The most likely culprit for a bitter taste in your tzatziki is the cucumbers. Cucumbers, particularly the seeds and skin near the stem end, can contain compounds called cucurbitacins which create a bitter flavour. Here are some tips to avoid bitterness in your tzatziki: 1. Choose the Right Cucumbers: Opt for varieties known for their low bitterness, such as English or Persian cucumbers. 2. Taste Test: Before grating the whole cucumber, taste a small slice. If it's bitter, consider using another one. 3. Remove Seeds & Ends: Remove the seeds and the stem end of the cucumber before grating, as these areas tend to be more bitter.