What I would suggest is using colorimetric methods such as Lowry to quantify protein content first and then using Kjeldahl or alternatively Dumass to get total Nitrogen. From the data obtained,  the factor could be calculated. 

CHNS you can try and we used few cases. It is quick but not accurate as like kjeldahl.

Read the attached papers.

Best regards

Esam H. Mansour 

Article Gross Chemical Profile and Calculation of Nitrogen-to-Protei...

Article Gross chemical profile and calculation of nitrogen-to-protei...

Data Gross chemical profile and calculation of nitrogen-to-protei...

Thank you dear all for your answers. I think that the more reliable method is to use total amino acid analysis by HPLC and then deduce nitrogen recovered from this AAs by the percent of nitrogen in each amino acid.

But, is there any other simple and not expensive methods? 

Best regards.

Mourad

Thank you Pr. Howgate for your explicit answer and I'm sorry for the delay. But, have you any idea on the deproteinization procedure of fish by-products and on the determination of degree of deproteinization of these biomass, I think that the use of a specific conversion factor for each kind of tissues is necessary.

Best regards

Mourad

Currently the most employed methods to extract proteins from fish by-products are enzymes and ISP (isoelectric solubilization adn precipitaiton), such methods allow to recover proteins that can be used afterwards.

If you are looking for total deproteinization, methods involving the use of acids and alkalis are the most commonly used; however, these methods produce alterations in proteins and its use can be affected.

In  y previous work I have been using Dumas method fro determining total nitrogen recovery and total nitrogen remaining in the bones, scales, etc.

Dear Dr.Carlos and Pr. Howgate

Thank you for your valuable idea, but how to know that my sample is completely deproteinized?

Hi Mourad:

I think the best way to determine if your sample is completely deproteinized is running an amino acid profile analysis. You can analyze nitrogen content, but you won't be sure if ammonium, amines or other nitrogenated compounds remain.

Regards

OK, thank you Carlos

I'm quite sure now that AAs analysis is the most convenient method.

Best regards

Thank you Pr. Howgate

I see that there are many analysis to carry on so that we can meet our needs.