Modern candling system do not only determine if hatching eggs contain an embryo by the amount of light that can pass through the egg at 18 days, but also if the embryo inside is still alive or not, by measuring the heart beat of the embryo or the temperature of the eggs compared to the surrounding eggs. As death embryos obviously do not have a heart beat or produce heat, it can not only determine fertile from infertile or early dead, but also if the embryo at a later stage has died, with or without contamination.

This new technology is very valuable for modern hatcheries, but we sometimes see that it is not very accurate and rejects more embryos than just the dead ones. This often has to do with the temperature of the eggs.

Embryos at 18 days of incubation react as if they are "cold blooded": their metabolism is related to their temperature. That means that if the temperature of the eggs is going down, their metabolism and with it their heat production and heart beat rate go down as well. Already in the beginning of the previous century people have observed that there is a lineair relation between the temperature of the egg and the heart beat.

This means that when we use heart beat as an indicator to determine if an embryo is alive or not, we need to make sure that the temperaure of the egg remains at the required level. For an embryo at 18 days of incubation it is no problem if its temperature drops for several hours or even a day. It can survive levels as low as 20-25 degrees without any problem, once it warms up again it will just continue its development, and often even hatches better as the period of lower temperature will take away part of the heat stress that can build up. 

But if the temperature of the embryo drops and its heart beat slows down (as if it is hybernating) the "heart beat" based candling systems will not detect a live embryo anymore and will remove much more embryos than they should.

This means that with such a candling system a strict process of transferring is important. If we take out eggs from the setter but leave the trolleys in front of the candling machine for too long, especially when the room temperature is low, when there is more air velocity, a draft from the air inlets or when there is water sprayed, the results can be dissappointing.

This temperature control can be quite critical: even after half an hour we sometimes already see an effect, if the conditions are not favourable. And this effect can be severe, in extreme situation we can see up to 10% of embryos being wrongly classified as mid dead.

An accurate candling system based on heart beat or temperature monitoring has many advantages but requires a strict protocol for transfer to avoid mistakes. These mistakes are not the fault of the candling machine by itself, but with these kind of systems we have to make sure that technology respects the biology.