谢谢各位的建议和讨论，我发现英文也有讨论，苦味不止来自枣核，也来自外皮

Dried jujubes often turn bitter after boiling due to compounds in the skin and seeds, specifically saponins and flavonoids. While they are naturally sweet, over-boiling or using dried, aged fruit can concentrate these bitter compounds. To reduce bitterness, remove the seeds, shorten boiling times, or wash them thoroughly.