As a
perception that is universally positive, sweetness is unique. Moreover, sweetness
is typically liked outside of its usual context of a food or drink. This is in
contrast to all other sensory signals. Salt is great when attached to chips,
red is terrific when painted onto a Ferrari, and Gb peerless when part of your favourite tune. But out of context,
they loose much of their appeal. Not so, sweet taste. In an earlier discussion (see March, 2012: The genetics of cat food),
I talked about the ability of sweet tastes to elicit smiles from human and
animal newborns reflecting the adaptive priority given to our ability to sense carbohydrates.
Even if
preference is always shown for sweetness, it, like every other sensory quality,
are liked to greater and lesser extents, in part depending on the type of food
or beverage. The role of learning here is obvious – even if we have taken sugar
in our tea or coffee for decades, we can quickly learn to like less or even no
sugar through repeated exposure. Hence, given different histories of food
consumption, we would expect individuals to vary in how sweet foods or
beverages should ideally be.
But what
is less considered is whether there might be consistent patterns of individual
differences in sweetness preferences that either can’t easily be explained by,
or may even precede, food choices. Such patterns in bitterness perception and
preference have become familiar in recent years through research on the compound
6-n-propylthiouracil or PROP (see for example [1]). Moreover, the
idea of subsets of consumers similarly showing reliable differences in their
liking for sweet tastes isn’t new. In 1970 [2], Rose-Marie
Pangborn described patterns of variations in both sweet and salty taste
preferences. Many of us would identify with the idea of having a “sweet tooth”
and research on ‘sweet liking’ has indicated that this is around two thirds of
us. Meanwhile, the remaining 30% are known as ‘sweet dislikers’, but this is a
misnomer, since it really reflects a preference for sweetness per se that peaks at a lower optimal than
it does for the sweet likers – not an actual dislike.
One
approach to quantifying the sweet liker/disliker dimension has been to simply
obtain the preferred sucrose concentration using a forced-choice procedure. While
this is useful and reliable as a measure, it will miss differences in patterns
of liking that become evident across a range of sucrose concentrations. Distinct
groups of sweet likers and dislikers can also be segregated by patterns of
increasing or decreasing preferences for the sweetness of sucrose
concentrations. Typically, sweet likers show a monotonic increase as sweetness
increases, reaching an asymptote at or close to the highest concentration. Dislikers,
on the other hand, often show a steep decline that begins at relatively low concentrations.
But it is also clear that some individuals fail to be especially impressed by
sweetness at any concentration, instead showing a moderate liking that is
insensitive to variations in concentration. These patterns, and the proportions
of consumers contained in each, are yet to be definitive, and what’s missing in
all the research is estimates of how people vary obtained from large population
samples.
The
important issue arising from categorizing consumers as sweet likers, dislikers
or ‘indifferents’ is whether or not classification into groups predicts
anything useful. After all, within a culture, there is more or less general
agreement about optimal sweetness levels: fruit should be sweet, but beer
shouldn’t. Such ‘agreement’ can be seen
as the consequence of cumulative experience of the typical products that are on
the market. Nevertheless, given the prevailing sweetness levels of foods and
drinks, do some consumers reject some products because they are just too sweet?
In other words, does classifying consumers into sweet liking groups predict which
of the available foods will be chosen? And if it does, what are the
implications for public health?
As we
stand, none of these questions can be answered with any certainty. But
increasingly, there are research findings that point to a tentative conclusion
that sweet liking might a key determinant of food preferences. Julie Mennella
and her team recently evaluated their method of assessing sweet liking in a
large sample of adults, adolescents and children [3]. Amongst other
findings, they noted a significant positive association between preferred
concentrations sucrose in solution and the sugar content of their favourite
breakfast cereal.
Since
our foods likes (and dislikes) are learned, the importance of the sweet liking
dimension is also illustrated by findings that it can influence preference
development. In this study [4], a novel odour was
repeatedly paired in solution with the sweet taste of saccharin. This ought to
produce a liking for the odour, since the “positive feelings” for the sweetness
are transferred to the odour, a process known as evaluative conditioning. And
so it did – but only for the sweet likers. Consistent with their classification
as individuals who do not find sweetness especially rewarding, the sweet
dislikers failed to develop a liking for the novel odour. Since evaluative
conditioning is thought to be a very common mechanism for food preference
development, the implications of such a finding are critical in helping us
understand how food preferences – and hence food choices - can vary within
populations.
Just one
of the obvious next questions, given the energy provided by carbohydrate
sweeteners, is whether or not sweet liking is a risk factor for obesity. Again,
we need much larger samples of consumers from which ask this question, but
Mennella’s study, plus another one from her lab, failed to find a relationship
between body weight (BMI) and preferred sucrose concentration.
Unlike
the case with PROP sensitivity, for which variations in specific genes are
implicated, there is little strong evidence one way or another that sweet
liking is determined by early food experiences, or genetics, or a combination
of both. However, Mennella also provides
interesting data that bear on the question of the origin of the sweet liker
dimension. Her sample was analysed in
terms of demographic factors including race, education, income and sex. Her
black participants preferred a significantly higher sucrose concentration, as
did those with a lower income. Boys preferred a higher concentration than
girls, and children higher sweetness than adults. None of these findings rule
out a role for taste genetics, but it is important to note that variations in
sweet liking reflects hedonics - and not sensitivity to - sweetness. In other
words, sweetness of equal intensity is liked differently. Mennella herself
notes that black mothers often feed sugar water during infancy and suggests
that this is one plausible origin of the greater liking for higher sweetness
levels in this group.
If it
seems premature to conclude that sweet liking will be an important predictor of
food choices, the research to date nevertheless supports a view that this
variable is worth further consideration. As with variations in PROP sensitivity and
the recently reported thermal tasting, we don’t yet know what ultimate benefits
measurement of sweet liking will bring, but understanding the impact of person-to-person
differences in consumers’ patterns of sensory and hedonic responses is unlikely
to be wasted effort. Sweet liking could
be the factor that mediates the tortuous path between responses to products in
consumer evaluations and what products consumers actually choose to eat.
