Appetite 75 (2014) 165–172

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Appetite journal homepage: www.elsevier.com/locate/appet

Research review

Colour, pleasantness, and consumption behaviour within a meal Betina Piqueras-Fiszman ⇑, Charles Spence Crossmodal Research Laboratory, Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, England, UK

a r t i c l e

i n f o

Article history: Received 3 December 2013 Received in revised form 8 January 2014 Accepted 10 January 2014 Available online 22 January 2014 Keywords: Colour Variety Sensory-specific satiety Habituation Boredom Consumption behaviour

a b s t r a c t It is often claimed that colour (e.g., in a meal) affects consumption behaviour. However, just how strong is the evidence in support of this claim, and what are the underlying mechanisms? It has been shown that not only the colour itself, but also the variety and the arrangement of the differently-coloured components in a meal influence consumers’ ratings of the pleasantness of a meal (across time) and, to a certain extent, might even affect their consumption behaviour as well. Typically, eating the same food constantly or repeatedly leads to a decrease in its perceived pleasantness, which, as a consequence, might lead to decreased intake of that food. However, variation within a meal (in one or several sensory attributes, or holistically) has been shown to slow down this process. In this review, we first briefly summarize the literature on how general variety in a meal influences these variables and the major theories that have been put forward by researchers to explain them. We then go on to evaluate the evidence of these effects based mainly on the colour of the food explaining the different processes that might affect colourbased sensory-specific satiety and, in more detail, consumption behaviour. In addition, we also discuss the overlap in the definitions of these terms and provide additional hypothesis as to why, in some cases, the opposite pattern of results has been observed. Ó 2014 Elsevier Ltd. All rights reserved.

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boredom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensory-specific satiety (SSS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensory habituation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Focus of the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food colour and sensory-specific satiety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food colour monotony and intake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product variety display and consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The colour of contextual factors and consumption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction There is now a very large body of empirical evidence to support the claim that increased variety in terms of the gustatory, olfactory, and/or oral-somatosensory attributes of a meal, or food selection, results in increased consumption (see Renner, 1944, for early work, ⇑ Corresponding author. E-mail addresses: betina.piqueras-fi[email protected], [email protected] (B. Piqueras-Fiszman). http://dx.doi.org/10.1016/j.appet.2014.01.004 0195-6663/Ó 2014 Elsevier Ltd. All rights reserved.

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and Hetherington, Foster, Newman, Anderson, & Norton, 2006; Sørensen, Møller, Flint, Martens, & Raben, 2003; and Wadhera & Capaldi-Phillips, 2014, for more recent reviews). Variety within a meal (regarding the components holistically, not specific sensory attributes within the same food product or category) can be determined in several ways, for instance, as the number of unique items, as the mix of unique items, and as a pairwise difference between items. It is worth noting that the way in which the items are presented affects the perceived variety and that this (the perceived, but not the actual variety) can also affect consumption. For instance,

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Levitsky, Iyer, and Pacanowski (2012) observed that the energy intake was lower when they presented mixed pasta and stir-fry vegetables than when the two components were presented separately, thus suggesting that segregating food into discrete units increases energy intake by increasing the perceived variety of foods that are available for consumption (see also Redden & Hoch, 2009). Variety can also be presented within the same food category; that is, by presenting systematic alternations of specific sensory attributes, such as the texture, the flavour, or the colour, of a single product category (e.g., as found in a flavoured yoghurt assortment pack). People eat more when given a variety of different foods to choose from in a meal setting, as compared to when they are given only a single foodstuff to consume (see Epstein, Temple, Roemmich, & Bouton, 2009; Rolls, Rolls, & Rowe, 1982a; Rolls, Rowe, & Rolls, 1982c; Rolls, Rowe, et al., 1981). Several different mechanisms have been put forward to account for this behavioural change. In the context of marketing, the three underlying mechanisms for variety-seeking behaviour that have been outlined are: Curiosity (a cognitive response), boredom (with the choice process), and attribute satiation (a sensory perceptual response; see Van Trijp, Hoyer, & Inman, 1996). It could be argued that within the context of a meal, these mechanisms (and particularly the latter two) are more closely interrelated (since they influence behaviour over a narrower timeframe; that is, over minutes or hours rather than over weeks or months). These two concepts, together with other closely related phenomena, will be described in this introductory section (see Haws & Redden, 2013, for an account of consumers’ selfcontrol). These are an essential contribution to the understanding of the effect that colour has on consumption behaviour in humans, which is the topic that will be covered in the main body of this review. Boredom ‘Boredom’ is frequently mentioned as a cause of low, or declining, consumption for those products that were once, initially, (highly) accepted. That said, Zandstra, Weegels, Van Spronsen, and Klerk (2004) argue that it is not always clear what is meant by this complex concept, since the specific definition of boredom would appear to vary from one researcher to the next. Furthermore, as yet, no standard method of measuring boredom has been developed. In the literature, boredom has often been studied via exposure testing. This involves the repeated consumption of a product over a period of weeks or months, although some researchers have observed boredom with the multiple alternations of the foods within a meal (Brondel, Lauraine, et al., 2009). Zandsra et al. defined two distinct types of boredom: One is boredom with the product, which is thought to be a neural/physiological response resulting in a decrease in actual liking attributable to a consumer’s satiation with specific attributes of the food that has been consumed (a response that encompasses sensoryspecific satiety – SSS, described below). The second type of boredom is with the concept. This is more of a cognitive response, involving, as it does, a decrease in the desire to eat a specific food repeatedly. Therefore, it could be argued that SSS constitutes a part of the liking dimension of boredom. In order to illustrate the difference between boredom (with the concept) and SSS, let’s take an example from an experiment reported by Zandstra, de Graaf, and Van Trijp (2000). In this study, three different groups of participants consumed a meat sauce once a week for dinner at home over a period of 10 weeks. The ‘monotony’ group received the same flavour of meat sauce for the duration of the entire study; an ‘imposed variation’ group received one of three different flavours of meat sauce each week in a random order, while the ‘free choice’ group were allowed to pick any one of the three flavours of the meat sauce

