Humans and other animals learn to associate flavours with aspects of consuming foods,
which can result in acquired liking or aversion for that flavour. Two main processes of
learning have been proposed to be critical: flavour-flavour (FFL) and flavour-nutrient
(FNL) learning. This thesis addresses two main research questions primarily in the
context of FNL; firstly, does liking for a flavour acquired through FNL persist once
energy has been removed? It has been suggested that acquired flavour liking is resistant
to extinction, but there are conflicting results within the human literature, which has
concentrated on FFL. Studies One and Two explored this but failed to demonstrate
acquired liking, although they tentatively suggested that extinction did not occur for
acquired liking as pleasantness ratings remained stable after removal of energy. The
second research question investigated whether liking acquired through FNL was
modulated by expectations. Study Three manipulated viscosity of a yoghurt drink to
determine if this impacted upon FNL, as thicker products have been shown to signal
higher energy content. Expectations were influenced by viscosity, but with little impact
upon pleasantness ratings and little evidence that FNL was enhanced. Studies Four and
Five used labelling to influence expectations regarding a yoghurt-based breakfast.
Study Four found that when no information was provided, liking changed as predicted
from FNL. Contrary to prediction, when congruent information about energy content
was provided, this acquired liking was not demonstrated, and ratings remained stable
across sessions. Study Five did not replicate this finding, with pleasantness ratings in
line with FNL literature. Addition of a hedonic label actually resulted in decreased
pleasantness of the breakfast over time, suggesting a contrast effect with the flavour not
delivering what was expected. Methodological limitations are recognised, with
measurement of liking and contingency awareness discussed as potential explanations
for weaker findings.

Flavor-flavor and flavor-nutrient associations can modify liking for a flavor CS, while flavor-flavor associations can also modify the sensory experience of the trained flavor. Less is known about how these associations modify behavioral responses to the trained CS. To test this, 60 participants classified as sweet likers were divided into five training conditions with a novel flavor CS. In the flavor-flavor only condition, participants consumed the target CS in a sweetened, low-energy form, with energy (maltodextrin) but no sweetness added in the flavor-nutrient only condition and both energy and sweetness (sucrose) in the combined flavor-flavor, flavor-nutrient condition. Comparison groups controlled for exposure to the CS, and repeat testing. Training was conducted in a hungry state on four non-consecutive days. To test for acquired changes in evaluation and intake, the flavor CS was processed into a low-energy sorbet, which was evaluated and consumed ad libitum on test days before and after training. Liking for the flavor CS increased only in the sucrose-sweetened condition, but intake increased significantly in both high-energy conditions. In contrast, rated sweetness of the sorbet increased in both sucrose-sweetened and aspartame-sweetened conditions. These findings suggest that liking changes were maximal when flavor-flavor and flavor-nutrient associations co-occurred, but that behavioral changes were specific to flavor-nutrient associations.

Monosodium glutamate (MSG) is known to enhance liking for the flavor of savory foods, but whether associations between flavors and effects of MSG lead to changes in subsequent liking and intake for the flavor alone is unclear. To test this, 32 volunteers evaluated and consumed a novel savory soup with no added MSG before and after four training sessions where the same soup was consumed either unchanged (Control) or with added MSG. The addition of MSG during training increased both pleasantness and savory character of the soup and resulted in a larger increase in rated pleasantness of the soup in the MSG-trained relative to control condition when the soup was re-evaluated Post-training without MSG. There was also a significant increase in voluntary soup intake Post-training after the soup had been paired with MSG but not in the Control condition, and rated hunger increased more after tasting the soup Post-training in the MSG-trained but not Control condition. These findings demonstrate that co-experience of a savory flavor and MSG can result in increased subsequent liking and intake for the flavor in the absence of MSG, and possible explanations for how MSG reinforces learning are discussed.

In flavour¿flavour learning (FFL), repeated co-experience of a novel flavour and a hedonically valanced flavour can lead to changes in liking for the novel flavour. Some studies suggest that restrained eaters are insensitive to FFL, while disinhibited eaters may over respond to hedonic stimuli. Here we assessed FFL through associations between food-related odours and sweet and bitter tastes to explore further how eating attitudes influence FFL. 52 women, pre-selected to be high or low on the restraint and disinhibition factors from the Three-Factor Eating Questionnaire (TFEQ), evaluated two novel odours orthonasally before experiencing retronasal pairings of one odour with sucrose and the second with quinine before orthonasal re-evaluation of the odours. At post-training, overall liking increased for the sucrose-paired odour but this effect was greater in women with high scores on the TFEQ disinhibition factor. Liking for the quinine-paired odour decreased equally in all groups. The sucrose-paired odour was rated as sweeter, and quinine-paired odour more bitter at post-training and these acquired sensory qualities were unaffected by restraint or disinhibition factors from the TFEQ. Overall, these data found no evidence of impaired FFL in restrained women, but instead found greater responsiveness to hedonic qualities of sucrose in women scoring high on the disinhibition factor, consistent with suggestions that this factor identifies individuals with heightened hedonic sensitivity to food.

