Esters flavour compounds

Problems Pear ester (43) is an important industrial flavouring compound with a pear-like taste and smell. Consider all possible Wittig disconnections and choose a reaction which should give the right geometrical isomer. 

Fig. 7.1 Some aliphatic esters that are important flavour compounds in fruits and vegetables that mainly contribute with fruity odours...

Guava is native to Central America. It was distributed into other parts of tropical and subtropical areas such as Asia, South Africa, Egypt, and Brazil by the early seventeenth century [49]. Some examples of impact-flavour compounds have already been identified in guava -ionone [58], terpene hydrocarbons [63], and esters [43] could be mentioned. 

Mango is one of the most popular and best known tropical fruits [44] and possesses a very attractive and characteristic flavour. Some authors reported great differences in flavour compounds (including esters, lactones, monoterpenes, and sesquiterpenes) [14]. 

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]. 

Of course, during processing of fruit juices hydrolysis effects may occur, leading to decreased amounts of ethyl 2-methylbutanoate. However, its enantiomeric purity remains unchanged, whilst the corresponding 2-methylbutanoic acid is found as the (S)-enantiomer (99.5% or more) [33-37]. Consequently, the detection of racemic 2-methybutanoic acid (or the corresponding esters) definitely proves the addition of a synthetic (so called nature-identical) flavour compound. 

The biotechnological production of flavour compounds is particularly focused on esters and lactones. Lipase from Mucor miehei is the most widely studied fungal lipase [30-35]. Esters of acids from acetic acid to hexanoic acid and alcohols from methanol to hexanol, geraniol and citronellol have been synthesised using lipases from Mucor miehei, Aspergillus sp., Candida rugosa, Rhizopus arrhizus and Trichosporum fermentans [32-37]. 

Methylbutanoates and methylbutyl esters are essential flavour compounds in fruit flavours they can be produced biotechnologically as mentioned before. Chowdary et al. [33] have described the production of a fruit-like flavour isoamyl isovalerate by direct esterification of isoamyl alcohol and isovaleric acid in hexane with the help of Mucor miehei lipase immobilised on a weak anion-exchange resin. 

It is well-known that in plant tissues certain amounts of flavour compounds are bound as non-volatile sugar conjugates. Most of these glycosides are jS-glu-cosides, but there are other glycones like pentoses, hexoses, disaccharides and trisaccharides too [46]. Acylated glycosides and phosphate esters have also been reported [47, 48]. Information about the analysis of glycosides can be found in the work of Herderich et al. [49]. 

Fig. 23.1 Microbial routes from natural raw materials to and between natural flavour compounds (solid arrows). Natural raw materials are depicted within the ellipse. Raw material fractions are derived from their natural sources by conventional means, such as extraction and hydrolysis (dotted arrows). De novo indicates flavour compounds which arise from microbial cultures by de novo biosynthesis (e.g. on glucose or other carbon sources) and not by biotransformation of an externally added precursor. It should be noted that there are many more flavour compounds accessible by biocatalysis using free enzymes which are not described in this chapter, especially flavour esters by esterification of natural alcohols (e.g. aliphatic or terpene alcohols) with natural acids by free lipases. For the sake of completeness, the C6 aldehydes are also shown although only the formation of the corresponding alcohols involves microbial cells as catalysts. The list of flavour compounds shown is not intended to be all-embracing but focuses on the examples discussed in this chapter... 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

In some aspects, supercritical fluids, which represent a state between the gaseous and liquid phases, have properties resembling those of non-polar solvents in being adequate for biotransformations of hydrophobic compounds. Although the use of supercritical fluids is not restricted to hydrolases, the use of this class of enzymes, especially lipases, dominates [3, 4]. Esters represent the main flavour compounds produced by this process [5]. 

Many cheeses contain the same or similar compounds but at different concentrations and proportions chromatograms of some cheese varieties are shown in Figure 10.25. The principal classes of components present are aldehydes, ketones, acids, amines, lactones, esters, hydrocarbons and sulphur compounds the latter, e.g. H2S, methanethiol (CH3SH), dimethyl sulphide (H3C-S-CH3) and dimethyl disulphide (H3C-S-S-CH3), are considered to be particularly important in Cheddar cheese. The biogenesis of flavour compounds has been reviewed by Fox et al. (1993, 1996a) and Fox, Singh and McSweeney (1995). 