each week. In the monotony group, an increase in boredom was observed together with a decline in people’s acceptance ratings, thus supporting the idea that specific product attributes can change long-term consumer preferences. However, those participants in the imposed variation group reported being more bored with the food than in the free-choice group. This result would appear to indicate that boredom with the product (note that the authors also refer to this as SSS, Zandstra et al., 2000) is not only influenced by the specific sensory attributes of the food itself, but also by psychological and situational factors beyond sensory and hedonic determinants. Sensory-specific satiety (SSS) As mentioned in the earlier section, SSS constitutes a part of boredom with the product (which involves repeated exposure). The term SSS is typically used in order to refer to the decrease in the pleasantness of a food that results from eating that food (ad libitum or to satiation), with a lower decrease in the pleasantness of other foods that have not been consumed (or eaten to satiation) within the meal (Hetherington & Rolls, 1996; Hetherington, Rolls, & Burley, 1989; Rolls, 1985). This means that people become sated with specific foods after having been presented with them (and typically consuming them) repeatedly, and hence start to find the food less appealing/pleasant. Note here that chewing without swallowing has also been shown to give rise to significant SSS effects, thus demonstrating that actual consumption of the food is not actually a necessary prerequisite when it comes to eliciting the effect (e.g., Nolan & Hetherington, 2009; Rolls & Rolls, 1997; Smeets & Westerterp-Plantenga, 2006).1 In addition, Higgs, Williamson, and Attwood (2008) have demonstrated that it is not necessary to remember having eaten a food in order to exhibit SSS to that food. In their study, they tested whether two densely amnesic patients with bilateral damage to the medial temporal lobes (who lacked memory for recent eating) would report SSS, comparing the results to a control group (n = 8). The results showed a decline in the rated liking of a food consumed to satiety from both groups, whereas only the amnesic patients showed hyperphagia (abnormal appetite and excessive ingestion of food). This finding seems to suggest that cognitive processes based on memories of having eaten do not underlie SSS (though memories of what has been eaten on the day have been shown to inhibit participants’ subsequent consumption of snacks, relative to their consumption in a condition in which the participants were asked to recall the lunch eaten on the previous day or other nonfood related memories; Higgs, 2002, 2008). To date, many studies have focused on the effect of SSS on the intake of consumers from a holistic point of view. One consequence of SSS is that people will, for instance, typically eat less of a given food if it is presented by itself than if it is presented together with a selection of other foods (e.g., Brondel, Romer, et al., 2009). Other studies, meanwhile, have either focused on the consumption of micronutrients or macronutrients instead (e.g., protein, carbohydrates; e.g., see Johnson & Vickers, 1992; Vandewater & Vickers, 1996). Several studies have demonstrated the contribution made by the individual sensory parameters that influence the perception of food in humans. Researchers have, for instance, explored the effect of the texture (Guinard & Brun, 1998), odour (Guinard, Caussin, Campo Arribas, & Meier, 2002; Rolls & Rolls, 1997), flavour (Maier, Vickers, & Jeffrey Inman, 2007; Romer et al., 2006), and, ultimately, 1 Note that SSS is related to alliesthesia, the phenomenon by which sensory stimuli can arouse pleasant or unpleasant sensations according to the internal state of a person, such as the hunger state (Jiang et al., 2008). So, for instance, the same food can be rated as either pleasant or unpleasant depending on whether a person is hungry or satiated, leading to positive or negative alliesthesia, respectively).

