Chemical test : Fehling’s solution
To test for the presence reducing sugars to the juice, a small
amount of Fehling’s solution is added and boiled in a water bath.
During a water bath, the solution progresses in the colours of
blue (with no glucose present), green, yellow, orange, red, and
then brick red or brown (with high glucose present ).A colour
change would signify and the presence of glucose.
Sucrose(table sugar) contains two sugars (fructose and glucose)
joined by their glycosidic bond in such a way as to prevent the
glucose isomerizing to aldehyde, or the fructose to alpha-
hydroxy-ketone form. Sucrose is thus a non-reducing sugar
which does not react with Fehling’s solution.(Sucrose indirectly
produces a postive result with Benedict’s reagent if heated with
dilute hydrochloric acid prior to the test, although after this
treatment it is no longer sucrose.) The products of sucrose
decomposition are glucose and fructose, both of which can be
detected by Fehling’s as described above.By comparing the time
required for completion of fermentation of equal amounts of
different substances containing starch the rates of
fermentation can be compared.
Addition of yeast
In wine making, yeast is normally already present on grape
skins. Fermentation can be done with this endogenous “wild
yeast,” but this procedure give unpredictable results, which
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depend upon the exact types of yeast species present. For this
reason, a pure yeast culture is usually added, this yeast quickly
dominates the fermentation. Baker’s is the common name for
the strains of yeast commonly used as a leavening agent in
baking bread and bakery products where it converts the
fermentable sugars present dough into carbon dioxide and
ethanol. Baker’s yeast is of the species Saccharomyces
cerevisiae, which is the same species commonly used in
alcoholic fermentation, and so is also called brewer’s yeast.
Pasteur’s salt
Pasteur’s salt solution is prepared by dissolving ammonium
tartarate , 10.0g; potassium phosphate, 2.0g; calcium
phosphate, 0.2g; and magnesium sulphate, 0.2g dissolved in
860ml of water. The Pasteur’s salts in a solution act as a buffer
to any acids the yeast may create. Since yeast only converts
sugar (most likely sucrose or glucose) to ethanol under
anaerobic conditions, and it is reasonable to assume that there
will be no oxygen present in the laboratory, some acetic acid is
created as a result. The Pasteur salts act as buffers to the
acidity so that the proteins in the yeast do not become
denatured.
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EXPERIMENT
Aim:
To compare the rates of fermentation of wheat flour,
gram flour, rice flour potato juice and carrot
juice and determine the substance which has the
highest rate of fermentation amongst the various
samples taken.
Requirement
a. Chemical requirement
Pasteur’s salts
Yeast
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Fehling’s solution
Test tubes
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Beaker
Bunsen burner, tripod stand watch glass
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PROCEDURE
1.Take 5gm of wheat flour in 100ml conical flask and add
30 ml of distilled water.
2.Boil the contents of the flask for about 5 minutes.
3.Filter the above contents after cooling; the filtrate
obtained is wheat flour extract.
4.Take the wheat extract into a conical flask and add 5ml
of 1% aq.NaCl solution.
5.Keep this flask in a water bath maintained at a
temperature of 50-60 degree Celsius and add 2ml of malt
extract.
6.After 2 minutes take 2 drops of the reaction mixture and
add to dilute iodine solution.
7.Repeat the above step after 2 minutes. When no bluish
colour is produced the fermentation is complete.
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8.Repeat the above steps for gram flour too.
9.Take 5.0ml of carrot juice and potato juice in two clean
250ml conical flask and dilute with 50ml of distilled
water separately.
10.Add 2.0gram of Baker’s yeast and 5.0 ml of solution of
Pasteur’s salts to the above conical flasks.
11.After 10 minutes 5 drops of the reaction take the
mixtures from the flask and add to a test tube containing
2ml of Fehling reagent. Place the test tubes in a boiling
water bath for about 2 minutes.Note the colour of the
solution or precipitate.
12.Record the total time take for completion of
fermentation.
Precautions
❖All apparatus should be clean and washed
properly.
❖The flask should not be rinsed with any of the
solution.
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Observation
Time required for the fermentation__
Wheat flour--10 hours
Gram flour--12.5 hours
Potato juice--13 hours
Carrot juice--20 min
Conclusion
Carrot juice with the highest content of sucrose
among the given samples takes the least time to
get fermented
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Bibliography:
Wikipedia – The free encyclopedia - (http://en.wikipedia.org)
Comprehensive Practical Chemistry manual
www.icbse.com
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