1
Determining Acidity of Foods
I. Purpose/Objective:
The purpose is to identify the normality of a prepared sodium hydroxide solution by titrating
samples of KAP. With the known normality of the base solution, samples of cheese and juice were
titrated to measure their titratable acidity and pH values. Also, samples of three types of juices,
two types of cheeses, and four solutions of vinegar and varying sugar concentrations were tasted to
compare their perceived acidity to sweetness ratios.
II. Introduction:
Many foods contain weak organic acids, which help to promote taste, color and longevity of
the foods. These acids stabilize foods and act as buffers, in addition to being antioxidants, pH
adjusters, and nutrients. In order to evaluate the acids present in foods, the food industry uses a
standard base to titrate into the acidic solution to determine the amount of acid present in the
sample.
In the base and acid reaction, the base accepts the hydrogen ions that the acid is donating.
When the pH reaches neutral, there is an equal amount of base and acid present in the solution.
This point, the equivalence point, determines the amount of acid originally present in the sample.
In this experiment, these concepts relating to food acids are used in titrating different juice and
cheese samples.
III. Procedure:
This laboratory procedure is found in the “Principles of Food Composition, Laboratory
Manual, FST 101A” (Fall 2012) Lab 2A, Preparing a Standard Base Solution, pages 14-16 and Lab
2B, Total Acidity and pH of Liquid and Solid Foods, pages 18-20. Modifications for part A of this
experiment include titrating 2 mL additions of NaOH until pH 11.5 is reached and determining two
vials of KAP that are different in weight. Modifications for part B of this experiment include no
titration of vinegar; the volume of cheese solution to be titrated is 50 mL; titrating the cheese
solution at 0.1 mL NaOH per addition until pH 9, then at 1.0 mL NaOH per addition until pH 12; a
solution of juice and water to be titrated is 25 mL juice and 25 mL bottled water; no filter is to be
used in the titration of juice; and titrating the juice and water solution with 0.5 mL NaOH per
addition until pH 6.0, then 0.1 mL NaOH per addition until pH 9.0, and 1.0 mL NaOH per addition
until pH 11.5.
2
Data/Result:
Table 1: Group 10’s starting and ending volumes of NaOH and ending pH for the titrations of the
first 3 known KAP samples and the 2 unknown KAP samples.
Titration Weight of KAP (g)
Starting NaOH Volume (mL)
Ending NaOH Volume (mL)
Ending pH
1 (vial 253) 0.3161 0.10 14.3 6.82 2 (vial 310) 0.3309 0.00 15.1 6.92 3 (vial 306) 0.3765 0.00 17.0 6.93
unknown 1 (vial 541) See Table 3 0.00 14.2 6.86 unknown 2 (vial 414) See Table 3 0.00 16.6 6.92
Table 2: Group 10’s pH vs. volume of NaOH for the 4th known KAP sample titration.
Volume NaOH (mL) pH ∆pH/∆Vol Volume NaOH
(mL) pH ∆pH/∆Vol
0.00 4.01 13.0 5.7 0.240 0.50 4.07 0.120 13.5 5.76 0.120 1.00 4.14 0.140 14.0 5.86 0.200 1.50 4.25 0.220 14.5 6.01 0.300 2.00 4.28 0.060 14.6 6.06 0.500 2.50 4.37 0.180 14.7 6.07 0.100 3.00 4.45 0.160 14.8 6.1 0.300 3.50 4.52 0.140 14.9 6.13 0.300 4.00 4.49 -0.060 15.0 6.18 0.500 4.50 4.66 0.340 15.1 6.22 0.400 5.00 4.75 0.180 15.2 6.27 0.500 5.50 4.79 0.080 15.3 6.32 0.500 6.00 4.85 0.120 15.4 6.4 0.800 6.50 4.88 0.060 15.5 6.45 0.500 7.00 4.98 0.200 15.6 6.51 0.600 7.50 5.02 0.080 15.7 6.56 0.500 8.00 5.04 0.040 15.8 6.61 0.500 8.50 5.09 0.100 15.9 6.73 1.20 9.00 5.18 0.180 16.0 6.84 1.10 9.50 5.22 0.080 16.1 6.95 1.10 10.0 5.26 0.080 16.2 7.09 1.40 10.5 5.31 0.100 16.3 7.89 8.00 11.0 5.37 0.120 16.4 8.78 8.90 11.5 5.47 0.200 18.4 11.39 1.31 12.0 5.52 0.100 20.4 12.04 0.325 12.5 5.58 0.120
Table 3: Group 10’s NaOH concentration and unknown KAP calculated and true weight.
