A level playing field – the risks and rewards of salt

Salt reduction has been embraced by the food industry for a considerable time. Many solutions have been developed, often specific to a certain product category, reflecting the many specific challenges for food producers, with no one-size-fits-all solution as yet available. Further investigations are continuing to be carried out by ingredient suppliers and food manufacturers. While reformulating products, it is important to understand the ingredient interactions, and to consider the sensory perception, food safety, processing and quality aspects.

Salt has its own taste that we are all very familiar with, and is classified as one of the five basic tastes along with sweet, sour, bitter and umami. Although other chemicals can also give a salty taste, sodium chloride is generally recognised as a 'pure' salty taste. Salt, like the other basic tastes, forms the foundation for the overall flavour response, created by a combination of the basic tastes, odour volatiles and trigeminal (irritant) response.

Salt is also important as a flavour enhancer in foods, in particular for savoury but also in sweet food such as chocolate. Removal of salt may make food bland and unappetising.

Salt also tastes sweet at low concentrations, suppresses bitterness, gives a feeling of fullness or thickness in foods and, in some products, is important for its visual crystal appearance. At higher concentrations, salt stimulates salivation.

Ways and means

Many food producers and industries have been successful with the reduction of salt by stealth. The idea of this is a very gradual reduction, allowing consumers' taste buds to become accustomed to the lower level of salt in the food without rejecting it. Stealth could be applied for products with strong brand loyalty or, alternatively, an industry-wide approach could be followed. The hypothesis behind stealth is that following a lower-salt diet, adaptation takes place and preferences shift towards lower levels of salt.

There are many salt replacers available. Of these, the most widely and most commonly used is potassium chloride, and many products that are on the market use this as a partial replacer.However, potassium chloride does not have the same flavour profile as sodium chloride, particularly as it gives a bitter taste perception when applied at higher concentrations.

Another approach to reduction is to use salt enhancers. These are substances that do not have a salty taste in themselves but enhance a salty taste when used in combination with sodium chloride. A range of ingredients are reported to act as salt enhancers, including amino acids, monosodium glutamate, lactates, yeast products and other flavourings. Taste enhancers work by activating receptors in the mouth and throat, which helps compensate for the salt reduction and enhances flavour.

Flavour companies have been actively researching the effect of salt in different application areas. When salt level is reduced, some flavours can effectively build back the aspects that are missing. It is also possible to use other fermentation-derived materials to boost richness and depth of flavour when sodium is lacking.

A different tactic to reduce salt could be odour-induced saltiness enhancement by a salty-congruent odour - essentially, this means selected odours may be used to compensate for lower salt levels. The five human senses used for food evaluation - sight, taste, smell, touch and hearing - interact with each other and can trick our us in the perception of certain stimuli. It has been suggested that saltiness could be enhanced, for example, by cheese odour (Pioneer et al, 2004) and soy sauce (Djordjevic et al, 2004).

In some cases, rating of the saltiness of food is affected simply by the name of the products, even prior to tasting. In a study by Lawrence (2009), anchovy and bacon items were considered to be the most salt-associated food names.

A gentle dissolve

Similar to the other basic tastes, we perceive salt with our taste buds, which are distributed over our tongue, palate and throat. The salt particles must be dissolved before we can perceive them. In liquid products, salt is thus more readily perceived than in solid products, where the food needs to be suitably chewed and mixed with our saliva in order for the salt to reach our taste buds.

A study by Phan et al (2008) demonstrates that the quantity of salt released from a soft cheese is higher than the quantity of salt released from a hard cheese. The maximum concentration of released sodium appeared higher for model cheeses with a lower fat-content:water ratio, whereby for this study with processed cheese, the sodium release during mastication appeared particularly enhanced by the water content in the matrix structure and limited by the presence of fat.

In addition, the same study showed that the amount of sodium released from model cheeses was variable between assessors, and ranged between 5 and 30% of the total quantity in the model cheeses. Assessors with high salivary flow rate and high masticatory performance experienced a higher sodium release and perceived saltiness more readily.

Any salt in the food that is not well mixed with the saliva will thus be swallowed without perceiving it. Using ingredients that dissolve readily - starch-based thickeners, for example - could enhance salt perception in the food.

As well as the enhancement of salt perception by the amount of dissolved sodium chloride in our saliva, the rate at which this dissolution occurs can also alter our perception of saltiness. Rapid dissolution can intensify saltiness in some foods and thus reduce the levels of sodium chloride required.

In solid form, the rate of dissolution of sodium chloride can be controlled by a difference in the exposed surface area of the salt crystals - the particle size and the crystal structure. Dendritic salt, for example, has voids throughout the crystal, thus drastically increasing exposed surface area and increasing the rate of dissolution (Bravieri, 1983).

A study examining particle size was carried out by Leatherhead Food Research (Angus et al, 2005) comparing the saltiness perception of different sodium chloride structures when applied to crisps over time. The sodium chloride types used in this research included table salt (figures 3a and 3b, above), salts with different structures, fine salts and freeze-dried salts with amorphous type structures.

The results from this time-intensity experiment confirmed that smaller particle sizes have a more rapid release profile and a higher overall salt intensity. Maximum salt intensity was reached sooner for the smaller particle sizes. Consumers tend to prefer a mixture of salts, possibly due to extended release of the salt.

In another study by Leatherhead Food Research, a 50:50 mixture of freeze-dried amorphous sodium chloride and table salt on crushed crisps was preferred to either table salt or freeze-dried amorphous sodium chloride alone (Narain et al, 2006).

See the value of salt

The position of salt is important to understand its full implications and to prevent consumers from swallowing large proportions of the sodium chloride in food products without detection by the palate. This principle could potentially also be applied to emulsion systems, by modifying the emulsion structure and creating double-emulsion systems, for example.

Simple oil-in-water emulsion containing salt would have all the salt dissolved in the external aqueous phase, while a simple water-in-oil emulsion would have all the salt dissolved in the internal aqueous phase. The sensory perception of salt from these two systems is different: in the oil-in-water emulsion, the salt is directly in contact with the palate; in the water-in-oil emulsion, it is the oil that is in contact with the palate, not the aqueous phase. In the latter case, the water-in-oil emulsion inverts in the mouth to an oil-in-water system, and the salt can then be detected by the palate. However, this takes some time and is influenced by the host food, so the salty perception may be reduced.

A double-emulsion system could be prepared in water to create water-in-oil-in-water emulsion. Maximum saltiness would thus be obtained when all the salt is dissolved in the external aqueous phase, which will have a higher concentration of salt in the external aqueous phase than a regular oil-in-water solution with the same salt content and would therefore likely be perceived more salty.

This article was authored by Cindy Beeren of Leatherhead Food Research on behalf of IFIS, producer of the FSTA research database, used by scientists and researchers worldwide.