Dextrins, iodine test

Note The color obtained should be recorded, as it is a definite clue to the compn of the dextrin sample. The iodine test can indicate the extent of hydrolysis dextrin has gone through, and the soly characteristics of the dextrin... 

Although the Tilden-Hudson test is very useful it has certain drawbacks in precise work, (a) The exact time at which needles appear is difficult to determine, (b) Enzyme activity is slowed but not stopped by the addition of iodine solution in fact, it is possible to add starch to a mixture of enzyme and iodine solution and observe the gradual formation of the crystalline dextrin-iodine complexes, (c) The apparent activity depends very much on the presence of such foreign materials as salts, n-glucose, maltose, etc., which may be present in the enzyme solution. In some enzyme preparations which seem to have fair activity, the needles are never observed. 

Fig. 17-—Iodine test for Schardinger dextrins. Microscopic appearance typical of the early stage of the decomposition of starch by B. macerans amylase. Under higher power the crystals are seen to be blue hexagons.

Fig. 23.—Radioautograph of paper chromatogram separation of mixture obtained by treating radioactive n-gliicose and inactive dextriii with R. marcrans amylase. Gi, Ga, Os, etc., represent n-glucose, maltose, maltotriose, etc. a, 0, y represent the positions to wliicdi the Sehardinger dextrins move. Note that the Schardinger dextrins them.selves do not become radioactive. After the chromatogram was sectioned, inactive a-dextrin was isolated from the band between G4 and Gs and identified by means of the iodine test.

Iodine test n. Test for the detection of starches and dextrins. It involves the addition of a small amount of a solution of iodine in potassium iodide to an aqueous solution of the suspected substance. Starches give purpose or blue colors according to the degree of hydrolyzation, whereas erythrodextrin gives an orange or reddish-brown coloration. 

When the consumption of iodine is the same in two successive tests and a sample of the mixture is no longer coloured by iodine the saccharification is complete. Usually 75-80 per cent of the starch taken is converted into sugar. The rest of the starch is only broken down into dextrins, which are, however, also saccharified in the course of the subsequent fermentation. The volume of the mash is measured in a cylinder, and from the result of the final maltose titration the sugar content is calculated. For the C02-determination 10 c.c. are retained (cf. p. 402). 

Solubility.— The poly-saccharoses differ from the sugars in the absence of a sweet taste, in their non-crystalline character and in their general insolubility. Inulin and dextrin are soluble in water, glycogen is soluble to an opalescent liquid, while starch and cellulose are insoluble. In hot water starch forms a colloidal solution or emulsion, known as starch paste. Starch reacts with a solution of iodine and gives a beutifiul blue color. This is a characteristic reaction for starch and is used as a qualitative test, especially in microscopic examination. Dextrin exists in several forms, one of which known as erythro-dextrin, gives a red color with iodine. 

Test solutions of dextrose, sucrose, lactose, and dextrin with a very dilute solution of iodine (light straw-yellow in color). 

Preparation and Properties of Dextrin (section 377).— Heat about 5 grams of starch for one-half hour at 220°-225° in an oil-bath or air-bath. Pour the product into a mortar, add 2 cc. of water, and notice the adhesive quality of the mixture. Add 25 cc. of water and grind with a pestle. If there is a residue of starch which has not been converted into dextrin, filter through a folded filter. Use 2 cc. of the solution to determine whether the product reduces Fehling s solution. Test 2 cc. of the solution with a drop of iodine solution and note the color. 

Add to the rest of the solution three times its volume of alcohol. Filter off the precipitated dextrin and wash twice with alcohol. Dissolve a little of the precipitate and test its reducing power with Fehling s solution. Determine the color produced by iodine solution. How does the color compare with that obtained during the intermediate stages of the hydrolysis of starch by acid (experiment 152g page 124). 

Or Mix 500 parts potato starch with 1500 parts of cold distilled water and 8 parts of pure oxalic acid place this mixture in a suitable vessel on a water-bath, and heat until a small sample tested with iodine solution does not produce the reaction of starch. When this IS found to be the case, immediately remove the vessel from the water-bath, and neutralize the liquid with pure carbonate of lime. After having been left standiug for a couple of days the liquor is filtered, and tho clear filtrate evaporated upon a water-bath until the mass has become quite a paste, which is removed by a spatula, and, haring been made into a thin cake, is placed upon paper aud further dried in a warm place 220 parts of pure dextrine are thus obtained. (5a AT). 29250... 

Cellulose dissolves in concentrated sulphuric acid when the solution is diluted with water and boiled, soluble carbohydrates are formed, which vary in complexity from soluble celluloses and dextrins to dextrose, the final product of hydrolysis. When unsized paper is left in contact for a moment with sulphuric acid diluted with about one-fourth its volume of water, the surface of the paper is converted into a colloidal modification of cellulose called amyloid. After washing in water and dilute ammonia the paper is found to have a smooth surface, and to have been markedly toughened by the treatment. The so-called parchment paper is prepared in this way. Amyloid is colored blue by iodine the reaction is used as a test for cellulose. 

An important test for the presence of starch is the reaction that occnrs between iodine, I2, and the coiled form of amylose. The product of the reaction is deep bine in color (see I Fignre 7.16) and is thonght to consist of the amylose helix filled with iodine molecules (see Figure 7.15B). This same iodine reaction is also widely used to monitor the hydrolysis of starch. The color gradnally fades and finally disappears, as sfarch is hydrolyzed by ei-fher acid or enzymes to form dextrins (smaller polysaccharides), fhen maltose, and finally glucose. The disappearance of the blue iodine color is thought to be the result of the breakdown of the starch helix. 

Distinguish between glycogen and dextrin by saturating the original solution with solid ammonium sulphate. Glycogen is com pletely precipitated dextrin is not. Filter. Test the filtrate with iodine, a red-brown colour indicates dextrin. 

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