Fundamental Principles of Food Microbiology and Processing

CONTAMINATION OF FOODS

• General Contamination Sources: Foods can be contaminated from plants, soil, air, water, sewage, animals, handling, and processing.

• Plant Contamination: Plants carry a typical surface flora and can be contaminated by outside sources. Parasitic diseases in plants carry the specific pathogen.

• Animal Contamination: Animals possess surface flora plus internal flora. Organisms are released through excretions and secretions. Animal parasitic diseases introduce pathogens to the food chain.

CONTAMINATION FROM PLANTS

• Natural Surface Flora: Varies by plant but typically includes:

  • Species: Pseudomonas, Alcaligenes, Flavobacterium, and Micrococcus.

  • Lactic Acid Bacteria: Lactobacillus brevis, Lactobacillus plantarum, Leuconostoc mesenteroides, Leuconostoc dextranicum, Streptococcus faecium, and Streptococcus faecalis.

  • Others: Bacillus species, yeasts, and molds.

• External Sources: Exposed plant surfaces are added to by soil, water, sewage, air, and animals.

• Growth Conditions: Post-harvesting increases in microbial numbers occur if conditions for growth are favorable.

• Persistence Factors: Microorganisms that persist on plant products do so because they can adhere to surfaces (resisting washing) and obtain nutritional requirements from the plant environment. Notable examples include lactic acid bacteria and some yeasts.

• Common Plant Bacterial Pathogens: Genera including Corynebacterium, Curtobacterium, Pectobacterium, Pseudomonas, and Xanthomonas.

CONTAMINATION FROM ANIMALS

• Primary Internal Sources: Flora of the respiratory tract and the gastrointestinal (GI) tract.

• Surface Sources: Hides, hooves, hair, feathers, and feet contain high numbers of organisms from soil, manure, feed, and water.

• Skin/Surface Organisms: Micrococcus, Staphylococcus, and beta-hemolytic Streptococci. Staphylococci can transfer from the respiratory tract or skin to carcasses and raw products.

• Enteric Organisms: Feces and fecal-contaminated products contain enteric organisms like Salmonella. Salmonellosis in animals contaminates derived foods (pig/beef carcasses).

• Infectious Disease Transmission: Improved animal husbandry has reduced these, but foodborne agents include Brucella, Mycobacterium tuberculosis, Coxiella, Listeria, Campylobacter, beta-hemolytic Streptococci, Salmonella, enteropathogenic Escherichia coli, parasites, and viruses.

• Insects and Birds: Cause mechanical damage to fruits/vegetables, introducing microbes and initiating spoilage.

• Animal Feed: Silage is a source of Listeria monocytogenes for dairy/meat animals. Dry feed spreads Escherichia, Clostridium, and Enterococcus throughout the environment.

• Animal Hides: Organisms from hides/udders contaminate raw milk, containers, and handlers. Common genera: Acinetobacter, Alcaligenes, Bacillus, Clostridium, Corynebacterium, and Enterococcus.

CONTAMINATION FROM SEWAGE, SOIL, AND WATER

• Sewage: Untreated domestic sewage ("night soil") used as fertilizer contaminates raw plants with gastrointestinal pathogens, coliform bacteria, anaerobes, enterococci, and viruses. Contaminated water affects seafood.

• Soil: Contains the greatest variety and large total numbers of microorganisms. Soil dust and particles are carried by air and water.

  • Important Soil Genera: Bacillus, Clostridium, Enterobacter, Escherichia, Micrococcus, Alcaligenes, Flavobacterium, Chromobacterium, Pseudomonas, Proteus, Streptococcus, Leuconostoc, Acetobacter, actinomycetes, and iron bacteria.

• Water:

  • Bacterial Content: Ground water (wells/springs) contains few to several hundred bacteria per milliliter ($ml$). Surface water varies, reaching thousands per $ml$ after rain.

  • Public Health Aspect: Water must be safe ($0$ pathogens). Indicator bacteria like Escherichia coli (intestinal origin) are distinguished from Enterobacter aerogenes (soil/plant origin).

  • Chlorination: Practiced if sanitary quality is doubted. Residual chlorine ranges from $0.025$ to $2$ or more parts per million ($ppm$).

