X. Possible Reinforcing Effects of Coffee, Independent of Caffeine Content
Even though there has been no demonstration yet of the possible reinforcing effects of coffee that are unrelated to caffeine, the smell and flavor of coffee and the social environment that usually accompanies a coffee break or an after-dinner coffee should not be totally neglected as factors in everyday coffee drinking. The possible effect of some other constituents of coffee has not been extensively explored, but there are some suggestive data.
Similar amounts of work on an ergometer were spent for caffeine capsules, regular and decaffeinated coffee and only the placebo capsules were considered not to be worth any effort (Griffiths et al., 1989). In a field study, a switch from filter coffee, to which the subjects were accustomed, to decaffeinated instant coffee supplemented with different amounts of caffeine, decreased the number of cups of coffee consumed per day slightly but significantly, regardless of the amount of caffeine (Höfer and Bättig, 1994a). In parallel, the ratings for the pleasantness of these substitutes for the habituated filter coffee decreased strongly but also independently of the caffeine content.
In another field study, this general finding was confirmed (Höfer and Bättig, 1994a). Two groups of 21 female regular coffee drinkers participated in the experiment. Both groups started with a 3-day baseline period with drinking of filter coffee. After this initial period one group obtained 50 mg of caffeine in tablets, whereas the other group received decaffeinated instant coffee for the following 3 days. As in the first study, less instant coffee was consumed than filter coffee, but the number of tablets taken instead of coffee decreased even more, resulting in a decrease of saliva caffeine by about 50%. During the second 3 days, the subjects had to rate six times per day their desire for coffee. This desire increased considerably and in a highly significant manner in the group given caffeine tablets but remained unchanged in the group given decaffeinated instant coffee, although this group, in contrast to the group consuming the caffeine tablets, experienced considerable symptoms of caffeine withdrawal. All measures returned to baseline values on a 7th day of the experiment when the subjects were allowed to drink again the filter coffee they were accustomed to.
All these results suggest that the type of drink, and even the type of coffee, is a significant factor in the subject's preference for coffee. In particular, coffee drinkers were not attracted by caffeine capsules, except, possibly, to relieve withdrawal effects. It is conceivable that the low liking of the capsules can in part be related to the fact that a warm drink in itself produces a number of physiological effects (Quinlan et al., 1997). Interestingly, some of the effects of hot water are influenced by caffeine, but the type of beverage and the presence or absence of milk modifies the overall response (Quinlan et al., 1997). For example, the addition of milk appeared to have positive mood effects and to cause reduced anxiety. Conversely, liking for the taste and aroma of coffee might be acquired through the process of classical conditioning, involving association of these orosensory cues with the psychopharmacological consequences of caffeine ingestion.