KBP Chemistry of Cooking

Kimia Bahan Pangan

Chemistry of Cooking

• Why is my toast brown ?
• What happens to meat when it is cooked ?
• What causes the odour of roasted meats ?
• what causes the flavour of roasted coffee ? 
• Does cooking introduce harmful by products ? 

Let's compare

• Grilling (BBQ) gas or with hard wood coals : char broiled

• Oven broiled
• Oven roasting (baked)
• Boiled
• Steamed
• Microwave

What is known for sure

• BBQ (Grilling) of high fat content meats at high temp produces Polynuclear aromatic hydrocarbons (PNAH's)
• Planar molecules : intercalate into the major groove of the DNA double helix

• Can be cancer inducing if DNA directed synthesis of protein gets out of control

First cancerous lesions

• Observed in chimney sweeps in UK in 1700's and workers in coal tar industry
• Exposure to soot on skin
• PNAH's common in soot from burning of coal

That BBQ steak flavour

• 15 different PNAH's have been isolated from the outer layer of charcoal broiled steak
• 8 microgram of 1,2-benzopyrene per kilo of steak
• Arise from decomposition of fat that drips on to the glowing charcoal and the subsequent vaporization of the hydrocarbons and deposition on the surface of the meat

Other possible carcinogens

• Heterocyclic amines (HCA's) added to list of known carcinogens in 2005
• Arise from reaction of creatine (an amino acid found in muscle) and carbohydrate
• Higher temps from grill, frying or oven broiling increase the concetrations

How to minimize these risks

• Use lean meats or remove fiat
• Cook at lower temperatures (ie allow coals gas grill move food to an upper rack
• Use merinades (olive oil or citrus based)
• Avoid overcooking

Marination : also denaturation

• Long time (days) exposure to acid (in vinegar) will denature some protein and tederize some meat
• Also kills salmonella, but not E coli

Cooking fish on the grill

• Leave skin on and do most of the grilling skin side down-easly separated when fish is cooked
• Use cedar grilling planks to impart a rich smoky flavour-keeps fish moist and no charring

Happy BBQ's (a summer tradition)

• Should be a treat. not every night ! Enjoy !

In general

• Long slow cooking (baking, roasting) are best due to lower temperature used
• Minimizes formation of potential carcinogens
• BUT, real dangers from undercooked food containing harmful bacteria (E coli : can be fatal almost immediately)
• Hamburger (large surface area) and poultry

Let's think positively about cooking

• Where do those wounderful aromas come from (ie, baking bread). coffee brewing, cookies from the oven and the traditional Sunday roast beef dinner ( with oven browned potatoes, carrots etc)

Traditional (for some)

The Maillard browning reaction

• Louis-Camille Maillard investigated this ~1910
• Reaction between an amino acid (in protein) and a sugar (from starch)
• Accelerated in a basic environment : amino group becomes non protonated

What is the mechanism ?

• The N atom of the amino acid is nucleophilic (ie it is seeking a partially positive target)
• The C = O in the open form of sugars (aldohexoses and aldoketoses) ie glucose and fructose has a partially positive C, due to the fact that O is more electronegative than C

Nucleophilic addition

• Lone pair of electrons on N "attacks" partially positive C of C=O group

 Can you name this compound ?

• Probably not !
• 2-acetyl-3,4,5,6-tetrahydropyridine
• Oops-it is a heterocyclic amine (HCA)

Acrylamide structure

• Planar, thus a potential DNA intercalator

 Note the similarity to acrylamide structure

• So..........could acrylamide be generated by the Maillard reaction ?
• Yes ! (J. Agr. Food Chem. 53, 4628-4632 (2005)

Glucose reacts with Asparagine

• N attacks C=O !

 Many Steps later .. a bit of ..

• Exact mechanism unknown for acrylamide formation
• Many other products !

What about acrylamide ?

•  No evidence yet of human cancer induction
• Considered a probable carcinogen based on animal studies
• First noted in Swedish study in 2002
• Particularly in deep fried foods
• Low levels suggest not a critical issue

Maillard reactions of Tryptophan

• In turkey : reacts with glucose to produce a glycoside

 Tryptophan : glycoside

Maillard reaction in roasted nuts

• Almost all nuts are roasted before
• consumption; kills bacteria and increases flavor
• Peanut roasting has been studied in detail due to widespread allergies
• Allergy is protein induced
• Some of the Maillard products may increase the allergenicity of peanuts

Summary

• Maillard reaction produces hundreds of compounds which are responsible for pleasant odour and taste of protein containing foods.
• It also produces trace amounts of acrylamide (the price we pay for flavor !)

Other Browning reactions

• Maillard "browning" often accompanied by carmelization if more carbohydrate (sugar) is present
• Carmelized onions : both protein and sugar naturally present in the onion (Demo).
• Heating of sucrose alone can cause carmelization (browning) (Demo).