Thermal and physical properties of bakery products

This article reviews the measurement techniques, prediction models, and data on thermo-physical properties of bakery products: specific heat, thermal conductivity, thermal diffusivity, and density. Over the last decade, investigation has focused more on thermo-physical properties of nonbread bakery products. Both commonly used and new measurement techniques for thermo-physical properties reported in the publication are presented with directions for their proper use. Data and prediction models are tabulated for the range of moisture content and temperature of the bakery products.

Dynamic oscillation measurements of starch networks at temperatures above 100 degrees C

Small deformation oscillatory studies were performed on wheat flour paste with a starch content of 75.4%. Work focused on temperatures above 100 degrees C in an effort to seek molecular understanding of such high-temperature processes as bakery operations which are characterised by evaporation of water. The moisture content of the sample decreased from about 32% at 100 degrees C to 6.5% at 130 degrees C. Viscoelastic spectra produced a sigmoidal profile with a disproportionate viscous element also seen in the glass transition of semiamorphous synthetic polymers and high sugar/polysaccharide mixtures during cooling. It is argued that the loss of water upon heating reduces the available free volume between neighbouring chain segments, thus generating a high-density thermoplastic melt suspending granule fragments. The configurational rearrangements of the disordered chains contribute mainly to an energy-dissipating process, as observed in the vitrification of cooled high-solids systems. The equation of Williams, Landel, and Ferry was modified with a 'moisture term' in order to describe the temperature function of viscoelasticity.

First- and second-approximation calculations in the relaxation function of high-sugar/polysaccharide systems

The distribution function of Maxwellian relaxation times (phi) was derived from the small- deformation dynamic properties of high-sugar/agarose, /deacylated gellan and /high-methoxy pectin mixtures. First-approximation calculations of phi employed the time derivative of the experimentally measured storage and viscous moduli, with the two traces converging at the theoretically predicted slope of - 0.5. Second-approximation calculations were based on phi, as derived by the first approximation, being a simple power function of relaxation times (tau(-m)). The slope m was measured at various points and used to derive correction factors for shifting the relaxation function to the second approximation. Thus, values of phi calculated from G' and G" were brought into satisfactory agreement, particularly in the recorded portion of the glass transition region. Once the function phi is accurately determined, it can be readily used for calculations of other viscoelastic properties.