  • Economic Aspect: Water used in processing must have acceptable taste, odor, color, clarity, and chemical composition (hardness/alkalinity).

  • Water-Borne Defects:

    • Gas-forming coliforms in cheese.

    • Alcaligenes viscolactis and Enterobacter aerogenes cause ropiness in milk.

    • Pseudomonas putrefaciens causes surface taint in butter.

    • Achromobacter, Alcaligenes, and Pseudomonas cause slime in cottage cheese.

CONTAMINATION FROM AIR

• Presence: Air does not have a natural flora; microbes are present by accident via dust, lint, dry soil, or moisture droplets (coughing/sneezing).

• Persistence: Organisms resistant to desiccation live longest. Cocci are generally more numerous than rod-shaped bacteria; spores are uncommon in dust-free air.

• Common Air Microbes: Mold spores, asporogenous chromogenic yeasts, Acinetobacter spp., Arcobacter spp., and Mycobacterium spp.

• Variable Factors: Movement, sunshine (kills microbes), humidity, and location. Numbers range from less than $1$ per cubic foot on mountaintops to thousands in dusty air.

• Treatment: Artificial removal via filtration (cotton, fiber glass, activated carbon), UV lamps, or chemical treatment.

HANDLING AND PROCESSING CONTAMINATION

• Personnel: Food handlers transfer microbes from soil, water, nasal cavities, mouth, skin, and GI tract. Common genera: Bacillus, Campylobacter, Clostridium, Corynebacterium, Escherichia, and Staphylococcus.

• Utensils: Cutting blocks, knives, and grinders can lead to a build-up of constant contamination levels. Same bacterial genera as personnel are involved.

FOOD SPOILAGE TYPES

• Definition: A metabolic process making food undesirable via changes in sensory characteristics (visual, textural, olfactory, flavor).

• Mouldiness and Whiskers: Aerobic surface growth. Chief bread mould is Rhizopus nigricans (white cottony mycelium with dark sporangia). Others: Neurospora sitophila (black), Penicillium expansum (red), Aspergillus niger (greenish/purplish brown).

• Rots: Black rot of eggs (Alternaria); watery soft rots (Pseudomonas marginalis); others include blue mould rot and downy mildew.

• Ropiness: Formation of viscous sticky material ($slime$). Examples: Bacillus subtilis, Leuconostoc mesenteroides. In bread, caused by mucoid variants of Bacillus species (spores survive baking).

• Sliminess: Bacterial growth on moist surfaces (meat, fish). Surface slime of meat caused by Pseudomonas, Streptococcus, Bacillus, Micrococcus. Serratia marcescens produces red pigment.

• Putrefaction: Anaerobic protein breakdown into foul-smelling compounds ($amines, H_2S, ammonia$). Organisms: Clostridium, Pseudomonas putrefaciens, Proteus.

• Fermentative Spoilage: Yeasts convert sugar to alcohols and $CO_2$. "Blown cans" result from gas-producing fermentation. "Bone taints" involve fermentation near the bone.

• Souring: Defect producing sour odor/taste from acids ($formic, acetic, butyric, lactic$). Caused by Streptococcus lactis, Streptococcus faecalis, lactobacilli.

• Colour Change: Serratia marcescens (red), Sarcina lutea (yellow), Pseudomonas fluorescens (green fluorescence). Greening of sausages linked to Leuconostoc mesenteroides.

• Rancidity: Undesirable flavors in fatty foods from oxidation or microbial action.

SPOILAGE OF SPECIFIC FOODS

Milk and Cream

• Souring: Clean or aromatic acidity produced by Streptococcus lactis. At $37$ to $50^{\circ}C$, S. thermophilus and L. bulgaricus dominate. Thermophilic spoilage by Lactobacillus thermophilus occurs above $50^{\circ}C$.

• Gas Production: "Stormy fermentation" caused by coliforms and Clostridium spp. (producing $H_2$ and $CO_2$).

• Proteolysis: Bitterness from peptides. Pseudomonas fluorescens proteinase survives pasteurization. Bacillus cereus causes sweet curdling.

