Principles of Food Science: Food Microbiology Flashcards

Fundamentals of Food Microbiology

• Definition of Microbiology: The scientific study of microscopic organisms, known as microorganisms, which are generally too small to be seen with the naked eye. These include bacteria, yeast, fungi, algae, viruses, and other microscopic life forms.
• Definition of Food Microbiology: A specific branch of microbiology focused on the study of microorganisms and their beneficial and harmful effects on the safety and quality of both processed and raw foods.
• Scope of the Field: Food microbiology is broad, encompassing the study of microbes that cause food spoilage and foodborne illnesses, as well as those essential for producing fermented foods like cheese, yoghurt, and bread.

Principal Types of Microorganisms

• Bacteria:     * Description: Single-celled prokaryotes featuring a cell wall, cell membrane, and genetic material ($DNA$).     * Nature: Can be beneficial or harmful.     * Examples: $Escherichia\,coli$, $Lactobacillus$, $Streptococcus$.
• Fungi:     * Description: Eukaryotic organisms with cell walls made of chitin.     * Subtypes: Includes molds (e.g., $Rhizopus$, $Penicillium$, $Aspergillus$), yeasts, and mushrooms.
• Protozoa:     * Description: Single-celled eukaryotes that lacks a cell wall.     * Motility: They move using pseudopodia, cilia, or flagella.     * Examples: $Amoeba$, $Paramecium$, $Giardia$.
• Algae:     * Description: Eukaryotic, mostly aquatic organisms containing chlorophyll.     * Function: Perform photosynthesis, vital to food chains and oxygen production.     * Examples: $Chlorella$, $Spirogyra$, $Diatoms$.
• Yeasts:     * Description: Single-celled fungi that typically reproduce by budding.     * Application: Crucial in fermentation processes.     * Examples: $Saccharomyces\,cerevisiae$, $Candida$.
• Viruses:     * Description: Extremely small entities, not made of cells. They consist of genetic material ($DNA$ or $RNA$) enclosed in a protein coat.     * Replication: Can only replicate inside living host cells.     * Examples: $HIV$, Influenza virus, Hepatitis virus.

Categorization of Microbes in the Food Industry

• Microbial Requirements: Microorganisms are living cells requiring a food source and a living environment within their physiological tolerance limits. Human food sources are highly vulnerable to bacterial growth because they are nutritionally dense.
• Mechanism of Alteration: Microbes alter food by secreting enzymes into the food substrate and absorbing small, soluble nutrient molecules. This enzymatic action leads to changes in appearance and texture.
• Spoilage Bacteria:     * Description: Usually not harmful but cause food to deteriorate.     * Impact: Development of bad smells, tastes, or textures.     * Examples: Sour milk, slimy meat, spoiled vegetables.
• Beneficial Bacteria:     * Role: "Good bacteria" used in food production and fermentation (Probiotics).     * Examples: Yoghurt, cheese; specifically Lactic Acid Bacteria ($LAB$) like $Lactobacillus\,acidophilus$ which converts lactose into lactic acid to thicken milk.
• Pathogenic Bacteria:     * Role: Harmful bacteria causing foodborne illness.     * Characteristics: Often do not change the color, odor, or feel of the food.     * Examples: $Salmonella$, $E.\,coli$, $Listeria$, $Clostridium\,botulinum$.

Probiotics and Beneficial Applications

• Probiotics: Beneficial bacteria supporting digestive health and overall wellness.
• Top 10 Probiotic Foods:     1. Yogurt: Dairy fermented with strains like $Lactobacillus$.     2. Kefir: Fermented milk drink with diverse probiotic ranges due to kefir grains.     3. Miso: Paste containing strains of $Bifidobacterium$ and $Lactobacillus$.     4. Kimchi: Spicy fermented vegetables (cabbage) from Korea.     5. Sauerkraut: Fermented cabbage rich in $Lactobacillus$ (unpasteurized is preferred).     6. Tempeh: Fermented soybeans providing protein and probiotics.     7. Kombucha: Fizzy, tangy fermented tea.     8. Pickles: Must be fermented in brine (not vinegar) to offer $Lactobacillus$.     9. Lassi: Yogurt-based Indian drink.     10. Buttermilk: Liquid left after churning yogurt, used as a digestive aid.
• Health Benefits:     * Fights antibiotic resistance and toxins.     * Improves digestive health, immunity, and liver function.     * Promotes healthy hair, skin, nails, and cholesterol levels.     * Supports mood, memory, and energy levels.

