CLIL for Chemistry 2 - Didattica della · PDF fileCLIL for Chemistry ... inverted sugar,...

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MODULE 02

CLIL for Chemistry - Associazione culturale CHIMICARE

AUTHOR: Teresa Celestino THE CARBOHYDRATES How food and beverages can be analysed utilizing chemical and physical properties of the carbohydrates.

Moduli CLIL per i l quinto anno degl i Ist ituti Tecnici e dei Licei

OVERVIEW

GENERAL INFORMATION VOCABULARY WORDS

ADDRESSED TO STUDENTS 18-19 YEARS OLD CLASS TIME: 10 HOURS OR MORE ENGLISH LEVEL: B1/B2 (EUROPEAN FRAMEWORK)

Carbohydrates, monosaccharide, disaccharide, polysaccharide, mutarotation, glycoside, glycosidic bond, epimer, epimerization, anomer, Fisher projection, Haworth projection, glucose, gluco-furanose, gluco-pyranose, aldose, ketose, pentose, hexose, fructose, ribose, xylose, arabinose, mannose, galactose, ribulose, maltose, lactose, sucrose, saccharose, homoglycan, heteroglycan, glycogen, amylose, amylopectin, murein, inulin, chitin, agarose, carrageenan, cellulose, fibril, microfibril, starch, amyloplasts, cyanogenic glycosides, optical rotation, polarimetry, cherries, apricots, peaches, cassava, bitter almonds, cherry laurel, flax seed, picric acid, photosynthesis, twigs, buds, rhizomes, tubers, Lugol’s iodine, Fehling’s solution, inversion of sucrose, inverted sugar, pastry-making, popping process, microwave, povidone–iodine solution, overwrap, susceptor, toothpick, gelatinisation, retrogradation …..

CONTENTS

- MONOSACCHARIDES - DISACCHARIDES - POLISACCHARIDES

LABORATORY EXPERIENCES

- DETECTION OF HCN RELEASED FROM PLANTS - TESTING FOOD FOR STARCH - TESTING FOOD FOR SUGARS - INVERSION OF SUCROSE - THE POPPING PROCESS

OBJECTIVES SKILLS

BY THE END OF THIS MODULE, STUDENTS SHOULD BE ABLE TO DEFINE VARIOUS TYPES OF CHEMICAL COMPOUNDS AND EXPLAIN IN ENGLISH LANGUAGE (SPEAKING OR WRITING) SOME OF THEIR CHEMICAL AND PHYSICAL PROPERTIES IN THE BIOLOGICAL SYSTEMS. THEY SHOULD BE ABLE TO CARRY OUT EXPERIMENTS RELATED TO FOOD&BEVERAGES ANALYSIS AND HOMEMADE FOOD PROCESSING, USING AN ENGLISH TECHNICAL LANGUAGE.

COMMUNICATION IN ENGLISH LANGUAGE (NOTE TAKING, ORAL AND WRITTEN EXPOSITION, INCLUDING SUMMARIZATION) COMPREHENSION (LISTENING, READING) OBSERVATION - MANIPULATION EXPERIMENTATION (CONDUCTING DATA ANALYSIS)

SUBJECTS USEFUL HINTS

ANALYTICAL CHEMISTRY PHYSICAL SCIENCE GENERAL SCIENCE

In this module, practical activities have more space than the theory, which needs to be investigated by additional lessons and researches, in order to answer the questions posed. Questions sheets could be used by the teacher to assign homework or as exercise to assess students’ level. If the teacher uses questions sheets during the lesson time, it is advisable to organize students two by two or in groups of three.


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INTRODUCTION The carbohydrates are carbonyl compounds that also contain several hydroxyl groups. They include monosaccharides, oligosaccharides and polysaccharides. These compounds are important components of food. Many organisms use polysaccharides as building materials. In the gut, oligosaccharides and polysaccharides are broken down into monosaccharides (the glucose is the form in which carbohydrates are distributed by the blood of vertebrates). Then, the glucose is utilized to obtain energy by the glycolysis or converted into other metabolites. The liver and muscles store glycogen as a polymeric reserve carbohydrate. Oligosaccharides and polysaccharides are often covalently bound to lipids or proteins, making glycolipids and glycoproteins respectively (they are, for example, in the cell membranes). MONOSACCHARIDES The most important natural occurring monosaccharide is the D-glucose. The Figures 1, 2, 3 show different form of representation of the glucose by: Fisher projection, Haworth projection, conformation (pag 35, fare relative domande).

