Physical properties of oils and fats
The analysis of the physical properties of oils and fats allows us to understand the behavior and characteristics of these elements, as well as their differences. For this, will be analyzed:
- The crystallization
- The melting point
- The viscosity
- The refractive index
- The density
- The solubility
- The plasticity
- The emulsifying capacity
Here we provide more detail on each of these.
Fats differ from oils in their degree of solidification at room temperature, since in these conditions the oils are in a liquid state (not crystallized) while the fats are in the solid (crystallized) state.
The proportion of crystals in fats have great importance in determining the physical properties of a product. Fats are considered solid when they have at least 10% of their crystallized components.
The fat crystals have a size between 0.1 and 0.5 μm and can occasionally reach up to 100 μm. The crystals are maintained by Van der Waalls forces forming a three-dimensional network that provides rigidity to the product.
An important feature of fat is its crystalline polymorphism, since mono-di and triglyceride crystallize in different crystalline forms (α, β, β’).
Form α (vitreous state)
- Appears when the fat solidifies by a quick method.
- The crystals formed are of the hexagonal type and are organized randomly in space.
- It occurs when the cooling is slow or if the tempering is carried out at a temperature slightly below the melting point, this form being the most stable of all.
- In the β form, tricyclic crystals are formed oriented in the same direction.
- The β form is typical of palm oil, peanut, corn, coconut, sunflower, olive and lard.
- It is produced from the tempering above the melting point of the α form.
- In the β-form, orthorhombic crystals are formed which are oriented in opposite directions.
- The β’form is typical of modified partial cottonseed oil, fats, fats and modified lard.
Both α, β and β’form have a melting point, an X-ray diffusion pattern and a refractive index.
The more double bond there is, the crystallization with which it tends to be liquid is hindered.
The melting point of a fat corresponds to the melting point of the β form, which is the most stable polymorphic form and is the temperature at which all the solids melt.
When short chain or unsaturated acids are present, the melting point is reduced.
The melting point is of great importance in the processing of animal fats.
The melting points of pure fats are very precise, but since fats or oils are made up of a mixture of lipids with different melting points we have to refer to the melting zone, which is defined as the melting point of the component of fat that melts at a higher temperature.
The viscosity of a fat is due to the internal friction between the lipids that constitute it. It is generally high due to the high number of molecules that make up a fat.
By increasing the degree of unsaturation the viscosity decreases, and when the length of the chain increases the fatty acids components also increases the viscosity.
The refractive index of a substance is defined as the ratio between the speed of light in air and in matter (oil or fat) that is analyzed.
Increasing the degree of unsaturation increases the refractive index, and when the length of the chain increases the refractive index also increases, that is why it is used to control the hydrogenation process.
As the temperature increases, the refractive index decreases.
The refractive index is characteristic of each oil and fat, which helps us to perform a quality control on them.
This physical property is of great importance when it comes to designing equipment to process grease.
Density decreases when fats dilate when going from solid to liquid
When the fats melt, their volume increases and therefore the density decreases.
For the control of percentages of solid and liquid in commercial fat, dilatometric curves are used.
Solubility has great relevance in the processing of fats.
Fats are fully soluble apolar solvents (benzene, hexane …).
Except for phospholipids, they are completely insoluble in polar solvents (water, acetonitrile). They are partially soluble in solvents of intermediate polarity (alcohol, acetone)
The solubility of fats in organic solvents decreases with increasing chain length and degree of saturation.
Phospholipids can interact with water because the phosphoric acid and the alcohols that compose them have hydrophilic groups.
Generally the surface tension increases with the length of the chain and decreases with temperature. Surface tension and interfacial tension decrease with ease with the use of surfactant agents such as monoglycerides and phospholipids.
It is the property that has a body to preserve its shape by resisting a certain pressure.
The plasticity of a fat is caused by the presence of a three-dimensional network of crystals inside which liquid fat is immobilized.
For a grease to be plastic and extensible there must be a ratio between the solid and liquid part (20 -40% solid state fat), the nets must not be tight and their crystals must be in α form.
The plastic fats act as a solid until the deforming forces that are applied break the crystal lattice and the grease passes to behave like a viscous liquid and therefore can be smeared.
The emulsifying capacity is the capacity in the water/oil interface allowing the formation of emulsion.
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