VEGETABLE AND FRUIT DEHYDRATION 155 



Determination of Starch (Vegetables) 



Starch determinations are often useful in evaluating the quality and 

 maturity of starchy vegetables and may be valuable in considering 

 their drying characteristics. Nielsen's method {37) has proved rapid 

 and useful and is described as follows : 



Apparatus: Photoelectric colorimeter with test-tube adapter, a dozen or more 

 matched test tubes, red filter, blender, balance (analytical or torsion sensitive 

 to 0.01 gm. ) , air oven. 



Reagents: Perchloric acid (2.7 parts by volume of 72 percent reagent-grade 

 solution plus 1 part water), sodium hydroxide (2 N), acetic acid (2 N), 

 potassium iodide solution (10 percent), potassium iodate (0.01 N), sodium 

 thiosulfate (0.1 N), ethyl alcohol, ether, phenolphthalein (0.1 percent in 90 

 percent alcohol). 



Procedure: A 100- to 200-gm. sample of the fresh, frozen, or canned vegetable 

 (or equivalent amount of dehydrated material rehydrated by heating one-half 

 hour or longer in an amount of water necessary to give the 100-gm. sample) 

 is placed in the disintegrator cup with an equal weight of water, and the 

 instrument is allowed to run at high speed for 3 to 4 minutes. Two or 3 

 grams of the ground sample, depending upon starch content, is weighed directly 

 into a 30-ml. beaker on a torsion balance. One ml. of water (or none if 3 

 gm. was taken) is added, and then exactly 3.7 ml. of perchloric acid, prepared 

 as directed above, is slowly added with thorough stirring with a glass rod, so 

 that there will not be momentary high concentrations of the acid in any portion 

 of the sample. The mixture is allowed to stand with occasional stirring for 

 about 10 minutes. If the product is not viscous, the simplest method of stirring 

 is to whirl the contents of the beaker during the addition of the acid as well as 

 in the later stirrings. 



After standing, the mixture is made up to 25, 50, or 100 ml. with distilled 

 water, depending upon starch content, and then poured into a suitable test tube 

 to settle. A 1-ml. aliquot of the supernatant liquid is pipetted into a 100-ml. 

 beaker and 6 ml. water added. A drop of phenolphthalein is added and the 

 solution is brought to a pink color with a few drops of 2 N sodium hydroxide. 

 Now 2.5 ml. of 2 N acetic acid, 0.5 ml. of 10 percent potassium iodide, and 5 

 ml. of 0.01 N potassium iodate are added accurately, and the solution is allowed 

 to stand at least 10 minutes with occasional stirring. Dilute to 25, 50, or 100 ml., 

 depending upon starch content. The color is estimated in a photoelectric 

 colorimeter with a red filter having a transmission range from 640 to 700 milli- 

 microns. The colorimeter should be set at zero absorption or the readings cor- 

 rected with a blank containing all of the reagents. If the filtrate used in 

 developing the starch-iodine color is turbid, an extra blank reading should 

 be made if precise results are desired. This can be done by discharging the 

 blue color with a few drops of 0.1 N sodium thiosulfate and comparing the 

 turbid solution against the first blank with its iodine color discharged by 

 thiosulfate. 



The percentage of starch is calculated from a curve prepared from the 

 colorimeter readings of a known range of starch concentration. For solution 

 depths of about one-half inch, the best range of starch concentrations is from 

 to 3 mg. per 50 ml., preferably 1 mg. 



Soluble starch cannot be used for standardization. If accurate results are 

 desired, starch prepared from raw unblanched material similar to that which 

 is to be analyzed should be used. Equal weights of starch from different prod- 

 ucts, such as potatoes and peas, do not give the same amount of color with 

 iodine ; therefore one type of starch cannot be used as a standard for all products. 



Since factors have now been established among the various starches, a single 

 standard, such as potato starch, suffices. If a series of analyses are to be 

 carried out on a given product and only relative results are desired, then 

 potato starch, which is easily prepared, can be used to prepare a standard 

 curve. 



Disintegrate the raw product in the mixer with an equal weight of water. 

 Separate out. the fibrous material by washing the ground pulp through a 60- 

 mesh screen. Now place the material that passed through the screen in a large 

 beaker or pan and stir with a large volume of water. Allow the starch to settle 

 and decant off the water. Repeat this process until the starch is free of extra- 

 neous material. Now wash the starch with alcohol and ether and dry in an oven 