Unknown KAP Sample [NaOH] (N) Calculated KAP
Weight (g) True KAP Weight (g)
Vial 541 0.1078 0.3126 0.3151 Vial 414 0.1078 0.3645 0.3722
3
Table 4: Sensory tastings of juices, vinegar, and cheeses.
Rank In Order of Acidity Rank In Order of Sweetness Preference
Juice Orange>Apple>Grape Grape>Apple>Orange Apple Vinegar D>S>C>B B>C>S>D B Cheese Cheddar>Mozzarella Mozzarella>Cheddar Cheddar
Table 5: Group 10’s pH vs. volume NaOH for apple juice titration.
Volume NaOH (mL) pH ∆pH/∆Vol Volume NaOH
(mL) pH ∆pH/∆Vol
0.00 3.77 10.9 7.55 1.30 0.50 3.82 0.100 11.0 7.68 1.30 1.00 3.88 0.120 11.1 7.84 1.60 1.50 3.94 0.120 11.2 7.95 1.10 2.00 4.05 0.220 11.3 8.04 0.900 2.50 4.13 0.160 11.4 8.17 1.30 3.00 4.21 0.160 11.5 8.23 0.600 3.50 4.29 0.160 11.6 8.35 1.20 4.00 4.37 0.160 11.7 8.45 1.00 4.50 4.49 0.240 11.8 8.53 0.800 5.00 4.61 0.240 11.9 8.62 0.900 5.50 4.70 0.180 12.0 8.68 0.600 6.00 4.81 0.220 12.1 8.74 0.600 6.50 4.90 0.180 12.2 8.85 1.10 7.00 5.01 0.220 12.3 8.92 0.700 7.50 5.15 0.280 12.4 9.01 0.900 8.00 5.25 0.200 13.5 9.50 0.445 8.50 5.46 0.420 14.5 9.84 0.340 9.00 5.70 0.480 15.5 10.09 0.250 9.50 6.03 0.660 16.5 10.39 0.300 9.60 6.08 0.500 17.5 10.57 0.180 9.70 6.24 1.600 18.5 10.75 0.180 9.80 6.31 0.700 19.5 10.89 0.140 9.90 6.36 0.500 20.5 11.00 0.110 10.0 6.46 1.00 21.5 11.11 0.110 10.1 6.61 1.50 22.5 11.19 0.080 10.2 6.67 0.600 23.5 11.27 0.080 10.3 6.76 0.900 24.5 11.33 0.060 10.4 6.88 1.20 25.5 11.40 0.070 10.5 6.95 0.700 26.5 11.44 0.040 10.6 7.09 1.40 27.5 11.48 0.040 10.7 7.25 1.60 28.5 11.51 0.030 10.8 7.42 1.70
4
Table 6: Group 10’s pH vs. volume of NaOH for cheddar cheese titration.
Volume NaOH (mL) pH ∆pH/∆Vol Volume NaOH
(mL) pH ∆pH/∆Vol
0.00 4.99 3.30 7.37 0.700 0.10 5.18 1.90 3.40 7.46 0.900 0.20 5.25 0.700 3.50 7.56 1.00 0.30 5.32 0.700 3.60 7.63 0.700 0.40 5.42 1.00 3.70 7.70 0.700 0.50 5.51 0.900 3.80 7.77 0.700 0.60 5.61 1.00 3.90 7.81 0.400 0.70 5.74 1.30 4.00 7.88 0.700 0.80 5.91 1.70 4.10 7.96 0.800 0.90 5.93 0.200 4.20 8.04 0.800 1.00 6.04 1.10 4.30 8.11 0.700 1.10 6.08 0.400 4.40 8.19 0.800 1.20 6.14 0.600 4.50 8.28 0.900 1.30 6.18 0.400 4.60 8.39 1.10 1.40 6.27 0.900 4.70 8.45 0.600 1.50 6.34 0.700 4.80 8.52 0.700 1.60 6.42 0.800 4.90 8.6 0.800 1.70 6.49 0.700 5.00 8.64 0.400 1.80 6.56 0.700 5.10 8.69 0.500 1.90 6.59 0.300 5.20 8.72 0.300 2.00 6.66 0.700 5.30 8.77 0.500 2.10 6.71 0.500 5.40 8.80 0.300 2.20 6.74 0.300 5.50 8.84 0.400 2.30 6.78 0.400 5.60 8.91 0.700 2.40 6.81 0.300 5.70 8.96 0.500 2.50 6.87 0.600 5.80 9.00 0.400 2.60 6.91 0.400 6.80 9.68 0.680 2.70 6.97 0.600 7.80 10.8 1.12 2.80 7.06 0.900 8.80 10.59 -0.210 2.90 7.10 0.400 9.80 11.24 0.650 3.00 7.18 0.800 10.8 11.58 0.340 3.10 7.26 0.800 11.8 11.84 0.260 3.20 7.30 0.400 12.8 12.01 0.170
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Table 7: Section 5 Group 7’s pH vs. volume of NaOH for mozzarella cheese titration.
