Food Preparation and Nutrition: Science of Cooking Notes

The Maillard Reaction

• Protein foods develop a brown color when heated due to protein denaturation.
• Occurs in dry-heat applications.
• Caused by the amino acids in the food.
• Produces a desirable flavor when not overdone.
• Maillard reaction is the browning that occurs when amino acids (proteins) are cooked with sugars; also called ‘non-enzymic browning’.
• Gives fried and baked foods their appealing color and flavor compounds.
• Occurs on the outer surface of bread during baking, and the browning of meats when roasted, fried, or grilled.

Gelatinisation: Starch

• Starch particles do not dissolve in water; they form a suspension.
• If the suspension is not stirred, the starch particles sink and lump together.
• When heated to $60°C$, starch particles begin to absorb water and swell.
• When heated to $80°C$, particles absorb up to 5 times their volume of water, eventually bursting and thickening the liquid.
• Starch granules absorb water as they soften and cook, holding the liquid in place.
• Molecules join together, helping food hold its shape.
• Occurs at different temperatures for different types of starch.
• Root-based starch breaks down faster in the presence of heat, sugar, and acids.

Foaming and Emulsions

• Emulsion is a mixture of two substances not usually soluble; the objective is to suspend one ingredient in the other.
• A vinaigrette is an example of a water/oil emulsion; shaking disperses vinegar into oil.
• Butter is a water/oil emulsion.
• Vegetable oils do not dissolve in water.
• Shaking oil and water together forms an emulsion where tiny droplets of one liquid spread through the other.
• Emulsions are thicker (more viscous) than the oil or water they contain.
• Useful in salad dressings and ice cream, as well as cosmetics and paints.
• Two main types of emulsion:
  • Oil droplets in water (milk, ice cream, salad cream, mayonnaise).
  • Water droplets in oil (margarine, butter, skin cream, moisturizing lotion).

Emulsifier

• If an emulsion is left to stand, a layer of oil will form on the surface of the water.
• Emulsifiers are substances that stabilize emulsions, stopping them from separating out.
• Egg yolk contains a natural emulsifier.
• Mayonnaise is a stable emulsion of vegetable oil and vinegar with egg yolk.
• Emulsifier molecules have two different ends:
  • A hydrophilic end ('water-loving') that forms chemical bonds with water but not with oils.
  • A hydrophobic end ('water-hating') that forms chemical bonds with oils but not with water.
• Mayonnaise, hollandaise, and forcemeats are examples of permanent emulsions.
• Oil and vinegar dressing is a temporary emulsion that must be mixed constantly.
• Other common emulsifiers are mustard, glace de viande, and butter (fat).

Dextrinisation

• When a starch (e.g., flour) is cooked in dry heat (e.g., oven), dextrins are produced.
• Dextrins are a monosaccharide (a type of sugar), making them sweeter than starch.
• Dry heat (oven/grill) causes starch to change color, texture, and flavor.
• Starch changes to dextrin.
• Dry heat turns starch into sugar called dextrin.
• Toasting bread causes heat to convert the outer layer of starch to dextrin, which is why toast has a slightly sweet taste.

Caramelisation

• When a sugar is heated in dry heat (e.g., oven), it gradually turns brown.
• This adds to the flavor and golden color of food products.
• Sugar (sucrose) is heated and melts to a syrup.
• The syrup boils.
• The sucrose molecules break up, and water molecules are formed.
• Water evaporates, the syrup gets thicker, and changes to a golden-brown caramel.
• It will eventually burn and become bitter if cooked for too long.
• Foods that contain natural sugars (e.g., onions) will caramelize.
• Caramelization Stages & Temperatures:
  • Thread: Binding agent for fruit pastes - $106-112°C$
  • Softball: Fudge, Fondant, Creams, Penuche, Maple - $112-116°C$
  • Firmball: Caramels - $118-120°C$
  • Hardball: Marshmallows - $121-130°C$
  • Soft Crack: Butterscotch and Toffee - $132-143°C$
  • Hard Crack: Peanut Brittle - $149-154°C$
  • Light Caramel: Glazes, Coating Agent - $160-170°C$
  • Dark Caramel: Glazes, Coating Agent - $177°C$

