Introduction [top of page]

• limitations of time and space
  
• budget
  
• equipment
  
• instructor comfort level
  
• student experience or ability
  
• teacher and student interest
  
• local industries
  

Description [top of page]

Objectives [top of page]

Timeline [top of page]

• Unit expectations explained
• Journal requirements explained
• Students put into groups
• Topic selection and distribution of lab materials
• Proposal sheet

• Food safety guest speaker

  -county home extension agent

  -public health inspector

• Collect and discuss proposal sheets

• Lab setup

  -Each group will set up and begin their experiment

  -Remind the students of lab procedure and aseptic technique

• Anaerobic fermentation lecture or video
  

  -Video-The History of Cheese Making (Wisconsin Milk Marketing Board)
  

• Lab work and research
  

  -Use the library to find information for your project
  

• Fermentation of yeast lab/demo
  

  -Teacher choice of demo or lab experiment
  

• Research
  

• Research
  

• History of fermentation lecture/discussion
  

  -discussion of history
  

  -student discussion on how cheese making began
  

  -student discussion of expectations from their lab
  

• Field trip or speaker on careers in biotechnology
  

  -local hospital
  

  -local laboratory
  

  -local veterinarian
  

  -area tech school with a bioscience technician program
  

  -area college with a biotechnology or food science degree
  

  -local fermentation industry (cheese plant, kraut plant, bakery)
  

• Food safety case studies

  -Please consult "Safe handling Beyond the Retail and Wholesale Shelf" Unit 11 pages 1-15 from Food Science, Safety and Nutrition by the National Council for Agricultural Education
  
  

  -Case studies are included in this reference which can be read and discussed in class or written about in student journals. These case studies discuss the following topics:
  
  

  ·Staph outbreak at a convention
  

  ·Botulism in chili peppers killing twelve
  

  ·Salmonella poisoning from restaurant
  

  ·Clostridium poisoning meals on wheels
  

  ·Listeriosis from coleslaw in Canada
  

  
  

  -Class discussion or lecture on consumer responsibilities such as food purchasing safety, storage, preparation and serving, proper cooking temperatures, leftovers safety and how fermentation relates to these safety issues.
  

• Work on presentations

• Presentations
  

• Presentations
  

• Evaluation/closing

  -Students will be asked to write a response to several questions. The questions will ask the students to synthesize the information from their lab with other groups and teacher presented information. The students will also have to evaluate their lab project and propose future research.

Teacher Background [top of page]

The use of natural biological processes to obtain useable products is certainly not new. Since recorded history, microbes have been involved in the preparation and processing of items in the daily diet of humans.

Lacking any knowledge of microorganisms, or of ways in which contamination of food by them could be avoided, man learned to live with microbially infected foods. Usually the actions of these microbes ultimately made the food unacceptable, either by altering the appearance or odor of the food to a point which it was no longer appetizing or by producing poisonous toxins, some of which were lethal. Occasionally, however, microbial infections of food materials made it appear more appetizing and the taste enhanced. Ultimately, microbial infections of these foods were exploited, so the fermented foods and beverages now form a large and important sector of the food industry. Today the main groups of microbes involved in the industry include the yeasts, molds, and bacteria.

Nobody knows exactly when cheese making began, but legend generally has it that its origin lies in the Middle East. A Bedouin, preparing for a journey across the desert, filled his skin pouch with ewe's milk for refreshment along the way. After hours in the hot sun, and weary from the jostling ride on the camel, the Bedouin opened the pouch made from the dried stomach of a sheep only to discover that the rich milk was no more. In its place lay a thin watery fluid surrounded by a thick white mass - whey and curds. Having nothing else to drink, he tried the liquid and found it tasted good; then he nibbled at the gummy curds and was equally pleased with the discovery. Arriving at his destination, he shared the remaining curds with his tribesmen, who were no less satisfied then he. Thus, quite by accident, cheese was introduced into man's diet. Today, the manufacture of cultured dairy products represents the second leading fermentation industry (next to alcoholic beverages), accounting for approximately 20% of all fermented foods produced world wide.

While no such legend exists for the discovery of sauerkraut, it undoubtedly was also discovered by accident and trial and error methods.

In the days before refrigeration facilities became available, a number of techniques were devised for preserving seasonally produced vegetables. One of the most efficient of these involved packing vegetables tightly in a vessel with salt or brine. This technique is thought to have originated in the Orient where, even today, it continues to be used extensively. Only in the last 60 years has it been shown that this method of preserving vegetables involves a microbiological fermentation.

