Indole, also called Benzopyrrole, an aromatic heterocyclic organic compound occurring in some flower oils, such as jasmine and orange blossom, in coal tar, and in fecal matter. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. It can be produced by bacteria as a degradation product of the amino acid tryptophan. It occurs naturally in human feces and has an intense fecal smell. This off flavour occurs in beer due to contaminant coliform bacteria during the primary fermentation stage of beer brewing. At very low concentrations, however, it has a flowery smell, and is a constituent of many flower scents (such as orange blossoms) and perfumes. Natural jasmine oil, used in the perfume industry, contains around 2.5% of indole. Indole also occurs in coal tar. The participation of the nitrogen lone electron pair in the aromatic ring means that indole is not a base, and it does not behave like a simple amine.
Indoles are important precursors for other substances made within the human body and are, therefore, researched and used in lifestyle and medical applications. The compound was officially discovered in 1866 by a scientist working with the properties of zinc dust who reduced oxindole from the zinc dust into an indole. After the discovery, indoles became important constituents of the textile industry, and as more research was conducted, the larger role that indoles played within the human body system was realized. The indolic nucleus in substances like tryptophan and auxin has led to a better understanding of their mechanism within the body.
Press GT-10-4 gas chromatography method for the determination with polar column.
Using the polar column method in GT-10-4 gas chromatography to determine the content of indole.
Control of Bacterial Processes
As an intercellular signal molecule in both gram-positive and gram-negative bacteria, indole regulates various aspects of bacterial physiology, including spore formation, plasmid stability, resistance to drugs, biofilm formation, and virulence. Indole has been shown to control a number of bacterial processes such as spore formation, plasmid stability, drug resistance, biofilm formation, and virulence. Indole may have anticarcinogenic activity.
Commonly synthesized from phenylhydrazine and pyruvic acid, although several other procedures have been discovered, indole also can be produced by bacteria as a degradation product of the amino acid tryptophan.
LD50 1000 mg/kg(test with rat orally).
FEMAmg/kg：soft drinks:0.26；cold drinks:0.28；candy:0.50；bakery product :0.58；pudding:0.02～0.40.
Moderate limit (FDA§172．515，2000)
It is the shiny flaky white crystals, and would turn into dark colors when it exposed to light. There would be a strong unpleasant odor with high concentration of indole, but the flavor would change into oranges and jasmine after highly diluted (concentration <0.1%). It has the melting point of 52~53 ℃ and the boiling point of 253~254 ℃. It is soluble in alcohol, ether, hot water, propylene glycol, petroleum ether and most of the non-volatile oil, insoluble in glycerin and mineral oil.
Natural indole are widely contained in neroli oil, orange oil, lemon oil, lime oil, citrus oil, peel oil, jasmine oil and other essential oil.
(1) According to the GB 27 60-96 , indole can be used as flavouring agent and mainly used for preparing the essence of cheese, citrus, coffee, nuts, grape, strawberry, raspberry, chocolate, assorted fruit, jasmine and lily etc.
(2) It can be used as the reagent for the determination of nitrite, can also used in the manufacture of perfume and medicine.
(3) It can be used as the raw material of perfume, pharmaceuticals and plant growth hormone.
(4) Indole is the intermediate for the indole acetic acid and indole butyric acid.The indole acetic acid and indole butyric are plant growth regulator.
(5) It can be widely used in the manufacture of the essences of jasmine, lilac, orange blossom, gardenia, honeysuckle, lotus, narcissus, ylang, orchid and prynne etc. It is usually combined with the methyl indole to imitate the artificial civet. The extremely few of the indole can be used in chocolate, raspberry, strawberry, bitter orange, coffee, nuts, cheese, grapes and fruit and other fruity essential oil.
(6) Indole is mainly used as spices, dyes, amino acids and the raw materials of pesticide. Indole itself is a spice commonly used in producing the essences of jasmine, lilac, lotus flowers, orchids and other flower flavor. The usage is generally in a few thousandths.
(7) It can be used for verifying the gold, potassium and nitrite and manufacturing jasmine-type fragrance. It can also be used in pharmaceutical industry.
Indole has an almost floral odor when highly purified. Otherwise,
it exhibits the characteristic odor of feces. It is not very stable on
exposure to light (turns red). Indole may be obtained from the 220
- 260°C boiling fraction of coal tar or by heating sodium phenylglycine-o-carboxylate with NaOH, saturating the aqueous solution
of the melt with C 02, and finally reducing with sodium amalgam;
can be prepared also by the reduction of indoxyl, indoxyl carboxylic acid, or indigo.
Indole has an unpleasant odor at high concentration, odor becomes floral at higher dilutions
white crystals with an unpleasant odour
Colorless to yellow scales with an unpleasant odor. Turns red on exposure to light and air. Odor
threshold of 0.14 ppm was reported by Buttery et al. (1988).
