Praise be to Allah.
There is nothing wrong with eating foods that contain L-cysteine (E920) no matter what its source is, even if it is from pigs or animal or human hair, because it does not remain in its original condition; rather it is transformed into another substance, therefore it becomes pure and lawful.
But if it is proven that harm is caused by that, then it is not allowed to use it, because of the harm it causes.
If we assume that this substance is taken from an impure source such as pigs, and is not transformed into another substance, then if it was in a very small amount that is absorbed (into other ingredients), there is nothing wrong with eating the food that contains it.
This has been explained previously in fatwa no. 102749.
In Fataawa al-Majlis al-Urubbi li’l-Ifta’ wa’l-Buhooth (fatwa no. 34) it says:
In the list of ingredients of some foods there appears the letter E followed by a number. It is said that this means that it contains a substance manufactured from pork fat or pig bones. If that is proven, what is the Islamic ruling on those foods?
What is referred to by the letter E followed by a number are additives of which there are more than 350. They are either preservatives, colourings, flavour enhancers, sweeteners, or other things.
They are divided, according to their origins, into four categories:
1. Ingredients of an artificial chemical origin
2. Ingredients of plant origin
3. Ingredients of animal origin
4. Ingredients that are dissolved in alcohol
The ruling thereon is that they do not have any impact on the permissibility of the food or drink, for the following reasons:
With regard to the first and second categories, that is because they come from a permissible source and there is no harm caused by using them.
With regard to the third category, it does not remain in the same state as when it was taken from its animal source; rather it has been subjected to a chemical process that has changed its nature completely, as it has been transformed into a new, pure substance. This change affects the Islamic ruling on that substance. If it was itself prohibited or impure, then its transformation into a new substance means that it comes under a new ruling, as in the case of alcohol, if it is turned into vinegar, then it becomes good and pure, and as a result of this transformation the ruling on alcohol no longer applies to it.
With regard to the fourth category, in most cases this refers to colourings, which are usually used in very small quantities, so that they are absorbed into the final products, and this is overlooked.
So whatever foods and drinks contain any of these substances in their list of ingredients remain permissible as they originally were, and there is nothing wrong with the Muslim consuming them.
Our religion is easy, and it forbids us to overburden ourselves; making excessive enquiries into such matters is not something that Allah, may He be exalted, or His Messenger have enjoined upon us. End quote. From Fiqh an-Nawaazil by Dr Muhammad al-Jeezaani (4/263-267).
It says in the recommendations of the Ninth Medical Fiqh Council:
Additives in food and medicine that come from impure or prohibited sources become Islamically permissible in one of two ways:
1. Istihaalah (process of transformation)
2. Istihlaak (process of absorption).
That occurs when the prohibited or impure substance is mixed with another substance that is usually pure and lawful, to such an extent that the characteristic of impurity and impermissibility is removed from it, from a shar‘i point of view. That is when the substance that is mixed and overwhelmed loses its characteristics of taste, colour and scent, so that the substance that is overwhelmed is absorbed into the dominant substance, and the ruling applies to the dominant one.
Examples of that include the following:
1. Additives that are used in a form that is diluted in alcohol, and used in very small quantities in the food or medicine, such as colourings, preservatives and emulsifiers.
2. L-cysteine and cholesterol, which are derived from impure sources without undergoing a process of transformation (istihaalah). It is permissible to use them in food and medicine in very small amounts that are absorbed into the dominant mixture that is lawful and pure.
3. Porcine enzymes such as pepsin and all other digestive enzymes and the like, that are used in negligible amounts that are absorbed into food and medicine.
End quote from Majallat al-Majma‘ al-Fiqh al-Islami (10/1431)
And Allah knows best.