MOlECULAR GASTRONOMY: Part III - Investigation and Models (TBC)

PART THREE:  INVESTIGATIONS AND MODELS

THE SECRET OF BREAD:

·         The two main components of wheat flour are starch granules which swell up with water and proteins which form a glutinous network as dough is needed.

·         These proteins are called gluten and they form a “viscoelastic network of proteins that becomes elongated by pulling and then partially reverts to its initial form when the tension is relaxed”.

·         Specifically, this glutinous network of dough is made up of prolamins which are water insoluble wheat proteins.  There are two types of prolamins:  gliadins (one single protein chain) and glutenins (composed of several protein chains held together by two covalently bonded sulfur atoms).

·         Glutenins have a central domain containing 440-680 amino acids formed of short repeated sequences and flanked by two terminal domains containing cysteines.

·         In 1998, it was found that chains of prolamins bond together through dityrosine bonds which increased during kneading.  Two types of dityrosine bonds form: dityrosine (two benzene groups are linked by the C atom of the –OH group) and isodityrosine (the two benzene groups are linked by an oxygen atom on one –OH group bonding to the C atom in the –OH group in the other tyrosine).

·         The presence of peroxidase in bread has also been correlated with the formation of these dityrosine bonds.

·         Oxidizing compounds like ascorbic acid and potassium bromated also increase the number of dityrosine bonds.

·         The author notes that perhaps the amount of dityrosine bonding between prolamine chains can be used as a measure of the quality of gluten and the dough.

YEAST AND BREAD

·         “Bread owes its flavor to fermentation.”

·         The flavor of bread comes from the fermentation products of saccharomyces cerevisiae.

·         There are three different methods of making bread with or without yeast and with or without fermentation.

·         Direct yeast fermentation – (most common) dough composed of flour, water, yeast, and salt is kneaded for 20 minutes, allowed to ferment for 45 minutes, then divided into lumps, fermented again for another 1 hour and 40 minutes and then baked at 250 C for 30 minutes.

·         Sponge method – same as the first method above except the dough is pre-fermented in a semi-liquid state (water is combined with a smaller quantity of the flour and allowed to ferment for several hours before the rest of the flour is added to turn it into a dough with the right consistency for bread-making).

·         Sourdough method – a starter (sourdough) is created by cultivating beforehand a natural microflora composed of yeast and lactic bacteria; this starter is then used to start the fermentation process in the bread dough.

·         Sponge method yields twice and the sourdough method yields 20 times the acetic acid obtained by direct fermentation.  The sourdough method also produces lactic acid.

·         Fermenting dough using yeast produces 3-hydroxy-2-butanone, 3-methyl-1-butanol, and 2-phenylethanol (gives the odor of wilted rose).

·         Without yeast, ordinary bread is more abundant in monounsaturated and polyunsaturated aldehydes and alcohols such as pentanol and benzyl alcohol, probably resulting from the oxidation of lipids in flour.

·         To study the chemical transformations in bread dough, scientists also looked at uncooked dough under various conditions and carried out chemical analysis through solvent extraction and chromatography and also human detection of smells.  They found that:

·         There was a general increase in different alcohols, ketones, esters, and lactones but a decrease in aldehydes.

·         With yeasts, more alcohols are formed.

·         With longer and faster kneading by mechanical means, flours produce hexanol which gives a stale, oily smell.

CURIOUS YELLOW

·         An egg yolk consists of concentric layers of varying shades of yellow because of the variation in the amount of yellow pigment produced during the day and during the night based on the “rhythm of feeding” by the hen.

·         Yolk is a mixture of granules suspended in a “plasma” phase.  It is about half water, a third lipids, and 15% proteins [by mass or volume?]. 

·         The granules are composed of low-density lipoproteins and high-density lipoproteins.  The LDL’s form a gel at about 70 C and cause the yolk to set during cooking.

·         Yolks are used in making mayonnaise and in this process the emulsion quality is important.  Some  the factors affecting the emulsion quality observed are below:

o   The plasma proteins are completely water soluble at all pH’s and salt concentrations.  The granules have low solubility at pH 3 but becomes more soluble as the pH goes to neutral in a low-salt solution (“sodium ions replace calcium ions, which establishes bridges between the granular proteins inside the granules, with the result that these proteins are released”).

o   The solubility properties are important in the emulsification process.  The other factor is the movement of oil droplets in water; less movement creates a more stable emulsion.  At pH 3, there is minimal movement in the plasma (salt concentration has no effect).  The emulsions from granules are affected by the acidity and the salt concentration.

o   Proteins are better at preventing any upward movement by oil droplets than do the phospholipids.

GUSTATORY PARADOXES

·         “The environment of aromas affects our perception of them.”

