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Chemistry of Flavour

Learn more about the chemistry of flavour.
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Flavour is the sensory impression of food or other substances, determined mainly by taste and smell. Working together with our sense of smell, around 10 000 taste buds in our tongue elicit various sensations. Flavour is a complicated subject and it is very rare for just one substance to be responsible for the flavour of a particular food or drink. Normally a cocktail of low-boiling organic compounds are present. For example, chocolate contains around 300 flavour components. One area of current interest is understanding why certain flavours go together, for example, strawberries and cream. Perhaps both components have many important aroma compounds that are the same, or similar?
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It is also interesting to understand why different people have different reactions to the same food or drink - remember the Marmite marketing slogan ‘love it, or hate it’? The taste, aroma and mouth feel all contribute to the overall
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flavour of a food or drink: two of which are chemical senses, the taste and the aroma. The expectations of the taster, the look of the food or drink, and the order in which foods are consumed all play a contributing role to the overall flavour and, when combined with chemical phenomena occurring when you taste something, the science of flavour becomes an incredibly complicated system. Located on the tongue, the taste buds are where a person’s sense of taste originates. Each taste bud contains many taste cells; taste cells are long and thin and stretch from the opening of the taste bud in the mouth, to the nerve cell.
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There are several taste cells in every taste bud, which are able to recognise five main tastes - sweet, salty, bitter, sour and umami. Umami has only recently been acknowledged as a separate taste. It is the taste of savoury things, like cheese and meat, and the taste comes from the amino acid L-glutamic acid. Each taste cell reacts to different taste-provoking molecules, or tastants. Sweet, bitter and umami taste signals trigger a response without entering the taste cell. The tastants are small molecules that bind to the receptor proteins that are on the surface of the taste receptor cell. The binding is very specific, as the tastant has exactly the right shape for interaction with the receptor protein.
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Once bound, the receptor protein changes shape, which initiates a series of chemical reactions that leads to the release of a neurotransmitter that activates nerves to the brain, registering the taste. Salty and sour taste signals operate differently. Ions, such as Na+ and Cl- in sodium chloride (or common salt), can bind to ion channels, which are proteins that assist the ions entering inside the taste receptor cell. This leads to neurotransmitters being released that activate nerves to the brain, registering it as a salty or sour taste. In acidic foods and drinks, the acids (such as citric acid) liberate hydrogen ions, or protons, that bind to the ion channels.
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While humans have only five primary tastes, there are a much larger number of aroma receptors. When a food is placed into the mouth, food vapours reach the nasal lining and interact with the aroma sensors, which are combined with the taste sensors to give a food or drink a specific flavour. In taste sensors, the taste bud (a specialised sense organ) is attached to nerve cells. Whereas aroma sensors are nerve cells themselves, which are directly connected to the brain. Without smell foods and drinks tend to taste bland and have no flavour. Research is still being undertaken on aroma sensors and how they bind to compounds, as the process is still not fully understood.
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The last contributing factor to flavour is the mouth feel of a food or drink. As well as taste sensors (taste buds) in the mouth, there are also other sensors that measure the mouth feel; reacting to pressure, temperature and irritating substances, picking up heat, cold, textures and the feel of carbonated drinks on the tongue, among others. All of these contribute to the mouth feel and, hence, the flavour of the final food or drink. Overall, there are three main contributors to the flavour of a substance; taste, aroma and mouth feel. Throughout this week we will look at the main flavour components of beer, tea and coffee and see how different organic compounds contribute to the final taste of a brew.

Favourite Smells

A survey of 2000 UK adults in 2015 showed that freshly baked bread was their favourite smell, followed by bacon, newly-mown grass then coffee. Interestingly, other than newly-mown grass, they are things that can help make a home more warm and welcoming. Food and drink smell featured heavily in the poll with orange, coconut, Christmas cake, lime, and doughnuts proving popular. Cherry, marzipan, popcorn, and wine also made the top 50 as well as cheese and biscuits.

At the other end of the spectrum, bins were named the most disliked smell, along with drains, body odour, sewage, vomit and rotten milk and food.

Researchers found that 60% of people believed there are certain smells that remind them of particular people or places, while 70% associated a smell with memory.

What is your favourite food or drink smell and does it remind you of anything?

For the thiol fans, you might like to know that according to the 2000 Guinness Book of World Records, ethanethiol (CH3CH2SH) is the ‘smelliest substance in existence’.

Smells Good?

Aside from foods and drinks, perhaps one or more of these everyday fragrances will interest you? (Organic compounds that contribute to the aroma are shown in brackets.)

New car smell (toluene, ethylbenzene, styrene, xylenes)

Wet dog smell (benzaldehyde, phenylethanal, ethanal, phenol, 2-methylbutanal)

Halitosis or bad breath (methanethiol, hydrogen sulfide, dimethyl sulfide)

Rain (palmitic acid, stearic acid)

Old books (benzaldehyde, vanillin, ethylbenzene, toluene, 2-ethylhexanol)

Newly-mown grass ((Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexenyl ethanoate, methanol, ethanol)

Headline News – Love It or Hate It

Researchers at York have discovered a potential link between eating Marmite and activity in the brain. It appears that Marmite increases levels of a specific neurotransmitter – known as GABA (γ-aminobutyric acid, H2NCH2CH2CH2CO2H) – in the brain. This is likely due to the high concentration of vitamin B12 in Marmite. So, dietary intervention may affect GABA imbalances, which are associated with a variety of neurological disorders.

Tongues Detect Odours

New research indicates the tongue detects not just taste, but also odours. In this work, human taste cells responded to fragrances, including a clove-scented compound called eugenol (C10H12O2), even though the concentration of these substances was below the level necessary to trigger a taste response. It opens up the possibility of using odours to trick us into healthier eating. For example, adding a very low concentration of an odour to a food could make us think it is sweeter than it is – thereby reducing the need for sugar and helping tackle the obesity crisis.

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Exploring Everyday Chemistry

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