Black Spots on Peeled Potatoes

Once potatoes are peeled, dark coloured spots (often black) may be seen.
These black spots result from tissue damage which causes a discolouration reaction. The black spots are only visible after peeling as they are below the surface.

Factors involved in the development of black spots:

• potato variety – some varieties are more susceptible than others eg Anya
• dry matter – potatoes with high dry matter are more susceptible to black spots eg Setanta.  Low dry matter potatoes eg. ‘Kestrel’, ‘Nadine’ and ‘Picasso’ bruise less easily
• soil nutrients – low potassium increases blackspot susceptibility
• size and shape of potato
• condition of soil at harvest – dry stony soil will cause more bruising , which leads to black spots
• rough handling at picking time causes more damage, bruising and black spots

How black spots develop:

Once the potato is damaged, biochemical reactions occur in the cells, which include a colour reaction.  It takes 10 – 20 hours for the black spots to start to appear, and several days for them to fully develop.

But this colour reaction can be slowed by:

a. cooling the potato down – as the enzymes work quicker at high temperatures & are inactivated at low temperatures

b. high humidity – this minimises water loss from the potatoes and reduces black spots

To reduce the occurrence of black spots:

• treat potatoes very gently when handling them
• if the potato is stored correctly the bruise will not develop

Bruises in potatoes are like those in apples, they are only in the damaged cells.  The bruise does not spread.  But stored potatoes with very high dry matter can develop black spots during storage, especially if they are allowed to dry out.

Black spots on potatoes are unsightly so it is worth trying to prevent them.