0610 · 21.2Biotechnology

Yeast respiration in bread and biofuel

0:00A loaf of bread, a glass of fruit juice and lactose-free milk seem like very different products. Yet each can be made using a living organism or an enzyme that carries out a useful chemical reaction. Biotechnology puts these biological processes to work for us. By the end of this video, you will understand how these products are made, why enzyme washing powders work, and how microorganisms produce useful substances inside industrial fermenters.

0:24There are two connected ideas to keep in mind. Sometimes we use whole living cells, such as yeast cells, and collect something they produce. Sometimes we use an enzyme without needing the whole organism that made it. In both cases, success depends on choosing the right biological process and providing conditions in which it works effectively. We only need a brief reminder about enzymes and respiration. Enzymes are proteins that function as biological catalysts, speeding up reactions without being used up.

0:50Each enzyme acts on particular substances called substrates, and its activity depends on temperature and acidity. Respiration involves chemical reactions in cells that break down nutrient molecules and release energy, so living cells need suitable nutrients as well as suitable conditions. Let us begin with yeast, a single-celled fungus. When yeast respires anaerobically, it releases energy from glucose without using oxygen. The products are ethanol and carbon dioxide, so the word equation is glucose produces ethanol and carbon dioxide.

1:20Yeast uses the released energy for its own activities, while we can make use of either of those products. For ethanol production, yeast is supplied with a sugar-containing material and kept under conditions without oxygen. Its anaerobic respiration converts the sugar into ethanol and carbon dioxide. The ethanol is then separated from the mixture and can be used as a biofuel, a fuel made from biological material. The important distinction is that yeast produces the ethanol, while burning that ethanol later releases energy for our use.

1:50In bread-making, we want the other product of the same reaction. Yeast is mixed into dough containing flour, water and available sugars, then the dough is left in a suitably warm place. As the yeast respires anaerobically, carbon dioxide bubbles become trapped in the dough. These bubbles expand the dough, causing it to rise and giving the finished bread its lighter texture. Here is a worked example. Bread dough contains yeast and sugar, but it rises very slowly in a cold room.

2:15First, identify the useful product, which is carbon dioxide rather than ethanol. Next, connect the low temperature to slower enzyme-controlled reactions in the yeast, so carbon dioxide is produced more slowly. A suitably warm place can speed up rising, but excessive heat can denature enzymes and kill the yeast. When the dough is baked, heating helps the trapped gases expand before the bread structure sets. The high temperature kills the yeast, and the ethanol produced during anaerobic respiration evaporates during baking.

2:42This is why ordinary bread is not an alcoholic product. For examinations, the essential chain is anaerobic respiration by yeast, production of carbon dioxide, and carbon dioxide causing the dough to rise.

Enzymes in juice and milk

2:54Now let us move from whole yeast cells to extracted enzymes. Fruit tissue contains pectin, which helps hold neighbouring plant cells together. Pectinase breaks down this pectin, making crushed fruit tissue break apart more easily. As a result, more juice can be extracted, and breaking down pectin also helps produce clearer juice. The enzyme is not creating juice from nothing. The juice is already present in the fruit, but some remains trapped within its tissues.

3:17Breaking down pectin makes that liquid easier to release and separate from the remaining solid material. This is a useful example of biotechnology improving the amount of product obtained from the same starting material. Lactose-free milk uses a different enzyme called lactase. Lactose is the sugar naturally present in milk, and some people do not produce enough lactase to digest it effectively. During production, lactase breaks down lactose into the simpler sugars glucose and galactose.

3:43The milk therefore contains the products of lactose breakdown rather than the original lactose. Notice the difference between breaking a substance down and physically removing it. Lactase does not filter lactose out of milk, it catalyses its chemical breakdown. The enzyme can be added to milk, or milk can pass over lactase held in place on a support. Either way, the essential explanation is the same. Lactase converts lactose into glucose and galactose, producing milk suitable for people with lactose intolerance.

Biological washing powders

4:12Biological washing powders contain enzymes that help break down substances in stains. Proteases break down proteins, so they are useful for protein-containing stains such as egg. Lipases break down fats, while amylases break down starch. The resulting smaller molecules are easier to remove during washing, so the enzyme chosen must match the material in the stain. The word biological does not mean that the powder contains living microorganisms washing your clothes.

4:38The useful ingredients are enzymes, usually produced by microorganisms before being added to the detergent. These enzymes can work effectively at relatively low washing temperatures, depending on their formulation. However, sufficiently high temperatures can denature them, changing their active sites so that their substrates no longer fit properly. How could we investigate whether an enzyme improves stain removal? Use equal-sized pieces of the same fabric with equal amounts of the same protein-containing stain.

5:05Place them in equal volumes of detergent solution, with protease in one solution and no enzyme in the other. Ideally, use the same detergent formulation apart from the enzyme, because two unrelated commercial powders could differ in several ingredients. Keep temperature, detergent concentration, washing time and agitation the same, so enzyme presence is the only intended difference. After washing, compare the remaining stain using the same assessment method, such as photographs taken under identical lighting and judged against a fixed scale.

