Introduction

Microorganisms have been used for centuries for food preservation and to improve or change its taste. Evidence exists that yogurt, which is milk fermented by bacteria, has been around for over 4000 years. Today, many of the foods we eat are the result of microorganisms acting on foods for a specific and desired effect. Some examples of fermented foods (in addition to the previously mentioned yogurt) are wine/beer, sausage, sauerkraut/kimchee, bread, and cheese. The industrial use of microorganisms in food production began in earnest in the late 19th century, when pure cultures of bacteria were grown specifically for that purpose.

Figure 1: Propionibacterium shermanii cultures are inoculated into milk that will be turned into Swiss cheese (pictured). P. shermanii generate carbon dioxide, which forms gas bubbles in the solidifying cheese and creates the “holes” in the cheese. The bacteria also generates propionic acid; this adds to the distinctive taste of Swiss cheese.

Fermentation

Fermentation is important for the production of a variety of dairy products. Cheeses are typically made using lactic acid-producing bacteria that aid the coagulation of the milk protein casein into the curd. The curd is then further treated with a different bacteria (depending on the desired final cheese) to produce distinctive tastes and aromas. Cheeses can be produced with either aerobically or anaerobically growing bacteria. The characteristic holes in Swiss cheese are produced by specific bacteria that generate carbon dioxide and creates gas bubbles in the cheese. Buttermilk, sour cream, kefir, and yogurt are also examples of foods resulting from the fermentation of milk by bacteria. You will be using milk and commercially available yogurt (with live active bacterial cultures) to make homemade yogurt in one of the experiments outlined below.

Figure 2: How do we get a chocolate bar from cacao seeds? Microorganisms of course!

A variety of microorganisms are responsible for the production of chocolate. Chocolate is made from cacao seeds that have been inoculated with yeasts and bacteria from a variety of sources (both environmental and human introduced). Chocolate seeds are very bitter initially and are not suitable for eating. The yeasts and bacteria each grow under specific conditions by metabolizing different components of the seeds. Yeasts digest the sucrose present in the seeds, convert it to glucose and fructose and generate ethyl alcohol. The accumulating alcohol kills the yeast and lactic acid and acetic acid bacteria can now flourish. Lactic acid bacteria metabolize glucose for energy while acetic acid bacteria metabolize ethyl alcohol for energy. As the seeds are dried, molds grow and can use the byproducts of the lactic acid and acetic acid bacteria for energy. The microorganisms in chocolate production all aid in turning an inedible product to an edible food.

Figure 3: What do beer, bread, and cheese have in common? They all depend on microbes for their production.

Yeast (Saccharomyces cerevisiae) is used in baking, especially for bread. The yeast ferments the sugars in the flour under aerobic conditions (kneading the dough introduces air into it to favor aerobic, rather than anaerobic, fermentation). Aerobic fermentation generates carbon dioxide, which creates gas bubbles in the bread, just like in Swiss cheese. However, the microorganism responsible for generating the carbon dioxide is different in bread and Swiss cheese. In addition to yeast, sourdough bread also has cultures of Lactobacillus in it. These lactic acid bacteria produce acids, which lower the pH of the bread and give it its characteristic tart (sour) flavor.

Figure 4: A USDA inspector examining beef at a US processing plant. The inspector determines whether the food has been processed to minimize the presence of bacteria. Meat and poultry that pass are stamped with a USDA inspection mark made from a foodgrade vegetable dye.

Saccharomyces cerevisiae is also used for the production of most alcoholic drinks, including beer, wine, sake, and distilled beverages. In all cases, the yeast converts the sugar present in the starting material, which can be grains (beer, whiskey, sake), potatoes (vodka), molasses (rum), fruit (wine), to alcohol. Anaerobic, rather than aerobic, fermentation is preferred because this encourages the generation of ethyl alcohol (ethanol). Vinegar is made by inoculating anaerobically fermented starting material with the aerobic acetic acid bacteria (for example, Acetobacter and Gluconobacter). The aerobic bacteria oxidize the anaerobically produced ethanol to acetic acid, which is vinegar (sour wine).

Health Hazards

While there are a wide number of foods that are produced with microorganisms, the introduction of harmful organisms to food presents a serious health hazard. Even bacteria that normally colonize the human digestive system (such as E. coli) can cause severe illness or even death if ingested. Food processing plants have a number of ways to reduce the presence of harmful bacteria in food. The U. S. government, under the Food and Drug Administration (FDA) and the Department of Agriculture (USDA), establish safety standards and station inspectors at food processing locations and restaurants to ensure safe practices are being used. Consumers are cautioned against holding foods at the “danger zone” of bacterial growth: between 40 and 140 °F. Bacteria grow rapidly at these temperatures; therefore, foods should kept at temperatures below 40 °F (refrigerated or frozen) and meats especially should be cooked to at least 145 °F (it is recommended that poultry is cooked to 165 °F).

Food-borne illness is due to consuming contaminated foods; the contamination can be due to a microbial pathogen or toxin produced from a microbe. While there are numerous different food-borne illnesses, the first symptoms of many include nausea, vomiting, abdominal cramps, and diarrhea. Among the most commonly encountered microbial pathogens are Listeria monocytogenes, Staphyloccus aureus, Bacillus cereus, Bacillus anthracis, Clostridium botulinum, Clostridium perfringens, members of the Salmonella and Campylobacter genus, Escherichia coli (especially type O157:H7), Hepatitis and Rota viruses, Prions, different species of tape-worms and roundworms, and protozoa (including Toxoplasma and Sarcocystis).

Figure 5: The food production chain encompasses all the steps food travels from the farm to the consumer. Contamination can occur at any of these steps. Vigilance at each point is critical to ensure the safety of the food we eat.

Illnesses caused by these, and other, pathogens can be limited to a single person who ingests the contaminated food, or can lead to a more widespread outbreak of the same illness when multiple people consume the same contaminated food. Many outbreaks are local in nature, such as when food from a restaurant is not held at the proper temperature, allowing bacteria to grow to disease-causing levels. However, increasingly there have been food-borne illness outbreaks that affect people in many different geographical areas, due to improper food handling and/or preparation during processing. For example, from late July through October of 2011, Listeria monocytogenes contaminated cantaloupes grown on a farm in Colorado led to infections in 146 people (including 30 deaths) from 28 states. The Centers for Disease Control and Prevention (http://www.cdc.gov/outbreaknet/outbreaks.html ) lists 16 multistate foodborne illness outbreaks in 2011, with Salmonella species accounting for 11 outbreaks, Escherichia coli accounting for 4, and Listeria monocytogenes accounting for 1 (the aforementioned contaminated cantaloupes).

Introduction