Many causes. Deterioration of the normal chemical constituents. Microbial growth. Contamination with any number of things.
The vast majority of known chemicals have no odor at all. The chemical compounds that have an odor are generally small organic molecules with up to 15 or so carbons, and a limited number of polar functional groups. Above that range, the odors (if any) are faint and the molecules are too heavy to evaporate quickly. Pheromones, perfume bottom notes, and musks are examples. Oily liquids become viscous, then waxy solids.
The worst off-odors from microbial growth derive mainly from breakdown of the product’s protein and fat/oil, not the carbohydrates.
Petroleum hydrocarbons – usually refined into distinct fractions or ‘cuts’ in narrow boiling point ranges. The smallest hydrocarbons are practically odorless (methane, ethane, propane and butane) and they are gases at room temperature. Above 10 carbons, their petroleum odors become fainter and heavier. Paraffin waxes have very little odor.
Pesticides: Most pesticide molecules are too large or have too many polar functional groups to possess any odor. Some exceptions are the sulfur-containing (phosphorothio) compounds, and some of the smallest volatile organochlorines (i.e. fumigants). Many familiar insecticides are diluted down to a usable concentration in petroleum solvents or distillates. They are fully effective in highly dilute solutions, often 1% active ingredient. For example, in bug sprays for home and garden use, what we are smelling is the petroleum “carrier” 90+% inert non-active ingredient. Most of the pyrethrin canned bug spays have petroleum alkane distillates in the C11 to C14 boiling point range and have no aromatics or PAHs and not many alkenes (as measured by GC and GC-MS).
Sulfur odors and sources: In food, usually comes from metabolites of the two sulfur-containing amino acids, methionine and cysteine, The sulfides in onion & garlic come from cysteine; the sulfurous odor in spoiled food is partly methylthiopropanol, a Strecker degradation product of methionine. Sulfur dioxide preservative added to packaged dried fruits. Mixed thiophenes from crude petroleum. A few of the phosphorothio- (organophosphorus) pesticides have a sulfur odor, often from trace level impurities. Less commonly encountered are other elements also in the Periodic Table’s Group 16 column right below sulfur (selenium and tellurium). H2S (swamp & sewer gas, rotten eggs smell, the simplest smallest sulfur compound, chemically analogous to H2O). In natural gas for stoves etc, small mercaptans or thiols are added so that dangerous leaks are immediately detected.
Lipids – fats/oils. Off-odors derive mainly from fatty acid breakdown and oxidation. PUFAs are most susceptible because of all the double bonds.
Nitrogenous compounds – proteins, amino acids, amines, heterocycles
Terpene breakdown products (mainly monoterpenes), turpentine as a non-polar solvent and cleaner, Pine-Sol, isoprenes. Very well characterized in countless orange juice shelf life studies going back 50 years and more. Pine forest studies (atmospheric breakdown). Some are unstable during low temperature distillation. In many cases with the flavor oils used to make drinks, the monoterpene hydrocarbons (C10H16) are selectively removed since they’re the quickest to breakdown and don’t contribute much to the overall aroma.
Musty odor – 3 main chemical families
Dan Harring (54) 