Why is some lettuce (iceberg, romaine) bitter? Is it age? Just the crop?
Lettuce develops a bitter flavour in response to stress during its growth cycle, e.g. not enough water, or fertilizer, or temperatures that are too high. It also gets bitter as it matures and starts to flower, so bigger lettuces are more likely to be bitter.
For me, any greens with a jagged edge and greens that have a red tinge are bitter to my taste.
I believe bitter is the natural state of plants. It discourages animals from eating the leaves of a plant in the wild. It takes farmers a lot of effort to breed the bitterness out.
So, to paraphrase seinfeld, "It’s a gamble going in’?