Bacterial etiology of ropy and slimy dairy products:

• Pseudomonas fragi produces fruity and ester-like off-flavors.
• Escherichia coli produces gas, acid, and barny off-flavors.
• Alcaligenes viscolactis is an obligate aerobe that synthesizes extensive extracellular capsular slime and exopolysaccharides. Because of its aerobic requirement, it proliferates predominantly at the surface layer of stored milk and cream, causing severe ropiness and sliminess where the cream layer forms long, viscous, mucilaginous strings when lifted.
• Enterococcus / Enterobacter faecalis causes acid and bitter defects.

Quantitative calculation of original milk volume across serial dilution plates:

• Dilution $10^{-2}$:
  Each 1 ml of a $10^{-2}$ dilution contains:
  $1\text{ ml} \times 10^{-2} = 0.01\text{ ml of original milk}$.
  Plated in duplicate (2 plates):
  $2 \times 0.01\text{ ml} = 0.02\text{ ml}$.
• Dilution $10^{-3}$:
  Each 1 ml of a $10^{-3}$ dilution contains:
  $1\text{ ml} \times 10^{-3} = 0.001\text{ ml of original milk}$.
  Plated in duplicate (2 plates):
  $2 \times 0.001\text{ ml} = 0.002\text{ ml}$.
• Total volume of actual milk represented in all plates:
  $\text{Total Volume} = 0.02\text{ ml} + 0.002\text{ ml} = 0.022\text{ ml}$.
  Evaluating options:
• ml is total pipetted liquid volume.
• 0.2 ml
• 0.22 ml corresponds to $10^{-1}$ and $10^{-2}$ dilutions.
• 0.022 ml matches the exact calculated volume.

Mechanism of late gas blowing defect in semi-hard and hard cheeses:
Assertion (A) is true: Clostridium tyrobutyricum (an anaerobic endospore-forming bacterium originating from silage) is universally recognized as the primary causative agent of 'late blowing' (late gas defect) in ripening semi-hard and hard cheeses (such as Emmental, Gouda, and Edam) occurring several weeks to months into ripening.
Reason (R) is false: In ripening cheese, lactose is completely depleted by starter lactic acid bacteria within the first few days of manufacture. Clostridium tyrobutyricum does NOT ferment lactose; instead, it specifically ferments lactic acid (lactate) via butyric acid fermentation:
$2\text{ Lactic Acid} \to 1\text{ Butyric Acid} + 2\text{ CO}_2 + 2\text{ H}_2$
The enormous pressure exerted by insoluble hydrogen ($H_2$) and carbon dioxide ($CO_2$) gases tears the cheese matrix, producing irregular cracks, cavities, splits, and severe foul rancid butyric off-flavors.