• Ropiness: Surface ropiness (Alcaligenes viscolactis at $10^{\circ}C$) or throughout the milk (Enterobacter aerogenes, Streptococcus lactis var. hollandicus).

• Fat Changes: Oxidation/hydrolysis by Pseudomonas, Proteus, Alcaligenes, Bacillus, Micrococcus, and Clostridium (lipolytic genera).

• Color Changes: Blue milk (Pseudomonas syncyanea), Yellow milk (Flavobacterium), Red milk (Serratia marcescens).

Fruits and Vegetables

• Juices: Spoilage by yeasts ($15.6$ to $35^{\circ}C$). Lactic acid fermentation of sugars by Lactobacillus brevis and Leuconostoc mesenteroides.

• Cereal/Bread: Moisture $< 13\%$ prevents growth. Bread mould by Rhizopus stolonifer. Ropiness in bread by Bacillus subtilis variants creates yellow/brown soft sticky crumbs and odors of overripe melons.

Meat and Fish

• Meat Aerobic Spoilage: Surface slime (Pseudomonas-Alcaligenes group), pigment changes (bloom loss), and whiskers (moulds like Mucor mucedo and Thamnidium).

• Meat Anaerobic Spoilage: Souring (volatile acids) and True Putrefaction (Clostridium producing $H_2, CO_2, H_2S, amines$).

• Fish Spoilage: Faster than meat due to low acidity and autolysis. Stale fishy odor due to trimethyl amine. Chilling at $0$ to $-1^{\circ}C$ is required. Salt fish spoiled by halophilic Serratia or Micrococcus.

Eggs

• Bacterial Rots:

  • Green Rots: Pseudomonas fluorescens (fluoresces under UV).

  • Black Rots: Proteus (yolks blackened, mud-colored contents, $H_2S$ odor).

  • Red Rots: Serratia.

• Fungal Spoilage: Pin-spot moulding (Penicillium - yellow/blue/green; Cladosporium - black); fungal rotting (mycelium penetrates pores).

PHYSICAL METHODS OF MICROBIAL CONTROL

Heat

• Thermal Death Point (TDP): Lowest temperature killing all microbes in a $10$-minute exposure.

• Thermal Death Time (TDT): Length of time needed to kill all microbes at a given temperature.

• Autoclaves (Moist-Heat): Standard settings are $121^{\circ}C$ or $132^{\circ}C$ at $15$ to $20\,psi$ for $20$+ minutes. Kills endospores.

• Pasteurization:

  • High-temperature short-time (HTST): $72^{\circ}C$ for $15$ seconds.

  • Ultra-high-temperature (UHT): $138^{\circ}C$ for $2$ or more seconds (allows non-refrigerated storage).

Low Temperature and Pressure

• Refrigeration: $0^{\circ}C$ to $7^{\circ}C$; inhibits metabolism.

• Freezing: Below $-2^{\circ}C$; stops growth.

• High-Pressure Processing (Pascalization): $100$ to $800\,MPa$. Kills vegetative cells but not necessarily endospores. C. botulinum endospores require $116^{\circ}C$ ($240^{\circ}F$).

Desiccation and Radiation

• Desiccation: Drying (jerky, raisins).

• Lyophilization: Snap-freezing then vacuum sublimation.

• Osmotic Pressure: Addition of salts or high sugar (e.g., $80\%$ sucrose in honey) lowers water activity.

• Ionizing Radiation: X-rays, gamma rays (from cobalt-$60$ or cesium-$137$). Damages DNA with double-strand breaks. Marked with the Radura symbol.

• Nonionizing Radiation: UV light; causes thymine dimers; used for surface sterilization.

SANITARY DESIGN OF FOOD PROCESSING PLANTS

Three Broad Principles

1. Provide zones of control.

2. Control temperature and moisture.

3. Design to facilitate sanitation.

Eleven Principles for Sanitary Design

1. Hygienic Zones: Physical separation of raw processing, clean rooms, and packaging.

2. Controlled Flows: Managing movement of personnel (e.g., color-coded uniforms) and materials.

3. Water Accumulation: Designing floors/walls to promote drainage; avoiding pooling.

4. Temperature/Humidity HVAC: Controlling air dew point to prevent condensation.

5. Air Flow Qualities: Positive air pressure in processing; negative pressure in restrooms. Air filters at $0.1\,micron$ remove bacteria/viruses.

