First please note sweet potato and yams are not the same - they are different vegetables only distantly related.
It's consistently a source of confusion.
I write about this in a food tip as part of a food tip series I post for my local community garden.
Here is the link;
https://sites.google.com/site/qhuccommunitygarden/peter-s-food-tips-from-our-gardenPlease scroll down the page and click on the food tip; I for Ipomoea Bata (the scientific name for sweet potato).
I regularly consume sweet potato mostly of the common orange fleshed variety.
Recently I purchased a number of orange fleshed sweet potatoes that were on special.
To my surprise as this hasn't happened to me before is the sweet potatoes were bitter.
I thought this was very unusual but the sweet potatoes had probably been sitting around for months. I picked them from a selection of sweet potoates that were showing signs of spoilage. I will be a little more careful next time when selecting sweet potatoes.
Clearly it's not very pleasant biting into a sweet potato which is bitter when your expectation is that it will be sweet.