Monday, August 20, 2012

Perceived Flavour (or Flavor if your spell it without an u)

As chefs, cooks, food producers, product developers, and scientists it helps to understand flavour perception, particularly if we want to develop foods that are widely liked and nutritionally sound. What follows is a basic overview of how we perceive the flavour of foods – the information should provide an interesting basis for your own discoveries about flavour.

The positive expectations of consuming a food begin when we visualize it and handle it, but the majority of sensory information comes when we take the first bite.  The flavour of a food is a combination of three independent sensory systems are activated, 1/taste, 2/smell, and 3/oro-nasal somatosensations (irritation, thermal, texture) making flavour a multi-sense experience. 



The term ‘taste’ is often used when ‘flavour’ is the more appropriate word. This is because flavour is a combination of others senses beside taste.  It is widely accepted that there are five basic taste qualities (sweet, sour, salty, bitter, and umami), yet there are many more flavours experienced when we eat a food or drink a beverage.  The sense of taste is housed in the mouth and the majority of taste receptor cells (TRCs) are organised into rosette-like structures called taste buds, which are embedded in folds or lingual bumps called papillae located on our tongue, others are located in other areas of the mouth and throat.  The taste system is stimulated when a food or beverage is placed in the mouth, the food is manipulated by the teeth and tongue, saliva is mixed with the food, new surfaces are created as the food is broken down, and during this process non-volatile compounds in the foods are in contact and stimulate TRCs. The chemically sensitive apical end of a TRC is a small membrane region near the tongue surface.

The importance of taste to overall flavour is illustrated in the development of high intensity (HI) sweeteners to replace sucrose.  For a HI sweetener to perceptually mimic sucrose, it must match the dimensions of flavour: quality, intensity, time course, and location.  HI sweeteners (e.g., aspartame, neotame, sucralose, saccharin…) match sucrose for quality (sweet), and the intensity of sweetness can be matched.  We have unconscious knowledge of the time course and location dimensions of sucrose that enables them to discriminate between a sucrose solution and a HI sweetener solution matched for intensity. The time course of sensation differs between sucrose and HI sweeteners as the sweetness of HI sweeteners tends to linger in the mouth longer than the sweetness of sucrose.  In addition, some HI sweeteners activate bitter taste.  The overall problem is that HI sweeteners match some of the flavour dimensions of sucrose, but not others.  Flavour dimensions are important because consumers have developed a flavour preference for sucrose, and while they do not cognitively assess time course and location differences elicited by HI sweeteners, they recognize it is not sucrose.  For a product to be successful, the consumer has to relearn and like the ‘new sweetness’.    

A common misrepresentation of taste is the often-recited theory of a tongue map (this was due to a mistranslation of a German PhD thesis into a text book back in the early 1900’s).  The tongue map states that the tip of the tongue is sensitive to sweet, the back of the tongue is sensitive to bitter, the sides to salt and sour (no mention of umami in the tongue map).  However, all taste qualities can be experienced at all sites in the oral cavity that contain TRCs.  To convince yourself that areas of the tongue respond to all qualities, dip the tip of your tongue into solutions of tonic water or strong coffee to assess bitterness, honey to assess sweetness, salt water to assess saltiness, lemon juice to assess sourness, and consommé to assess umaminess.  As you will find, all five qualities may be elicited from the tip of the tongue. 



The immense diversity of flavours we associate with foods are primarily derived from the volatile compounds (released into the air) that are released in the oral cavity when food or liquids are chewed and swallowed.  The scientific importance of the sense of smell was recently highlighted when Axel and Buck were awarded the 2004 Nobel Prize in Medicine for their work on olfactory receptors (ORs) and organisation of the olfactory system. There are approximately 1,000 genes for ORs in the mammalian genome making it the largest family of G-protein coupled receptors, but in humans on only approximately 36% of the genes remaining functional.  If the volatile compound has a structure recognized by the receptor, the a signal is sent to the processing regions of the brain and we perceive an aroma quality (e.g., rose, caramel, cut grass).  There are two routes to activate the sense of smell, orthonasal and retronasal. First, orthonasal, this is active sniffing or the act of breathing through the nose.  Think about walking past a bakery and the aromas that are coming from it – you are not eating the baked goods, but you can smell them. Second, retronasal, this is when the volatile compounds released from the food in the mouth take a passage to the nose at the back of the mouth, top of the throat.  Both are used when you are tasting beer or wine, the assessor will first actively sniff the product before placing in the mouth, then when in the mouth close the lips and breath through the nose. Retronasal is associated with the flavour of the food or beverage as the food is in your mouth and the senses of taste and somatosensory (see below) are involved, as well as the sense of smell.

