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Abstract not available

Effect of water migration on physico-chemical properties of bread during storage was investigated. When breadcrumbs were stored with crust, moisture and aw decreased significantly, whereas for those stored without crust, these properties remained unchanged (no moisture loss from crumb). Addition of glycerol retarded amylopectin recrystallization but increased breadcrumb firmness suggesting that breadcrumb firming in this case was not directly related to amylopectin recrystallization. Biopolymers in bread might be locally dehydrated (osmotic dehydration) in the presence of hygroscopic solutes. Stress-strain curves showed the typical sigmoidal shape described by three-parameter empirical model with C1, C2, and C3 representing scale factor, prominence of the shoulder and densification level, respectively. C1 correlated well (r 2 = 0.96) with firmness, whereas recoverable work decreased over the first 3 days of storage but did not show the effect of glycerol and crumb moisture loss effect. The self-diffusion coefficient (D) of water (by PFG-NMR) in fresh breadcrumbs increased with increasing glycerol content at the same moisture content suggesting an increased water partitioning with glycerol as compared to a starch and gluten matrix and more water was free to diffuse. Although dependent on moisture, the diffusion coefficient did not proportionally change with moisture content during storage suggesting other factors (water-biopolymer interactions, geometry, restricted diffusion in complex domains etc.) might be related. 13C CP/MAS NMR peak intensity indicating relative amount of 13C solids was found to increase with increasing storage time in all cases. A lower 13C CP/MAS NMR peak intensity was observed in breadcrumb stored without crust than that of breadcrumb stored with crust at given glycerol content because less disordered-to-ordered chain rearrangement occurred. This confirmed that an amylopectin ordered structure (observed by DSC) was formed contributing to rigid crystalline solids ( 13C CP/MAS NMR). Data confirmed that the hardening effect of added glycerol during storage was unrelated with the amylopectin recrystallization. Therefore, the hardening of bread during storage in this case was related to other changes such as local dehydration and stiffening of the amorphous domains. Future investigation is warranted on the molecular origins of these changes. This could lead to further development in engineering physical properties of many bakery products.

Analytical instruments based on ultrasonic spectroscopy have been recently developed to measure the size distribution and droplet concentration of emulsions. Nevertheless, there are a number of problems that currently limit the practical application of these instruments to food emulsions: (i) ultrasonic measurements often have to be made at different temperatures; (ii) many food emulsions are concentrated; and (iii) many food emulsions are flocculated. The objective of this study was therefore to investigate the influence of temperature, droplet concentration and droplet flocculation on the ultrasonic properties of emulsions, hence developing ultrasonic techniques that could be applied to food emulsions. In addition, this study also aimed at testing the efficiency of the newly developed ultrasonic scattering theories (UST) for describing ultrasonic properties of flocculated and concentrated emulsions. The influence of temperature was investigated by measuring ultrasonic velocity and attenuation coefficient of emulsions of different compositions and droplet sizes at 5 to 50°C. The ultrasonic properties of emulsions were sensitive to temperature as well as to composition and droplet size, and could be described by UST. The effect of droplet concentration (0–50 wt%) and diameter (0.2–0.58 μm) on ultrasonic properties was determined in non-flocculated oil-in-water emulsions. The traditional UST gave poor predictions of ultrasonic properties of emulsions at high droplet concentration, small droplet size and low ultrasonic frequency, while an extended UST, which incorporated the thermal overlap effects, gave much better predictions under the same conditions. The effect of flocculation on ultrasonic properties of emulsions was studied over a range of frequency (1–120 MHz) using 5% oil-in-water emulsions, flocculated by either depletion or an electrostatic mechanism. A new theory that took into account the thermal overlap effects could well explain the ultrasonic properties of flocculated emulsions. As flocculation increased, the attenuation coefficient decreased at low frequency due to the thermal overlap effect, while at high frequency the attenuation coefficient increased due to the scattering effect. The usefulness of ultrasonic techniques in characterizing actual foods was tested using salad dressing and chicken. The ultrasonic techniques could provide valuable and accurate information about the composition of these foods.