Fats & Oils
Fruits & Vegetables
Hydrocolloids, Vegetable Gums
Meat, Fish, Poultry
Frequently Asked Questions
OBJECTIVES The learner shall be able to -
- identify the function of crystals in candies and frozen dessert.
- list factors that affect the production of ice and sugar crystals.
- produce a product with sugar crystalline structure.
- produce a product with ice crystalline structure.
- identify other crystals in foods and their affect upon quality.
Crystallization of sugars and salts can be a problem or an advantage in a variety of products. The most common crystal, sugar, is a component in many candies. The grape juice and wine industry have some concerns regarding tartaric acid crystals. Certainly, salt and the crystal size is also a major "food item". Another sugar crystal, lactose, may play a role in nonfat dry milk. the crystallization of lactose in a glassy state will make nonfat milk difficult to disperse. If too great amount of milk solids are added to a frozen dessert one may get a gritty texture due to the lactose crystals.
Candies can be divided into two groups, crystalline and noncrystalline. Crystalline candies include fudge, fondant and any other candies which have crystals as an important structural component. Divinity is a crystalline candy but is a special case as the crystals are dispersed in a foam. Noncrystalline candies include caramels, brittles, taffies, marshmallows and gum drops. Marshmallows and gum drops are also special classes of candies as they contain a gelling substance.
Frozen desserts are not only crystalline systems, but usually are both
suspensions and foams as well. When cream is used in the mix, an emulsion
is also involved. Pure liquid (water) changes from a liquid to a solid
state at 0C. For water to change to ice the removal of 79.9 cal/g is required.
Frozen desserts contain solutes dissolved in water to form a solution.
This lowers the freezing point and the amount the freezing point is lowered
is dependent on the concentration and extent of ionization of the solutes.
For example, one gram molecular weight of sugar (non-ionizing) per liter
of water lowers the freezing point only half as much as one gram molecular
weight of salt (ionizing to form two ions).
Overall, regardless of the type of crystal, the following factors may impact crystallization:nature of crystallizing substance
rate of cooling
degree of agitation
impurities and/or interfering agents
nature of containers
size and previous history of samples
Candies are made of sugar (sucrose), water or other liquid and usually some interfering agent(s). Butter, milk, cocoa and corn syrup are commonly used as both crystal interfering agents and flavoring. Candy begins when the water or other liquid is supersaturated with the solute, usually sucrose. Supersaturation occurs when more sugar is present than can be dissolved at that temperature. By heating the solution above the boiling point of water the solute concentration becomes greater. A supersaturated solution is formed when this solution, after heating to a high temperature, is allowed to cool undisturbed. Upon cooling the sugar recrystallizes into several small crystals or forms one large amorphous mass. For crystallization to occur, nuclei must form and solute must be added from the solution to these nuclei. Usually these nuclei form spontaneously but sometimes are "seeded" to the cooked mixture to initiate crystallization. The size of the resulting crystals depends on the number of nuclei, rate and temperature of crystallization, agitation and impurities in the solution.
Crystallization is a complex process with many interrelated factors. The nature of the crystallizing substance is important for crystallization, although not as obvious in candy making as sucrose is almost always the substance under discussion. The rate of crystallization is the speed at which nuclei grow into crystals. This rate is dependent upon the concentration of the solute in the solution as a more concentrated (more supersaturated) syrup will crystallize more rapidly than a less concentrated syrup. At a higher temperature the rate of crystallization is slow and becomes more rapid at a lower temperature. Agitation distributes the crystal forming nuclei and hastens crystallization.
Impurities in the solution usually delay crystallization and in some cases such as caramels may prevent crystal formation. Fat and protein decrease the number and size of crystals through the interference of their masses with the orientation of the sucrose molecules. Corn syrup also has this interfering role; however, additionally it serves to enhance the solubility of sucrose and thus decreases its tendency to crystallize. Cream of tartar as an added ingredient in a candy formula serves indirectly to decrease the rate of crystallization as well as crystal size. It does this through its ability to hydrolyze sucrose into its invert sugar. This not only forms two sugars of greater solubility than sucrose, but it gives agents which enhance the solubility of sucrose.
A supersaturated or supercooled solution forms nuclei. This process
involves the uniting of atoms. Solution or liquids at the freezing point
form nuclei spontaneously or by a process called seeding. Allowing a
solution to stand causes the formation of nuclei in various places spontaneously.
