MOLECULAR GASTRONOMY: Part II - The Physiology of Taste

PART TWO:  THE PHYSIOLOGY OF FLAVOR

In Part II, the focus is on the physiological factors that affect how our bodies respond to food to detect its flavor (through taste molecules and odorants), as a form of medicine or to provide and supplement missing nutritional requirements like salt, and prepare our digestive system to absorb food products for subsequent metabolism.  It also looks at the anatomy and physiology of taste receptors in papillary cells in the tongue and how the different tastes affect each other’s interaction with the tongue receptors. It differentiates the way the receptors chemically transfer information about taste to the brain, relates the experimental process by which the fifth type of taste (umami) was discovered, and provides an explanation for why our mouth feels like it’s on fire when we eat spicy foods. It also devotes a couple of brief chapters on how food allergies and its possible link to respiratory allergies and developments in detection of and differentiation between pathogenic and benign strains of listeria.

FOOD AS MEDICINE:

·         Mammalian evolutionary adaptation has resulted in larger animals having better detection methods for sugars (one exception is the coucang, a prosimian insensitive to sucrose).  Primates including humans, with their large body mass, are known to be very sensitive to sweet tastes.  There are environmental modulators, however, that affect the level of this sensitivity.  Pygmies who occupy fruit-rich tropical forests have a lower sensitivity for glucose and sucrose while inhabitants of savannas have a higher sensitivity to detect smaller sugar concentration in plants in grasslands.

·         There is also an interesting variation in sensitivity to sweetness among species.  African non-human primates can detect the protein monellin which is 100,000 sweeter than sucrose but American primates cannot.  The same difference exists for another protein sweetener, thaumatin.  In America, no protein sweetener has been found.

·         Vertebrates are able to detect sodium chloride in food sources and salt deprivation triggers an instinct to seek them out.  Examples of these include horses that seek out salt-licks and a type of monkey who leaves its natural tree habitat to feed on the leaves of a plant known concentrate more salts than others.

·         Tannins inhibit protein digestion because they form complexes with them.  Some animals eat clay and other soil substances because they are able to absorb the unwanted tannins.

·         Bitter taste generally warns the eater of something potentially toxic.  However, not all toxic foods compounds are bitter, e.g. dioscin, found in yams, is a tasteless, toxic alkaloid.

·         Chimpanzees are known to eat the bitter plant vernonia amygdalina that contains steroidal glycosides for treating gastrointestinal problems.

TASTE AND DIGESTION

·         In this chapter, the author looks at some results of studies that correlate the taste and/or ingestion of a particular food with stimulating metabolic processes.  Some examples include:

·         A small amount of sugar placed on the tongue activates the liver to immediately release glucose.

·         In 1960, scientists at College de France observed the release of the hormones glucagon (responsible for the release of glucose) and insulin (responsible for the metabolism of glucose) when taste receptors are stimulated by saccharin.

·         Similarly, a group of scientists sought to find out if reflexes to metabolize protein are activated by the taste of proteins.  Earlier studies showed the ingestion of monosodium glutamate acts as a signal for the body to start absorbing proteins.  To study the role of monosodium glutamate, in 1991 these scientists carried out an experiment to test metabolic response to ingestion of MSG as measured by thermogenesis (not associated with locomotion).  They found that, in rats, there was only a very weak hormonal response detected for those ingesting MSG. When the rats were fed MSG solutions while eating, there was a considerable increase in the amount and rate of thermogenesis associated with food intake, suggesting that the MSG acted as a protein “saccharin” even though the meal was mostly carbohydrates, not protein.

TASTE IN THE BRAIN

·         Studies designed to determine which parts of the brain respond to certain tastes, scientists found that:

·         There is no single taste center in any part of the brain that is specifically linked to a particular taste.

·         The “perception of taste is lateralized in a way that is analogous to language use and motor activity”.

·         “The brain constructs a global sensation through the synthesis of signals coming from various types of receptors.” [The author explains that in a study comparing responses to molecules that have only one taste and to molecules that have both taste and astringency, the activated areas were ‘analogous’.  I am not sure what he meant by that.]

PAPILLARY CELLS

·         The author describes the sensation of taste starts when the taste molecule (the book likes the adjective “sapid” which AOD defines as “having a strong, pleasant taste”) binds to receptors or ion channels in the membrane of a papillary sensory cell (papillary: a small rounded protuberance on a part or organ of the body, AOD).  This binding triggers a series of reactions that cause a change in electrical potential.  A signal is sent to the brain exciting neurons and delivering the taste message. 

·         The binding of hydrogen ions (sour) and sodium ions (salty) directly change the electric potential because of their electrical charge. 

·         Compounds of sweet, bitter, and other tastes bind to protein receptors on the cell membrane in contact with the outside of the cell.  This binding triggers the release of second messenger molecules inside the cell.  The taste molecules bind only weakly allowing a more rapid turnover of taste molecules.  One of these membrane protein receptors is gustducin which was found to be similar to the protein receptor transducin found in the eye.  Transducin, found in the rods and cones of the eye, has been detected in the papillary taste receptor cells.

