Chapter 6 Food Microbiology
Microbes have had a considerable impact on foods. These impacts include:
- Food spoilage
- Foodborne illnesses
- Food production
- As foods themselves
6.1 Food Spoilage
This occurs as a result of microbes growing on food, hence altering food visibly and rendering it unsuitable for consumption.
Food spoilage also involves the predictable succession of microbes; toxins are sometimes produced, and different foods undergo different types of spoilage processes.
Intrinsic and extrinsic factors may also affect food spoilage:
6.1.1 Intrinsic factors
These are factors within the food itself.
In the case of carbohydrate-rich foods, food spoilage is predominated by mold.
Mold degrades food via hydrolysis (tomatoes are particularly suscpetible to degradation by mold), during which very little odor is produced and ergotism may develop from the consumption of mold-infested foods.
Ergotism is caused by hallucinogenic alkaloids by Claviceps purpurea and may result in death. Ergotism was also associated with the Salem witch trials and began way back as early as 857 AD:
“a Great plague of swollen blisters consumed the people by a loathsome rot, so that their limbs were loosened and fell off before death.”
— Some quote from 875 AD
Food degradation by bacteria may occur in foods high in proteins or fats (e.g., milk)
Putrefaction is the proteolysis and anaerobic decomposition of proteins that result in foul-smelling amines.
Unpasteurized milk is spoiled via acid-producing bacteria and putrefaction, whereas butter goes bad via short chain fatty acid formation (hence resulting in rancid butter):
Figure 6.1: Unpasteurized Milk and Butter Gone Bad
Furthermore, there are also other intrinsic factors at play:
6.1.1.1 pH
This impacts the composition of the microbial community and hence, the types of chemical reactions that may occur when microbes grow in food. A low pH favors the growth of yeast and mold.
6.1.1.2 Water availability
In general, a lower water content inhibits microbial growth.
6.1.1.3 Oxidation-reduction potentials
This is altered by cooking; a lower redox potential gives rise to more bacteria and anaerobes being present in the food.
6.1.1.4 Physical structures (of food)
Grinding and mixing food (e.g., minced meat) distributes microbes and promotes their growth; the outer skin of fruits and vegetables slows microbial growth.
6.1.1.5 Antimicrobial substances
Compounds found in nature may also have anti-microbial properties:
| Compounds | Foods |
|---|---|
| Coumarins | Fruits and Veggies |
| Lysozyme | Cow milk and eggs |
| Aldehydic and Phenolic Compounds | Herbs and spices |
| Allicin | Garlic |
| Polyphenols | Green and black teas |
6.1.2 Extrinsic factors
There are several extrinsic factors that influence the degradation of food:
6.1.2.1 Temperature
A lower temperature retards the growth of microbes.
6.1.2.2 Relative humidity
A more humid environment promotes microbial growth.
6.1.2.3 Oxygen concentrations
An oxygen-rich environment promotes microbial growth. Hence, modified atmosphere packaging (i.e., MAP) is used to shrink wrap and vaccum pack food into controlled atmospheres.
6.2 Controlling Food Spoilage
The modern era of food microbiology was established by Louis Pasteur in 1857.
The goal of food preservation is to eliminate or reduce the population of spoilage and / or disease-causing microbes while maintaining the quality of food.
Liquids like wine, water, beer, juices, soft drinks, and other liquids are usually filtered - this may preserve their aroma and flavor.
6.2.1 Temperature
Refridgeration at 5 degrees Celsius retards, but does not stop microbial growth. Growth at -10 degrees Celsius has even be recorded!
Food can also be heated in special containers at 115 degrees Celsius for 25 to 100 minutes, hence killing spoilage microbes (but not necessarily all microbes in food).
Spoilage of canned foods can usually be a consequence of underprocessing, spoilage of the food prior to being canned, and the leakage of contaminated water into cans during the cooling process.
Figure 6.2: Improper Processing of Carrots in a Factory
6.2.2 Pasteurization
Pasteurization kills microbes and substantially reduces the amount of spoilage microorganisms in food.
Different pasteurization procedures heat the food for different lengths of time; a shorter heating time improves the flavor of the food.
6.2.3 Availability of water
Lyophilization to produce freeze-dried foods is commonly employed to eliminate bacterial growth.
