Oral malodor is considered to derive primarily from the anaerobic degradation of proteinaceous materials by oral microorganisms. Proteins are readily available from saliva, exfoliated epithelial cells, food debris, blood and crevicular fluids, and possibly postnasal drip. The constituent proteins are hydrolyzed by proteolytic enzymes of bacterial origin, yielding free amino acids, which can then be further broken down. Some of the molecular by-products of this process are particularly foul-smelling.

Deglycosylation: Salivary mucins are large glycoproteins comprised of a long protein core surrounded by carbohydrate side chains in a bottle brush-like structure. Unlike ordinary polypeptides, their proteolytic degradation requires the prior removal of their carbohydrate side chains, or deglycosylation

Mucin  →  Protein  →  Amino acids  →  Putrefactive end products

One of the key enzymes in the deglycosylation process is β-galactosidase. Our research has shown that β-galactosidase activity in saliva highly correlated with malodor ratings of 64 subjects (Sterer et al. 2002). Furthermore, addition of β-galactosidase or β-galactosidase producing bacteria (i.e., Streptococcus salivarius) to a mucin incubation mixture promoted mucin putrefaction by Porphyromonas gingivalis (Sterer and Rosenberg 2006).

The proteolytic process described above results in the breakdown of available oral proteins (or glycoproteins) into free amino acids. These free amino acids serve as nutrients for oral microorganisms that do not grow on carbohydrates (asaccharolytic), and their metabolites are important pH modulators and precursors for various molecules such as iron-scavenging siderophores. Amino acids are degraded via different metabolic pathways such as deamination, decarboxylation, and various oxidation–reduction processes, yielding different by-products, as described below.

Volatile fatty acids: The enzymatic cleavage of the amino group from various amino acids occurs at a pH range of 6–7 and results in the production of volatile fatty acids, and ammonia (NH₃) (Table 3.1).

Amines: Decarboxylation of nitrogen-containing amino acids occurs in the mouth mainly at pH 6.5 (Gochman et al. 1959) and serves also as a means for the microorganisms to regulate pH conditions. The decarboxylation of these amino acids results in the production of various amine compounds (Table 3.2).

Indoles and Phenols: Production of indoles and phenols occurs as a result of the metabolism of various aromatic amino acids (Table 3.3).

Volatile sulfide compounds: Production of sulfur containing compounds results from sulfate reduction or the metabolism of sulfur containing amino acids:

Cysteine  →  Hydrogen sulfide

Methionine  →  Methyl mercaptan

Many of the above compounds, as well as various other VOCs, have been detected in saliva samples, saliva headspace, tongue-coating samples, and breath samples. These samples were analyzed using different techniques varying from classic colorimetric techniques to modern liquid and gas chromatography. These reported compounds are listed in Table 3.4, highlighting those that have been implicated in oral malodor production and stating their odor characteristics and thresholds: