Electrostatic Interactions Opposites attract is a good way to think of the interaction between molecules that are charged. Molecules that have one or more charged atoms will be attracted to an oppositely charged group on another molecule. Proteins have many different kinds of functional groups, in which several have the potential to be charged, including carboxylic acids (─COO− ) and amines (─NH3 ). Electrostatic interactions govern the behavior of the milk protein, casein. Molecules of casein have carboxylic acid groups that coat each milk fat droplet with negative charges. Because of the negative charges, the fat droplets in milk will repel one another, reducing the possibility of aggregation of the droplets and curdling of the milk. Thus the key electrostatic interaction, in this case, is repulsion or lack of an interaction, which allows the fat to remain suspended in the milk liquid. Hydrophobic Interactions Hydrophobic interactions are forces that are of particular importance for food molecules that are in a water (aqueous) environment. Plant and animal tissues are rich in water. Animal muscle is made of nearly 70% water, while plant water content ranges from 75 to 90% of total mass. Thus, the proteins, sugars, fats, and other compounds in our bodies and plants are constantly exposed and surrounded by water molecules.
Compounds that have a charge (full or partial) will interact with the water molecules via hydrogen bonding or electrostatic‐like interactions; they easily dissolve and remain suspended in this water or aqueous environment. However, some molecules, like fats, have no charge and cannot hydrogen‐bond or be involved in electrostatic interactions. These molecules tend to clump or aggregate together to “hide” from the water surroundings; this phenomenon is called the hydrophobic effect. Molecules (or regions of molecules) that have no charge and do not participate in hydrogen bonds are considered nonpolar; the hydrophobic interaction brings these molecules together to “avoid” interacting with water molecules. Why does this interaction take place? Consider two hydrophobic molecules.
When first placed into water, each hydrophobic molecule becomes surrounded by a shell or cage of water molecules. Why does the water form a cage? Because there are minimal favorable interactions (such as hydrogen bonding or electrostatic interactions) between the hydrophobe and the water, any water molecule that does interact organizes itself in the caged format to reduce the number of water molecules that have to interact with the hydrophobe. This allows more water molecules (in the entire solution) to remain in a disordered or random array. The scientific term for disorder or randomness is entropy. The more entropy within the system, the better. Thus, in this type of a system, entropy can be increased further through a “clumping” of all of the hydrophobic molecules together.