These crystals grow from these nuclei and because the number of nuclei
formed are small, they grow to large sizes. Seeding is done by adding
crystals of the same material to a solution to start the crystallization
process. These added crystals serve as nuclei. Agitation of the solution
increases the number and the rate of nuclei formation. In water crystallization
the nuclei and crystallizing substance is the solvent of the system.
The larger portion of reported datum have dealt with the crystallization
of solute from dispersions.
The rate of crystallization is the rate the crystallizing substance becomes
a solid crystal. It is determined by the time required for a drop from
one temperature to another. Other components contribute to decreasing
or increasing crystallizing rate. Rapid crystallization and agitation
during the freezing are a must for the formation of small crystals.
Solutes decrease the freezing point or the point of crystallization of water. Thus, the greater the solute concentration, the slower the rate of crystallization. Rate of crystallization is also decreased during the solidification of water by colloids and suspensions in a mixture. Fats, milk or egg proteins or stabilizers and emulsifiers would be in this category. These stabilizers and emulsifiers help keep the ice crystals small by absorbing some of the free water to form a gel. The gel prevents the growth of large ice crystals due to its stiffness and internal structure. Their physical presence interferes with crystal growth.
In frozen dessert mixes, air is incorporated during agitation in the freezing process. The incorporation of the air causes an increase in volume of the mix. This increase in volume is called "overrun". The whipping ability of the mix is increased by the presence of nonfat dry milk solids, egg yolks and emulsifying agents. Whipped evaporated milk, beaten egg whites and whipped cream help incorporate the air in still frozen desserts. Fat decreases the percent overrun in ice creams especially when present as large fat globules or clumps. Homogenization of the fat reduces the particle size and therefore may change overrun.
The quality of frozen desserts is evaluated by the texture (mouth feel), consistency (hardness or softness) and body. A smooth, creamy texture, a consistency that is neither too hard or too soft and a body that is not too watery and compact nor too viscous and spongy when the frozen dessert starts to melt are the qualities that determine a desirable frozen dessert.
Freezing Point: Making Sherbet
Sherbet at home, you'll need to use both sugar and salt.
The sugar is added to water and other ingredients to sweeten the mixture, which is currently at room temperature (27 degrees C). Adding sugar lowers the point at which the mixture will freeze from 0 degrees C to -2 or -3 degrees C. It's necessary to lower the freezing temperature of the mixture in order to get the fine crystal structure of frozen sherbet.
Ice surrounds the mixture in an outside container. When you begin to turn the crank, the temperature of the ice is -5 degrees C. As the ice begins to melt, you add rock salt to it, which dissolves in the water and lowers the freezing point of the remaining ice. Remember that for every gram molecular weight of salt you add, you'll lower the freezing temperature of the ice by -1.8 degrees C. And you'll need this lower freezing point to turn the mixture in the center into a frozen sherbet.
What causes the ice to begin melting in the first place? The heat energy from the temperature of the sherbet mixture, and the temperature of the room. Turning the crank on the machine keeps this heat energy evenly distributed throughout the sherbet mixture.
This heat melts more ice, which dissolves more rock salt, which continues to lower the freezing point. The melting lowers the level of ice, so you must replenish the ice and rock salt to freeze the mixture throughout. When the temperature drops to -12 degrees C, the temperature of the mixture inside will be -3 degrees C and your sherbet will be frozen.
