Explanation
The physical properties of food and food forms are closely related. In this module, we look at the relationship between them, first by covering the methods for assessing the physical properties of food according to its form, with various examples of such foods given. Then, we look at the various factors that change the physical properties and forms of food.
Explanation
To understand the relationship between the physical properties and forms of food, we need to have clearly defined terminology.
According to clinicians certified by the Japanese Society of Dysphagia Rehabilitation (JSDR), the physical properties of food are defined narrowly as mechanical properties. The term "food form" represents the external appearance and internal structure of food. Also, because the physical properties of food are affected by its form, the relationship between the two is often expressed by the term "texture".
Explanation
Food comes in many forms and has physical properties conforming to these forms. As the diagram shows, food form is divided into 3 categories. Food in each form is further classified as homogeneous or heterogeneous.
Explanation
This slide shows form-appropriate methods for measuring the physical properties of liquid foods. In terms of dispersion state, liquid foods exist in various forms from homogeneous to heterogeneous. Here we see the measurement methods suited to each form. Measurement of viscosity with a Brookfield viscometer and texture measurement are explained in slides 6 and 7 respectively. Cone-plate rotational viscometers and dynamic viscoelasticity measurement are limited to homogeneous liquid foods and are not explained here.
Examples of foods corresponding to each form are shown at the bottom. Liquid food forms range from homogeneous foods like water, through foods that are midway between homogeneous and heterogeneous such as jelly beverages containing gels (jelly) with loose structures, to liquid foods with solids dispersed in them such as corn soup containing corn kernels.
Explanation
Classifications of liquid food based on physical properties are easier to understand when represented in terms of viscosity. The terms "smooth" and "sticky" express viscosity. Homogeneous sols which follow Newton's law of viscosity are called Newtonian fluids; typical examples include juice and water. Non-Newtonian fluids, the viscosity of which depends on the measurement conditions, are represented as apparent viscosity. One example of a non-Newtonian fluid is potage. Newton's law of viscosity is an equation that represents the relationship between shear velocity, γ, and shear stress, S; the proportionality constant is viscosity, η.
Explanation
A Brookfield viscometer (photo) rotates a spindle set inside a beaker (at least 6 cm in diameter) containing roughly 200 mL of a sample and measures the resistance of the sample transmitted to the spindle as torque. Depending on the viscosity of the sample, the spindle may need to be changed. One widely used model allows the rpm of the spindle to be set to 4 different speeds. The convenience of this viscometer is that it's also capable of measuring the viscosity of heterogeneous fluids. Unlike a cone-plate viscometer, a Brookfield viscometer displays rpm rather than shear rate. The figure shows measurements for samples of 2 texture modified foods: one modified with guar gum and one modified with starch.
Explanation
Textural characteristics are often measured with a device like the one seen here. The plunger is set, a sample is loaded in the sample container to a thickness of 15 mm, and constant-rate compression is performed twice. On the right, we can see a texture profile analysis plot. Photo 1) shows the plunger at its lowest point, at which peak 1 is at its maximum value (h1); this represents hardness. Photo 2) shows the plunger raised after having compressed the sample. As the arrow shows, force is working in a negative direction; the area represented by a3, that is, workload, can be calculated as adhesiveness. C and D are conversion factors for hardness and adhesiveness, respectively, and are device-specific values. Cohesiveness is calculated by dividing the area under peak 2 (a2) by the area under peak 1 (a1). "Hardness" and "adhesiveness" represent the hardness and viscosity of foods, respectively; while "cohesiveness" represents the binding force within foods.
Explanation
Here we see methods for measuring the physical properties of semi-solid foods depending on the form of foods. In terms of dispersion state, semi-solid foods exist in various forms from homogeneous to heterogeneous. Here are the measurement methods suited to each form. See Slide 7 for measurement of texture.
Examples of foods corresponding to each form are shown at the bottom. Semi-solid foods range from homogeneous emulsions like mayonnaise, through foods that are midway between homogeneous and heterogeneous like scrambled eggs made from half-boiled chicken eggs mixed together, to semi-solid foods that can be considered liquids with solids dispersed in them like zen-gayu (rice porridge with a 1:5 ratio of rice to water).
