Culinary Science with Prof. Dr. Sibel Ozilgen: 5 Kitchen Experiments

The freezing point is the temperature at which a liquid changes to a a solid. When a solute, a substance such as sugar and salt that dissolves in a solvent, is added to water, it physically interrupts the intermolecular forces in the solvent. 

A solution freezes at a lower temperature than the pure solvent. This phenomenon is called freezing point depression. Foods freeze below 0°C (32°F) because they naturally contain solutes, such as fibers, fats, vitamins, and sugar.

The temperature at which a compound changes from a liquid to a vapor is called the boiling point. Boiling occurs when vapor pressure inside the liquid is equal to the atmospheric pressure. When a nonvolatile solute, such as sugar or salt, is added to the liquid, the vapor pressure is reduced. 

This means water molecules will need more kinetic energy to escape from the surface of the liquid. Therefore, the solution needs to be heated to a higher temperature to boil. This phenomenon is called boiling point elevation. The boiling points of solutions are higher than that of the pure water. Therefore, our foods boil at higher temperatures than pure water.

In general, foods that have sour taste, such as citrus fruits, kiwi, and tomato are considered acidic. Acidity is measured with a scale called pH (potential of hydrogen) that ranges from 0 to 14. Foods with a pH value of 7 are considered neutral, lower than 7 are acidic, and above 7 are basic.

Low-acidity foods have pH values higher than 4.6, while high-acidity foods have pH values equal to 4.6 or lower. Acidic foods contain increasingly higher concentrations of positively-charged hydrogen ions. Diluting with water, for example adding water into an orange juice decreases the concentration of hydrogen ion in the medium, hence the acidity. 

Analyzing the textural changes in spinach as a function of freezing rate is interesting. The size and amount of the ice crystals in foods can contribute to several sensory attributes, such as texture and mouthfeel. In general, a slow freezing rate results in large ice crystals. 

Large ice crystals in foods may rupture the food cells and may cause a loss of natural juice found in cells upon defrosting. The foods get dry and wilted. Large ice crystals also may give a very gritty and lumpy structure to some frozen foods, such as ice creams and frozen desserts. However, rapid freezing results in a large number of ice crystals. These ice crystals are usually smaller in size. Therefore, they cause less cell ruptures than large ice crystals. Rapid freezing is usually favored for the food freezing processes, vegetable and meat freezing being the primary examples.

The rheological differences between different starch types is interesting. Starch is a polysaccharide (long chains of glucose units) which is a major component of many plants. Seeds, cereals, roots, and the tubers are the main sources of starch. 

The most common starches in culinary processes are corn (maize), rice, wheat, potato, and tapioca (cassava). Thickening, body and texture formation, binding, coating, and moisture retention are the primary functions of starches in cooking processes. Chemical structure of the starch and the size of the starch granule determine the viscosity of the starch gel, its thixotropic (shear thickening) properties, the rate of gelatinization, the strength of the gel, and the gelatinization temperature. Each starch type has its own chemical structure and they are not interchangeable. Using the correct type of starch is important in culinary processes.

Observing the effects of mechanical force on egg white protein is interesting. The structure of the proteins can be affected by processing conditions. 

The most common physical and chemical factors affecting the protein structure in foods during food preparation and processing include: heat treatment, mechanical treatment (whisking, mixing, pounding), salt addition, pH change, enzymatic activities, sugar addition. For example, when raw egg whites, cream, and milk are beaten they form foams since, their protein structures unfold and the air bubbles are incorporated into the network formed by re-association of denatured protein molecules.