Influence of temperature, air flow rate and slice thickness on egg drying rate and protein content under forced convection

Abstract

The availability of egg products for consumption is hindered by spoilage and breakages during processing. Forced convection drying serves as one of the methods of solving these challenges. However, the influence of temperature, airflow rate and slice thickness on forced convection drying of eggs is inadequately documented. Therefore, drying of eggs was investigated under forced convection process. The mode of airflow and the food material was the continuous concurrent type. Raw eggs were boiled at 90oC for 17 minutes to harden them. The hardened eggs were deshelled and cut into 10 mm thick slices without separating the egg yolk and egg white. Forced convection drying of sliced hardened eggs was done at thicknesses of 10 mm, 20 mm and 30 mm. Drying temperatures were set at 35oC, 40oC and 45oC. Airflow rates were set at 0.09 m³/s, 0.12 m³/s and 0.15 m³/s. Protein content for each sample was determined using Kjeldahl method. L9 Taguchi orthogonal array technique was used to determine the optimal combination of airflow rate, drying temperature and egg slice thickness that gave the highest drying rate and protein content. Nine thin layer-drying models were fitted to the experimental data to determine the model that predicted drying process with minimal variations between experimental and predicted results. An analysis of variance at 5% level of significance showed that air drying temperature had significant influence on the drying rate and protein content. The drying rate increased with increased air drying temperature. The drying rate increased from 0.67 g/g min to 0.75 g/g min with rise in temperature from 35oC to 45oC respectively. Protein content dropped from 54.6% to 47.5% with increase in temperature. Increasing drying airflow rate increased drying rate from 0.67 g/g min to 0.76 g/g min at 0.09 m³/s and 0.15 m³/s. The protein content decreased from 54.4% to 48.3% at 0.09 m³/s and 0.15 m³/s. Increasing airflow rate had significant influence on protein content (p<0.05). Thickness increment led to decrease in drying rate from 0.72 g/g min to 0.68 g/g min at 10 mm and 30 mm. Lower slice thickness had lower protein content of 47.9% while the highest thickness had protein content of 53.8% due to high heat transfer in the lower thickness that thermally affect proteins. Increasing thickness had insignificant influence on protein content (p>0.05). The optimal combinations for drying rate were slice thickness of 10 mm, flow rate of 0.15 m³/s and temperature of 45oC. Slice thickness of 30 mm, airflow rate of 0.15 m³/s and temperature of 35oC were the optimal combinations for protein content. Page, Modified Page, and Aghbashlo et al. were found suitable for predicting drying processes of boiled eggs. However, Page model was most superior of the three with the highest R2 of 0.9991, lowest x2 and RMSE of 0.0001 and 0.0012 respectively.