It is unsatisfactory to fry or deep-fry in oil that isn't hot enough, and since the addition of room-temperature food will surely lower the temperature of a hot oil, it stands to reason that you should not cook too much food at any one time. The less food you add to the oil, the smaller the drop in oil temperature and the quicker the temperature recovery time. When deep-frying, use at least six volumes of oil for each volume of food.
You can also help minimize the problem of the deep-frying oil dropping below its desired temperature by preheating your oil about 15oF above its optimal deep-frying temperature. If the ideal temperature is 375oF, preheat to 390oF (but no higher because you would risk ruining the oil's cooking effectiveness by chemically altering its molecular structure). A temperature of 400oF or higher taxes even an oil with a 450oF smoke point. It also increases the number of fat molecules that vaporize into the air and make a greasy landing on your kitchen cupboards.
Here are two other tips. Use a thick-gauge pan for better heat retention. Warm your ingredients to at least room temperature before you add them to the hot oil. Otherwise, the oil temperature may drop into the "greasy" 300o to 325oF zone - and will take time to return to the proper frying temperature.
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