Evaluation of the consistencies of full-fat and low-fat Greek yogurt by back extrusion using a mesh probe.
Yogurt is a popular fermented dairy product world wide. There are currently different forms of yogurt in the market, such as stirred, set, frozen, and liquid yogurt. Processing variables in the manufacture of yogurt control its textural properties. Variability in total solids content (casein or fat), heat treatment, pH, cultures, and incubation temperature produce different textures of yogurt. An increase in total solids content, particularly casein and whey protein, forms firmer yogurts and the tendency for whey separation is minimised. Yogurt firmness, however, decreases with increasing fat levels due to the fat globules disrupting the protein network.
Yogurts can be classified as pseudo-plastic materials (contains a yield stress that needs to be exceeded for flow to be initiated). As such, depending on the form of yogurt, the product can be either a visco-elastic fluid (as with stirred and drinking yogurt) or a visco-elastic solid (as with set yogurt). The viscosity of yogurt can increase by the use of stabilisers that also function to prevent whey separation in fruit yogurts.
The CT3 back extrusion test using the mesh probe can determine yogurt consistency and flowability. As the mesh probe compresses the sample over a specified distance, the sample is deformed and compressed to pack more tightly into the remaining space available (under the descending mesh probe). When the sample becomes more compact with limited air pockets, the measured force increases and the extrusion commences.
The back extrusion test is generally useful for viscous products. In addition, these products can also be tested directly in their packaged containers from the production line.
Samples can be tested from their original containers or from a standard size back extrusion container (40 mm in diameter) 75% full. The samples must be tested immediately after removal from storage (refrigerator) so that test temperatures are consistent for all samples. Ensure that the sample surface is level so that, when the test commences, the mesh probe will be in full contact with the sample surface without having penetrated the sample to any extent.
Note: The chosen distance for extrusion depends on the depth of the container. Typically a chosen depth should not exceed 75% of the depth of the container; otherwise the instrument may be overloaded if the probe comes into contact with the base of the container.
Fluctuations on the graph may be observed as the probe compresses the sample. This is the result of air pockets in close proximity to the probe. Generating air pockets should therefore be kept to a minimum when filling the extrusion container.
When optimising test settings, the hardest sample is better tested first in order to anticipate the maximum testing range required for subsequent samples. This will ensure that the force capacity covers the range for other future samples.
Graphs show the consistencies of full-fat and low-fat yogurt using a mesh probe.
Figure 1 shows the force required to extrude the contents of full-fat and low-fat Greek style yogurt through and over the probe. Tests have been carried out at room temperature.
Figure 2 shows force vs. distance for the mesh extrusion of full-fat and low-fat Greek style yogurt tested at room temperature.
When a trigger force of 10 g has been attained at the sample surface, the mesh probe proceeds to compress the sample to a depth of 25 mm (or other specified distance). During this time, the sample is deformed and compressed to pack more tightly into the remaining space available (under the descending mesh probe). When the sample becomes more compact with limited air pockets, the force increases and the extrusion commences. When the force increases to a maximum point, a plateau is observed indicating the force required to continue extrusion. This maximum force is a measure of sample firmness and the area under the peak a measure of consistency (work done to hardness1). The higher the force value, the more viscous the sample. The low-fat Greek yogurt has a higher consistency and firmness than the full-fat Greek yogurt.
As the probe returns to its starting position, the negative load values on the graph result from back extrusion. This gives an indication of the adhesiveness/cohesiveness and resistance of the sample. The maximum negative force on the graph indicates sample adhesive force; the more negative the value the more “sticky” the sample. The area under the negative part of the graph is known as the adhesiveness (the energy required to break probe sample contact) and can give an indication of the cohesive forces of the molecules within the sample. The higher the value, the more energy required to break the probe/sample contact as the probe withdraws from the sample. Full fat yogurt is more adhesive than low fat yogurt.
The back extrusion test can be used as a quality control tool to ensure consistency in production and also in R&D and Product Development in monitoring the effects of formulation or process control changes.
Mean hardness, work done and adhesive values for two samples of full fat and low fat yogurt are shown below: