After-cooking darkening: the affliction of cooked potatoes

The discolouration that blights cooked potatoes has plagued the potato industry for many years. This phenomenon, known as after-cooking darkening, is one of the most undesirable characteristics and has been reported from every potato-growing area in the world. It takes place, as the term implies, when cooked potatoes are exposed to air and produces coloured areas ranging from grey though blue and purple to black.

Although after-cooking darkening has no effect on the flavour or nutritional value of potatoes, it bestows an unpleasant appearance and is an unwelcome side effect of cooking. Boiled and steamed potatoes are most affected but fried, dehydrated and reconstituted-rehydrated potatoes are also marred.

Research has already established a strong genetic trait to after-cooking darkening but the underlying molecular mechanisms are unclear. So, scientists from Canada have compared the proteomic profiles of potato tubers to try and identify proteins that are implicated in after-cooking darkening.

Gefu Wang-Pruski and Fanming Kong from Nova Scotia Agricultural College, Truro, with Devanand Pinto from the NRC-Institute for Marine Biosciences, Halifax, and Patrick Murphy from Dalhousie University, Halifax, examined six sets of potatoes from a breeding family clonally propagated by the Agriculture and Agri-Food Canada Potato Research Centre.

The research led to a set of 30 proteins which correlated with after-cooking darkening

The research team proposed that wound response took place in conjunction with an increase in polyphenols (chlorogenic acid) synthesis, leading to darkening.

Although this is a small sample set, knowledge of the proteins that are associated with after-cooking darkening will help in the development of potato varieties that will be more resistant to darkening without affecting the other desirable properties.