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the appearance of the food on SSS (see Rolls, Rowe, & Rolls, 1982b; Wadhera & Capaldi-Phillips, 2014), while keeping all of the other sensory parameters constant. It is the different aspects of the colour-related appearance of food (variety of colour, display of the elements, etc.) that we will describe in later sections of this review. It is worth noting that several authors have observed that repeated consumption of staple foods does not seem to affect liking (Hetherington, Pirie, & Nabb, 2002; Kamen & Peryam, 1961; Meiselman, de Graaf, & Lesher, 2000) and induce less boredom than foods with particular, or intense, sensory characteristics (Touyarou, Sulmont-Rossé, Gagnaire, Issanchou, & Brondel, 2012). Staple foods are defined as those that are eaten regularly and in such quantities as to constitute the dominant part of the diet and supply a major part of an individual’s energy needs. These are generally cereal products and are culture-specific (e.g., bread in France, and rice in Southeast Asia). This also reflects a distinction between the two components of food reward: liking and wanting. According to Berridge (2009), the incentive salience ‘wanting’ does not require understanding of causal relations by an individual about the hedonic outcome, and can be triggered even without an individual’s conscious awareness. Interestingly, Redden (2008, Study 3) demonstrated that SSS also depends on how much people’s attention is drawn to the repetition of a feature. In his study, people were served jelly beans of 5 different flavours; in one condition these were labelled with a number, while in the other condition, they were labelled by their flavour. While participants seeing the specific flavour labels enjoyed the experience significantly more and indicated a greater desire to continue eating jelly beans, somewhat unexpectedly, they did not perceive greater variety. However, these flavour labels reduced people’s perceptions of repetition, even with only five different types. These findings therefore suggest that repetition and variety may not simply be polar opposites of each other and that satiation depends on perceptions of repetition more than on variety, but this may not hold in all settings. Sensory habituation One final mechanism to be introduced here is response habituation, which is generally thought to underlie SSS. Epstein, Robinson, Roemmich, Marusewski, and Roba (2010) suggested that SSS represents an example of the more general phenomenon of habituation, although it has been argued that the two terms are often used interchangeably in the context of food consumption (Epstein et al., 2009). Furthermore, Redden (2008) has also mentioned that this term is used to refer to ‘adaptation’, to express the idea that people get used to experiences as they repeat them). Response habituation comprises a decrease in the responsiveness to a particular stimulus with repeated or prolonged exposure. In the context of food consumption, habituation provides a theoretical model that can be used to understand why continuing to eat the same food results in a reduction of the motivation to consume that food (Epstein et al., 2009). Importantly, however, habituation also involves the immediate recovery of the habituated response back to the original level (this is known as dishabituation). If the stimulation is changed, or else continued in a different environmental setting (Marlin & Miller, 1981), the habituated responding will tend to show at least a partial recovery; that is, the change in stimulation serves to restore the level of responding to the original habituating stimulus (Epstein et al., 2009). As a general rule, therefore, any response that can be habituated can also be dishabituated. Recently, Havermans (2012) examined whether SSS for a specific food can be dishabituated in a study in which the participants received bite-sized portions of a test food to consume repeatedly (which, as expected, resulted in the participants showing SSS

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to that food). After SSS had been established, the participants either ate a different test food repeatedly for 10 min or else played a computer game (i.e., Tetris), depending on the experimental condition. It was expected that the consumption of another food would result in a dishabituation of SSS to the original test food. However, Havermans failed to observe any such effect. It would therefore, appear that one can delay SSS (see also Brondel et al., 2009; Hetherington et al., 2006), but one cannot instantly dishabituate it. Nevertheless, as Havermans has argued (see also Meillon, Thomas, Havermans, Pénicaud, & Brondel, 2013), and as we concur, the finding that SSS is stimulus specific but not subject to dishabituation does not imply that SSS is not an example of habituation. In another study, Hetherington and colleagues (2006) allocated their participants to one of two groups. In one (food focus), the participants ate chocolate. However, their chocolate consumption was repeatedly interrupted while they rated their enjoyment of the chocolate. In the other group (food distraction), the participants also ate chocolate, but were repeatedly asked to rate both chocolate and cheese crackers. Pleasantness ratings for the chocolate decreased more rapidly in the food focus group than in the food distraction group. Hetherington et al. therefore concluded that the variety effect partly comprises the delaying of SSS. Clearly this delay did not require exposure to a wide variety of different foodstuffs, but just an interruption in the constant presentation (monotony) of chocolate. Focus of the review Thus far, we have outlined the main theories of the interacting mechanisms that explain how variety in food affects consumption. Although we are aware that the literature available is much broader than that reviewed above, our aim has simply been to provide the reader with the main notions (and illustrate them with some examples). In the sections that follow, we review the evidence that is relevant to each of the hypothesized accounts that have been put forward to explain how colour in food influences its pleasantness and consumption (see Table 1 for a summary). We will cover the effect of colour on SSS, the impact of colour monotony on intake, and the effect of the display and number of colours on consumption. Here, it is worth noting that while the colour of the food itself might be expected to have the largest impact on people’s consumption behaviour, the colour of the plateware, packaging, and even the colour of the environment in which food and drink are consumed have also been shown to exert an effect on the perception, and hence likely ultimately also on consumption (see Spence & Piqueras-Fiszman, in press, for a review). The evidence demonstrating these effects will also be briefly reviewed. Food colour and sensory-specific satiety Answering the question of whether colour per se might influence SSS is made all the more complex since, in nature, foods of different colours normally vary in terms of their other sensory qualities (i.e., taste and smell) as well. This means that it is difficult to dissociate empirically the effect of colour on SSS from the influence of other factors (such as the flavour) unless, that is, the stimuli that are used can be altered only in terms of their colour. Although there is remarkably little published research that actually supports the claim that colour can be used to influence SSS, it has often been suggested in the literature that it does (e.g., Rolls et al., 1982c). Indeed, many contemporary reviews of multisensory flavour perception refer to the phenomenon of colour-based sensory-specific satiety as if it were a well-established empirical phenomenon (e.g., see Martin, 2004; Stevenson, 2009). At present, though, we would like to suggest that this claim should be treated a little more cautiously. Certainly, the only study that we are aware of that has

