Objectives The roasting flavor quality of sweet potato were studied by analyzing the contents of main nutrition components and key volatile substances under different potassium (K) fertilizer application rates.

Methods A microplot field experiment was carried out in Agronomy Experimental Station of Shandong Agricultural University with two sweet potato cultivars of Yanshu 25 and Beijing 553. Four K fertilizer application level treatments were designed, including K₂O 0, 12, 24 and 36 g/m². The contents of main quality indices in root tubers at harvest and the contents of volatile substances after roasting were measured. The roasting flavor of root tubers was graded as well.

Results The application of K fertilizer increased the contents of soluble sugar, starch, soluble protein, vitamin C, β-carotene and cellulose of root tubers in different extents. The concentration of diacetone alcohol, phenylacetaldehyde, nonanoic acid, 4-vinylguaiacol, butyl butyrate, unknown-1, pentadecanoic acid, phytol, geranylgeranio were all increased, and flavor scores of the two varieties were consequently increased. The highest contents of nutrient components and compounds mentioned above all achieved in treatment K₂O 24 g/m². The result of stepwise regression and path analysis indicated that the key factors that improving the roasting flavor were the contents of fructose, glucose, amino acids, soluble proteins, amylopectin and cane fructose. The main volatile substances that deciding the roasting flavor included butyl butyrate, phenylacetaldehyde and unknown compounds.

Conclusions The application of potassium fertilizer is effective to increase the nutrient quality and roast flavor of sweet potatoes. In addition, proper K nutrition could increase the volatile substances that deciding the roast flavor like butyl butyrate and phenylacetaldehyde, which is also one of the reasons of better flavor. Under this tested condition, the optimum rate of K₂O is 240 kg/hm².