MOLECULAR GASTRONOMY: Part I - Secrets of the Kitchen

PART ONE:  SECRETS OF THE KITCHEN

In Part 1, the author devotes mini-chapters first to explore the validity of tradition cookbook recipe tips and instructions and, second, to propose a science-based explanation either based on published studies or by conducting actual experiments.  Some of the more interesting observations and results are listed below:

·         In Making Stock, the author finds that, contrary to traditional recipe instructions, the rate of juice loss by meat is the same regardless of whether it is placed in coldwater first and allowed to boil or it is plunged directly into boiling water.  The author cited an experiment where similar size meat pieces were cooked for an hour, one starting out in cold water the other in boiling water.  After one hour, he found no discernible difference between the mass lost due to liquid loss.

·         In Hardboiled Eggs, the author explores a method to keep the yolk centered after cooking.  The yolk is an emulsion of water and lipids which are less dense than water.  The white is composed of mostly water and proteins and therefore more dense than the yolk.  When left in one position while cooking, the less dense yolk gradually rises to the top.  Rolling the egg around while cooking will keep the yolk centered.

·         Egg white is 90% water and about 10% proteins [by mass, I assume].  Upon heating, the proteins unfold and eventually denature trapping the water and forming a gel-like structure.  The extent of unfolding and denaturation and loss of water determines whether the cooked egg white will remain runny or rubbery.  Most of the proteins in the egg start to coagulate at a temperature below the boiling point of water at 68 C.   In the egg white, the ovotransferrin protein starts to coagulate at 62 C while proteins in the egg yolk remain “liquid” and does not start to harden until 68 C.  Above these temperatures, more water starts to leave the yolk and the egg white resulting in a harder and more rubbery texture. 

·         Quiches, Quennelles, and Puff Pastries:  The puffing up process is mostly due to vaporized water and only little due to expansion of egg.

·         Echaudes and Gnocchi:  In this chapter, the author addresses the question “Is it true that when they float to the surface of the cooking water they are done?”  To answer this question, the author first investigated what makes them rise in the first place and discovered that it must be trapped steam or water vapor that causes its density to decrease causing it to float.  The author then placed two different size dough pieces in water and found that at the point that they have risen to the surface, the smaller dough piece has a higher temperature than the larger dough piece suggesting cooking times that depend on the size and not necessarily signaled by its rising to the surface.

·         The Well-Leavened Souffle:  “Water evaporates upon contact with the heated sides of the ramekin and causes the soufflé to rise.”  Contradictory to common belief, only 20% of the rise in volume of soufflés is due to the expansion of trapped air.  The majority is due to evaporation of water in the milk and eggs that causes the increase in pressure and subsequent rise in volume.  Puncture a soufflé and steam can be seen rising off the top.  Another factor that affects the rising of a soufflé is the degree to which the eggs were whipped. Stiffer egg whites cause more rising because of its better ability to trap the vaporized water within the protein matrix.

·         Quenelles and Their Cousins:  Quenelles are a “small seasoned ball of pounded meat or fish (OAD)”.  In this chapter, the author seeks to find out the best way to cook quenelles, best meaning “the greatest possible tenderness with sufficient firmness”.  Both of these properties rely on the type of gel that forms from the meat or fish protein.  The firmness of a gel is determined by the following parameters:  storage time of the solution, rate of heating, maximum cooking temperature, protein concentration, acidity, and salt concentration.  Some findings that correlate with optimal firmness (firmness was measured using a penetrometer but there was no mention of the quantitative definition using this instrument only that these parameters led to “sufficiently firm and elastic gel” from trout)

o   The maximum protein concentration for optima firmness is about 10 g/L

o   Heating within the temperature range of 70 – 80 C

o   Heating rate of 0.25 C per minute

o   A pH of about 5.6 (the ionization states of the acidic and basic side groups of the amino acids determine bonding of the gel with water)

·         Fondue:  The author tries to find out the best wine and cheese for a non-flapping fondue.  A fondue is just cheese heated with wine.  A “successful fondue is necessarily an emulsion a dispersion of microscopic droplets of fat in water solution”.  Some interesting chemical descriptions:

o   Milk is an emulsion of an aqueous phase, casein proteins, and fast.  The casein are held together by calcium salts (especially phosphates) forming micelles that surround fats with kappa-caseins on the surface interacting with the aqueous phase.

o   The rennet added in cheesemaking contains an enzyme that “detaches a part” of the casein causing the micelles in milk emulsion to aggregate into a gel trapping the fats.

o   WELL-AGED CHEESE:  Peptidases in well-aged cheese help in pre-breaking up of the casein and other proteins into smaller fragments that are more easily dispersed in water.  These proteins stabilize the emulsion process with water causing a smoother, more viscous mixture (“which is why a Camembert fondue will always turn out well”).

o   WINE:  Dry, very acidic, and fruity wines that contain high concentrations of tartaric, malic, and citric acids because their anions are good at chelating calcium ions which help separate the casein micelles, releasing constituent proteins that help stabilize the emulsion.

o   SODIUM BICARBONATE:  Adding this helps in deprotonating the acids in wine to release the basic anions that can chelate more calcium ions.

