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.

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).

SPANISH HAMS

·         Since 1970’s, it’s been known that the aroma of certain foods like cheese is due to degradation products of proteins and lipids. 

·         In the 1990’s, similar analysis was done and similar findings resulted when the transformation of proteins and lipids at every curing step for ham was studied.  They found two types of reactions responsible for producing the molecules of aroma:

o   Maillard reactions between amino acids and sugars during prolonged storage periods which cause darkening

o   Strecker degradations which are reactions between amino acids and acids like fatty acids from lipid degradation.  These produced aldehydes that contribute to the aroma.

§  The aldehydes play a role in the Maillard reaction forming products that slow down the process by which fats turn rancid.

·         Analysis of the volatile molecules from Spanish hams, Spanish chemists found:

o   both linear alkanes from the decomposition of lipids and ramified (branched) alkanes “a consequence of the distinctive acorn-based diet on which Iberian pigs feed”.

o   Linear aldehydes produced by Strecker reactions and reactions associated with unsaturated fatty acids turning rancid

FOIE GRAS

·         “It melts less and tastes better when it is cooked immediately after the geese are slaughtered.”

·         Did not care much for this chapter.

ANTIOXIDANT AGENTS

·         The autoxidation of fatty acids is responsible for fats turning acid upon contact with air. The steps involved are:

·         Light breaks the –C-H bonds of a lipid

·         Unstable –C free radicals are formed

·         The C radicals react with oxygen in the air to form –COO free radicals

·         The –COO radicals react with other –C-H to create a new –C radical that propagates oxidation

·         Steps to slow down and minimize this chain reaction are protecting the food from oxygen in the air and light and using antioxidants (substances that inhibit oxidation).

·         Some naturally occurring antioxidants found in food are tocopherols (vitamin E) in virgin oil and ascorbic acid (vitamin C) in lemon.

·         The food processing industry uses phenols as antioxidants.  Phenols and their esters (aromatic in its chemical nature and stable) because of their aromaticity (renders stability) are able to inhibit or slow down oxidation because when they react with a radical and lose an electron and become radical themselves, the delocalized nature of the unpaired electron creates less reactive radicals.  Thus, the radical reaction is not propagated at the same rate to sustain a chain reaction.

·         In the 1990’s, scientists came up with a way to test the antioxidant properties of some compounds.  In this test, oxygen is bubbled into dodecane, a lipophilic solvent.  Both the fat and the antioxidant compound to be tested are then dissolved in the dodecane at 110 C.  In a specific test using gas chromatography, they determined that it takes 3 hours for half of the methyl linoleate (test fat) to be oxidized. They then test the antioxidant “power” of other compounds by measuring how much the oxidation half-life of methyl linoleate is reduced or increased.

·         Using this test, they were able to compare the antioxidant power of plant phenol acids with 4 antioxidants commonly used in the food processing industry:  butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), 2-tertbutylhydroquinone (TBHQ), and propyl gallate.  In general, they found that the antioxidant ability of a compound is correlated with the number of hydroxyl groups around the benzene ring and the degree of stabilization by delocalization of electrons.

·         Various extracts from plants were tested and they confirmed the presence of antioxidant compounds in rosemary, sage, cloves, and ginger.  Sage, in particular, was found to contain 6 powerful antioxidant compounds: carnasol, carnosic acid, and isorosmanol in large quantities and also rosmadial, rosmanol, and epirosmanol.

TROUT

·         Filet of fario trout contains 65-70% water, 20-24% proteins, and 2-12% lipids [by mass, I am guessing].

·         The pink color of filets are due to carotenoid molecules (astaxanthin and canthaxanthin).  The color of the fish does not seem to be a good indicator of the quality of its filet because the color is mostly due to the fish’s diet.

·         Fish meat has a different protein structure from that of other animal meat.  Animal muscle usually is composed of long fibers wrapped with collagen and collected in bundles which are also sheathed in collagen.  “The cooking of meat therefore involves a delicate compromise between hardening, which results from the coagulation of the proteins contained in these cells, and tenderizing, a consequence of separation and dissociation of the collagen molecules”.  Fish meat contains a small amount of collagen and their muscle fibers are arranged in sheets with only the surface protected by collagen.  Lipids located within the muscle sheets keep the sheets together and determine the texture of the flesh.  Trout flesh is known to be firmer than other freshwater fish like carp and catfish.

