Effects of Blanching Method on Sugar and Protodioscin Contents of White Asparagus Spears

In Japan, white asparagus spears are grown using two blanching methods to block sunlight: the traditional soil-mound and the film-cover methods. The objective of this study was to investigate the influence of the film-cover and soil-mound methods on components of white asparagus spears related to flavor, especially sweetness and bitterness. We investigated the effects of the two blanching methods on sugar (fructose, glucose, and sucrose) and protodioscin (a major furostanol saponin in white spears) contents in white spears by conducting a spring field survey and an experiment using winter-forcing cultures. No significant differences were observed in sugar content or composition in spears cultivated by the two methods in either the field survey or the experiment. Protodioscin content tended to be higher in spears blanched by the soil-mound method than in spears blanched by the film-cover method in both the field survey and the experiment. These results suggest that differences in the flavor of white spears grown using the two blanching methods are caused mainly by bitterness associated with saponin content, and saponin content may be influenced by soil-borne stresses.