An operational amplifier circuit with improved feedback factor is provided that includes an input impedance, an operational amplifier, and a current conveyor. Within a rotational range for a cylindrical workpiece to rotate from a present rotational phase to a target rotational phase in the retraction grinding, target grinding resistances in respective rotational phases are generated based on residual grinding amounts in the respective rotational phases of the cylindrical workpiece. HMB may be added to food products comprising a source of amino-nitrogen enriched with large neutral amino acids such as leucine, isoleucine, valine, tyrosine, threonine and phenylalanine and substantially lacking in free amino acids.