Correlating Microstructure with Texture of Particulate Biopolymer Gels

Doctoral thesis, 1995

The microstructure of particulate biopolymer gels has been studied to comprehend the effect of microstructure on textural properties. Particulate structures have been characterized at different levels by using light microscopy, scanning electron microscopy, and transmission electron microscopy. Various microstructural parameters have been defined for particulate structures. Methods for quantification have been developed and particulate structures have been quantified. Two different systems have been analysed: dispersions of starch particles and particulate protein gels. The microstructure of particulate gels has been related to texture, as measured instrumentally and by sensory perception. Two protein systems were chosen as particulate gel model systems, (-lactoglobulin and whey protein, WPC. Both proteins form gels on heating and particulate gels are formed around the iso-electrical point (∼ pH 5.2). Fine-stranded gels were formed at high pH, above pH 6, and at low pH, below 4, when no salt was added. In order to comprehend the size concept in microstructural terms, such as individual particles or clusters, particles in a dispersion were analysed. The size of industrially produced spherical starch particles was estimated by using light microscopy in combination with image analysis. Methods for size estimation were analysed taking clustering and orientation of clusters into account. Quantification of microstructural parameters facilitated the structural analysis of a large amount of samples and the statistical correlation of structure with the perceived texture. A stereological approach was used to estimate the size of pores and particles in the particulate gels. The size was estimated by the star volume. Three-dimensional features, such as strand characteristics and clustering phenomena, have been defined and quantified in an objective manner by the trained human experts on a microstructural panel. The microstructural data was correlated to textural data. It was found that the microstructure can explain texture, as measured instrumentally or perceived sensorily. Microstructural parameters of particulate protein gels can be divided into two groups: overall network characteristics (pore size and particle size) and strand characteristics ('mode of aggregation'). The test principles of analysing texture can also be divided into two groups: destructive tests and non-destructive tests. The texture analysed by using a large force, either when perceived sensorily (chewing or the appearance of a newly cut surface) or when measured instrumentally (tensile tests), was sensitive to the overall network dimensions. The texture analysed by using a small deformation, either as perceived sensorily by pressing lightly with the forefinger or as measured viscoelastically, was shown to be sensitive to the strand characteristics of the particulate gels.