Physiology & Behavior, Vol. 50, pp. 1181-1186. © Pergamon Press plc, 1991. Printed in the U.S.A.
0031-9384/91 $3.00 + .00
Starch Flavor: Apparent Discrimination Between Amylopectin and Amylose by Rats ISRAEL RAMIREZ
Monell Chemical Senses Center, Philadelphia, PA 19104-3308
R e c e i v e d 30 April 1991 RAMIREZ, I. Starchflavor: Apparent discrimination between amylopectin and amylose by rats. PHYSIOL BEHAV 511(6) 11811186, 1991.--Rats given a choice between dilute suspensions of corn amylopectin and corn amylose generally preferred amylopectin. The preference threshold for amylopectin was lower than the preference threshold for amylose (0.1% and 0.5%, respectively). Two sources of evidence indicate that the difference in preference for these two types of starch is due to an off-taste component in corn amylose rather than to an ability to discriminate between amylopectin and amylose per se: 1) rats given a choice of purified amylopectin and amylose from potato did not show a significant preference, and 2) aqueous extracts of amylose reduce preference for water and amylopectin, respectively. Extensive washing of corn amylose with ammonia-methanol, water and methanol did not completely remove the off-taste of corn amylose. Despite the difference in off-taste, rats trained to avoid amylopectin also avoided amylose. It is proposed that starch has two flavor components: a component due to starch itself that induces preference, and a component due to impurities that reduces preference. Taste
Flavor
Starch
Preference
Conditioned flavor aversion
HUMANS can taste only one class of carbohydrates, simple sugars. Rats, on the other hand, can sense sugars, maltooligosaccharides, and starch (6, 10, 11). Rats' preference and detection thresholds for starch and maltooligosaccharides are as low as or lower than thresholds for sucrose (6,8). Furthermore, rats can discriminate among these classes of carbohydrates. Thus rats trained to avoid sucrose do not avoid maltooligosaccharides (Polycose brand), and rats trained to avoid starch do not avoid maitooligosaccharides (7,8). Whether these are the only classes of carbohydrate flavors recognized by rats has not been ascertained. The present report evaluates rats' ability to discriminate between two different kinds of starch. The word starch refers to a heterogeneous family of glucose polymers linked by et-l,4 bonds and having high molecular weights (1,2). Perhaps the most important factors differentiating starch types are chain length and degree of branching. Amylose contains glucose units that are mainly joined in a linear fashion; it has a molecular weight of roughly 106 daitons. Amylopectin contains many or-l,6 bonds, which give the molecule a branched (tree-like) structure; it has a molecular weight of around 10 s daltons. Most natural starches are mixtures of amylose and amylopectin [see (1,2) for reviews]. It has recently been suggested that rats can discriminate between powdered amylopectin and amylose (11). This is surprising because the insolubility of uncooked starch should make it difficult for animals to sense chain length or degree of branching; there is no known way to get information about such interior details merely from the surface of starch granules (1,2). Several technical problems hamper acceptance of this proposed discriminative ability. The starches used in the previous study were obtained from different species of plants (amylopectin from maize and amylose from potato) (11); perhaps rats can
detect slight corn or potato odors present in these starches. The amylopectin used in the previous study was apparently raw starch from waxy maize (see below), whereas the amylose was probably fractionated starch, i.e., starch that had been treated with heat, solvents or both [see (1)]; perhaps rats prefer raw over fractionated starch. A better way to examine this issue is to use raw starches derived from a single species that differ in the type of starch present. Plant breeders have been able to produce varieties of corn that are high or low in particular starch types. Waxy maize starch is nearly pure amylopectin (4). Amylomaize has about 70% amylose and 30% short-chain amylopectin (1,13). The present series of experiments examined rats' ability to discriminate between these two starch types. Some experiments examined the preferences shown by untrained rats. Preference tests alone cannot establish whether the differences between the starch types are quantitative or qualitative; perhaps amylopectin has a more intense flavor than amylose. Therefore, one experiment assessed possible qualitative differences in starch flavor by training rats to avoid amylose and testing intake of amylopectin. GENERAL METHOD
Animals Rats were housed individually in hanging stainless steel cages, maintained on a 12/12-hour light/dark cycle at 23-27°C, and given food and tap water ad lib, except as indicated below. The food used, Purina Laboratory Chow (#5001), contains about 49% carbohydrate, most of which is starch. The rats were maintained in the laboratory for at least one week after arrival to permit them to acclimate to their surroundings before testing.
