Food Microbiology

Food Spoilage

• Food spoilage results from microbial activities that degrade food quality, rendering it unsafe or unappetizing for consumption. This deterioration is often visible through changes in texture, odor, and appearance.

• Microbes such as bacteria, molds, and yeasts can cause spoilage by producing enzymes that break down food components. Some microbes are pathogenic, leading to severe illnesses when contaminated food is consumed.

Food Poisoning Examples

• Salmonella outbreaks have been linked to contaminated eggs, particularly those from large-scale poultry farms in California. Symptoms can range from mild gastroenteritis to severe systemic infections.

• A food poisoning outbreak at a New South Wales Central Coast Hotel in Australia (12/07/2022) highlighted the risks of improper food handling and storage in commercial settings.

• Three deaths were linked to Listeria monocytogenes detected in NHS (National Health Service) desserts in the UK, raising concerns about food safety standards in healthcare facilities, symptoms include fever, muscle aches, and gastrointestinal issues.

• Major retailers such as Kohl's, Woolworths, Aldi, and IGA have recalled salad products following E. coli detection, pointing to widespread contamination issues in the fresh produce supply chain, symptoms includevere abdominal cramps, diarrhea (often bloody), and vomiting.

• Our food supply is frequently contaminated with pathogenic organisms, emphasizing the need for stringent food safety practices to prevent harm, illness, or death.

Beneficial Microbes

• Microbes play a crucial role in the preparation of foods like cheese, kimchi, wine, soy sauce, sausages, bread, tempeh, chocolate, beer, and yogurt through fermentation, they are the start of many culinary products.

• Microorganisms such as yeasts and bacteria contribute unique flavors, textures, and aromas that enhance the sensory appeal of these foods; lactic acid bacteria in yogurt, for example, produce lactic acid, which gives yogurt its tart flavor and thick texture.

Course Sections

• Food Spoilage

• Foodborne Microbial Diseases

• Fermented Foods and Beverages

Course Objectives

• Understand microbiology and its role in food safety and production.

• Gain basic knowledge of microbial properties, growth characteristics, and survival mechanisms in various food environments.

• Understand how microbes cause food spoilage and how to prevent or control it.

Food Microbiology Defined

• Food microbiology involves the study of interactions between microorganisms, food, and humans. This field examines both the beneficial and detrimental aspects of microbial activity in food systems.

• These interactions can be characterized as "The Good, The Bad, and The Ugly."

  • The Good: Beneficial food products like cheese and wine, where microbes contribute desirable flavor and texture profiles.

  • The Bad: Illnesses caused by consuming food contaminated with pathogenic microbes, leading to foodborne diseases.

  • The Ugly: Food spoilage that makes food unappealing due to microbial degradation.

Course Objectives

• Upon completion of this course, students will be able to define microbiology and explain its fundamental principles.

• Upon completion of this course, students will be able to gain basic knowledge of microbial properties, including their structure, metabolism, and genetic characteristics. You will also learn about the factors influencing microbial growth and survival.

• Upon completion of this course, students will be able to describe the mechanisms by which microbes cause food spoilage, including the enzymatic degradation of food components and the production of undesirable metabolites.

Impact of Microbes on Food Supply

• Food Spoilage and Waste: Microbes cause food spoilage, leading to waste, resulting in economic losses and environmental concerns. According to the FAO, 30% to 50% of food produced worldwide is wasted due to spoilage.

• Food Safety: Pathogenic microbes contaminate food, causing foodborne illnesses. These illnesses can range from mild discomfort to life-threatening conditions.

• Food and Beverages: Positive impact through the production of fermented foods. Fermentation processes enhance flavor, texture, and nutritional value.

• Production of Food Ingredients: Microbes are used to produce vitamins, colorings and additives that enhance food quality and appeal.

  • Example: Rennet, traditionally from calf stomachs, is now often produced by microbes for cheese making. This microbial rennet is crucial for cheese production on a large scale.

• Probiotic Benefits: Probiotic organisms (e.g., Lactobacillus in yogurt) improve health, promoting digestive health and boosting the immune system.

  • Probiotic means "good for life," reflecting their health-promoting properties. Bacteria such as Lactobacillus and Bifidobacterium are commonly added to foods to create probiotic-rich products.

• Biocontrol Agents: Microbes or their components are used as antimicrobial or anti-pest agents in agriculture, reducing reliance on synthetic chemicals.

• Analysis and Quality Assurance: Microbes are analyzed to control food quality and prevent deterioration using techniques like microbial culturing and PCR-based detection methods.

Microbes: What Are They?

