Butter is a multiphase emulsion comprising of crystalline fat, fat globules, and a dispersed aqueous phase in continuous oil phase. In addition to taste, customers perceive texture, spreadability and appearance as important characteristics of butter. Spreadability and hardness are inversely related to each other, and are two of the most measured properties of butter. Both properties are strongly temperature-dependent, and are largely influenced by the cooling rate after churning and variation in each region or season due to the diet of cows.
Rheology can be used to characterize and optimize the texture of butter. The shear modulus, which is in relation with the stiffness of a product, is measured as a temperature function by oscillatory testing. Yield stress is the stress required for the butter to deform plastically, its spreadability. Advanced rheometers, like the Kinexus rotational rheometer, possess sophisticated axial capabilities to investigate the tack and hardness of butter.
A comparison between the melting and spreading characteristics of a normal butter, made only from milk fat, and a spreadable butter containing vegetable oil, is performed using rheology. Vegetable oil minimizes the melting temperature and stiffness of the spreadable butter after it is taken from fridge.
An evaluation of butter samples was carried out at 4 - 35°C, using a small-amplitude oscillatory test and axial test. A Kinexus rotational rheometer, coupled with a Peltier plate cartridge and a roughened plate-plate measurement system, was utilized to make measurements, involving sequences that are pre-configured in rSpace software. To subject the samples to consistent thermal and loading protocols, a standardized loading sequence was utilized. A single frequency strain-controlled temperature ramp test was carried out at 4 - 35°C at 2°C/minute rate, utilizing strain in the linear viscoelastic region. To determine tack and hardness, normal force response was measured using a fresh sample with 1 mm dimension on an axial compression-decompression cycle at a temperature of 4°C.
Findings and Discussion
Small-amplitude oscillatory tests are non-destructive, and indicate deviations in a complex microstructure with respect to time or temperature without deconstructing the microstructure. Parameters such as G', the elastic (storage) modulus, and G", the viscous (loss) modulus, are commonly measured in correspondence with the stiffness of solid-like and liquid-like components of the samples, where the total stiffness is expressed in terms of the complex modulus, G* = v(G'2 + G"2).
The phase angle (ä) denotes the phase difference between measured stress and applied strain, and is used to quantify the structure with regard to its viscoelastic characteristics. Phase angle of a liquid-like material is greater than 45°, whereas that of the solid-like material is below 45°.
Figure 1. Temperature melting profiles of spreadable (bottom) and normal butter (top) performed at 2°C/minute ramp rate.
The single frequency oscillatory temperature ramp results for both of the butter samples are shown in Figure 1. At a temperature of 4°C, the stiffness of the normal butter is higher than that of spreadable butter with respect to G'. This explains the usability of the spreadable butter direct from fridge, as lower G' values are expected in correspondence with low yield stress. Phase angles of both samples are very low, less than 10°, denoting the solid-like nature of both samples while in fridge, where the spreadable butter has more elasticity.
The increase in temperature leads to a reduction in modulus values, which indicates softening of the structure, largely associated with crystalline fat melting. The modulus value decrement is highly significant for the normal butter, where there is an approximate reduction in G' value by 10 MPa at 4 - 20°C, in comparison to 0.5 MPa for the spreadable butter. This melting transition is in correspondence with a peak in the phase angle, which is more pronounced for normal butter at a slightly higher temperature than the the spreadable butter.
Figure 2 illustrates the axial compression-decompression test performed on the samples, which involves squeezing the samples between two plates and then separating the plates for a continual recording of normal force response. Deformation and yielding of the samples during compression are related to their spreadability and hardness. The sticky or tacky characteristic of the samples during decompression will indicate the tendency of the butter to stick to the knife when spreading.
Figure 2. Schematic diagram of axial measurements made using Kinexus rotational rheometer.
Normal force profiles for both samples with respect to axial deformation are illustrated in Figure 3. The force needed to compress the normal butter by 1 mm is 30 N, for spreadable butter it is just 6N, indicating that the spreadable butter can be more easily yielded and deformed than the normal butter. The peak tensile force generated by the normal butter during decompression is -10 N. However, sharp peaks were not observed for the spreadable variant. This indicates that normal butter has a high tendency of sticking to the knife while spreading.
Figure 3. Axial measurements of compression and tack performed using the Kinexus rotational rheometer.
Rheometers can be used for various measurements to characterize and compare different butter samples according to texture, spreadability and microstructure. They can also be used to complete various tests, including; single frequency oscillatory test to analyze changes in viscoelasticity and stiffness in relation to temperature, and axial test to determine tackiness and hardness of butter during use.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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