Why shouldn’t you crowd a pan with food when frying?

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 hot oil, it stands to reason that you should not cook too much food at 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. Another way to minimize the problem of the deep frying oil dropping below its desired temperature is by preheating the oil about ten degrees above the optimal temperature. 

Use a thick gauged pan for better heat retention. Do not deep fry cold foods, warm the ingredients to at least room temperature before you add them to the hot oil.