Salt, meat and infrared

Cheese and charcuterie (cured meats), often featuring in French meals (!), are foods for which the manufacture and flavor rely on the use of salt, NaCl, to transform a raw material (milk or meat) through one or several stages. This involves procedures known and used for centuries, the original purpose being to extend the shelf life of food; a whole salami or a wheel of gruyere can be kept without refrigeration for much longer than a steak or a liter of milk.
However, it is now accepted that something that tastes good is, unfortunately, not necessarily good for the health and scientists are looking at ways of reducing the amount of salt used, while, at the same time, not compromising on the nutritional qualities and flavor of the product. In a nutshell, doctors recommend reducing one’s daily salt intake to 6 g, compared to 8 - 10 g at present.

Adding less salt to a vegetable dish will just alter its taste, but doing the same to pork products, for instance, could lead to much more serious consequences, with health implications (higher bacterial contamination risks) and manufacturing factors, the salt concentration directly influencing the water retention and texture of the product.

The research group led by Thierry Astruc is studying in detail, at the cell level, the modifications that result from changes in salt concentration (variations in ionic strength*) on meat. As meat is composed of adipose cells, connective tissue and several types of muscle fiber, its heterogeneity makes it all the more difficult to characterize these modifications. For the same ionic strength, some components will not be affected; others will become denatured or even solubilized.
The mechanisms involved are still not well understood. Apart from a purely fundamental interest (why one type of fiber resists better than another), understanding these mechanisms would allow the amount of salt to be adapted to muscle characteristics, since certain meat products (e.g. cooked ham) are prepared by separating the muscles and reconstituting the finished meat product as required.
However, the long-term aim of this INRA group is also to look at new ways of preserving the quality of products at the same time as reducing salt levels needed in their manufacture: to find a salt substitute that would increase the ionic strength without affecting the flavor of the product and at the same time reduce the risk of hypertension!

The experimental protocol followed by Thierry Astruc and his group was as follows: Samples were prepared in the laboratory in Clermont Ferrand. Very thin transverse muscle sections, so thin that several sections could be cut through the same cells, were “sliced” in series.
Out of the series of sections obtained, some were used to identify the different components of the muscle sample by specific staining. Others, not chemically treated, were analyzed with infrared spectroscopy on theSMIS beamline at SOLEIL. All had been previously incubated in varying concentrations of NaCl. Sample staining helped to identify the areas of interest, i.e. the structures and cell types on which the infrared light beam would be directed. This was repeated for each salt concentration under study.

As the beam on the SMIS is very narrow (5 x 5 µm), scientists can “target” the space between cells or even inside a given cell, this precision not attainable with an infrared source from “conventional” laboratory equipment. It is therefore possible to take several measurements at different places in a particular cell, to obtain an average value for this cell.
The brilliance of the beam also reduces the time required to take the measurements; a not insignificant benefit given the number of areas under analysis, each repeated over a series of sections.

Changes in the peptide bond, by which amino acid components of proteins are assembled, are measured by infrared microspectroscopy (which detects amide bonds) in relation to salt concentration. These changes reflect the possible denaturation of proteins present in the area being analyzed. They can therefore be followed over several cell types, showing differences in behavior depending on ionic strength.

Figure 1: example of an infrared spectrum of each of the muscle components studied in this work. A muscle mainly consists in 3 types of tissues: (1) fat (lipids); (2) muscle fibres: contractile fibres made of actin and myosin polymers (myofibrillar proteins); (3) elastin and collagen, structural proteins. Perimysium is a tissue that assembles the muscle fibers in fasciculi (“bundles”).
 

The first results with SMIS have been very encouraging and Thierry Astruc would like to go on using this beamline. However, he is also planning other experiments, this time on DISCO, to measure fluorescence in the UV field. The aim would be to follow the chemical changes in tissues by studying an auto-fluorescent molecule that loses its fluorescence when destroyed, for example after the tissue is cooked.
He is also interested in X-rays: X-ray fluorescence would allow salt to be localized on the tissue slices and to understand the heterogeneity of salting sometimes observed in muscles (connective tissue and lipids are thought to slow down the diffusion of salt).
Results that will soon show the complementarity of SOLEIL’s techniques and beamlines and that you will be able to access on our website as soon as they are published.


* Ionic strength: one of the main factors influencing ion activity in aqueous solution. Ionic strength depends on the concentration and the number of electric charges of the different ions present in a solution.