with Other Food Constituents
5.5 Natural and Synthetic Flavorings
Aromatized food has been produced and con-sumed for centuries, as exemplified by confec-tionery and baked products, and tea or alcoholic beverages. In recent decades the number of
aro-Table 5.39. Use of aromas in the production of foods Product group Percentage (%)a Non-alcoholic beverages 38
Confectionery 14
Savoury productsb, snacks 14
Bread and cakes 7
Milk products 6
bSalty product line like vegetables, spices, meat.
matized foods has increased greatly. In Germany, these foods account for about 15–20% of the total food consumption. A significant reason for this development is the increase in industrially pro-duced food, which partly requires aromatization because certain raw materials are available only to a limited extent and, therefore, expensive or be-cause aroma losses occur during production and storage. In addition, introduction of new raw ma-terials, e. g., protein isolates, to diversify or ex-pand traditional food sources, or the production of food substitutes is promising only if appro-priate aromatization processes are available. This also applies to the production of nutraceuticals (cf. 19.1.3).
Aroma concentrates, essences, extracts and in-dividual compounds are used for aromatization.
They are usually blended in a given proportion by a flavorist; thus, an aroma mixture is “composed”.
The empirically developed “aroma formulation”
is based primarily on the flavorist’s experience and personal sensory assessment and is supported by the results of a physico-chemical aroma anal-ysis. Legislative measures that regulate food aromatization differ in various countries.
At present, non-alcoholic beverages occupy the first place among aromatized foods (Table 5.39).
Of the different types of aroma, citrus, mint and red fruit aromas predominate (Table 5.40).
5.5.1 Raw Materials for Essences
In Germany, up to about 60% of the aromas used for food aromatization are of plant origin and,
Table 5.40. Types of aroma used Aroma type Percentage (%)a
Citrus 20
thus, designated as “natural aroma substances”.
The rest of the aroma compounds are synthetic, but 99% of this portion is chemically identical to their natural counterparts. Only 1% are synthetic aroma compounds not found in nature.
5.5.1.1 Essential Oils
Essential (volatile) oils are obtained prefer-entially by steam distillation of plants (whole or parts) such as clove buds, nutmeg (mace), lemon, caraway, fennel, and cardamon fruits (cf. 22.1.1.1). After steam distillation, the essen-tial oil is separated from the water layer, clarified and stored. The pressure and temperature used in the process are selected to incur the least possible loss of aroma substances by thermal decomposition, oxidation or hydrolysis.
Many essential oils, such as those of citrus fruits, contain terpene hydrocarbons which contribute little to aroma but are readily au-tooxidized and polymerized (“resin formation”).
These undesirable oil constituents (for instance, limonene from orange oil) can be removed by fractional distillation. Fractional distilla-tion is also used to enrich or isolate a single aroma compound. Usually, this compound is the dominant constituent of the essential oil.
Examples of single aroma compounds iso-lated as the main constituent of an essential oil are: 1,8-cineole from eucalyptus, 1(−)-menthol from peppermint, anethole from anise seed, eugenol from clove, or citral (mixture of geranial and neral, the pleasant odorous
compounds of lemon or lime oils) from lit-seacuba.
5.5.1.2 Extracts, Absolues
When the content of essential oil is low in the raw material or the aroma constituents are destroyed by steam distillation or the aroma is lost by its solubility in water, then the oil in the raw material is recovered by an extraction process. Examples are certain herbs or spices (cf. 22.1.1.1) and some fruit powders. Hexane, triacetin, acetone, ethanol, water and/or edible oil or fat are used as solvents.
Good yields are also obtained by using liquid CO2. The volatile solvent is then fully removed by distillation. The oil extract (resin, absolue) of-ten contains volatile aroma compounds in excess of 10% in addition to lipids, waxes, plant pig-ments and other substances extractable by the chosen solvent. Extraction may be followed by chromatographic or counter-current separation to isolate some desired aroma fractions. If the sol-vent used is not removed by distillation, the pro-duct is called an extract. The odor intensity of the extract, compared to the pure essential oil, is weaker for aromatization purposes by a factor of 102to 103.
5.5.1.3 Distillates
The aroma constituents in fruit juice are more volatile during the distillation concentration process than is the bulk of the water. Hence, the aroma volatiles are condensed and collected (cf. 18.2.10). Such distillates yield highly concentrated aroma fractions through further purification steps.
