8 BULLETIN 1055, U. S. DEPARTMENT OF AGRICULTURE. 
Oil can not be used indefinitely without being renewed. After 
prolonged use of the oil the chips do not brown well and take too 
long to cook. Much foreign matter has been absorbed by the oil, 
which can not be removed by the most careful straining. It should 
be thrown away or used for soap grease. The common practice in 
potato-chip factories is to replace what is used up in the cooking 
process by adding fresh oil. This, however, should not be continued 
indefinitely, entirely fresh oil being required at least every few days. 
The most successful potato-chip factory which was visited makes a 
practice of renewing the oil every second or third day. The small 
particles of potato are skimmed out after every batch of chips is re- 
moved and all the oil carefully filtered each night after the close of 
business. New oil is added as needed during the day, and every sec- 
Fic. 5.—Soaking the different lots of potatoes in pans of cold water preparatory to frying 
into chips. The last lot is still under the faucet of running water. 
ond or third night the contents of the kettles are emptied and sold 
for soap grease. The result is a high-quality potato chip that will 
keep sweet for weeks. Some manufacturers make no effort to renew 
their oil entirely, simply adding fresh oil as needed. Asa result the 
oil is never entirely sweet; and the old, worn-out oil which is always 
present affects the ease of cooking, the flavor, and the keeping quality 
of the chips. 4 
In the refined cottonseed oil sold for cooking purposes there is re- 
markably little of the foreign matter from which the oil 1s expressed. 
Small particles of such foreign matter act as ferments if left in the 
oil and set free fatty acids which make the oil turn rancid and lower 
its smoking point. Chips can be cooked at a higher temperature in 
oil that is free from such impurities than in oil that has not been 
as highly refined. The small particles of fried potato or of the plant 
from which the oil is expressed burn and smoke at a comparatively 
low temperature and impart a scorched flavor to the oil, which is 