In cheese making, the mystery surrounding the nomad's discovery can be easily explained. The four essentials of cheese making were acting together that memorable day in the desert: milk plus a slight churning motion coupled with heat and rennet (the product of an enzyme produced in the membrane lining of ruminate animals stomach). The cheese discovered by the Bedouin was probably what we would call cottage cheese or cheese curds. Similar legends attend the origin of aging, curing or ripening which has lead to the many various cheese flavors we have today.

We now know that in the production of sauerkraut, lactic acid bacteria proliferate in the brine. These bacteria produce acids which lower the pH. The combined action of the salt and acid lowers the activity of enzymes responsible for the breakdown of vegetable tissue. At the same time oxidative changes in the tissues are inhibited and thus prevent spoilage.

Although respiration and breathing are often thought of as the same, they are in fact two different processes. Breathing is the exchange of gases between an organism and its external environment. Respiration occurs within all living cells. Cellular respiration involves breaking the chemical bonds of organic molecules and releasing energy that can be used by the cells.

Most students are not familiar with fermentation which occurs in some of the less complex organisms such as bacteria and yeasts. Fermentation reactions are anaerobic, proceeding without oxygen being present. Anaerobic reactions involve cellular food products and/or glucose sugar as their reactants. And without oxygen they can produce combinations of ethyl alcohol (C₂H₅OH), carbon dioxide (CO₂), and lactic acid (C₂H₄OCOOH) as their products.

We have used the products of anaerobic respiration (fermentation) to our advantage, supplying ourselves with food, drink, and even fuel for automobiles. Yeasts are used as tiny "fermentation factories" producing carbon dioxide and alcohol. Certain bacteria and molds ferment milk, producing carbon dioxide and lactic acid.

It has been stated that the fermentations are the result of growth of bacteria, yeasts, molds, or combinations of these. Stated more precisely, the changes that occur are caused by the enzymes liberated by these microorganisms. Some foods usually said to be fermented are actually cured by the enzymes naturally inherent in the foods. Throughout the centuries fermentation has been one of the most important methods for preserving food; It still remains one of the most important methods. Relatively few people, however, are aware that the many food products consumed regularly are prepared and/or preserved by fermentation processes.

It is essential to understand that the lactic acid bacteria produce acid which in effect inhibits the growth of many other organisms. Most species convert sugars to acids, alcohol, and carbon dioxide. The fermentative yeasts produce ethyl alcohol and carbon dioxide from sugars. They require oxygen for growth but not for fermentation. The molds have the greatest array of enzymes, are aerobic, and will grow on most foods to produce various types of digestion.

The changes that occur during fermentation of foods are the result of the activity of enzymes. The enzymes arise from three sources: Those that are produced by the microorganisms that are involved in the fermentation, those that are native to the food, and those that are produced by the microbial flora that happen upon the unfermented food. A good fermentation is one in which the enzymes produced by the fermentative microorganisms play the primary role.

There are relatively few pure culture fermentations. An organism that initiates a fermentation will develop until its byproducts of growth inhibit further growth and fermentation. During this initial growth period other organisms develop. They in turn are followed by other more tolerant species. This succession of growth of different species may be referred to as a natural sequence of growth. The use of starters or inocula should be based upon these facts. In general, growth will be initiated by bacteria, followed by yeasts and then molds, if conditions are suitable for growth of these microorganisms.

Now let us try to relate these biological processes to biotechnology. What is biotechnology? "In the broadest and simplest terms, biotechnology is defined as the collection of industrial processes that involve the use of biological systems."(Harlander, 1991).

We have been using bacteria, yeasts, and molds for centuries to produce a host of fermented foods including buttermilk, yogurt, sour cream, butter, cheese (over 700 kinds), pickles, sauerkraut, sausage, breads, crackers, pretzels, doughnuts, grape nuts (you thought it was a cereal brand name?), wines, beer, spirits, soy sauce, coffee, cacao, vanilla, tea, citron, ginger, and more.

Biotechnology is also used in some food processing related areas including processing aids, ingredients, rapid detection systems, and biosensors. Enzymes acting as protein catalysts, are used extensively in the food processing industry to control texture, appearance, and nutritive value, and for the generation of desirable flavors and aromas. Because they are isolated from plants, animals, or microorganisms, their availability is dependent upon the availability of the source material. Using genetically engineered microorganisms for the production of enzymes eliminates the need to rely on source materials while ensuring a continuous supply of enzymes.(Harlander, 1991).

