Food and Industrial Microbiology

Introduction

This topic explores the role of microbes in food spoilage, preservation, fermentation, and industrial biotechnology. Microorganisms are both a challenge and a resource in food science, affecting food safety, quality, and production of valuable products.

Food Spoilage and Preservation

Food Spoilage

• Food spoilage refers to microbial changes that render food unfit or unpalatable for consumption.

• Food contamination or food poisoning is caused by the presence of human pathogens in food.

• Microbes can digest food, create toxic or unpleasant materials, and cause diseases.

Food Preservation Methods

• Eat fresh food before microbes can grow.

• Heat treatment:

  • Pasteurization: Quick heat kills most microbes. Example: Louis Pasteur discovered pasteurization, originally experimenting with beer.

  • Sterilization via heat: Cooking, UHT, or pressure-cooking (autoclave) for canning kills all microbes.

• Remove excess water: Drying, salting, or concentrating sugars inhibits microbial growth.

• Use acids: Vinegar, lactic acid, citric acid lower pH and inhibit spoilage organisms.

Factors Affecting Food Preservation

• Temperature: Lower temperatures slow microbial growth. Example: Refrigeration at 0°C, 2°C, or 8°C affects spoilage rates (see graph below).

• pH: Lower pH inhibits microbial growth. Example: Foods at pH 4.0 spoil more slowly than those at pH 5.0.

Microbes and Food

Fermentation Industry

• Fermentation dates back to 5000 BCE.

• Modern fermentation uses bacteria in large vessels to produce enzymes and other products.

Microbes as Food

• Fungi:

  • Mushrooms are fungal fruiting bodies (e.g., Agaricus bisporus: button mushrooms, portobellos).

  • Yeasts such as Saccharomyces cerevisiae are used for nutritional yeast and yeast extract.

• Algae:

  • Red algae (Porphyra): nori for sushi.

  • Brown algae (Macrocystis): alginate as a thickener.

• Bacteria:

  • Cyanobacterium Spirulina is used as a protein-rich food supplement.

Fermented Foods Overview

Fermentation Process

• Fermentation is the treatment of food with microbes.

• Microbes digest food, preserving it and making it inedible for predatory microbes (e.g., alcohol, acid buildup).

• Fermentation makes food more digestible by breaking down indigestible fibers.

• Microbes add nutrients (e.g., vitamins) and flavors.

Types of Fermentation

Acidic Dairy Fermentation

• Bacteria coagulate milk to form curds.

• Lactose is fermented to lactate, making cheese and yogurt suitable for lactose-intolerant individuals.

• Proteases (rennet) digest milk protein, forming harder cheeses.

• Further fermentation produces flavors and CO2 gas (e.g., Propionibacterium).

• Penicillium mold is used in blue and soft cheeses.

Acidic Vegetable Fermentations

• Soybeans:

  • Rhizopus mold makes tempeh.

  • Aspergillus mold makes miso and soy sauce.

• Cabbage, cucumbers, olives:

  • Pickling involves fermentation in brine (high salt) by Leuconostoc (e.g., sauerkraut, pickles, kimchi).

Alkaline Fermented Foods

• Soybeans: Bacillus bacteria make Natto.

• Eggs: Bacillus bacteria make Pidan (Century eggs).

Ethanolic Fermentation

• Bread: Saccharomyces cerevisiae (baker's yeast) ferments pyruvate to ethanol and CO2.

  • CO2 causes bread to rise; ethanol is removed by baking.

• Wine: Saccharomyces cerevisiae (brewer's yeast) ferments grape sugars to alcohol.

  • White wine: grape skin removed; red wine: skin retained.

  • Malolactic fermentation by Oenococcus oeni bacteria softens wine acidity.

• Beer: Saccharomyces cerevisiae ferments grains, breaking down starch into maltose.

  • Secondary fermentation products give beer unique flavors.

  • Alcohol destroys bacterial contaminants.

Food Spoilage and Pathogens

Food Spoilage Mechanisms

• Microbes destroy food before it is eaten, digesting it without providing nutrition.

• Microbial by-products cause:

  • Acids: sour taste

  • Oxidation of fats: rancidity

  • Decomposition of proteins: putrefaction

  • Alkalinity: bitter taste

  • Toxic pathogens: food poisoning

Pathogens Contaminating Food

• Salmonella enterica: Example outbreak in 2008 from peanut products.

• Listeria monocytogenes: Psychrotrophic bacterium that invades intestinal epithelium, causing listeriosis.

• Clostridium botulinum: Produces botulinum toxin, which inhibits synaptic vesicle fusion, causing flaccid paralysis. Example: Botulism outbreaks from improperly processed foods (e.g., bulging cans, baked potatoes wrapped in foil).

Industrial Microbiology

Microbial Exploitation for Products

• Microbes are used to produce vaccines, pharmaceuticals, human proteins, and organic compounds (e.g., xanthan gum from Xanthomonas campestris).

• Synthesis in E. coli:

  • Recombinant DNA technology allows insertion of human genes into bacteria for protein production.

  • Bacteria and eukaryotes modify proteins differently.

• Recombinant genes on plasmids:

  • Enable large-scale protein production.

Industrial Fermenters

• Fermenters provide optimal conditions for microbial growth: pH, osmolarity, aeration, temperature.

• Growth phase management:

  • Some products are made in log phase, others in stationary phase.

• Supply sterile nutrients, remove product and waste, and purify the final product.

Biotechnology Applications

• Messenger RNA Vaccines:

  • Produced using plasmid DNA, transformed into E. coli, and purified for vaccine use.

• Insulin Production:

  • Insulin is a hormone for blood sugar regulation.

  • Originally purified from animal pancreases; now produced by recombinant E. coli expressing the human insulin gene.

  • Recombinant insulin is functionally equivalent to human or animal-derived insulin.

Summary Table: Microbial Roles in Food and Industry

Key Terms

• Pasteurization: Heat treatment to kill most microbes.

• Fermentation: Microbial conversion of organic substrates to acids, alcohols, or gases.

• Recombinant DNA: Technology for inserting foreign genes into organisms.

• Psychrotrophic: Microbes that grow at low temperatures.

• Pathogen: Microorganism that causes disease.

Conclusion

Microbes play essential roles in food spoilage, preservation, fermentation, and industrial biotechnology. Understanding their biology enables safe food production and the development of valuable products such as medicines and vaccines.