·         The taste of vinegar is modified when a lot of sugar is added to it even though its pH is unchanged.  This is because, as addressed in an earlier chapter, our perception of one type of taste may be enhanced or diminished by the presence of another taste.

·         The same type of interaction and effect on each other takes place in olfactory receptors.

·         When food is placed in the mouth, the water-soluble taste molecules first have to diffuse through saliva before reaching the taste receptors.  Odorant molecules first have to vaporize to make their way toward the nasal olfactory receptors.  Their variation in solubility in water results in an uneven distribution inside the mouth and its cavities.  Thus the study of aromas of food necessarily involves knowledge of the movement of molecules between liquid (polar and nonpolar) and gaseous phases.

·         To study these factors, one experimental set-up involved measurements of the distribution molecules in the various phases that exist in an oil-water mixture (water, oil, air above water, air above oil). 

·         One finding is that, when the molecules are first dissolved in the oil, their vapor move to the air above the oil, diffuse through the water, and then move to the air above the water [not perfectly clear about this description by the author].

·         Also, they observed that “transfer was more rapid from oil to water than from water to oil in the case of the esters and ketones but more rapid from water to oil than from oil to water in the case of alcohols and aldehydes”.  This was a surprising result as alcohols, for instance, are water-soluble.

·         Odorant molecules must first penetrate the mucus layer before arriving at the olfactory receptors and become dissolved in the hydrophobic phase of the cell membrane.

·         When human noses were used to detect the aromas, there was a difference from those detected by the non-human detectors in the case of 1-octane-3-ol (mossy smell), benzaldehyde (almond smell), and acetophenone (beeswax smell).  There was no difference however with linalool (gives the odor of lavender and bergamot)

·         “The presence of water vapor can affect the perception of an aroma.”

THE TASTE OF FOOD

·         “The texture of vinaigrettes determines their odor.”

·         Odorant and taste molecules can bind to odorless starch and proteins.  Adding too much flour to sauces can make it tasteless.

·         “In homogenous phases such as solutions, the release of odorant molecules depends on the viscosity of the system.”

·         Foods are dispersed systems:  foams (air bubbles trapped in solids or liquids), emulsions (oil droplets dispersed in water), and suspensions (solids in liquids).  Odorant and taste molecules must escape both the dispersed particles and the “solubilizing” medium.

·         A study was done by two agronomical and nutrition institutes in Dijon to investigate how odorant molecules are released.  They studies vinegar-based sauces.

·         Composition of vinegar-based sauces:  the aqueous phase consists of wine vinegar, lemon juice, and salt, sunflower oil emulsified with the help of whey proteins, and a mixture of xanthan (a polymer obtained by microbial fermentation of glucose) and starch to stabilize the sauce.  Odorant molecules were added:  isothiocyanate in the oil phase (hint of mustard) and phenyl-2-ethanol and ethyl hexanoate (rose and fruity notes).

·         “Although the acid taste was preponderant, the tasters struggled to describe the other sensations.”  However, some observations: the overall odor, the taste and odor of the egg, the mustard odor, and the butter taste increased but the citrus odor decreased as the size of oil droplets increased.

·         Results of analysis of volatile molecules in the air above the sauces:  lower concentrations of water-soluble components detected as the oil droplet size decreased and more abundant oil-soluble molecules.

LUMPS AND STRINGS

·         Flour is made up mostly of starch.  There are two types of polymers in starch, linear amylase and branched amylopectin.  Amylose is soluble in hot water while amylopectin is not.  When hot water is added to flour, the amylase dissolves while water permeates within the amylopectin molecules causing granules that swell up and form a gel (starch paste).  This gel slows down and even stops the diffusion of water into the center of the granules and lumps are formed.

·         Pre-soaking gelatin helps the separation and pre-dissolving of protein layers which prevents the formation of strings (bound protein polymers that water cannot penetrate through) when making gelatin.

FOAMS

·         “The stability of foams depends on the arrangement of the proteins at the interface between the water and air.”

·         The stability of foam depends on the formation of small enough bubbles so that the surface tension is stronger than gravitational forces which cause the air to rise and the water to fall.  To stabilize foam, the viscosity of the liquid phase should be increased and the absorbent films should have good drainage properties.  In protein foams, the film’s integrity and strength are affected by intramolecular and intermolecular forces between and within protein molecules.  This complex network of interactions makes it difficult to study the effects of proteins on foam stability.  They found, however, that the concentration of soluble proteins does not have much an effect.  Insoluble proteins that fold in complex ways are hard to study.  Nevertheless, they observed that for globular and nonglobular proteins, interfacial tension increases with the concentration of the foam proteins.