A simple experiment demonstrates the influence the sense of smell has on flavour perception; if you taste a grated apple and onion it is very easy to distinguish between the two, yet with your nose plugged (index finger and thumb pinching your nostrils closed) it is near impossible.  When the nose is plugged, there is no airflow over the olfactory epithelium effectively removing aroma from the overall flavour and we must distinguish between the two samples by taste and texture attributes alone. 



Irritant and textural sensations are also perceptual components of flavour.


Free endings of individual nerve fibers innervating both the oral and nasal mucosa have sensory receptors that respond to both heat and cold both of which evoke thermal and pain sensations.  The oro-nasal nerve fibers are not independent sensory systems, but a component of the pain and temperature fibers that occur throughout the body.  Our mouth and nose (and other mucus membrane regions) are particularly sensitive to certain chemical irritants due to a porous skin surface allowing chemicals to diffuse through the protective barrier to the nerve endings beneath.  A common feature of oro-nasal chemical irritation is the delayed response of sensation relative to that of taste or smell, due to the time taken for the chemicals to diffuse through tight junctions or epithelium to engage receptors on the nerve fibers- think about eating a chilli pepper and the time taken for the heat (and pain) to build.

There are a number of chemicals that are capable of activating irritant sensations and different adjectives to describe the sensations; the burn of chili pepper, the warmth of ethanol, the tingle of CO2, the pungency of wasabi.  While there is not diversity of flavours we associate with the sense of smell, what we experience from chemical irritants often adds to the complexity of flavour.


The importance of texture in flavour perception should not be underestimated.  Our mouth contains nerve endings that respond to touch, pressure, and vibration and we have muscles, tendons, and joints that convey information on chewing foods to flavour processing areas of the brain.  The first bite and manipulations of the food are the most important in assessment of texture.  When you bite into fresh bread, the first few jaw movements collapse the food structure and provide information about the quality and flavour of the bread.  As the process of chewing continues the texture of the food changes as particle size of the food is reduced and a bolus suitable for swallowing is formed with the addition of saliva.  The thought of eating entirely puréed foods would not only modify the pleasure of eating but also cause problems identifying the foods you eat as texture conveys important characterizing information.

Both sight and hearing are involved in flavour perception.  You have heard the saying ‘you first taste with your eyes’, to an extent it is true, sight sets up expectation and expectation can be persuasive!  Think about an orange flavour in a red jelly, 9 out of 10 people will pick the flavour as strawberry or raspberry – a flavour we associate with red coloured fruit.

Finally hearing, which is very much associated with the perceived quality of food.  Think about a wilted celery stick, it lacks the crispness (basically sound) expected and makes the celery stick less liked even though the taste and smell components are identical to a crisp celery stick.  The same situation will occur with potato crisp, if they lack the crispness and noise expected when eating, they are less liked.

Flavour is fun – my daughter Hannah and I experiment with Jelly Belly candies – close your eyes, your partner in the flavour experiment places the Jelly Belly in your mouth.  Next, try to identify the flavour.  Or if you are by yourself,  you can block your nose with thumb and index finger, place the Jelly Belly in your mouth, use the sense of taste first, what do you experience?  Then release your nostrils, breath out through them and experience the flavour of the Jelly Belly. 

Flavour is complex, but every day, chefs, product developers and occasional cooks manage to use the knowledge they have to produce foods we all consume. While an in depth understanding is not essential to produce foods, it is an interesting topic area and understanding how the senses work together to produce flavour may help you produce new and interesting versions of foods you like.

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