5.5.1.4 Microbial Aromas
Cheese aroma concentrates offered on the market have an aroma intensity at least 20-fold higher than that of normal cheese. They are produced by the combined action of lipases and Penicil-lium roqueforti using whey and fats/oils of plant origin as substrates. In addition to C4–C10 fatty acids, the aroma is determined by the presence of 2-heptanone and 2-nonanone.
5.5 Natural and Synthetic Flavorings 395
5.5.1.5 Synthetic Natural Aroma Compounds In spite of the fact that a great number of food aroma compounds have been identified, economic factors have resulted in only a limited number of them being synthesized on a com-mercial scale. Synthesis starts with a natural compound available in large amounts at the right cost, or with a basic chemical. Several examples will be considered below.
A most important aroma compound world-wide, vanillin, is obtained primarily by alkaline hydrolysis of lignin (sulfite waste of the wood pulp industry), which yields coniferyl alcohol. It is converted to vanillin by oxidative cleavage:
(5.45) A distinction can be made between natural and synthetic vanillin by using quantitative13C analy-sis (cf. 18.4.3). The values in Table 5.41 show that the13C distribution in the molecule is more mean-ingful than the13C content of the entire molecule.
The most important source of citral, used in large amounts in food processing, is the steamdistilled oil of lemongrass (Cymbopogon flexuosus). Cit-ral actually consists of two geometrical isomers:
geranial (I) and neral (II). They are isolated from the oil in the form of bisulfite adducts:
(5.46) The aroma compound menthol is primarily syn-thesized from petrochemically obtained m-cresol.
Table 5.41. Site-specific13C isotopic analysis of van-illin from different sources
Source R (%)ain R(%)atotal
CHO Benzene ring OCH3
Vanilla 1.074 1.113 1.061 1.101 Lignin 1.062 1.102 1.066 1.093 Guaiacol 1.067 1.102 1.026 1.089
aR (13C/12C) was determined by site-specific natural isotope fractionation NMR (SNF-NMR). Standard de-viation: 0.003–0.007.
Thymol is obtained by alkylation and is then fur-ther hydrogenated into racemic menthol:
(5.47) A more expensive processing step then follows, in which the racemic form is separated and 1(–)-menthol is recovered. The d-optical isomer substantially decreases the quality of the aroma (cf. 5.3.2.4).
The purity requirement imposed on synthetic aroma substances is very high. The purification steps usually used are not only needed to meet the stringent legal requirements (i. e. beyond any doubt safe and harmless to health), but also to remove undesirable contaminating aroma compounds. For example, menthol has a phenolic off-flavor note even in the presence of only 0.01%
thymol as an impurity. This is not surprising since the odor threshold value of thymol is lower than that of 1(−)-menthol by a factor of 450.
5.5.1.6 Synthetic Aroma Compounds
Some synthetic flavorings which do not occur in food materials are compiled in Table 5.42. Except for ethyl vanillin, they are of little importance in the aromatization of foods.
Table 5.42. Synthetic Flavoring Materials (not naturally occurring in food)
Name Structure Aroma description
Ethyl vanillin Sweet like vanilla
(2 to 4-times stronger than vanillin)
Ethyl maltol cf. 5.3.1.2 Caramel-like
Musk ambrette Musk-like
Allyl phenoxyacetate Fruity, pineapple-like
α-Amyl cinnamic-aldehyde Floral, jasmin and lilies
Hydroxycitronellal Sweet, flowery,
liliaceous
6-Methyl coumarin Dry, herbaceous
Propenylguaethol (vanatrope) Phenolic, anise-like
Piperonyl isobutyrate Sweet, fruity, like
berry fruits
5.5.2 Essences
The flavorist composes essences from raw mate-rials. In addition to striving for an optimal aroma, the composition of the essence has to meet
food processing demands, e. g., compensation for possible losses during heating. The “aroma formulation” is an empirical one, developed as a result of long experience dealing with many problems, disappointments and failures, and
5.5 Natural and Synthetic Flavorings 397
is rigorously guarded after the “know-how”
is acquired.
5.5.3 Aromas from Precursors
The aroma of food that has to be heated, in which the impact aroma compounds are generated by the Maillard reaction, can be improved by increasing the levels of precursors involved in the reaction. This is a trend in food aromatization.
