Evolution of the Rheological Behavior of Palm Oil

Abstract

Palm oil is dark yellow to red with a sweet taste and violet scent. Under the action of oxygen in the air, palm oil discolors due to the oxidation of carotene. The sample at different temperatures was recorded at different shear rates. Based on the rheograms (graph of the average shear stress versus shear rate), the studied ones proved to be Newtonian fluid. The shear range used did not significantly affect the absolute viscosities of the palm oil at different temperatures. The absolute viscosities of palm oil have decreased with increasing temperature and can be equipped with an Arrhenius-type relationship.

1. Introduction

The palm tree (Elaeis guineensis) is part of the palm family. The fruit has an ovoid shape and dark red to purple or even black. It consists of two distinct parts: the pulp from which palm oil is obtained (palm oil) and a hardwood shell, in which there is a white and oily core. From this is obtained palm kernel oil (palm oil), which differs from palm oil both in properties and in its chemical composition.

Palm oil is dark yellow to red, due to its high carotene content, with a sweet taste and violet smell. Under the action of oxygen in the air, palm oil discolors due to the oxidation of carotene.

The oil obtained from the core of the palm kernels is white to yellowish in color, with a characteristic smell and a slightly sweet taste. Palm oil is native to West Africa, where it has been extracted from the fruits of tropical palm (other than coconut) for over 5000 years. Until our century, this oil was produced on a small scale in African villages and only after

1920 became an important export product. Unrefined palm oil (“virgin” or red) is one of the richest natural sources of carotene, it is also rich in vitamins E and K, coenzyme Q10, and antioxidants. Although it has a high smoke point (230°C), palm oil is not recommended for frying, because after the first use the antioxidants are destroyed, and after four uses the carotene also disappears. It is used on an industrial scale as an ingredient for bakery and pastry products, for margarines and candies (especially potato).

Agricultural units are of different sizes and can be classified as small, medium, and large scale (estates). There are three varieties of palm trees: Dura, Pisifera, and Tenera. Dura, the main variety, has been found in orchards for decades and was the main source of palm oil long before modern methods of cultivating palm oil were introduced to Africa in the second quarter of the twentieth century. Small fruits are without shell. Tenera also contains small fruits that peel easily to release palm kernels. Tenera palm kernels are smaller than Dura, although Tenera bunches are much larger than Dura bunches.

Palm oil is obtained from the fleshy part that surrounds the seeds, by crushing and pressing. The seeds are separated, and after being broken and peeled, they can be processed in turn, resulting in palm kernel oil, which represents about 10% of the total production of oil obtained from palm, the kernel oil is processed separately because the composition and uses are different.

Palm oil and its liquid fraction—olein, are used worldwide as cooking oil and margarine. Also, palm fats are incorporated into mixtures used for the manufacture of various foods as well as in the preparation of household products.

Originally from Africa, where there are still wild palm plantations and used for over 5000 years for food purposes, palm oil ranks third in world consumption of oils and fats, after animal fats and soybean oil.

It is generally accepted that palm oil originates in the rainforest region of the West African rainforest. The main belt passes through the southern latitudes of Cameroon, Ivory Coast, Ghana, Liberia, Nigeria, Sierra Leone, Togo, and the equatorial region of Angola and Congo. The processing of palm fruits into edible oil has been practiced in Africa for thousands of years, and the oil produced, very colorful and aromatic, is an essential ingredient in the traditional cuisine of West Africa.

In the fifteenth century, the tree was introduced to other parts of Africa, Southeast Asia, and Latin America. In Malaysia, the tree was introduced in 1870 as an ornamental plant, and the first commercial plantations appeared in 1917. In the 1960s, the production of plum oil was considerably expanded to reduce the economic dependence on rubber and coffee.

Today, Malaysia is the largest producer and exporter of palm oil in the world. In 1995, world production of palm oil was 15.3 million tons, of which Malaysia produced 7.8 million tons, or 51.6% of world production. In 2000, Malaysia produced 8.6 million tons of crude palm oil, of which over 8 million tons were exported.

Palm plantations occupy an area of over 2.5 million ha in Malaysia. The bunches are harvested manually and the transport is mechanized, and the processing of fruits and seeds is done in modern installations, of high technology and productivity.

Due to the high fat content of palm fruits and high productivity, the price of palm oil is usually below the price of soybean oil. Palm oil and its liquid fraction olein are used worldwide as cooking oil, shortening, and margarine. Palm fats are also incorporated into mixtures used in the manufacture of various foods as well as in the preparation of household products.

Palm oil is rich in carotenoids (pigments found in plants and animals), from which derives its deep red color and major component of semisolid.

