SALT CONCENTRATION AND Aw OF FISH PRODUCTS
Richard Chivers asked on 6 September for the Aw of smoked salmon of 3.5%
salt concentration in the water phase. Lasse Jørgensen replied on the 7th
quoting a formula relating salt concentration and Aw based on studies at his
laboratory. The Aw of salted, undried, fish products can be estimated from
salt concentration with a considerable degree of accuracy because the Aw is
almost entirely determined by the salt concentration in the water phase. The
contribution of the intrinsic solutes in fish muscle to the colligative
properties of the tissue is negligible. When using a relationship between
salt concentration and Aw from a literature reference care must be taken in
using the correct units for the salt concentration based on the analytical
results. The analysis gives the water and the salt contents based on the wet
sample and usually expressed as g/100g of sample. One way of expressing the
salt concentration is the ratio of salt content to water content times 100.
This gives the weight of salt in 100g of water. The other way of expressing
concentration is the ratio of salt content to (salt plus water) content.
This gives the weight of salt in 100g of solution. In Britain at least it is
customary to use the latter form, g/100g of solution. For reasons I will
come to later, I believe Lasse used the former convention, g/100g of water.
For converting salt concentrations to Aw I use Table 8.3 in the book by R A
Robinson & R.H. Stokes, 'Electrolyte Solutions', London, Butterworths, 1959.
This lists molality (not a spelling mistake) of NaCl solutions and the
corresponding Aw. From these data a quadratic prediction equation to
estimate Aw from molality can be calculated. Molality is the number of moles
of solute in 1000ml of water; molarity is the number of moles in 1000ml of
solution. NaCl concentration expressed as g/100g of water multiplied by 10
and divided by the formula weight of NaCl, (58.44), gives molality. For
concentration expressed as g/100g of solution, molality is given by salt
concentration/(100-salt concentration) times 1000 divided by 58.44. The
table in Robinson & Stokes can then be used to estimate Aw.
Using a spreadsheet I have calculated the molality and Aw of NaCl solutions
expressed in both the conventions. The paper cited by Lasse in his message
gives a quadratic equation for relating salt concentration and Aw based on
measurements of samples. The estimated Aw's from this equation are almost
identical to what I estimate from the R&S data when based on concentration
expressed as g/100g of water. The paper does not give the basis for
expressing salt concentrations, but based on this closeness I assume it is
g/100g water. The paper compares the results with those of Chirife & Resnik,
J. Fd Sci, 1984, 49, 1486-1488,but I do not have a copy of this paper to
check the concentration units.
For those interested in detail, and perhaps in checking my calculations, I
attach a copy of an Excel spreadsheet with the data and calculations. The
leftmost columns reproduces the data from the R&S table, and the
coefficients of the quadratic regression. Columns H-J show the molalities
and Aw's calculated from the R&S data for set concentrations expressed as
g/100g water. Columns K-M are the equivalent for concentration expressed as
g/100g of solution. Column N shows the Aw's calculated from the expression
in the paper cited by Lasse and using concentration expressed as g/100g
water.
Ken Hildebrand asks in a message of 7 September if Lasse's Aw-WPS
relationship holds true in the presence of sugar in the cure. Sucrose is not
nearly as effective as NaCl in exerting colligative effects. NaCl
contributes up to two entities towards its colligative properties depending
on degree of dissociation which in turns depends on concentration, so its
effective molecular weight as far as colligative properties are concerned is
around 30. Sucrose does not dissociate in solution and its formula weight is
342 so it has only about 9% as effective as an equal concentration, wt/wt,
of NaCl. A approximate correction for the contribution of sucrose in
lowering Aw is to calculate its molality from its concentration in the
sample and add a tenth of this to the molality of the NaCl before estimating
the Aw. A 5% sucrose concentration in the water phase, (molality of 0.015),
would be approximately equivalent to adding 0.001 to the NaCl molality. Some
of the samples studies by Lasse were cured with salt and sucrose, but,
judging from the scatter diagram in the paper, the Aw's of these are on the
same Aw/concentration line as the samples cured with salt only.
Peter Howgate
----- Original Message -----
From: Lasse Vigel Jørgensen <lvj@dfu.min.dk>
To: 'Richard Chivers' <richard@fishonline.co.uk>; seafood
<seafood@ucdavis.edu>
Sent: Thursday, September 07, 2000 9:15 AM
Subject: SV: Smoked salmon, pH/Aw
Dear Richard,
The initial pH of cold-smoked salmon is 6.0-6.2 and during storage it might
drop to 5.7 or stay at 6.0-6.2 depending on what kind of spoilage occurs. I
have recently studied the relationship between aW and water phase salt in
cold-smoked salmon and showed that the following equation express this
relation:
aW = 1 - (0.005 * WPS) - (1.3*10E-4 * WPS*WPS)
If you intested you can read more about it in the following referneces.
Jørgensen, L.V., Dalgaard, P. and Huss, H.H. (2000a) Multiple compound
quality index for cold-smoked salmon (Salmo salar) developed by multivariate
regression of biogenic amines and pH. Journal of Agricultural and Food
Chemistry, 48. 2448-2453.
and my Ph.D. -thesis that will soon be available:
Jørgensen, L.V. Spoilage and Safety of Cold-Smoked Salmon. Ph.D.-thesis.
Danish Institute for Fisheries Research, Dept. of Seafood Research.
fish@dfu.min.dk.
Regards
Lasse Vigel Jørgensen, M.Sc. Food Sciences
Danish Insitute for Fisheries Research
Department of Seafood Research
Technical University of Denmark, B221
DK-2800 Kgs. Lyngby
E-mail: lvj@dfu.min.dk
Phone: +45 45 25 25 68
Fax: +45 45 88 47 74
> -----Oprindelig meddelelse-----
> Fra: Richard Chivers [SMTP:richard@fishonline.co.uk]
> Sendt: 6. september 2000 13:19
> Til: seafood
> Emne: Smoked salmon, pH/Aw
>
> Does anyone have figures for the pH and Aw ranges for smoked salmon which
> has a minimum salt content of 3.5% in the water phase please?
>
> Thanks
>
> Richard Chivers
> Fisheries Consultant
> Seafood Audit International
> www.fishonline.co.uk <http://www.fishonline.co.uk>
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