Figure 1 Figure 2 Figure 3

Fischer projection Ring forms (Haworth projection) Conformation

Some important conversions related to sugars (monosaccharides) are listed below, using D-glucose as an example (see Figure 4): Mutarotation The mutarotation involves the cyclic form; it is the reaction that interconverts anomers into each other (in the α-anomer the OH at the chiral center C-1 and the CH2OH group lie on the same side of the ring ; in the β-anomer the OH at the C-1 and the CH2OH are on different sides). Glycoside formation When the anomeric OH of the glucose reacts with an alcohol eliminating water, it yields an α–methylglycoside. The glycosidic bond is not a normal ether bond, because of its particular properties.


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Reduction and oxidation The reduction at C-1 produces an alcoholic group (see the open-chained form in the Figure 1), making the sugar alcohol sorbitol. Oxidation of the aldehyde group at C-1 goves an intramolecular ester (generally called lactone, gluconolactone if the sugar glucose is involved). When glucose is oxidized at C-6, glucuronic acid is formed, a strongly polar compound playing an important role in some biotransformations. Epimerization In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D- mannose (the reaction isn’t shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epimers, and their interconversion is called epimerization. Esterif ication The hydroxyl groups of monosaccharides can form esters with acids. In metabolism, phosphoric acid esters such as glucose 6-phosphate are very important.

Figure 4 ‐ Reactions of the monosaccharides

Monosaccharides are classified according to the number of C atoms (into pentoses, hexoses, etc.) and according to the chemical nature of the carbonyl function into aldoses and ketoses (Figure 5).


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Figure 5 ‐ Classification of the monosaccharides

DISACCHARIDES When the anomeric hydroxyl group of one monosaccharide is bound glycosidically with one of the OH groups of another, a disaccharide is formed. As in all glycosides, the glycosidic bond does not allow mutarotation. Since this type of bond is formed stereospecifically by enzymes in natural disaccharides, they are only found in one of the possible configurations (α or β). The best known disaccharides are maltose, lactose, sucrose, shown in the following Figures 6, 7, 8.

Figure 6 – Maltose Figure 7 – Lactose Figure 8 ‐ Sucrose

Questions Sheet 1. What is the meaning of the letters D or L used in the monosaccharide nomenclature? …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………. 2. Give the R,S configuration of each stereogenic center in the following Fischer projection. Is this a D or L sugar?

3. What is the meaning of “pyranose” or “furanose” forms? Select the β-pyranose form of the following aldohexose derivative in Haworth projection


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4. The following sugar derivative is written in Haworth projection. Select a Fischer projection of the open form of this compound.

5. Which is the most stable aldohexose? Why?

6. Connect the two columns:

Fructose Glucose Galactose Arabinose Mannose Ribulose Xylose

Pentose Hexose

Ketose Aldose

3. Explain the meaning of the term “mutarotation”. ……………………………………………………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… 4. What is the difference between anomers and epimers? ……………………………………………………………………………………………………………………………………………………………………………………………………… 5. Classify each of the following monosaccharides as the α or β anomer

6. Classify each of the following disaccharides as having an α or β glycosidic bond.


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POLYSACCHARIDES Polysaccharides formed from only one type of monosaccharide are called homoglycans, while those formed from different sugar constituents are called heteroglycans. Both forms can exist as either linear or branched chains. Many molecules consist of α13, α14, α16, β14, etc… linked monomer residues. For example, glycogen and amylopectin are both branched homoglycans α14 linked glucose residues . In glycogen, on average every 8th to 10th residue carries — via an α16 bond — another 1,4-linked chain of glucose residues ( Figure …). On the contrary, murein is a linear heteroglycan, with a more complex struxture.

