Dic: strain (as a particular quality)

■ WAY OF SAYING SOMETHING】[singular] formal. △an amount of a feeling that you can see in the way someone speaks, writes, paints etc

• a strain of bitterness in Young's later work.

■  You can use strain to refer to a particular quality in someone's character, remarks, or work.

• There was a strain of bitterness in his voice. 
• this cynical strain in the book.

■

1. 7(a). The tone, tenor, or substance of a verbal utterance or of a particular action or behavior: spoke in a passionate strain.
2. 7(b). A prevailing quality, as of attitude or behavior.

..... LDOCE, COBUILD, The American Heritage