Cracking the salt challenge
By Dr. Aaron Goh
According to a recent survey, Malaysians eat too much salt. Salt is associated with high blood pressure, which is bad for the heart, but many find food with too little salt simply bland and tasteless.
However, reducing salt intake in food is not just a matter of people resisting the urge to add generous amounts of salt when cooking, or diners breaking the habit of adding salt to everything they eat.
Indeed, scientists around the world are trying to decipher the mysteries of salt and helping to crack the salt challenge. They are working hard to find out what goes on when we eat, and what approaches we can take to cut down on our salt intake.
The chemistry of salt
There are many types of salt, but the one we commonly refer to and which is used in foods is made up of two components – sodium and chloride. Hence, our ‘common salt’ is also known as sodium chloride. It is the sodium component that needs to be reduced in our diet.
The sodium and chloride components bind tightly together in solid crystals that we sprinkle on our foods. When we dissolve the salt in water, the components (called ions) become detached from each other and each can move around in the liquid, much like an ant scouting for food.
The ions are very small. If we use the length of a swimming pool to represent the thickness of our hair, then the size of the ions is equivalent to the thickness of a hair in the swimming pool!
Tastebuds and taste
Food scientists divide ‘flavour’ into two components, that is, ‘taste’ which consists of sweet, salty, sour, bitter and umami; and ‘aroma’.
The tastebuds scattered on our tongues contain receptors that are responsible for our sense of taste while ‘aroma’ is sensed in the nose. When we have a cold and a blocked nose, we can still determine whether a food is sweet or salty, but we cannot detect the overall flavour of the food as less aroma can reach the nasal passage.
When the tastebuds are stimulated, the stimulation is transmitted to the brain and the information is ultimately used to make judgements about the acceptability and taste of the food.
We do not fully know yet how the salty taste is detected in the tastebuds, but it is largely due to receptors which specifically detect sodium ions. This is the reason why our common salt is made of sodium and not other metal components like calcium.
The rest of the salty taste detection is thought to involve movement of ions through the membranes of the tastebuds. This is where the chloride component plays an important role in salty taste.
Compared to other salts that contain sodium, such as sodium acetate, which is used to give flavour to salt and vinegar crisps, sodium chloride is more salty. This is because of the smaller size of the chloride ion which can move about more easily than the bulkier acetate ion. The movement of the chloride ions enables the sodium ions to move easily too.
When we first put salty food into our mouths, the taste is normally quite distinct. However, after eating the same food for a while, we ‘forget’ about the taste of the food. If we then eat other food that is less salty, we will find that the food is much less salty than the first one.
An important phenomenon called ‘adaptation’ is at work here. Adaptation refers to the reduction in taste when the tastebuds are exposed continuously to the same stimulation. When a different stimulation is introduced, the sensitivity of the tastebuds is either reduced or enhanced.
The phenomenon of adaptation was exploited in many ways in the olden days, such as to reduce the unpleasant taste of medicines through prior tasting of vinegar. Nowadays, adaptation has been successfully implemented in some countries to lower salt in various food products. The salt content of products is reduced gradually so that consumers are not aware of the changes and get used to the new taste and lower salt content over time.
More sophisticated techniques of adaptation are also currently being experimented on by scientists. By designing foods that deliver salt in pulses rather than continuously, scientists are hoping that the effect of adaptation can be reduced so that the sensitivity to salt is increased.
From food to tastebuds
In the mouth, sodium ions in the salt have to travel from the food to the tastebuds to trigger salty taste.
Since only a portion of the salt in food reaches the tastebuds, scientists are interested to maximise the amount of salt that does reach the tastebuds. This involves looking at how the salt is dissolved, and how it is mixed and transported to the tastebuds.
In the solid form, the salt crystal can be found in various sizes and shapes. The crystals that provide higher salty taste tend to dissolve faster in the mouth, so more salt is available to trigger the tastebuds.
In liquid form, some foods can mix better with the saliva. Good mixing allows the sodium ions to reach the tastebuds rapidly. Solid or very thick liquid food can taste less salty than food that flows more easily. This is because a more solid food needs to be broken down first and pushed around by the tongue before it can reach the tastebuds.
The distribution of salt in the food matrix is also important and its effect has been demonstrated in a few types of foods.
For example, some low-salt cereals remain salty despite having cores which have low salt. The salty taste comes from a coat of salt that is applied only to the surface. By not having more salt in the core, the amount of salt can be reduced.
In soups with chicken pieces, a higher salty taste is obtained if the salt is concentrated in the pieces rather than in the liquid. Bread made up of high and low salt layers can achieve a higher salty taste than bread with uniformly distributed salt.
Some foods contain fillers to fill up the volume so that less salt is required. An example is air bubbles, which have been used to reduce not just salt but also fats in foods like mayonnaise.
Salt substitutes and enhancers
One of the most common approaches to reducing salt is by replacing it with a substitute such as potassium chloride.
The biggest problem with these salt substitutes is that they increase the bitterness and, in some cases, the sourness of food. If you have had the unfortunate experience of tasting paracetamol, then potassium chloride tastes like a salty paracetamol.
Because of the off-tastes, there is a limit to how much salt substitutes can be used. Nowadays, salt enhancers are also used together with salt substitutes.
Salt enhancers can mask the bitterness of salt substitutes so that the bitterness is not detected. In medicines, using sweeteners to mask bitterness is quite common, but in foods, using sweeteners is not a healthy or practical choice.
Salt enhancers in foods with reduced salt help by providing other taste and aroma to help re-balance the overall flavour. Although not considered salt enhancers, adding herbs and spices to boost aroma can also lead to an increase of salty taste. Some salt enhancers also increase stimulation of the tastebuds to enhance salty taste. Typical salt enhancer include yeast extracts, amino acids and MSG, which provide a savoury taste.
Addressing the salt challenge
Although a proper balance of sodium level is necessary for our body to function properly, for taste preference rather than physiological needs, we now consume much more salt than our body requires, sometimes consuming more than 10g of salt per day. As a comparison, the World Health Organisation (WHO) has set a target of 5g per day as part of a balanced diet.
Reducing sodium in foods is a pressing matter and scientists from different disciplines are trying to understand better the mechanisms underpinning taste detection and perception, search for compounds that can replace sodium chloride, and engineer new food matrices where release or detection of the salt can be manipulated.
As for the rest of us, developing a general awareness of the need to reduce our daily sodium consumption will help lower our risks of developing hypertension and related heart diseases.
Although we may be able to control the salt that we physically add to our foods, we should also be aware that a large proportion of our salt intake comes from processed foods that we buy from supermarkets or restaurants. Simple measures like keeping a note on sodium content on food labels (1g sodium = 2.5g salt) may go a long way towards ensuring that we have a balanced and healthy diet.
Dr. Aaron Goh is an academic staff of Curtin Sarawak’s School of Engineering and Science. He is an expert in the mechanical strength of materials, in addition to having a keen passion for food. He has vast working experience in food microstructures and human sensory perception and is currently leading a Ministry of Science, Technology and Innovation (MOSTI)-funded research project on salt reduction in foods. Dr. Goh can be contacted at +60 85 443939 or by e-mail to firstname.lastname@example.org.