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