates (more than about 8 s^-1) were used to analyze the influence of different temperatures on viscosity of rapeseed oil under constant shear rate. The analytical result shows that the rising temperature results in the corresponding gradual decrease of viscosity of rapeseed oil at each constant shear rate (Fig.5). However, the viscosities at 40 and 60 ℃

Fig.5 Relationship between temperature and viscosity of rapeseed oil at constant shear rate

are almost the same. This result reveals that viscosity of rapeseed oil is not correlated with the variation of shear rate at a special constant temperature, but is negatively correlated with temperature at a special constant shear rate. Therefore, when rapeseed oil is used as the raw of edible oils and industries, the working procedures at high temperature will not influence its rheological characteristic distinctly, and the temperatures of 40 and 60 ℃ should be priorly used.

1) The refined rapeseed oil is successfully prepared by machinery pressing and deep refining procedures from matured rapeseeds.

2) The viscosities of rapeseed oil at gradient temperatures (including 20, 40, 60 and 80 ℃, respectively) are almost constant when the dynamic shear rate is bigger than 8 s^-1. The rising temperature results in the corresponding gradual decrease of viscosity of rapeseed oil at each special constant shear rate.

3) This result reveals that viscosity of rapeseed oil is not correlated with the variation of shear rate at a special constant temperature, but is negatively correlated with temperature at a special constant shear rate.

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(Edited by YANG Hua)

Foundation item: Project(2007CB210201) supported by Major State Basic Research Development Program of China; Project(07JJ6053) supported by Hunan Provincial Natural Science Foundation of China

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

Corresponding author: LUO Ying-she, Professor; Tel: +86-731-5623376; E-mail: lys0258@sina.com