Figure 9 ‐ Glycogen

Figure 10 ‐ Murein

Other important polisaccharides are amylose, inul in, chit in and those derived from algae (e.g. agarose and carrageenan). Two glucose polymers of plant origin are of special importance among the polysaccharides: cellulose (β14 linked polymer) and starch (mostly α14 linked). Cellulose is the most abundant organic substance in nature. Naturally occurring cellulose is extremely mechanically stable and is highly resistant to chemical and enzymatic hydrolysis. These properties are due to the conformation of the molecules and their supramolecular organization. The unbranched β14 linkage results in linear chains that are stabilized by hydrogen bonds within the chain and between neighboring chains. Elementary fibril is an association of 50-100 cellulose molecules with a diameter of 4 nm; about 20 elementary fibril form a microfibril. Starch, a reserve polysaccharide widely distributed in plants, is the most important carbohydrate in the human diet; it is composed by amylose and amylopectin In plants, starch is present in the chloroplasts in leaves, as well as in fruits, seeds, and tubers. In these plant organs, starch is present in the form of microscopically small granules in special organelles known as amyloplasts. Some 15–25% of the starch goes into solution in colloidal form when boiling is prolonged. This proportion is called amylose (“soluble starch”). Amylose consists of unbranched α14 linked chains of 200–300 glucose residues. Due the α configuration at C-1, these


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chains form a helix with 6–8 residues per turn. Unlike amylose, amylopectin, which is practically insoluble, is branched. On average, one in 20–25 glucose residues is linked to another chain via an α16 bond.

Figure 11 ‐ Structural motifs of cellulose

Figure 12 – Starch: structural motifs of amylose and amylopectin

Resources from “Color Atlas of Biochemistry”, J. Koolman and K.H. Roehm (second edition, 2005)


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Questions Sheet 1. Connect the two columns: Glycogen α14 Starch β14 Cellulose α16 Murein Amylose Amylopectin 2. Write in short origin and use of inulin, chitin, agarose e carrageenan ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….... …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

Read the information below trying to understand the referring substance. Connect every number (on the left) with one or more letters (from left to r ight)(e.g.: 1 e, f). 1 . Physical properties a. OSHA PEL: 15 mg/m3 (total dust)

2. Storage requirements

b. ACGIH TLV: 10 mg/m3

3. Hazardous characteristics c. If inhaled, coughing, choking, shortness of breath.

4. Principal target organ(s) or system(s)

d. In the eyes, irritation.

5. Typical symptoms of acute exposures

e. Fine, white, water-insoluble powder

6. Exposure limits

f. Vapor pressure at 20 °C: negligible

7. Additional remarks

g. Incompatible with air, when dispersed. When starch powder is dispersed in the air, for example as a “cloud” of starch dust, it explodes violenty if it is ignited by a flame or spark. A spark from a static electrical charge that develops as a consequence of the dispersion is often the source of ignition in such explosions. When heated in bulk form however, starch burns and chars slowly. h. Except for inhalation involving the respiratory system, laboratory exposures of sufficient magnitude to adversely involve target organs or systems are not foreseeable. i . Store with other chemicals in a cool, dry, well-ventilated general storage location. l . Starch is a mixture of the carbohydrate polymers, amylose and amylopectin. In the laboratory, starch from potatoes is commonly used; however, this CLIP pertains to starch in finely powdered form potatoes or any other source. m. SHA PEL: 5 mg/m3 (respirable fraction)

Abbreviations: OSHA PEL: Occupational Safety and Health Administration ACGIH TLV : American Conference of Governmental Industrial Hygienists – Threshold Limit Value CLIP: Chemical Laboratory Information Profile From: Jay A. Young, “CLIP: Chemical Laboratory Information Profile” - Journal of Chemical Education, Vol. 85 - No. 10, October 2008.

Express your considerations about it: ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ List the necessary precautionary measures: ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


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The teacher reads slowly the fol lowing paragraph. Students are organized two by two: every student l istens and takes notes; then, the couple of students write a short summary.

Cyanogenic glycosides A disproportionately large number of the most important human food plants is cyanogenic. The accumulated research of numerous people working in several different disciplines now allows a tenable explanation for this observation. Cyanogenesis by plants is not only a surprisingly effective chemical defence against casual herbivores, but it is also easily overcome by careful pre-ingestion food processing, this latter skill being almost exclusive to humans. Moreover, humans have the physiological ability to detoxify cyanide satisfactorily, given an adequate protein diet. It appears that early in the domestication of crop plants the cyanogenic species would have been relatively free of pests and competitive herbivores, as well as having good nutritional qualities, and thus ideal candidates for cultivation by the first farmers. Fruit having a pit (such as cherries, apricots and peaches, cassava, bitter almonds, cherry laurel, flax seed, among many others) slowly release hydrogen cyanide from cyanogenic glycosides. From: “Plant Biodiversity and Health” Comenius course – Laboratory session (Faculty of Pharmacy, University of Barcelona, July 2008). Post: http://urtoefficace.linxedizioni.it/tag/acido-cianidrico/ (in Italian)

The teacher reads slowly the fol lowing brief paragraph. Students are organized two by two: every student l istens and takes notes; then, both students answer the related questions.