Esters also constitute a group of important flavour compounds. They are the main aroma components found in fruits (apples, pears,. ..). For example, bananas contain 12-18 ppm acetates. The price of the pure flavour compounds, when isolated from fruit, can range between 10,000 and 100,000 US /kg In the past, research has been carried out by our group about the microbial production of fruity esters by the yeast Hansenula mrakii and the fungus Geotrichum penicillatum [10]. A fermentation was developed whereby fusel oil was continuously converted into a mixture of 3-methylbutyl acetate (isoamyl acetate) and 2-methylbutyl acetate, the character impact compounds of banana flavour. 

In recycled PP, 61 compounds were detected and 35 of them were identified. Many peaks showed very low separation levels making their identification difficult. In virgin and recycled PP the following compounds were identified Ethylbenzene and xylene were found only in the recycled resin. Present in both virgin and recycled PP were a large number of branched alkanes and n-alkanes between Qg and C25. Octadecanoic acid, methyl ester and dibutyl palmitate, which is a typical compound used in the cosmetic industry, were found only in the recycled PP. Amines such as hexamine, 3-ethyl and NIN"N" trimethyl dipropylene triamine were identified in both PP materials. Carboxylic acids and ketones were absent in both polymeric materials and so were fragrance or flavour compounds [113]. 

The majority of volatiles in wines are, however, fermentation compounds, starting with the so-called higher alcohols and going on to important flavouring compounds such acetates as of higher alcohols and the ethyl ester of C4-C10 fatty acids for the fruity scents. The free fatty acids, when in abundance, contribute to the goaty flavour (Meilgaard, 1975). 

The berry or the small fruits consist of strawberry, raspberry, blackberry, black currant, blueberry, cranberry and elderberry. The volatiles responsible for the flavour of small fruits are esters, alcohols, ketones, aldehydes, terpenoids, furanones and sulfur compounds (Table 7.3, Figs. 7.1-7.7). As fruit ripen, the concentration of aroma volatiles rapidly increases, closely following pigment formation [43]. 

Sugars, acids and aroma compounds contribute to the characteristic strawberry flavour [85]. Over 360 different volatile compounds have been identified in strawberry fruit [35]. Strawberry aroma is composed predominately of esters (25-90% of the total volatile mass in ripe strawberry fruit) with alcohols, ketones, lactones and aldehydes being present in smaller quantities [85]. Esters provide a fruity and floral characteristic to the aroma [35,86], but aldehydes and furanones also contribute to the strawberry aroma [85, 87]. Terpenoids and sulfur compounds may also have a significant impact on the characteristic strawberry fruit aroma although they normally only make up a small portion of the strawberry volatile compounds [88, 89]. Sulfur compounds, e.g. methanethiol. 

Important aroma compounds of black currant berries have been identified mainly by GC-O techniques by Latrasse et al. [119], Mikkelsen and Poll [115] and Varming et al. [7] and those of black currant nectar and juice by Iversen et al. [113]. The most important volatile compounds for black currant berry and juice aroma include esters such as 2-methylbutyl acetate, methyl butanoate, ethyl butanoate and ethyl hexanoate with fruity and sweet notes, nonanal, /I-damascenone and several monoterpenes (a-pinene, 1,8-cineole, linalool, ter-pinen-4-ol and a-terpineol) as well as aliphatic ketones (e.g. l-octen-3-one) and sulfur compounds such as 4-methoxy-2-methyl-butanethiol (Table 7.3, Figs. 7.3, 7.4, 7.6). 4-Methoxy-2-methylbutanethiol has a characteristic catty note and is very important to blackcurrant flavour [119]. 

The kiwi fruit is a cultivar group of the species Actinidia deliciosa. More than 80 compounds have been identified in fresh and processed kiwi [137]. Methyl acetate, methyl butanoate, ethyl butanoate, methyl hexanoate and ( )-2-hexenal have the most prominent effect on consumer acceptability of kiwi fruit flavour [137-140]. The volatile composition of kiwi fruit is very sensitive to ripeness, maturity and storage period [138, 139]. Bartley and Schwede [140] found that ( )-2-hexenal was the major aroma compound in mature kiwi fruits, but on further ripening ethyl butanoate began to dominate. Ripe fruits had sweet and fruity flavours, which were attributed to butanoate esters, while unripe fruits had a green grassy note due to ( )-2-hexenal [140]. The most important character-impact compounds of kiwi fruits are summarised in Table 7.4. 

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