The new technologies have allowed researchers to target the genetics of plants, animals, and microorganisms and to manipulate them to our food production advantage. What might be in store for tomorrow's food advancement? Predictions include:

1. Environmentally hardy food-producing plants that are naturally resistant to pests and diseases and capable of growing under extreme conditions of temperature, moisture, and salinity.
   
   
2. An array of fresh fruits and vegetables, with excellent flavor, appealing texture, and optimum nutritional content, that stay fresh for several weeks.
   
   
3. Custom designed plants with defined structural and functional properties for specific food-processing applications.
   
   
4. Cultures of microorganisms that are programmed to express or shut off certain genes at specific times during fermentation in response to environmental triggers.
   
   
5. Strains engineered to serve as delivery systems for digestive enzymes for individuals with reduced digestive capacity.
   
   
6. Cultures capable of implanting and surviving in the human gastrointestinal tract for delivery of antigens to stimulate the immune response or protect the gut from invasion by pathogenic organisms.
   
   
7. Microbially derived, high-value, "natural" food ingredients with unique functional properties.
   
   
8. Microsensors that accurately measure the physiological state of plants; temperature-abuse indicators for refrigerated foods; and shelf-life monitors built into food packages.
   
   
9. On-line sensors that monitor fermentation processes or determine the concentration of nutrients throughout processing.
   
   
10. Biotechnologically designed foods to supply nutritional needs; meat with reduced saturated fat, eggs with decreased levels of cholesterol, and milk with improved calcium bioavailability.(Harlander, 1991)
    

Sausage:

1. Semi-dry fermented sausages
   • Summer sausage: Pediococcus cerevesiae and Lactobacillus plantarum

2. Dry fermented sausages
   • Pepperoni: Pediococcus cerevesiae and Lactobacillus plantarum
   • Genoa salami: Micrococcus spp. mixed with either Pediococcus cerevesiae or Lactobacillus plantarum

Lactic acid is produced, lowering the pH of the sausage to preserve and flavor.

Enzymes:

1. Chymosin

   Replaces rennet in over 80% of the cheese produced. Rennet is extracted from the stomachs of milk-fed veal calves. The supply is linked to the veal market, causing wide shifts in availability of the rennet.

   Recombinant Chymosin is produced by an E. coli. The gene for chymosin was transferred from a calf to E. coli.

2. Amylase

   Amylase is the enzyme that will break starch into its separate glucose components. Amylase is used in the brewing industry for malting and used in baking.

3. Glucose isomerase

   The enzyme glucose isomerase converts or isomerizes glucose (an aldehyde) to fructose (a ketone) which is a sweeter product. The fructose is 1.8 times sweeter than glucose, so less is needed for the same taste.

4. Pectinase

   Pectinase is the enzyme that breaks down pectin, a polysaccharide found in fruit. Pectinase is used to remove particulate matter, or clarify, fruit juices.

5. Glucose oxidase

   Glucose oxidase is an enzyme used in the production of dried egg whites. When drying egg whites the glucose present in the white can react with amines in a reaction known as Maillard browning. This will cause the dried egg whites to turn brown. The addition of glucose oxidase will bread down the glucose and prevent the glucose from reacting and causing the off color of the egg whites.

To produce these recombinant enzymes, the gene is first transferred into bacteria. The bacteria are then grown in fermenters. The enzyme is purified and sold.

Other additives:

1. Commercial Gums: (Dextran, Gellan, Rhansan, and Welan)

   thickeners and stabilizers

2. Xanthan gum

   Xanthan gum is a polysaccharide produced by the bacterium Xanthomonas campestris on the cell wall. Xanthomonas campestris occurs naturally on the leaves of plants in the cabbage family. Commercial xanthan gum is produced by aerobic submerged fermentation. The bacteria are mixed with sugar, a nitrogen source, trace elements, and other growth factors in a large stainless steel tank. During fermentation aeration, agitation, pH and temperature are precisely controlled. After fermentation the solution is pasteurized to kill all bacteria. The gum is used in many products including cakes, muffins, ice cream, sherbet, sour cream, salad dressings, sauces, gravies, syrups, and toppings. Taken from "Xanthan Gum, 5th Edition," by Kelco, 500 W. Madison, Suite 3180, Chicago, IL 60661, phone 1-800-535-2687.