HARD SAUSAGE

·         In this chapter, the author looks at characterizing the molecular composition of sausage to understand their aromatic qualities.

·         Using GC-MS, scientists detected about 100 organic compounds produced by enzymes and fermentation agents in meat.

·         It was determined that the flora used to age sausages play a big role in producing aromas.

·         In an experiment, 6 mixtures of acidifying and aromatizing bacteria were used in preparing 30 sausages (5 samples each mixture).  Some findings (the aroma was determined by trained testers based in some agreed to terms for describing aromatic properties):

·         Oxidation of lipids played a “preponderant role in determining aromatic qualities”:

o   Rancid smell is correlated with aldehydes, alkanes, and alcohols

o   Good sausage smell is correlated with methyl ketones and methyl aldehydes

o   Degradation of sugars “favors the development of vinegar odors produced by acetic acid or of butter aromas produced by 2,3-butanediol”.

·         Conclusion: aromatic quality of sausages depends on the quality of the strains used in the maturation process.

·         Other factors that affect the aromatic quality are length of the curing process and the type of packaging.  Drying loses some of the aroma because some of the volatile organic compounds evaporate with the water but it also concentrates the salts which bring out the flavor.

·         In studies of the mechanisms of aromatization, they found traces of pepper (terpenes), garlic (sulfur-containing molecules), and brandy (esters formed by the reaction of ethyl alcohol with fatty acids produced by salting).

TO BE CONTINUED

MOLECULAR GASTRONOMY: Part III - Investigations and Models (To be con't)

PART THREE:  INVESTIGATIONS AND MODELS

THE SECRET OF BREAD:

·         The two main components of wheat flour are starch granules which swell up with water and proteins which form a glutinous network as dough is needed.

·         These proteins are called gluten and they form a “viscoelastic network of proteins that becomes elongated by pulling and then partially reverts to its initial form when the tension is relaxed”.

·         Specifically, this glutinous network of dough is made up of prolamins which are water insoluble wheat proteins.  There are two types of prolamins:  gliadins (one single protein chain) and glutenins (composed of several protein chains held together by two covalently bonded sulfur atoms).

·         Glutenins have a central domain containing 440-680 amino acids formed of short repeated sequences and flanked by two terminal domains containing cysteines.

·         In 1998, it was found that chains of prolamins bond together through dityrosine bonds which increased during kneading.  Two types of dityrosine bonds form: dityrosine (two benzene groups are linked by the C atom of the –OH group) and isodityrosine (the two benzene groups are linked by an oxygen atom on one –OH group bonding to the C atom in the –OH group in the other tyrosine).

·         The presence of peroxidase in bread has also been correlated with the formation of these dityrosine bonds.

·         Oxidizing compounds like ascorbic acid and potassium bromated also increase the number of dityrosine bonds.

·         The author notes that perhaps the amount of dityrosine bonding between prolamine chains can be used as a measure of the quality of gluten and the dough.

YEAST AND BREAD

·         “Bread owes its flavor to fermentation.”

·         The flavor of bread comes from the fermentation products of saccharomyces cerevisiae.

·         There are three different methods of making bread with or without yeast and with or without fermentation.

·         Direct yeast fermentation – (most common) dough composed of flour, water, yeast, and salt is kneaded for 20 minutes, allowed to ferment for 45 minutes, then divided into lumps, fermented again for another 1 hour and 40 minutes and then baked at 250 C for 30 minutes.

·         Sponge method – same as the first method above except the dough is pre-fermented in a semi-liquid state (water is combined with a smaller quantity of the flour and allowed to ferment for several hours before the rest of the flour is added to turn it into a dough with the right consistency for bread-making).

·         Sourdough method – a starter (sourdough) is created by cultivating beforehand a natural microflora composed of yeast and lactic bacteria; this starter is then used to start the fermentation process in the bread dough.

·         Sponge method yields twice and the sourdough method yields 20 times the acetic acid obtained by direct fermentation.  The sourdough method also produces lactic acid.

·         Fermenting dough using yeast produces 3-hydroxy-2-butanone, 3-methyl-1-butanol, and 2-phenylethanol (gives the odor of wilted rose).

·         Without yeast, ordinary bread is more abundant in monounsaturated and polyunsaturated aldehydes and alcohols such as pentanol and benzyl alcohol, probably resulting from the oxidation of lipids in flour.

·         To study the chemical transformations in bread dough, scientists also looked at uncooked dough under various conditions and carried out chemical analysis through solvent extraction and chromatography and also human detection of smells.  They found that:

·         There was a general increase in different alcohols, ketones, esters, and lactones but a decrease in aldehydes.

·         With yeasts, more alcohols are formed.

·         With longer and faster kneading by mechanical means, flours produce hexanol which gives a stale, oily smell.

