Effect of cooking temperature on acetic acid content of vinegar

Kitchen chemistry and cooking science is one of the most wide and major application of Chemistry. All the food products that we use starting from different spices to food additives are organic compounds and thus cooking is basically an organic reaction they go through. For example, change of color of green vegetables while cooking, souring of milk, caramelization of sugar are all examples of organic reactions. Thus, changes in chemical composition of substances during cooking has always been a major topic of interest in Chemistry. Due to the global pandemic, when access to school laboratory was not there, selecting and identifying a appropriate topic for the Chemistry Internal Assessment became an issue especially when I was determined to opt for an IA that uses experimental data. So, I chose to base my investigation on chemicals that are used in our daily life. Focusing on the fact that compounds change their molecular structure as well as composition during cooking, I decided to understand how it happens and what factors may impact it. I chose to deal with vinegar as that is one of the most widely used ingredients for cooking. Heating is an essential condition of cooking. Almost all foods that we prepare involves thermal exposure of chemicals in some or the other way. So, I decided to explore how exposure to temperature would alter the chemical composition of vinegar and to what extent.

How does the molar concentration of ethanoic acid (CH₃COOH) in mol dm^-3 within vinegar depends on the temperature to which it is heated temperature, determined using acid base titration with a primary standard NaOH solution?

Vinegar is a commercially used aqueous solution of ethanoic acid. As per FDA (Food and Drug Adminstration), vinegar must contain ethanoic acid within the range of 4.00% to 8.00% (volume by volume). It means that 100 cm³ of vinegar should contain 4.00 cm³ to 8.00 cm³ of ethanoic acid. The most widely used vinegar is Apple Cider Vinegar which is named based on the fact that it is made from apple. Vinegar is used as a souring agent and is used in marination of meats as it aids in the breaking down of polypeptide chains into smaller uses and thus makes it easier for the meat to become tender. Researches reports that vinegar has various medical benefits as well. Journals of medical researches reveals that direct consumption of vinegar may be beneficial to treat health conditions like hyperacidity, Type-II Diabetes and so on.

Vinegar is prepared in a biological process. The biochemical pathway consists of preparing vinegar involves two enzymatic pathways:

• Alcoholic fermentation of glucose:

C₆H₁₂O₆ (glucose) ----→ 2 C₂H₅OH (aq) +2 CO₂ (g)

Glucose undergoes fermentation to form ethanol and release carbon dioxide. This step is a breakdown process where a large organic molecule of glucose decarboxylates (loses CO₂) to produce ethanol.

• Oxidation of ethanol formed from fermentation:

2 C₂H₅OH (aq) + O₂ (g) ---→ 2 CH₃COOH

Here ethanol when exposed to oxygen in air oxidizes to ethanoic acid. The oxidation number of C increases from -2 in ethanol to 0.

The first step is anaerobic as it happens in absence of oxygen while the second step needs presence of oxygen. Both the steps involve the use of enzymes secreted from the microbes present within the fruit which undergoes fermentation. As both of them are enzyme catalyzed reactions, they occur at an optimum temperature and also requires a definite pH level. The ethanoic acid made is diluted and a dilute solution of vinegar containing ethanoic acid at 4% to 8% by volume is then used as vinegar.

Ethanoic acid is a weak organic acid and reacts with the strong base NaOH to produce the salt- Sodium ethanoate and water. This is a neutralization reaction and can thus be used as a basis of the analytical process to deduce the concentration of ethanoic acid from an acid base titration.

CH₃COOH (aq) + NaOH (aq) -------→ CH₃COONa (aq) + H₂O ...........(equation - 1)

The salt formed is a basic salt as ethanoic acid is a weak acid and NaOH is a strong base. Thus, the pH at equivalence point will lie above 7.00 and thus phenolphthalein can be used as a suitable indicator for this titration. Here, the analyte is ethanoic acid and the titrant is NaOH. Thus, the color changes from colorless (in acidic medium) to pink (in basic medium).

The vinegar solutions will be heated to various temperatures using a water bath. After being heated, the solutions were titrated with a solution of NaOH of known concentration. The burette reading will be used to calculate the number of moles of NaOH consumed and thus the moles of ethanoic acid present. This will then be used to determine the concentration of ethanoic acid. Following this, a scatter plot will be used to elucidate the correlation of temperature and concentration of ethanoic acid in vinegar.

