Yeast fermentation for bioethanol

When oxygen is absent, yeast breaks down sugar by anaerobic respiration, releasing energy and producing ethanol and carbon dioxide. This process is called fermentation.

In bioethanol production, the useful product is ethanol, which can be used as a biofuel. Carbon dioxide is also produced during fermentation, but it is not the product sought for fuel.

Yeast fermentation in bread-making

Yeast in bread dough breaks down sugar without oxygen. This is anaerobic respiration, also called fermentation. It produces carbon dioxide and ethanol.

The carbon dioxide forms bubbles that become trapped in the dough. As gas collects, the bubbles expand, making the dough expand and rise.

Ethanol is also produced, but it is the trapped carbon dioxide—not the ethanol—that makes the dough rise.

Pectinase in fruit juice production

Fruit contains pectin, a substance that helps hold its cells together. Pectin can make it harder to extract juice and can leave the juice cloudy.

Pectinase is an enzyme that breaks down pectin. When pectinase is added to fruit, it breaks down the pectin holding the cells together, helping to release more juice. Breaking down pectin also makes the extracted juice clearer.

Using pectinase therefore increases the juice yield: the amount of juice obtained from the fruit.

Enzymes in biological washing powders

Biological washing powders contain enzymes that help remove stains. An enzyme breaks a substance in a stain into smaller molecules, which are generally more soluble and can be washed away more easily.

Different enzymes act on different stain substances:

• Proteases break down proteins into smaller molecules, including amino acids.
• Lipases break down fats into fatty acids and glycerol.
• Amylases break down starch into smaller sugars.

A powder containing a mixture of enzymes can therefore help remove stains made from different substances.

Investigating the effect of enzymes

The contribution of enzymes can be investigated by comparing a powder containing enzymes with an otherwise equivalent powder without enzymes. Use pieces of the same fabric with equivalent stains: the stain substance, amount and area should be as similar as possible.

Wash one stained piece with each powder, keeping conditions such as the amount of powder, water volume, temperature, washing time and washing method the same. Compare how much stain remains after washing. Repeating the comparison helps show whether the result is consistent.

If the enzyme-containing powder consistently removes more of the stain, the difference provides evidence that its enzymes helped clean it. Keeping the other conditions the same makes it less likely that the difference was caused by something other than the enzymes.

Lactase in lactose-free milk production

Milk contains a sugar called lactose. The enzyme lactase breaks lactose down into two simpler sugars: glucose and galactose.

When milk is treated with lactase, the enzyme breaks down its lactose before the milk is consumed. This reduces the lactose content and produces lactose-free milk. The glucose and galactose remain in the milk.

Some people do not produce enough lactase to digest lactose. Lactose-free milk is suitable for them because the lactose has already been broken down.

Large-scale production in fermenters

A fermenter is a vessel in which microorganisms, such as bacteria or fungi, are grown in large quantities to produce useful materials. The microorganisms receive nutrients, and conditions such as temperature, pH and oxygen supply are controlled to support their growth and production.

The useful product may be a substance made by the microorganisms and collected from the culture, or it may be the microorganisms themselves. The product is then collected and, where necessary, purified.

Products made in fermenters

Human insulin can be made by genetically modified bacteria. The bacteria are grown in a fermenter, and the insulin they produce is collected and purified.

Penicillin is an antibiotic produced by the fungus Penicillium. The fungus is grown in a fermenter, and penicillin is obtained from the culture.

Mycoprotein is made by growing fungi in a fermenter. In this case, the useful product is the protein-rich fungal biomass itself, rather than a substance collected from the culture.

Control of fermenter conditions

A fermenter provides a controlled environment in which microorganisms grow and produce a desired product. Conditions must remain suitable for the microorganisms and the enzymes involved in growth and production.

• Temperature: The temperature is kept within a suitable range. If it is too low, enzyme-controlled reactions are slow. If it is too high, enzymes can be damaged, reducing their activity.
• pH: The pH is kept within a suitable range because unsuitable pH values reduce enzyme activity or damage enzymes.
• Oxygen: Oxygen is supplied when the microorganisms need it for aerobic respiration, which releases energy for their activities.
• Nutrients: Nutrients must remain available so microorganisms can grow and form the desired product.
• Waste products: Waste must be prevented from building up to levels that inhibit the microorganisms.

These conditions are monitored and adjusted as needed. This maintains a suitable environment for the microorganisms and allows production to continue effectively.