Volume NaOH (mL) pH ∆pH/∆Vol Volume NaOH
(mL) pH ∆pH/∆Vol
0.00 5.44 1.70 7.72 1.40 0.10 5.64 2.00 1.80 7.83 1.10 0.20 5.81 1.70 1.90 8.01 1.80 0.30 5.92 1.10 2.00 8.06 0.500 0.40 5.93 0.100 2.10 8.10 0.400 0.50 6.19 2.60 2.20 8.29 1.90 0.60 6.26 0.700 2.30 8.41 1.20 0.70 6.39 1.30 2.40 8.48 0.700 0.80 6.50 1.10 2.50 8.56 0.800 0.90 6.63 1.30 2.60 8.77 2.10 1.00 6.71 0.800 2.70 8.85 0.800 1.10 6.84 1.30 2.80 8.93 0.800 1.20 7.01 1.70 2.90 9.10 1.70 1.30 7.09 0.800 3.90 10.52 1.420 1.40 7.23 1.40 4.90 11.47 0.950 1.50 7.37 1.40 5.90 11.93 0.460 1.60 7.58 2.10 6.90 12.18 0.250
Table 8: Data for each juice’s end point volume, equivalent pH, Brix, and concentration of NaOH.
Group Juice NNaOH pHEquivalent Brix (oBx)
Veq-Vinitial (mL)
1 0.1083 8.59 12.2 31.20 2 0.1064 8.92 12.2 26.70 3 0.1065 8.06 12.2 28.00 4
Orange
0.1082 7.47 12.2 30.30 5 0.1088 8.27 15.9 23.80 6 0.1090 7.48 15.9 20.23 7 0.1078 7.30 15.9 21.30 8
Grape
0.1081 8.18 15.9 24.80 9 0.1049 5.71 11.6 10.00
10 0.1078 7.25 11.7 10.70 11 0.1061 8.20 11.7 9.000 12
Apple
0.1077 7.79 11.7 12.50
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Table 9: Data for each cheese’s initial pH, equivalent pH, Brix, end point volume, weight, and the
concentration of NaOH.
Group Cheese pHinitial pHEquivalent NNaOH Brix (oBx)
Veq-Vi (mL)
Weight of Cheese (g)
1 5.62 8.54 0.1083 0.8 6.80 12.10 3 5.50 8.29 0.1065 0.9 5.10 12.50 9 5.60 7.53 0.1049 1.1 4.30 12.25 7 5.44 8.93 0.1078 1.3 2.90 12.10 5 5.64 8.57 0.1088 0.9 4.90 12.00
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Mozzarella
5.43 8.25 0.1061 1.3 2.80 12.21 2 5.54 8.40 0.1064 1.2 7.80 12.02 4 5.66 8.18 0.1082 0.8 8.90 12.64 6 5.66 8.88 0.1090 0.9 10.0 12.20 8 5.73 8.07 0.1081 1.2 9.50 12.00
10 4.99 7.56 0.1078 0.8 3.50 12.00 12
Cheddar
5.16 8.85 0.1077 1.2 5.90 12.10
Table 10: Data for percent acidity, equivalent pH, Brix, Brix/titratable acidity, and taste ranking for
each juice and cheese using average data in each group.
