Formol titration

The formol titration is not influenced by removal of CCP, suggesting that aNH2-groups of lysine are not involved. 

Gould, I. A. and Frantz, R. S. 1945. Some relationships between pH, titrable acidity, and the formol titration in milk heated to high temperatures. J. Dairy Sci. 28, 387-399. 

Alternate Protocol 3 Formol Titration to Determine Degree of Hydrolysis B2.2.7... 

The ninhydrin reaction (see Basic Protocol 1), the TNBS reaction (see Alternate Protocol 1), the fluorescamine reaction (see Alternate Protocol 2), and formol titration (see Alternate Protocol 3) all evaluate released amino groups by comparing the amounts of free amino groups before and after hydrolysis. The first three methods are spectro-photometric techniques, whereas the fourth is a potentiometric technique. The first and second are chromogenic techniques, whereas the third is fluorometric. These techniques are usually performed as time-course experiments. As the hydrolysis reaction proceeds, aliquots (samples) of the reaction are taken periodically and treated with a test reagent. Products of this reaction are proportional to the amount of free amino groups at each time point. 

Protein hydrolysis, evaluating degree of fluorescamine reaction, 146-147 formol titration, 147-148 ninhydrin reaction, 142-144, 153 pH-stat, 149-151, 153 TCA solubility index, 152 TNBS reaction, 144-146... 

Formol titration is a method that estimates amino groups by titration with NaOFI and a phenolphthalein indicator (Vakaleris et ah, 1960). Addition of formaldehyde to the neutralized mixture reduces the pFI by making the amino groups less basic. The amount of NaOFI required to retitrate the mixture has been used as an indicator of proteolysis. Another titrimetric method relies on the increase in buffering capacity of the cheese during ripening and has been applied to study proteolysis in Swiss cheese (Lucey et ah, 1993). 

Taylor, W.H. (1957) Formol Titration An evaluation of its various modifications. Analyst, 82, 488-498. 

An alternative, simpler, procedure for improving the inflexion in the neutralization of an amino-acid is to add formaldehyde to the solution although this does not affect the acid-titration curve, the one for alkaline titration is changed, as seen in Fig. 107. The effect of the formaldehyde is to increase the strength of the ammonium ion acid which is being titrated, and so the pH inflexion at the equivalence-point becomes much more obvious. This is the basis of the formol titration of amino-acids discovered by Sorensen (1907) approximately 10 per cent of formaldehyde is added to the solution which is then titrated with standard alkali using phenolphthalein as indicator. In the presence of thii concentration of formaldehyde the pH-neutralization curve has a sharp inflexion in the region of pH 9, and so a satisfactory end-point is possible with the aforementioned indicator. 

XIII. Formol Titration of Amino Nitrogen. J. biol. Chem. 139, 1 (1941). 

Formol titration and other methods of determining terminal amino groups are also used to measure the average molecular weight of protein hydrolysates, but they are imprecise because —NH2 groups of amino acid side chains also react and the molecular masses are usually underestimated. 

Formol titration after the addition of formaldehyde to a solution of the sample at pH 8-9, the free amino acids ate determined by titration with sodium hydroxide solution. 

Another kind of titration after transformation is provided by the formol titration of a-amino acids (Sprensen s method). We have already seen that it is impossible to titrate them in water with sodium hydroxide, because of their macroscopic ionization constant on the order of 10 -. The reaction with the hydroxide ion is not sufficiently quantitative to be used as a titration reaction ... 

The proteolytic enzyme of papain is active in both acid and alkaline media. Bullock and Sen recommended casein as substrate for formol titrations. The importance of activation, preferably with cysteine hydrochloride, was established and the follov/ing assay proposed. 

---