·         “If papillary cells function like the cells in the eye, then transducin and gustducin activate an enzyme that diminishes the production of cyclic adenosine monophosphate.”  In the absence of this second messenger molecule would either “modify the ion channels of the cell membrane and associated enzymes or disrupt the exchange of calcium ions between the inside and outside of the cell”.  Experiments with mice born without the ability to make gustducin (through gene inhibition) showed normal reactions to salty and sour (binding of sodium and hydrogen ions cause direct change in the electric potential with no need for the secondary messenger) but much weaker response to bitter and sweet molecules (quinine sulfate and denatonium benzoate for bitter and sucrose for sweet).  The researchers speculate that the sensing of bitter and sweet was not completely removed because transducin was still present.

HOW SALT AFFECTS TASTE

·         “Salt transforms and softens bitter and sweet flavors.”

·         Adding salt to an aqueous solution causes an increase in ionic strength which makes it easier for “odorant” molecules to vaporize from solution, “amplifying odor which is an important part of flavor”. The effect of salt was studied using sodium, lithium, and potassium chloride and also sodium aspartate. 

·         In an experiment to determine salt “selectively filter tastes, weakening unpleasant ones while enhancing pleasant ones”.  They used solutions containing varying amounts of one or more of the following: urea for bitter, sucrose for sweet, and sodium acetate for sodium ions (without adding too much saltiness).  Results:

·         Sodium acetate reduced the bitterness of urea much more effectively than sucrose did.  A mixture containing all three was sweeter and less bitter than a mixture without the sodium acetate.

·         The sweetness of a sugar solution was increased by the addition of the sodium acetate “probably because the salt offsets the weakening of the sweet intensity caused by the bitterness of the urea”.  In subsequent tests, they did indeed show that adding sodium acetate does not increase the sweetness of sucrose if no urea is present.

·         Other studies confirmed that that sodium ions “selectively suppress bitterness (and probably other disagreeable tastes as well) while intensifying agreeable tastes.

·         The author muses that perhaps adding salt to our foods became habitual because of their ability to enhance its flavor beyond just making it salty.

DETECTING TASTES

·         “Discovery of a molecular receptor for a fifth taste.”

·         In 1908, a Japanese scientist from the Imperial University of Tokyo “established that glutamate produced a particular sensation that was neither salt, sugar, sour, or bitter”.

·         “The detection of tastes is important because it signals satiety.  One does not cease eating because one’s stomach is full; one stops because the brain, alerted by the sensory system, notifies the organism that a sufficient quantity of food has been consumed.”

·         Glutamate is a neurotransmitter (exchanged between neurons, released from one and binds to another).  University of Miami scientists looked for the same neuronal protein receptor for glutamate in the taste cells and found a truncated form of it establishing a taste receptor for glutamate.  In the truncated form (less 300 amino acids), the glutamate binds only weakly to this taste cell protein receptor version because it is missing one of two binding sites.  They also found that the glutamate concentrations needed to activate this truncated form are similar to the taste thresholds in mice.

·         Toward the end of the chapter, the author noted substantial evidence that confirms that this protein is the receptor associated with the 5^th umami taste.

BITTER TASTES

·         Several types of bitterness have been discovered.

·         In 2000, a team of biologists discovered a large family of receptors for the bitter taste.

·         Two University of Miami scientists sought to investigate the responses of this family of receptors to different bitter compounds.  The activation of taste receptor cells occur with the release of calcium ions.  By injecting a coloring agent that binds to calcium ions, they were able to observe the change in distribution of the calcium ions under a confocal microscope that can image deep tissue cells.  The results:

·         Cycloheximide triggered a “strong but transient variations in the concentration of calcium ions in taste receptor cells”.

·         Denatonium  benzoate, sucrose octaacetate, phenythiocrbamide, and quinine “produced weaker but prolonged reactions lasting several minutes”.  The intensity of the response to each compound correlated with the concentration of the molecules.

·         Only 18% of the 374 cells tested responded to one or more of the five bitter molecules at moderate concentrations.

·         Of the 18%, the highest portion (14%) of cells responded to cycloheximide and the smallest (1.6%) to sucrose octaacetate.

·         This study showed also that different parts of the tongue are not specific to particular tastes, contrary to a commonly held view.

HOT UP FRONT

·         “Why spicy foods burn the mouth”

·         UCSF scientists study the receptor for capsaicin (active principle in chili peppers, paprika, and cayenne).

·         Exploration of the effects of morphine on the brain and identification of receptors for morphine and its derivatives led scientists to speculate that if these receptors exist, there must be naturally produced molecules in the body analogous to morphine.  These are the endogenous opioids, along with the “regulatory system for suppressing pain”.