• Most microbes are single-cell organisms and are found in various environments on Earth.

• Microbes are very small, typically smaller than 10 micrometers and not visible to the naked eye, requiring microscopes for observation.

• Microbes Commonly Encountered in Food:

  • Bacteria (1-2 micrometers): Single-celled organisms that can multiply rapidly under favorable conditions.

  • Filamentous Fungi (Mold) (5-20 micrometers): Multicellular organisms that form visible colonies on food surfaces.

  • Yeast (10-20 micrometers): Unicellular fungi that are involved in fermentation processes.

• Less Common Microbes:

  • Bacterial Viruses: Also known as bacteriophages, these viruses infect and replicate within bacteria.

  • Animal Viruses (e.g., Hepatitis A): Viruses that can contaminate food and cause human illnesses.

  • Protozoa: Single-celled eukaryotic organisms that can cause parasitic infections.

  • Parasites: Organisms that live in or on a host organism and derive nutrients from it.

  • Algae: Photosynthetic organisms that can contaminate seafood and produce toxins.

Types of Microbes in Food

• Bacteria

  • Common shapes include rod-shaped (bacillus) and spherical (cocci). These shapes influence bacterial behavior and identification.

  • Less common shapes include spiral or curved, which are observed in specific bacterial species.

• Yeast

  • Vast majority of yeast typically have an oval shape and are larger than bacteria.

  • Reproduce by budding, forming scars on the cells, which can be observed under a microscope.

  • Mother and daughter cells are terms used to describe yeast cells undergoing reproduction.

• Fungi (Mold)

  • Form colonies from spores, which are resistant structures that allow fungi to survive adverse conditions.

  • Different species create different colony morphologies (shapes, sizes, colors), aiding in their identification.

  • Penicillium is a genus used to make penicillin, an antibiotic that inhibits bacterial growth.

• Viruses

  • Coronavirus: Has spike proteins to attach to respiratory cells. These proteins mediate the entry of the virus into host cells.

  • Viruses cannot reproduce themselves; they rely on a host cell and its machinery for replication.

  • Viruses are host-specific, meaning they can only infect certain types of cells or organisms.

  • Bacteriophages: Viruses that attack bacteria; can cause "stuck fermentation" in yogurt production, but can also be used to control pathogens like Salmonella in poultry. This application is known as phage therapy.

Microbial Contamination

• Food contamination is almost inevitable because microorganisms are ubiquitous (everywhere) and can be found in virtually every environment.

• They are found in soils, sediments, rivers, water, on plants and animals, and in intestinal tracts, increasing the likelihood of food exposure.

• Our food are biological materials that naturally come into contact with microorganisms during production, processing, and distribution.

• Microbial cells and mold spores are present in the air, facilitating their dispersal and contamination of food surfaces.

• Food goes through processing, storage, transportation, and retail display, providing many opportunities for contamination at various stages of the food supply chain.

• Contamination may not be bad if the organisms are unharmful and present in low numbers, posing minimal risk to human health.

Microbial Growth Curve

• Lag Phase: Initial microbial level is low with almost no growth as the microbes acclimate to the new environment, adapting to the available nutrients and conditions. The goal is to keep this phase as long as possible to prevent spoilage by inhibiting microbial reproduction.

• Exponential Phase: Microbial numbers increase rapidly due to multiplication, with cells dividing at an accelerated rate under optimal conditions.

• Stationary Phase: The growth rate slows, and the population becomes stagnant as nutrients are used up and toxic metabolites are produced, limiting further growth.

• Decline Phase: The microbial population starts to decline as most cells die due to starvation, accumulation of toxic compounds, or unfavorable environmental changes.

Microbial Growth Requirements

• Nutrients: Like humans, microbes need:

  • Carbon Source: Sugars, polysaccharides (starches), fats. Microbes prefer simple sugars because they are easier to metabolize for energy.

  • Nitrogen Source: Ammonia, amino acids (from proteins). These are essential for synthesizing proteins and nucleic acids.

  • Vitamins and Minerals: Act as cofactors for enzymes and are necessary for various metabolic processes.

  • Water: Crucial for dissolving nutrients and facilitating biochemical reactions within cells.

• Suitable Environment:

  • Temperature: Too cold inhibits growth, too hot kills microbes. Optimal temperature ranges vary depending on the type of microorganism.

  • Acidity: Microbes don't like acid, which is why yogurt has a longer shelf life than milk. Acidic conditions inhibit the growth of many spoilage and pathogenic bacteria.

  • Air/Oxygen: Some microbes need air/oxygen (aerobes), so vacuum packaging is used to inhibit their growth, while others thrive in the absence of oxygen (anaerobes).

