01. Introduction
02. Grammar/Activities:
03. Reading Section
04 Listening Section:
05. Speaking Section:
06. Writing Section:
07 Practice TOEFL tests

Listening Test 1.11


Professor – Drying or desiccation is known to preserve bio-material including food. In freeze-drying, water is evaporated in a frozen state in a vacuum by means of sublimation–the process of changing a solid to a vapor without first changing it to a liquid. In 1811, John Leslie had first caused freezing by means of the evaporation and sublimation of ice. In 1813, William Wollaston demonstrated this process to the Royal Society of London. It does not seem to have occurred to either Leslie or Wollaston to use sublimation for drying. That distinction goes to Richard Altmann, a German histologist who dried pieces of frozen tissue in 1890. Later in 1903, Vansteenberghe freeze-dried the rabies virus. In 1906, Jacques-Arsène d’Arsonval removed water at a low temperature for distillation. Since water removal is the essence of drying, d’Arsonval is often credited with the discovery of freeze-drying, but the first clearly recorded use of sublimation for preservation was by Leon Shackell in 1909. His work was widely recognized and he freeze-dried a variety of biological materials. The first patent for freeze-drying was issued to Henri Tival, a French inventor, in 1927. In 1934, William Elser received patents for a modern freeze-drying apparatus that supplied heat for sublimation. In 1933, Earl W. Flosdorf had freeze-dried human blood serum and plasma for clinical use. The subsequent efforts of Flosdorf led to commercial freeze-drying applications in the United States. With the freeze-drying technique fairly well-established for biological products, it was a natural extension for Flosdorf to apply the technique to the drying of foods. As early as 1935, Flosdorf experimented with the freeze-drying of fruit juices and milk. An early British patent was issued to Franklin Kidd, a British inventor, in 1941 for the freezing and the freeze-drying of foods. An experimental program on the freeze-drying of food was also initiated at the low-temperature research station at Cambridge University in England but until World War II, freeze-drying was only an occasionally used scientific tool. It was the desiccation of blood plasma from the frozen state performed by the American Red Cross for the U.S. armed forces that provided the first spectacular, extensive use of freeze-drying. This work demonstrated the vast potential of freeze-drying for commercial applications. In 1949, Flosdorf published the first book on freeze-drying, which laid the foundation for freeze-drying of foods and remains one of the most important contributions to large-scale operations in the field. In the book, Flosdorf described the freeze-drying of fruit juices, milk, meat, oysters, clams, fish fillets, coffee and tea extracts, fruits, vegetables, and other products. Flosdorf also devoted an entire book to the describing of the equipment used for both batch and continuous processing and he discussed cost analysis. The holder of more than 15 patents covering various aspects of freeze-drying, Flosdorf dominated the move towards commercialization in the United States. Simultaneously, researchers in England were developing freeze-drying applications under the leadership of Ronald I.N. Greaves. The food crisis during World War II had led to the recognition that dried foods cut the cost of transport, storing, and packaging foods in times of emergency. Thus, in 1951, the Ministry of Food Research in England was established at Aberdeen, Scotland. Scientists at Aberdeen developed a vacuum contact plate freeze-dryer that improved product quality and reduced the time required for rehydration–replacement of the water removed in the freeze-drying process so that the food can be used. In 1954, trials of initial freeze-drying followed by the ordinary process of vacuum drying were carried out. The abundance of membranes within plant and animal tissues was a major obstacle to the movement of water vapor, thus limiting the drying rate. In 1956, two Canadian scientists developed a new method of improving the freeze-drying rate for steaks by impaling the steaks on spiked heater plates. This idea was adapted in 1957 by interposing sheets of expanded metal instead of spikes between the drying surfaces of the frozen food and the heating platens. Because of the substantially higher freeze-drying rates that it achieved, the process was called accelerated freeze-drying. In 1960, Greaves described an ingenious method of freeze-drying liquids. It involved continuously scraping the dryer layer during its formation. This led to a continuous process for freeze-drying liquids. And during the remainder of the 1960s, freeze-drying applications proliferated with the advent of several techniques for controlling and improving the effectiveness of the freeze-drying process. Flosdorf’s vision and ingenuity in applying freeze-drying to foods has revolutionized food preservation. He was also responsible for making a laboratory technique a tremendous commercial success. Freeze-drying is important because it stops the growth of micro-organisms, inhibits deleterious chemical reactions, and facilitates distribution and storage. Freeze-dried foods are easily prepared for consumption by adding water, rehydration. When freeze-dried properly, most foods either raw or cooked can be rehydrated quickly to yield products that are equal in quality to their frozen counterparts. Freeze-dried products retain most of their nutritive qualities and have a long storage life even at room temperature. Freeze-drying is not, however, without disadvantages. The major disadvantage is the high cost of processing. Thus, to this day, the great potential of freeze-drying has not been fully realized. The drying of cell-free materials such as coffee and tea extracts has been extremely successful but the obstacles imposed by the cell membranes in foods such as fruits, vegetables, and meats have limited the application to expensive specialty items such as freeze-dried soups and to foods for armies, campers, and astronauts. Future economic changes may create a situation in which the high cost of freeze-drying is more than offset by the cost of transportation and storage.