Diagram of respiration and fermentation

CELLULAR	ALCOHOLIC	LACTIC ACID
RESPIRATION	FERMENTATION	FERMENTATION
GLUCOSE	GLUCOSE	GLUCOSE
|	|	|
|	|	|
V	V	V
PYRUVIC ACID	PYRUVIC ACID	PYRUVIC ACID
|	|	|
|	|	|
V	V	V
CO2	CO2	LACTIC ACID
+	+	+
WATER	ALCOHOL	2 ATP
+	+
38 ATP	2 ATP

Laboratory Exercises [top of page]

Root Beer: [full lab procedure]

• demonstrate the action of yeast on a mixture of sugar, water and flavorings through aerobic and anaerobic respiration.

• materials:

  bottles--washed and sterilized

  wine corks or caps and crowns

  stirring spoon

  large (20 liter or 5 gallon) enamel kettle or pot--DO NOT USE ALUMINUM!!

  *59 mL Schillings Root Beer Concentrate

  *2.27 kg sucrose (table sugar)

  *19 l Chlorine-free water

  *Containers to measure out needed volumes of materials--your choice--see procedures

  *9.5 g yeast dissolved in 236 mL warm water

  * use proportions to suit needs

• Possible variations:

  flavorings other than Shillings Root Beer Extract

  amount of sucrose

  amount of root beer extract

  quick rise yeast vs. normal

  amount of yeast

  different sources of sugar (ie. dextrose)

• (Nancy Heitel, et. al., 1988)

Making Cheese: [full lab procedure]

• prepare cheese from milk and buttermilk

• materials:

  500 mL whole milk

  50 mL buttermilk

  hot plate

  thermometer

  fine-mesh cheesecloth

  cotton twine

  labels

  50 mL, 500 mL and 600 mL measuring devises

  50 mL, 500 mL and 600 mL containers

• variations:

  milk--goat, sheep, 2%, 1%, skim, cream enriched

  rennin/rennet addition at day 1, step 1

  specific bacteria inoculation

  mold inoculation at day 1, step 1

  addition of flavorings, salt, colorings

Kimchee: [full lab procedure]

• using 2 liter bottles to study lactic acid fermentation

• note to teacher:

  Cabbage should be kept covered with its own juices at all times. Gas bubbles will escape each day as the lid is pressed down onto the cabbage. This gas is produced as bacteria grow on the sugary contents of the Chinese cabbage juice in the salty solution. As pickling proceeds, there will be an increase in and change in acidity. Windows to air out the classroom would be nice to have when doing this lab!! Let students know that anaerobic lactobacilli are found almost everywhere in our environment.

• materials:

  Chinese cabbage (cut into 5-7 cm chunks)

  1 red hot chili pepper, chopped

  2 cloves garlic, thinly sliced

  3 tsp. non-iodized (pickling) salt

  one 2 liter soda bottle

  large plastic lid (petri plate lid)

  pH indicator paper

  small plastic pipette

• variations:

  different cabbage types

  change seasonings or ingredients

  place bottles in various temperatures

• reference:

  1990 Fast Plants and Bottle Biology Projects, Department of Plant Pathology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706

Sauerkraut: [full lab procedure]

• lactic acid fermentation of cabbage

• note to teachers:

  Containers used in making kraut should be cleaned and rinsed well. Crocks should have shiny glaze to the surface, and not be cracked or chipped. Metal containers re definitely not to be used. If plastic is used, only use food- grade plastic. They dyes in nonfood plastic are not intended for consumption and should not be used for food.

• materials:

  cabbage sliced into thin strips

  non-iodized salt

  container (Consider the fact that cabbage will require anaerobic conditions while fermenting in this container. If a fermenting crock is you container of choice, be careful that it is not chipped or cracked. Food grade sturdy plastic pails are excellent containers. Do not use metal containers of any type.)

• variations:

  different types of cabbage can be used

  cabbage can be cut into different sizes to see how size varies the resulting product

  spices or seasonings can be added for a variety of kraut flavors

  sauerkraut can be canned for long-term storage

• references:

  Mennes, Mary E., "Make Your Own Sauerkraut", Food Science, University of Wisconsin-Madison, Food Management Specialist, UW-Extension

Fermenting Power of Bread Yeasts: [full lab procedure]

• observing the raising quality of different types of bread yeast

• materials:

  50 or 100 mL graduated cylinder (to measure 30 mL distilled water)

  100 mL graduated cylinder, greased with vaseline

  flour

  two brands, A and B, of active dry yeast (either A or B should be a yeast cake)