Group
% acidity (g/100mL juice) (g/weight g of
cheese)
pH Brix (oBx)
Brix/ Titratable Acidity
Taste Ranking (1=most
preferred)
1 0.8656 8.59 12.2 14.1 2 0.7277 8.92 12.2 16.7 3 0.7639 8.06 12.2 16.0 4 0.8398 7.47 12.2 14.5
Orange Juice
average 0.7992 8.26 12.2 15.3
2
5 0.7770 8.27 15.9 20.5 6 0.6617 7.48 15.9 24.0 7 0.6890 7.30 15.9 23.1 8 0.8045 8.18 15.9 19.8
Grape Juice
average 0.7331 7.81 15.9 21.8
3
9 0.2813 5.71 11.6 41.2 10 0.3094 7.25 11.7 37.8 11 0.2561 8.20 11.7 45.7 12 0.3611 7.79 11.7 32.4
Apple Juice
average 0.3020 7.24 11.7 39.3
1
1 1.605 8.54 0.800 0.498 3 1.223 8.29 0.900 0.736 9 0.9955 7.53 1.10 1.11 7 0.6815 8.93 1.30 1.91 5 1.153 8.57 0.900 0.781
11 0.6535 8.25 1.30 1.99
Mozzarella Cheese
average 1.052 8.35 1.05 1.17
2
7
2 1.797 8.40 1.20 0.668 4 2.193 8.18 0.800 0.365 6 2.396 8.88 0.900 0.376 8 2.220 8.07 1.20 0.540
10 0.8157 7.56 0.800 0.981 12 1.385 8.85 1.20 0.866
Cheddar Cheese
average 1.801 8.32 1.02 0.633
1
Figure 1: Titration curve and the first derivative for the fourth KAP titration.
Figure 2: Titration curve and first derivative for the apple juice titration.
-2
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20 22
pH o
r Firs
t Der
ivat
ive
Volume NaOH (mL)
Titration First Derivative
0
2
4
6
8
10
12
0 5 10 15 20 25 30
pH o
r Firs
t Der
ivat
ive
Volume of NaOH (mL)
Apple Juice Titration
Apple Juice First Derivative
Equivalence Point
Equivalence Point
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Figure 3: Titration curve and first derivative for the cheddar cheese titration.
Figure 4: Titration curve and first derivative for the mozzarella cheese titration.
IV. Calculations:
€
[NaOH] =
weight KAP (g)
204.22gmol
*1000meqmol
Volume NaOH added
-2
0
2
4
6
8
10
12
0 2 4 6 8 10 12
pH o
r Firs
t Der
ivat
ive
Volume of NaOH (mL)
Cheddar Cheese Titration
Cheddar Cheese First Derivative
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6 7
pH o
r Firs
t Der
ivat
ive
Volume of NaOH (mL)
Mozzarella Cheese Titration
Mozzarella Cheese First Derivative
Equivalence Point
Equivalence Point
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Example: For Group 10, KAP Titration 1 Data (see Table 1)
€
[NaOH] =
0.3161 g
204.22gmol
*1000meqmol
14.2 mL NaOH
[NaOH] =.001548 mol*1000meq
mol14.2 mL NaOH
[NaOH] = 1.548 meq14.2 mL NaOH
[NaOH] = 0.109 N
€
weight KAP (g) = NNaOH *VNaOH
1000meqmol
*204.22gmol
Example: For Group 10, unknown KAP 1 (see Tables 1 and 3)
€
weight KAP = .1078 N *14.2 mL
1000meqmol
* 204.22gmol
weight KAP = 1.53 meq
1000meqmol
*204.22 gmol
weight KAP =312.6 meq*g
mol1000meq
molweight KAP = 0.3126 g
€
pKa = pH - log [A_ ]
[HA]
Example: For Group 10, pKa of lactic acid in cheddar cheese. (see Tables 6 and 9)
€
pKa = pH - log [A_ ][HA]
pKa = pHmidpoint when [HA] = [A_ ]pH equivalent = 7.56 at 3.5 mL NaOH
pHmidpoint=6.49 +6.56
2, pH = 6.49 at 1.7 mL NaOH and pH = 6.56 at pH 1.8 mL
pHmidpoint= 6.53pKa = 6.53
€
% acidity = Nacid *EqWt * 0.1L100mL
10
Example: For Group 10, % acidity of apple juice. (see Table 8 and 10)
€
% acidity = Nacid * EqWt * 0.1 L100 mL
% acidity = VNaOH * NNaOH
Vacid* EqWt * 0.1L
100 mL
% acidity =0.01070 L *0.1078 eq
L.025 L
*67.05 geq * 0.1 L
100 mL
% acidity = 0.001153 eq0.025 L
*67.05 geq * 0.1 L
100 mL
% acidity = 0.04614 eqL *67.05 g
eq * 0.1 L100 mL
% acidity = 0.3094 g100 mL
€
oBxtitratable acidity
=oBx
% acidity
Example: For Group 10, oBx/titratable acidity of apple juice (see Table 10)
€
oBxtitratable acidity
=11.7 oBx0.3094
oBxtitratable acidity
= 37.8
V. Discussion:
For the unknown weights of the two KAP samples, the calculated weight for vial 541 was
0.3126 g and for vial 414 0.3645. These values were close to their actual weights of 0.3151 g and
0.3722 g, respectively. The differences in values could be caused by not adding enough NaOH to
the solution, thus undershooting the equivalence point, or could be due to mass of KAP lost in
transfer in making the solution.