·         Similarly, UCSF scientists thought that if humans evolved to eat spicy foods, we must have receptors in our organs for endogenous molecules that signal pain.  They carried out a study that isolated RNA messengers from neurons for detecting spiciness in the mouth, synthesized the corresponding DNA molecules, and inserted them in various cell cultures to see if and how capsaicin molecules will bind to the proteins formed.  By observing to which protein/s the capsaicin bonded, they were able to identify the part of the DNA that codes for the capsaicin receptor.  It was shown that the binding of the capsaicin to the receptor causes ion channels (primarily for calcium) to open.  Measurements also showed a correlation between the intensity of the cellular response (and presumably correlated with neuronal response) and concentration of capsaicin.  Capsaicin can bind to nerve cells (on their surface or inside) because they are fat-soluble (hard to diminish the mouth’s response with water because of this).

·         The lowering of the response threshold to capsaicin by people who eat a lot of spicy foods is attributed to the loss of sensory fibers (due to inundation with calcium ions?).

·         Rapid increase in temperature results in the same response by the capsaicin receptors in triggering ion currents which is perhaps why spiciness is associated with the sensation of “heat”.  This ion channel turns out to be both a chemical and thermal sensor.

THE TASTE OF COLD

·         “Cooling and heating the tongue arouse the perception of tastes, even in the absence of food.”

·         “Variations in the temperature of the tongue alone are enough to cause tastes to be perceived.”

·         This phenomena is attributed to the proximity of taste receptors and thermal sensors on the tongue.

MASTICATION

·         Mechanical process which food is broken down into smaller particles and rebound by saliva into a “compact mouthful” small enough for easier swallowing but large enough to minimize going down the windpipe.

·         The smaller bits also increase the surface area exposed to digestive juices in the stomach.

TENDERNESS AND JUICINESS

·         The “texture” of food is not the same as the “consistency” which is a microstructural property.

·         In this chapter, the author discusses an experiment wherein meats where prepared under different conditions of storage and maturation to see how they affect the “tenderness” property.  Mechanical measurements, sensory evaluations, and electromyographic measurements all showed that the most tender meats resulted from the longest aging period.

MEASURING AROMAS

·         The flavor of food involves the contributions of odorants, volatile food molecules that travel from the mouth upward to the nasal receptors as food is chewed.

·         Characterization of these odorants and analysis is difficult because their contribution to flavor involves interaction with saliva and their mixing inside the mouth before making their way to the nasal receptors.

·         Scientists have used mass spectrometry to analyze volatile substances above food but also the breath of subjects as they chew.

·         For example, in subjects eating gelatin, they found that ethylbutyrate found in fruits such as strawberries was released during mastication for about a minute and ethanol was released for a longer period of time because of its water solubility.

·         Acetone was also detected at every respiratory cycle because this compound is released as the product of fatty acid metabolism in the liver and is naturally found in breath.

·         In general, they found that the concentration of odorants and the time it took to reach maximum depended on the mastication rate:  fast masticators released the smallest amount of odorants, probably because the gelatin is swallowed in larger pieces faster.

A TABLE IN THE NURSERY

·         In this chapter, the author provides an account of the results of studies focused on observing the eating habits of small children.  Some general observations were:

·         Children in this study reinforced what most parents have observed that young children prefer starches and meats.  There was no pronounced selectivity toward any one kind of meat (roast pork, turkey, leg of lamb, and organ meats were on the menu.)

·         They generally avoided foods that tend be hard and fibrous and also those that are bitter.

·         The author notes that conditioning causes children to associate the feeling of fullness with eating high-calorie nonsweet dishes like fatty foods while culture is responsible for them developing  taste for food that have a strong taste,

FOOD ALLERGIES

·         In this chapter, the author notes the increasing rate of development of food allergies.  While the reasons for this are not completely known (at the time of writing), it is speculated that the development of a food allergy may be linked to the development of respiratory allergies to plant pollens.  This crossing over of reactions may results from “shared epitopes” which are molecular fragments that are the target of the immune system.  (“The antibodies produced by the human organism against a pollen antigen sometimes react also against molecules from a very different source.”)

·         Studies done on subjects with known to allergies to milk found that 92% of the serums contained the immunoglobulin E, the antibody specifically directed against beta-casein (one of four types of casein).  Immunologists later found that immunoglobulin E also responded to human beta-casein.  In comparing the structures of bovine and human beta-casein, it was found that they share 50% of their amino acids, a common domain that has a helical form, and another common domain that contains the primary phosphorylation sites in beta-casein.  The immunologists were able to determine that it is the phosphorylation process that affects the allergenic character of beta-casein.

PUBLIC HEALTH ALERTS

·         The topic of this chapter is the challenge of developing efficient and accurate tests to distinguish dangerous versus benign strains of food microorganisms.  For instance, two types of listeria have been identified:  the pathogenic listeria monocytogenes and the harmless listeria innocua.

·         An in vitro test has been developed wherein a sample of the microorganisms is placed in a culture of human intestinal cells.  It is then observed for any ruptures in the cell layer in response to the presence of bacteria.  Through this test, they discovered that the pathogenic listeria monocytogens consisted of different strains, only certain ones of which were virulent to a small degree or not at all.

·         Other research being done involves analyzing how certain food products and processing techniques may present an environment conducive to the growth of pathogenic strains.  Some of these conditions include humidity, acidity, and storage time.

THE END