Sucrose and fructose have qualitatively different flavors to rats

Previous studies suggest that rats might be able to discriminate between sucrose and fructose, but no previous study has examined this possibility in much detail. Rats were conditioned to avoid either sucrose or fructose by injecting them with lithium chloride when they drank these substances. Control rats were given the same injections but were not exposed to either sugar during training. After training, the rats were given a choice of fructose vs. sucrose. Data from control rats provided information about the relative taste intensity of the sugars. If the sugars possess only a single gustatory quality, control rats should prefer the sweeter sugar; under this assumption, sucrose appears to be two-four times sweeter than fructose. The two sugars share a common taste because rats trained to avoid sucrose avoided fructose when the fructose concentration was much greater than the sucrose concentration. Nevertheless, the two sugars are discriminable because, when the apparent sweetness of the sugars was matched, rats showed a greater aversion to the sugar they were trained to avoid. Aversions to sucrose and fructose also generalized to maltodextrins, but sucrose may have a somewhat greater maltodextrin flavor than does fructose. It is proposed that the biological function of maltodextrin taste is to allow animals to sense the ratio of glucose to fructose in foods.