Biology 3ii - Microorganisms

People used to think that life could spontaneously generate from non-living material. But later evidence supported the theory of biogenesis - the theory that living things are created from other living organisms.

It used to be believed that substances IN food were changed into microbes which made the food go off. Spallanzani boiled two sets of broth to kill the microbes, then sealed one flask and left the other one open - only the open one went off. This showed microbes got into the food from the air - but opponents just said it meant air from outside the flask was necessary to start the change.

However, this theory that fresh air caused substances in food to change into microbes was disproved by Schwann, who showed that meat would not go off in air, if the air was heated first to kill microorganisms.

Pasteur heated two flasks of broth, both left open to the air. But one flask had a curved neck so that bacteria from the air would settle in the loop, and not get into the broth. This broth stayed fresh unlike the other, which proved that it was MICROBES and NOT the air which was causing it to go off.

A culture of bacteria is added to milk.

The bacteria produces solid curds in the milk.

The curds are seperated from the liquid whey.

More bacteria are sometimes added to the curds, and the whole lot is left to ripen.

Moulds are added to give blue cheese its colour and taste.

The milk is heat treated first to kill off any bacteria that might be in it, then cooled.

A starter culture of bacteria is then added. The bacteria ferment the lactose sugar (present in the milk) to lactic acid.

The acid causes the milk to clot and solidify into yoghurt.

Sterilised flavours e.g fruit are then added.

Yeast is a single-celled fungus. It is a microorganism.

Yeast can respire with or without oxygen. Anaerobic respiration of glucose by yeast (fermentation): glucose -> ethanol + carbon dioxide +ENERGY

aerobically (produces lots more energy) -                                                       glucose + oxygen -> carbon dioxide +water +energy

Yeast is used to in dough to make nice, light bread. The yeast converts sugars to carbon dioxide and some ethanol. It is carbon dioxide that makes the bread rise. As the carbon dioxide expands, it gets trapped in the dough, making it lighter.

Beer is made from grain - usually barley.

The barley grains are allowed to germinate for a few days, during which the starch in the grains is broken down into sugar by enzymes. Then the grains are dried in a kiln. This process is MALTING.

The malted grain is mashed up and water is added to produce a sugary solution with lots of bits in it. This is then sieved to remove the bits.

Hops are added to the mixture to give the beer its bitter flavour.

The sugary solution is then fermented by yeast, turning the sugar into alcohol.