Question

In a plating experiment of milk, 1 ml of each \(10^{-2}\) and \(10^{-3}\) dilutions were plated out in duplicate. The volume of the actual milk sample used in the experiments is

• A. 4 ml
• B. 0.2 ml
• C. 0.22 ml
• D. 0.022 ml

Hint:

Duplicate means multiplying by \(2\).
- Volume from \(10^{-2}\) plates: \(2 \times 0.01 = 0.02\text{ ml}\).
- Volume from \(10^{-3}\) plates: \(2 \times 0.001 = 0.002\text{ ml}\).
- Sum: \(0.02 + 0.002 = 0.022\text{ ml}\).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
In quantitative food microbiology, serial dilution is used to reduce high bacterial concentrations to countable levels.
The volume of the actual raw milk sample represented in any agar plate is determined by multiplying the aliquot volume plated by the corresponding dilution factor.
Key Formula or Approach:
For any plate: \[ \text{Actual sample volume} = \text{Volume plated (ml)} \times \text{Dilution factor} \] The total volume of raw milk used across the entire plating experiment is the sum of the actual milk volumes in all plates: \[ \text{Total Volume} = \sum_{i=1}^{n} (\text{Actual sample volume})_{i} \]

Step 2: Detailed Explanation:
The plating experiment consists of two dilutions plated in duplicate (a total of \(4\) plates):
1. Dilution \(10^{-2}\) (Plated in duplicate):
- Plate 1 volume of actual milk = \(1 \text{ ml} \times 10^{-2} = 0.01 \text{ ml}\)
- Plate 2 volume of actual milk = \(1 \text{ ml} \times 10^{-2} = 0.01 \text{ ml}\)
2. Dilution \(10^{-3}\) (Plated in duplicate):
- Plate 3 volume of actual milk = \(1 \text{ ml} \times 10^{-3} = 0.001 \text{ ml}\)
- Plate 4 volume of actual milk = \(1 \text{ ml} \times 10^{-3} = 0.001 \text{ ml}\)
Now, sum these individual volumes to find the total volume of raw milk analyzed across all four plates: \[ \text{Total Volume} = 0.01 \text{ ml} + 0.01 \text{ ml} + 0.001 \text{ ml} + 0.001 \text{ ml} \] \[ \text{Total Volume} = 0.02 \text{ ml} + 0.002 \text{ ml} = 0.022 \text{ ml} \]

Step 3: Final Answer:
The total actual volume of the milk sample utilized across the four plates is \(0.022 \text{ ml}\).