Consequently, food preservation happens as a result of free-water loss and an increase in solute concentrations.
6.2.4 Chemical-based methods
GRAS agents are Generally Recognized as Safe and include organic acids, sulfite, ethylene oxide gas, ethyl formate, and sodium nitrite (which inhibits spore formation in meats and forms nitrosamines).
The pH and salt content of the food also influence the effectiveness of chemical preservatices.
6.2.5 Radiation
6.2.5.1 Radapperitization
Gamma radiation (i.e., ionizing radiation) is used to extend the shelf life or sterilize meats, seafoods, fruits, and vegetables while keeping the aforementioned foods non-radioactive.
This process also kills microbes in foods by producing peroxides from water (as peroxides oxidize cellular components).
6.2.5.2 e- beams
e- are electrically generated, so such a procedure can be turned on only when needed.
While e- beams do not generate radioactive waste, they also do not penetrate as deeply as gamma radiation.
6.2.6 Microbial product-based inhibitions
Bateriocins are bacteriocidal proteins that are active against related species; some bacteriocins dissipate the proton motive force of susceptible bacteria, form pores in the plasma membrane, or inhibit protein or RNA synthesis.
Nisin from Lactococcus lactis is used in low-acid foods to inactive Clostridium botulinum during the canning process.
Bacteriophages that kill Listeria monocytogenes are also sprayed onto ready-to-eat meats prior to packaging.
6.2.7 Packaging
The gases that food are stored in may also affect microbial growth. Shrink-wrapping and vaccum technology not only make food impermeable to oxygen gas, but superoxide radicals also inhibit the growth of microbes.
Polylactic acids are a greener alternative to plastic made from wood and corn. Such acids are also embedded with nisin and are slowly released.
6.3 Food-Borne Illnesses
There are two primary types of food-borne diseases: food-borne infections and food intoxications.
Food-borne infections may be carried out by pathogens like the norovirus, E. coli, Campylobacter jejuni, Salmonella, and Listeria monocytogenes.
Both food-borne illnesses and intoxications can be transmitted via bad hygeiene and the fecal-oral route. Fomites (inanimate objects that transmit infectious organisms from one individual to another) are also important.
6.3.1 Food-borne infections
These infections can arise as a result of ingesting a pathogen. Thereafter, the pathogen grows in the gastrointestinal tract, invades the tissues, and releases toxins.
Raw foods are an important source of food-borne infections.
Listeriosis in particular is much more hazardous to pregnant women, the young, and the elderly, and is responsible for the largest meat recall in the US.
Hence, those at risk should not consume refrigerated smoked meats, deli meats, and undercooked hot dogs.
6.3.1.1 Typhoid Mary
In 1907, Mary Mallon was the first “healthy carrier” of typhoid fever in the US. She worked as a cook, and the most common way to spread typhoid was through food and water sources.
In total, 47 people were infected and three died. She was later captured and forced to live in isolation.
Mary did appeal in the end; her appeal was successful and she was set free on the condition that she did not work as a cook.
Five years later, there was an outbreak of typhoid fever at a maternity hospital that claimed two lives.
6.3.1.2 Clostridial food poisoning
The bacteria C. perfringens and C. botulinum have the potential to cause serious food poisoning. Both bacteria are anaerobic and spore-forming. Hence, canning and cooking can kill vegetative cells, but not vegetative spores!
As a general rule of thumb, at least a 100 million C. perfringens cells are required for food poisoning. The bacterium sporulates in the gut and begins producing endotoxins thereafter.
While poisoning starts 7 - 15 hours after ingestion, most patients recover after 24 hours; food poisoning by C. perfringens is rarely fatal.
6.3.2 Food-borne intoxications
These intoxications occur as a result of ingesting toxins in foods in which microbes have grown.
Symptoms of food-borne intoxications show up shortly after the food has been consumed (as no microbial growth is required).
Some common food-borne intoxications include botulism, C. perfringens poisoning, and B. cereus food poisoning.
Furthermore, aflatoxins found in fungus-infected grain and nut products are shown to be carcinogenic (i.e., cancer-causing). Fumonisins from fungus-infected corn are also carcinogens.
