Food as a Substrate for Microorganisms

FOOD AS A SUBSTRATE FOR MICROORGANISMS

Factor	Description	Effect on Microbial Growth	Examples / Notes
1) pH (Hydrogen-ion concentration)	Each microbe has minimum, optimum, maximum pH	Low pH inhibits bacteria; yeasts & molds more tolerant	Fruits, pickles, soft drinks resist bacteria
Foods with low pH (<4.5)	Natural or fermentation acidity	Prevent bacterial spoilage	Spoilage mainly by yeasts & molds
Buffering capacity	Some foods resist change in pH	Strong buffers allow longer microbial growth	Milk (high buffer), fruit juice (low buffer)

 

Microbe Type	Preferred pH	Outcome
Bacteria	Near neutral (6–7)	Grow widely in many foods
Yeasts	Slightly acidic (4–5)	Fruit juices, fermented foods
Molds	Wide pH range (2–8.5)	Spoilage in highly acidic foods

 

2) Moisture (Water Activity – aw)	Meaning	Effect	Examples/aw Range
Water Activity (aw)	Amount of available water (0–1)	Higher aw → faster growth	Meat, milk: 0.98–1.00
Low aw	Water unavailable due to solutes, drying, freezing	Delayed or no microbial growth	Honey, biscuits: <0.60

 

Microorganism Group	Lowest aw they can grow at
Most bacteria	0.91
Most yeasts	0.88
Most molds	0.80
Halophilic bacteria	0.75
Xerophilic molds	0.65
Osmophilic yeasts	0.60

 

How aw is Reduced	Effect on Microbes	Examples
High sugar content	Osmophilic yeasts survive	Jams, jellies
High salt content	Halophiles survive	Meat curing
Drying / dehydration	Stops most microbes	Cereals, spices
Freezing (ice forms)	Water unavailable	Frozen foods

 

3. Oxidation-Reduction Potential (Eh)	Type Supported	Examples
High Eh (oxidizing)	Aerobes	Surface of meat, fruit juices
Low Eh (reducing)	Anaerobes	Canned foods, vacuum packs
Intermediate Eh	Facultative	Lactic acid bacteria

 

Effect of Food Structure on Eh	Outcome
Whole tissues → low Eh	Supports anaerobic growth inside
Cutting/grinding → more oxygen	Faster aerobic spoilage

 

4) Nutrient Content	Microbial Use	Examples
Carbohydrates (sugars)	Energy source	Yeasts → fermentation
Proteins & amino acids	Energy & nitrogen	Proteolytic bacteria spoil meat
Fats (after hydrolysis)	Energy	Lipolytic molds and bacteria
Minerals & vitamins	Growth factors	Egg white has avidin → binds biotin, inhibits microbes needing biotin

 

5) Inhibitory Substances	Where Found	Effect
Lysozyme	Egg white	Destroys bacterial cell walls
Benzoic acid	Cranberries	Natural preservative
Lactoperoxidase, lactenins	Fresh milk	Inhibit bacteria
Phenols (from smoking)	Smoked meats/fish	Antimicrobial
Alcohol from yeast	Wines	Inhibits bacteria

 

Microbial Inhibitors Produced by Microbes	Produced by	Acts Against
Nisin (bacteriocin)	Streptococcus lactis	Clostridia, spoilage bacteria
Propionic acid	Propionibacteria in cheese	Inhibits molds
Organic acids	Lactic acid bacteria	Pathogens & spoilage microbes

 

6) Biological Structure	How it protects food
Egg shell, nutshell, fruit skin	Blocks microbial entry
Fish scales, meat fat cover	Reduces surface spoilage
Whole tissues (undamaged)	Low microbes internally

 

7) Combined Effects (Hurdle Concept)	Explanation	Example
Multiple factors together inhibit microbes	pH + aw + temperature + preservatives	Preventing Clostridium botulinum in canned foods