Victoria Sponge Cake - Chemical Reactions

• Ingredients: 225g margarine, 225g caster sugar, 4 eggs, 2 tsp vanilla extract, 225g self-raising flour.
• Cream the butter and sugar together until pale and fluffy.
• Beat in the eggs, a little at a time, and stir in the vanilla extract.
• Fold in the flour, adding a little extra milk if necessary, to create a batter with a soft dropping consistency.
• Divide the mixture between the cake tins and gently spread out with a spatula.
• Bake for 20-25 minutes, or until golden-brown on top and a skewer inserted into the middle comes out clean.
• Relevant processes:
  • Gelatinization
  • Dextrinisation
  • Caramelization
  • Denaturation & Coagulation

Chemical Agents & Mechanical Methods

• Chemical Agents
  • Baking powder
  • Bicarbonate of soda
  • Yeast
• Mechanical Methods
  • Sieving
  • Creaming
  • Whisking
  • Folding & rolling
  • Rubbing in

Enzymic Browning

• Enzymic browning is a chemical process that occurs in some fruits and vegetables, causing them to discolor, usually turning a brown color.
• Caused by an enzyme called polyphenol oxidase.
• Can be seen in fruits such as apricots, pears, bananas, grapes, and avocados, and vegetables such as aubergines, potatoes, and lettuce.
• Once the fruit or vegetable is cut, some of the cells are opened up to the air.
• The enzyme polyphenol oxidase reacts with the oxygen in the air, resulting in the fruit & vegetables turning brown.
• Enzymic browning in fruits and vegetables is not desirable and can be controlled with a range of methods.

Protein Denaturation

• Natural proteins are shaped like coils or springs.
• When exposed to heat, salt, or acid, they denature (their coils unwind).
• Proteins are made up of individual units called amino acids.
• Protein molecules are often folded into compact ‘bundles’ so that they take up less space.
• They contain oxygen, carbon, hydrogen, and nitrogen.
• Chemical bonds in the protein hold it together and stop it from unfolding.
• The structure of protein changes when heat or mechanical action (beating) is applied.
• This causes the tangled-up protein molecules to unravel.
• Denaturing proteins makes them more palatable and easier to digest.
  *Denaturation via Heat
  *Hydrogen bonds are broken by increased translational and vibrational energy.
  *Example: Coagulation of egg white in fried egg.
  *Denaturation via Acids
  *Acids disrupt the hydrogen bonds by releasing hydrogen ions when dissolved in water.
  *Example: Adding lemon juice to cream or vinegar added to poached eggs.
  *Denaturation via Air
  *Air bubbles disrupt the hydrogen bonds.
  *Example: Whisking eggs for a whisked sponge.
  *Denaturation via Mechanical Agitation
  *Air bubbles disrupt the hydrogen bonds.
  *Example: Whisking egg whites to make meringue.

Protein Coagulation

• When proteins denature, they tend to bond together, or coagulate (set), and form solid clumps.
• An example of this is a cooked egg white, which changes from a transparent fluid to an opaque solid.
• As proteins coagulate, they lose some of their capacity to hold water, which is why protein-rich foods give off moisture as they cook, even if they are steamed or poached (juice from meat).
• Some heat-induced denaturation is reversible through cooling.
• Roasted foods should be allowed to rest before carving; as the temperature falls, some of the water (“juice”) that was forced into spaces between the proteins is reabsorbed, and the food becomes moister.
• Denatured proteins are easier to digest than natural proteins.
• Process where liquids change state to form a semi-solid or solid mass due to the clumping of protein strands.
• Denatured protein molecules are larger and take up more space.
• They knock into other denatured protein molecules and start to join together in large groups – called coagulation.
• The protein molecules trap and hold water from the food in pockets between them.
• As coagulation continues, the appearance and texture of the food changes…
• If a food containing protein is overcooked:
  • The coagulated protein molecules tighten up and squeeze out the water they were holding.
  • Overcooked meat or fish is dry and chewy, and overcooked scrambled egg is rubbery and chewy.