Institute of Food Research
Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK
www.ifr.ac.uk Tel: +44(0) 1603 255000 Fax: +44 (0)1603 507723
Taste and Flavour
- Recognise that some people are more sensitive to bitter tastes than others
- Give examples of foods with bitter tastes that some people dislike
- Explain why some people are able to taste bitter compounds but others are not
- Describe the inheritance of the PTC tasting trait (TAS2R38 gene)
- Suggest reasons for populations with different numbers of supertasters
Key words: evolution, homozygous, dominant, allele, genotype, phenotype
There is a single gene that codes for a protein (receptor) found in our taste buds known as taste receptors, type 2 (TAS2R38). If a person has this protein receptor then they are able to detect the bitter taste of a chemical called phenylthiocarbamide (PTC). PTC can bind to the receptor and the person will be able to taste the chemical. If the individual does not have the gene, and hence the protein is not present, PTC cannot bind to the taste buds and the person cannot taste it.
Research has shown that TAS2Rs are coupled to the G protein gustducin. Researchers use two synthetic substances, phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP) to study the genetics of bitter perception. PTC and PROP can taste very bitter to some people, but are virtually tasteless to others. Among the tasters, some are so-called “supertasters” to whom PTC and PROP are extremely bitter. The variation in the ability to taste substances amongst people has been determined to be a genetic trait. This has led to great interest from geneticists and anthropologists investigating diverse groups around the world.
It is also of interest to evolutionary biologists and nutritionists since many naturally bitter compounds also happen to be toxic but so too are foods high in antioxidants such as broccoli or coffee. Those individuals with the ability to taste PTC may have acquired this trait through an evolutionary protective mechanism that now puts them off healthy foods such as green vegetables.
Being able to taste PTC is a dominant trait. About 2/3 to 3/4 of people in western cultures are able to taste it, while 1/3 to 1/4 will not. Individuals who are homozygotes for the dominant allele are commonly described as “supertasters”. Having two copies of the gene means that they produce more proteins or more binding sites for PTC.
The proportions of tasters and ‘non-tasters’ varies in different parts of the world. It has been reported that 100% of Native Americans, about 70% of westerners, and 50% of Aboriginal peoples are tasters and supertasters are more likely to be African-American, Asian or female.
A wide variety of responses to PTC have been described and may elicit tastes such as sour, sweet, salty and occasionally more obscure tastes.
Tasters are more likely to avoid bitter foods such as green vegetables, coffee and tea as well as smoking
There are about 30 genes in total that code for different bitter taste receptors in mammals.
Phenylthiourea papers, taste bitter to seven out of ten people, and sodium benzoate papers taste sweet, salty, bitter, or tasteless to different people.
Tongue rolling starter
The ability to roll the tongue is a genetic trait and this is commonly used in class to start topics on genetics. It is a good way to start this lesson to introduce the topic of genetics to pupils or remind pupils about what they know and elicit their level of knowledge. It’s not just the way you roll your tongue that is decided by your genetics, your sense of taste is determined by the genes you inherit from your parents.
Genetic taste test strips
4 Test strips are available
o Sodium Benzoate
Pupils should each be provided with a control strip first. This will enable pupils to distinguish between the residual taste of the paper strips and the potential taste of the test substances. Results of the taste tests should be collated either on a whiteboard or flipchart so that pupils can calculate the percentage of tasters or super tasters in the class. This data can then be compared to the results found in different ethnic groups, genders or in the results from other investigations.
Do not inform pupils of the possible taste prior to the test as suggestion and prior expectations can have a strong influence on the perceived taste. It is also beneficial, if possible, that all pupils try the taste strips at the same time and do not react too strongly to any tastes they experience as this may put off other pupils from even trying the strips. Despite this some people find the tastes extremely strong and it is also wise to ensure that there are sweet flavoured drinks or water on hand for those pupils who have a strong taste sensation. One end of the taste test strips should be placed in the middle of the tongue. Saliva is often required and there is often a delay before pupils will be able to taste the strips, if at all.
Dispense taste test strips while wearing gloves to prevent any transmission of germs and as with all lab activities involving food ensure pupils wash their hands prior to the activity (not just after the activity). Pupils should record the intensity of the taste as none, weak or strong as well as describing the taste according to one of the five basic tastes, sweet, sour, salty, bitter or umami.
Repeat with PTC, thiourea or sodium benzoate.
Pupils can be classified as non tasters if they did not taste anything with PTC. Weak tasters or supertasters according to the strength of their reaction.
Ask pupils if their results correlate with the number of papillae (taste buds) they could locate on their tongue (see activity Taste Buds).
Confirm with pupils who could not taste PTC that they had identified taste buds on their tongue and then enquire of the class possible reasons for some individuals not being able to taste PTC yet having taste buds (if required hint back to the starter activity).
At this point you can provide more information about the genetic traits of supertasters and the response to PTC.
Depending on the age and ability of the pupils you may want to ask them to draw conclusions from the results about the type of genes or alleles associated with the supertaster trait.
What might be the genetic advantage or disadvantage of being a super-taster?
Is being a ‘supertaster’ related to liking or disliking marmite?
Are ‘supertasters’ more sensitive to stimulation of the trigeminal nerve by chopping onions?
Health and Safety Information
It is recommended by CLEAPSS that pupils do not taste more than two PTC strips.
It is strongly recommended that children and their parents do not participate in this activity together.
The ability to taste PTC is a trait which is inherited from parents.
The results of the taste tests could lead to anxiety to parents and children who participate
together in this activity. As a dominant trait, and one that can produce a strong reaction in homozygous individuals, a lack of response in either parent or adult and a strong response in the other could be interpreted as a lack of relatedness. This assumption may not take into account other genetic variations including in TASR38, age or lifestyle such as smoking.
However, as this activity could cause undue distress it is recommended that situations that could lead to parents and children both taking part are avoided. It is also sensible to take precautions against pupils taking the taste test strips home.
Survival Rivals – A question of taste http://survivalrivals.org/a-question-of-taste/about
DNA learning centre – Using a single-nucleotide polymorphism to predict bitter tasting ability