Biochemical metabolites responsible for dairy flavor defects:

• Putrid odour in butter is caused by Pseudomonas putrefaciens hydrolyzing surface proteins and fat, producing volatile isovaleric acid and alkylamines $\to$ (II).
• Frothiness/yeastiness in sour cream is caused by lactose-fermenting yeasts (Kluyveromyces marxianus) and heterofermentative bacteria producing copious acid and gas ($CO_2$) $\to$ (I).
• Phenolic/medicinal/carbolic flavour in in-bottle sterilized milk is caused by heat-resistant sporeformers (Bacillus circulans) producing bromophenols or cresols $\to$ (IV).
• Malty flavour in milk is caused by Lactococcus lactis subsp. lactis biovar maltigenes decarboxylating and reducing leucine into 3-methyl butanal and 3-methyl butanol $\to$ (III).

Classical ecological succession of microorganisms in unpasteurized raw milk held at room temperature:

• Step 1 (A): Lactococci / Streptococci (Lactococcus lactis) initiate fermentation of lactose to lactic acid, dropping pH down to ~4.5, which inhibits their own further growth.
• Step 2 (C): Highly acid-tolerant Lactobacilli (Lactobacillus bulgaricus / L. casei) proliferate in the acidic environment, fermenting residual lactose and driving pH down to 3.5-4.0.
• Step 3 (B): Acid-tolerant yeasts and film-forming moulds (such as Geotrichum candidum) colonize the surface, metabolizing and oxidizing lactic acid into $CO_2$ and $H_2O$, progressively neutralizing acidity and raising the pH back towards neutrality.
• Step 4 (D): Once acidity is neutralized, putrefactive and proteolytic sporeformers (Bacillus and Clostridium) and lipolytic pseudomonads degrade the casein and fat, culminating in putrefaction.

Microbiological spoilage of evaporated canned milk:

• Bacillus cereus causes sweet curdling without severe gas production.
• Bacillus subtilis causes proteolytic sweet curdling and soft clot formation.
• Bacillus panis is associated with ropey bread defects.
• Bacillus megaterium (and related gas-producing aerobic sporeformers) is historically recognized in dairy bacteriology for causing coagulation in canned evaporated milk accompanied by gas production (bulging cans) and a characteristic cheesy or putrid odor resulting from extensive proteolysis and volatile acid formation.