Formol index for nitrogen

[Claude Jolicoeur]

I was wondering if someone was aware of (or has tried) a test for
evaluation of the EAN (Easily-assimilable nitrogen) in a must. This
test is called Formol index and has been developed for wine. Here is
what I can read in "Handbook of Enology, The Microbiology of Wine and
Vinifications":
"Aerny's (1996) formol index provides a simple estimate of the free
amino acid and ammonium cation content. A formol index of 1
corresponds to 14 mg of amino nitrogen per liter. [...] The formol
method provides a quick, simple assessment of available nitrogen
deficiency by assaying NH4+ and free amino acids, except proline. It
should be more widely used to monitor ripeness and fermentation."

The method is also described in the Spanish book by Pereda Rodriguez
on cider. This seems particularely interesting, for example to
evaluate the EAN before and after keeving...

Claude

[Andrew Lea]

On 26/08/2012 23:43, Claude Jolicoeur wrote:
> I was wondering if someone was aware of (or has tried) a test for
> evaluation of the EAN (Easily-assimilable nitrogen) in a must. This
> test is called Formol index and has been developed for wine.

Claude,

The formol titration for amino nitrogen fruit juices is quite well
established and the original procedure by Sorensen goes back to 1907
(Wikipedia). (Sorensen was the man who gave us the concept of pH so all
hail to him!) Here is a recent procedure for grape juice
http://www.moundtop.com/yan/FormolTitration.pdf There are many many
variants of this procedure and different ways of expressing the 'index'.

It needs a fair degree of chem lab skill to make it work well and it is
only ever an indication of free amino nitrogen because it relies on some
quite complex chemistry with the formaldehyde (see for example
http://www.jbc.org/content/99/3/767.full.pdf). You can buy automated
equipment to do it now
http://www.hannainst.com/usa/prods2.cfm?id=007001&ProdCode=HI%2084433

It has its uses though and from the 1980's became part of the German RSK
system for detection of adulteration in fruit juices which is where I
first met it. We used to do it routinely in my authenticity lab. It is
still sometimes used in technical papers on apple juice composition. The
RSK range for formol index in apple juice is 2.5 - 10 whereas for grape
juice is 10 - 30, so the apple juice titres are quite low which makes it
more difficult to do accurately in apple juice (both expressed as ml of
0.1N NaOH per 100 ml of solution).

I have not seen its use in connection with keeving experiments although
I know for a fact that the formol method was in use at the INRA cider
research group at Rennes (Le Rheu) in the 70's, so they may have done
so. I suspect that after keeving the values obtained would be very very
low (they are low in apple to start with) so it might not be very
accurate. Also it only measures free amino nitrogen and not thiamine
which is another key limiting factor for yeast growth.

A final point for the hobbyist is that formaldehyde solution (formalin)
is classed as a carcinogen so it might be difficult to obtain nowadays
outside of a controlled laboratory environment. It is increasingly
difficult in the current regulatory situation for the 'man in the
street' to get hold of once common laboratory chemicals for private use.

It would be a lovely idea to test it out in conjunction with keeving
experiments to see if it has value. I have often thought of it but never
done so! I have a copy of the Spanish book you mention, but as far as I
can see although he describes the formol method he does not describe any
actual use for it nor what to do with the figures obtained!

Hope this background is useful.

Andrew


--
Wittenham Hill Cider Portal
www.cider.org.uk

[Claude Jolicoeur]

Andrew Lea wrote:
> It would be a lovely idea to test it out in conjunction with keeving
> experiments to see if it has value. I have often thought of it but never
> done so! I have a copy of the Spanish book you mention, but as far as I
> can see although he describes the formol method he does not describe any
> actual use for it nor what to do with the figures obtained!

Thanks so much Andrew!
I am really surprized that you "have thought of it but never done so"!
It would be interesting to know the % reduction after a keeve, or
would it bring it near zero?

As of the Spanish book, the author does give numbers corresponding to
a poor and a rich must, so I guess from the measured data, one can
have an idea as to where he is standing.

Claude

[Andrew Lea]

On 27/08/2012 16:59, Claude Jolicoeur wrote:
>
> Thanks so much Andrew!
> I am really surprized that you "have thought of it but never done so"!
> It would be interesting to know the % reduction after a keeve, or
> would it bring it near zero?

UK data says about 50% reduction (soluble nitrogen measured by Kjeldahl)
http://www.cider.org.uk/beech_larsrept1950.pdf I think the French data
I've seen is similar though I don't have it to hand.

>
> As of the Spanish book, the author does give numbers corresponding to
> a poor and a rich must, so I guess from the measured data, one can
> have an idea as to where he is standing.

Yes you are right he does give numbers (120-150 mg/L 'rich' and 50 mg/L
'poor'). They are comparable to the figures in this French paper
http://www.scientificsocieties.org/jib/papers/2008/G-2008-0602-546.pdf
see Figure 4C (obtained by Kjeldahl) where nitrogen drops from 130 mg/L
to 30 mg/L after fermentation. When I was at Long Ashton we reckoned 100
mg/L N (Kjeldahl) was about right for a good slow fermentation and that
300 mg/L was way over the top (at that time a lot of juices from
juvenile fertilised dwarf trees were coming in at that sort of level).