Food Solids

Amorphous and Crystalline Solids and the Properties of Solid Foods Background All matter, including food, can exist in different physical states. Some of these states, such as the crystal (ice), liquid (water), and vapor (steam) states of water (H2O), are truly stable states that all matter can exist in (these are called thermodynamic phases by chemists). Other states, such as the hard/dry (glass) and soft/viscous (rubber or melt) states of noodles and pasta and hard candies, are not true stable states characteristic of all matter but are rather peculiar to materials such as foods and most plastics that can form non-crystalline (amorphous) solids.

Stable (thermodynamic) phases Water, like all pure substances, can exist in crystalline solid (s), liquid (l), and vapor states (v). These states are stable over specific temperature ranges. We can represent this schematically as follows (Tm is the melting temperature and Tb is the boiling temperature). H2O(s) T  Tm

 Tm = 0C

H2O(l) Tm  T  Tb

 Tb = 100C

H2O(v) T  Tb

The phase transitions connecting these different states of water have characteristic features. 1) The transition occurs at a specific and defined temperature (0, 100C). 2) Two phases can co-exist (ice plus water, boiling water plus steam) only at the phase transition temperature. This means, for example, that liquid water in equilibrium with ice is always at exactly 0C under one atmosphere of pressure, and that boiling water (water in equilibrium with steam) is always at exactly 100C under one atmosphere pressure. 3) The transition between phases involves a large flow of heat into (ice to water; water to steam) or out of (water to ice; steam to water) the material. 4) Each transition involves a large change in molecular order: liquids have less molecular order than crystals; vapors have much less molecular order than liquids. This is actually the most salient feature of phase transitions. True phase transitions involve a change in the molecular order of a material.

Glass and Rubber: States of Matter Some materials, such as sugars, carbohydrates, proteins, and most synthetic polymers (plastics), do not readily form crystals. In these materials, crystallization is often “frustrated” by either the high viscosity of the liquid (sugars) or by entaglements of the polymer that prevent molecules coming together in the proper orientation to form crystals. In these materials, rapid cooling does not result in crystallization. As the temperature of these liquids drops below the melting temperature (Tf), the liquids become more viscous. This gradual increase in viscosity as the temperature decreases eventually generates a liquid whose viscosity is so high that the liquid behaves like a soft solid; this soft solid is in a rubbery state. The rubbery state of a solid is flexible and soft but can support its own weight. As the temperature decreases, however, the rubber will undergo a distinct transition in which the viscosity will increase dramatically (perhaps by greater than 1000-fold in a temperature interval of 10-20C) to form a glassy solid that is hard and brittle. The temperature (midpoint) of this transition is called the glass transition temperature Tg. The two states have the following characteristic properties. Glassy solid: hard and brittle; stable to chemical and physical change; nearly complex absence of molecular motion (other than vibration); complete absence of macroscopic flow. Rubbery solid: viscous soft and flexible; unstable to chemical and physical change (albeit slow); presence of molecular motion; macroscopic flow is slow but occurs. If the material is a polymer the state is called rubbery; if not, it is called a supercooled liquid. The glass transition temperature is an important temperature for the material because it determines many of the physical properties and the chemical stability of the material. For a plastic, the glass transition temperature will determine whether the plastic is glassy (hard) or rubbery (soft) under the conditions of normal use (room temperature); an application requiring a hard, rigid material will need a polymer with a Tg > room temperature, while an application requiring a soft, flexible material will need a polymer with a Tg < room temperature. For foods, the Tg is also an appropriate reference temperature for stability. Foods are stable for long periods of time at T < Tg (years perhaps) and are stable for much shorter periods at T > Tg (hours to days). Examples include noodles and other pasta products, breakfast cereals, dried soups, puddings, and other mixes, and corn, potato, and other chips. The Tg also influences the texture of the food. Glassy foods (those with Tg > temperature of consumption) are crispy and brittle (potato chips); rubbery foods are soft and flexible (cooked pasta). The glass transition temperature of foods is very sensitive to the amount of water in the food. The more moisture, the lower the Tg; the less moisture, the greater the Tg. For this reason, dry foods are usually glassy at room temperature while moist foods are usually rubbery. The hydration of dry foods will dramatically lower their Tg; the effect is such that addition of about 20% water (by weight) can lower Tg by over 100C. This is why potato and corn chips will become soft and flexible if allowed to absorb moisture from the air (on a humid day), why glassy breakfast cereals become soft and flexible in milk, and why pasta and noodles become soft and flexible after boiling in water for 5-10 minutes.

