4) I saperi tradizionali della gondola veneziana
1.2 Gli approcci metodologici della Scuola di Chicago e suoi sviluppi in Sudamerica
Objective:
After reading this unit, you should able to Know what crystallization is.Know to create nucleate.
Know how to control crystallization in food.
Know the physical chemistry keywords.
Reading 1: What is crystallization?
Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering crystallization occurs in a crystallizer. Crystallization is therefore an aspect of precipitation, obtained through a variation of the solubility conditions of the solute in the solvent, as compared to precipitation due to chemical reaction.
TASK 1: COMPREHENSION QUESTIONS
Answer the questions below1. Please name three sources in which crystallization can occur?
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2. Based on the reading, how many kinds that precipitation can be classified?
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2. Do you think that crystallization is the process in which a homogeneous system becomes heterogeneous system?
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4. In chemical engineering, where does crystallization occur?
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Reading 2
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Crystals are solids in which the atoms are arranged in a periodic repeating pattern that extends in three dimensions. While all crystals are solids, not all solids are crystals. Materials that have short-range rather than long-range ordering, like glass, are noncrystalline solids. A noncrystalline solid is often referred to as an amorphous solid. Many materials can form solids that are crystalline or amorphous, depending on the conditions of growth. In addition, some materials can form crystals of the same composition but with differing arrangements of the atoms forming different three-dimensional structures. Other materials can have the same three-dimensional structure but appear different in shape when viewed under the microscope. To make sense of this, and to understand the nature of crystals and how they are identified requires some knowledge of crystals and their structure. The study of crystal structure is called crystallography and is described in a number of standard references.
Crystallization from solution can be thought of as a two-step process. The first step is the phase separation, or "birth," of new crystals. The second step is the growth of these crystals to larger sizes. These two processes are known as nucleation and crystal growth, respectively. Analysis of industrial crystallization processes requires knowledge of both nucleation and crystal growth. A supersaturated solution is required for crystallization to occur. A supersaturated solution is not at equilibrium. In order to relieve the supersaturation and move towards equilibrium, the solution crystallizes. Once crystallization starts, however, the supersaturation can be relieved by a combination of nucleation and crystal growth. It is the relation of the degree of nucleation to crystal growth that controls the product crystal size and size distribution, and is, therefore, a crucial aspect of industrial crystallization processes.
TASK 2: TRUE OR FALSE
Decide whether the following statements are true (T) or false (F).
1. A crystal or crystalline solid is a solid material whose constituent atoms (molecules, or ions) are arranged in an orderly repeating pattern extending in all three spatial dimensions.
2. Non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal.
3. Both saturated and unsaturated solutions have a ability to form crystallization.
4. Crystallization process includes two processes, first is nucleation and then is crystal growth.
5. Crystal size depends on the supersaturation degree of the solution.
6. Supersaturated solution is not at equilibrium condition and tends to change to equilibrium state.
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Reading 3. Homogeneous and Heterogeneous Nucleation
Nucleation can be either homogeneous, without the influence of foreign particles, or heterogeneous, with the influence of foreign particles. Generally, heterogeneous nucleation takes place more quickly since the foreign particles act as a scaffold for the crystal to grow on, thus eliminating the necessity of creating a new surface and the incipient surface energy requirements.
Homogeneous Nucleation
Nucleation generally occurs with much more difficulty in the interior of a uniform substance, by a process called homogeneous nucleation. The creation of a nucleus implies the formation of an interface at the boundaries of a new phase. Homogeneous nucleation is the process of generating a new phase from an old phase whose free energy has become higher than that of the emerging new phase. Nucleation occurs via the formation of small embryos of the new phase inside the large volume of the old phase. Another prominent feature of nucleation is metastability of the old phase, i.e., the transformation requires passage over a free energy barrier. This is easily understood by considering the free energy changes associated with the formation of the nucleus. The statement that the free energy per molecule of the new phase is less than that of the solvated phase only applies to the bulk of the new phase. The surface is a different matter.
Because the surface molecules are less well bound to their neighbors than are those in the bulk, their contribution to the free energy of the new phase is greater. The difference between the free energy per molecule of the bulk and that of the surface is referred to as the interfacial free energy. (It is sometimes called the surface free energy, but, strictly speaking, this term should be reserved for surfaces in contact with vacuum.) The interfacial free energy is always a positive term and acts to destabilize the nucleus. As a consequence, at very small size when many of the molecules reside at the surface, the nucleus is unstable. Adding even one more molecule just increases the free energy of the system. On average, such a nucleus will dissolve rather than grow. But once the nucleus gets large enough, the drop in free energy associated with formation of the bulk phase becomes sufficiently high that the surface free energy is unimportant, and every addition of a molecule to the lattice lowers the free energy of the system. There is an intermediate size at which the free energy of the system is decreased whether the nucleus grows or dissolves, and this is known as the critical size. This phenomenon is referred to as the Gibbs-Thomson effect. Of course, if the supersaturation is high enough, the critical size can be reduced to less than one growth unit. Then the barrier vanishes and the old phase becomes unstable so that an infinitesimal fluctuation of an order parameter, such as density, can lead to the appearance of the new phase. The rate of generation and growth of the new phase is then only limited by the rate of transport of mass or energy.
93 Heterogeneous Nucleation
The presence of a foreign surface can be used to exert even greater control over nucleation because, quite often, the interfacial energy between a crystal nucleus and a solid substrate is lower than that of the crystal in contact with the solution. This is because the molecules in the crystal can form bonds with those in the substrate that are stronger than the bonds of solvation. Because the enthalpic contribution to the free energy comes primarily from chemical bonding, stronger bonds lead to a smaller interfacial free energy.
