I have bought a 3 liter deep fryer. There was a 1.5 liter one available too but I wanted a larger basket. This 3 liter deep fryer requires 2.5 liters oil minimum. I had no idea about this. This quantity seems too much.
Can I use less? Has anyone any idea what would happen if I use less than the recommended? If I cannot use less than the minimum what do I have to do to make it last longer?
If you need a larger basket to fry the amount of food you want to cook, you need more oil as well. If you don't always want to make that much food (or use that much oil) then you need a smaller vessel for deep-frying, such as a second fryer or a small wok or kadai.
If you put less oil in than the minimum it may not cover the food and/or the oil could overheat, depending on how the fryer manages its temperature settings.
Overheating oil can be dangerous and will also burn your food while leaving it raw inside. All around, something to avoid.
You can save and reuse frying oil. There are some good tips at the sites below.
Let it cool fully then filter out food particles (I use coffee filters and a funnel for this; it can be a slow drain, be patient)
Refrigerate used oil to retard bacterial growth
Stop using it if it looks weird or smells funny!
http://www.seriouseats.com/2013/09/ask-the-food-lab-how-many-times-can-i-reuse-fry-oil.html
http://extension.psu.edu/food/preservation/faq/questions-and-answers-about-re-using-frying-oil
http://www.epicurious.com/expert-advice/you-can-reuse-frying-oil-article
The whole point of deep frying is to use large amounts of oil. You don't want to just coat the food. You really need to have the food swim in a mass of oil much larger than the mass of the food. Else the food will cool down the oil locally and you won't get the desired crust.
So no, as long as you are using a large basket (which holds a large amount of food), you can't do proper deep frying with a small amount of oil. A consumer style deep fryer is probably already on the low side of the correct oil/food ratio.
I've used two different home-kitchen deep fryers over the last 18 years or so; one of them was essentially a kettle with a hidden heating element, and the other has a removable heating element that sits under the frying basket.
The early one I used, the type with the hidden heating element, really has no minimum other than the basic rule-of-thumb that want at least 3x-4x the volume of oil to food being cooked, so that the oil doesn't cool down too quickly. The basket was the other factor, but if I remember correctly, my basket got pretty close to the bottom of the pan, so that wasn't a major consideration.
A more serious model that I now use, which has a feature for filtering/draining oil, a more convincing thermostat, and electronic timers, has some more serious constraints, notably a minimum oil content of about 2.5 liters, and a maximum of maybe a tad over three liters. The minimum is pretty simple to explain: If you use less than 2.5 liters, none of the food will have enough room to float above the basket. You probably won't even cover the heating element without at least a liter of oil in there.
Not covering the heating element has some serious consequences, namely the risk of the element glowing and eventually over-heating (based on the thermostat probe placement), oil spatter, and the more practical one of not being able to use the basket.
The maximum fill line is an approximation designed to reduce the risk of oil boil-over. Having dealt with a oil boil-over on a friteuse meant for stovetop use in my starving student days in Germany, I can tell you the consequences are unpleasant. I had scars to prove it. High water content in some foods leads to furious bubbling, and if those bubbles manage to carry oil over the side of the pan, bad things happen.
I'd dare say a small amount of oil can work if there is ample,controlled resupply of heat - why would it make much difference if it is coming from the vessel wall or more oil.
However, I found that deep friers are just not designed for that, relying on the oil fill as thermal mass a lot... even or especially the small "fondue" style ones*.
TL;DR for the rest of post: Advice on deep frying without using a fryer. Use at your own risk.
You are better off with a chip pan/wok/pot and a thermometer for the kind of job where you just want to oil-blanch an ingredient and/or want to minimise oil usage because you do not want to reuse it (eg you want to fry tofu in fresh oil because it tends to be a magnet for the worst flavors it can get from old oil, you want to deep fry something with a strongly spiced and/or unruly rub/breading/batter on it, you want to fry ingredients like carrots or red bell pepper that will discolor the oil, you want to heat the oil very high initially to account for a temperature drop).
The largest plate on even a normal electric hob is around 2000W - same as a medium sized home deep fryer, and pots and pans are optimized to transfer these 2000W to what is in them.
If you are not confident with open deep frying, have someone who is work with you for a while - yes it is hazardous, but manageably so - never leave the setup unattended. especially if no food in oil, monitor thermometer closely. always have something made of metal (nothing else) in reach that can cover the pot. always assume something can boil over. keep clutter away from your work area. use really high-heat oil - peanut, refined safflower, soy, rice bran; not some unspecified compound oil with a 180°C warning. be especially careful with induction, can heat a pot with small amounts of liquid in them alarmingly quick. keep the lid off as long as you are heating. if working on gas, ask someone who has experience in doing so for additional advice (overboiling into the burner is an obvious danger!), i'm electric...
*the small ones that are partially plastic in construction - never managed to set a grease fire, but I think I would be far more at a loss to control one with these things than in a pot...