·         Heating (in the process of cooking) causes the muscle proteins to coagulate increasing the mechanical resistance of the muscle fibers.

COOKING TIMES

·         Because cooking is “fundamentally a transformation of foods by heat”, the amount of tenderness and juiciness in cooked meat depend on the amount of cooking time.

·         Media for heating include gases, liquids, solids, and waves.

·         Cooking by gas includes smoking, drying, braising, steaming, and oven roasting.

o   In smoking and drying, cooking is slow because the temperature of the heated air is not much above the room temperature.

o   Cooking by steaming is faster because the food receives both the kinetic energy of the steam and the energy released by condensation.

o   In oven cooking, the air can be heated to much higher temperatures.

·         How much time to cook?  Meat twice as thick takes 4 times the cooking because both the distance the heat travels and the quantity of meat to be heated are doubled.  “For a spherical body, the cooking times is proportional to the mass raised to the power of two-thirds, a relationship described by a curve that flattens out after an initially rapid rise.”

·         The chemical transformations taking place in the braising method of cooking are:

·         “…At 40°C (104° F) meat becomes opaque because the proteins in it, initially folded into a ball, begin to unfold before they coagulate (thus becoming denatured); at 50°C (122°F) the muscle fibers begin to contract; at 55°C (131°F) the fibrillar part of myosin (a protein that, along with actin, is essential for muscle contraction) coagulates, and collagen (a protein that gives meats their toughness) begins to dissolve; at 66°C (151°F) various other proteins coagulate; at 70°C (158°F) myoglobin no longer fixes oxygen, causing the inside of meat to turn pink; at 79°C (174°F) actin coagulates; at 80°C (176°F) the cell walls are ruptured and the meat becomes gray; at 100°C (212°F) water evaporates; and at temperatures higher than 150°C (302°F) so-called Maillard (and other) reactions produce brown and flavorful results.”

THE FLAVOR OF ROASTED MEATS

·         Fats are found to participate in Maillard reactions that produce compounds that make up the “chief aromatic components of heated foods”.

·         Maillard reactions occur between sugars and amino acids from proteins.  They produce compounds that give favor to bread crust, roasted aroma of meats, beer, and chocolate and form melanoidins which give food a characteristic brown color.

·         In addition to Maillard reactants, chemists have found that phosphate sugars, nucleotides, peptides, glycopeptides and organic acids are also precursors of volatile compounds in cooked meats.

·         ROLE OF PHOSPHOLIPIDS:  Earlier studies showed that removing triglycerides from meats did not alter its odor after cooking but removing phospholipids did to one of roasted meat and biscuit.

·         Phospholipids contain polyunsaturated fatty acid residues that can undergo oxidation and they also contain a molecular portion soluble in water which can react with oxygen also.

·         To study the role of phospholipids in the Maillard reactions, scientists set-up a simple reaction in which cysteine (sulfur-containing amino acid chosen because it contains sulfur which is involved in the production of cooked meat aroma) is reacted with ribose (“a sugar known for its activity in cooking that can be released in nucleotides [?]).  They then added either fatty acids found in phospholipids (linoleic acid, palmitic acid, and ethanolamine) or the principal phospholipids in meats (phosphatidylcholine and phosphatidylethanolamine) and the mixtures were heated to 140 C.

·         They used gas chromatography to study the product profiles focusing on heterocyclic compounds (with meaty taste) and products of lipid oxidation.

·         Findings:

·         Phospholipids play a greater role in producing the aroma of cooked meats and these aromas come from two effects:

·         Carbonylated compounds which create a fatty aroma (from the oxidation of fatty acids)

·         Interaction of lipids and their degradation products with the direct and intermediate products of Maillard reactions

·         “Further analysis showed that the odors of the modeled systems resulted more from a disturbance of Maillard reactions than from lipid oxidation.  Although lipids do not come into contact with compounds dissolved in the aqueous phase, phospholipids, because of their polar head, are partially soluble and can react with the intermediate products of Maillard reactions.”

TO BE CONTINUED