  • Antimicrobial Factors: Natural substances (e.g., benzoate in berries, compounds in garlic and ginger) or artificial preservatives, and salt. These factors can inhibit microbial growth or kill microorganisms.

Microbial Growth

• Microbial growth refers to multiplication (increase in numbers) and the subsequent formation of colonies or populations.

• Bacteria: Grow by binary fission (one cell splits into two), a process that results in exponential growth under favorable conditions.

• Filamentous Fungi: Grow by hyphae extension (like tree branches), forming a network of interconnected filaments called mycelium.

• Yeast: Grow by budding (forming daughter cells that detach), a process where a new cell grows out of the parent cell.

• Different Microbes Grow Differently in Our Food

  • Viruses

  • Human Viruses: Do not grow; they are passive carriers because they need host-specific cells from humans, and not fish. They rely on human or animal hosts for replication.

  • Bacterial Viruses (Bacteriophage): Can grown by invading host cells and replicating within bacteria, leading to bacterial cell lysis.

Consequences of Microbial Growth

• Breakage: Microbes break down complex compounds (proteins, polysaccharides) into simpler components through enzymatic activity.

• Absorb: Microbes absorb the individual building blocks or molecules into the cells to obtain nutrients and energy.

• Metabolize: Absorb nutrients through their cell walls and metabolize them through various biochemical pathways.

• Excrete: Byproduct that will later impact the original food, altering its properties and characteristics.

• Consequences or properties of our food will change (chemical composition, physical property, and sensory property), affecting its overall quality and safety.

Microbial Metabolism

• Releases metabolites (waste products) into the food, altering its chemical composition and sensory properties.

• Alters chemical composition, physical properties, and sensory properties of the food, affecting its quality and safety.

• Changes can be:

  • Good (e.g., wine production), where microbes produce desirable flavors and aromas through fermentation.

  • Bad (e.g., terrible smell caused by hydrogen sulfide due to meat spoilage), rendering the food unappetizing and unsafe.

  • Ugly (toxins released by pathogens), posing a risk to human health and causing foodborne illnesses.

Factors Influencing Microbial Growth and Survival

• Properties of the food (intrinsic factors):

  • Physical and chemical composition, including pH, water activity, and nutrient content.

• Processing factors (extrinsic factors):

  • Heating, freezing, drying, adding preservatives to inhibit microbial growth and extend shelf life.

• Storage conditions (extrinsic factors):

  • Temperature, packaging, and atmosphere control to minimize microbial growth and spoilage.

• Properties of organisms (implicit factors):

  • Tolerance to cold, heat, acid; need for oxygen. These characteristics determine the ability of microbes to survive and grow in specific food environments.

• Interactions between microbes:

  • Antagonistic: Competing for nutrients, leading to inhibition of one species by another.

  • Symbiotic: Mutually beneficial interactions that enhance the growth of both species.

  • Coexistence: Different species can coexist without affecting each other's growth.

Impact on Sensory Properties

• Changes in:

  • Appearance (color, turbidity, sediment), making the food unappetizing or unacceptable.

  • Texture (loss of structure, sliminess, gassiness), altering the mouthfeel and acceptability of the food.

  • Odor: Production of foul odors due to microbial metabolism, indicating spoilage.

  • Flavor (taste plus smell), leading to undesirable or off-flavors in the food.

Microbial Spoilage of Food

• A sensory judgment; food is spoiled when it is no longer suitable for consumption due to changes in appearance, texture, odor, or flavor.

• Subjective and varies by person and culture, depending on individual preferences and cultural norms.

• Economic consequences of shelf life and spoilage are very important to the food industry, influencing production, distribution, and retail practices.

  • A very major part of the food industry is about how do we maximize the shelf life of our product while maintaining its quality and safety.

Introduction to Foodborne Microbial Illness

• Consumers become ill after consuming food or beverages contaminated with specific microorganisms or their toxins, leading to various health problems.

• Type of illness, severity of symptoms, and associated factors vary with the organism and the health status of the consumer, ranging from mild discomfort to life-threatening conditions.

• Foodborne microbial illnesses have a long history, with records dating back to ancient civilizations, highlighting the ongoing importance of food safety.

Forms of Foodborne Microbial Diseases

• Infection: Live pathogenic microbial cells are ingested and grow in the body, causing illness through their proliferation and invasion of host tissues.

• Intoxication: Microbes grow in food and release toxins that cause illness when ingested (e.g: botox). These toxins can affect various organ systems and lead to different clinical manifestations.