Effect of processing conditions on physical properties of a milk fat model system: Microstructure |
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Authors: | M L Herrera R W Hartel |
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Affiliation: | (1) Department of Food Science, University of Wisconsin-Madison, 1605 Linden Dr., 53706 Madison, Wisconsin |
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Abstract: | The effect of processing conditions on the microstructure of three blends of 30, 40, and 50% high-melting fraction Mettler
dropping point (MDP)=47.5°C] in the lowmelting fraction (MDP=16.5°C) of milk fat was studied. The effect of cooling and agitation
rates, crystallization temperature, chemical composition of the blends, and storage time on crystalline microstructure (number,
size, distribution, etc.) was investigated by confocal laser scanning microscopy (CLSM). To improve resolution, a mix of Nile
blue and Nile red dyes was dissolved in the melted samples in proportions that did not modify the nucleation kinetics. Samples
were then crystallized by cooling (0.2 or 5.5°C/min) to crystallization temperature (25, 27.5, and 30°C). After 2 h at crystallization
temperature, a slurry was placed on a microscope slide and samples were stored 24 h at 10°C. During this period, more material
crystallized. Slowly crystallized samples (0.2°C/min) formed different structures from rapidly crystallized samples (5.3°C/min).
Crystals were sometimes diffuse and hard to distinguish from the liquid. Samples were darker as a result of this solid-mass
distribution. However, rapidly crystallized samples had well-defined crystals and seemed to be separated by a distinct liquid
phase. These crystals were not in touch with each other as was the case for slowly crystallized samples. Higher agitation
rates led to smaller crystal size due to enhanced nucleation. Larger crystals were formed when crystallization occurred at
higher temperatures. Storage time resulted in an increase of crystal size. Larger crystal size and structures with more evident
links had a more elastic behavior with higher elastic modulus E’. |
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Keywords: | Confocal laser scanning microscopy microstructure milk fat model system processing conditions |
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