In this series of ten blogs, various parts of the GCSE Science, IGCSE Biology specifications will be explored through the context of food. As well as assisting students revising for their GCSE and IGCSE exams, the blogs also provide an every-day context for science which all readers should find accessible, interesting and useful.
Beer and cheese: what a typically British delicious combination, more ancient than we know and still tantalising our taste-buds today. This food and drink owe their existence and flavour to the action of enzymes and micro-organisms on raw ingredients.
The ingredients of beer are barley, water, hops and yeast. Yeast is the micro-organism.
The first stage of making beer involves malting the barley. This process involves keeping barley grains moist and warm to start them germinating. This activates enzymes in the grain which turn insoluble starch into soluble sugar.
The next stage is the mash. Hot water is added to the barley and the enzyme activity continues in the malted barley grains and the sugars dissolve in the hot water making it sweet (and smell delicious).
The hot, sweet water is then boiled, which destroys the enzymes – they denature at high temperatures. Enzymes are made of protein folded into a globular shape. Heat energy causes their structure to break down, and means they can no longer carry out their chemical reaction. During the boil, hops are added which give flavour and act to preserve the beer.
Finally the yeast is added to the hoppy, malty liquid when it has cooled sufficiently. The yeast is a fungus which feeds on sugars in the liquid. They respire anaerobically, converting sugar into alcohol and carbon dioxide. A few weeks later, the liquid is much less sweet, still hoppy, a little alcoholic and slightly fizzy: beer!
The raw ingredients for cheese are milk, salt, a starter culture of bacteria and rennet which contains an enzyme.
Milk is sterilized by heating and the starter culture of bacteria is added once the milk has cooled. The bacteria can be the same as is found in live yoghurts. The bacteria respires (feeds on) sugars in the milk and excretes lactic acid. The acid causes the milk to separate into curds and whey.
Rennet is added – its enzyme acts on proteins in the curds, changing their structure and turning the curds into a gel. Further heating destroys the bacteria and denatures the enzymes leaving the curds sterile.
The gel is kneaded with salt, pressed to squeeze out the last of the liquid whey, and then left at a steady cool temperature (a cave for example) wrapped up in cloth or coated in wax until the cheese – cheddar, for example – is mature and tasty.