  Saccharomyces cerevesiae, young streak culture (solid medium)

  square sheets of brown wrapping paper

  buffered methylene blue stain:

  mix 1 part of 1:5000 methylene blue and 1 part of a phosphate buffer solution (99.25 mL of 0.2 M KH₂PO₄ to 0.25 mL of 0.2 M Na₂HPO₄) to give pH or 4.6

  microscope

  microscope slides and coverslips

• variations:

  different brands of yeast

  different forms of yeast

  rapid rise vs. normal

  different temperatures

• references and resources:

  Boyd, Robert F. 1988. General Microbiology 2nd ed. Times Mirror/Mosby College

  Frazier, W.C., and D.C. Westhoff. 1978. Food Microbiology, 3rd ed. McGraw-Hill, Inc., New York

  Grula, Dr. Mary M., Oklahoma State University, Department of Botany/Microbiology, Stillwater, OK 74078- 0289

Yogurt: [full lab procedure]

• simple lactic acid fermentation

• materials:

  food grade containers, washed

  food grade thermometer

  hotplate

  beaker tongs

  2 cups milk

  1/3 cup nonfat dry milk

  2 tablespoons plain yogurt (Old Home Black Label works well)

  pH paper or meter

  microscope slides

  Bunsen burner

  inoculating loop (a toothpick will work)

  crystal violet stain

  microscope (oil immersion if available)

• possible variations:

  type of milk (whole, 2%, skim, chocolate)

  different temperatures for incubation

  different brands of yogurt for starter culture

  different levels of starter

  different levels of non-fat dry milk

• adapted from:

  "Bacteria in Yogurt" pages 44-47 of Laboratory Experiments in Biotechnology and Related Areas Volume III: Experiments with Microorganisms, Dept. of Biological Sciences, Mankato State University, Mankato, MN, 1988

  "Bacteria in Yogurt" pages 5-6 of The BioNet Booklet, Volume 3, Spring 1995, BioNet c/o Rod Johnson, Eau Claire North High School, 2700 Mercury Ave, Eau Claire, WI 54703

  "Homemade Yogurt, Sour Cream and Buttermilk" by M. Wagner, R. Bradley, and M. Mennes, University of Wisconsin Extension publication B2768, available from Agricultural Bulletin, Rm 245, 30 N. Murray St., Madison, WI, 53715, 608-262-3346

Yeast Fermentation, Variation in CO₂: [full lab procedure]

• glucose consumption and CO₂ production by yeast

• materials:

  15 mL plastic centrifuge tubes with caps

  7% yeast solution (4 packages yeast and 400 mL water)

  5% glucose solution (20 grams glucose in 400 mL water)

  TES-TAPE (available at Wal-Mart and pharmacies)

  large beakers (250 to 500 mL)

  water bath at 40 degrees C

  permanent fine point lab markers

• possible variations:

  regular vs. Rapid-Rise yeast

  levels of sugar

  type of sugar

  other temperatures (ice water, boiling water)

• adapted from:

  "Fermentation, Respiration, and Enzyme Specificity: A Simple Device and Key Experiments with Yeast," by L. Reinking, J. Reinking, and K. Miller, The American Biology Teacher, Vol. 56, March 1994, pp. 164-168.

Alternative labs:
· "Dinner Date with a Microbe" by A. Gillen and R. Williams, The American Biology Teacher, May 1993, pages 268-274. Provides kitchen microbiology recipes and health information for yogurt, sauerkraut, and root beer.
· Silos and Sauerkraut from The AgriScience Institute and Outreach Program published by the National Association of Biology Teachers, 1994, pages 5-1 to 5-28. Bottle biology labs for making mini silos and fermentation chambers.

Evaluation Guidelines [top of page]

• The daily journal writings will be evaluated on the answer to the daily focus question, and a detailed description of all lab procedures and observations. The journal must also include self evaluations, group evaluations, definitions to vocabulary words, and summaries of at least three recent articles related to their topic.
  
  
• The oral presentations will be evaluated on the delivery and content. The content must include procedures, results, and a description of two careers related to the topic area.
  
  
• The written presentation must include a more detailed description of the information from the oral presentation plus a section describing the importance of fermentation to the economy of the area.
  
  

Click below for copies of each form

• Unit Evaluation Sheet
• Unit Guidelines
• Lab Proposal Sheet
• Journal Information Sheet
• Journal Page
• Glossary Page
• Sample Focus Questions
• Sample Vocabulary

National Standards [top of page]

• Benchmarks for Science Literacy

  3A Technology and Science:

  Technology usually affects society more directly than science because it solves practical problems and serves human needs...

