Experiment 16

Mix 1/4 tsp. starch in 1/4 c. cold water. Filter through filter paper. Test the filtrate and also the residue on the paper with iodine for starch. Did the starch pass through the filter? Is starch soluble in cold water?

Experiment 17

Mix 1/4 ts. starch in \ c. cold water. Heat to boiling. Filter. Test as above. Did any of the starch pass through the filter? Is starch at least partly soluble in boiling water ? Examine some of the cooked starch under the microscope. Has any change taken place in the form of the granule ?

Experiment 18

Pour boiling water directly upon dry starch. Examine lumps. What caused them to form?

Experiment 19

Mix starch with cold water and stir into boiling water. Did lumps form in the mixture ? Give one method by which lumps may be avoided when starch is used to thicken a mixture.

Dextrin

Before starch can be absorbed into the blood, to be utilized in giving heat and energy to the body or to be stored as fat, it must first be changed to a soluble carbohydrate or form of sugar. Before the final change to sugar takes place an intermediate product is formed called dextrin. This has the same chemical formula as starch (C6H10O5)n, but possesses different properties.

Dextrin may be formed (1) by the application of heat to dry starch, as in browning of flour; (2) it is the first change that occurs when the enzymes, ptyalin of saliva, amylopsin of the pancreatic juice, or diastase of sprouting grains, convert starch into sugar.

Identification Test For Dextrin

Brown some flour without burning it, add water; shake well. Add a drop of iodine. The reddish brown color which results is a characteristic test for dextrin.

Experiment 20

Mix some browned flour or well-browned toast with water. Let stand awhile. Filter. Test filtrate for dextrin.

Is dextrin soluble in cold water? Does it differ from starch in this respect ?

Experiment 21

Test the brown crust of bread, and also the white crumb of the center of the loaf, for dextrin.

Which is the more soluble, hence the more digestible, the crust or the crumb of bread ?

Cellulose

Cellulose forms the basis of the cell structure of plants. Cotton and linen fiber are nearly pure cellulose. Paper consists largely of cellulose. Cellulose, as a rule, is hard and dense and resists the action of the digestive juices, so that when taken as a food, it is excreted without having been changed by the digestive juices. The cellulose of young and tender plants may be in part digested. Cooking tends to soften cellulose, thus making vegetables, cereals, and fruits more digestible; and it also disintegrates the starch granule, making the starch available as food.

While having but little nutritive value, cellulose is very beneficial in the diet, as it is a mechanical stimulus to the action of the large intestine, thus tending to prevent constipation. It also absorbs and dilutes the waste products formed during digestion. Uncooked cellulose should not be given in large amounts to young childrem