·         Roasting Beef:  “Allowing meat to rest after cooking causes the juices that have been retained in its center to flow outward to the dry periphery.”

o   At 70 C, ferrous ion in myoglobin is oxidized to ferric iron which turns the meat “pink”. At 80 C, the cell walls break down releasing myoglobin which reacts with oxygen and turns it brown.

·         Seasoning Steak:  In this chapter, the author tries to find out whether it makes a difference to add salt before, during, or after grilling steak.  No discernible differences as far as the retention of water due to osmosis.  During cooking, using x-ray analysis, they found that salt sprinkled on the surface actually “passes out of the meat during cooking”.

·         Wine and Marinades: The author tests whether red wine or white wine marinade results in a more tender beef.  Red wine was the winner.  They tested the hypothesis that the higher concentration of polyphenols which react with protein which presumably leads to a more tender meat.  Using a solution containing tannic acid representing polyphenols, organic acids, and ethanol, they were able to replicate the same result as the red wine.  It is thought that the polyphenols reacting with the protein create a hardened surface that can retain more juices.

·         Color and Freshness:  The author tests different ways to prevent discoloration in fruits and vegetables.

o   Darkening of fruit when exposed to air is caused by the oxidation of polyphenols to quinones by the action of the enzyme polyphenol oxidase producing brown pigment.  The ascorbic acid in lemon and other citrus fruits help reduce the browning because the ascorbic acid is more easily oxidized protecting the polyphenols.  In addition, he following three methods are used to slow the browning process:

§  Refrigeration slows down the polyphenol oxidase enzyme

§  Pasteurization denatures the polyphenol oxidase (may degrade the fruit)

§  Vacuum packing protects prevents contact with oxygen

§  Storing in a nitrogen or carbon dioxide atmosphere removes oxygen (done in food processing industry)

o   Good question for 1B.  Give the following passage to student and ask student to identify what part of the rate equation is affected by each of the techniques given to prevent browning:  “Various methods are used to prevent the darkening of vegetables and fruits that have been sliced or chopped—often their lot in the kitchen. Freezing and refrigeration slow but do not prevent the action of enzymes. Pasteurization, a more radical procedure that inactivates the enzymes, cannot be applied to all fruits and vegetables, for it often degrades their texture and color. Finally, vacuum packing—sealing fruits and vegetables in containers from which the oxygen has been drawn out—prevents the appearance of brown compounds; alternatively, nitrogen and carbon dioxide atmospheres sometimes are used in the food processing industry.”

o   Other chemical ways of reducing the browning:

§  A small amount of naturally occurring salicylhydroxamic acid inhibits the formation of polyphenol oxidase in apples and potatoes.

§  Bentonite, a protein-absorbent clay, also inhibits the enzyme.

§  Gelatin, activated charcoal, and polyvinyl pyrrolidone have been used to extract phenols from wine and beers but can affect the properties of these drinks

§  Sulfites bond with quinones and converts them to colorless sulfoquinones.  In wines, sulfur dioxide and sodium metabisulfite are used to prevent darkening.

§  Cysteine and its derivatives, other natural compounds in honey, figs, and pineapple and synthetic compounds are being studied (at the time of the book’s writing) as well.