5:33Repeat the comparison and calculate a mean stain score to reduce the effect of variation. Wear eye protection, avoid inhaling detergent powder, and follow the laboratory safety instructions. To investigate temperature instead, use the same biological detergent at several controlled temperatures. Keep the other variables unchanged, and include an enzyme-free comparison at each temperature if possible. This matters because temperature can affect stain removal even without enzymes. If you observe better cleaning at one temperature, your conclusion should follow the evidence rather than assuming that hotter water must always make enzymes work better.

Fermenters and controlled conditions

6:07The same biological principles can be used on a much larger scale inside a fermenter. This is a vessel in which bacteria or fungi are grown under controlled conditions to produce useful products. A nutrient-containing liquid provides materials for growth, and the microorganisms carry out the reactions we want. The vessel allows manufacturers to control conditions throughout a large culture, rather than simply leaving microorganisms in a container. Different microorganisms produce different useful products.

6:33Genetically modified bacteria can produce human insulin, which is collected and purified for medical use. The genetic modification process is covered in the next video. The fungus Penicillium produces the antibiotic penicillin. Another fungus, Fusarium, is grown to produce mycoprotein, a protein-rich food made from fungal material. These examples also show that the useful product is not always the same kind of thing. Insulin and penicillin are substances produced by microorganisms and then separated from the culture.

7:03For mycoprotein, the fungal biomass itself is harvested and processed into food. Biomass means the living material produced as the microorganisms grow, rather than just a chemical released into their surroundings. Despite its name, a fermenter does not automatically mean that conditions are anaerobic. Industrial fermenters are also used for processes in which microorganisms require oxygen for aerobic respiration. Oxygen supplies must therefore match the needs of the organism and the production process.

7:31Do not transfer the oxygen-free conditions used for ethanol production to every other example of biotechnology. Temperature is controlled because microbial growth and product formation depend on enzyme activity. If the culture becomes too cold, enzyme-controlled reactions slow down and production may decrease. If it becomes too hot, enzymes can denature and microorganisms may die. Respiration releases heat, so a cooling jacket removes excess heat and helps maintain a suitable temperature, monitored by a temperature probe.

7:58The level of acidity or alkalinity must also be controlled because enzymes work best within a suitable range. As microorganisms use nutrients and produce substances, they can change the acidity of their surroundings. A sensor detects these changes, and acid or alkali can be added to restore the required conditions. The aim is not always neutrality, but the optimum conditions for that microorganism and process. For an aerobic culture, a supply of oxygen allows aerobic respiration to release energy for growth and production.

8:29Air can be introduced near the bottom of the fermenter, and a stirrer helps distribute oxygen and nutrients through the liquid. Without mixing, some regions might have plentiful oxygen while others have too little. Mixing also helps distribute heat, making conditions more even throughout the culture. The nutrient supply must provide substances needed for energy release and growth. These include an energy source such as glucose, along with suitable sources of nitrogen and mineral ions.

8:53If an essential nutrient runs out, growth or production can become limited even when temperature and oxygen are suitable. Nutrient delivery is therefore controlled to meet the needs of the culture. Waste products must also be managed because their accumulation can inhibit growth or alter the culture conditions. Carbon dioxide from aerobic respiration can leave through a gas outlet, while other unwanted products may need to be removed through controlled withdrawal of culture liquid.

9:17Harmful build-up would reduce production rather than help it. The required method depends on the microorganism and the particular manufacturing process. Finally, the equipment and nutrient medium are sterilised before use, and incoming air is filtered. This reduces contamination by unwanted microorganisms that could compete for nutrients or spoil the product. Temperature, acidity, oxygen, nutrients and waste management all work together rather than independently. A plentiful food supply cannot compensate for enzymes damaged by excessive heat or an aerobic culture starved of oxygen.

Fermenter exam answers and recap

9:49Consider an exam-style question that asks you to explain why a fermenter used to grow an aerobic fungus needs cooling and a supply of oxygen. Begin by linking cooling to heat production and enzyme function, then link oxygen to respiration. A precise answer would say that respiration by the microorganisms releases heat. Cooling maintains a suitable temperature and prevents enzyme denaturation. Oxygen is needed for aerobic respiration, which releases energy for growth and production.

10:15Watch for three important mix-ups across this topic. Carbon dioxide makes bread dough rise, whereas ethanol is the useful product for biofuel production. Pectinase breaks down pectin, lactase breaks down lactose, and protease breaks down proteins, because enzymes act on particular substrates. Finally, a fermenter is a controlled growing vessel, not a guarantee that oxygen is absent. Let us bring everything together. Biotechnology uses living organisms or enzymes to carry out useful processes, whether we want ethanol, rising dough, clearer juice, cleaner fabric or lactose-free milk.

10:50Large-scale production uses fermenters to provide suitable conditions for bacteria and fungi producing substances such as insulin and penicillin, or biomass such as mycoprotein. Once you identify the biological reaction, the useful product and the conditions needed, these applications become connected ideas rather than separate facts to memorise.