Visualization of intracellular ice crystal formation using x-ray micro-computed tomography / Nurzahida Mohd Zaid

Thesis (Master of Engineering Bioprocess) -- Universiti Malaysia Pahang - 2010

Bibliography : p. 157-158

This study demonstrates the capability of X-ray Micro-computed Tomography (XMT) technique to characterise the internal ice crystal microstructure of freeze-dried samples (strawberry). The method requires the fruit being frozen under specific freezedrying temperature of the samples to remove frozen water before scanning to indicateice crystal and internal structure of the samples. Results are presented for the 2-D ice crystals formed within the samples. The dendrite spacing (size, volume and width) of ice crystals is related to freezing condition of the samples. Only 1% small intercellular voids distribution of the total 99% volume in fresh strawberry samples as compared to 72% void or pore distribution in freeze-dried samples. Other than that, the average width size of ice crystal for two different operating freezing (-20oC and -80oC) temperatures for strawberry samples was 0.15 to 0.29 mm and 0.29 to 0.40 mm respectively. The overall results indicate that the ice crystal distribution within samples were diverse with the axial distance of the material from its cooling surface. The ice crystal size is bigger when the fruits were far from the cooling surface. At the investigated conditions, the comparisons with air-drying technique had been done on jackfruit and the study on those fruits had been limited to image visualization for validation and nutrient analysis. Samples dried in the freeze dryer were seen to retain their shape much better than air dried samples which underwent shrinkage and change in color. Samples dried in freeze dryer possessed less dense structures and consequently displayed more favorable rehydrated textural properties than the air-dried equivalents. Furthermore the total phenolic (TP), and ascorbic acid (AA) of the fresh and freezedried samples had been investigated. This method had preserved 3.65 mg/mL AA concentration in freeze-dried strawberry samples with ultrasonic pre-treatment and no AA had been detected in the air-dried strawberry samples. Total phenolics (TP) concentrations was also evaluated and compared to the TP content in fresh samples that were frozen and stored at -20oC. The average TP content of frozen, freeze-dried and airdried strawberries are 270.5, 231.0, and 28.7 mg/100 g of fresh weight, respectively.