Milk to Curd
Converting milk into curd involves a series of internal actions facilitated by the addition of a starter culture, typically containing lactic acid bacteria. Here’s a detailed explanation of the process:
Heating the Milk
Purpose: Heating milk to about 85-90°C (185-194°F) serves multiple purposes:
Destroys Pathogens: Eliminates harmful microorganisms that could interfere with fermentation.
Denatures Proteins: Changes the structure of milk proteins, especially casein, making them more amenable to coagulation.
Cooling the Milk
Purpose: After heating, the milk is cooled to a temperature suitable for the starter culture, typically around 40-45°C (104-113°F). This is the optimal temperature range for the bacteria to thrive and ferment the milk.
Inoculating with Starter Culture
Purpose: Adding a starter culture, which contains live bacteria like Lactobacillus and Streptococcus species, introduces the necessary microorganisms for fermentation.
Actions:
Bacterial Growth: The bacteria begin to multiply and metabolize lactose (milk sugar) into lactic acid.
Acidification: As lactic acid accumulates, the pH of the milk decreases.
Fermentation
Purpose: The fermentation process is where milk transforms into curd.
Actions:
Casein Coagulation: The lowered pH causes casein proteins, which are normally dispersed in milk, to coagulate. This forms a gel-like network that traps fat and other components, leading to the formation of curd.
Texture Development: The curd solidifies into a semi-solid form due to the coagulation of casein, resulting in the separation of whey (the liquid part).
Ripening
Purpose: After curd formation, it is left to ripen for a few hours. This stage allows the development of flavor and texture.
Actions:
Further Fermentation: During ripening, bacterial enzymes continue to act on the curd, enhancing its flavor and texture.
Acid Concentration: The acidity continues to rise slightly, leading to the curd becoming firmer and tangier.
Cooling and Storage
Purpose: After the desired texture and flavor are achieved, the curd is cooled to stop further fermentation.
Actions:
Preservation: Cooling helps in preserving the curd and maintaining its consistency. It is then stored in a refrigerator to prevent spoilage.
Summary of Internal Actions:
Protein Denaturation: Heating milk denatures proteins, preparing them for coagulation.
Lactic Acid Production: Starter cultures ferment lactose into lactic acid, lowering the pH.
Protein Coagulation: Acidification causes casein proteins to coagulate, forming curd.
Flavor and Texture Development: Bacterial enzymes continue to enhance flavor and texture during ripening.
Cooling and Storage: Final cooling and storage maintain the curd’s quality.
This process of converting milk to curd is a combination of biochemical reactions and physical changes that result in the transformation from liquid milk to a semi-solid dairy product with a distinct taste and texture.
The conversion of milk to curd involves several key chemical changes:
Protein Denaturation
Process: When milk is heated, proteins, particularly casein and whey proteins, undergo denaturation.
Chemical Change: Denaturation involves the unfolding and restructuring of protein molecules due to the application of heat. This makes proteins more susceptible to interactions that lead to coagulation.
Lactose Fermentation
Process: Starter cultures, primarily containing lactic acid bacteria such as Lactobacillus and Streptococcus species, ferment lactose (the sugar in milk).
Chemical Change: Lactose is converted into lactic acid through bacterial fermentation. The chemical reaction is:
pH Decrease
Process: As lactic acid accumulates, the pH of the milk decreases.
Chemical Change: The decrease in pH results in the formation of more acidic conditions. This acidic environment is crucial for the next step in curd formation.
Protein Coagulation
Process: The decrease in pH leads to coagulation of casein proteins.
Chemical Change: Casein proteins, which are normally dispersed in milk, aggregate to form a gel-like network. This occurs due to the isoelectric point of casein (pH ~4.6), where casein molecules aggregate to minimize their solubility. The main reaction involves:
Whey Separation
Process: As casein coagulates, it forms curds and separates from the whey.
Chemical Change: The curd is composed of coagulated casein proteins, fats, and other suspended solids, while whey consists of water, lactose, soluble proteins, and minerals. The separation is a result of the coagulation process that removes casein from the liquid state.
Flavor and Texture Development
Process: During ripening, bacterial enzymes act on the curd to develop flavor and texture.
Chemical Change: Proteolysis (breakdown of proteins) and lipolysis (breakdown of fats) occur:
Proteolysis: Bacterial enzymes break down casein proteins into smaller peptides and amino acids, contributing to flavor development.
Lipolysis: Fats in the curd are broken down into fatty acids and other compounds, further influencing flavor.
Summary of Chemical Changes:
Protein Denaturation: Heating alters the structure of milk proteins.
Lactose Fermentation: Lactose is converted into lactic acid by bacteria.
pH Decrease: Lactic acid lowers the pH of milk.
Protein Coagulation: Acid-induced aggregation of casein proteins forms curd.
Whey Separation: Coagulated proteins and fats separate from the liquid whey.
Flavor and Texture Development: Enzymatic reactions further modify proteins and fats.
These chemical changes collectively transform liquid milk into curd, altering its structure, texture, and flavor.
