Microbial Food Spoilage

Microbial Food Spoilage

• Microbial spoilage results in microorganism growth or enzyme release in food

Detectable Parameters Associated with Spoilage

• Detectable spoilage parameters involve changes in color, odor, texture, slime, gas, foam, or liquid accumulation
• Understanding spoilage factors is crucial for identifying the cause and devising its control

Sequence of Events in Food Microbial Spoilage

• Spoilage requires contamination
• Followed by an environment that supports microorganism growth
• Improper storage temperatures, allowing enough time for microbes to grow to detectable levels

Significance of Microorganisms

• Raw and processed foods contain molds, yeasts, and bacteria
• Bacteria and yeasts result in quick spoilage
• Molds are more common when bacteria/yeast growth is poor, or food is stored long-term
• Anaerobic packaging reduces mold and yeast spoilage but not anaerobic bacteria

Microbial Number

• To cause noticeable spoilage, microorganisms must multiply to a "spoilage detection level"
• Bacteria and yeasts need to reach around 107 cells/gram/milliliter, or /cm² from the start
• Detection levels range from 106 to 108 cells/gram/milliliter, or /cm²
• Slime is evident at >108 cells/gram/milliliter, or /cm²
• H2S, certain amines, and H2O2 are detectable at low microbial loads
• Lactic acid production is noticeable at higher loads

Predominant Microorganisms in Food Spoilage

• Unspoiled, non-sterile food has various genera, but spoilage is dominated by one or two types
• The generation time of microbial species is significantly longer in food compared to microbiological broth

Important Food Spoilage Microorganisms

• Food Spoilage Molds

• Predominantly on spoiled solid foods with low water activity (0.7 to 0.8)

• Grow in pH 3 to 8

• Some produce mycotoxins

• Includes Mucor, Rhizopus, Penicillium, Aspergillus, and Botrytis

• Food Spoilage Yeasts

• Cause spoilage in high-sugar/salt foods

• Grow in fruits, low pH juices, and on meats/cheeses

• Includes Saccharomyces spp., Zygosaccharomyces, Candida, and Dekkera (or Brettanomyces)

• Food Spoilage Bacteria

• Psychrotrophic Bacteria

• Grow at ≤5°C, multiply at 10°C to 25°C, even higher

• Aerobic means psychrotrophic aerobes dominate

• Anaerobic means anaerobic and facultative anaerobic bacteria predominate

• Low-heat without post-heat contamination means psychrotrophic thermoduric bacteria cause spoilage

Thermophilic Bacteria

• Bacteria that thrive at high temperatures
• Grow in 40°C to 90°C, with optimal growth from 55°C to 65°C
• Spores germinate during warm storage
• Post-heat contamination can occur with temperatures at 50°C
• Key species are Pediococcus acidilactici, Streptococcus thermophilus, Bacillus, and Clostridium

Aciduric Bacteria

• Grow quickly at pH ≤ 4.6
• Associated with acidic foods like fruit juices, pickles, salsa, dressings, mayonnaise, and fermented sauces

Key Aciduric Bacteria

• Heterofermentative Lactobacilli include Lactobacillus fructivorans, Lactobacillus fermentum, Leuconostoc mesenteroides
• Homofermentative Lactobacilli include Lactobacillus plantarum, Pediococcus acidilactici

Significance of Foods in Microbial Spoilage

• Foods' susceptibility varies with intrinsic factors

Food Types

• Perishable foods spoil in days, need refrigeration/freezing
• Semi-perishable foods have long shelf life, lasting weeks/months
• Non-perishable foods have very long shelf life of lasting months/years

Food Nutrients

• Microbial growth links to metabolism, causing spoilage
• Carbohydrates, polysaccharides, trisaccharides, disaccharides, monosaccharides, and sugar alcohols
• Proteinaceous compounds like proteins and peptides
• Non-protein nitrogenous compounds such as amino acids, urea, creatine, trimethylamine oxide
• Lipids like triglycerides, phospholipids, fatty acids, and sterols

End Products of Microbial Metabolism in Food Spoilage

• Nutrients cause unwanted changes like color alteration, texture degradation, gas accumulation, odor, slime, spoilage

Contributing Factors in Microbial Spoilage of Food

• Type and composition of product
• Processing, contamination during processing
• Storage conditions, likely use temperatures
• Nature of product packaging

Normal Microbiological Quality of Foods and Its Significance

• Under regular conditions, food harbors naturally occurring/introduced microorganisms
• Relative abundance depends on food conditions
• Microbial load results from contamination/subsequent growth

Raw and Ready-to-Eat Meat Products

• Contain microorganisms from skin, hair, feathers, gastrointestinal tract, surrounding environment
• Carcasses have 10¹ to 10³ bacterial cells per square inch, with low-level enteric pathogens
• Microorganisms found at low levels include Salmonella, Yersinia enterocolitica, Campylobacter jejuni, Escherichia coli, Clostridium perfringens, Staphylococcus aureus
• Bird carcasses have high Salmonella risk from fecal matter exposure
• Microorganisms from carcasses and processing equipment harbor chilled raw + ground meat
• Predominant microorganisms are psychrotrophs like Lactobacilli, Leuconostocs, Pseudomonas
• Ground meat has 10⁴ to 10⁵ microorganisms per gram, some with Salmonella

Milk

• Low-Heat Processed meats use heat to 160°F (71°C)
• Heat kills most microorganisms, but thermoduric bacteria survive
• Post-heating microbial levels range from 10¹ to 10² / gram
• More processing may cause microorganisms
• Vacuum-sealed/modified-atmosphere storage can increase bacterial numbers affecting safety/shelf life

Raw and Pasteurized Milk

• Both raw and pasteurized milk have diverse bacteria so both need refrigeration due to microbial growth
• Raw milk bacteria from udder, animal surfaces, feed, water, and storage equipment
• Healthy udders have types like Micrococcus, Streptococcus, and Corynebacterium
• Raw milk typically contains