Bacteria in Dairy Products

• Microorganisms, especially bacteria, significantly influence the quality and safety of dairy products.

Definition of Bacteria

• Bacteria are single-celled microorganisms visible only with a microscope.
• All life processes occur within a single cell.
• Most bacteria are prokaryotes, differing from plant and animal cells (eukaryotes) by lacking a true nucleus and possessing a unique cell wall.
• Bacteria exist wherever life exists, with some beneficial for nutrient conversion and food fermentation, while others cause spoilage and disease.
• Individual bacteria are identified by Genus and species.
• Bacteria are classified by appearance, structure, metabolism, growth characteristics, nutrient needs, growth temperatures, oxygen requirements, ability to use specific substrates, and metabolic by-products.
• Thousands of bacterial species exist, but only certain groups are important to the dairy industry.

General Characteristics

• A microscope with 1000X magnification is needed to view bacteria, measured in microns (1 micron = 1/1000 mm = 1/25,000 inch).
• Cells are typically stained to enhance visibility under a standard light microscope.
• Milk smears are stained with a "milk-stain," while bacteria grown on agar or in broth are stained with simple (e.g., methylene blue) or complex stains (e.g., gram-stain).
• Bacteria show a variety of shapes, sizes, and arrangements that help define them.

Common Bacteria in Milk and Dairy

• Cocci: Spherical cells, 0.4-1.5 microns, occurring as single cells, pairs, chains, or clusters (e.g., Streptococcus, Staphylococcus).
• Bacilli: Rod-shaped, 0.5-30 microns, occurring as single cells, pairs, or chains (e.g., Lactobacillus, Bacillus, Pseudomonas).
• Other: Spiral, helical, or club-shaped rods of varied sizes, generally less common in milk (e.g., Campylobacter (spiral), Corynebacterium (club)).

Gram-Stain Reaction

• Bacteria are classified as gram-positive or gram-negative using the gram-stain procedure.
• Gram-staining differentiates bacteria under a microscope using Crystal Violet (blue) and Safranin (red) stains.
• Gram-positive bacteria stain blue, while gram-negative bacteria stain red, based on cell wall structure.
• Gram-variable organisms may stain blue or red depending on conditions and are often truly gram-positive.
• Gram-positive bacteria include Bacillus (rod), Streptococcus (cocci), and Staphylococcus (cocci).
• Gram-negative bacteria include Pseudomonas (rods) and E. coli & other coliform bacteria (rods).
• Generalizations include gram-negative bacteria not surviving pasteurization, bacteria that survive are gram-positive (but not all), gram-negative bacteria spoil milk faster under refrigeration than gram-positive, and specific antibiotics are more effective against gram-positive bacteria.

Endospore Formation

• Endospores are protective structures that enable bacteria to survive adverse conditions.
• Under unfavorable conditions (e.g., nutrient scarcity), vegetative growth stops, and spores form within the cell.
• During sporulation, a thick coating develops, encasing the cell’s genetic material.
• Spores forming inside a cell may appear as swollen, possibly clear areas.
• Bacterial spores resist heat, drying, nutrient deprivation, chemicals, and sanitizers.
• Spores remain dormant for extended periods and can germinate to return to an actively growing state when conditions become favorable.
• Spores are activated into growth by heat or other triggers.
• Bacillus, Paenibacillus, and Clostridium are common gram-positive, spore-forming rods important to dairy.
• Spores are found in soil, manure, and other environmental sources.

Bacterial Reproduction (Growth)

• Bacteria reproduce via binary fission, where one cell divides into two.
• Bacterial growth is the increase in cell numbers or mass.
• Growth rate is the change in cell numbers or mass per unit time.
• Generation time is the time for a bacterial population to double, varying with the organism and environmental conditions.
• Optimum conditions can result in generation times as short as 10-20 minutes.
• Less favorable conditions, like low temperatures, extend generation times.
• If a bacterial cell reproduced every hour, it would result in ~17,000,000 cells in 24 hours.
• During division, cells may not fully separate, leading to pairs (diplococci), chains (streptococci), tetrads, or clumps.

Bacterial Growth Curve

• When bacteria enter a new environment, they undergo a lag phase (adjustment period).
• The lag phase is followed by the exponential/logarithmic growth phase.
• Growth then levels off to a stationary phase as the environment changes (e.g., nutrients deplete).
• Cells eventually die off during the death phase.