Molecular Order and Molecular Motion Liquids are both similar to and different from solids. Both are condensed phases: the molecules in liquids and in solids are in contact with one another (compare a solid or liquid with a gas or vapor in which the molecules are not in contact). But the phases are obviously different: liquids can flow while solids cannot. The molecular origins of the differences between liquids and solids lie in two aspects: molecular order and molecular motion. Molecular order. Liquids are disordered. The molecules in a liquid, although in contact, do not exhibit any long range molecular order (they exhibit short range order just due to the fact that they are in contact and thus, on average, all molecules are the same distance from their neighbors). Liquids typically become solids by crystallizing. In the crystal, the molecules are packed into regular three-dimensional arrays; imagine rows of desks arranged in three dimensions. Each molecule is fixed in space due to specific and exact interactions with its neighbors. There is long range order: molecules a great distance apart have specific spatial relationships with one another. In amorphous solids, either glassy or rubbery, molecules are not in regular arrays; the solids are not crystalline. Amorphous solids are thus liquid-like in that they have the molecular order (or, rather, disorder) of a liquid. Molecular motion. The molecules in a liquid are in rapid motion; liquids can flow because the molecules can move in space with respect to one another. This type of motion in space is referred to as translational motion. The molecules in a crystal, however, cannot move in space; they thus exhibit no translational motion. Molecules can also rotate about their centers of gravity and they can vibrate if they have two or more atoms (the more atoms in a molecule the more ways that the molecule can vibrate). Due to the constraints of the regular contacts present in a crystal, the molecules cannot rotate. They can, however, vibrate in place. In amorphous glassy solids the molecules move as they do in a crystalline solid: they can vibrate in place but they cannot rotate and cannot move through space (translate). Amorphous glassy solids are thus crystal-like in that the molecules in them cannot move by rotation or translation. At the glass transition temperature the molecules begin to undergo rotational and translational motion; this is the physical basis for the increase in viscosity and the onset of flow that occurs at the Tg. We can thus say clearly and unambiguously that amorphous solids are like liquids in that their molecules are disordered; and that glassy solids are like crystals in that their molecules cannot rotate or translate. The ambiguous and complex nature of amorphous solids makes them truly an unusual state of matter.

Glassy and Crystalline Foods Class Discussion Goal of the exercise The group will examine several common solid foods, discuss their physical properties, determine whether the molecules in them are in the crystalline, glassy, or rubber states, and discuss how the physical state influences their stability, their texture, and their quality.

Materials Hard candy (lemon drops, life savers, etc.) Chocolate (Hershey's kisses, chocolate bar, etc.) Crackers Table salt Table sugar Soft plastic item (child's toy, etc.) Hard plastic item (plastic fork/spoon/knife, toy, etc.) Rubber bands Butter or margarine: both hard (cold) and soft (room temperature) Noodles or pasta: both uncooked and cooked Breakfast cereal (Chex, Grape Nuts, Shredded Wheat, etc.) Potato or corn chips Glass item (drinking glass, etc.) (Other items as appropriate based on your understanding of the module) Magnifying glass or dissecting microscope

Class Discussion Although all of the materials in the list above are solid, they obviously have very different physical properties. What are these physical properties? Discuss with your class the physical properties of these materials by asking them what words describe their physical properties. The following terms should be mentioned (as well as some more): Hard Brittle Soft Flexible (pliable) Elastic Opaque or clear Fragile Some of these materials are hard and brittle (hard candy, hard plastic, uncooked pasta, chips, etc.). Some are soft and flexible (soft plastic item, rubber band, cooked pasta). Some change their physical properties when wet (pasta/noodles, chips, breakfast cereal); some do not (drinking glass, rubber band, etc.)

Next, ask the class which of these materials are crystalline and which are amorphous. The key is below: Hard candy (lemon drops, life savers, etc.): amorphous glassy sugar (sucrose, etc.) Chocolate (Hershey's kisses, chocolate bar, etc.): crystalline milk fat and cocoa butter Crackers: amorphous carbohydrate and protein Table salt: crystalline NaCl Table sugar: crystalline sucrose Soft plastic item (child's toy, etc.): amorphous rubbery synthetic polymer Hard plastic item (plastic fork/spoon/knife, toy): amorphous glassy synthetic polymer Rubber bands: rubbery polymer (origin of the term rubbery) Butter or margarine Hard (cold): crystalline milk fat (large fraction but not all) with some liquid Soft (warm): mostly liquid milk fat with small fraction of crystalline Noodles or pasta: Uncooked: amorphous glassy carbohydrate Cooked: amorphous rubbery carbohydrate Breakfast cereal (Chex, Grape Nuts, Shredded Wheat): glassy amorphous carbohydrate Potato or corn chips: glassy amorphous carbohydrate Glass item (drinking glass, etc.): glassy SiO2 (origin of the term glass) If you observe the table salt or sugar under the magnifying glass, the regular crystalline order is apparent: these substances are regular crystals (albeit small). If you observe any of the other materials under the magnifier, there is no evidence of any crystalline order. Final note: the whitish powder (bloom) that forms on chocolate is a result of heat abuse. When chocolate is stored at a temperature greater than melting point of the lowest melting fat, the liquid fat can migrate to the surface and recrystallize. The powder is thus tiny crystals of this fat. Alternatively, if water condenses on cold chocolate it can dissolve sugar that will then coat the chocolate; when the water evaporates, the sugar will crystallize into tiny crystals on the surface.