Sodium reduction in muscle foods: Analytical methods for measuring sodium and changes in the food matrix during sodium reduction in muscle foods.

Salt (NaCl) is the world’s most established food additive, because of its excellent
preservative effects, its positive effect on technological and sensory properties, and low
cost. This combination of factors has resulted in salt being used at higher levels than
necessary in many food products, particularly processed foods. However, a high
consumption of sodium is also associated with an increased risk of high blood pressure,
which has been found to be a significant contributor to the development of cardiovascular
diseases and strokes in humans. In addition, a high salt intake has been linked to increased
risk of stomach cancer, renal stones, and decreased bone mineral density. For these reasons,
the WHO recommends a decrease in sodium consumption, to a total of 5 g NaCl per person
per day (2 g Na per day). To reach the WHO target, the food industry therefore has to reduce
the sodium content in their products by 30 -50%. A reduction of this magnitude could lead to
challenges with regards to processing, physicochemical properties, yield, sensory attributes,
texture, and food safety of the products. Partial replacement of the sodium salt by salt
replacers and other ingredients will therefore be necessary.
Given this, the main objectives of the present thesis are 1) to evaluate different
analytical methods for measuring sodium in a food matrix, and to evaluate changes in this
food matrix as a function of sodium reduction; and 2) to increase the theoretical
understanding of the effect of salt and salt replacers on food sensory and physicochemical
properties. The first objective is of importance given the high focus on the necessity of salt
reduction in food products, and the increased use of salt replacers such as potassium
chloride (KCl) in its stead. This makes it necessary to find new rapid techniques for
determining the sodium content in products. For the purposes of this study, the following
four methods were evaluated: (1) Impedance spectroscopy, (2) LF-NMR T2 relaxation method
(LF-NMR), (3) Computer Vision, and (4) sodium ion-selective electrode. The methods were
applied to food matrixes containing different amounts of salt (NaCl), and the salt replacers
KCl and magnesium chloride (MgCl2). The model products consisted of minced fish prepared
from hake (Merluccius paradoxus/capensis) and haddock (Melanogrammus aeglefinus).
These products had different amounts of pure salts and mixtures of salts added beforehand,
and were analyzed using the different methods above. Knowledge transfer from research in
lab-scale to industry requirements for commercialization has been of utmost importance in
i the work with this thesis. Cooked ham of pork and fish pudding were therefore prepared
with a more complex composition of ingredients in order to investigate the effect of salt
reduction on physicochemical and sensory properties of the final products as well.
The work in this thesis has demonstrated that the method (1) impedance
spectroscopy could be a potential technique for monitoring aw in low-salt low–fat fish mince,
in that it is able to indirectly predict the amounts of salt in the sample. The (2) LF-NMR
measurements on both raw and cooked fish mince added low levels of salt (0-3%), showed
that this method was sensitive to the resultant changes in protein structure of the products
due to the salt addition. The results of this thesis furthermore confirmed that the (3)
multimodal machine vision system (Computer Vision) showed changes in lightness as a
function of reduced salt content in cooked ham, and that the (4) sodium ion selective
electrode is another good method for direct measurement of sodium in low-salt low-fat food
products.
The extractability of salt soluble proteins (SSP), were found to increase with higher
concentration of salt in the products. The results indicated that Na+ can partially be replaced
with K+ and Mg2+ without changing the solubility of proteins. However the addition of Mg2+
should only be used in small amounts, due to the negative effect on protein solubility at 0.55
M MgCl2. Freezing and thawing of haddock fillets furthermore decreased the solubility of SSP
and influenced the physicochemical properties in cooked fish minces, adding other types of
cations than Na+ did not compensate for these changes.
This work also confirmed that the solubility of proteins affect the quality in the final
product. This was demonstrated by both the Water-holding capacity (WHC) and breaking
force increasing with higher levels of SSP in the study on fish pudding, for instance. A linear
relation between breaking force and WHC was also found. In the model product made of
haddock mince, with added water and different amount of pure salts, salt reduction led to
an increase in moisture, reduced WHC, increased cooking loss and a decrease in breaking
force. The pH also increased with decreasing salt content. Replacing NaCl with pure KCl had
no, or only a small, effect on the pH, moisture, breaking force, WHC and cooking loss in the
model products. Replacing NaCl with an equal molar concentration of MgCl2, however,
affected several of the physicochemical properties and in particular WHC, cooking loss and
pH. 
In light of this, and in summation, a partial replacement of Na+- ions with K+ - ions is
possible without changing the protein solubility and the physiochemical properties in fish
products. A reduction of salt from 1.0 down to 0.6%, without any salt replacers, affected the
taste experience more than the physicochemical properties in fish pudding. Based on the
investigated factors in this study a 40% sodium reduction is possible in fish pudding using
high mineral permeate as a salt replacer. The study on cooked ham, however, showed that
only a 25% replacement of Na+ - ions with K+- ions was possible without changing the quality
in the final product. However, when the sodium was reduced by more than 35% (< 2.1% salt
in the product), the salt reduction influenced both the sensory and physicochemical
properties negatively