In a broad sense, the physical properties of food may be defined as those properties that lend themselves to description and quantification by physical rather than chemical means. Their importance stretches from product handling to processing to consumer acceptance. Most of the early advances in this area have been made in the context of agricultural products with much credit due to agricultural engineers, quality control personnel and agriculture endorsed universities. More recently, food process engineers and food scientists interested in this area have made significant contributions. Although much still needs to be researched and endorsed, these efforts have resulted in a sizable body of organized knowledge.
Physical properties of food are aspects such as colour, structure, texture, rheology and interfacial properties, and composition. There is a range of instrumental methods for objectively characterising and measuring food structure and physical properties. These are useful for applications such as new product development, benchmarking, reformulation and specification.
These properties include:
- Color: Although only a few definitive studies have been done on the effect of color on consumer acceptance, there is much experimental evidence that it is one of the most important quality attributes. It can signal a high quality product (such as the golden yellow of orange) or can alert the consumer to a potential physiological danger (such as green processed meat). Also, color has important psychological connotations that can influence the mood of and emotional state of humans.
- Volume, Density and Surface Area: Volume and density measurements of liquid foods present no special problems, other than the proper control of temperature at which the measurements are made. The situation is more complicated with solid foods, especially those of a porous nature. Volume of agricultural products, especially those exhibiting an irregular shape, is usually determined by water displacement.
- Structure: The structure of food influences texture. Examples include porous products such as aerated foods and bakery products where the bubble structure affects softness, and starch-based snacks where it affects crispiness.
- Size and Shape: We can differentiate here, two general cases: one, food products such as agricultural commodities, in which the shape and size can be differentiated with the naked eye. Two, food powders such as ground coffee, salt and milk powder in which the differentiation of shape and size can be best done with the aid of magnifying glasses. They influence packaging, distribution of stresses when forces are applied and processibility.
- Texture: Food texture is an important sensory attribute as it affects the way food tastes and how it feels in the mouth. The texture depends on the rheological properties of the food and evaluation involves measuring the response of a food when it is subjected to forces such as cutting, shearing, chewing, compressing or stretching.