Is starch flavor unitary? Evidence from studies of cooked starch

Chimpanzee food preferences, associative learning, and the origins of cooking

A recent report suggested that chimpanzees demonstrate the cognitive capacities necessary to understand cooking (Warneken & Rosati, 2015). We offer alternate explanations and mechanisms that could account for the behavioral responses of those chimpanzees, without invoking the understanding of cooking as a process. We discuss broader issues surrounding the use of chimpanzees in modeling hominid behavior and understanding aspects of human evolution.

Baclofen-induced reductions in optional food intake depend upon food composition

Baclofen reduces intake of some foods but stimulates intake or has no effect on others. The reasons for these differences are not known. The present study examined effects of baclofen when composition, energy density, preference, presentation and intake of optional foods varied. Semi-solid fat emulsions and sucrose products were presented for brief periods to non-food-deprived rats. In Experiment 1, fat and sucrose composition were varied while controlling energy density. In Experiment 2A, schedule of access and the number of optional foods were varied. In Experiment 2B, the biopolymer (thickener) was examined. Baclofen reduced intake of fat and/or sugar options with different energy densities (1.28-9kcal/g), when presented daily or intermittently, and when intakes were relatively high or low. However, the efficacy of baclofen was affected by the biopolymer used to thicken the options: baclofen had no effect when options were thickened with one biopolymer (3173), but reduced intake when options were thickened with another biopolymer (515). Baclofen failed to reduce intake of a concentrated sugar option (64% sucrose), regardless of biopolymer. Based upon these results, caution is urged when interpreting results obtained with products using different thickening agents. Systematic research is needed when designing products used in rat models of food intake.

Fat emulsion composition alters intake and the effects of baclofen

Thickened oil-in-water emulsions are useful model foods in rat studies due to their high acceptance and similarity to foods consumed by humans. Previous work from this laboratory used oil-in-water emulsions thickened with a biopolymer blend containing starch. Intake and effects of baclofen, a GABA-B agonist that decreases fat intake and drug self-administration, were reported, but the contribution of starch was not assessed. In the present study, intake and effects of baclofen were assessed in rats using emulsions prepared with two fat types (32% vegetable shortening, 32% corn oil) and thickened with three biopolymer blends. One biopolymer blend contained starch and the other two did not. Daily 1-h intake of the vegetable shortening emulsion containing starch was significantly greater than the other emulsions. When starch was added to the emulsions originally containing no starch, intake significantly increased. Baclofen generally reduced intake of all emulsions regardless of starch content and stimulated intake of chow. However, effects were more often significant for vegetable shortening emulsions. This report: (1) demonstrates that products used to prepare thickened oil-in-water emulsions have significant effects on rat ingestive behavior, and (2) confirms the ability of baclofen to reduce consumption of fatty foods, while simultaneously stimulating intake of chow.

Great apes prefer cooked food

The cooking hypothesis proposes that a diet of cooked food was responsible for diverse morphological and behavioral changes in human evolution. However, it does not predict whether a preference for cooked food evolved before or after the control of fire. This question is important because the greater the preference shown by a raw-food-eating hominid for the properties present in cooked food, the more easily cooking should have been adopted following the control of fire. Here we use great apes to model food preferences by Paleolithic hominids. We conducted preference tests with various plant and animal foods to determine whether great apes prefer food items raw or cooked. We found that several populations of captive apes tended to prefer their food cooked, though with important exceptions. These results suggest that Paleolithic hominids would likewise have spontaneously preferred cooked food to raw, exapting a pre-existing preference for high-quality, easily chewed foods onto these cooked items. The results, therefore, challenge the hypothesis that the control of fire preceded cooking by a significant period.

Stimulus specificity in flavor acceptance learning

Infusing rats with 6% carbohydrate whenever they drank a test fluid stimulated their intake of 0.03-0.3% saccharin, 0.9% sodium chloride, and 0.5% oligosaccharide solutions within 1 to 3 days, but did not greatly increase intake of cherry-flavored water, 2% saccharin, 0.45% sodium chloride, monosodium glutamate, 0.025% sucrose octaacetate, or 0.4% maltodextrin, when food was available adlib. In general, it was more difficult to detect an effect with aversive stimuli, but it was possible to detect a significant effect with one such aversive flavor by either increasing the number of animals to permit detection of a small effect, or by using a preference test. For most stimuli, however, rats that had been trained with food available and then tested when food deprived showed significant increases in fluid intake, compared to rats that had been given the same flavor without carbohydrate infusions, even when plain water was infused in all rats during the food deprivation test. The following forms of evidence indicate that the increased intake is due to a form of Pavlovian conditioning. It tends to increase over repeated trials and to persist for a day after the carbohydrate infusions have been discontinued. Indeed, in one experiment, increased intake even persisted for a day when the infusion conditions were reversed, such that control rats were infused with carbohydrate and experimental rats were infused with water. Increased ingestion is attenuated by preexposure to the infusion in the absence of an appropriate taste solution. Increased ingestion is attenuated by preexposure to the taste solution in the absence of infusions (i.e., latent inhibition). Although increased intake is a form of associative conditioning, it is unlike malaise-conditioned taste aversions because taste aversions were conditioned to monosodium glutamate, cherry flavor, and saccharin with equal ease. The ease with which infusion increased intake is influenced by at least three different factors, degree of aversiveness, stimulus intensity, and rats' innate predisposition to acquire certain stimulus-response relationships more readily than others.