Sugar solutions and polarimetry

Sugar solutions can be analyzed by polarimetry, a method based on the interaction between chiral centers and linearly polarized light. It can be produced by passing normal light through a special filter (a polarizer). A second polarizing filter of the same type (the analyzer), placed behind the first, only lets the polarized light pass through when the polarizer and the analyzer are in alignment. In this case, the field of view appears bright when one looks through the analyzer. Solutions of chiral substances rotate the plane of polarized light by an angle α, either to the left or to the right. When a solution of this type is placed between the polarizer and the analyzer, the field of view appears darker. The angle of rotation, α, is determined by turning the analyzer until the field of view becomes bright again. A solution’s optical rotation depends on the type of chiral compound, its concentration, and the thickness of the layer of the solution. This method makes it possible to determine the sugar content of wines, for example.

Figure 13 – The polarimetry allows the determination of the α and β

glucose content.


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Questions Sheet 1. Observe the Figure 13 (on the left side) and comment upon the three steps of the instrumental analysis. - Step 1 …………………………………………………………………………………………………………………………………… - Step 2 …………………………………………………………………………………………………………………………………… - Step 3 …………………………………………………………………………………………………………………………………… 2. Observe the Figure 13 (on the right side). Explain the meaning of the graph shown. ……………………………………………………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………

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The teacher reads slowly paragraph-by-paragraph. Students are organized two by two: every student l istens and takes notes; then, both students write together a summary of every paragraph. The day after, teacher asks students to expose the summary oral ly, without reading.

The starch

Foods as rice, potatoes or bread contain a large amount of starch. Produced in plants by the photosynthesis of carbon dioxide, starch granules are made out of glucose polymers and serve as energy stores. Towards the end of the growing season, starch accumulates in twigs of trees, close to the buds. It is also found in fruits, seeds, rhizomes and tubers. Starch granules are very suitable for such long-term storage, because of their compactness, relative dryness and high stability.

In order to increase its digestibility, starch needs to be cooked, so it becomes water-soluble and edible. The transformation of raw starch in hot water is called gelatinisation: the granules swell and burst, forming a paste. During cooling, the starch paste often thickens due to a phenomenon called retrogradation. Gelatinisation and retrogradation affect the behaviour of starch-containing systems.

Consequently, starch is excellent for modifying the texture of many home-cooked foods (for example, to thicken sauces), and has also been used for centuries for other purposes, including the manufacture of paper (sizing), glues or fabric stiffener. Today, new applications of starch are emerging, including low-calorie dietary fibres, biodegradable packaging materials, thin films and thermoplastic materials. From: Dominique Cornuéjols, “Starch: a structural mystery” – Science in school, Issue 14, p. 22-27, Spring 2010 http://www.scienceinschool.org/repository/docs/issue14_starch.pdf


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LABORATORY DETECTION OF HCN RELEASED FROM PLANTS Materials: Filter Whatman 3 mm Sodium carbonate Water Picric acid (1,0 g) Plant material (about 1 g or more)* Test glass tubes Cork or rubber stoppers Xylene (0,5 ml) Mixer machine Bath at 60 °C * Each group has 3 test tubes and uses the drugs corresponding to the number written on the tube: Test tube n. 1: cassava pulp (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 2: cassava pulp (raw) 3: flax seeds 4: cassava bark (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 5: cassava bark (raw) 6: peach seeds 7: cassava pulp (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 8: cassava pulp (raw) 9: flax seeds 10. cassava bark (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 11: cassava bark (raw) 12: tapioca (cassava starch)

13: cassava pulp (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 14: cassava pulp (raw) 15: peach seeds 16: cassava bark (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 17: cassava bark (raw) 18: flax seeds 19: cassava pulp (about 10 g) boiled in c. a. 200 ml of water for 30 minutes 20: cassava pulp (raw) 21: flax seeds Procedure: The picric acid paper was prepared beforehand by dipping filter Whatman 3 mm in a water solution of 1,0 g of picric acid in 100 ml of 10% (w/v) sodium carbonate. The paper was allowed to air dry and was cut into strips about 1 cm by 10 cm. Store dried strips avoiding sunlight contact. Finely chop or crush with knife or hands a small quantity of plant material (about 1 g) and place it in a test glass tube that can be sealed with a cork or rubber stopper (even with cotton). Put one end of the stopper to hold a picrate paper strip. If plant material is dry, moisten with about 0,5 ml of xylene, mix by the mixer machine; add 3 ml of water using the mixer again and allow hydrolyzing several minutes in stoppered tube in a bath at 60 °C degrees. If the paper changes from yellow to brick red within 30 minutes, HCN is present. The redness is proportional to


