Which one of the following microbes causes acidification and curding of milk ?
- A.Lactic Acid Bacillus
- B.Clostridium botulinum
- C.Vibrio cholerae
- D.Saccharomyces cerevisiae
Correct answer
Explanation
The correct answer is A, Lactic Acid Bacillus. Lactobacillus turns the lactose sugar of milk into lactic acid, which sours the milk and makes it set into curd.
When a little curd is added to warm milk, the Lactobacillus bacteria in it multiply and ferment lactose into lactic acid. The rising acidity makes the milk protein casein coagulate, so the liquid milk sets into a thick curd. The bacteria also improve the nutritional quality of curd by raising its vitamin B12 content, and in the stomach they help check harmful microbes. The same group of bacteria is used to make yoghurt, buttermilk and cheese.
B is wrong: Clostridium botulinum produces the botulinum toxin, which causes botulism, a serious form of food poisoning. C is wrong: Vibrio cholerae is the bacterium that causes cholera. D is wrong: Saccharomyces cerevisiae is baker's or brewer's yeast, used to make bread rise and to ferment sugar into alcohol, not to curdle milk.
Exam tip: milk to curd = Lactobacillus (lactic acid); bread and alcohol = yeast (Saccharomyces cerevisiae).
Practice Questions
View allA negative work is done when an applied force F and the corresponding displacement S are
- A.perpendicular to each other.
- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
Show answer
Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.
Read the full explanation →Which one of the following statements about a satellite orbiting around the Earth is correct ?
- A.Satellite is kept in orbit by remote control from ground station.
- B.Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.
- C.Satellite requires energy from solar panels and solid fuels for orbiting.
- D.Satellite does not require any energy for orbiting.
Show answer
Once a satellite is placed in orbit with the right speed, the Earth's gravity keeps it going round without any engine.
Read the full explanation →Which one of the following is not a conservative force ?
- A.Frictional force
- B.Electric force
- C.Gravitational force
- D.Spring force
Show answer
Friction is a non-conservative force because the work it does depends on the path taken and the energy it removes is lost as heat.
Read the full explanation →A microphone converts
- A.electrical signals to sound waves
- B.sound waves to electrical signals
- C.microwaves to sound waves
- D.sound waves to microwaves
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A microphone is a transducer that changes the pressure variations of sound into a matching electrical signal. In a common dynamic microphone, sound waves make a thin diaphragm vibrate.
Read the full explanation →When a light ray passes through from air to water with a non-zero angle the ray will be
- A.bending towards the normal
- B.bending away from the normal
- C.propagating in straight line
- D.reflected towards the opposite direction
Show answer
Light slows down when it enters water from air, so a ray that strikes the surface at an angle bends towards the normal.
Read the full explanation →