Ordinary potatoes and sweet potatoes have a very similar consistency when raw, yet sweet potatoes roast in about half the time. What is the reason for this?
• Without laboratory analysis any explanation is speculative, but given that potatoes and sweet potatoes have roughly the same density and thermal conductivity, a few variables will be relevant.
The first is the nature of the cell walls of the internal tissue. More resilient walls rich in cellulose could retain their structure longer. A second, more likely, cause would be intercellular cement with more pectins that would hydrolyse and soften faster. The third, perhaps most probable explanation, would be the difference in the organelles in the cells that contain the starch: the amyloplasts. It’s possible that starch in the sweet potato amyloplasts is less resistant to heat and richer in soluble sugars.
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An interesting question arises from the sweet potatoes that I knew as a child; they were not the fleshy varieties of today, but larger, lumpier and above all, starchier. They were known as “wurgpatat”, meaning “choke sweet potato” in Afrikaans, because the floury texture was harder to swallow, rather like a baked potato. I preferred them, but they have gone out of fashion. I wonder whether they took longer to roast?
Jon Richfield, Somerset West, South Africa