156 



THE POTATO. 



same circumstances. 



table : 



The results are contained in the foJowins 



In the particular circumstances under which this experiment wa? 

 made, therefore, it is obvious that the Rohan variety contained the 

 larjrest quantity of nutritive matter and starch. 



Potatoes which have been exposed to a temperature a few degrees 

 below the freezing point of water, undergo so great a change in their 

 texture, that it becomes difficult afterwards to extract the starch 

 which they contain. They besides acquire so disagreeable a flavor, 

 as all the world knows, that cattle sometimes refuse to eat them. 

 After having ascertained that a potato has the same chemical com- 

 position before and after congelation, M. Payen examined the starchy 

 substance under the microscope, and found that the starch obtained 

 from a frozen tuber presented itself in compound granular masses, 

 four or five times the size of the largest natural grains of starch. 

 The pulp which remained upon the sieve in the preparation of this 

 starch, was formed by a collection of cells, for the most part full of 

 starch. It would therefore appear, that in consequence of the 

 changes of volume of the fluid successively congealed and liquefied, 

 the adhesion between the cells was destroyed : they become separa- 

 ble with the slightest force, and merely part one from another by 

 the action of the grater without being torn ; the larger number re- 

 main unbroken and still filled with starch. This fact enables us to 

 understand how potatoes, which have been frozen, will yield nearly 

 the whole of their starch if they be treated before they are thawed. 

 The cells then sealed up by the congealed water resist sufficiently 

 to be broken by the teeth of the grater. Potatoes which have been 

 i'rozen, are generally less farinaceous, at the same time that they have 

 a decidedly sweet taste, which, according to M. Payen, is owing to 

 vegetation having already made some progress in the tubers before 

 congelation ; and we know that during germination there is always 

 a formation of sugar at the expense of the fecula. Frosted potatoes 

 have always a disagreeable taste, and a most unpleasant smell, so 

 that in many places they are thrown upon the dunghill. The effect 

 of the frost, in fact, is to set the juices which are enclosed in the 

 tissue of the potato at liberty, and the higher temperature which ac- 

 Isoinpanies and follows i thaw , exposes these juices to be acted upon 