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Table 1 Summary of studies that have focused on colour as part of the factors modifying selection and consumption. Effect

Stimuli

Reference

Colour variety affects SSS and intake Colour monotony (but different flavours) affects intake Colour monotony (same flavour) affects intake The structure of an assortment and perceived colour variety affects selection (and intake)

Smarties Yoghurt Potato crisps M&Ms, jelly beans, plastic toys

Rolls et al. (1982c) (Study 1) Rolls et al. (1981) (Study 3) Geier et al. (2012) Kahn and Wansink (2004)

SSS: sensory-specific satiety.

provided direct empirical evidence for the existence of colourbased SSS was conducted by Rolls et al. (1982c; Experiment 1). It is therefore worth considering this experiment in some detail. These researchers investigated whether increasing nothing more that the variety of colour in a meal would influence their participants’ consumption behaviour in a laboratory setting. The secondary hypothesis underlying this research was that people might consume more when the food that they were given to eat (sugarcoated chocolate candies; Smarties, Rowntree Mackintosh Ltd.) was more varied in terms of its colour than when they were only allowed to eat Smarties of a single hue (that happened to be their preferred one). The 24 teenagers who took part in this study were allowed to consume their preferred-coloured Smarties (note that these coloured Smarties all physically taste the same)2 over four consecutive seven-minute experimental sessions. This procedure was repeated on three separate days. On one day, the children were only allowed to eat Smarties of their favourite colour. On another day, they ate Smarties of a different colour in each of the four seven-minute sessions, while on a third day they ate a mixture of all four colours of Smarties in each session. The results demonstrated that the drop in participants’ ratings of the pleasantness of their favourite-coloured chocolates after having eaten Smarties of just that colour was greater than the drop in rated pleasantness for the other three colours of Smarties that had not been eaten.3 It was on the basis of this result that Rolls et al., and several other researchers thereafter, concluded that colour-based SSS does, in fact, exist (e.g., see Rolls & Rolls, 1997; Stevenson, 2009, p. 198). However, it is impossible to know what the effect of simply looking at Smarties of a particular colour might have had on pleasantness ratings (obviously, for the colour-based SSS account it should not have had any effect at all). One thing to note about Rolls et al.’s study is that the authors do not report pleasantness ratings at the intermediate time-point (i.e., after 14 min), nor do they report what happened to the pleasantness ratings at any interval in either the successive or simultaneous conditions. As such, it is also impossible to know whether the taste ratings for the preferred colour also declined in either of these conditions. We believe that there are both theoretical and empirical grounds for being cautious about extending the phenomenon of SSS to the visual modality (that is, to the case of foodstuffs that vary only on the basis of their colour and nothing else). It is important to remember that the majority of researchers argue that gustation, olfaction, and oral-somatosensation (trigeminal

2 Many people in the UK, where Rolls et al.’s (1982c) study was conducted, believe that differently-coloured Smarties taste different (see Levitan, Zampini, Li, & Spence, 2008). What is more, such beliefs can affect people’s ability to discriminate between pairs of Smarties that actually have the same/different flavour. Given this potential illusory difference in flavour between Smarties of different colours, one might question whether the effect of colour-specific satiety reported in Rolls et al.’s study, might not, in fact, actually reflect a form of flavour-specific satiety effect instead (just that in this case the flavour difference happened to be illusory – induced by the colour of the food itself). In fact, 14 out of 24 of Rolls et al.’s participants reported that the Smarties tasted different upon debriefing. 3 Rolls et al. (1982c, p. 412) mention that they were able to replicate their findings in a follow-up experiment with 13–14 year old boys where the Smarties replaced the last course of lunch.

