Culprit

"Hello," Ning greeted the owner of the mill, the man called Miller. He had decided to find out if his hunch was correct or not.

"Hello. Do you need something?" the old man asked. 

"Yes, just some answers," Ning said. "Is your mill incapable of separating the ryegrass from the wheat?"

The old man was taken aback for a moment and finally realized that he had seen Ning just a few minutes ago. 

"You were with that little miss from the Golden Flour, weren't you? What do you want? Do you want to drag our name down? It was an honest mistake," the old man said. 

Ning walked up to the old man and put his arms around his shoulder. he asked," Was it?" 

The old man's eyes changed and he replied, "No, it wasn't a mistake."

Ning's eyes brightened; his hunch was right. 

"Then you did it knowingly?" he asked.

"Yes, master. I personally added the ryegrass flour to the wheat this morning," the old man said with no hesitation.