Flavour compounds, defined

Non-volatile flavour compounds in the sense of this chapter are defined as molecules which cause one of the sensory impressions sweet, bitter, salty, sour or umami (Fig. 23.2). 

Process flavourings are more related to cooking than to chemical synthesis. The reactions that occur are chemically very complex with hundreds of volatiles and nonvolatiles being formed. The final process flavouring is defined by the sum of the sensory effects of all aroma-active volatiles and taste-active compounds. 

The most difficult problem in flavour research is to interpret the results of the volatile analysis, which gives information on the identity and the quantity of the volatile compounds collected from a given product. Many volatile compounds are not flavour-active, i.e. they cannot be detected in the olfactory system, while others may even in trace amounts have significant effects on flavour owing to their low odour-threshold values that is defined as the minimum concentration needed to produce an olfactory response. Consequently, the most abundant volatiles are not necessarily the most important contributors to flavour. Much... 

An aliquot of the extract which was used for the first GC-O experiment is diluted with the solvent, usually as a series of 1+1 or 1+2 dilutions and each dilution is analysed by GC-O. This means that in each GC run the assessor records the retention time of each odour along with a descriptor of that odour. This procedure is continued until no odorants are perceivable. The highest dilution at which a compound can be smelled is defined as its flavour dilution (ED) factor. The ED factor is a relative measure, and is proportional to the OAV of the compound in air. 

A flavouring substance is a defined chemical component or substance with flavouring properties (Tab. 3.3). Various synonyms are in use, e.g. flavour substance, flavour chemical, flavour(ing) component, flavour(ing) compound, flavouring agent, aroma compound, aroma chemical, and others. Since flavour includes both taste and odour, a flavouring substance may be a substance that causes either taste or odour impressions, or both. 

Contrary to microbial flavour generation directly in the food by starter cultures, the technical bioreactions for flavour production with micro-organisms do not use the complete food raw material as substrate. Isolated and purified single components of food are used as substrates for the micro-organisms. Examples are butterfat from butter, proteins from meat, carbohydrates from plant food materials. Microbial material syntheses may lead to chemically defined pure substances (cf. chapter 3.2.1.1.2). It is also possible to obtain complex mixtures of different compounds. Polysaccharides, natural colours and also complex flavour extracts belong to this category. Figure 3.17 outlines the principle of such processes. 

Micro-organisms also have a potential to form fruity components like alcohols, esters, lactones, ketones and terpenes. These substances are, however, also obtainable by chemical syntheses as well as by relatively simple extraction from the natural fruit. Additionally, fermented fruit flavours in many cases are not very typical for a certain fruit species and therefore may be used only as raw materials for compounded flavours. Presumably this is the reason why the corresponding processes have not yet been transferred onto an industrial scale. This is not true for the fermentative generation of well defined single substances which may serve as natural flavour raw materials for compounded flavours (cf. chapter 3.2.1.1.2). 

At this point, it is important to clarify a few terms that are often used in the context of this chapter. Flavour is usually divided into the subsets taste and smell, which are perceived in the mouth and the nose, respectively [3], The terms aroma and odour are not well defined and often used as synonyms. Qdour is best reserved for the smell of food before it is put into the mouth (nasal perception) and aroma for the retronasal smell of food in the mouth. In this paper, we mainly use the terms aroma and taste , as well as flavour comprising sensory notes imparted by both volatile and nonvolatile compounds (odorants and tastants). In the public domain, however, taste is often used as a synonym for flavour . Proper definitions of these terms are ... 

Natural flavouring Consisting of simple substances, chemically defined, or of compounds extracted from vegetables, or sometimes animal substances, natural or manufactured, suitable for human consumption, exclusively by physical means. 

Artificial flavourings Aromatic substances, chemically defined which are not found in natural products, and which are obtained by organic synthesis, and compounded or not with natural flavourings or nature-identical flavourings. 

Chapter 3 deals with the raw materials which are of interest for the flavour sector. The topics range from chemically defined flavouring substances, both of natural and synthetic origin, to flavouring preparations and source materials, such as complex natural extracts, essential oils and juices. Eurthermore, process flavourings and nonflavouring compounds which are important for food technology are also presented. 

One promising approach might be the application of defined starter cultures that are capable of growing rapidly and that are highly competitive in the environmental conditions under which the product is kept Therefore, numerous attempts have made especially to ferment cabbage to sauerkraut, but also to ferment cucumbers and olives by defined starter cultures. Especially in the case of sauerkraut, most of the experiments performed with pure cultures or mixtures of pure cultures led indeed to the desired uniform, sufficient and rapid acidification of the products. But they lacked the typical rounded sauerkraut flavour, which essentially is due to the lack of certain fermentation by-products or the insufficient decomposition of compounds characteristic for the respective raw material (Kandler, 1981). Sauerkraut with perfect sensorial characteristics was obtained with selected strains of L. mesenteroides, which simultaneously improved the uniformity of sauerkraut from different batches. However, even in this case, so far no such benefits could be achieved compared to spontaneous fermentation that would economically justify the cost of the production of starter cultures (Buckenhueskes Hammes, 1990). 

In flavourings that are not chemically defined compounds, but mixtures of many substances, the following can be distinguished ... 

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