Due to its economic importance as a high-yield source of edible and technical oils, palm oil is now grown in plantations in most countries with rich rainfall (minimum 1600 mm / year) in a tropical climate, 10° from the Equator.

The fruits consist of an outer skin (exocarp), the pulp (mesocarp), which contains palm oil in a fibrous matrix, a central part consisting of a shell (endocarp), and the core, which contains oil quite different from the oil palm, which resembles coconut oil.

This is equivalent to a yield of 5 tons of oil/ha/year (except for palm kernel oil), which far exceeds any other source of edible oil (Table 1).

Bunches/fruits	Composition
The weight of the bunches	23−27 kg
Fruits/Bunches	60–65%
Oil/Bunches	21–23%
Core/Bunches	5–7%
Mezocarp/Bunches	44–46%
Mesocarp/fruit	71–76%
Core/fruit	21–22
Shells/fruits	10–11

Table 1.

The composition of the bunches of palm fruits.

However, high yields are rarely achieved in practice due to unfavorable climatic conditions. Rainfall is irregular in Central Europe and West Africa and therefore the tree suffers from water problems. Expensive management of labor and production factors, imported fertilizers, pesticides, and harvesting equipment are also a difficulty that slows down plantation production.

The oil can be obtained from various types of plants. Large amounts of oil are obtained from the seeds, but also from the fleshy part of the fruit, as is the case with olive or palm oil. The most important crops of oil plants are in tropical and subtropical areas, and in temperate areas soybeans, flax, rapeseed, mustard, sunflower are grown.

Palm oil is native to Africa where there are still wild palm plantations, and the oil is obtained by simple, traditional methods. It has a long history of use for food purposes, with archeological evidence dating back to 5000 years ago. It is produced from the fruit of the palm tree called Elaeis Guinneis, native to West Guinea and ranks third in world consumption of oils and fats, after animal fats and soybean oil. The palm tree matures 3 years after planting and annually produces 10–12 bunches, each weighing 20–30 kg. A bunch can have 1000–3000 fruits the size of an elongated cherry that is made up of the fleshy part and white core kernels [1].

The approximate concentration of fatty acids (FAS) in palm oil is given in Tables 2 and 3.

Acids found	Content
C16 palmitic acid (saturated)	44.3%
C18 stearic acid (saturated)	4.6%
Myristic acid C14 (saturated)	1%
C18 oleic acid (monounsaturated)	38.7%
C18 linoleic acid (polyunsaturated)	10.5%
Other fatty acids	0.9%

Table 2.

The fatty acid content of palm oil.

Acids found	Content
C12 lauric acid (saturated)	48.2%
Myristic acid C14 (saturated)	16.2%
C16 palmitic acid (saturated)	8.4%
Capric acid C10 (saturated)	3.4%
C8 caprylic acid (saturated)	3.3%
C18 stearic acid (saturated)	2.5%
C18 oleic acid (monounsaturated)	15.3%
C18 linoleic acid (polyunsaturated)	2.3%
Other fatty acids	0.4%

Table 3.

Fatty acid content of palm kernel oil.

There are many rheological models for interpreting data. For most fluids, the most widely used model is a three-parameter model, described by the Herschel-Bulkley equation [2, 3, 4, 5]:

$$\mathrm{\tau }={\mathrm{\tau }}_0+\mathrm{K}{\left(\mathrm{d\gamma }/\mathrm{dt}\right)}^{\mathrm{n}}$$E1

In this model, τ₀ represents the voltage or the effort of flow, signifying the minimum voltage that must be applied to initiate the flow. Obviously, being a voltage it is measured in Pascali (1 Pa = 1 N. m⁻²). K is the consistency coefficient, the value of which depends on the nature and temperature of the fluid. It is measured in Pa. sⁿ, formally being a viscosity. The flow behavior index is the exponent of the shear rate, n. It is a dimensionless quantity, which depends on the nature of the fluid, its value being very little influenced by temperature.

The Herschel-Bulkley model is simplified and will take the form of the Ostwald model or the law of power:

$$\mathrm{\tau }=\mathrm{K}{\left(\mathrm{d\gamma }/\mathrm{dt}\right)}^{\mathrm{n}}$$E2

Fluids for which the flow behavior index is unitary, as in Newtonian fluids, but there is flow effort (or tension) (τ_o > 0), are called plastic fluids or Bingham plastic. For such a material, its behavior is like an elastic solid when the stress is below the value of the flow stress (or stress) τ_o. For voltages greater than the flow voltage, the mathematical expression for the model is [6, 7, 8]:

$$\mathrm{\tau }={\mathrm{\tau }}_0+{\mathrm{\eta }}_{\mathrm{p}}\left(\mathrm{d\gamma }/\mathrm{dt}\right)$$E3

This mathematical expression of the Bingham model derives from the Herschel-Bulkley equation when n = 1, and K = ηp. Given that the flow behavior index is unitary, the unit of measurement for the consistency index (K) is Pa, that is, the unit of measurement for viscosity. This is why K is replaced by ηp.

