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
For Teachers
Taste Basics
Learning objectives:
- Recognise the five basic tastes
- Describe the five basic tastes
- Distinguish between tastes
- Give examples of foods with the five basic tastes
- Describe tastes other than the five basic tastes
Key words/ Word cloud
taste, bitter, salty, sweet, umami, sour, fatty, metallic, taste bud
Background
Basic tastes are detected by taste receptor cells or ‘Taste Buds’ on your tongue, throughout your mouth. Each taste bud has 50-150 taste receptors sensitive to certain chemicals. When these chemicals dissolve in the saliva in your mouth, a signal is sent straight to your brain.
The basic tastes are the commonly recognized tastes that you will be familiar with including bitter, salty, sour, sweet, and a fifth taste termed umami Taste is detected by sensory organs called taste buds, found on the upper surface of the tongue and other parts of the mouth. Umami (the flavour of certain glutamates, variously described as savoury, meaty, or brothy) has long been known in Asian cooking, but has only recently been found to have its own taste receptors.
Receptors that recognise a large number of specific chemicals have been identified. The specific chemicals that can be detected include sodium, potassium, chloride, glutamate and adenosine.
There is scientific debate concerning the existence of basic tastes as identified by chemical reactions between receptors for individual molecules and the perception of taste as a combination of experience, cultural expectations and psychological states.
The final ‘flavour’ experienced by an individual may be somewhat different to the biochemical stimulus that an individual is exposed to.
The sense of taste is termed gustation.
There are Five basic Tastes
Salt
The taste of salt is due to sodium chloride (and to a lesser degree other salts). Sodium (Na+) and other salt ions can pass through ion channels in the membranes of taste receptor cells on the tongue. This triggers an action potential leading to an electrical signal along a sensory nerve fibre. The taste of salt can be blocked by an ion-channel inhibitor called amiloride, indicating that this is the biochemical basis of the taste of salt. The sensitivity to the taste of salt can differ depending on factors such as salt depletion and dehydration. Salt can also modify other senses of taste increasing their potency. This explains why tomatoes have a ‘fuller’ flavour with the addition of salt.
Sour
The taste of sour is due to acids like vinegar, lemon juice or malic acid found in ‘sour sweets’. Sour tastes are detected by ion channels similar to those for the taste of salt. Hydrogen ion channels detect the concentration of acids or hydrogen ions (H+ ions) in food. Hydrogen ions are capable of passing through the amiloride-sensitive sodium channels, but they can also inhibit potassium channels and prevent hyperpolarisation of taste receptor cells. The combination of depolarisation by passage through the ion channels as well as the inhibition of potassium channels causes the sensation of sourness.
Sweet
The sensation of sweetness is produced by sugars, sugar substitutes and some proteins. Aldehydes and ketones are also often found to be sweet. These substances are detected by receptors that are joined to the G protein gustducin, found on the taste buds. There are a variety of similar receptors for sweetness and more than one type needs to be activated to perceive the sweet taste.
The lowest concentration of sugars that can normally be detected are about 10 millimoles per litre for sucrose and 30 millimoles per liter for lactose (see activity Taste Threshold).
Bitter
Bitter tastes are initially unpleasant and often require a period of exposure to ‘acquire a taste’ for foods that are very bitter such as beer, coffee or quinine found in tonic water.
Bitter tastes may indicate toxins or spoiled food. Most toxic plants and many poisons taste bitter and evolutionary biologists think that a dislike of bitter tastes evolved as a defence against accidental poisoning. There are many bitter foods and beverages are not at all harmful and even healthy, such as brussel sprouts, but their taste is off-putting to many people.
There are 25 different types of ‘bitter’ receptors. Research has shown that TAS2Rs (taste receptors, type 2) coupled to the G protein gustducin are responsible for the human ability to taste bitter substances (see activity Taste Genetics). They are identified not only by their ability to taste for certain “bitter” ligands, but also by the morphology of the receptor itself (surface bound, monomeric).
The bitterest substance known is a synthetic chemical called denatonium. It is often added to toxic substances to prevent accidental ingestion. It is odourless but creates a strong taste response that may even lead to vomiting creating the added benefit of stimulating the regurgitation of any consumed toxins.