Explanation
Classifications of semi-solid foods based on their physical properties are easier to understand when represented in textural terms such as "soft" and "hard". Soft semi-solid foods look like gels at a glance but flow when stirred and trickle down slowly from a spoon. Hard foods include foods such as mashed potatoes which are paste-like, can maintain their form, and can be molded into various shapes.
Explanation
This slide shows form-appropriate methods for measuring the physical properties of solid foods. In terms of dispersion state, semi-solid foods exist in various forms from homogeneous to heterogeneous. Here we see the measurement methods suited to each form. Measurement of breaking properties is not explained here, as such measurement often applies to mastication and so yields values similar to those for textural properties.
Examples of foods corresponding to each form are shown at the bottom. One example of a homogeneous solid food is silken tofu. However, different types of tofu have different forms; the process for making firm tofu, unlike silken tofu, involves hardening tofu gel with a coagulant, amassing the gel, and shaping it while wringing out the water in it, resulting in a somewhat heterogeneous form. Hamburger steak, which is made by heating and coagulating minced meat, onions, and other ingredients, exists in a form in which solids are hardened by proteins.
Explanation
The physical properties of solid foods can be classified in terms of texture as "soft", "hard", or "very hard". Solid foods exist in a wide variety of textures: soft solid foods collapse under their own weight when removed from their molds; hard solid foods must be chewed, and very hard solid foods are especially chewy. What all of these solid foods have in common is that they can't return to their original shape once that shape is lost.
Explanation
One factor that changes the physical properties of food is temperature. The graph on the left shows how the consistency (viscosity) of white sauce changes according to temperature. At 60°C, white sauce is smooth, but its viscosity increases roughly 3-fold when measured at 20°C. A classic example of a solid food that undergoes major changes in consistency due to temperature is gelatin jelly, which has a melting point of 22-25°C. When left out at room temperature, the initial temperature of gelatin jelly increases and it loses hardness, so temperature control is important for gelatin jelly.
Explanation
Another factor that changes the forms and physical properties of foods is the addition of hydrocolloids. Hydrocolloids are the main ingredients in gelling agents and thickening agents added to food. Because of their ability to improve the texture of food when added in small quantities, hydrocolloids are also referred to as "texture improvers".
Explanation
Homogeneous liquid foods are partially mixed with saliva and swallowed as-is. Heterogeneous liquid foods are mixed with saliva and transformed into heterogeneous liquid boluses. Semi-solid foods are also mixed with saliva as a result of mastication (tongue, hard palate, etc.) and are transformed into heterogeneous semi-solid boluses. Solid foods are mixed with saliva during mastication and transformed into heterogeneous semi-solid boluses. No matter the form of a bolus, its hardness is lower than that of the food itself (see Slide 15).
Explanation
This figure gives a specific example of changes in the properties of a solid food during eating. When the hardness of boluses of pork loin in 4 different forms (non-tenderized meat, meat tenderized with baking soda, meat layered in thin slices, and restructured meat in which ground meat is bound together with starch) was measured immediately before swallowing, the hardness of each form was roughly identical. As these results clearly demonstrate, humans chew food to adjust its physical properties, making it easier to swallow.
References
- Szczesniak, A.S., Classification of textural characteristics, J. Food Sci.,28, 385, 1963
- Tomoko Takahashi and Hiro Ogoshi, Effect of Thickener Characteristics on the Swallowing of Liquid Food, Journal of the Society, 50, 333-339, 1999
- Hiro Akabane, Hamako Osawa, Nobuko Nakahama, Rhealogical Properties of White Sauce during Cooking and Cooling Processes, Journal of the Society, 30, 845-850, 1979
- Tomoko Takahashi, Yoshie Nakagawa, Yukihiro Michiwaki, Aki Kawano, Miki Suzuki, Keiko Wada, Hiro Ogoshi, Textural Properties of Meat and Human Chewing Movements Involved, Journal of the Society, 55, 3-12, 2004
Recommended readings
- Hiro Ogoshi & Hiroko Sinagawa: Konko to Chori no Science, Gakubunsha, 2010
- Nobuko Nakahama, Hiro Ogoshi, Hatsue Moritaka: The Rheology and Texture of Food, IK-Publishing, 1997
- Shokukaigo Kenkyukai Ed:Considering swallowing disorders., Kazan, 2007
