Evaluation of the spreadability/firmness of margarine by a forward extrusion test through a 6 mm hole.
Spreadability is the ease with which a sample spreads out over a surface in a thin even layer. The measurement of firmness (by a separate penetration test) and spreadability are highly correlated. Consequently, the spreadability values of a lipid rich sample will decrease with increasing viscosity of the oil, which also increases the measured hardness of the margarine. Also affecting hardness are the fatty acid composition and storage temperature. Saturated animal fats (as used in butter) result in a firmer product. Lower temperatures will also increase fat solidity.
The texture of margarine at room temperature is based upon a mixture of crystallized and liquid fat. The crystal network provides a rigid solid until a deforming stress is applied. When the deforming stress exceeds the network’s yield value, work softening begins. This network is also responsible for viscoelastic behavior.
Using the CT3 Texture Analyser and the forward extrusion cell, the firmness and spreadability of the sample can be quantified. By extruding the sample through a 6 mm hole, the sample is forced to flow at a high shear rate. The force required to perform the extrusion process is a measure of the yield stress in the sample. The firmness (hardness) and hardness work done values provide a way to evaluate spreadability. The higher the hardness and work-done values, the less spreadable and firmer the sample.
Figure 1 shows the force required to extrude a premium brand of margarine through a 6 mm hole at room temperature. The maximum force value on the graph is a measure of sample firmness. The area under the graph from the start of the test to the maximum force value is a measure of hardness work done. As the plunger withdraws from the sample, the negative part of the graph is generated. The maximum negative value is a measure of sample adhesive force (the maximum force required to overcome the attractive forces between the sample and the plunger). The area above this negative peak is a measure of sample adhesiveness (energy required to pull sample away from the plunger as the plunger withdraws from the sample).
Figure 2 shows the force vs. distance for the extrusion of a premium brand of margarine through a 6 mm hole. This is an alternative option for displaying the results. The maximum force value on the graph at the target distance (20 mm) is a measure of sample firmness. The area under the graph from the start of the test to the target distance (20 mm) is a measure of hardness work done. The negative peak is a measure of adhesive force and the area above this peak a measure of sample adhesiveness.
When a trigger force of 50 g has been attained at the sample surface, the plunger proceeds to penetrate the sample at a test-speed of 1 mm/s during which time the sample is deformed and compressed to pack into the decreasing space available. When the sample is compacted, the force is seen to rapidly increase and the extrusion begins. As the sample is extruded, a plateau on the graph is observed which is the maximum force required to continue flow.
The maximum force value is a measure of sample firmness; the higher the firmness value, the less spreadable the sample. The area under the graph from the start of the test to the maximum force value (see Figure 1) or target distance point (see Figure 2) is a measure of hardness work done.
When the probe withdraws from the sample at the return speed of 1 mm/s, a negative peak is generated. The negative peak is a measure of adhesive force (the force required to overcome the attractive forces between the sample and the plunger surface with which it comes into contact); the more negative the value, the more “sticky” the sample. The area above the negative peak is a measure of sample adhesiveness (the energy required to break plunger/sample contact as the plunger withdraws from the sample).