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the HCN contents. HCN reacts with sodium picrate producing isopurpurate (red color). Cautions: Picric acid is explosive and toxic. Modern safety precautions recommend storing picric acid wet. Dry picric acid is relatively sensitive to shock and friction, so laboratories that use it store it in bottles under a layer of water, rendering it safe. Glass or plastic bottles are required, as picric acid can easily form metal picrate salts that are even more sensitive and hazardous than the acid itself. Hydrocyanic acid developed irritates seriously the eyes and the respiratory tract. Avoid all contact! Wear safety goggles or eye protection in combination with breathing protection. From: “Plant Biodiversity and Health” course – Laboratory session (Faculty of Pharmacy, University of Barcelona, July 2008). Post http://urtoefficace.linxedizioni.it/tag/acido-cianidrico/

Questions Sheet 1. Observe the following draft related to the laboratory experience and explain it.

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2. Describe the experiment’s steps related to the following pictures.

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mmmmm m ……………………………………………………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………


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TESTING FOOD FOR STARCH Iodine reacts with starch to form starch/iodine complex which has a blue-black colour. The appearance of blue-black colour confirms the presence of starch in the given food sample.

Materials: Test tubes Test tubes support Glass rods Lugol’s iodine Water Food samples Dropper Knife Pestle and mortar Funnel and gauze Procedure: Prepare the referring sample: introduce a small quantity of soluble starch in a test tube. Add some ml of water in order to solubilize it; then, add from 2 to 5 drops of Lugol’s Iodine. Wait the formation of a blue complex intensely colored. Mince finely every food sample and put the same quantity in every test tube labeled with the corrisponding number.

Add 5-10ml of water filling all the test tubes until the same level. Mix well. Add some drop of Lugol’s iodine by the dropper. Note in a table the color of every test tube content, comparing with the referring sample. Write the results in the following table:

FOOD COLOR after adding Lugol’s iodine

POSITIVE NEGATIVE Pasta

Sugar (saccharose)

Salt (sodium chloride)

Wheat flour

Bread

Rice

Milk

Vegetables

Fruits

Cautions: Elemental iodine is toxic if inhaled; it is also a skin irritant. Lugol's solution is capable of causing tissue damage if the exposition is prolonged.


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Observations …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… Conclusions …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… Questions 1. Describe the composition of the Lugol’s iodine …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………. 2. Why does Lugol’s iodine react with starch? …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ……………………………………………………………

TESTING FOOD FOR SUGARS

Fehling's solution is a chemical test used for monosaccharides. It is always prepared fresh in the laboratory, made initially as two separate solutions, Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulfate, while Fehling's B is a clear and colorless solution of aqueous potassium sodium tartrate and a strong alkali (commonly sodium hydroxide). Fehling's can be used to determine whether a carbonyl-containing compound is an aldehyde or a ketone . This test works with reducing sugars.

Materials: Test tubes Test tubes support Glass rods Fehling’s solution Water Food samples Dropper Knife Pestle and mortar Bunsen burner Beaker

Procedure: Prepare the referring sample: put in the test tube 2 ml of glucose solution, add 4 ml of Fehling’s A e B, shake the test tube and warm it. Observe and take notes. Mince finely and put the same quantity of every food sample in a labeled test tube.


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Add 5-10 ml of water (fill all the test tubes until the same level) and mix well. Add some drops of Fehling’s A and Feheling’s B by the dropper. Observe the color of the content in every test tube. Heat the test tubes in a double boiler (using a beaker). Wait some minutes and take note of the changing colors. Complete the table.

FOODS COLOR

obtained by adding Fehling’s solution

COLOR after some

minutes POSITIVE NEGATIVE

Pasta Sugar (saccharose)

Salt (sodium chloride)

Wheat flour Bread Rice Milk Vegetables Fruit juices

Notes: It is opportune to filter the fruit juice in order to eliminate the suspended particles . Add 3 or 4 ml of vinegar to 10 ml of milk before testing lactose in the milk. So, the proteins can precipitate. Filter and carry out the test utilizing the clear solution. If sugars are in small quantity, the final content could begin cloudy green rather than yellow or red. Either sugars or proteins react with the copper sulfate, so in the starting phase sugars could be masked by proteins’ reaction. For this reason, it is necessary a prolonged heating of the test tube until the formation of thebrick- red precipitate.

