FRUIT AND VEGETABLES: TUBERS
Diagram
Two starchy tubers from different plant families
Potato
A stem tuber from the nightshade family. Starchy, mild, with thin skin and many eyes. The white potato.
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Sweet potato
A root tuber from the morning glory family. Sweeter, with orange flesh rich in beta-carotene, the source of vitamin A.
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The key point
Both are starchy tubers used the same way in cooking, but they come from different families and differ in sugar and vitamin A.
Figure 1: Potato is a stem tuber from the nightshade family; sweet potato is a root tuber from the morning glory family. Drawn in our own style from Sinha et al., Handbook of Vegetables and Vegetable Processing, potato and sweet potato chapters. Note: this source is chapter level.
Introduction
Potato and sweet potato are both starchy tubers, and both are boiled, baked, fried and mashed in the same ways, but they are different plants. A potato is a stem tuber from the nightshade family, the family of tomato and aubergine. A sweet potato is a root tuber from the morning glory family. That difference in family goes with differences in sugar, in colour, and in vitamin A: the orange flesh of sweet potato is rich in beta-carotene, which the body turns into vitamin A.
Sinha et al. cover potato and sweet potato in separate chapters of the Handbook of Vegetables and Vegetable Processing. This article follows that book, and it states plainly that the source is chapter level.
Quick comparison
Table 1: Potato compared with sweet potato
| Aspect | Potato | Sweet potato |
|---|---|---|
| Plant family | Nightshade family | Morning glory family |
| Part eaten | Stem tuber, a swollen underground stem | Root tuber, a swollen root |
| Skin | Thin, with eyes, the buds of the stem | Thicker, smoother, with no eyes |
| Flesh colour | White, cream or yellow | Orange, purple or white, with orange the most common |
| Taste | Mild and starchy, not sweet | Sweeter, because it holds more sugar |
| Sugar | Low in sugar | Higher in sugar than potato |
| Vitamin A | Little, because the flesh is not orange | Rich in beta-carotene, which the body turns into vitamin A |
| Starch | High in starch | High in starch, with more sugar alongside it |
| Cooking | Boiled, baked, fried, mashed, roasted | Boiled, baked, fried, mashed, roasted, in the same ways |
| Storage | Kept cool and dark, away from light, or it turns green | Kept cool and dry, and bruises more easily than potato |
Scientific explanation
Definition
A potato is the stem tuber of the potato plant, a member of the nightshade family, and it is a swollen underground stem with eyes, which are the buds of the stem. A sweet potato is the root tuber of the sweet potato plant, a member of the morning glory family, and it is a swollen root with no eyes. Both are starchy tubers, and both are cooked in the same ways, but they come from different families and different plant parts.
Working principle
A potato tuber is a stem, so it carries eyes that can sprout into new plants, and its skin turns green in light as it makes chlorophyll and the bitter compound solanine. A sweet potato tuber is a root, so it has no eyes, and its orange flesh holds beta-carotene, the orange pigment that the body turns into vitamin A. Both store starch as their energy reserve, and cooking softens the starch and makes it digestible. The sweet potato holds more sugar alongside its starch, so it tastes sweeter when cooked.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
The books used here name no single standard laboratory procedure for telling the two apart. The distinction is made from the form: eyes on the skin mark a potato as a stem tuber, and orange flesh with no eyes marks a sweet potato as a root tuber. The books give no worked food example with numbers for this comparison, so none is given.
Factors affecting readings
Variety changes the flesh colour, the sugar and the starch in both tubers. Growing conditions and storage change the sugar: cold storage can sweeten potato, and light turns potato skin green. Cooking method changes the texture of both, and overcooking makes both fall apart.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Potato is used for chips, mashed potato, roast potato and baked potato.
- Sweet potato is used for baked sweet potato, fries, mashes and pies.
- Potato is used in soups, stews and curries as a starchy base.
- Sweet potato is used where a sweeter, orange fleshed tuber is wanted.
- Both are used as staple starchy foods around the world.
Advantages and limitations
- Potato: advantage: mild and versatile, and it suits every cooking method. Limitation: little vitamin A, and the skin turns green in light.
- Sweet potato: advantage: rich in beta-carotene, the source of vitamin A, and sweeter in taste. Limitation: bruises more easily than potato, and its sweetness does not suit every dish.
- Potato: advantage: stores well in the cool and dark. Limitation: sprouts from its eyes in storage.
FAQ
Are potato and sweet potato related?
No. Potato is from the nightshade family and sweet potato is from the morning glory family, so they are not close relations.
Why is sweet potato orange?
Its flesh holds beta-carotene, an orange pigment that the body turns into vitamin A.
Can sweet potato replace potato in a recipe?
It can, but it is sweeter and softer, so the dish will taste and feel different.
Why does potato turn green?
Light makes the skin form chlorophyll and solanine, so potatoes must be stored in the dark.
References and further reading
- N. K. Sinha et al. (eds.), Handbook of Vegetables and Vegetable Processing, Wiley, 2010 (Google Books page read).
