All-Purpose Potatoes are those whose starch content is “medium” — 15-17% starch.
The main feature promoted about them is that, in theory, with them you can have just one potato on hand for multiple uses.
Some people say that while they’re safe to use for anything — nothing disastrous will happen — that nothing spectacular will, either. Your potato chunks for potato salad will stay together, but only just — and your mashed potato will be heavy and pasty.
Some say they have no particular flavour, either, in general.
Disregard those who tell you that All-Purpose Potatoes “mash well.” Some of the most indifferent mash you’ve had has probably been made with “all-purpose potatoes.” And if ever you’ve been almost reduced to tears by having potatoes crumble apart on you for your potato salad, it’s likely because you were sold a bag of potatoes just labelled “table potatoes” or “all-purpose.”
Some of the better All-Purpose Potatoes are Desirée and Wilja.
Ilam Hardy is an All-Purpose Potato but it can be better for some purposes rather than others depending on when it was harvested. If Ilam Hardy Potatoes were harvested early, they are waxy, and therefore better as a boiled potato or in salads. If they are harvested late, they are much better for mashing and baking, because more of their sugars will have had time to convert to starch.
While this is true of some All-Purpose Potatoes, it’s not true of all of them.
Some All-Purpose Potatoes develop more definite qualities after long storage. For instance, during storage, they may tip more to the floury side, and become better at least for mashing and baking, but sadly, worse as a boiling potato.
North American potato producers liked to sell potatoes as “all purpose” to avoid complicating things in consumers’ minds, and because they felt the consumer just wanted to have one potato in the house for everything. Perhaps they also wanted to brand their potatoes geographically — for instance, “Idaho Potatoes” — instead of having the consumers think first of floury or waxy. Now, though, some potato producing regions are realizing that the downside to that approach is that their potatoes become a “generic” commodity, attracting those kinds of generic prices. Consequently, some North American producers are beginning to actually sell their potatoes as distinct-purpose potatoes — “for baking”, “for mashing”, “for salads” — that can command a higher price. So, even though they may not be changing their labelling with the express purpose of making their potatoes more “end-user friendly”, North Americans trying to cook with potatoes still come out ahead, anyway.
European potato breeders may refer to All-Purpose Potatoes as “Cooking Type B”.