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136 PROCEEDINGS OF INTERNATIONAL SCIENTIFIC CONFERENCE ON APPLIED BIOTECHNOLOGYextraction process. The effect of dried black pepper powder size on oil recovery yield is shown in Figure 4. Figure 4. Effect of size of raw material on essential oil recovery yield of black pepper, (A) Oil volume (mL), (B) Oil yield (%). * indicates statistically significant differenceIt is seen in Figure 4A that decreasing the size of black pepper powder from 2-3 to 0.5 mm resulted in the increase of oil volume from 5.2 to 7.6 mL for 240 min extraction. This level was surprisingly decreased to 5.3 mL when the size of black pepper powder further decreased to 0.2 mm (Figure 4A). Similarly, the oil yield obtained from the material with the medium size of 0.5 mm resulted in the highest level of 2.18% (Figure 4B). For the other black pepper sizes of larger than 0.5 mm or smaller than 0.5 mm, the yields were determined in the range of 1.49-1.92% which were significantly lower than that of the 0.5 mm size material (paired p values < 0.05). 3.4. Effect of raw material size The effect of material to water ratio on essential oil yield is shown in Figure 5.