Changes in sugar and starch concentrations in parsnip (Pastinaca sativa L.) during root growth and development and in cold storage

Summary

Low-temperature sweetening of parsnip (Pastinaca sativa L.) roots, which may occur both pre-harvest and post-harvest, is a well-known phenomenon, but is poorly understood. ‘White King’ parsnip roots were analysed over two seasons (2008 and 2009) during field growth and over six months cold storage at 1ºC for their dry matter content, as well as their starch, sucrose, glucose, fructose, and malto-oligosaccharide (MOS) concentrations and for their changes in amylolytic enzyme activities (2009 only). In 2008, when the air temperature fell below 5ºC for 12 d in the last 8 weeks before harvest, the concentration of starch decreased significantly (P ≤ 0.05) and the concentration of total sugar increased significantly. In 2009, when temperatures did not fall below 5°C during the last 8 weeks before harvest, the concentrations of starch (339 + 17 mg g^–1 DW) and total sugars (168 + 12 mg g^–1 DW) did not change significantly. However, in both years, during 24 weeks in cold storage at 1ºC, starch levels were almost completely depleted, whereas sucrose concentrations and, to a lesser extent, MOS concentrations increased at least two-fold. Glucose and fructose concentrations also increased significantly during 24 weeks in cold storage, but to a much lower level than sucrose or MOS. Total amylolytic activity and α-amylase activity doubled during the first 8 weeks in cold storage, whereas β-amylase activity did not change significantly. As products of α-amylase activity, changes in MOS concentration paralleled the changes in α-amylase activity. The accumulation of sucrose and MOS may raise the culinary quality of cold-stored parsnip.