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The physicochemical properties of the starches from three varieties of sweet potatoes were investigated. The crystalline structures of Koganesengan and Minamiyutaka starches were of the A type and that of Norin‐2 starch was of the Ca type. Their amylograms showed pasting temperatures of 73–75°C and maximum viscosities of 500–580 BU. 3‐Methyl‐1‐butanol lowered their pasting temperatures without altering their maximum viscosities. The amylose and organic phosphorus contents of the starches were 17.2–19.0% and 117–139 ppm, respectively. The amyloses comprised branched molecules of average degree of polymerization 3400–4400 with 10–13 chains on average and were hydrolyzed 72–73% by β‐amylase. The main amylopectins had average chain lengths of 21–22, β‐amylolysis limits of 55–56% and organic phosphorus contents of 117–144 ppm, of which 75–81% was attached to C–6 of the glucose residue and the remainder to C–3. Another type of amylopectin (10–15% of the starch), which had higher iodine affinity, was isolated.

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Both amyloses and amylopectins from sago starches with low (280 BU, low‐v) and high (735 BU, high‐v) viscosities on amylography differed in molecular structures. The low‐v amylopectin (average chain length, c.I., 22) was a smaller molecule with a slightly higher amount of long chains than the high‐v amylopectin (c.I., 22). The number‐and weight‐average degrees of polymerization of the low‐v amylose were 2490 and 5090, respectively, which were half those of the high‐v amylose. The low‐v amylose had a lower molar fraction of branched molecules with a shorter inner chain length than the high‐v amylose, but both branched molecules had a similar number (17–19) of chains on the average. Both starch granules had the same crystalline type (Ca) and amylose content (24%).