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.

Statement (I) is false: The Moseley keeping quality test is NOT a sterility test; it is a keeping quality and post-pasteurization contamination test for pasteurized fluid milk to predict refrigerated shelf-life.
Statement (II) is false: In the Moseley test, samples are stored at refrigeration temperatures ($7.2^\circ\text{C}$ or $45^\circ\text{F}$, not $30^\circ\text{C}$) for $5\text{ to }7\text{ days}$, after which standard plate count (SPC) is performed at $32^\circ\text{C}$ for $48\text{ hours}$ and compared with the initial day-0 SPC.

Fruity flavour defects in milk and dairy products (such as Cheddar cheese or pasteurized milk) are caused by volatile ethyl esters formed by enzymatic esterification of free fatty acids with ethanol, catalyzed by microbial esterases (predominantly from Pseudomonas fragi). Ethylbutanoate (ethyl butyrate) and ethyl hexanoate are the primary volatile esters imparting sweet, apple- or pineapple-like fruity aromas.

Option analysis:

• methylbutanol: Responsible for the malty flavor defect produced by Lactococcus lactis biovar maltigenes.
• methoxy formazine: Chemical intermediate, not a dairy fruity odorant.
• Ethylbutanoate: Correct. Key ethyl ester responsible for fruity off-flavour in dairy products.
• Trimethylamine: Responsible for fishy off-flavour in stored butter and milk.