Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2008
Publication Date: 4/2/2008
Citation: Haagenson, D.M., Klotz, K.L., Campbell, L.G. 2008. Impact of storage temperature, storage duration, and harvest date on sugarbeet raffinose metabolism. Postharvest Biology and Technology. 49:221-228. doi:10.1016/j.postharvbio.2008.02.007 Interpretive Summary: Raffinose is a carbohydrate impurity that often accumulates in sugarbeet roots late in the production season and during postharvest storage. During processing, raffinose slows processing and increases the loss of sucrose to molasses, thereby increasing the cost of sugar production. Although storage temperature has been shown to effect postharvest raffinose accumulation, little is known about the biological mechanisms that affect raffinose accumulation. In this manuscript, raffinose accumulation and the expression and activity of enzymes that contribute to raffinose synthesis and degradation were examined in stored sugarbeets with respect to harvest date, storage temperature and storage duration. Results demonstrate that raffinose accumulation is affected by a complex interaction of harvest date, storage temperature and storage duration that differentially affected raffinose accumulation in the above-ground root crown and the below-ground portion of the root. Raffinose accumulation was generally unrelated to the expression or activity of any biosynthetic or degradative enzyme activity, suggesting that these activities are not likely to have a major role in determining raffinose levels in sugarbeet roots during storage.
Technical Abstract: Raffinose negatively impacts sugarbeet (Beta vulgaris L.) processing by decreasing extractable sucrose yield and altering sucrose crystal morphology which reduces filtration rates and slows processing. Although increased raffinose concentrations have been observed during cold storage, the physiological and biochemical mechanisms associated with raffinose accumulation in sugarbeet are poorly understood. The objective of this study was to characterize the impact of storage temperature, storage duration, and harvest date on raffinose metabolism. Field-grown sugarbeets were harvested 7 Sept., 27 Sept., and 26 Oct. 2004, and stored for 2, 10, and 18 wk at 2 or 6°C. Raffinose concentrations were approximately double at two weeks of storage, nearly 3-fold higher at 10 wk, and decreased slightly at 18 wk. Delaying harvest date increased raffinose concentration at harvest (0 wk), but decreased concentrations at 18 wk of storage. Storage temperature did not affect crown raffinose concentrations, but root tissues stored at 2ºC had 19% higher raffinose concentrations than those stored at 6ºC. Biosynthetic or catabolic enzyme activities accounted for less than 15% of the variation in raffinose content in storage, although a small positive correlation (r=0.28) between raffinose synthase activity and raffinose concentration in root tissues was observed. Galactinol synthase was highly expressed in tissues collected in late October and at 2 wk of storage, and alpha-galactosidase activity increased 55% in roots stored for 18 wk at 6ºC. The results of this study demonstrate that the factors contributing to sugarbeet raffinose accumulation in storage are complex and that raffinose metabolism is impacted by storage duration, harvest date, and storage temperature.