Modified eggshell powder using thermal treatment and its application in Ca-fortified dog biscuits

Due to the high global consumption of eggs, eggshell has become as one of the top domestic, agricultural and industrial wastes. This study determined eggshell characteristics after boiling at 95 °C and steaming at 121 °C, with additional heat treatments using hot air at 200 °C, microwaving at 900 W and infrared at 1050 W. Boiling in water for 60 min inhibited spoilage and pathogenic microorganisms that was the equivalent of steaming at 121 °C for 15 min. Scanning electron microscopy revealed that heat treatments on dried eggshell power modified the pore size and the accumulation of particles on the powder surface. From the X-ray diffraction pattern, all eggshell powder samples presented a peak at 29.40° demonstrating a crystallographic lattice of calcium carbonate with crystallinity in the range 90.20–91.05%. The calcium releasability of the control sample was 205.17–208.40 mg/L. Further treatment using hot air for 10–20 min increased the calcium releasability of the boiled and steamed eggshell powders to 219.95–225.50 and 230.35–305.20 mg/L, respectively while the microwave treatment for 2 min increased the calcium releasability of the boiled and steamed eggshell powders to 230.85 and 244.60 mg/L, respectively. The infrared treatment did not improve the calcium releasability of the sterilized eggshell powders. Up to 2% eggshell powder could be added to the dog biscuit dough. The fortified calcium biscuits contained 507.12 mg calcium/100 g of biscuit, while the Ca-to-P ratio was 1.94:1, which is within the recommended range for dog food.