·         Softening Lentils:  In this chapter, the author looks at the role of bicarbonate in cooking lentils. 

o   Effect of pH: To do this, the author carried out an experiment in which he cooked lentils in three identical pans filled with distilled water, basic water using bicarbonate, and acidic water using vinegar.    The results:  distilled water – lentils just cooked, acidic water – lentils hard as pebbles, and basic water – lentils falling apart.  The hardness of lentils is primarily due to the pectin “glue” holding the tissue together.  To soften this tissue, the pectin must come apart.  Pectin is a protein containing carboxylate side groups  In an acidic solution, these carboxylate groups are protonated and neutral which removes the repulsive forces between the chains.  In a basic solution, the carboxylate groups are deprotonated and negatively charged causing repulsive forces that help keep the pectin chains apart causing tissue deterioration.

o   Effect of calcium ions in hard water:  Experiment done cooking lentils in distilled water (cooked) and in water that has calcium ions added (still hard).  The calcium ions are thought to bind phytic acid molecules and pectin molecules which causes more cohesion.  Monovalent sodium ions do not have the same effect.  Temperature also affects the cooking process, not surprisingly the cooking time is inversely proportional to the temperature.  Plotted data on firmness as function of time, however, shows an elbow which suggests that a rapid preliminary softening as water seeps into the beans followed by a slower rate associated with the gelling of the starch.  Lastly, it has been shown that at temperatures above 86 C, there are more lentils that have fallen apart and those that have softened but retained their form.

·         Souffleed Potatoes:  How to avoid the greasiness of deep frying in making this dish is the topic for this chapter.  Souffleed potatoes are puffed up slices of potatoes deep fried in oil.  The puffing up process results from the vaporization of water in the potatoes.  Potatoes are about 78% water and 17% starch (by mass?) which is denser than oil and water.  When water inside becomes steam, its pressure prevents oil from seeping inside.  The lowered density causes it to rise in the oil.  More oil will adhere to the cooked potato when the surface is rough, the oil is reused, and oil is not removed from the surface (as the water vapor condenses, oil will get sucked inside).

·         Preserves and Preserving Pans:  In this chapter, the author looks at the why unplated copper pans are recommended for cooking fruit preserves.   Fruits used for making jams and preserves can be as acidic as pH 3. 

o   Effect of copper (II) ions:  When cooked in an unplated copper pan, the copper metal is oxidized into copper (II) ions.  Based on an experiment carried out by the author, addition of copper (II) salt results in a firmer preserve gel as the copper (II) ions are thought to help the pectin protein molecules to bond with each other forming a stronger and tighter matrix for the trapped liquid.

o   Effect of other metal ions:  silver salts cause raspberries to turn a little bit white, copper ions cause them to turn red-orange, and tin ions cause a purple tinge.

·         Saving a Crème Anglaise:  To prevent curdling in crème anglaise and similar preparations, add a pinch of flour. The starch molecules in flour swell as they absorb water and release amylase chains which get in the way of protein movement thus stabilizing them and preventing aggregation into clumps.

·         Grains of Salt:  It was found that:  water will take the same amount of time to boil (within experimental precision) with or without salt, meat will lose and absorb about the same amount of liquid (+/- 1 gram, based on mass before and after measurements) regardless of whether the water is salted or not, no effects on eggshells either, nor on vegetables as the surface is covered with a waxy layer that prevents osmosis.

·         Of Champagne and Teaspoons:  The author set out to test whether inserting a teaspoon inside the neck of a bottle prevents bubbles from escaping.  Experiments were done by an interprofessional committee on champagne.  Degassing was measured using weight loss for three different samples under the same conditions:  corked, open, and with a teaspoon in the neck. 

o   Pressure measurement results:  only 10% drop in the pressure for the corked bottles compared to 50% for both the open bottles and the bottles with a teaspoon. 

o   Mass measurement results:  identical mass loss for open bottles and the bottles with a teaspoon and zero for corked bottles. 

o   These experiments showed that the extent of degassing depend on: pressure above the surface of the liquid, the amount of suspended particles, and the roughness of the inner surface of the bottle [both suspended particles and rough surface acting as “nucleation” point for bubbles?]

·         Coffee, Tea and Milk:  How best to cool down a hot beverage?  Heat transfer can take place by convection, conduction, or radiation.  Radiation of heat by a body per unit time is proportional to the 4^th power of the body’s absolute temperature (Stefan’s law): the hotter the coffee, the more heat is radiated per unit time.  Some experimental results: 

o   Using the addition of 7.5 cL room-temperature milk to cool down 20 cL of 100 C coffee:

o   10 minutes to go from 100 C to 55 C.

o   4 minutes to 55 C if milk is added only after coffee has cooled down to 75 C

o   Placing a teaspoon to facilitate heat radiation: no significant effect.

o   Stirring has two effects: 1) the movement of liquid exposes more surface area for heat transfer to cold air to take place and 2) accelerates the expulsion of the higher kinetic energy molecules (they are not recycled back into the liquid) lowering the average kinetic energy and thus the temperature.

o   Blowing is more efficient than stirring:  6 C/minute loss versus 3.5 C/minute loss for stirring.