This article includes the study of the rheological behavior of palm oil at temperatures between 40 and 100°C and shear rates between 3 and 120 s⁻¹ [9, 10, 11, 12].

2. Material and methods

The rheological behavior of palm oil was determined using a Haake VT 550 Viscotester developing shear rates ranging between 3 and 120 s⁻¹ and measuring viscosities from 10⁴ to 10⁶ mPa.s when the HV₁ viscosity sensor is used. The accuracy of the temperature was 0.1°C. The physicochemical properties of palm oil are given in Table 4.

Physical and chemical properties	Stock
Melting point (°C)	34.2
Relative density (25°C)	0.8937
Refraction index (50°C)	1.455
Moisture and impurities (%)	0.1
Iodine index	53.3
Saponification index (mg KOH/g)	208.2
Hydroxide index (mg KOH/g)	22.9
Acidity index (mg KOH/g)	26

Table 4.

Physical and chemical properties of palm oil [8].

3. Results and discussion

Figures 1–7 show the rheograms of the palm oil. As can be seen in the rheograms, the shear stress increases with increasing shear rate and has a linear dependence.

Figure 1.

Dependence shear stress versus shear rate for palm oil at temperature 40°C.

Figure 2.

Dependence shear stress versus shear rate for palm oil at temperature 50°C.

Figure 3.

Dependence shear stress versus shear rate for palm oil at temperature 60°C.

Figure 4.

Dependence shear stress versus shear rate for palm oil at temperature 70°C.

Figure 5.

Dependence shear stress versus shear rate for palm oil at temperature 80°C.

Figure 6.

Dependence shear stress versus shear rate for palm oil at temperature 90°C.

Figure 7.

Dependence shear stress versus shear rate for palm oil at temperature 100°C.

In addition to the rheological models found in the literature, this article proposes two rheological models found on the basis of experimental data (Tables 5 and 6):

Temperature, ^0C	Value of parameters of the theoretical model described by Eq. (4)		R^2
	B	A
40	40.8707	19.8184	0.9999
50	45.5434	11.2528	0.9995
60	38.8396	9.6028	0.9997
70	38.9844	8.9793	0.9994
80	38.2732	8.4015	0.9989
90	39.5068	7.7240	0.9989
100	33.0540	7.5880	0.9996

Table 5.

The temperature, value of parameters of the model described by Eq. (4), coefficient correlation for olive oil.

Temperature, ^0C	Value of parameters of the theoretical model described by Eq. (5)			R^2
	A	B	C
40	−2.1256E7	2.1256E7	−1.0726E6	0.9999
50	8226.2569	−8194.2029	667.7298	0.9995
60	−9428.5756	−9428.5756	921.9126	0.9997
70	5055.5401	−5031.0308	499.1903	0.9998
80	3581.1580	−3561.7138	361.8749	0.9999
90	3159.7222	−3138.4422	344.0841	0.9998
100	5327.3930	−5303.8596	638.4288	0.9999

Table 6.

The temperature, value of parameters of the theoretical model described by Eq. (5), coefficient correlation for olive oil.

$$\mathrm{\tau }=\mathrm{A}+\mathrm{B}\left(\mathrm{d\gamma }/\mathrm{dt}\right)$$E4

$$\mathrm{\tau }=\mathrm{A}+\text{Bexp}\left[\left(-\mathrm{d\gamma }/\mathrm{dt}\right)/\mathrm{C}\right]$$E5

Applying model (4), the correlation coefficients have values between 0.9989 and 0.9999, which demonstrates that this model correctly describes the rheological behavior of unadditive palm oil used as a biodegradable agent.

Model (5) has the values of the correlation coefficients between 0.9995 and 0.9999, dedi, and it correctly describes the rheological behavior of palm oil. The two models were obtained by linear and exponential fitting of the experimental data.

4. Conclusions

The article proposes two rheological models obtained by linear and exponential fitting of unadditive palm oil. The range of shear rates at which the oil was studied is between 3.3 and 120 s⁻¹ and temperatures between 40 and 100°C over the entire range of shear stresses. The rheological models found accurately describe the rheological behavior of palm oil.