Flavour / flavouring compounds thresholds

Compositional analyses of flavour compounds in fresh and processed fruits are often of limited value because it is clear that compounds with very low aroma thresholds can have dominant effects upon fruit flavour. Moreover after processing, the compounds with the primary influence on flavour may change. Quantification of individual aroma compounds is also problematic requiring high resolution gas chromatographs linked to ion-trap or related detectors (HRGC/MS) although flame-ionisation detection is often more convenient. 

The sulfur components ethyl S-(+)-2-methylbutanoate and dimethyl trisulfide (with 0.006 and 0.01 pg/L odour thresholds in water, respectively) were reported as impact-flavour compounds in fresh Hawaiian pineapple essence prepared by solvent extraction. The major volatile components were methyl and ethyl esters [59]. 

The following review focuses on the composition of flavour compounds in spirit drinks, their origin, and their sensory attributes like odour quality and threshold value. Important information on flavour-related aspects of technology, like distillation and ageing, as well as the main categories and brands of spirits to be found on the national and international markets are summarised. Finally, aspects of sustainability in the production of distilled spirits are discussed. 

Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

Compound Flavour quality Threshold (mgL water) Threshold (mgT ethanol solution) Typical concentration [mgL (40% v/v)] ... 

In combination with the threshold of a flavour compound in a given matrix, it is possible to calculate the OAV value using the following formula ... 

Hydroxy-2-butanone (acetoin) is a characteristic constituent of butter flavour used for flavouring margarine and can be obtained as a by-product of molasses-based and lactic acid fermentations [49, 71]. The closely related 2,3-butanedione (diacetyl) has a much lower organoleptic threshold than acetoin and is an important strongly butter-like flavour compound in butter and other dairy products [72] in buttermilk, for instance, the diacetyl concentration is only about 2-4 mg [73]. a-Acetolactate (a-AL) is an intermediate of lactic acid bacteria mainly produced from pyruvate by a-acetolactate synthase. In most lactic acid bacteria, a-AL is decarboxylated to the metabolic end product acetoin by a-AL decarboxylase (ALDB) [71] (Scheme 23.5). 

Other flavouring compounds found in reasonable quantities are 14(3098) pen-tanal, 21 l-methyl-l/f-pyrrole, 23(3584) 1-penten-3-ol, 26(2633) (B)-(+)-limonene, 36 cis-2-heplenal, 40(2782) nonanal, 42(2805) l-octen-3-ol and 44(2362) decanal. The compound described as the character-imparting compound in bell peppers is 2-isobutyl-3-methoxypyrazine. This very powerful odour-ant has a threshold in water of 1 part in 10A12 (Govindarajan and Sathyanarayana, 1986). 

Interestingly, the detection threshold of different flavour compounds in water also varies considerably with a grapefruit compound being an astonishing low value of 2 parts in 10 ". So the human sensory system has evolved a highly specialised sensitivity to, amongst other things, citrus compounds. 

The associated analytical instruments separate out some of the chemical compounds in a complex odour according to a physical property, for example, their solubility, mass, mass to charge ratio etc. Reference [1] provides an overview of these different analytical instruments. The disadvantages with these types of instruments for detecting odours are that they are relatively slow (minutes per reading), large, expensive and, more importantly, simply separate out the different constituents of a compound rather than provide a measure of their relative olfactory intensity. Indeed, the smell of a complex odour may well be dominated by a few key flavour compounds which occur below the detection threshold of even these analytical instruments - typically ng/ml. 

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... 

The root of parsnip Pastinaca sativa) is eaten boiled or baked. The major classes of compounds identified in raw and cooked parsnip are monoterpenoids, aliphatic sulfur compounds, and 3-alkyl-2-methoxypyrazines [35]. To the best of our knowledge, no investigations have been performed to elucidate the character-impact compounds in parsnip by modern GC-O techniques however, it has been suggested that volatile compounds such as terpinolene, myristicin and 3-sec-butyl-2-methoxypyrazine maybe important contributors to the flavour of parsnip owing to either their high concentrations or their low threshold values, or both [35]. 