Requirements and Conditions for Growth

• Bacterial species vary widely in their growth condition requirements.
• Nutrient availability, pH, moisture, oxygen, inhibitors, and temperature influence bacterial growth.
• These factors are interactive, not independent.

Nutrient Requirements

• Most bacteria have similar basic nutrient requirements.
• Energy Source: Carbohydrates, proteins, lipids.
• Carbon Source: Carbohydrates, proteins, lipids, carbon dioxide.
• Nitrogen Source: Proteins, peptides, amino acids, ammonia, nitrates.
• Vitamins: Primarily water-soluble B-vitamins.
• Minerals, Metal Ions & Salts: Potassium, phosphorus, calcium, magnesium, iron.
• Milk supplies sufficient nutrients for a large selection of microorganisms.
• Bacteria utilize proteins, fats, carbohydrates, and vitamins in milk for growth and metabolism.
• Different species vary in enzymes needed to break down milk components.
• Microbial growth, increased numbers, and enzyme activity cause measurable changes in milk and the development of by-products that affect the product beneficially (e.g., cheese fermentation) or harmfully (e.g., milk spoilage).

pH Requirements

• Acidity and alkalinity are measured on a pH scale of 0-14, with 7 being neutral.
• Most bacteria grow best at neutral or slightly higher pH, though it varies.
• Most bacteria do not grow below pH 4.0 or above pH 10.0.
• Normal milk has a pH around 6.7, allowing many bacteria to grow.
• Cultured dairy products have lower pH values.
• Dairy Starter Cultures (Lactic Acid Bacteria or LAB) produce lactic acid and grow at pH 4.0-5.0.
• LAB are used to ferment milk for products like yogurt, buttermilk, and cheese.
• Culturing milk prevents many spoilage or harmful bacteria from growing.
• LAB may produce enough acid to limit their own growth.
• LAB can be wild contaminants that cause spoilage (e.g., excess acid/gas).
• Yeast and mold grow over a wider pH range, causing spoilage in cultured milk products.

Moisture or Available Water

• Bacteria need high levels of available moisture, expressed as water activity (aw).
• Pure water has an aw of 1.00; fluid milk is close.
• Sugars and salts bind water, lowering available water.
• Some cheeses have water activities less than 0.90 due to concentration and salt binding.
• Drying lowers available water (aw ~ 0.20).
• Most bacteria need a water activity greater than 0.91 and won't grow/spoil foods lacking water.
• Fluid milk spoils easily, while milk powder can be stored unrefrigerated long-term.
• Yeast and molds need less water, spoiling cheeses and jams/jellies.

Oxygen Requirements

• Some bacteria require oxygen, while others cannot grow in its presence.
• Oxygen can be toxic to certain bacteria.
• Aerobic: Require oxygen for growth.
• Anaerobic: Grow only without oxygen (oxygen may be lethal).
• Facultative Anaerobic: Grow with or without oxygen.
• Milk contains dissolved oxygen, supporting aerobic and facultative anaerobic growth.
• Strict anaerobes rarely grow in milk.
• Some bacteria (e.g., certain starter cultures) are “microaerophilic” and grow best in lower oxygen levels.
• Cheese may have a reduced oxygen environment, allowing anaerobic bacteria to grow, causing defects (e.g., late gas-blowing).
• Clostridium botulinum is an anaerobe producing a deadly toxin, rarely associated with dairy.

Influence of Inhibitors

• Chemical substances inhibit (bacteriostatic) or kill (bactericidal) bacteria.
• Examples include drugs/antibiotics, lactoferrin (in raw milk), carbon dioxide, lysozyme, sanitizers, organic acids, preservatives (e.g., potassium sorbate), and natural inhibitors (e.g., nisin).

Temperatures for Growth

• Optimum growth temperature is when generation time is shortest.
• Each bacterium has minimum and maximum growth temperatures.
• These ranges vary and overlap.
• Thermophilic: "Heat Loving“- Min: 104°F (40°C), Max: 140°F (60°C), Opt: 122-131°F (50-55°C).
• Mesophilic: Medium Temps - Min: 41°F (5°C), Max: 122°F (50°C), Opt: 86-98oF (30-37oC).
• Psychrophilic: "Cold Loving“- Min: 32°F (0°C) or less, Max: 77°F (25°C).