|Food Item||Crystal Type||Crystal Role, Development and/or Inhibition in Foods|
|Fondant ||sugar ||Fondant is a two-base system: sucrose crystals in syrup. For good crystal formation the syrup needs to be sufficiently supersaturated. The presence of acid as an interfering agent makes both the final temperature and the cooking time critical for optimum crystal size. The temperature at which agitation begins is critical for preventing formation of too large crystals. Once agitation has begun it must be constant until the end point is reached or large crystals may be formed. |
|In solution the K2H2 tartrate gives the juice a cloudy appearance and an acid flavor. Bulk storage at low temperatures (less than 40F) allows for precipitation of an crystals prior to packaging. Precipitation can be brought about in as little as 24 hours using rapid freezing, thawing, and filtering. Storage is in metal, wooden or cement containers with a protective coating; glass carboys or stoneware jugs. Packing is in glass bottles or specially enameled cans. Ion exchange and cold stabilization facilitate precipitation of the crystals to improve flavor and maintain clarity. |
|Sherbet ||ice ||The ice crystals give the frozen dessert its hardness. Sherbet is a two-phase system: ice crystals in syrup. In sherbets the amount of sugar is very high to counteract the acid which serves to lower temperature at which the crystals form. Sugar also acts as an interfering agent to help keep the crystal size small. Constant vigorous agitation during freezing also helps to keep crystal size small. Addition of non-fat milk solids provides another interfering agent producing small crystals and a finer product. If nonfat milk solids are used they increase the product's viscosity and favor the incorporation of air bubbles as the mix is agitated during freezing. The fruit juice causes milk solids to thicken and thus interfere with the development of large crystals. Additionally, if cold and concentrated enough the lactose may crystallize out of solution. |
|Shortening ||polymeric alpha, beta, beta prime and intermediate ||Plastic fats are a two-phase system: crystal fat surrounded by oil. Fat crystals are relatively large and the spaces between them are large. A solid, phase, of 5-35% of the volume will give a plastic fat. Small crystals rapid cooling and rapid agitation. Large crystals -slow cooling, no agitation. Melting point of fat changes with each change in type of crystal. Homogeneity causes transformation to coarse crystals. |
amorphous: has no crystalline structure. Amorphous candy are those such as lollipops, marshmallows, caramels.
anhydrous: a solid containing no water bound to the molecule as in a hydrate or not water of crystallization.
boiling point: the temperature at which a liquid vaporizes.
is the temperature at which the atmospheric pressure is equal to the vapor pressure of a liquid and an equilibrium is established.
candy: origin is evidently from the Latin candidus, glittering white (candied sugar having that characteristic), or again it may have originated form Khand, the Sanscrit for the sugar cane. However that may be, there is only one accepted defintion of the word in cookery, and that is "boiled sugar with more or less flavouring," The process of making candy must not be confounded, although it is oftenis, with crystallising sugar.
crystalline: the aggregation of molecules of a substance in a set, ordered pattern, forming individual crystals. Crystalline candy generally has sugar crystals.
crystallization: the formation of crystals from the solidification of dispersed elements in a precise orderly structure
crystallize: is to coat and impregnate (fruit or petals) with sugar as a means of preserving them: a box of crystallized fruits.
the formation of crystals from the solidification of dispersed elements in a precise orderly structure
frozen dessert: have a base either of cream, plain or whipped, custard, fruit juice, or water.
ice cream: is a plain frozen dessert medium to high in milkfat and milk-solids-not-fat. With or without small amounts of egg products. Without visible particles of flavoring materials. With the total volume of color and flavor less than 5 percent of the volume of the unfrozen ice cream mix. Regulatory limitations say not more than 0.5% edible stabilizer. Not less than 10 percent milkfat. Not less than 20 percent total milk solids.
ice milk: is low in fat, with and or without small amounts of egg products. It may or may not have chocolate, fruits, nuts, or other flavor materials. From a regulation viewpoint it must have no more than 0.5% edible stabilizer, not less than 2 % nor more than 7% milkfat. Not less than 11% total milk solids.
levulose: is also called fructose or fruit sugar. Is a monosaccharide with the chemical formula C6H12O6.
melting point: Often characteristic of a particular chemical and used as a means of identification. Particularly valuable as an index of purity, as impurities lower the melting point.
saturated solution: Saturated fats are those in which all carbons contain a hydrogen, and therefore, no double bonds exist. In general, fats that contain a majority of saturated fatty acids are solid at room temperature, although some solid vegetable shortenings are up to 75 percent unsaturated. Some common fatty acids in foods include palmitic, stearic and myristic acids.
saturation: indicates the level at which a solute is dissolved into a solution at a specific temperature.
sherbet is a frozen dessert of ice of water (or milk) given a slight body with gelatin or egg white.
sugar: Although the term is commonly used to refer to table sugar or sucrose, there are a large number of sugars, e.g. fruit sugar (fructose), grape (glucose), which are monosaccharides; malt sugar (maltose), milk sugar (lactose), which are disaccharides; and also higher multiples.
Updated: Wednesday, May 23, 2012.