Milk going sour is a chemical change. This transformation involves fundamental alterations to the milk’s composition, resulting in new substances. The process cannot be easily reversed to its original state once it has occurred.
What Defines a Chemical Change?
A chemical change, also known as a chemical reaction, occurs when one or more substances are transformed into new substances. This process involves the rearrangement of atoms, leading to a change in chemical properties or composition. Unlike physical changes, which only alter a substance’s form or appearance without changing its chemical identity, chemical changes create new products that possess different characteristics from the original materials. For instance, burning wood is a chemical change because it produces ash, smoke, and gases, which are entirely new substances. In contrast, melting ice into water is a physical change, as the water molecules remain chemically the same, only changing their state from solid to liquid.
The Transformation Within Souring Milk
The souring of milk begins with the activity of microorganisms, primarily lactic acid bacteria. These bacteria naturally present in milk consume lactose, the sugar found in milk, as their energy source. Through a metabolic process called fermentation, they convert this lactose into lactic acid. The production of lactic acid increases the acidity of the milk, dropping its pH level.
This increased acidity then affects the milk’s proteins, particularly casein, which makes up about 80% of milk proteins. Casein proteins normally exist as suspended structures called micelles, contributing to milk’s smooth texture and opaque appearance. As the lactic acid concentration rises and the pH drops below 4.6, these casein proteins undergo a process called denaturation, where their three-dimensional structures unfold. This unfolding causes the proteins to coagulate, forming curds that separate from the watery component known as whey.
Observable Signs of Milk’s Chemical Shift
The chemical transformations within souring milk lead to noticeable changes in its appearance, smell, and taste. The change in odor is a key sign, as new chemical compounds, including lactic acid, give the milk a sour smell. The taste shifts from sweet to tangy or sour due to the increasing concentration of lactic acid.
Beyond sensory changes, the texture of milk also undergoes alteration. The coagulation of casein proteins results in the milk thickening and forming visible curds, separating from a liquid. These observable changes—altered smell, sour taste, and curdled texture—confirm that new substances have formed and the milk’s original chemical composition has been fundamentally altered.