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0031-9384/91 $3.00 + .00
Starch Flavor: Apparent Discrimination Between Amylopectin and Amylose by Rats ISRAEL RAMIREZ
Monell Chemical Senses Center, Philadelphia, PA 19104-3308
R e c e i v e d 30 April 1991 RAMIREZ, I. Starchflavor: Apparent discrimination between amylopectin and amylose by rats. PHYSIOL BEHAV 511(6) 11811186, 1991.--Rats given a choice between dilute suspensions of corn amylopectin and corn amylose generally preferred amylopectin. The preference threshold for amylopectin was lower than the preference threshold for amylose (0.1% and 0.5%, respectively). Two sources of evidence indicate that the difference in preference for these two types of starch is due to an off-taste component in corn amylose rather than to an ability to discriminate between amylopectin and amylose per se: 1) rats given a choice of purified amylopectin and amylose from potato did not show a significant preference, and 2) aqueous extracts of amylose reduce preference for water and amylopectin, respectively. Extensive washing of corn amylose with ammonia-methanol, water and methanol did not completely remove the off-taste of corn amylose. Despite the difference in off-taste, rats trained to avoid amylopectin also avoided amylose. It is proposed that starch has two flavor components: a component due to starch itself that induces preference, and a component due to impurities that reduces preference. Taste
Flavor
Starch
Preference
Conditioned flavor aversion
HUMANS can taste only one class of carbohydrates, simple sugars. Rats, on the other hand, can sense sugars, maltooligosaccharides, and starch (6, 10, 11). Rats' preference and detection thresholds for starch and maltooligosaccharides are as low as or lower than thresholds for sucrose (6,8). Furthermore, rats can discriminate among these classes of carbohydrates. Thus rats trained to avoid sucrose do not avoid maltooligosaccharides (Polycose brand), and rats trained to avoid starch do not avoid maitooligosaccharides (7,8). Whether these are the only classes of carbohydrate flavors recognized by rats has not been ascertained. The present report evaluates rats' ability to discriminate between two different kinds of starch. The word starch refers to a heterogeneous family of glucose polymers linked by et-l,4 bonds and having high molecular weights (1,2). Perhaps the most important factors differentiating starch types are chain length and degree of branching. Amylose contains glucose units that are mainly joined in a linear fashion; it has a molecular weight of roughly 106 daitons. Amylopectin contains many or-l,6 bonds, which give the molecule a branched (tree-like) structure; it has a molecular weight of around 10 s daltons. Most natural starches are mixtures of amylose and amylopectin [see (1,2) for reviews]. It has recently been suggested that rats can discriminate between powdered amylopectin and amylose (11). This is surprising because the insolubility of uncooked starch should make it difficult for animals to sense chain length or degree of branching; there is no known way to get information about such interior details merely from the surface of starch granules (1,2). Several technical problems hamper acceptance of this proposed discriminative ability. The starches used in the previous study were obtained from different species of plants (amylopectin from maize and amylose from potato) (11); perhaps rats can
detect slight corn or potato odors present in these starches. The amylopectin used in the previous study was apparently raw starch from waxy maize (see below), whereas the amylose was probably fractionated starch, i.e., starch that had been treated with heat, solvents or both [see (1)]; perhaps rats prefer raw over fractionated starch. A better way to examine this issue is to use raw starches derived from a single species that differ in the type of starch present. Plant breeders have been able to produce varieties of corn that are high or low in particular starch types. Waxy maize starch is nearly pure amylopectin (4). Amylomaize has about 70% amylose and 30% short-chain amylopectin (1,13). The present series of experiments examined rats' ability to discriminate between these two starch types. Some experiments examined the preferences shown by untrained rats. Preference tests alone cannot establish whether the differences between the starch types are quantitative or qualitative; perhaps amylopectin has a more intense flavor than amylose. Therefore, one experiment assessed possible qualitative differences in starch flavor by training rats to avoid amylose and testing intake of amylopectin. GENERAL METHOD
Animals Rats were housed individually in hanging stainless steel cages, maintained on a 12/12-hour light/dark cycle at 23-27°C, and given food and tap water ad lib, except as indicated below. The food used, Purina Laboratory Chow (#5001), contains about 49% carbohydrate, most of which is starch. The rats were maintained in the laboratory for at least one week after arrival to permit them to acclimate to their surroundings before testing.
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