Do potatoes become toxic if not eaten right away?


Do potatoes become toxic if not eaten right away?

It is not true that potatoes should be eaten as soon as they are cooked and that, even if kept in the fridge, they become toxic.

The origin of the false belief of having to eat freshly cooked potatoes is probably due to the fact that sometimes, after cooking, the potatoes can darken. However, their possible gray-black color does not change their flavor or nutritional value. This phenomenon does not always occur and is independent of the type of cooking: whether they are boiled, fried or roasted.
It is just a simple chemical oxidation reaction: the iron contained in potatoes reacts with chlorogenic acid, a phenolic substance that defends potatoes from harmful microorganisms, has antioxidant and metabolic regulation properties, and is contained in green coffee, in various types of fruit, in aubergines and tomatoes.
In the presence of oxygen, the iron-chlorogenic acid complex oxidizes, giving the potatoes a dark color, perhaps unpleasant to the eye of the consumer, but absolutely not dangerous.
The iron content present in potatoes depends on the soil where they are grown, while that of chlorogenic acid is determined by the variety of the vegetable itself. Other substances present in potatoes, such as citric acid, can contribute to making them less dark, but their content depends on the environmental and cultivation conditions.
Therefore, due to all these variables, it is not always guaranteed that potatoes turn black, especially if stored well after cooking.
More than the dark portion that occurs as a result of the oxidation reaction, it is much more important to pay attention to so-called "anti-nutritional" factors, such as glycoalkaloids, which can be present in all plants as they are produced by the plant itself as mechanism of protection and defense and which can be toxic to human health, especially if ingested in excessive quantities. In the potato, glycoalkaloids are represented by α-caconine and α-solanine. In the tubers, that is, in the potatoes we eat, they are concentrated above all in the peel, in particular, in the tubers exposed to the sun and in the old ones, wrinkled and with many sprouts.
Common cooking techniques, such as boiling or microwaving, do not seem to make substantial changes in the concentration of glycoalkaloids, while frying, thanks to the high temperature reached during cooking, manages to decrease their concentration. The only sure way to reduce the amount of these toxic substances is to remove the skin from the tuber, both on the raw product and after cooking. In any case, in the commercial varieties the glycoalkaloid content found is less than 100 mg / kg or in any case less than 200 mg / kg of fresh pulp, a precautionary limit set by the FAO / WHO commission of experts.
We can, therefore, rest assured in consuming potatoes, even the leftover ones, as long as they are properly stored:

  • when they are still raw, they must be kept in a dark and dry place, and must be consumed before they germinate
  • once cooked, they must be stored in the refrigerator being careful not to let too many days pass before consuming them to avoid the growth of microorganisms or bacteria

To be totally safe and avoid toxicity risks, it is better to remove the peel!

1. Wang-Pruski G and Nowak J. Potato after-cooking darkening. American Journal of Potato Research. 2004; 81: 7–16

2. Wang-Pruski G. The Canon of Potato Science: 47. After-cooking Darkening. Potato Research. 2007; 50: 403–406

3. Crocco, S. Potato sprouts and greening potatoes: Potential toxic reaction. JAMA. 1981; 245: 625


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