Food Flavours
Definition Of Flavour:
What is flavour? There are two main definitions of flavour which depend upon the viewpoint of the definer. Flavour can refer to a biological perception, such that it is the sensation produced by a material taken in the mouth, or flavour can refer to an attribute of the material being perceived. The attribute is the aggregate of the characteristics of the material that produces the sensation of flavour. Flavour is perceived principally by the aroma receptors in the nose and taste receptors in the mouth. However, flavour descriptors, such as hot, pungent and biting, are also given to sensations received by the general pain, tactile, and temperature receptors in the mouth, nose and eyes.
Whether flavour refers to the chemicals responsible for the stimulation or the biological receptor stimulation itself, is immaterial to the consumer of foods. Consumers consider flavour one of the three main sensory properties decisive in their selection, acceptance, and ingestion of a particular food. The other two sensory properties are appearance and texture (Figure 1.1). We are all familiar with the basic five senses: sight (eyes), taste (tongue), odour (nose), hearing (ears) and touch (fingers, mouthfeel). The sense of touch, giving mouthfeel, can be broken down into three sensations: pressure, trigeminal and kinaesthesis. Pressure represents the feeling when force is applied over the surface of the food, trigeminal refers to a pain sensation and kinaesthesis denotes feedback from masticatory muscles during chewing.
Classification Of Food Flavours:
Flavours can be classified by the general sensations that one feels when eating different foods. Flavour comes from three different sensations: taste, trigeminal and aroma (odour). It is generally agreed that taste sensations are divided into four major categories: saltiness, sweetness, sourness and bitterness. However, some Japanese scientists also include a fifth category called umami (savoury) that can be represented by the flavour of glutamate. Trigeminal sensations give us the descriptors of astringency, pungency and cooling. Both taste and trigeminal sensations occur upon contact with food in the mouth, as most substances which produce these flavours are non-volatile, polar, and water-soluble. For aroma sensations to occur, an aromatic compound must be sufficiently volatile to allow detection at a distance. The physical interaction between the volatile compound and the receptor site occurs in the nasal passages. Those molecules that reach the olfactory receptors, either via the nasal passage or oral passageway, trigger the odorous sensations.
However, food flavourants are usually classified by the food sources in which they are normally detected because more than one flavour sensation is usually triggered by a food flavourant. Given a specific flavourant, the food industry wants to know what type of image the average consumer will envision when he or she encounters it. For example, celery flavourant (from an extract of celery seed) used in a soup is bitter with a floral aroma, but to an average consumer this flavourant just elicits the thought of celery soup.
The problem with using food sources to classify flavours is that flavours may vary with the history of the food source. For example, fresh cabbage has a quite different aroma than cooked cabbage and sauerkraut is a vastly different olfactory and gustatory experience! Thus, classifying flavours by food source is somewhat arbitrary, with the processing method frequently denoted in the descriptive name of the flavour.
Chemical Compounds Responsible for Flavour:
The many different possible flavours are due to interactions of chemical compounds with taste, trigeminal or aroma receptors. The characteristic task (including trigeminal stimulations) of a food is normally related to a single class of compounds. But an odour is usually elicited by a combination of volatile compounds each of which imparts its own smells. Differences in characteristics of certain aromas can be equated to the varying proportions of these volatiles. However, some substances contain trace amounts of a few volatile compounds that possess the characteristic essence of the odour. These are called character-impact cmpounds.
Difficulties Of Flavour Chemistry Research:
There are many problems facing the flavourist trying to relate chemical compounds to flavours. Flavour compounds can be found in any class of chemical compounds: neutral compounds, acids, nitrogen and sulphur compounds, compounds with high volatility, compounds with low volatility, etc. These compounds are susceptible to chemical changes of various kinds. For example, aldehydes are easily oxidized to acids; amines may complex with metal ions; in the presence of acids, terpenes rearrange and isomerize; exposure to light may cause photo-oxidation or rearrangements; and polymerizations of unsaturated compounds do occur. These transformations are a real concern during the collection and concentration of foods for flavour determinations, particularly because there is such a low quantity of flavour compounds in foods. The instability of many compounds generates artefacts during the isolation of flavours, e.g., when flavours are isolated by bubbling air through oils, the triglycerides are oxidized and so new flavours are formed. Flavours also change with time and processing conditions, e.g., freshly squeezed orange juice is easily distinguished from juice reconstituted from frozen concentrate, juice that is bottled or canned and even from freshly squeezed orange juice that has been allowed to stand at room temperature open to the air for an hour or two.
Objectives Of Flavour Chemistry Research:
Given all the difficulties expressed above, why do people accept the challenge of flavour research? Someone who is willing to untangle all the complexities of food flavours is a significant asset to the food industry. If the chemical composition of flavours is understood, then better control of these flavours is possible. If we can understand the mechanisms of their formation, then we can suppress unwanted flavours and enhance the desired ones.
During the growing and processing of foods, off-flavours occasionally develop. It is important to the quality of these foods to try to prevent or at least retard this process. Sometimes fresh flavours need to be restored to processed foods, so research is required to determine what gave the unprocessed food its original fresh flavour and what happened to this under the conditions of processing. Other times, processing creates flavours, so a thorough understanding of the reactions that produce flavour compounds can help the food processor to improve the flavour of his foods by accelerating these reactions. Occasionally, added flavour compounds not naturally found in certain foods will improve their flavours. In this way, new food items are created.
There are many different flavour-related jobs in the food industry, which include work in creation, analysis, applications and general flavour science. The creative role is very specialized. The primary focus of the creative ‘flavourist’ is to create flavours by combining pure chemicals or any of thousands of plant, microbial and/or animal extracts together, much like a painter mixes colours on a palette. The flavourist continuously builds on previous knowledge, as an artist, to blend flavouring materials in new ways. It is the flavour applications expert who takes that blend and applies it to a variety of different canvases. The applications expert is concerned with the flavour quality of the overall product. He or she must have a strong food science background, understanding both food chemistry and processing and their effects on foods.
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