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Biotech in a Bag

By Barbara Kolb

Type of Entry:

  • Hands-on

Target Audience:

  • Biology


Biotechnology is the application of principles of Biology for useful purposes. Currently , Biotechnology includes the use of DNA for such diverse applications as forensics, organism classification, and genetic engineering to change the genetic structure of an organism. Biotechnology also includes the production of foods, cosmetics, pharmaceuticals, diagnostics, and purified macromolecules for use in research and industry. Any useful application of Biology is Biotechnology.

The basis of biotechnology is often fermentation biochemistry, as yeasts, molds, or bacteria must be cultured in order to yield a high titer of the product desired. Ancient methods of food preservation included allowing the natural oxidation of sugars in order to preserve or produce a desired food for later use. Grapes rot and turn to compost, milk is fresh for only a few hours without proper refrigeration, and raw flour is useless to humans. Using the fermentation technology, large genetic engineering, pharmaceutical, agricultural, food and diagnostics companies are synthesizing their products in huge quantities, heretofore unavailable without tremendous costs involved.

Three experiments, which can now safely, conveniently, and inexpensively be performed in a self-locking baggie are submitted. In each of these, the use of Biotechnology is demonstrated. In the first, yeasts will anaerobically oxidize grape sugars to produce ethanol and carbon dioxide. The second experiment demonstrates denaturing protein in milk, and the third experiment uses yeasts again for fermentation.



Students carry out a series of experiments using self-locking plastic baggies. Each experiment demonstrates a phenomenon or principle of biotechnology.

ANAEROBIC RESPIRATION OF FRUCTOSE (Using yeasts to ferment grape sugar)

Purpose: The purpose of this experiment is to observe the results of cellular respiration of yeasts, using grape sugar as a substrate and yielding carbon dioxide and ethanol.

Procedure: Use two self-locking plastic baggies, one which has perforations (for keeping vegetables fresh), and the other a heavy-duty freezer bag. Size may vary, but quart sizes work well. Mass 5 grams of dried bakers yeast (Saccharomyces) and place inside the heavy-duty freezer bag. Tape a pH indicator strip toward the top of the perforated bag (outside or inside). Then insert the perforated baggie inside the heavy-duty bag. Mass 150 g of grapes, and, using a pH indicator strip, squeeze one grape to produce enough grape juice to test the pH of the grape. Record the pH. Into the perforated bag place all the grapes. Close the perforated bag. Carefully squeeze out any air in the bags. Close the heavy-duty baggie.

Crush the grapes through the baggie, using fingers to crush each grape, to release the grape juice. Mix the grape juice with the dry yeast, without opening the bag. Place the bags into a warm environment (25 degrees C), and observe for the next hour. Note the foam and gas filling the baggie. Allow some juice to touch the pH strip and record the pH. Record other observations. Open the bag and test the gas produced with a glowing splint or lighted match. The carbon dioxide gas produced should extinguish the splint. By carefully massing each ingredient, many quantitative experiments may be performed, such as measuring the volume of gas produced, the percent alcohol produced, or the sugar used. Using a refractometer or a densitometer (or substitute an antifreeze tester), the original juice can be tested for sugar content, and the resulting product tested for alcohol content. Any fruit can be substituted for the grapes. (Students should not taste any product). A variable could be the temperature, to determine the best temperature for yeast enzyme activity. Another modification might be to keep the experiment for a day, then open the bag and allow air to reach the juice. After a few days, note the vinegar smell (acetic acid) as wine is converted to acetic acid due to bacterial action. Test pH with another indicator strip.

Analysis: Yeasts use fruit sugars as a substrate for cellular respiration. In the absence of oxygen, glycolysis continues as fermentation, resulting in alcohol production. Sweet grapes (20% sugar in the juice) produce 11% alcohol, after which the yeasts die, due to the high level of alcohol and waste product buildup. Opening the bag to air borne Acetobacter bacteria to complete the conversion of alcohol to vinegar.


Purpose: The purpose of this experiment is to observe the process of changing a soluble protein (casein) into an insoluble partially denatured product, by the process of fermentation.

Procedure: Place a perforated baggie inside a heavy-duty baggie so that the perforated baggie fits snugly inside the other bag. Measure 250ml of fresh milk and pour into the perforated bag. Carefully place the bags containing milk into a microwave (do not close bags, stand upright inside a microwave safe container) and heat to 40 degrees C. Add 25 grams of blue (or roguefort) cheese. Place in a warm place overnight. Observe coagulation of milk. Pull perforated bag out, and allow to drip. Inside should be the denatured milk protein (curds), and in the heavy-duty baggie remains water, and undenatured protein (whey).

Modifications: An alternative method of cheesemaking is to heat the milk to scalding in the microwave, then add 25ml of vinegar, swirl, note the coagulation, hang the perforated bag to drip. This method produces ricotta cheese.

Analysis: Cheese may be made in one of two ways - by microorganism fermentation or by chemically changing the pH of the substrate. Since milk contains protein, unlike grapes, the resulting product is solid, indication a change in protein structure.


Purpose: This experiment is designed to observe the process of fermentation to capture the gas produced for the useful purpose of breadmaking.

Procedure: Lightly grease the inside of a self-locking microwave-safe baggie. Add 150 grams of high gluten flour (bread flour), 10g yeast, 10g sugar, 1g salt, and enough 40 degree C water to mix. Close the bag and mix by hand. Allow to rise, then knead through the bag once more and allow to rise again. Open the bag. Then cook the bread in the microwave, for approximately 2 minutes, turning once during baking.

Modifications: Bread may also be baked in a conventional oven, in the bag, if the bag is an oven-bag.

Analysis: Yeasts use sugar and flour as a substrate to produce alcohol and carbon dioxide in fermentation. In the process of cooking, alcohol is driven off, and the carbon dioxide is held within the wheat particles. High gluten flour (as in bread flour) prevents carbon dioxide from leaving the bread by forming a glutinous mass to capture the gas.

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