Stimulation of fluid intake by maltodextrins and starch

Maltodextrin solutions are more potent stimulators of fluid intake than are starch suspensions, even though these two substances are both glucose polymers. When rats were offered a choice of two fluids, one containing maltodextrin and the other containing the same concentration of starch, rats preferred maltodextrin; this preference increased over successive days for rats given 7.4% or 10% carbohydrate but not for rats given 1% carbohydrate. Rats offered fluids containing artificial flavors mixed with maltodextrin and starch acquired preferences for flavors paired with 10% or 7.4% maltodextrin but not for flavors paired with 1% maltodextrin. In an attempt to determine whether this greater preference for maltodextrin is due to an innate flavor preference, rats were given a choice of maltodextrin containing sucrose octaacetate vs. plain starch; these rats initially preferred starch but subsequently ingested more maltodextrin flavored with sucrose octaacetate than plain starch. Direct evidence for a postingestive factor was obtained by giving rats intragastric infusions of either maltodextrin, starch, or vehicle whenever they drank a dilute saccharin solution. Both carbohydrates stimulated fluid intake, but maltodextrin did so more rapidly (within 1 day rather than the 2 days of training required with starch infusions). These results indicate that some difference in the postingestive effects of maltodextrin and starch contributes to the greater intake-stimulating effect of maltodextrin. The postingestive factor responsible for the difference between maltodextrin and starch has not been identified, but it is not the amount of utilizable energy because rats prefer maltodextrin over starch even when the starch suspension contains more calories than does the maltodextrin solution. It is proposed that the postingestive factor differentiating these substances is rate of digestion.

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.

Glucose polymer taste is not unitary for rats

The present studies examined rats' responses to two maltodextrin preparations. One maltodextrin, a maltooligosaccharide, had an average chain length of 4.4 glucose units. The other maltodextrin was a maltopolysaccharide, having an average chain length of 22. In 24-h preference tests, rats strongly preferred 1% and 5% maltodextrin over water, regardless of the type of maltodextrin offered. When given a choice of two maltodextrins, rats preferred the maltooligosaccharide, but the degree of preference was influenced by the rats' previous experience with maltodextrins. Conditioned flavor aversion experiments were conducted to determine whether rats detect qualitative flavor differences between the these two maltodextrins. Rats trained to avoid one maltodextrin also avoided the other maltodextrin. Nevertheless, rats could be trained to drink maltooligosaccharide but avoid maltopolysaccharide; these rats showed no reliable tendency to respond to the intensity of maltooligosaccharide flavor. Therefore, maltodextrins of varying chain length differ more in flavor quality than in flavor intensity. This difference in flavor quality is not attributable to known sweet and starch flavors because neither maltodextrin contained much glucose and because rats trained to avoid the polysaccharide did not avoid starch. Because rats can discriminate between solutions containing only 0.5-1% maltodextrin, their ability to discriminate among carbohydrates must be far more acute than that of humans.

Rats discriminate between starch and other substances having a similar texture

Studies examined the contribution of textural factors to the ability of rats to sense starch. If rats sense the abrasive effects of starch suspensions, conditioned aversions to starch should generalize to substances having a similar texture. Two substances having a texture similar to that of starch were examined, polymeric dialdehyde (a product derived from starch) and inulin (a polymer of fructose). Rats were trained to avoid dilute suspensions of rice starch, potato starch, polymeric dialdehyde, or dahlia inulin, by injecting them with lithium chloride after they drank the test substance. Generalization tests revealed that rats trained to avoid rice or potato starch subsequently avoided corn starch, whereas rats trained to avoid polymeric dialdehyde or dahlia inulin did not avoid corn starch. Rats trained to avoid polymeric dialdehyde avoided this substance to a greater degree than did rats trained to avoid rice or potato starch. Rats trained to avoid dahlia inulin avoided this substance to a greater degree than did the rats in the other groups, but avoidance of dahlia inulin did not generalize to chicory inulin. Untrained rats showed little or no preference for dilute suspensions of polymeric dialdehyde or inulin, but did prefer starch over vehicle. It is concluded that tactile sensitivity does not play a large role in the perception of dilute starch suspensions.

Malaise can condition avoidance of high-viscosity fluids

Previous researchers have assumed that malaise-inducing drugs condition aversions to tastes more readily than to textural stimuli. The present report shows that rats given a single injection of lithium chloride after they have sampled a viscous fluid, subsequently showed greater avoidance of that fluid than did control rats. Rats learned this response even when the lithium chloride injection was given 60 min after they sampled the viscous fluid. The generalization gradient for viscous fluids is apparently very broad, because rats trained to avoid 11 or 95 cp fluids subsequently avoided 11 and 95 cp fluids to a similar degree. Nevertheless, this avoidance is based on textural cues because rats injected with lithium chloride after ingesting one viscous fluid subsequently avoided a wide variety of viscous fluids, including those made from carbohydrate polymers (algin, guar, xanthan), a modified carbohydrate (methyl cellulose), and a protein (methyl cellulose). However, the flavors of viscous gums may influence texture aversion.