Factors affecting bitterness perception and preference for coffee

The instinctive rejection of bitter taste has been crucial to the survival of our species and continues to influence food choices. In fact, the bitter taste receptors have evolved to protect the body from the ingestion of foods potentially harmful to health. However, some foods are widely consumed despite their high intensity of bitterness, for example coffee, one of the world’s most popular beverages. Coffee is drunk for its pleasant flavour and aroma as well as for the stimulatory properties arising from its caffeine content. Such properties, as well as the addition of sugar and/or milk, are most likely responsible for our learned preferences for coffee. The aim of this work was to study the relationship between the preference patterns for coffee and the perception of its bitterness. For this purpose, factors relating to both the product – degree of roasting - and the individual - bitterness perception and caffeine metabolism rate - were studied. First, the effect of the roasting temperature on coffee sensory profile and preference was investigated, initially by describing the sensory properties of coffee brews prepared with both under-roasted (140-165°C) and standard roasted (220°C) beans. In addition, seventy-five coffee consumers rated their liking for unsweetened and freely sweetened under-roasted coffee samples. Increasing the bean processing temperature positively affected both the perceived intensity of some sensory attributes specific to coffee and also consumers’ liking. In particular, coffee processed at temperature higher than 150°C were described by the terms “coffee”, “roasted”, “burnt”. Furthermore, no significant differences in the intensity of these attributes were found between samples processed at temperatures higher than 155°C and samples roasted at the standard temperature. In general, samples processed at temperatures higher than 150°C were preferred. However, a subgroup of subjects who liked the samples roasted at lower temperatures was found. These subjects were on average less neophobic than the others, that is, more inclined to consume unfamiliar/unconventional foods. Later, the effects of bitterness sensitivity and caffeine metabolism rate on perception of, and liking for coffee were examined. One hundred and thirty-five regular coffee consumers participated in the study. Subjects were characterised in terms of bitterness sensitivity (fungiform papillae – FP - number and sensitivity to the bitter compound 6-n-propylthiouracil –PROP-); caffeine metabolism rate (slow and fast metabolizers); attitudes and food behaviour (frequency – number of coffees per day- and modality - black, with sugar, with milk- of coffee consumption; preference for and familiarity with bitter foods; self perceived effect of caffeine intake). Caffeine metabolism was investigated by competitive immuno-enzymatic assay. Subjects provided saliva samples after 12 hours of caffeine abstinence (T0) and at 30 and 90 minutes after ingestion of caffeine (100 mg). The individual caffeine metabolism rate in 90 minutes was expressed as a caffeine metabolism index (CmI), computed as the ratio between the amount of residual caffeine in saliva 60 minutes after the adsorption peak and the amount of caffeine at the adsorption peak corrected with the baseline. Subjects rated their liking for six unsweetened and freely sweetened coffee samples selected on the base of their roasting degree, caffeine content and bitterness. The immersive test technique was applied to evaluate the effect of the context on coffee liking. In particular, just before starting the test, subjects were asked to describe the most preferred situation for consuming coffee (time of the day, place, company, etc.) and to imagine that they were doing coffee tasting in that particular situation. In a separate session, subjects evaluated the sourness, bitterness and astringency in the coffee samples. Psychophysical curves were constructed for caffeine and quinine-HCl over 6 concentrations. No significant relationships were found between the FP number, the PROP taster status and the caffeine metabolism rate. Subgroups of subjects were identified based on these indices and differences in preference and sensory responses were explored independently in the subgroups. Both FP density and PROP taster status affected taste sensitivity for bitter compounds in general (caffeine; quinine-HCl) and the use of sugar in coffee. In particular, subjects with a higher FP number rated the bitterness of caffeine and quinine-HCl solutions and sourness of coffees stronger than subjects with a lower FP number. They also added more sugar to, and gave higher liking ratings for, the coffee samples. While coffee liking was unrelated to PROP taster status, PROP non-tasters added more sugar to the coffee samples than did PROP super-tasters. However, super-tasters rated sourness, bitterness and astringency as stronger than non-tasters, both in coffee samples and in standard solutions. Caffeine metabolism was related to bitterness perception both in coffee samples and in standard solutions of caffeine, but not of quinine-HCl. In particular, subjects with a lower caffeine metabolism index rated the bitterness of coffees and of caffeine solutions as stronger than subjects with a higher caffeine metabolism index. Moreover, they added more sugar in coffees and consumed less coffees daily. In general, the results demonstrate that even in high coffee consuming cultures such as Italy, there are clear sensory variations in the coffees that are consumed. Coffee consumption and preference, in terms of frequency and modality, are influenced both by the sensory properties of coffee and the psychological, physiological and metabolic characteristics of consumers. In the coffee production process, the roasting degree is a crucial step towards the development of the sensory properties affecting consumer preference. Furthermore, both the physiological indices – FP number and PROP taster status – and the caffeine metabolism rate play a significant role in taste sensitivity for bitter compounds in general and also in the preference for coffee with or without sugar.