Reported occurring in several natural products as a complex compound that decomposes during enfleurage or
steam distillation yielding free indole; reported found in the essential oil from flower of Jasminum grandiflorum, in neroli oil and in
the oil extracted from flowers of bitter orange; also reported in the flowers of several plants: lemon, coffee, Hevea brasiliensis and
Randia formosa in the oil extracted from flowers of Jasminum odoratissinium L. and in the oil of Narcissus jonquilla. Also reported
found in apricot, mandarin orange peel oil, grapes, kohlrabi, French fried potato, crispbread, cheeses, butter, milk, milk powder,
boiled egg, fish oil, chicken, beef, pork, beer, rum, Finnish whiskey, red and white wine, coffee, tea, soybean, mushrooms, cauliflower, figs, rice, licorice, buckwheat, malt, wort, elder flower, clary sage, shrimp, okra, crab, clam, squid and green maté
Indole occurs in coal tar. It is used, underhigh dilution, in perfumery, and as an intermediatein organic synthesis.
Can be used in perfumes and in the synthesis of tryptophan.
In highly dil solutions the odor is pleasant, hence indole has been used in perfumery.
Indole is a flavoring agent that is a white, flaky crystalline product. it
has an unpleasant odor when concentrated and a flowery odor when
diluted. it is soluble in most fixed oils and propylene glycol and
insoluble in glycerin and mineral oil. it is obtained from decomposi-
tion of a protein.
indole: A yellow solid, C8H7N, m.p.52°C. Its molecules consist of a benzenering fused to a nitrogen-containingfive-membered ring. It occurs insome plants and in coal tar, and isproduced in faeces by bacterial action.It is used in making perfumes.Indole has the nitrogen atom positionednext to the fused benzenering. An isomer with the nitrogentwo atoms away from the fused ringis called isoindole.
Obtained from the 220 to 260°C boiling fraction of coal tar or by heating sodium phenyl-glycine-o-carboxylate with
NaOH, saturating the aqueous solution of the melt with CO2 and finally reducing with sodium amalgam; can be prepared also by the
reduction of indoxyl, indoxyl carboxylic acid or indigo.
Aroma threshold values
Detection: 140 ppb
The Journal of Organic Chemistry, 55, p. 580, 1990 DOI: 10.1021/jo00289a036Chemical and Pharmaceutical Bulletin, 35, p. 1823, 1987 DOI: 10.1248/cpb.35.1823
Indole is classified under the volatile flavor compounds (VFCs). It is known to play significant role in various biological functions such as anti-inflammatory, anticonvulsant, cardiovascular and antibacterial activities.
Low to moderate toxicity was observed inexperimental animals resulting from oral orsubcutaneous administration of indole. Theoral LD50 value in rats is 1000 mg/kg. It is ananimal carcinogen. It caused tumors in bloodand lungs in mice subjected to subcutaneousadministration.
Taste at 0.3-2 ppm
Indole was detected in jasmine flowers (Jasminum officinale), licorice (Glycyrrhiza
glabra), kohlrabi stems (Brassica oleracea var. gongylodes), and hyacinth flowers (Hyacinthus
orientalis) at concentrations of 42 to 95, 2, 1.33, and 0.24 to 3.45 ppm, respectively. Indole also
occurs in tea leaves, black locust flowers, corn leaves, petitgrain, and yellow elder (Duke, 1992).
A liquid swine manure sample collected from a waste storage basin contained indole at a
concentration of 4.8 mg/L (Zahn et al., 1997).
Biological. In 9% anaerobic municipal sludge, indole degraded to 1,3-dihydro-2H-indol-2-one
(oxindole), which degraded to methane and carbon dioxide (Berry et al., 1987). Heukelekian and
Rand (1955) reported a 5-d BOD value of 1.70 g/g which is 65.4% of the ThOD value of 2.48 g/g.
Chemical/Physical. The aqueous chlorination of indole by hypochlorite/hypochlorous acid,
chlorine dioxide, and chloramines produced oxindole, isatin, and possibly 3-chloroindole (Lin and
The indole is metabolized in a mineral salt medium
inoculated with 9% anaerobically digested nitrate-
reducing sewage sludge, resulting in the sequential
occurrence of four structurally related compounds:
oxindole, isatine, dioxindole, and anthranilic acid.
Indole is metabolized by fungus via indoxyl (3-hydroxyindole), N-formylanthranilic acid, anthranilic
acid, 2,3-dihydroxybenzoic acid, and catecol, which is
further degraded by an ortho cleavage.
Indole is oxidized to 3-hydroxyindole (indoxyl) which is conjugated with glucuronic and sulphuric acids before excretion. The sulphate conjugate seems to be the main product in rabbits and, even with relatively large doses of indole, the sulphate conjugation always exceeds that of glucuronic acid(Williams, 1959).
It can be further purified by sublimation in a vacuum or by zone melting. The picrate forms orange crystals from EtOH and has m 175o. [Beilstein 20 II 196, 20 III/IV 3176, 20/7 V 5.]
Indole causes oxidative damage to membranes.