The concept of learned satiety (LS) suggests that associations between the sensory quality and post-ingestive effects of foods may lead to acquired control of meal-size. Although a recent study appeared to support LS since participants learned to eat more of a flavoured cereal with lower energy density (ED) after repeated experience, suggesting that they adjusted voluntary intake to ensure adequate energy was consumed, the large serving portion used in training may have lead to over-satiation. To investigate this further, groups of 12 men were assigned to one of four conditions based on the trained serving portion (150 or 300 g) and presence or absence of cues to differentiate high and low ED versions. In the absence of sensory cues, neither mass consumed nor rated pleasantness differed between high and low ED conditions either before or after training, resulting in greater energy intake in the high ED condition. When sensory cues differentiated ED, intake increased significantly post-training in both the high ED condition trained with the small portion and low ED condition trained with the large portion, and flavour pleasantness changed similarly. Moreover hunger increased significantly after the food was tasted in both conditions where intake increased. These data provide further evidence that learning can moderate meal-size dependent on energy content, but suggest that these changes are driven by changes in flavour liking rather than LS.

It is well established that the hedonic quality of the flavour of food (i.e. palatability) is an important determinant of short-term food intake. Detailed analysis of changes in appetite within meals suggested that palatability effects are generated through orosensory reward mechanisms which translate the hedonic evaluation of orosensory stimulation by food into enhanced motivation to eat, indicated by increased hunger in the early stages of meals with palatable foods. However, the hedonic quality of food is in a large part a learned response. Two distinct but interacting types of association may partly underlie the acquisition of palatability. The first, flavour¿flavour learning, is based on associations between new flavour elements and existing liked or disliked components such as sweet tastes, while the second, flavour-consequence learning, is based on associations between flavours and post-ingestive effects of nutrients. While many studies have demonstrated that both these associations can increase liking, only recently have we been able to assess whether these acquired likes also stimulate intake. Our recent finding that flavour¿nutrient associations can stimulate intake suggest that energy-dense foods may promote active as well as passive over-consumption, while new data also suggest enhanced appetite through flavour-flavour associations. The implications of these observations for understanding overeating in the context of the current obesity crisis is explored.

Repeated retronasal exposure to novel food odours with sweet and bitter tastes can lead to subsequent changes in orthonasal liking and sensory experience of the taste-paired odours. PROP tasters have been reported to rate low concentrations of saccharin as more bitter and less sweet than do PROP non-tasters. We therefore predicted that acquired liking for, and sweetness and bitterness of, odours conditioned by association with saccharin would vary depending on PROP taster status. 87 volunteers evaluated two novel odours before and after co-experience of one odour with 4 mM/l saccharin and the second with water. PROP taster status was assessed from the intensity of 3.2 mM PROP relative to 1.0 M NaCl, and sweet-liker status from liking ratings for 0.21 and 0.83 M/l sucrose and 0.0004 M/l and 0.0010 M/l saccharin. Liking for the saccharin-paired odour increased in sweet likers but decreased in sweet-dislikers. Overall liking change also varied with PROP taster status, with both odours rated less pleasant post-training in the super-taster and taster but not non-taster groups. The saccharin-paired odour was rated sweeter post-training, regardless of PROP taster or sweet-liker status. PROP super-tasters rated the saccharin-paired odour as more bitter post-training, in-line with enhanced bitterness of saccharin in this group. These data confirm that sensory and hedonic changes operate independently olfactory conditioning, and are influenced by individual differences in sensitivity to bitter and sweet stimuli.

Previous research suggests that women scoring high on dietary restraint may be insensitive to flavourflavour learning, but no study has yet explored this using the olfactory conditioning paradigm. Accordingly, 56 women who were sweet likers were classified as either high or low on both the Three Factor Eating Questionnaire restraint and disinhibition scales. They evaluated two odours before and after disguised pairings of one odour with 10% sucrose and the other with 0.01% quinine. Liking for the quinine-paired odour decreased post-training, with no effects of restraint or disinhibition. In contrast, the increase in liking for the sucrose-paired odour was significantly greater in women classified as scoring high in disinhibition, but was unaffected by restraint. Sweetness of the sucrose paired odour increased, and bitterness of the quinine-paired odour decreased, similarly in all groups. These data suggest that sensitivity of restrained eaters to flavour-based learning may result from their attitude to the food used as reinforcer rather than some basic failure in the learning process, and also suggest that women scoring high on disinhibition may show heightened sensitivity to hedonic cues.

Repeated pairings of novel food-related odours with sweet tastes can result in enduring changes in sweetness of the odour alone, but have less consistent effects on odour liking. Variation in ability to taste propylthiouracil (PROP) might account for this, since PROP supertasters (ST) have been reported both to experience stronger sweetness intensity and to be more likely to dislike sweetness than do PROP nontasters (NT). Alternatively, individual differences in liking for sweetness may transfer to sweet-paired odours independently of PROP sensitivity. To explore this, evaluations of sucrose, saccharin, and PROP solutions were used to classify 92 volunteers as either sweet likers or dislikers and as PROP ST, NT, or medium tasters (MT). Changes in pleasantness of odours that had been paired with the taste of saccharin increased in sweet likers but decreased in dislikers. Odour sweetness increased regardless of PROP taster or sweet liker status. PROP ST rated saccharin as more bitter than did other taster groups and also showed greater increases in acquired bitterness of the saccharin-paired odour. Overall, these data suggest that individual differences in evaluation of saccharin reliably predict subsequent changes in evaluation of saccharin-paired odours, with hedonic changes corresponding with liking for sweet tastes and sensory changes reflecting differences in sensory quality between PROP taster groups.