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

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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

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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

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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

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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

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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.