Thermomechanical analysis of baked products

Dynamic mechanical thermal analysis (DMTA or DMA) has been used to analyze glass transitions in foods (cheese, casein, gluten, soy isolates, starches and chocolate) [131,132]. 7 g is measured as the middlepoint of the change in elastic (E ) or loss (E ) moduluses, as well as the loss peak in tan 5, as a function of temperature. The modulus drop at 7g encompasses two or three orders of magnitude the DTMA determination of 7 g is therefore much easier than by DSC, since the relevant heat capacity change at 7g can be very small [132]. 

Dynamic mechanical thermal analysis can be used to produce a state diagram, which can be useful to evaluating textural changes in food processes and during the shelf life. It is commonly accepted [125,133-137] that the texture and the shelf life of the product are determined by the water content. 

Bread crumb texture during ageing and shelf-life is one of the most investigated phenomena in rheological analysis of food products. Many studies are based on steady measurements, mostly in steady compression mode. Some approached the dynamic measurements, both in shear, and in compressive oscillatory mode. As 

In DMA experiments, a sinusoidal stress is applied to the sample on increasing temperature.The response strain frequency is either in phase or out of phase with the stress depending on the viscoelastic properties of the system considered. These authors pre-cooled the sample down to -90 °C and then heated it up to 200 °C at 2 °C/min stress was applied at various frequencies (up to 20 Hz) in a three point bending test. As for the fresh sample, the trace presents a sharp drop of E and E and a peak of tan 5. This main transition is attributable to a first order transition (ice melt) that occurred simultaneously with a change in heat capacity (a second order transition, glass transition). 

Moisture loss below freezable water range was mainly responsible, according to Vittadini et al. [145], for the thermomechanical behaviour of prebaked pizza dough reheated with a microwave oven as detected with DMA in three-point bending mode. Thermal transitions were associated to pizza texture characteristics thermograms presented two transitions a transition was detected at 60 40°C and was found responsible for the leathery texture in microwaved pizza, while the one detected at 0 30 °C was the dominating transition in the soft pizza reheated by conventional oven. 

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