Participants were recruited via the database of the Food & Biobased Research organization. All participants had a normal sense of smell as determined by the European Test of Olfactory Capabilities (ETOC). Twenty-four non-allergic and non-vegetarian participants were selected based on gender, age and weight. Vegetarians were excluded because some of the tests referred to meat products. One participant was a smoker. All participants signed consent forms to participate in the study. To test systematic effects of age and gender, participants were divided into two age groups, namely “young” (32.0 yrs. ± 10.3 SD) and “middle-aged” (51.1 yrs. ± 4.0 SD). Within each age group, equal numbers of males and females were recruited. Twenty-two participants (13 females and 9 males) completed the study. The study protocol was approved by the Social Science Ethics Committee of Wageningen University.
The study was carried out in the so-called ‘mood rooms’ located at the research facilities of the Restaurant of the Future in Wageningen, The Netherlands. The mood rooms are four identical test rooms (12 m2 in size) which are sparsely decorated and furnished, and equipped with cameras mounted in the ceiling to monitor the activity of participants, and vaporizers and air conditioners to control ambient conditions. Ambient temperature was held at 21°C. Three of the four rooms were used in this study, one for each aroma condition. One room was scented with the citrus aroma, one with the vanilla aroma and one room was kept/remained odorless. Participants were unaware of the presence of the vaporizers and aroma concentrations were kept at barely detectable levels. These levels had been determined in a pilot study in which groups of consumers were rotated through the rooms with different concentrations of the aromas in each room. Consumers were asked to indicate if they smelled an odor and, if they did, what kind of odor it was. With the results of this study we set the concentration levels of the ambient aromas. The selected concentrations were noticeable in the pilot study by most of the consumers, but only when their attention was drawn to the aromas.
Vaporizers (AllSens Geurbeleving, Oosterhout, The Netherlands) filled with natural aromatic citrus and vanilla oils (Voit Aroma Factory, Martinsried, Germany) were used to generate ambient aromas. Clean air generated by a programmable compressor was passed through the saturated headspace of the aroma vessel into the space occupied by the test participants in 2-sec pulses every 10 minutes. Together with the room ventilation this produced a relatively stable intensity during the session. A vaporizer filled with water was used as an odorless control.
Actual food choice of congruent and non-congruent foods
While participants were exposed to each of the ambient aroma conditions, they were presented with plates of small portions of foods and drinks. Every plate contained congruent, non-congruent, and neutral food and drink. The plates consisted of citrus-congruent (mandarin orange segments and orange juice), vanilla-congruent (vanilla cookies and milk) or neutral in relation to either aroma (cubes of cheese and mineral water). The participants were led to believe that the food and drinks were presented only for their convenience and that they were free to sample from them during the session,except during the response time test. Consumption of food during exposure was measured by tallying food present at the beginning and end of the session. Similarly, the drinks were weighed at the beginning and end of the session.
Physiological affective measurements
Heart rate and physical activity
These were monitored using the Polar S810 heart rate monitor and SenseWear BMS sensor system (BodyMedia Inc., Pittsburgh, PA. USA), respectively. The heart rate monitor monitors heart rate accurately  and consists of a receiver mounted in a wristwatch and a sensor placed with a belt around the chest. The Sensewear BMS sensor system is a commercially available sensor system designed to continuously monitor energy expenditure, activity and sleep efficiency accurately . The total energy expenditure is the amount of energy expended in calories averaged across a session and includes the basal metabolic rate of the participants. To compensate for differences in session durations, energy expenditures were converted into energy expenditures per minute (total energy expenditure divided by session duration, expressed in kcals/min). Physical activity levels were expressed in Metabolic Equivalents of Tasks or METs. A MET-score of 1 MET is the rate at which adults burn kcal at rest: this is approximately 1 kcal per kilogram of body weight per hour (expressed as 1 kcal/kg/hr). For example, METs for dancing, hiking and running equal approximately 4.5, 6, and 7.5.
Response time test
A computerized semantic decision test was developed in which participants had to indicate as quickly as possible whether a word presented on the monitor placed in front of them referred to something edible (food) or inedible (part of the landscape or the interior of a house). At the beginning of each trial, the participant placed the index finger of his/her dominant hand on a marker located at equal distances from the left and right arrow buttons on the computer keyboard. As soon as the test word appeared on the monitor, the participants moved the finger to either the left or right arrow button to indicate whether the test word referred to something edible or inedible. To counterbalance for possible response biases, half of the participants had to press the right arrow button for an “edible” response and the left-button for an “inedible” response. For the other half of the participants the buttons for “edible” and “inedible” were reversed. A computerized timer recorded the time between the presented word and the press on the left or the right arrow button. A total of 200 test words were displayed, with a break of 30 seconds after the first 100 words. Per participant and aroma condition, the median response times for the set of edible and inedible responses were calculated to reduce the influence of outliers.
Psychological affective measurements
The PANAS (Positive Affect and Negative Affect Schedule) questionnaire of Watson and Clark was used to assess each participant’s mood status . In this test participants rate their emotional status using a list of 20 emotions and a 5-point rating scale. The sub scores of the positive and negative mood scales were added and averaged .
Projective emotion test
This test consisted of 24 photographs of male and female faces with comparable neutral expressions. Each face was judged by the participant using a set of 12 positive and negative descriptors. The positive descriptors were “kind”, “enterprising”, “cheerful”, “open”, “reliable” and “warm”. The negative descriptors were “introvert”, “arrogant”, “tensed”, “shy”, “suspicious” and “discouraged”. The set of 24 photographs was divided into 3 subsets of 8 photographs, each subset having a comparable mix of male and female faces. Each subset was used in one of the three aroma conditions. Subsets and aroma conditions were randomized across participants. Ratings for each descriptor were averaged per participant across subsets.
After the third and final session, participants were asked to speculate on the purpose of the study and on any ambient aromas they might have noticed during the sessions. The participants could indicate whether they smelled an ambient aroma in week 1, 2 and/or 3 as well as indicate the aroma. When participants did not remember the ambient aromas of the rooms they had been in several weeks ago, they were allowed to smell the available aroma samples. Based on these samples they could choose the aroma that they thought was present in the room on a certain day.
Each participant participated in 3 separate 45-minute session held at the same time on the same weekday during 3 consecutive weeks. Per session, a single participant was exposed to one of the ambient aroma conditions (citrus, vanilla and odorless control) during which he/she performed a number of tests. Participants were not told anything about the ambient aromas; nonetheless, the order of the aroma conditions was randomized across participants. At the start of each session, participants were equipped with the heart rate and activity sensors, and were guided into the experimental room where they received additional instructions for each test via a computer monitor. During the next two minutes a mood test was conducted, followed by the projective emotion test (five minutes), the response time test (eight minutes), the second mood test (two minutes) and, finally, an interview (three minutes) in which participants were questioned regarding their general impressions of the test procedures and the test environment. During each session participants had access to congruent, non-congruent and neutral food and drinks and participants were told that these were free for them to consume (food choice test). At the end of the third and final session participants were debriefed.