Some of the precursors are added directly, while some precursors are generated within the food by the preliminary release of the reaction components required for the Maillard reaction (cf. 4.2.4.4). This is achieved by adding protein and polysaccharide hydrolases to food.
5.5.4 Stability of Aromas
Aroma substances can undergo changes during the storage of food. Aldehydes and thiols are es-pecially sensitive because they are easily oxidized to acids and disulfides respectively. Moreover, unsaturated aldehydes are degraded by reactions which will be discussed using (E)-2-hexenal and citral as examples. These two aldehydes are im-portant aromatization agents for leaf green and citrus notes. (Z)-3-Hexenal, an important con-tributor to the aroma of freshly pressed juices, e. g., orange and grapefruit (cf. 18.1.2.6.3), is con-siderably more instable than (E)-2-hexenal (Ta-ble 5.43) and, consequently, hardly finds applica-tion in aromatizaapplica-tion.
Table 5.43. Half-life periods in the degradation of C6 and C7aldehydes in different solvents at 38◦Ca Aldehyde Water/ Bufferb/ Triacetin
Ethanol Ethanol (8+ 2, v/v) (8 + 2, v/v)
n-Hexanal 100 91 86
(E)-2-Hexenal 256 183 71
(Z)-3-Hexenal 42 36 26
n-Heptanal 79 76 73
(E)-2-Heptenal 175 137 57
(Z)-4-Heptenal 200 174 64
aThe half-life period is given in hours.
bNa-citrate buffer of pH 3.5 (0.2 mol/l).
In an apolar solvent, e. g., a triacylglycerol, (E)-2-hexenal decreases much more rapidly than in a polar medium in which its stability exceeds that of hexanal (Table 5.43). It oxidizes mainly to (E)-2-hexenoic acid, with butyric acid, valeric acid and 2-penten-1-ol being formed as well. The reaction pathway to the C6− and C5− acids is shown in Formula 5.48.
(5.48) At the acidic pH values found in fruit, autoxida-tion decreases, (E)-2-hexenal preferentially adds water with the formation of 3-hydroxy-hexanal.
In addition, the double bond is iso-merized with the formation of low concentrations of (Z)-3-hexenal. As a result of its low threshold value, (Z)-3-hexenal first influences the aroma to a much greater extent than 3-hydroxyhexanal which has a very high threshold (cf. 18.1.2.6.3).
Citral is also instable in an acidic medium, e. g., lemon juice. At citral equilibrium, which consists of the stereoisomers geranial and neral in the ratio of 65:35, neral reacts as shown in Formula: 5.49.
It cyclizes to give the labile p-menth-l-en-3,8-diol which easily eliminates water, forming various p-menthadien-8-ols. This is followed by aroma-tization with the formation of p-cymene, p-cy-men-8-ol, and α,p-dimethylstyrene. p-Methyl-acetophenone is formed from the last men-tioned compound by oxidative cleavage of the
8-double bond. Together with p-cresol,
p-methylacetophenone contributes
apprecia-bly to the off-flavor formed on storage of lemon juice. Citral is also the precursor of p-cresol.
In citrus oils, limonene andγ-terpinene are also attacked in the presence of light and oxygen. Car-vone and a series of limonene hydroperoxides are formed as the main aroma substances.
5.5.5 Encapsulation of Aromas
Aromas can be protected against the chemical changes described in 5.5.4 by encapsulation.
Materials suitable for inclusion are polysaccha-rides, e. g., gum arabic, maltodextrins, modified starches, and cyclodextrins. The encapsulation proceeds via spray drying, extrusion or formation of inclusion complexes. For spray drying, the aroma substances are emulsified in a solution or suspension of the polysaccharide, which contains solutizer in addition to the emulsifying agent.
In preparation for extrusion, a melt of wall ma-terial, aroma substances, and emulsifiers is pro-duced. The extrusion is conducted in a cooled bath, e. g., isopropanol.
β-Cyclodextrins, among other compounds, can be used for the formation of inclusion complexes (cf. 4.3.2). Together with the aroma substances, they are dissolved in a water/ethanol mixture by heating. The complexes precipitate out of the cooled solution and are removed by filtration and dried. Criteria for the evaluation of encapsulated aromas are: stability of the aroma, concentration of aroma substance, av-erage diameter of the capsules and, amount of aroma substance adhering to the surface of the capsule.
(5.49)