stimulation) are the only senses that are integral to the perception of flavour (e.g., see Delwiche, 2004; ISO 5492, 2008). While it is certainly true that the visual attributes of a food can influence the multisensory perception of flavour via expectancy effects (e.g., a red-coloured aqueous solution being perceived as sweeter than when colourless, Johnson & Clydesdale, 1982; see PiquerasFiszman & Spence, in press, and Spence, Levitan, Shankar, & Zampini, 2010, for reviews), visual cues are not part of most published definitions of flavour (Spence, Auvray, & Smith, 2014). Given this fact, one might expect that people would respond somewhat differently to the introduction of variety into the taste, smell, or oral-somatosensory qualities of a food (such as to changes in its texture, shape, and/or temperature), since these changes will actually result in a change in the flavour of the food that is being consumed. By contrast, changing the visual (or perhaps even the auditory) attributes of a food may not have quite the same effect. Havermans and Brondel (2013) carried out a study in which they tested whether the mere presentation and suggestion of food variety would undermine SSS. Participants ate several bite-sized servings of a test food in order to induce SSS. Meanwhile, the participants in the experimental group were shown another food during the consumption of the test food. Relative pleasure of the test food was determined before and directly after its repeated consumption by means of pleasure ratings and participants’ ratings of their desire to eat that particular food. The relative pleasure of the test food decreased similarly in both groups, and so there was no evidence to suggest that the mere presence of another food during intake undermined SSS for a given test food. It would therefore appear that in order for SSS to occur, a real interruption in the perception of the tasted food has to have taken place. Interestingly, going back to Rolls et al.’s (1982c) study, there was no significant difference in participants’ consumption of Smarties as a function of the exposure condition, when the different coloured candies in successive courses were presented simultaneously or successively. However, over the intervening years, many authors have drawn from this study the erroneous conclusion that introducing variety in terms of colour into a ‘‘meal’’ (keeping all other factors constant) not only reduces the decrease in taste liking, but also enhances intake (e.g., Garber, Hyatt, & Starr, 2001, p. 14; Martin, 2004, p. 67; Rolls & Baylis, 1994, p. 5437; Stevenson, 2009, p. 198). To be absolutely clear, as yet, there is no evidence in the literature of SSS indicating that increased variety of colour per se (of foods belonging to the same product category) enhances intake. In another study, Rolls, Rowe, et al. (1981, Study 2) demonstrated that participants ate significantly more when three flavours of yogurt (hazelnut, blackcurrant, and orange) which were distinctive in taste, texture and colour were offered to them than when given just one of the flavours repeatedly across the sessions, even if the flavour was the participants’ favourite. However, when the participants were offered three flavours of yogurt (strawberry, raspberry, cherry) differing only in their flavour (the colour and texture were nearly identical for all three types), there was no enhancement of intake when the variety was offered in succession (Rolls, Rolls, Rowe, & Sweeney, 1981, Study 3). Although this study measured intake and not directly SSS (since pleasantness ratings

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were collected but were unfortunately not analysed), it could be hypothesised that SSS was not activated in the latter study since very likely vision prevailed over the flavour information (see Delwiche, 2012). Having said that, it should be recognised that if the yogurts would have had more distinct flavour profiles (the flavours were distinguishable, but nevertheless were all sweet and fruity), the likelihood of SSS being observed would have been greater. However, to date, this speculation awaits future research in order to be unequivocally resolved. Given that SSS for colour has not been shown to influence consumption, in the sections that follow, we will move on to review the various other ways in which colour variety (and its display) affect consumption. Food colour monotony and intake Another factor related to colour that can decrease a consumer’s liking, perhaps eventually even their intake of food is related to ‘monotony’ (Rolls, 1986): Repeated exposure to a food can lead to a reduction in the pleasantness of the appearance of that food (along, of course, with its other sensory attributes; see Hetherington, Pirie, & Nabb, 1998; Hetherington et al., 2002). This can eventually lead to decreased intake. However, it should be noted that at an unconscious level, monotony in a food is more likely to lead to an increase in mindless eating, mostly in snack foods, and during ‘‘incidental’’ eating episodes (note that, as mentioned in the Introduction, staple foods, which are often monotonous in colour and eaten on a daily basis, do not appear to induce as much boredom as other foods). If we focus our attention on the colour of the items, it could be the case that a variety of colour in a ‘‘meal’’ could increase people’s awareness of the amount that is being eaten, and hence act as a ‘‘stop’’ signal. For instance, Geier, Wansink, and Rozin (2012) conducted a couple of studies, using different groups of participants who ate from tubes of potato chips while watching a movie. In each study, the participants ate chips that were either identical (the control group) or which had red-coloured chips inserted at regular intervals (every xth chip; the treatment group). In both studies, intake was reduced by more than 50% when the red-coloured potato chips were present in between the normal yellowish ones. Geier et al. argued that their large effect moves in the opposite direction from either habituation or SSS (Rolls, Rolls, et al., 1981), which would have been expected to decrease intake on all of the control (unsegmented) participants. Instead, the researchers put forward three possible reasons as to why segmentation (i.e., interrupting the monotony) might have decreased people’s intake: (1) It focused their attention on the act of eating and the amount that the person had eaten (Chandon & Wansink, 2007); (2) It provided norms regarding portion-size (Geier, Rozin, & Doros, 2006); and (3) The colour segmentation interrupted the episode of ‘‘mindless’’ eating (Wansink, 2006). Product variety display and consumption A separate line of empirical research has demonstrated that the way in which foods that vary only in terms of their colour are laid out also influences acquisition behaviours. In particular, Kahn and Wansink (2004) reported a series of studies on both adults and children using foods (and also non-food products). These researchers demonstrated that increasing either the actual (or perceived) variety of flavour and/or colour in a selection resulted in a significant increase in consumption behaviour.2 In their first two studies, 36 children (6–7 and 9–10 years old) were observed to pick up, and eat, significantly more jelly beans (and coloured beads), in fact, more than twice as many, when the assortment presented 24 differently coloured items organised in separate compartments as

