MILK AND BACTERIA

Objective of the experiment

We want to verify the influence of the temperature on the increase of the bacteria that turn the milk acid.

Instructions

Half fill two vases with milk, hold one vase in refrigerator and leave the other one in a warm place. Observe what happens for a week.

Observations

Vase #1(held in a warm place) Vase # 2(held in refrigerator)

 

Vase #1
(held in a warm place)

Vase # 2
(held in refrigerator )

1th day

No change No change
2nd day
On the bottom of the vase there is a dense white substance and over it a yellow liquid that smells acid. No change
3rd day
The white substance has became more solid and has taken the shape of a round and crushed cheese; the yellow liquid is a little cleaner No change
4th day
 The separation of the white substance from the yellow liquid is complete; the taste of the "cheese" is bad, acid No change
5th day
/
 The milk is light yellow

 
2nd day - held in a warm place
 
4th day -held in a warm place
 

Conclusions

The observed changes are caused by the development of bacteria. The bacteria work better in a warm place.

BROTH AND BACTERIA

Objective of the experiment

Verify where the bacteria develop better and the broth go bad.

Instructions

Put in three cups the same amount of warm water and melt a bouillon cube in them . Add in a cup a teaspoon of salt and in the other one some vinegar. Leave the cups in a warm place for two days.

Observations

 
CUP #1 (only broth)
CUP #2 (broth + salt)
CUP # 3 (broth + vinegar)
1° day
The liquid is a little turbid; there is the typical smell of broth The liquid has not changed aspect; there is the typical smell of the broth There is a vinegar smell; the liquid has not changed its aspect
2° day
The broth has gone bad: it has became more turbid, with a yellowish colour and a sour smell The broth appeares denser but it keeps its colour The smell of vinegar is strong; the broth has not changed.

Conclusions

The changes observed are caused by the development of bacteria. The broth in the cup#1 is more easily attacked; the vinegar is more effective than salt for the conservation of the the broth.

School site Aristotle Redi Spallanzani Metamorphosis Microorganisms "The tiny animals"