To Study How Addition of Acid and Salt Affects Milk Protein Coagulation and to Find Out How Heating Changes the Colour and Flavor of Milk

TITLE

Milk

AIMS

To study how addition of acid and salt affects milk protein coagulation and to find out how heating changes the colour and flavor of milk.

INTRODUCTION

The main composition of milk is water, which makes up around 80-88% of milk depending on the type of milk. Inside milk, there are over 250 types of chemical compounds present for example milk sugar lactose, water soluble vitamins, proteins, lipids, carbohydrates and minerals. Milk composition differs in different breed of animals and depends on the diet and environment of the animal (Damadoran 2008; Mehas and Rodgers 2006).

The two main proteins present in milk, casein and whey protein are dispersed as colloids in milk. 80% of bovine milk protein are casein proteins. Casein interacts with calcium phosphate and with each other to form casein micelle, a unique, highly hydrated spherical complex. The normal pH of milk is around pH6.6. At this pH, the casein micelles have a net negative charge hence they are unable to aggregate as like charges repel. pH of milk can be lowered by the addition of acid. When milk pH drops to about 4.6, the casein will break down and form lumps that separate from the liquid. At this point, the milk is said to have coagulated (Damadoran 2008; Mehas and Rodgers 2006).

When salt is dissolved in water, salt such as NaCl will dissociate into Na+ and Cl- ions. These ions have the ability to interact with electrical charges on the surface of milk proteins. As a result of this, the conflicting electrical charges on the milk proteins are reduced to an absolute minimum, favouring the denaturation and coagulation of milk proteins (McWilliams 2008).

When fats and casein are removed from milk, whey protein can be found in the liquid remaining. Whey protein is also known as serum protein and is heat sensitive. With prolonged heating, the whey protein (α-lactoglobulins and β-lactalbumins) will precipitate gradually. Heating will cause this susceptible protein to change from their native tertiary state to the secondary state. This causes an increase in number of sulfhydryl(−SH) exposed, contributing to a “cooked” flavor in heated milk. The precipitated whey protein interacts with lactose in milk in a process known as Maillard reaction causing colour and flavor change in milk (McWilliams 2007).  

Raw milk obtained directly from cow needs to be processed for it to be safe for consumption for humans. The study on how acid, temperature and salt affects milk can be used as a guideline in food production in order to maintain the quality and nutritive values of milk during processing. Raw milk has to pasteurized in order to kill microorganisms present. The study on how heat affects milk can be used to determine the temperature and heating time to prevent milk protein coagulation. This study can also be used to determine the best milk processing method to prolong milk’s shell life. This study concentrates on how milk protein would react upon varying pH, temperature and salt addition.

MATERIALS

UHT full cream milk, vinegar, pH meter, paper towel, distilled water, glass rod x1, 100mL beaker x4, solid sodium chloride, magnetic stirring hot plate, sweetened condensed milk, universal bottle x2, timer x1, Colourflex spectrophotometer, electronic balance, safety goggles x1, 1 pair of disposable gloves, 1 pair of heat resistant gloves, measuring cylinder, test tube rack, water bath at 90°C

METHOD

Part 1:

1. The initial pH of milk and vinegar in the 2 separate beakers was measured using pH meter. The electrode in the pH meter was thoroughly washed with distilled water and wiped using paper towel before measuring each solution.
2. The pH value obtained was recorded.
3. The vinegar was poured into the beaker containing milk. Glass rod was used to stir the 2 solutions together.
4. The pH of the mixture was measured and recorded using pH meter.
5. The mixture was allowed to stand for 30 minutes and observations were made after 30 minutes.
6. pH of the mixture was measured after 30 minutes.

Part 2:

1. 10g of solid sodium chloride was measured using an electronic balance.
2. The weighed sodium chloride was poured into the beaker containing 40mL of milk. The solution was stirred using a glass rod until all the salt have dissolved.
3. The pH of the solution was measured and recorded using pH meter.
4. The pH of 40mL of milk with no salt added in a separate beaker was also measured and recorded.
5. The two beakers, one containing milk with salt and one containing milk only was placed on the magnetic stirring hot plate set at no. 4.
6. The two solutions were heated until the solutions were boiling. After the milk had boiled for 5 minutes, the two beakers were removed from the hot plate.
7. The pH of both solutions was measured after the solutions had cooled to room temperature.

Part 3:

1. 4mL of distilled water was measured using a measuring cylinder and poured into the universal bottle containing 2mL of sweetened condensed milk. The solution was stirred using a glass rod.
2. The pH of the diluted and undiluted sweetened condensed milk was measured and recorded using the pH meter.
3. The colour of the diluted and undiluted sweetened condensed milk was measured using the colourflex spectrophotometer and the readings obtained was recorded.
4. The diluted and undiluted sweetened condensed milk was sniffed to find out the flavor of the condensed milk.
5. The two universal bottles was placed in a thermostatically stable water bath at 90°C for 90 minutes.
6. After 90 minutes, the samples were removed from the water bath and was cooled down to room temperature.
7. The pH, colour and flavor of the diluted and undiluted sweetened condensed milk was determined by using pH meter, Colourflex spectrophotometer and by sniffing

RESULTS

Table 1: Table showing the pH and observation of milk, vinegar and mixture of milk and vinegar.

Table 2: Table showing the pH and observation of milk and milk containing salt before and after boiling

Table 3: Table showing the pH, colour and flavor of diluted and undiluted sweetened condensed milk before and after heating in water bath