Banana beer anyone?
SUMMARY
Most people enjoy an alcoholic drink from time to time. There are a lot of drinks which are a
mix of an alcoholic beverage and fruit juice. But why not make an alcoholic drink from fruit?
We are interested in an alcoholic beverage made from either bananas or lemons. That’s why
we have decided to test if we can get a useful amount of alcohol (ethanol) out of the
fermentation of the fruit with yeast. The fermentation will take place in an oxygen free
environment to maximize alcohol production. After testing we came to the conclusion that we
cannot get a useful amount of alcohol out of the fermentation of the fruit. The vol% was
below 1, and that’s too low for an alcoholic beverage. the cause of this is probably the acid in
the lemons, but we don’t know what caused the low amount of alcohol in the banana test.
Introduction
Alcoholic beverages are defined as
beverages that contain ethanol (C2H5OH).
This ethanol is almost always produced by
fermentation – the metabolism of
carbohydrates by certain species of yeast
under anaerobic or low-oxygen conditions.
Beverages such as wine, beer, or distilled
spirits all use yeast at some stage of their
production. Since we couldn’t find any
beverages produced through the
fermentation of bananas or lemons, we
think that something in the fruit
counteracts the production of ethanol. The
ethanol gained from fermentation usually
mainly comes from the conversion of
glucose. the chemical equation below
summarizes the fermentation of glucose,
whose chemical formula is C6H12O6.
→
2
+ 2 O-C-O
Experimental procedure and approach
We prepared 45 grams of lemon and
banana, and added 0,5 grams of yeast. We
put both mixtures into Erlenmeyer’s with a
tube through the cork and put the tube into
a upside down 50ml glass cylinder which
was suspended in water. This way any gas
would bubble up into the cylinder and we
could measure the amount of gas produced.
Here our results:
fruit
Amount
of yeast
Amount
of fruit
banana
lemon
0,5 g
0,5 g
45 g
45 g
Glucose
One glucose molecule is converted into
two ethanol molecules and two carbon
dioxide molecules. Fermentation of
glucose by yeast under anaerobic or lowoxygen conditions:
C6H12O6 → 2C2H5OH + 2CO2
Amount
of gas
produced
80 ml
66 ml
We distilled the product and measured the
amount of ethanol in the distillate
Now the hard part: measuring the amount
of ethanol in our distillates. The
determination takes place through a socalled indirect titration. First we added a
known surplus of acidified potassium
dichromate (K2Cr2O7), to our 100ml
mixtures. This oxidizes the ethanol through
two steps into acetic acid (see reaction 1).
The surplus dichromate is converted into
another oxidiser (see reaction 2). The
iodine produced is titrated with 0,10M
sodium thiosulfate (see reaction 3). after
the titration a starch solution is added to
create the blue colour (iodine + starch), to
better notice the inflection point.
Reactions:
1
CH3CH2OH
→ CH3CHO + 2 H+
+2e
followed by
CH3CHO + H2O
→ CH3COOH + 2
H+ + 2 e--------------------------------------------------+
CH3CH2OH + H2O → CH3COOH + 4
H+ + 4 ex3
2−
+
−
Cr2O7 + 14 H + 6 e → 2 Cr3+ + 7 H2O
x2
--------------------------------------------------+
3 CH3CH2OH + 3 H2O + 2 Cr2O72− + 28
H+ → 3 CH3COOH + 12 H+ + 4 Cr3+ + 14
H2O
2 Cr2O72− + 16 H+ + 3 C2H5OH → 4 Cr3+ +
11 H2O + 3 CH3COOH
results
With the information gained we could
calculate the amount of ethanol in our
solutions:
Thio used: 50,0 – 13,0 mL = 37,0 mL x
0,090 M = 3,33 mmol
3,33 mmol thio ↔ 1,665 mmol I2 ↔ 0,555
mmol Cr2O72−
10, 0 x 0,10 M Cr2O72− = 1,0 mmol – 0,555
mmol = 0,445 mmol reacted with C2H5OH
0,445 mmol Cr2O72− ↔ 0,6675 mmol
C2H5OH in 10,0 mL solution = 0,06675 M
0,06675 M x 46 g mol-1 = 3,07 g L-1
3,07 g L-1 / 0,80 g mL-1 = 3,8 mL alcohol
per litre = (3,8 mL / 1000 mL) x 100% =
0,38 vol%
We did the same with the distillate of the
lemon, which contained 0,36 vol% ethanol.
conclusion
0,38 vol% and 0,36 vol% are both very
low, so there is reason to believe that both
bananas and lemons contain substances
which counteract the production of
ethanol. This, however, requires further
testing.
Evaluation
Since we don’t know what caused the low
ethanol percentage, an interesting further
inquiry question would be: what causes the
low ethanol production when fermenting
bananas or lemons?
Colour change: yellow to green
Bibliography
2
Cr2O72− + 14 H+ + 6 I− → 2 Cr3+ + 3 I2 + 7
H2O
colour change: yellow to brown
3
2 S2O32− + I2 → S4O62− + 2 I−
http://www.everyoneweb.com/zaanlandsex
act/
http://scholierenlab.tudelft.nl/nl/forum/topi
c/biologie/alcohol-gisting/
http://www.10voorbiologie.nl/index.php?c
at=9&id=453&par=469&sub=474
http://en.wikipedia.org/wiki/Fermentation_
(wine)