Food materials possess physical properties. Physical properties are those properties that can be observed or measured without changing the chemical makeup of the material. Physical properties can give us clues about the chemical composition and processing characteristics of food. A few common properties are discussed here.
Freezing, Melting, and Boiling Point
Freezing point (Fp) is the temperature at which a liquid turns into a solid when it is cooled. Melting point (Mp) is the temperature at which a solid melts, and boiling point (Bp) is the temperature at which a liquid turns to vapor. A good reference to use to compare these temperature points against, is water. The freezing and melting point of water is 0 ºC (32 ºF) and it has a boiling point of 100 ºC or 212 ºF. Adding solids to water, such as sugars and salts will increase the boiling point. Therefore getting burned with hot syrup will cause much more injury than getting burned from hot water. Adding solids to water causes the freezing point to drop. That means more energy has to be removed from the liquid to cause freezing. We see this effect in ice cream where the presence of sugar causes the ice cream to remain soft at below-freezing temperatures without becoming too hard and icy. In the winter, salt is put on the road to reduce the freezing point of water. With the presence of salt, the water stays at a lower temperature without being able to freeze.
Alcohol has a lower boiling point than water causing it to evaporate quickly. For example, the boiling point of ethanol is 78.37 ºC (173.1 °F). Alcohols are used in hand sanitizers because they not only kill bacteria but evaporate and dry off your hand without the need to use a towel. Cooling oils have high boiling points, exceeding 300 ºC (572 ºF) in some cases. Hence they are used in frying. Getting splashed with hot oil can cause severe injury.
Heat Transfer
Heat transfer, as the name suggests is the ability of heat to be conducted through the food. This is important for cooking times and rates, and the ability of heat to penetrate and sterilize food effectively. Many factors affect heat transfer rates in food. For example,
- Specific heat capacity: A measure of the amount of heat that must be absorbed to change the temperature of the material by 1 degree. For example, water has a high specific heat capacity compared to fat. Therefore, butter will heat up faster in a microwave due to the lower heat capacity. Essentially, a lower heat capacity means that it uses less energy to heat up.
- Thermal conductivity: A measure of how well heat is carried across the thickness of a material. If the value is high, heat is transferred faster through the materials and slower if the value is low. The presence of air, lowers thermal conductivity, while higher water content and density increase thermal conductivity.
- Thermal diffusivity: A measure of the ability of a material to conduct heat relative to its ability to store heat. In other words, it’s the ability of a material to carry heat from one point to the next without getting hot easily. Ice and frozen foods are good examples of foods that have very high thermal diffusivity.
Size and Thickness
The size and thickness of fresh produce are influenced by genetics and the environment in which they are grown in. For example, tomatoes can be large or plum-sized depending on the variety. However, with each variety, size may vary depending on the availability of water, nutrients, and other environmental conditions affecting its growth. During processing, the size and thickness of food products can be manipulated to achieve specific goals. For example, reducing the cost of handling and transportation, improving food texture, and improving heat transfer rate. For example, tomato juice, fruit pulp, and minced meat take up less space and hence will cost less to transport. Milling of flours will produce finer particle-size grains which can affect the texture and mouthfeel of the products they make. Heat transfer rate, that is, the rate at which heat is carried across a material is affected by size and shape. Small particles and thinner thickness will facilitate faster heat transfer. This means that the food will cook faster and will kill more pathogens (e.g. bacteria and viruses) during the heating period.
Deformation
Food materials have the ability to fracture and be reduced into smaller particle sizes. This property is taken advantage of in grinding, pulping, and milling. The ability of a material to deform can give you an idea of its internal structure and moisture content. Hard and finer materials have fewer lines of weakness in them and therefore will take more energy to deform.
Density and Specific Gravity
Density is the ratio of the density of a material to its volume. It is calculated as mass divided by volume. The unit is g/cm3 or kg/m3. It tells you how compact a food material is. For example, a gooey candy bar will be much denser than a slice of bread that is very airy and spongy. Hence, density is influenced by how closely the material is packed together, and how much air, water, fats, and solids are incorporated. Foods containing more air will be lighter. For example, ice cream is light and fluffy (low density) due to air incorporation. In addition, the presence of milk fat which has a low density, also makes it feel light rather than compact. Removal of water from food increases the proportion of solids and hence compactness. Fruits are pulped and converted to fruit concentrates by removing some of the water which serves to increase density, increases rates of heat transfer, improves shelf life, and lower the cost of transportation of the material.
Specific gravity is a similar measure to density but it is unitless. It is calculated by dividing the density of the material by the density of water (1 g/cm3). Therefore the specific gravity of the material ends up being the same value as the density without adding the unit. The term specific gravity is generally used when working with liquids. For example in the beer industry, the specific gravity of wort extract from barley is monitored to determine alcohol content. The lower the specific gravity, the higher the alcohol content.
Refractive Index
The refractive index of a substance refers to its ability to bend light. Put a pen or a pencil in a glass of water. What do you notice? Does it appear as if the pen is bent? This is due to light refraction. As light passes from one medium (air) to the next (water), light speed slows down and bends. Hence the light reflecting off the pen to your eyes makes it look like the pen is bent too. The more dissolved solids present in food, the greater its influence on light bending. Hence, the refractive index of syrup will be greater than pure water. Refractive index is used to quantify the amount of sugar in a liquid food product using a refractometer. A liquid or semi-solid sample is placed in the refractometer where light passes through the sample. Depending on how much the light bends, we know how much sugar is present.
Water Activity (aW)
Water activity refers to how available water is in a food material. Water can either be in a bound state or a free form. If free, it is readily available to take part in chemical reactions or supply microorganisms with food to grow. Water activity ranges from 0 to 1 where 0 means no free water (bone dry) and 1 means maximum free water (pure water). The shelf life (how long it keeps before spoiling) of food is heavily influenced by aW. The graph below illustrates how the growth of microorganisms and certain chemical reactions are effected by aW.
Source: Labuza, T.P., S.R. Tannenbaum and M. Karel. 1970. Water content and stability of low moisture and intermediate moisture foods. Food Technology 24:543-550.
Viscosity
Viscosity is used when referring to the thickness of liquid foods. It is the internal resistance to flow. It’s basically how thick or how runny a liquid is. For example, water has a low viscosity (runny) compared to ketchup or corn syrup (thick). When studying viscosity, several flow behaviors are usually discussed. These include:
- Newtonian flow: Flow property where the material keeps the same thickness no matter how much you mix it e.g. water and most juices
- Pseudoplastic flow: The fluid becomes thinner the more you mix it e.g. yogurt
- Dilatant flow: The fluid becomes thicker the more you mix it e.g. corn starch suspension
- Bingham Plastic: A fluid that will flow only after a certain stress is added e.g. ketchup
- Viscoelastic flow: A fluid that can flow like a liquid and also has bouncing properties like an elastic. However, unlike true elastics, the fluid does not go all the way back to its original form after stretching.