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compared to when the assortment presented only 6 different colours. However, the difference in the number of colours did not have an effect on the amount chosen and their consumption when the assortment was disorganised (that is, when all of the colours were mixed into a single compartment). Similar results were also observed in adults (though this study only involved jelly beans, varying in terms of their flavour). In the most pertinent study for present purposes (Kahn & Wansink’s Experiment 3), the researchers used M&M candies or small spider toys varying only in terms of their colour (in this case a 6-colour range was presented). Again, 36 children of the same age range as in the first study were instructed by an adult to take as many as they wanted. There were three factors in the design of this study: The first factor was the organization of the assortment (organized vs. disorganized), the second was the number of replicates (a single six-cell tray vs. a 12-cell tray with double replicates), and the third (repeated) factor was the product class (M&Ms vs. spiders). The results revealed that the larger the tray the larger the quantities selected. However, this effect was only observed in the organized condition (where the kids picked up nearly 4 times as many items). These results cannot be explained simply in terms of an increase in the number of different colours chosen in the 12-cell organized assortment, as there were no differences between the numbers of different colours selected in each condition. In a fourth study (this time conducted with adults), the participants were given bowls of M&Ms. of either 7 or 10 colours and either presented with an equal distribution of colours (symmetric condition) or with 30% dark-brown M&Ms. (asymmetric condition). The results of this final study showed that only for the asymmetric assortments did increasing the actual colour variety from 7 to 10 lead to a significant increase in the quantity consumed (an increase of 77%). The fact that the same effects were reported for both food- (jelly beans and M&Ms) and non-food items (beads and toy spiders) in Kahn and Wansink’s (2004) study argues against an effect of colour variety specifically on consumption behaviour for food, but rather suggests an impact of visual variety on choice behaviour more generally (note that the authors did not report whether the children actually ate the sweets that they had chosen, though the adults did eat the food products). The influence of colour variety on people’s consumption behaviour highlighted by this research therefore likely operates via another mechanism, that is, the structure of an assortment (e.g., organization and symmetry or entropy) moderated the effect of actual variety on perceived variety. Kahn and Wansink further demonstrated that it is perceived variety that, in turn, influences consumption (or selection) quantities through anticipated consumption utility. That said, one has to be careful here, given that elsewhere in the literature it has been shown that giving people too much choice can also be demotivating, and hence could possibly reduce consumption behaviour (see Iyengar & Lepper, 2000). Redden and Hoch (2009) conducted a series of experiments that may help to explain why perceived variety increases consumption (or acquisition). They demonstrated (Study 3) that variety in an assortment can reduce its perceived quantity; this was shown in a study where the participants were asked to match the quantity of a sample of 52 g of brown M&Ms. contained in a transparent bowl by pouring M&Ms. into another transparent bowl, knowing that they would not consume them afterwards. In one condition, the M&Ms. given to pour were a mix of three colours, while in another condition, the M&Ms. had only a single colour. The results showed that participants poured 12% more into a bowl when the candy was multicoloured. Thus, beyond any separate effects variety might have on choosing an amount to consume, variety caused people to pour more candy into a bowl, presumably because variety made them perceive that they had placed less in the bowl in the first place. In another study (Study 4), the participants were asked to match quantities of M&Ms. (as in Study 3), but this time

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the number of colours in the sample bowl also varied (single colour vs. 4 colours). Not only were the results from Study 3 replicated, but there was also the predicted main effect of the number of colours in the sample, whereby people poured less when matching a sample that had multiple colours versus a single colour, presumably because a sample appeared to have less quantity when it had a variety of colours. Thus, it would appear as though a variety of colours reduces perceived quantity whether in the sample or the poured items, and, as a consequence, could lead people to estimate smaller serving portions and eventually to repeat servings. In line with the research on the arrangement of the colours present in foods, Debra Zellner and her colleagues (2010) have also studied whether balance and complexity of the food on a plate (defined here as an increase in the use of colour) would affect the attractiveness of the presentation. In addition, the willingness of the participants (68 undergraduates) to try the food, and their liking for it, was also assessed in four different visual conditions (monochrome-balanced, coloured-balanced, monochrome-unbalanced, and coloured-unbalanced). In this study, the food consisted of four slices of water chestnut, and four lines and a dot of tahini (the colour manipulation in the coloured conditions was achieved by artificially colouring the tahini). While the addition of colour increased the participants’ rating of the attractiveness of the balanced food presentation, it had no effect whatsoever on their rating of the unbalanced presentation. The participants were more willing to try the monochrome (plain light brown) than the colourful plates of food, but there was no effect of colour, or balance, on the participants’ overall liking of the flavour of the food itself. So, while manipulating the colour and balance in a dish may affect how attractive it looks, these results suggest that it does not necessarily alter how much people like the flavour of the food, nor how much they would eat if they had the possibility of eat this ad libitum or repeatedly.