Intrinsic Factors Affecting Microbial Growth

• Acidity ($pH$):     * Optimal Growth: Most bacteria prefer neutral $pH$ ($6.6-7.5$).     * Microbial Tolerance: Yeasts and molds are more acid-tolerant than bacteria and can grow at $pH < 4.6$.     * Stability: Foods with $pH < 4.5$ prevent pathogenic bacteria but remain susceptible to spoilage by yeast and molds.     * Common $pH$ Values:         * Limes: $1.8-2.0$         * Citrus fruits: $3.0-4.0$         * Milk/Fish/Meat: Neutral range ($6.0-7.0$         * Sweet Corn: $7.3$         * Egg White: $8.0-9.0$         * Fermented Shark: High alkalinity ($\sim 10.0-11.0$)
• Water Activity ($a_w$):     * Definition: The amount of free/unbound water available for microbial growth. It is a qualitative measure of energy status, distinct from total moisture content.     * Correlation: Higher $a_w$ promotes growth; lower $a_w$ inhibits it.     * Minimum $a_w$ Requirements:         * Most spoilage bacteria: $0.90-0.91$         * $Staphylococcus\,aureus$: $0.86$         * Most spoilage yeast: $0.88$         * Osmophilic yeast: $0.60-0.65$         * Xerophilic moulds: $0.65$     * Reduction Methods: Adding salt/sugar, drying, or freezing.
• Nutrient Content: Microbes require energy sources (carbohydrates, amino acids), nitrogen sources (peptides, proteins, ammonia), and growth factors (vitamins, minerals).
• Redox Potential ($E_h$):     * High $E_h$: Favors aerobic bacteria/molds (e.g., fresh meat surface).     * Low $E_h$: Favors anaerobic bacteria (e.g., canned foods).
• Natural Antimicrobials:     * Essential Oils: Eugenol (cloves), Allicin (garlic), Cinnamic aldehyde (cinnamon), Thymol (oregano).     * Animal Products: Lysozyme/conalbumin (eggs), Lactoferrin/lactoperoxidase (cow’s milk).

High-Risk vs. Low-Risk Foods

• Potentially Hazardous Foods (PHF): High water activity combined with neutral $pH$. They require strict temperature control.     * Examples: Raw meat/poultry, milk, cooked rice/pasta, seafood, egg products, cut fruits.
• Lower Risk Foods: Low water activity or high acidity. Usually shelf-stable.     * Examples: Jam (high sugar), pickles (acidic), biscuits (low moisture), salted fish.

Extrinsic Factors Affecting Microbial Growth

• Temperature:     * Psychrophiles: Grow at $0^{\circ}C$; Optimal below $20^{\circ}C$ (e.g., $Bacillus\,globisporus$).     * Mesophiles: Grow between $20^{\circ}C - 45^{\circ}C$; Optimal $30^{\circ}C - 40^{\circ}C$ (e.g., $E.\,coli$, $S.\,aureus$).     * Thermophiles: Grow at $45^{\circ}C$ and above; Optimal $55^{\circ}C - 65^{\circ}C$ (e.g., $Bacillus$, $Clostridium$).
• Relative Humidity ($RH$): Affects the surface $a_w$. High $RH$ can lead to moisture buildup and surface spoilage even in dry foods.
• Atmospheric Gases:     * Aerobes: Absolute requirement for oxygen (e.g., $Mycobacterium\,tuberculosis$).     * Anaerobes: Grow only in the absence of free oxygen (e.g., $Clostridium$).     * Facultative Anaerobes: Grow under both conditions (e.g., $Enterobacteriaceae$).
• Storage Time: The longer food is stored, the more microbes multiply, even under refrigeration.
• Microbial Interactions: Concepts of competition, mutualism, antagonism, and commensalism. Lactic acid bacteria inhibit others by lowering $pH$.

Food Spoilage Factors

• Signs of Spoilage: Changes in appearance (slime, discoloration), smell, texture, and taste.
• Major Spoilage Agents:     1. Microorganisms: Bacteria (rapid in neutral/alkaline foods), Yeasts (rapid in acidic foods like juices and honey), and Molds (spread via airborne spores; require oxygen).     2. Enzymes: Biological catalysts that speed up chemical reactions like ripening. Inactivated by temperatures above $\sim 70^{\circ}C$ through processes like blanching.     3. Insects: Worms, weevils, and fruit flies damage tissue and reduce nutritional value.

Foodborne Illness and Waterborne Disease

• Global Burden (WHO Estimates): 1 in 10 people fall ill annually; 420,000 deaths (1/3 are children under 5).
• Pathogenesis: Microbes cause illness by either infecting body cells or releasing toxins.
• Risk Groups: Pregnant women, young children, older people, and immunocompromised individuals.
• Pathogen Examples:     * Campylobacter: From unpasteurized milk/poultry; causes diarrhea, cramps, and vomiting.     * Listeria: From deli meats and raw produce; can progress to muscle stiffness and convulsions.     * Norovirus: Contaminated during preparation; causes sudden abdominal pain and nausea.     * Bacillus cereus: From starchy foods like rice left at room temperature.
• Waterborne Diseases: Caused by contaminated drinking/recreational water or animal waste.     * Most common: Typhoid fever, Dysentery, Hepatitis A, $E.\,coli$, and Salmonella.     * Prevention: Wash hands, drink treated water, eat properly cooked food, and clean water tanks regularly.

Outbreak Investigation and Management

• Types of Data Used:     1. Epidemiologic: Identifying patterns in timing and location; interviewing sick people to find common exposures.     2. Traceback: Reviewing distribution records from restaurants or stores to find a common point of contamination from farm to fork.     3. Food & Environmental Testing: DNA fingerprinting to link germs found in food production environments to those found in patients.
• Control Actions: Public warnings, company recalls of contaminated products, and temporary closure of implicated restaurants or production facilities.