6. Site Elements: Landscaping (bushes/trees away from building), door seals, and insect electrocutors.

7. Building Envelope: Sealing gaps/voids; using single-membrane roof materials.

8. Spatial Design: Leaving at least $30\,inches$ between machinery and walls for cleaning access ($360$-degree access).

9. Building Components: Using wear-resistant materials like epoxy on floors.

10. Utility Systems: Clean, uncomplicated piping and electrical layouts.

11. Integrated Sanitation: Planning for wash areas, steam systems, and Clean-In-Place (CIP) systems.

PUBLIC HEALTH AND SANITATION

• Definitions: Public health is the art/science of preventing disease and promoting health. Sanitation is the safe disposal of human excreta.

• Disease Prevalence: Half of urban populations in Africa, Asia, and Latin America suffer diseases from poor sanitation.

• Pathogen Load: $1\,gram$ of fresh feces from an infected person can contain $10^6$ viral pathogens, $10^6$ to $10^8$ bacterial pathogens, and $10^4$ protozoan cysts.

• Specific Diseases:

  • Diarrhoeal Diseases: $1.6$ to $2.5\,million$ deaths annually ($19\%$ of deaths in children under $5$ in sub-Saharan Africa).

  • Trachoma: Caused by Chlamydia trachomatis. Preventable blindness spread by Musca sorbens flies.

  • Helminths: Roundworms and hookworms transmit via open defecation.

  • Schistosomiasis: Schistosomes live in portal veins; eggs pass in urine (S. haematobium) or feces.

• Benefits: Privacy, convenience (avoiding snakes/pests), protection for women/girls, and economic savings from reduced health system costs.

MICROBIAL HAZARDS AND FOOD POISONING

• Biological Hazards: Bacteria (B. cereus, E. coli O157:H7, L. monocytogenes, etc.), Viruses (Hepatitis A, Norovirus), Parasites (Cryptosporidium, Taenia, Toxoplasma), and Prions (Mad Cow Disease or BSE).

• Foodborne Infection: Ingestion of live bacteria which grow in the human intestinal tract.

  • Invasive: Pathogens invade intestinal tissue (e.g., Shigella).

  • Enterotoxigenic: Pathogens produce toxin in the intestine (e.g., Vibrio cholerae).

• Foodborne Intoxication: Ingestion of pre-formed toxins in food. The microbe does not need to be alive at consumption.

Staphylococcus aureus (Staphylococcal Intoxication)

• Etiology: Gram-positive cocci; coagulase-positive strains produce enterotoxins (A, B, C1, C2, D, E). Types A and D are most common.

• Characteristics: Salt-tolerant ($10-20\%\,NaCl$); sugar-tolerant ($50-60\%$); Nitrate-tolerant. Growth range $14$ to $46^{\circ}C$; minimum $a_w = 0.86$; maximum $pH = 8$.

• Enterotoxin: Heat-stable; not destroyed by normal cooking. Best production at $40^{\circ}C$.

• Symptoms: Short incubation period of $2$ to $4$ hours. Vomiting, diarrhea, retching, nausea.

• Prevention: Personal hygiene of handlers (S. aureus natural habitat is skin/nasal cavity); adequate refrigeration; lowering food $a_w$ below $0.86$.

OTHER BIOLOGICAL HAZARDS AND MANAGEMENT

• Marine Biotoxins: Produced by harmful algal blooms; accumulate in bivalve molluscs.

• Biogenic Amines: Histamine and tyramine produced by bacterial decarboxylases.

• Mycotoxins: Secondary metabolites produce by fungi. Aflatoxins are carcinogenic and mutagenic (produced by Aspergillus spp.).

• Biofilms: Microbial communities in an extracellular matrix (DNA, proteins, polysaccharides).

• Food Safety Management: Identification of critical control points (HACCP). High-pressure homogenization is used but does not guarantee sterilization.