The colour of contextual factors and consumption While several studies have demonstrated the effect of the colour of the plateware/container (e.g., Gal, Wheeler, & Shiv, 2007; Harrar, Piqueras-Fiszman, & Spence, 2011; Piqueras-Fiszman, Alcaide, Roura, & Spence, 2012; Piqueras-Fiszman & Spence, 2012; Spence, 2012; Spence & Piqueras-Fiszman, in press; Stewart & Goss, 2013) and the packaging (Becker, Van Rompay, Schifferstein, & Galetzka, 2011; Piqueras-Fiszman & Spence, 2011; Spence & Piqueras-Fiszman, 2012) on people’s perception of the sensory attributes of food products (see Piqueras-Fiszman & Spence, in press, for a review), few studies have actually demonstrated that the colour of these extrinsic elements present during eating occasions actually exerts any influence on people’s consumption behaviour. Regarding the effect of certain colours of the extrinsic elements of food on people’s intake, it is worth considering the fact that the colour red has been shown to elicit avoidance motivation across a variety of behavioural contexts (e.g., Birren, 1963; Mehta & Zhu, 2009; though Singh, 2006, once reported that red stimulates appetite). Several studies have investigated the effect that the colour red has on the consumption of both snack foods and soft drinks (e.g., Genschow, Reutner, & Wanke, 2012). For instance, the participants in Genschow et al.’s study drank less from a cup with a red label than from a cup with a blue label. They also ate less snack food from a red plate than from a blue or white one. The authors concluded that red might function as a subtle stop signal that works even outside of a person’s focused awareness and may thereby reduce incidental food and drink intake (cf. Kulman, 2001, for a possible environment in which to apply such findings).4 4 Interestingly, blue is a colour that is also associated with avoidance motivations, but when it is part of the food (see Tysoe, 1985; Wheatley, 1973).

Meanwhile, Bruno, Martani, Corsini, and Oleari (2013) extended this line of research by looking further into this intriguing phenomenon, demonstrating that red plates reduced the consumption of food (and the use of hand cream), while, interestingly, the samples from all the plates were rated as being similarly liked. In addition, Bruno et al. also demonstrated that their results were not dependent on the Michelson (luminance) contrast nor on the colour contrast either, leaving them to suggest that it might simply be the meaning of avoidance associated with the colour red (of the plate) that was influencing people in those conditions. Elsewhere, it has been shown that people may be willing to pay significantly more for a bottle of wine at a wine tasting under certain room illumination colours as compared to others (Oberfeld, Hecht, Allendorf, & Wickelmaier, 2009; see also Wheatley, 1973, and Spence & Piqueras-Fiszman, in press; Stroebele & De Castro, 2004 for reviews of the influence of environmental factors on food consumption). In a study conducted in a winery on the Rhine (Oberfeld et al., 2009), tourists reported being willing to pay more for the wine that they were sampling when the normal white lighting was replaced by red. It would therefore seem that in terms of the lighting, the effect of the colour on consumer behaviour might have other consequences, which suggests that further research would be beneficial in this area in order to delve into the nature of these colour effects. It is also known that room colours affect temperature sensation. So, for example, in one study, at approximately 15 °C, participants felt cold in a room that had been painted blue–green but felt cold in a room that had been painted orange only when the temperature dropped to approximately 2 °C (see Stroebele & De Castro, 2004). Furthermore, Sester et al. (2013) demonstrated the effect of the room ‘‘warmth’’ as conveyed by the atmospheric elements (not modifying the temperature itself) on people’s drinks choices. While this study did not measure consumption, it would be expected to have an effect on intake as well. In a similar vein, Kim and Tokura (1998) exposed 10 healthy women to bright light of 5000 lx (‘‘Bright’’ condition) or to dim light of 200 lx (‘‘Dim’’ condition) from around 7:30 to 18:00 h, and then to 200 lx from 18:00 to around 23:30 h, while keeping the atmospheric temperature constant throughout the experiment (28 °C). The participants were instructed to select the most preferable colour of 41 differently coloured cloth samples every 5 min from 21:00 to 22:00. Most of the participants in the Dim condition preferred a warmer cloth colour. Since the same participants participated in the bright- and dim-light sessions (on different days), the authors concluded that the pleasure aroused by the same visual stimuli varied with the participants’ internal state (their measured temperature), which, in turn, depended on ambient light. In brief, this finding suggests that the light brightness to which subjects are exposed to can give rise to alliesthesia for stimuli that only vary in terms of colour. It could be hypothesized that this effect might be extended to the food domain, where simply the light colour (or intensity) of a room would modulate the liking (and perhaps the intake) of certain colour drinks, for example.

