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When deep frying sausages, chips, small pieces of coated chicken breast etc, a good rule of thumb seems to be when they are nicely browned and float to the top of the oil, that they are totally cooked through. The only exception to this seems to be anything coated in batter (especially with raising agents), which floats fairly quickly due to the trapped gasses.
Is this an accurate indication of the internal temperature and if so, what exceptions are there to this?
For foods that should not be consumed undercooked, no, floating is not a reliable mechanism for determining if the internal temperature of a food is within the safe zone. All it means is that a sufficient amount of water has been converted to steam and either trapped or expelled that the food has become more buoyant than oil.
Only really safe way to determine if the food is safe is to use a food thermometer. The good news is that cooking times in oil are very consistent. Once you know how long it takes to cook a piece of chicken of a given size, for example, if you maintain the oil temperature you can cook the rest of the chicken to be cooked. Always check the largest piece, and I recommend continuing to check the internal temperature of the largest piece in each batch though.
For food that can be eaten raw, usually they are cut into small enough pieces that you can use the external doneness to determine the internal doneness.