Algal toxins are found in shellfish and fish
6.4 Microorganisms and Food Production
Most food production involving microbes involve fermentation (chemical changes in food from microbial action). Some major fermentations used are lactic, propionic, and ethanolic fermentations.
6.4.1 Fermented milk
The majority of fermented milk products rely on lactic acid bacteria that belong the following genera:
- Lactobacillus
- Lactococcus
- Leuconostoc
- Streptococcus
Bacteria from these genera are all gram positive, tolerate acidic conditions, are non-spore forming, and are aerotolerant with a strictly fermentative metabolism.
6.4.2 Probiotics
These are live microorganisms that confer a health benefit to the host when administered in proper amounts (usually Lactobacillus and Bifidobacteria).
However, specific, individual requirements should be met!
Figure 6.3: Probiotic Development Process
6.4.2.1 Benefits of probiotics
Probiotics have several advantages:
- Immunomodulation
- Diarrhea control
- Anticancer effects (colon cancer in particular)
- Possible modulation of Crohn’s disease
- Treating enteric disease
- A pretty good alternative considering that antibiotic resistance is on the rise!
6.4.3 Making cheese
The Romans have exported and made a variety of cheeses, evidence for which dates back to 3100 BC.
There are at least 2000 distinct varieties of cheese with 20 general kinds (classified based on texture and hardness). Roman cheeses were all made from lactic acid fermentation.
Figure 6.4: The General Cheese Making Process
6.5 Meats and Fish
6.5.1 Fermented meat
Sausages are some of the oldest foods consumed by humans - sausage making began over 2000 years ago and is still a growing industry today!
Sausages helped early civilizations overcome the problem of food preservation: there were no refridgerators back then to preserve meat like we do now!
6.5.1.1 Effects of bacterial cultures on meat and meat products
Adding a curing agent and sugar under fermentation conditions stopped the growth of microbes in fermented sausages.
Injecting psychotropic lactic acid (even under aerobic conditions) in raw ham and ready-to-eat meats led to a 2 log reduction of L. monocytogenes during MAP conditions!
Furthermore, strains of lactic acid bacteria also help improve the shelf life and freshness of refrigerated meat (e.g., bacon and sausages).
Pediococcus starter bacteria produce acids and restrict the growth of any C. botulinum bacteria, consequently extending the shelf life of salted semi-processed raw meats.
6.6 Alcoholic Beverages
6.6.1 Wines and Champagnes
Enology is the art of producing wine:
Figure 6.5: General Wine-Making Process
As observed in the graphic, grapes are first crushed to separate and store the juice (must) before it is fermented.
The fresh must is then treated with a SO2 fumigant, and Saccharomyces cerevisiae or S. liposideus is then added for consistent results. The must is fermented for 3 - 5 days at 20 - 28 degrees Celsius.
Dry or sweet wines are made by regulating the initial must’s sugar content.
Racking is a process that removes sediments produced during fermentation.
6.6.2 Beers and ales
Cereal grains are used for fermentation here (instead of grapes).
Figure 6.6: General Beer Making Process
The malt refers to germinated barley grains that have been activated by enzymes. The mash refers to malt that has been mixed with water in order to hydrolyze starch to usable carbohydrates.
Mash heated with hops (components that lend flavor to the beer) not only add flavor to the beer, but also inactivates hydrolytic enzymes.
6.6.3 Bread
Baking bread involves the growth of Saccharomyces cerevisiae (i.e., baker’s yeast) under aerobic conditions.
Other microbes (e.g., Bacillus species) are used to make different varieties of bread (e.g., sourdough bread - this bread has a certain ropiness).
6.7 Microbes as Food
As the subtitle suggests, microbes themselves can also be food. Examples include:
- Mushrooms (e.g., Agaricus bisporus)
- Spirulina dried cakes
- Probiotics
- Lactobacillus acidophilus in beef cattle (which decreases the concentration of E. coli O157:H1)
- Bacillus subtilis in poultry (which reduces the amount of Salmonella and Campylobacter in poultry products)
6.8 Food Security and Technology
Singapore currently makes less than 5% of its food. Hence, the government has recently pushed to increase food production with less space and water.
Alternative food sources have also been considered: roof-top farming and insect farming in particular.
Many firms have also utilized microbes to make new foods (i.e., boutique food production, Sous Vide, etc).