Optimization of drying specification and protein losses of poultry litter during drying process using response surface methodology

Abstract

Poultry litter is a high protein by-product feed for ruminants but it needs to be dried for destruction of pathogens. Because drying affects the amount of protein, it is necessary to study the effects of different factors on protein content of poultry litter during the drying process. Energy consumption and drying effectiveness are other important factors in drying process. This research studied the effect of air temperature (60–80 °C), air relative humidity (8–18%), poultry litter depth (2–4 cm), and air velocity (2–3 m s–1) on drying effectiveness (Y1), specific energy consumption (Y2), and crude protein (Y3). The Box–Behnken design (BBD) was used to optimize the drying process. The results showed that the effects of the air temperature were more pronounced on the Y1 and Y2 compared to other variables and the air velocity was the most significant variable on the Y3. The minimum achieved values for the Y1 and Y2 were 1.67 min g-1 and 3.38 MJ kg-1, respectively and the maximum of the Y3 was 35.14%. These optimum values were attained at the air temperature of 80 °C, air relative humidity of 18%, poultry litter depth of 2 cm, and air velocity of 2 m s-1. Validation of experimental results with the empirical models was evaluated using adjusted R2 which were found to be 0.89, 0.92, 0.89 for Y1, Y2, and Y3, respectively. The comparison between the experimental and predicted values showed that the predicted values were in agreement with the experimental values in the uncertainty range of ±5%.