Cautions: Fehling’s solution is made with sodium hydroxide, caustic at high concentrations. Precautions should be taken as such not to come into direct contact with it. Another component, copper(II) sulfate, is toxic if ingested.


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Observations …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… Conclusions …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… Questions 1. Describe the preparation of the Fehling’s solution. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

2. How does the Fehling’s solution work? Write the reaction with a generic monosaccharide. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

3. Explain the result obtained when testing the sucrose. …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………


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INVERSION OF SUCROSE

Materials: Test tubes Test tubes support Sucrose Hydrochloric acid 37% Feheling’s solution Bunsen burner Procedure: Dissolve in 5 ml of distilled water one spatule of commercial sucrose in a test tube, one spatule in another test tube. Add to the second test tube 2 or 3 drops of hydrochloric acid 37%. Heat both the test tubes by the bunsen burner.

Add 3 ml of Fehling’s solution in every test tube.

Cautions: Hydrochloric acid is irritant and corrosive, use protective equipment.

When saccharose reacts with a strong acid, the molecule is separated into its two components, fructose and glucose. This process is called “inversion”. Inverted saccharose reacts with Fehling’s solution.

Observations ……………………………………………………………………………………………………………………………………………………………………………………… Conclusions ………………………………………………………………………………………………………………………… Questions 1. Why the term “inversion” is used? ………………………………………………………………………………………………………………………………………………………………………………………………………. 2. The inversion of sucrose can be home-made using, for example, lemon juice or vitamin C. Why these ingredients? …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 3. The inversion of sucrose is commonly used in pastry making laboratories. Why? ……………………………………………………………………………………………………………………………………………………………………………………………………… For more information, see: http://www.chefeddy.com/2009/11/invert-sugar/


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THE POPPING PROCESS

Materials: Two bags of “high-fat” microwave popcorn Microwave oven Scissors Small container Large bowl Toothpicks Clear, colorless, or light-colored cup Hot water Paper plate Gloves and goggles Povidone–iodine solution Procedure:

STEP 1. Remove the plastic overwrap from a bag of “high-fat” microwave popcorn. What purpose might the overwrap serve?

Nearly everyone has popped popcorn in a microwave oven. Microwave heating is the result of the friction created as polar molecules, such as water molecules inside an unpopped kernel of popcorn, oscillate at the microwave frequency. When a kernel bursts in response to the vapor pressure of superheated water, the hot gelatinous starch inside the kernel first expands and then cools to form a solid foam as water evaporates.

STEP 2. Examine the inner and outer surfaces of the bag. Locate the “This Side Up” and “Open This End” instructions on the outside of the bag and a dark rectangle (the susceptor), on the inside of the bag. The susceptor is vacuum-deposited aluminum on a polyester film. What purpose do you think the susceptor serves? STEP 3. Using a microwave oven, pop a second, unopened bag of “high-fat” microwave popcorn according to the package instructions. Carefully touch the susceptor; what do you notice about its temperature? Examine the adhesives that seal the top and bottom of the bag. Pour the contents into a large bowl. Record your observations. STEP 4. Fill a clear, colorless, or light-colored cup with hot water. Add about a dozen popcorn flakes from step 3, one at at ime, to the water. Listen! Look! Mix the water and flakes with a toothpick. Record your observations.

STEP 5. While wearing gloves and goggles, add a few drops of povidone–iodine solution to the flake/water mixture. Stir the mixture with a toothpick and allow it to stand for a few minutes. Starch turns blue-purple in the presence of iodine. Record your observations. What’s it happen?


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STEP 6. Transfer flakes to a plastic bag before disposal in a trash can. Pour the remaining liquid into a sink for disposal. Cautions: Be Safe! When popcorn is heated in a microwave oven, the bag and its contents get very hot. Use caution when handling the bag. Heating popcorn too long in a microwave oven can cause the popcorn and/or the bag to burn. Wear gloves and goggles when handling the povidone–iodine solution. From: Marissa B. Sherman and Thomas A. Evans, “Popcorn—What's in the Bag?” - Journal of Chemical Education, Vol. 83 - No. 3, March 2006.

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