VEGETABLE AND FBUIT DEHYDRATION 



161 



Table 14. — Methods of calculating ratios and coefficients of rehydration 



Determination made on 

 sample 



Ratio that may be cal- 

 culated 



Equation and calculation 



Wo 9.45 (weight of original 



sample) . 

 W 1.25 (weight of dehydrated 



sample). 

 Wr 7.50 (rehydrated weight)-- 



A 87.5 (percent water in the 



original sample). 

 B 5.3 (percent water in the dry 



sample). 





Wo 9.45 pounds 7.56 

 W 1.25 pounds 1 

 Wr 7.50 pounds 6.00 





Coefficient of restoration 

 . of weight. 



Rehydration ratio 



W 1.25 pounds 1.00 

 C 7.50 6.00 



Coefficient of restoration 

 of weight. 



Drying ratio . . . __ 



D 1.25 1 

 C 7.50X(100-87.5) = 



C 7.50 pounds, (drained weight 

 of rehydrated sample). 



D 1.25 pounds (weight of dry 

 sample used for rehydration 

 test). 



E 



(D-BD)IQO 1.25- (1.25X0.053) 



100-A 



(7.50) (12.5) 93.75 

 (1.25-0.07) 1.18 



(Rehydration ratio) 100 6X100 _ 



F 



Percent of water in the 

 rehydrated material. 



Coefficient 79.4 ' •"** 

 ' 50 -^ 18 X100-? 3 , 2 -84.3 





Sources of Error 



Needless to say, the technique of performing each test must be care- 

 fully standardized if comparable data of value are to result. The con- 

 trollable factors of rate of heating, time held in the water, temperature 

 of the water, ratio of water to sample, and the composition of the 

 water can be readily standardized. It is not too simple to standardize 

 the number of duplicated tests, the number of points on a given curve, 

 or the number of curves that must be developed in order to gain ade- 

 quate information concerning a sample. 



Effective heat control and accurate timing are of major importance. 

 The results shown in figure 70 (curve a) were obtained at room tem- 

 perature (78° F.) with yg-inch sliced carrots, which regained approxi- 

 mately 41 percent of their original moisture in the first 20 minutes in 

 the water. In the next 20 minutes they gained another 12 percent, 

 and in 50 minutes they had regained 58 percent. 



Other samples (curve b) at boiling temperature regained 69 percent 

 of their moisture in the first 20 minutes, 10 percent more hi the next 

 20 minutes, and at the end of 50 minutes they had regained a total of 

 81 percent. When soaked at room temperature before boiling (curve 

 c) for a half hour, 75 percent of the original amount of water was 

 regained during the first 20 minutes of boiling after soaking, and when 

 soaked for a full hour, 82 percent was regained. In other words, 

 soaking 1 hour and boiling 20 minutes was as effective in rehydration 

 as was 50 minutes of boiling without soaking. 



From this it is obvious that comparative tests are valuable only when 

 time and temperature at which the material is kept in water are 

 carefully controlled. Since all calculations of results are based on 

 the drained weight of the rehydrated sample, only careful weighing 

 and adequate draining will yield drained weights that are worth while. 

 Draining through strainers, cheese cloth, or by gravity through filter 

 paper yields rough approximations, but the Buchner funnel, with 

 gentle suction and careful timing, has given the best results obtained 

 thus far. 



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