Applied Dairy Microbiology Overview

Applied Dairy Microbiology

• Dairy microbiology is a branch of applied microbiology.

Milk Composition

• Milk is a liquid nutrient-rich food.
• Milk contains varying amounts of nutrients.
• Milk is primarily composed of water (87.4%) and solids (12.6%).
• Solids include lactose (5%), protein (3%), fat (3.5%), minerals (1%), vitamins (A, E, K, B, C) and enzymes (catalase, lipase, phosphatase).
• Milk is neutral with a pH of approximately 6.7.

Milk Contamination

• Milk is sterile when secreted from the udder.
• Milk can be contaminated before leaving the udder, even without mastitis.
• The first milk from the milking process should be discarded because it contains high bacteria numbers.
• Milk can be further contaminated during subsequent handling, transport and processing.
• Factors influencing milk contamination include animal health, milking method, farm environment, transportation, storage and pre-processing activities.

Sources of Milk Contamination

• Air, milking staff, animal skin, tools, water supplies and the udder (interior and exterior) are significant sources of milk contamination.
• Contaminating microbes in milk include bacteria, yeasts and fungi.

Microorganisms in Milk

• Microorganisms in milk can either be spoilage or pathogenic.
• Spoilage microorganisms belong to four physiological groups that produce lactic acid, propionic acid, butyric acid, or degradative enzymes.
• Lactic acid bacteria can create fermented dairy foods.
• Pathogenic microorganisms in milk can be infectious or toxin-producing.

Types of Bacteria in Milk

• (1) Lactic Acid Bacteria*

• Lactococci (e.g., Lactococcus lactis, Streptococcus lactis), Lactobacilli (e.g., Lactobacillus casei), and Leuconostoc (e.g., Leuconostoc lactis) ferment lactose to lactic acid.

• These are used to create dairy products such as yogurt.

• (2) Coliforms*

• Facultative anaerobes with optimum growth at 37°C.

• Coliforms are indicator organisms associated with pathogens.

• Coliforms can cause rapid milk spoilage by fermenting lactose, producing acid and gas and degrading milk proteins.

• They are killed by pasteurization.

• Escherichia coli is a type of coliform.

• (3) Spoilage Microorganisms*

• The main spoilage microorganisms are psychrotrophic organisms.

• Some psychrotrophs, such as Pseudomonas fluorescens and Pseudomonas fragi produce heat-stable proteolytic and lipolytic extracellular enzymes that cause spoilage.

• Other spoilage microorganisms like Bacillus, Clostridium, Cornebacterium, Lactobacillus, Micrococcus, and Streptococcus can grow in refrigerators.

• (4) Pathogenic Microorganisms*

• Hygienic milk handling and pasteurization lessen milk-borne diseases such as tuberculosis, brucellosis, and typhoid fever.

• Raw milk and inadequately processed dairy products can still pose a risk of infections.

• Examples of bacterial pathogens include Bacillus cereus, Listeria monocytogenes, Yersinia sp., Salmonella spp., Escherichia coli O157:H7, and Campylobacter jejuni.

Effect of Temperature on Milk Microbes

• Psychrophilic bacteria thrive in cold temperatures closer to freezing.
• Mesophilic bacteria grow at ambient temperatures.
• Thermophilic bacteria thrive at high temperatures above 65°C.
• Heat-resistant bacteria can survive pasteurization.
• Low temperatures inhibit acid-producing bacteria but activate protein-degrading bacteria.
• Medium temperatures allow acid-producing bacteria but reduce fat/protein degrading bacteria.
• High temperatures kill acid-producing bacteria/ degrade proteins.

Milk Preservation Methods

• Cooling: Cooling milk immediately after milking to around 4°C slows down microbial growth, especially mesophilic and psychrotrophic bacteria. This also extends milk's shelf life, but thermoduric bacteria can still survive at this temperature.

Pasteurization

• Pasteurization is a heat treatment process. It significantly reduces spoilage and pathogenic microbes, including Mycobacterium tuberculosis, by exposing to mild heat below boiling point (90-99%).
• It helps maintain nutritional components, specially vitamins and calcium.
• There are two main pasteurization methods: slow pasteurization (LTHM) and fast pasteurization (HTST).
• The slow method uses around 62.85°C for 30 minutes, and the faster method uses 71.75°C for 15 seconds.

Phosphatase Test

• Alkaline phosphatase (ALP) enzyme presence in raw milk is an indication of proper pasteurization.
• Complete pasteurization inactivates ALP to undetectable levels using traditional methods.
• Comparing heat resistance of enzyme ALP to that of pathogens determines milk safety.
• Methods for testing phosphatase include classic (visual) and rapid (test strips).

Microbes in Pasteurized Milk

• Thermophilic bacteria can survive pasteurization and cause spoilage if milk is stored improperly (e.g., Streptococcus thermophilus, Lactobacillus delbrueckii).
• Some thermoduric bacteria can withstand heat treatment. They use milk components including proteins and fats as nutrients leading to quality changes (e.g., Streptococcus spp, Enterococcus spp, Lactobacillus spp).
• Psychrophilic bacteria cause spoilage in milk after a few days and are not significantly affected by pasteurization. (e.g., members of genera Alcaligenes, Pseudomonas and Lactococcus).
• Spore-forming bacteria like Bacillus & Clostridium can tolerate pasteurization if present in sufficient amounts and may cause spoilage.
• Coliforms can be present, but do not harm pasteurized milk.

This quiz explores the fundamentals of applied dairy microbiology, with a focus on milk composition and contamination. Learn about the nutritional components of milk and the various factors that contribute to its contamination during the milking and processing stages. Test your knowledge on these crucial aspects of dairy science.