In comparing the total acidity and Brix, the values differed for each juice. The average percent
acidity of orange juice was 0.7992% and the average Brix was 12.2 oBx. For grape juice, the
average percent acidity was 0.7331% and the average Brix was 21.8 oBx. Apple juice had an
average percent acidity of 0.3020% and an average Brix of 11.7. This data did not correspond to
the sensory test. In sampling each juice, grape juice was found to be the least acidic (most sweet),
as found in the data. However, in tasting orange and apple juice, orange juice was found to be the
most acidic, though the data suggests apple juice to be more acidic. In comparing data to the
sensory tastings, the rank in order of acidity differs because people’s taste buds perceive acidity
and sweetness at different concentrations.
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The titration curve for the fourth KAP titration looks like an expected titration and first
derivative curve. However, the curves for the juice and two cheeses greatly differed. The
equivalence points are more difficult to discern when looking at the graphs. Also, the titration
curves for the two cheeses did not look like the expected titration curve. The differences in the
curves could be because the KAP was a total weak acid solution. The apple juice and the two
cheese solutions were a mixture of weak acids and other foodstuffs, which inhibited the NaOH
from fully reacting with only the acid.
The results of the titrations of the juices were overall fairly consistent. For the orange juice,
the highest pH value was found at the lowest volume of NaOH added. For grape juice, the pH
value as compared to the added volume of NaOH was fairly consistant. For apple juice, the lowest
pH value was found at a relatively average volume of NaOH added. Also, the highest pH value
was found at the lowest volume of NaOH added. An average pH value was found at the highest
volume of NaOH added. For the mozzarella cheese, each of the pH values compared to the
volumes of NaOh added were consistent. For the cheddar cheese titrations, the lowest pH value,
which was not very low when compared to other pH values, the volume of NaOH added was very
low when compared to the other volumes of NaOH added. These discrepancies in the data could
be due to errors in setting up the titration and when calculating the pH. For the cheeses, there were
different cheese weights used in the groups, and each prepared the cheese solution slightly
differently. Also, errors in calculating the pH could be found if the pH was written down before
the pH electrode was absolutely ready.
VI. Conclusions:
In this two-part laboratory solution, a standardized base solution was prepared and used to
titrate a weak acid solution and used to titrate acids in foods. Many foods contain acid, and the
amount of acid for foods can be found through titrations with a standard base solution. In titrating
the food samples, the amount of acid present is found when the concentration of hydroxyl ions
equals the concentration of hydrogen ions. In the experiment, three types of juices (each diluted
with water) and two types of cheeses (blended then strained to create a solution) were titrated with
the standard base solution, with the pH being tracked. Both members of group 10 both participated
in preparing the juice solution and the cheese solution. Both also titrated one of the solutions with
the base solution while the other recorded the pH.
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VII. Questions:
Part A:
1. When a standard base solution of NaOH reacts with CO2, it forms sodium carbonate in the
solution. This causes the solution to have an imprecise concentration, making it very difficult
to accurately titrate an acidic sample.
2. When the indicator changes color, the pH of the sample is at it’s equivalence point. This pH
provides a good estimate for the end-point of a titration of a weak acid by a strong base
because the concentration of hydroxyl ions equals the concentration of hydrogen ions.
3.
equivalence point = pH 9.42
equivalence point 1 = pH 30.30
equivalence point 2 = pH 47.50
0.00
5.00
10.00
15.00
20.00
25.00
0.00 10.00 20.00 30.00 40.00 50.00
pH o
r Firs
t Der
ivat
ive
Volume of NaOH (mL)
Acetic Acid Data
Acetic Acid Titration
First Derivative
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
pH o
r Firs
t Der
ivat
ive
Volume of NaOH (mL)
Phosphoric Acid Titration
Phosphoric Acid Titration First Derivative
13
Part B:
1. The average percent acidity and average Brix for orange juice and for apple juice fell between
the acceptable ranges. However, the average percent acidity and average Brix for grape juice
were below and higher than their respective acceptable ranges, if only considering tartaric acid
as the primary acid in grape juice.
2. pH and titratable acidity are related because as the titratable acidity increases, so does pH. This
is because the greater the amount of acid, the more base is required to neutralize it, making the
pH higher.