Banana (Musa sapientum L.) is one of the most common tropical fruits, and one of Central America s most important crops. It is grown in all tropical regions and is one of the oldest known fruits [45]. From a consumer perspective, bananas are nutritious with a pleasant flavour and are widely consumed throughout the world [57]. Esters predominate in the volatile fraction of banana (Fig. 8.2). Acetates are present in high concentrations in the fruit and generally possess a low threshold. Isopentyl acetate and isobutyl acetate are known as the two most important impact compounds of banana aroma. Alcohols are the second most important group of volatiles in banana extracts. 3-Methyl-1-butanol, 2-pentanol, 2-methyl-1-propanol, hexanol, and linalool are the alcohols present in higher concentrations in the fresh fruit [45]. 

Table 10.2 presents a summary of odour qualities, odour thresholds in water, and concentrations of some selected volatile compounds, which are characteristic flavour impact compounds, owing to their typical flavour quality and their rather low odour thresholds. These compounds are not formed during fermentation but originate from the raw material and contribute significantly to the typical flavour of a fruit. The components summarised in Table 10.2 are important compounds in wine and different fruits and are discussed later. 

Oxazoles have been found in relatively few cooked foods, although over 30 have been reported in coffee and cocoa, and 9 in cooked meat. Oxazolines have been found in cooked meat and roast peanuts, but not to any extent in other foods. 2,4,5-Trimethyl-3-oxazoline has been regularly detected in cooked meat [26], and when it was first identified in boiled beef [27] it was thought that the compound possessed the characteristic meat aroma however, on synthesis it was shown to have a woody, musty, green flavour with a threshold value of 1 mg/kg [28]. Other 3-oxazolines have nutty, sweet or vegetable-like aromas and the oxazoles also appear to be green and vegetable-like [28]. The contribution of these compounds to the overall aroma of heated foods is probably not as important as the closely related thiazoles and thiazolines. 

Owing to very low thresholds, volatile sulfur compounds (VSCs) usually have prime impact on food aromas they are found in lots of natural sources, including fermented foods (e.g. wine, beer, cheese), and act as both flavours and off-flavours [249, 250]. Although their biogenetic formation has been elucidated in detail, only few biotechnological processes with potential for commercial application have been reported. The sulfur-containing amino acids L-methionine and L-cysteine are the natural precursors of a wide variety of VSCs. Methanethiol is the most frequently found VSC in cheese and can be readily oxidised to other VSCs, such as dimethyl suMde and dimethyl disulfide, or... 

Flavour is a complex sensation, made up principally of smell and taste, but touch and hearing contribute as well. The human senses of smell and taste differ in sensitivity, between each other and depending on the nature of the component eliciting the sensation. Substances may have no impact at all (such as oxygen or carbon monoxide) or exhibit very low thresholds (such as 2 X 10 14g mL 1 water for the odour of bis-2-methyl-3-furyl disulfide).204 In general, odour thresholds are much lower than taste thresholds and so flavour tends to be dominated by odorous components, by substances able to reach the olfactory epithelium high up in the nose, that is, substances with at least some volatility. Hence, the emphasis on the volatile compounds derived from the Maillard reaction. 

The chemistry of the flavour of milk fat and butter is very complex, involving a large number of compounds contributing to the overall aroma and taste. Approximately 200 volatile compounds have been identified in milk fat (Schieberle et al., 1993). However, many of the volatile compounds are present at concentrations below their individual flavour threshold level, and the extent to which these compounds contribute to the overall flavour profile is not known fully. The perceived flavour of milk fat can be altered by a change in the concentration of individual volatile compounds. The principal factor that can change the concentration of the volatile compounds is the feeding regime of the cow (Bendall, 2001). 

At low levels (5 mg/L), diacetyl is considered to add complexity to wine aroma since it can impart positive nutty or caramel characteristics, although at levels above 5 mg/L it can resuit in spoilage, creating an intense buttery or butterscotch flavour, and is perceived as a flaw. Microbial formation of diacetyl is a dynamic process and its concentration in wine depends on several factors bacterial strain, pH, wine contact with lees, SO2 content (Martineau and Henick-Kling 1995 Nielsen and Richelieu 1999). The sensory threshold for the compound can vary depending on the levels of certain wine components, such as sulfur dioxide. It can also be produced as a metabolite of citric acid when all the malic acid has been used up. However, diacetyl rarely taints wine to levels where it becomes undrinkable. 

Other compounds released during autolysis are present in lower amounts, such as lipids and nucleic acids, but could play an important role in the sensorial character of the final wine. Lipids may affect wine flavour in that the fatty acids released could give rise to volatile components with low sensory thresholds, either directly or through derivatives such as esters, ketones and aldehydes (Charpentier and... 

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