Savouriness or Umami
The taste of savoury foods is the ‘fifth’ taste and is produced by free glutamates (forms of the glutamic amino acid). These glutamates are often found in protein rich foods such as meats and mushrooms as well as fermented and aged products such as soy sauce and parmesan.
In Japanese, the term umami (旨味, うまみ) is used for this taste sensation, whose characters literally mean “delicious flavour.” Umami is now the commonly used term by taste scientists.
Not every glutamate produces a savoury-like taste sensation and there is continuing investigation into the exact mechanism of how the savoury taste sensation is produced. Glutamate binds to a variant of G protein coupled glutamate receptors. Recently it has been discovered that the savoury taste is produced by interaction of NMDA and mGluR4, mGluR1 and taste receptor type 1(TRT1) receptors.
The additive monosodium glutamate (MSG), which was developed as a food additive in 1907 by Dr Kikunae Ikeda at the Imperial University of Tokyo, produces a strong savoury taste. Savoury is also provided by the nucleotides disodium 5’-inosine monophosphate (IMP) and disodium 5’-guanosine monophosphate (GMP). These are naturally present in many protein-rich foods. Together in certain quantities these three glutamates act to produce an even stronger umami taste.
The glutamate taste sensation is most intense in combination with sodium. This is one reason why tomatoes exhibit a stronger taste after adding salt. The molecular gastronomist Heston Blumenthal, along with Scientists at Nottingham University have even studied the relative distribution of umami taste in different parts of the tomato.
There may also be other basic tastes
Fatty
Fat is mainly detected by it’s texture. The taste of triglycerides (dietary fat molecules) may be due to fatty acids that are produced when the lipase enzyme in the mouth breaks down the triglycerides. A potential taste receptor called the CD36 receptor reacts to fatty acids and maybe a de facto fat receptor. CD36 has been found in mice, but probably exists in humans and other mammals as well. Mice with a genetic defect that prevents the CD36 receptor from working do not show the same urge to consume fatty acids as normal mice. They also do not release gastric juices in their digestive tracts in preparation of digesting fat. A better understanding of the biochemical activity of this receptor may lead to an understanding of cravings for fat and treatments for obesity.
Metallic
Scientists have found receptors for calcium and are searching for others. If you’ve ever put a cut finger in your mouth or bitten your lip you may have experienced a metallic taste in your blood.
Water
The taste of water can be hard to test for as most taste sensations are caused by substances in solutions and the presence of saliva. Water may have a sweet taste as a phenomenon known as “sweet water after-taste” occurs when the receptors that normally detect sweetness are blocked and water is tasted.
Polysaccharides
Molecules such as starch are detected by different receptors to those for sweet tastes.
It should be emphasized that there are no more than 5 distinctive tastes: salty, sour, sweet, bitter, and umami.
The 10,000 different scents which humans usually recognize as ‘tastes’ are often actually ‘flavour’ (see activity Taste and Flavour), which many people, who can smell, confuse with taste.
Activity
Pupils will be given a ‘taste’ of compounds that represent the five basic tastes. Sweet, salty, sour, umami and bitter. The taste solutions used are sugar, salt, citric acid, monosodium glutamate and flat tonic water. Pupils can test their sense of the five basic tastes by placing some of each solution on their tongue with a cotton bud. Subjects should be able to identify these equally well with or without holding the nose, because these flavours do not depend on smell.
They will all react differently to umami – some may hate it others may not be able to taste it at all and the taste will range from salty, meaty, fishy or vinegary and several other things too! This is not unusual we don’t all have the same ability to taste, and some of the differences between us are due to our genes. You could do a survey here and present the results.
What you will require
• cotton buds
• gloves
• plastic cups or bowls
• Lemon juice or citric acid
• Sugar
• Salt
• Tonic water or Coffee
• Monosodium Glutamate
Health and Safety
The taste solutions should be made up at a weak concentration. When carrying out taste tests cotton buds are recommended for applying taste solutions to the tongue. It is IMPORTANT to stress that people should not put cotton buds that have been in their mouths back into the taste solutions to prevent cross contamination. If the solutions are applied by a teacher or assistant they should wear rubber nitryl gloves to maintain hygiene and prevent allergic reactions.
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