Question

The following passage is adapted from an essay on the history of early twentieth-century pharmacology:

In 1915, chemist Alice Ball at the College of Hawaii developed a groundbreaking treatment for Hansen's disease (leprosy). Prior to her research, oil extracted from the seeds of the chaulmoogra tree (*Hydnocarpus wightianus*) was known to possess therapeutic properties, but it presented severe administration challenges. When ingested orally, the raw oil induced debilitating gastrointestinal nausea, causing patients to vomit before the medication could be absorbed. Subcutaneous injections of the unmodified oil were equally problematic; due to its extreme viscosity and insolubility in water, the oil pooled under the skin, creating painful lesions and sterile abscesses that refused to heal.

Recognizing that the active therapeutic agents were specific fatty acids within the oil—namely chaulmoogric acid and hydnocarpic acid—Ball sought a chemical technique to alter their physical properties without destroying their antibacterial efficacy. Working in a small laboratory equipped with a reflux apparatus and vacuum distillation units, she subjected the raw fatty acids to esterification. By reacting the fatty acids with ethyl alcohol in the presence of an acid catalyst, she converted the heavy, viscous triglycerides into lightweight ethyl esters.

However, a major chemical obstacle remained: separating the therapeutic ethyl esters from unreacted fatty acids and undesirable lipid byproducts. Ball discovered that by cooling the reaction mixture to exactly $5^\circ\text{C}$ in an ice-salt bath, the unreacted saturated fatty acids crystallized into solid precipitates, whereas the desired ethyl esters remained in liquid form. She then utilized a suction filtration assembly fitted with a Büchner funnel to isolate the liquid filtrate. Subsequent vacuum distillation yielded a purified, water-soluble ethyl ester derivative that could be safely injected into muscular tissue without causing subcutaneous clumping or systemic toxicity.

Despite her sudden illness and untimely death in December 1916 at age 24, Ball's supervisor, Dr. Arthur L. Dean, continued testing the derivative. However, Dean initially published the findings without crediting her discovery—a technique later coined the "Ball Method." It was not until 1922, when Dr. Harry T. Hollmann published a paper explicitly identifying Ball as the sole originator of the ethyl ester isolation procedure, that her contribution received formal scientific recognition. The Ball Method remained the primary treatment for Hansen's disease worldwide until the introduction of sulfone antibiotics in the 1940s.

According to the passage, what specific technique did Alice Ball use to isolate the liquid ethyl esters from the unreacted saturated fatty acids?