Monday, 2 November 2015

The Science of Taste

By David Morris

Structure of a taste bud

In 350 BC, the Greek philosopher Aristotle postulated that varying blends of two tastes, sweetness and bitterness, comprised all flavours. As the understanding of taste developed, it was shown that sourness and saltiness were also distinct tastes. In 1908 the Japanese chemist, Kikunae Ikeda, discovered a fifth subtle and now well established taste known as umami. Until recently, these tastes were universally accepted as the five fundamental detectable tastes that made up all flavours that can be experienced by humans.

The role of smell (olfaction) in flavour wasn’t fully appreciated until as late as 2004, when biologists Richard Axel and Linda Buck discovered the role of the nose in detecting and characterising odours. When you breathe in through your nose, millions of odorous molecules activate and inhibit olfactory receptors at the roof of the nasal cavity. Electrical impulses travel from these receptors to the brain, informing you of what you’re eating. This action is supported by taste buds in the mouth.

Molecules within certain foods can chemically activate senses that are responsible for the sensation of heat, pain or touch. This is known as chemesthesis. Despite being different from taste, chemesthesis can contribute to the flavour of a substance. Examples include the ‘tingle’ on the tongue from ingestion of carbonated drinks, the feeling of heat upon eating a chili pepper, and the cooling feeling from eating gum.

 Taste receptors are mounted on papillae, which are structures found on the front and back of the tongue, the roof and sides of the mouth, and even at the back of the mouth and throat. The idea of a ‘tongue map’ whereby certain areas of the tongue detect certain tastes is a myth, although the concentration of certain receptors may vary in different areas of the tongue.

           Saltiness and sourness are the simplest of the tastes. Sourness arises from the donation of hydrogen ions, which is a characteristic feature of acids. It has been postulated that detection of sour foods defends the body from ailments like indigestion. Similarly, saltiness arises from the dissociation of salt on the tongue, forming sodium and potassium ions. The taste response acts as a warning for your body to replenish electrolytes, but not too much, hence why we find salty foods appealing, but also repulsive in large amounts.
           Bitterness, umami and sweetness are more complicated tastes. They all come from the binding of larger, more complex molecules to large proteins in the taste buds that are typically responsible for signal transduction throughout the body. Bitterness receptors can bind with molecules that are usually toxins or poisons. In this way, the bitter taste warns your body against ingesting molecules that can harm your body.

           Similarly, umami receptors detect glutamate (a component of monosodium glutamate, or MSG, a popular salt substitute), giving off a subtle savoury taste. The subtlety arises because the glutamate is usually bound to sodium, giving off a more powerful salty taste. Glutamate is useful to your body as a neurotransmitter and a catalyst to metabolism, so like salt, the taste warns your body to replenish glutamate, but not excessively.

Tomatoes are rich in umami

           In July of this year, academics at the University of Purdue published a paper in Chemical Senses describing the existence of a sixth taste called oleogustus. Usually, when the body ingests oil or fat, it is in the form of triglyceride, a molecule comprised of three fatty acids chemically bound through a glycerol molecule. This triglyceride is too large to fit into oleogustus receptors and thus can’t be tasted. However, when the fat spoils, the triglyceride is broken down into glycerol and the fatty acids. The fatty acid then binds to the oleogustus receptors and acts as a warning that the food is no longer suitable for ingestion, hence it smells and tastes rancid.
           Each of the aforementioned tastes, as well as the olfactory and chemesthetic systems, have proven essential for the body to be able to consume the correct amount of different types of food, and to protect from ingesting harmful toxins and poisons.