Dehydration during storage affects carbohydrate metabolism and the accumulation of non-sucrose carbohydrates in postharvest sugarbeet roots

Authors: LAFTA, ABBAS - North Dakota State University; KHAN, MOHAMED, F.R. - North Dakota State University; Fugate, Karen

Submitted to: Journal of Agriculture and Food Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2020
Publication Date: 5/15/2020
Publication URL: https://handle.nal.usda.gov/10113/6936542
Citation: Lafta, A.M., Khan, M., Fugate, K.K. 2020. Dehydration during storage affects carbohydrate metabolism and the accumulation of non-sucrose carbohydrates in postharvest sugarbeet roots. Journal of Agriculture and Food Research. 2:100047. https://doi.org/10.1016/j.jafr.2020.100047.
DOI: https://doi.org/10.1016/j.jafr.2020.100047

Interpretive Summary: Sugarbeet roots produced in cold climates are stored in large piles prior to processing and are cooled by ambient winter air. These storage conditions dehydrate roots and increase the loss of sucrose during storage. How dehydration causes sucrose degradation to increase and how dehydration affects the production of impurities that are derived from sucrose and impede processing, however, are unknown. Therefore, research was conducted to determine the effect of mild and severe dehydration on the production and accumulation of glucose, fructose, and raffinose, three impurity compounds that are derived from sucrose, and of compounds that are involved in sucrose degradation. To obtain mildly and severely dehydrated roots that were typical of roots found in the interior and exterior of storage piles, roots were stored at high and low humidity levels for up to 28 days. Dehydration significantly altered the production and accumulation of raffinose, an impurity that dramatically slows factory operations. Under mildly dehydrating conditions, raffinose content increased by as much 3.5-fold; under severely dehydrating conditions, raffinose decreased by as much as 87%. Also affected by severe dehydration were compounds believed to have a role in the regulation of sucrose degradation. In contrast, glucose and fructose, collectively referred to in the industry as invert sugars, were largely unaffected by dehydration. From these results, it is concluded that raffinose production and accumulation are highly sensitive to root dehydration and that dehydration affects the concentration of compounds that regulate the rate of sugar degradation. Overall, this research provides new insight into the impact of environmental conditions within storage piles on raffinose accumulation and the mechanisms by which dehydration accelerates sucrose loss in stored sugarbeet roots.

Technical Abstract: Sugarbeet roots produced in cold climates are stored in large piles prior to processing and are cooled by ambient winter air. These storage conditions dehydrate roots, causing root respiration rate and sucrose loss in storage to increase. Dehydration likely alters carbohydrate metabolism due to a greater demand for respiratory substrates and may affect the accumulation of non-sucrose carbohydrates that impede processing. However, effects of dehydration on carbohydrate metabolism of postharvest sugarbeet roots have not been studied. Research, therefore, was conducted to determine the effect of mild and severe dehydration on concentrations of the non-sucrose carbohydrates, glucose, fructose, and raffinose, their biosynthetic precursors, and the concentrations of metabolites that are intermediates in respiratory sucrose catabolism through glycolysis and the TCA cycle. Roots were stored at 85 and 40% relative humidity for up to 28 d to generate mildly and severely dehydrated roots typical of roots in the interior and exterior of storage piles. Severe dehydration was associated with reductions in raffinose, galactinol, glucose 6-phosphate, fructose 6-phosphate, phosphoenolpyruvate, and pyruvate concentrations, and increases in inositol and malate concentrations, while mild dehydration was associated with increased raffinose concentration. Concentrations of other glycolytic and TCA cycle intermediates, as well as the invert sugars, glucose and fructose, were largely unaffected by dehydration. From these results, it is concluded that raffinose accumulation and biosynthesis is highly sensitive to root dehydration and that carbohydrate metabolism through glycolysis is altered by dehydration, presumably to provide increased substrates for respiration. Overall, this research provides new insight into the impact of environmental conditions within storage piles on raffinose accumulation and the mechanisms by which dehydration accelerates sucrose loss in stored sugarbeet roots.