Food microbiology application: selecting lactobacilli from cheddar cheese samples During enumeration and isolation in dairy microbiology, at what target pH should the culture medium be maintained to selectively favor the growth of lactobacilli present in cheddar cheese?

Introduction / Context:
Lactic acid bacteria such as Lactobacillus spp. are key ripening flora in hard cheeses (for example, cheddar). Selective recovery from mixed cheese microbiota requires controlling inhibitory factors like pH and salt so that lactobacilli outcompete non–lactic acid bacteria. This question checks practical food microbiology knowledge about the pH commonly used to enrich or select lactobacilli from cheddar cheese homogenates.

Given Data / Assumptions:

• The sample is cheddar cheese containing mixed microflora (starter and nonstarter lactic acid bacteria).
• We seek a medium condition that favors lactobacilli while suppressing competitors.
• Lactobacilli tolerate lower pH better than many Gram-negative organisms.

Concept / Approach:

Selective plating of lactobacilli uses slightly acidic media (about pH 5.3–5.4) that inhibit undesirable bacteria yet do not unduly stress Lactobacillus spp. Media such as acidified MRS or tomato juice agar are classically adjusted near pH 5.4. Values that are too high (for example, pH 6.3–6.5) reduce selectivity; values that are too low (for example, pH near 4.3) can slow or injure target cells.

Step-by-Step Solution:

Verification / Alternative check:

Dairy microbiology protocols routinely acidify MRS-derived media to approximately pH 5.4 for selective enumeration of lactobacilli; results are reliable across cheddar matrices with typical salt and moisture contents.

Why Other Options Are Wrong:

• 6.35: Too close to neutral; poor selectivity for lactobacilli.
• 4.35: Excessively acidic; may inhibit or injure even acid-tolerant lactobacilli.
• 5.75: Less selective than ~5.35; allows more competitors.
• 5.00: Stronger acid stress than necessary; risks reduced recoveries.

Common Pitfalls:

• Confusing growth pH range with selective pH; selection aims for competitive advantage, not maximal growth rate.
• Ignoring buffering capacity of cheese homogenates, which can drift plate pH upward if starting pH is too high.

Final Answer:

5.35