Kinetics of carbohydrate change during dehydration of d'Agen prunes
The kinetics of changes in the four major simple carbohydrates (i.e. glucose, fructose, sucrose and sorbitol) present in d'Agen prunes are followed during dehydration. The effect of using different drying temperatures (70-90°C) is investigated. During dehydration of the fresh plum three major regimes may be identified corresponding to the onset of a particular type of reaction. Firstly, acid hydrolysis of sucrose occurs with a rapid decrease in the amount of sucrose in the plum. This happens over a period of 1.5-6 h, depending on the temperature, and corresponds to a moisture loss of 40-50%. During this time, the amounts of both fructose and glucose (hydrolysis products) increase proportionally. The second stage is characterised by degradation of fructose and glucose via Maillard-type reactions with nitrogen-containing compounds in the plum. At this point there is little change in the amount of sorbitol which, because of its structure, does not undergo Maillard reactions. The third stage (occurring after 8-10 h at 80°C) is characterised by loss of all three monosaccharides, including sorbitol. This is due to the onset of caramelisation reactions, which only occur at very low moisture contents in this temperature regime. At the same time it is likely that Maillard reactions are still occurring. The onset of caramelisation at higher temperatures is not as clearly delineated from the start of Maillard reactions as it is at 80°C or lower.
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