ABSTRACT

The objective of the study is to, evaluate and optimize the process parameters for effective drying of two high energy forage plant species indigenous to South-east Nigeria for hay making. Two forage grasses which includes Andropogon tectorum (Gamba grass) and Pennisetum purpureum (Elephant grass) was identified and selected based on their proximate compositions.   The forage grasses were dried from 70.9 - 10 ± 3 % to 60 – 10 ± 1%  (w.b) for  Elephant and Gamba grass respectively using sun, hot air oven (40 -70 ^oC) and microwave oven (180 to 720 W) drying methods. In hot air oven drying method, increasing the drying temperature, rapidly increased the drying rate from 31.5 to 64.5 kg water/kg dry matter/hour for Elephant grass (Pennisetum purpureum) and from 5.8 to 51.0 kg water/kg dry matter/hour for Gamba grass (Andropogon tectorum) within the first one hour of drying. Similarly, increasing microwave power increased drying rate from 915.6 to 2790.2 kg water/kg dry matter/hour and 189.5 to 1432.3 kg water/kg dry matter/hour within the first 3 minutes of drying Elephant grass (Pennisetum purpureum) and Gamba grass (Andropogon tectorum), respectively. The result of analysis showed that specific energy consumption in hot air oven drying ranges between 12.2 to 14.7 kWh/ kg water, and 24.1 to 29.1 kWh/kg water for Elephant grass (Pennisetum purpureum) and Gamba grass (Andropogon tectorum) varieties, respectively. While the specific energy consumption in microwave drying is between 22.9 to 51.5 kWh/kg water and 33.2 to 81.5 kWh/kg water for Elephant grass and Gamba grass varieties respectively. Again, specific moisture extraction rate in hot air oven drying is between 0.65 to 0.94 kg water/kWh and 0.28 to 0.38 kg water/kWh for Elephant grass (Pennisetum purpureum) and Gamba grass (Andropogon tectorum) varieties respectively. Whereas, specific moisture extraction rate in microwave drying is between 6.3 to 10.0 kg water/kWh, and 2.2 to 8.9 kg water/kWh for Elephant grass (Pennisetum purpureum) and Gamba grass (Andropogon tectorum) varieties respectively. Also the nutrient content analysis showed that increasing drying temperatures and microwave radiation generally increased the dry matter from 29.4 to 94.5%  and crude fiber from  9.2 to 55.1%, whereas, the crude protein content decreased from 10.8 to 5.3% in the dried samples. Again, whereas total colour difference showed sharp variation with temperature changes, changes in microwave power level showed slight variation. There was a decreasing trend of rehydration ratio when the hot air oven drying temperatures and microwave power levels were increased. Using response surface methodology (RSM), reliable prediction equations were developed, and the following optimum drying conditions were indicated: maximizing drying rate resulted in a drying time of 1.4hours, drying temperature of 68.8  at a drying rate of 52.5 kg water/kg dry matter/hour for hot air oven drying method, whereas it gave a drying time of 0.083hours, drying power level of 720W and drying rate of 416.1 kg water/kg dry matter/hour in microwave method. The optimization of crude fiber (%) in hot air oven method resulted in a dry matter of 92.4, drying temperature of 70  with a crude fiber of 51.7%, while in microwave method, optimum conditions were achieved at drying power level of 360W, with 47.6% crude fiber and 93.13% dry matter values. Optimal value of 10.8 % crude protein in hot air oven was reached at drying temperature of 28.5^oC, and dry matter of 39.8 (%), while 11.03% crude protein was reached at drying power level of 59.5W and 49.50% dry matter in microwave method.