Step 1: Understanding the Problem:
The question asks for the primary visual and morphological method used to identify the presence of moulds (filamentous fungi) in food products.
Recognizing macroscopic differences between moulds, yeasts, and bacteria is essential for microbial analysis in food microbiology.
Step 2: Detailed Explanation:
• Mould Morphology: Moulds are multicellular, filamentous fungi whose vegetative and reproductive structures are highly characteristic.
• Mycelium Structure: The basic vegetative unit of a mould is a microscopic thread-like filament called a hypha. As these hyphae grow and branch out extensively over a food matrix, they form a visible, tangled web-like network known as a mycelium.
• Colony Appearance: On solid surfaces (such as bread, cheese, fruits, or agar plates), this mycelial growth manifests macroscopically as fuzzy, cottony, powdery, or velvety colonies that spread rapidly across the surface.
• Comparison with Other Microorganisms:
• Pinpoint colonies on nutrient agar: This is a typical morphological feature of unicellular bacteria (e.g., lactic acid bacteria) or yeasts, which do not form extensive filamentous networks.
• Acid production in broth: This is a metabolic activity characteristic of fermentative bacteria (such as Lactobacillus), detected via pH indicator color shifts, rather than a visual morphological cue.
• Gram-positive chains: This is a microscopic cellular arrangement under a Gram stain, characteristic of bacterial genera like Streptococcus, and is not used for macroscopic fungal detection.
Step 3: Final Answer:
Moulds are macroscopically identified in foods by their distinct fuzzy, spreading colonies characterized by a multicellular mycelium network, making option (C) the correct choice.