This Experiment Is Conducted to Study the Effect of Acid and Salt on Milk Protein Coagulation and to Examine Changes in Colour and Flavor of Milk During Heating

Title: Milk  

Aim: This experiment is conducted to study the effect of acid and salt on milk protein coagulation and to examine changes in colour and flavor of milk during heating.

Introduction:

Milk is liquid by the mammary glands of mammals that consist of 87%-88%water, carbohydrates such as disaccharide lactose, whey proteins and fats (Dunham, 1925). Casein is the most abundant protein within milk as it makes up to 80% of milk protein (Dunham, 1925).  While the rest of whey protein such as lactalbumins or beta-lactoglobulins (Dunham, 1925). Condensed milk is milk without water content thus longer time taken to spoil (Kannan and Jenness, 1961).

Curdling of milk involves denaturation and coagulation of milk proteins by decreasing pH of milk to the isoelectric point of casein (Annand, 1971).  At acid coagulation of milk, casein micelle properties are altered by a lowered milk pH (Lucey & Singh, 1997). This causes colloidal calcium phosphate to dissociate from the micelles and the negative charges in the casein micelles are neutralized with aggregation occurring as the isoelectric point of the casein micelle (pH6.4) is approached (Annand, 1971).

Factors affecting milk protein coagulation includes the addition of sodium chloride as it reduces the stability of casein molecules (Annand, 1971). When salt is heated with milk, the salt ions increases the collision of particles which leads to stronger hydrophobic bond thus causes denaturation of milk protein (Annand, 1971).

Heating condensed milk decreases pH, causes cooked flavor and browning of the milk (Ferretti, 1973).  Maillard reactions are of paramount importance for the flavour and colour of evaporated milk (Anema and Li, 2015). The “cooked” flavor of UHT milk is associated with the presence of a variety of Sulphur containing compounds while the “stale” flavor is characterized by the dissipation of these sulphur volatiles and an increase of the formation and presence of both methyl ketones and aliphatic aldehydes over time (Ferretti, 1973).

Materials

Part 1: UTH full cream milk, vinegar, pH meter, kim wipes, distilled water, 100mL beaker, glass rod

Part 2: UTH full cream milk, sodium chloride, magnetic stirring hot plate, paper towel, distilled water, 100mL beaker

Part 3: Sweetened condensed milk, universal bottles, test tube rack, timer, 90oC water bath, pH meter, distiller water, kim wipes, pippete, Colourflex spectrophotometer

Methods:

Part 1

1. 40mL of UHT full cream milk was poured into a 100 mL beaker and the pH and its initial appearance was recorded.
2. 20mL of vinegar was measured in a separated beaker and the vinegar was mixed into the milk.  
3. The mixture was stirred well and pH was measured.  
4. The mixture was left to stand for 30 minutes and the observation was recorded.

Part 2

1. 40mL of milk added into a 100mL beaker and added with 10g sodium chloride.  
2. The mixture was stirred to ensure all sodium chloride has dissolved in the milk.  
3. Then, the pH and appearance of milk were recorded.  
4. Next, the milk mixture was heated and boiled for 5minutes and cooled to room temperature.  
5. The pH of the milk was then measured and observation was recorded. Part 2 was repeated once again however without addition of sodium chloride.

Part 3

1. 2mL of sweetened condensed milk was pipetted into a universal bottle and diluted with 4mL of water and the pH of diluted milk was recorded.  
2. Then three respective 10mL of sweetened condensed milk was pipetted into three different universal bottle and closed with lid.  
3. The colour and flavor of the samples were recorded.
4. Next, the colour of milk was measure with Colourflex spectrophotometer by filling sample cup with ¼ content and measuring the L*, a* and b* readings.  
5. In addition, three universal bottles were placed in a rack and into 90oC water bath.
6. After 90minutes, the changes in pH, colour and flavor of sample was recorded.

Results:

Part 1

Part 2

Part 3