The main purpose of the investigation is to determine the molar concentration of ethanoic acid in vinegar. It can be done using a pH curve instead of an acid base titration. That would give us more accurate result. Moreover, concentration of ethanoic acid can also be determined spectrophotometrically by measuring the absorbance of the molecule at a wavelength at which it displays maximum absorbance. Since ethanoic acid is a colorless liquid, it would show absorbance in the ultra violet region and thus a UV-Visible spectrophotometer is required for this. However, the first method was discarded as a pH probe was not accessible and the lack of a UV-Visible spectrophotometer did not allow to use the second method.

In a medical study on the anti-bacterial and anti-glycemic effect of Vinegar, it was found that the increase of temperature reduces the concentration of ethanoic acid in vinegar. The study is titled as – “Vinegar: Medicinal Uses and Antiglycemic Effect” by Carol S. Johnston and Cindy A.Gaas published in the journal – “Medscape General Medicine”.

The temperature at which vinegar is heated has no correlation with the molar concentration of ethanoic acid in the vinegar.

As temperature increases, the molar concentration of ethanoic acid increases. With heating, the volume of water evaporates and thus the solution becomes more concentrated. Thus, molar concentration of ethanoic acid in vinegar would increase.

Temperature of Vinegar: A water bath was set to the required temperature and then the samples were incubated inside it. A thermometer was used for each sample to ensure the required temperature is reached. The cooking temperature usually differs from a range of 40°C to 100°C. So, the temperature measured were 20°C, 40°C, 60°C, 80°C and 100°C.

Concentration of Ethanoic Acid (CH₃COOH) in Vinegar: The vinegar samples were titrated against NaOH solution of known concentration. The change in the burette readings were recorded. Using the volume of NaOH required as obtained from the burette reading, the moles and the stoichiometric ratio in which ethanoic acid reacts with NaOH, the molar concentration of ethanoic acid was deduced. The molar concentration was measured in the unit – moldm^-3.

• Protective Lab equipment were worn such as gloves, lab coat, safety goggles and mask.
• Chemicals were handles with care to avoid spillage
• Foods and drinks were kept outside the lab
• Skin and eye contact with chemicals were avoided throughout.
• The boiling tube was held with the support of a test tube holder while taking it outside the water bath to avoid possible skin burns.

Ethical Issues:

• There were no significant ethical issues but the wastage of chemicals was kept to the minimum.

• There were no significant environmental issues.

Number of Moles = concentration × Volume = 1.00 × \(\frac{100}{1000}\) = 1.00 × 0.10 = 0.10 moles Mass of solid NaOH required = moles mass  = 0.10 × (23.00 + 16.00 + 1.01 + 1.01) = 4.00g

• A watch glass was put on the digital mass balance. The scale was now set to zero grams.
• A spatula was taken and solid NaOH of 4.00 ± 0.01 g was put on the watch glass and weighed.
• A beaker (250 cm³ ) was cleaned and then distilled water was put till 100 cm³ using a graduated measuring cylinder.
• The weighed solid NaOH of 4.00 ± 0.01 g was put inside the beaker.
• The solid particles were mixed thoroughly with the distilled water using a glass rod.
• Now, this solution was stored in a clean glass reagent bottle and labelled.

• A clean and dry burette was taken and rinsed with NaOH solution.
• The burette was filled with the 1.00 moldm^-3 NaOH solution using a funnel and to ensure that there are no bubbles, it was done slowly. The burette was filled till the mark of 0.00 cm³.
• A 100 cm³ conical flask was taken and 5.00 ± 0.05 cm³ of the vinegar solution was added to it using a graduated pipette.
• The flask was kept on a water bath and the temperature was set at 40.0°C. A laboratory thermometer was inserted inside the conical flask to monitor the temperature. The heating was continued till the temperature reached a mark of 40.0°C.
• After heating, the conical flask was kept on a mat on the table top to allow it to come down to room temperature.
• 2 drops of phenolphthalein was added to the same conical flask using a dropper.
• The vinegar in the flask was titrated against the NaOH solution running down the burette.
• The end point was detected by the appearance of a permanent pink color and the burette reading was noted.
• Steps 2-9 were repeated for four more times to obtain concordant readings.
• The same process was followed for other values of temperature- 60.00 ± 0.50°C, 80.00 ± 0.50°C and 100.00 ± 0.50°C. To get the reading at 20.00°C, the water bath was not used as the room temperature was 20.00° C.