role of bacteria in yoghurt production

Question 1

why are bacteria useful?

Answer 1

they are capable of producing complex molecules e.g. certain bacteria added to milk produces enzymes that turn the milk into yoghurt

Question 2

why are bacteria also useful?

Answer 2

they reproduce rapidly- the amount of chemicals they can produce can also rapidly increase

Question 3

what bacteria is used in yoghurt production?

Answer 3

lactobacillus

Question 4

what is yoghurt?

Answer 4

milk that has been fermented by certain species of bacteria- lactobacillus bulgaricus and streptococcus thermophiles

Question 5

what do the bacteria use?

Answer 5

lactose (milk sugar) to respire anaerobically producing lactic acid
this causes a fall in pH- denatures the milk proteins and causes them to coagulate (join together), thickening the yoghurt

Question 6

what happens to the equipment firstly?

Answer 6

it is sterilised to kill unwanted bacteria and to prevent chemical contamination

Question 7

what happens to the milk firstly?

Answer 7

it is pasteurised, heated at 85-95 for 15-30 minutes to kill all other, unwanted bacteria

Question 8

what could contamination with other bacteria cause?

Answer 8

could slow the production of the yoghurt by competing with the Lactobacillus for the lactose in the milk

Question 9

what could contamination with other bacteria cause for the taste of the yoghurt?

Answer 9

spoil the taste

Question 10

what happens to the milk once it is pasteurised?

Answer 10

it is homogenised- breaks up any fat globules

Question 11

what happens once the milk is homogenised?

Answer 11

it is then cooled to 40-45 degrees and the lactobacillus bulgaricus and streptoccus thermophiles bacteria are added

Question 12

what happens once the milk is cooled?

Answer 12

the mixture is then incubated at this temperature for several hours, allowing the bacteria to partially digest the milk sugars and respire anaerobically producing lactic acid

Question 13

what also happens to the Lactobacillus bacteria when it is incubating?

Answer 13

the Lactobacillus bacteria digests the milk proteins and ferments the sugar (i.e. the lactose) in the milk

Question 14

what does the Lactobacillus bacteria convert the lactose into?

Answer 14

lactic acid- this increases acidity sours and thickens the milk to form yoghurt

Question 15

why is the yoghurt then sampled?

Answer 15

to assess whether it is sufficiently coagulated

Question 16

what does the lowering of the pH allow?

Answer 16

helps to prevent the growth of other microorganisms that may be harmful- so acts as a preservative
means the yoghurt can be kept for longer (compared to fresh milk)

Question 17

why is the yoghurt stirred and cooled to 5 degrees?

Answer 17

to halt the action of the Lactobacillus bacteria

Question 18

what containers are the yoghurt packaged into?

Answer 18

sterile containers