  
  

  10I Discovering Germs:

  Pasteur wanted to find out what causes milk and wine to spoil

  
  

  12D Communication Skills:

  Locate information in reference books, back issues of newspapers and magazines, compact disks, and computer disks.

  Use tables, charts and graphs in making arguments and claims in oral and written presentations

  
  
• National Science Education Standards

  Content
  

  A--Science as Inquiry:

  Abilities of scientific inquiry

  
  

  B--Physical Science:

  Chemical reactions

  
  

  C--Life science:

  Cell biology

  
  

  F--Science in Personal and Social Perspectives:

  Science and technology in local, national and global challenges

  Personal and community health

  
  

  G--History and Nature of Science:

  Historical perspectives

  
  

  Assessment Standards
  

  A--Coordination with Intended Purposes:

  Deliberately designed

  Explicitly stated purposes

  
  

  B--Measuring Student Achievement and Opportunity to Learn:

  Achievement data collected focuses on the science content that is most important for students to learn

  
  

  C--Matching Technical Quality of Data with Consequences:

  The feature that is claimed to be measured is actually measured

  Assessment tasks are authentic

  
  
• Program Standards
  

  B--Curriculum:

  Inquiry is emphasized as a tool for learning science

  The curriculum connects to other school subjects

  
  

• National Resource Council Recommendations for Agricultural Education

  Ongoing efforts should be expanded and accelerated to upgrade the scientific and technical content of vocational education courses

  New curriculum components must be developed and made available to teachers addressing the sciences basic to agriculture, food and natural resources

  
  
• School-to-Work Initiative
  

  School Based Learning

  Career exploration to help students in the career identification process

  High academic content enabling admissions, if desired, to post-secondary education

  Applied and integrated technical and academic curriculum

  
  

References [top of page]

Biology, Visualizing Life--Teachers Resource Binder. Holt, Rhinehart and Winston, 1994.

Biology, Visualizing Life. Holt, Rhinehart and Winston, 1994.

Brock, Thomas D. and Michael T. Madigan (1988) Biology of Microorganisms, 5th Ed. Prentice Halls, Englewood Cliffs, New Jersey.

Coghlan, Andy. "An Explosive Start To Fast Maturing Cheese." New Scientist, March 16, 1991.

"Fermentation;" Modern Biology Laboratories, Holt, Rhinehart and Winston, Investigation 7.2, p. 45-48.

Frazier, William C., and Dennis C. Westhoff (1988) Food Microbiology 4th Ed., McGraw-Hill, New York

Harlander, Susan K. "Biotechnology - A Means For Improving Our Food Supply." Food Technology, April, 1991.

Harlander, Susan K. "Food Technology: Yesterday, Today and Tomorrow." Food Technology, September, 1989.

Harley, J. and L. Prescott, Laboratory Exercises in Microbiology, William C. Brown Publishers, 1993.

Heitel, Nancy, et al. "Production of Home Brewed Root Beer." Minnesota; Mankato State University, 1988.

Lindquist, John (1990) Bacteriology 102 Lab Manual. University of Wisconsin-Madison.

May, Ranee (1995) Special Topics in Food Science. University of Wisconsin-River Falls.

Muriel Mandell, Simple Kitchen Experiments. Sterling Publishing Company, 1993.

Oklahoma State University, Dr. Mary Grula, Fermenting Power of Bread Yeasts.

Pearl, Anita May, Cuttle, Constance and Deskins, Barbara B. Completely Cheese, Jonathan David Publishers, Inc. Middle Villiage, New York, 1978.

Pederson, Carl S. Microbiology of Food Fermentations. Westport, Connecticut; AVI Publishing Co, Inc., 1987.

Potter, Norman N., and Joseph H. Hotchkiss (1995) Food Science, 5th Ed. Chapman and Hall, New York.

Robinson, R.K. Ed. (1995) A Colour Guide to Cheese and Fermented Milks. Chapman and Hall, London.

Rose, A.H., Fermented Foods, Academic Press, New York, New York, 1982.

Towle, Albert, Modern Biology Holt, Rhinehart and Winston, New York, New York, 1989.

Varnam, Alan H. and Jane P. Sutherland (1994) Milk and Milk Products. Chapman and Hall London.

Wood, Brian J.B., Microbiology of Fermented Foods Vol. 1, Elsevier Applied Science Publishers, London and New York, 1985.

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