This may well be the key physical phenomenon that allows living organisms to delineate both location and orientation of crystallites. Clearly, the strength of bonding at the interface is strongly dependent on the structure and chemistry of the substrate surface. If the atomic structure of the substrate surface closely matches a particular plane of the nucleating phase so that lattice strain is minimized and, in addition, the substrate presents a set of chemical functionalities that promote strong bonding to the nucleus, then the enthalpic contribution to the interfacial free energy becomes small, and nucleation occurs preferentially on that crystal plane.
TASK 3: COMPREHENSION QUESTIONS
Answer the questions below1. What is homogeneous nucleation? What is heterogeneous nucleation? In which process, the nucleation process occurs easily and rapidly?
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2. What is interfacial free energy?
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3. Normally, does the nucleus prefer to dissolve or growth? When does it growth?
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4. What is Gibbs-Thomson effect?
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5. Is the critical size larger or smaller in the higher solution?
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6. Compare the interfacial energy between a crystal nucleus and a solid substrate and that of the crystal in contact with the solution? Why?
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7. How can the living organisms delineate the location of crystallites?
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8. Which factors will affect the strength of bonding at the interface?
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Reading 4. Controlling Crystallization in Foods
In order to control crystallization, it is necessary to have an understanding of the phase behavior of the system, knowledge of nucleation and growth kinetics, and the effects of both formulation and processing conditions on these kinetic. However, this is often a difficult task in the food industry as crystallization is often extremely complex with many factors that can influence the final results. In addition, the food manufacturer does not always have access to some of the tools available for controlling crystallization due to product limitations. For example, use of certain ingredients that would modify crystallization may not be allowed under some code of identification for a product. For example, there is a standard of identity for chocolate that specifies the ingredients that can be used. In the US, chocolate may contain cocoa butter and up to a certain level of milk fat. No other fats can be added and still have the product labeled chocolate. Thus, the ability to modify cocoa butter crystallization in chocolate is limited to milk fat ingredients. However, in compound coating (imitation chocolates based on vegetable fats), a nucleator is oftern used to promote crystallization and improve throughput rates. The nucleator is typically a hardened (fully hydrogenated) vegetable oil with high melting point. It is also possible that use of a desired ingredient for controlling crystallization may produce an undesired taste or texture in the product. Thus ontrolling crystallization in foods may be extremely challenging at times.
In foods, two circumstances for controlling the formation of crystals can be distinguished: those where the crystals provide element of structure in the product and those where crystallization is a separation process.
In the first case, control of the correct number, size (and distribution), shape and polymorph of crystals is required to provide te desired processing characteristics, quality (texture, flavor, etc.), and appearance and/or shelf stability of the product. Here, the kinetics of nucleation and growth must be controlled, through proper choice of formulation and processing conditions, to give the desired crystalline micro-structure. Controlling crystallization in sugar products (sugar frosted cereals, fondant, panned candies, caramels, etc.), frozen foods (ice cream, frozen desserts, and other frozen products) and lipid-based products (tempering of chocolate, butter, margarine, shortenings, etc.) are examples of products where a certain crystal size distribution is required to give the desired attributes. For many products, the desired crystal size distribution is one that has a large number of very small crystals that provide smooth texture
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and excellent dissolution or melt-down properties. For example, the ice crystals in ice cream have a mean size somewhere between 35 and 45 µm with a range of sizes from 1 µm to over 100 µm. Over 109 crystals per liter of ice cream provide a significant cooling effect upon melting. Controlling formation of these crystals is extremely important because it is thought that we can detect ice crystals on the order of 50 µm on our palate. However, it is unclear how this threshold detection size relates to the quantitative size distribution in ice cream. The shape of the ice crystals is also important to product texture and mouth feel.
Ice crystals in ice cream have a rather sooth, rounded surface and this allows the ice crystals to flow across each other easily to give a smooth, creamy characteristic. If the ice crystals in ice cream had jagged edges, as are often found in ice crystals in popsicles, the texture of the ice cream would be much more brittle since the crystals would not flow across one another easily. Clearly, controlling the nature of the crystalline structure in food products is crucial to the desired product attributes.
In products containing lipids, control of the crystal polymorphic form is also necessary. Lipids form different crystalline structures, or polymorphs, depending on the nature of the fat and the processing conditions. Transitions from less stable to more stable polymorphs are also dependent on the composition and processing conditions. For example, tempering (or pre-crystallization) of chocolate is a process through which the chocolate is sequentially cooled and warmed to promote crystallization of cocoa butter into the desired polymorphic form. Controlling crystallization to produce the proper size distribution of this polymorph provides: (1) the desired contraction upon cooling (release from a mold); (2) snap or brittleness; (3) glossy surface appearance; and (4) stability to fat bloom.
TASK 4: COMPREHENSION QUESTIONS
Answer the questions below1. Please name some knowledge to control crystallization process?
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2. Food engineers can use almost any tool available to control crystallization process?
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2. Name two reasons which determine whether or not one control crystallization method can be used?
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3. In which cases, the correct number, size (and distribution), shape and polymorph of crystals need to be seriously considered?
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4. In order to produce smooth texture, what are the requirements of the nucleation?
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5. In order to produce smooth texture, what are the requirements of the nucleation?
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TASK 8: How to find good keywords
Find the keywords in reading paragraphs 3 and 4 TASK 9: Summary
In about 5 sentences, summarize the main idea in paragraphs 3 and 4 TASK 10: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 amorphous
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23
solvent
24 supersaturate 25 vacuum