General discussion and conclusions On the basis of the literature that has been reviewed here, there are several interrelated mechanisms (i.e., boredom, SSS, habituation, and monotony) by which altering the colour aspects related to food can modify our perception and motivation to eat certain foods. In this final section, we will summarise and add some thoughts about the existing and alternative theories that have been put forward to explain colour-based effects on food consumption. On the one hand, there is the notion that more visually pleasing foods are more likely to result in more pleasant tasting experiences. Most of the studies in which vision was not impeded, could

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have presented a halo effect by which the participants could have been influenced in their responding by the appearance of the foods rather than by the possible flavour when rating the food’s pleasantness over time (note that most of the studies mentioned in the first sections involved children, who are more sensitive to colour variety; see also Spence, 2012). On the basis of the literature reviewed here, it would appear challenging to separate out the effect of vision on the taste (or overall) pleasantness ratings. However, regardless of what participants are ‘‘really’’ rating, this effect has not necessarily been found to hold true after repeated exposures, since several putative mechanisms may be expected to decrease that perceived pleasantness, such as boredom with the concept, habituation, and/or specifically SSS (see Rolls, Rolls, & Rowe, 1982d). These mechanisms can start even when the food varies solely in terms of its colour (that is people would become bored of eating the same coloured M&Ms. which do not vary in terms of flavour, Rolls et al., 1982c), but also when colour (appearance) is kept constant while the flavour itself changes (as when people are presented with yoghurts of the same colour but which vary in terms of flavour; Rolls, Rolls, et al., 1981). Should such an effect of colour be clearly observed upon replication (that is, if colour alternations of a food, keeping all other sensory attributes constant, were found to reduce the decrease in liking or/and enhance intake of those foods; and vice versa) with a larger sample of foods, it could be framed in terms of the phenomenon of sensory dominance (Spence et al., 2010). These findings would therefore suggest that the colour of a food can by itself override our flavour experiences (and their pleasantness ratings) across time. This would certainly be an interesting research area to explore, since it could have important implications in the nutritional domain. For instance, colouring a food, which elderly people or children would quickly become bored with but nevertheless need in their diet, differently each day (or within a meal) could reduce their dislike, and perhaps increase the intake, of that needed food. However, increasing solely the variety of colour in a food/meal does not always appear to alter the amount of food that is consumed, contrary to the expectations based on other studies in which other types of SSS were manipulated (although it should be acknowledged that intake often presents a wide variance which lowers the statistical power and hence can make it harder to obtain significant effects). However, disentangling the underlying reasons as to why the perceived pleasantness of a product drops more pronouncedly when tasting a single coloured food repeatedly, as opposed to when consuming multicoloured foods, remains a challenge since other plausible explanations for the data can also be put forward. For instance, it would seem that people may become bored when the same colour (food or non-food) is repeatedly presented, and that such an effect may, at least in part, be accounted for in terms of ‘monotony’ rather than SSS (Hetherington et al., 2002). Consequently, while the evidence in support of the existence of increased consumption when people are presented with increased variety regarding the taste, aroma, texture, and shape is now well-supported, researchers should be cautious (until further empirical evidence has been collected) before asserting the existence of an equivalent effect induced by nothing more than increased colour in a meal. Here, it is important to remember that colour variety may also make the consumers more aware of their intake, mostly in situations in which people eat mindlessly (and mainly when they eat snack foods; Geier et al., 2012). Therefore the account for the decrease in intake seems to be quite different depending on the eating occasion and the product presented. In this review, we have also seen how the components of a meal are displayed affects how much colour variety is perceived, therefore that is another factor that should be taken into account when measuring the effect of colour (and its variety) in consumer behaviour.

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One final point to note here is that the studies on the effect of product colour in a meal (explicitly, increased colour variety) that have been published to date have primarily involved children (e.g. Kahn & Wansink, 2004; Rolls et al., 1982c; Walsh, Toma, Tuveson, & Sondhi, 1990). There is therefore a need for more research involving adults to be able to make the effects and hence the conclusions more generalizable to the population as a whole (see Spence, 2012). It is certainly noticeable how colour variety appears to be a more commonly utilized marketing tool in foods that are targeted at children (Tolliday, 2012), but we believe that there is an interesting opportunity to exploit the findings reviewed here in adults in order to deliver more global nutritional benefits (cf. Kulman, 2001). There may also be interesting avenues to pursue looking at the variety in product colouring and the design of product packaging (cf. Deng & Srinivisan, 2013). The fact that there is a substantial literature on other sensory aspects of SSS and consumption but little focusing on colour (see Table 1) calls for more research to control at a more individual level the effects observed, extend the stimuli to other foods, and test a broader spectrum of participants. In addition, it would be interesting to measure consumers’ evolution of pleasantness appraisal being measured as well for the appearance aspect of the food (perhaps measuring this indirectly with other methods, including neuroimaging or electrophysiological techniques) and try to determine the extent to which differences in colour (or differences in other sensory attributes) trigger visual sensory dominance when the hypothesised interrelated factors, such as boredom, monotony, SSS, and habituation, are being measured.

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