CURIOUS YELLOW

·         An egg yolk consists of concentric layers of varying shades of yellow because of the variation in the amount of yellow pigment produced during the day and during the night based on the “rhythm of feeding” by the hen.

·         Yolk is a mixture of granules suspended in a “plasma” phase.  It is about half water, a third lipids, and 15% proteins [by mass or volume?]. 

·         The granules are composed of low-density lipoproteins and high-density lipoproteins.  The LDL’s form a gel at about 70 C and cause the yolk to set during cooking.

·         Yolks are used in making mayonnaise and in this process the emulsion quality is important.  Some  the factors affecting the emulsion quality observed are below:

o   The plasma proteins are completely water soluble at all pH’s and salt concentrations.  The granules have low solubility at pH 3 but becomes more soluble as the pH goes to neutral in a low-salt solution (“sodium ions replace calcium ions, which establishes bridges between the granular proteins inside the granules, with the result that these proteins are released”).

o   The solubility properties are important in the emulsification process.  The other factor is the movement of oil droplets in water; less movement creates a more stable emulsion.  At pH 3, there is minimal movement in the plasma (salt concentration has no effect).  The emulsions from granules are affected by the acidity and the salt concentration.

o   Proteins are better at preventing any upward movement by oil droplets than do the phospholipids.

GUSTATORY PARADOXES

·         “The environment of aromas affects our perception of them.”

·         The taste of vinegar is modified when a lot of sugar is added to it even though its pH is unchanged.  This is because, as addressed in an earlier chapter, our perception of one type of taste may be enhanced or diminished by the presence of another taste.

·         The same type of interaction and effect on each other takes place in olfactory receptors.

·         When food is placed in the mouth, the water-soluble taste molecules first have to diffuse through saliva before reaching the taste receptors.  Odorant molecules first have to vaporize to make their way toward the nasal olfactory receptors.  Their variation in solubility in water results in an uneven distribution inside the mouth and its cavities.  Thus the study of aromas of food necessarily involves knowledge of the movement of molecules between liquid (polar and nonpolar) and gaseous phases.

·         To study these factors, one experimental set-up involved measurements of the distribution molecules in the various phases that exist in an oil-water mixture (water, oil, air above water, air above oil). 

·         One finding is that, when the molecules are first dissolved in the oil, their vapor move to the air above the oil, diffuse through the water, and then move to the air above the water [not perfectly clear about this description by the author].

·         Also, they observed that “transfer was more rapid from oil to water than from water to oil in the case of the esters and ketones but more rapid from water to oil than from oil to water in the case of alcohols and aldehydes”.  This was a surprising result as alcohols, for instance, are water-soluble.

·         Odorant molecules must first penetrate the mucus layer before arriving at the olfactory receptors and become dissolved in the hydrophobic phase of the cell membrane.

·         When human noses were used to detect the aromas, there was a difference from those detected by the non-human detectors in the case of 1-octane-3-ol (mossy smell), benzaldehyde (almond smell), and acetophenone (beeswax smell).  There was no difference however with linalool (gives the odor of lavender and bergamot)

·         “The presence of water vapor can affect the perception of an aroma.”

THE TASTE OF FOOD

·         “The texture of vinaigrettes determines their odor.”

·         Odorant and taste molecules can bind to odorless starch and proteins.  Adding too much flour to sauces can make it tasteless.

·         “In homogenous phases such as solutions, the release of odorant molecules depends on the viscosity of the system.”

·         Foods are dispersed systems:  foams (air bubbles trapped in solids or liquids), emulsions (oil droplets dispersed in water), and suspensions (solids in liquids).  Odorant and taste molecules must escape both the dispersed particles and the “solubilizing” medium.

·         A study was done by two agronomical and nutrition institutes in Dijon to investigate how odorant molecules are released.  They studies vinegar-based sauces.

·         Composition of vinegar-based sauces:  the aqueous phase consists of wine vinegar, lemon juice, and salt, sunflower oil emulsified with the help of whey proteins, and a mixture of xanthan (a polymer obtained by microbial fermentation of glucose) and starch to stabilize the sauce.  Odorant molecules were added:  isothiocyanate in the oil phase (hint of mustard) and phenyl-2-ethanol and ethyl hexanoate (rose and fruity notes).

·         “Although the acid taste was preponderant, the tasters struggled to describe the other sensations.”  However, some observations: the overall odor, the taste and odor of the egg, the mustard odor, and the butter taste increased but the citrus odor decreased as the size of oil droplets increased.

·         Results of analysis of volatile molecules in the air above the sauces:  lower concentrations of water-soluble components detected as the oil droplet size decreased and more abundant oil-soluble molecules.

TO BE CONTINUED

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