Why do cold cooked potatoes sometimes taste bitter?

I have always enjoyed eating cold cooked potatoes. However, in recent years, I’ve noticed some potatoes develop a bitter aftertaste when left whole overnight. Some have a green tinge on the surface, which is a warning. These things seem especially noticeable in so-called “new” potatoes.

Answer:

Nothing is as constant as change, and this is no less true for potatoes. Cooking, cooling, and reheating all cause significant changes to the structure, altering the greening process and also how quickly the potatoes are digested and thus our blood-sugar response after eating them.

The “greening” occurs when potatoes are exposed to light in the field, on supermarket shelves or at home. Light stimulates the formation of green pigments, indicating that chlorophyll is being formed. There is nothing inherently wrong with chlorophyll; it is a harmless, tasteless natural compound that gives green plants their colour. Chlorophyll is like a tiny solar panel inside plant cells, capturing sunlight and using that energy to help the plant make its own food.

Chlorophyll is not a health concern but it is something of a “canary in the mine shaft”. It indicates there will also likely be an increase in the presence of glycoalkaloids in the potato, particularly one toxin called solanine.

The potato plant produces toxins such as solanine to protect itself from damage by insects, bacteria and fungi. It is one of the potato plant’s natural defences, and this compound is probably responsible for the bitter taste you notice in green potatoes. That taste is a warning to humans, because in high doses, solanine is toxic. Fortunately, reports of serious illness from solanine ingestion are rare.

Nevertheless, if a potato has visible greening, peel or cut off the green areas and any sprouts (eyes) before cooking. The cooking process will destroy the enzyme mechanism that produces chlorophyll and solanine, so once cooked, potatoes can no longer produce greening or bitter solanine.

However, cooking does not remove any existing green chlorophyll or solanine present in raw potatoes. In your case, it’s possible the greening was subtle enough that you didn’t notice it before cooking, so picked up the bitter taste and colour only after cooking.

Certainly, your observation that new potatoes are more prone to greening and the accompanying bitter taste is true. According to Potatoes New Zealand, new potatoes are more prone to these things.