不是薄荷心生凉不是柠檬心生酸
[bù shì bó hé xīn shēng liáng bù shì níng méng xīn shēng suān]
It reads as 'Neither mint nor lemon can cool or sour my heart.' Mint and lemons often metaphorically stand for refreshing or sharp experiences respectively which can also symbolically refer freshness (joy), and sharpness(sadness). So it expresses a state free from external emotional influences.