Transcript Browning Reactions File

CHAPTER 3 BROWNING REACTIONS
1. Introduction
Browning is a common colour change seen in food
during pre-preparation, processing or storage of food.
It occurs in varying degrees in some foods.
The colour produced range from cream or pale yellow to
dark brown or black.
Browning reactions observed in food may be classified
as enzymatic browning or nonenzymatic browning.
a) Enzymatic Browning
• Fruits such as apples, pears, peaches, apricots, and
bananas, and vegetables such as potatoes quickly turn
brown when their tissue is exposed to oxygen.
• Such oxygen exposure occurs when the food is sliced or
bitten into or when it has sustained bruises, cuts or other
injury to the peel.
• This “browning reaction” is related to the work of an
enzyme called phenolase (or polyphenoloxidase), a
conjugated enzyme in which copper is present.
Phenolase
• Phenolase is classified as an oxidoreductase.
• The substrates for phenolase are phenolic compounds
present in the tissues of the fruits and vegetables.
Phenolase hydroxylates monophenols to 0-diphenol and
oxidizes 0-diphenols to 0-quinones.
• The 0-quinones then enter into a number of other
reactions, which produce the “undesirable” brown
discolorations.
• Quinone formation is enzyme and oxygen-dependent.
• Once the quinones have formed, the subsequent
reactions occur spontaneously and no longer depend on
the presence of phenolase or oxygen.
Prevention
Enzymatic browning can be prevented or slowed in several
ways.
Immersing the “injured” food (for example, apple slices) in cold
water slows the browning process.
The optimum temperature for enzymes to act is
43ºC(109ºF).The lower temperature decreases enzyme activity,
and the water limits the enzyme’s access to oxygen.
Refrigeration slows enzyme activity even more, and boiling
temperatures destroy (denature) the enzyme.
A long-used method for preventing browning involves lowering
of pH to 2.5-2.7 by the addition of acids such as ascorbic acid,
malic or citric acid
Phenolase works very slowly in the acidic environment
created by the added acids.
In addition, the vitamin C (ascorbic acid) present in
lemon juice functions as an antioxidant.
It is more easily oxidized than the phenolic-derived
compounds, and its oxidation products are colorless.
b) Non Enxymatic Browning
1) Maillard reaction:
• The non enzymatic browning or Maillard reaction is a chemical
reaction between an amino acid and a reducing sugat, usually
requiring heat.
• When aldoses and ketoses are heated with amines, a variety of
reactions ensue, producing numerous compounds some of
which are flavours, aromas and dark coloured polymeric
material.
•
They may be produced slowly during storage and much more
rapidly at the high temperature encountered during frying
roasting or backing.
• The reducing sugar reacts with the amine to form a Schiff base (an
imines) which may cyclate to form glucosamine.
• In the case of glucose the Schiff base undergo a reaction called
Amadori rearrangement to give 1-amino-1-deoxy-D-fructose or
Amadori compound.
• The Amadori compounds are early intermediates in the browning
reaction sequence.
• Amadori compounds undergo transformation via different pathways
starting with four different intermediates formed from them.
• The result is a complex mixture of intermediates and products.
• The Maillard reaction occurs in three main steps:
• 1. Initial step- formation N glycoside: The carbonyl group of the
sugar reacts with the amino group of the amino acid, producing Nsubstituted glycosylamine and water
• 2. After formation of N glycoside the immonium ion is
formed and then isomerizes, this reaction is called
Amadori rearrangement and forms a compound called
ketosamine:
• 3. The ketosamine products then either dehydrates into
reductones and dehydro reductones, which are caramel,
or products -short chain hydrolytic fission products such
as diacetyl, acetol or pyruvaldehyde which then undergo
the Strecker degradation
and produce short-chain
hydrolytic fission products and brown nitrogenous
polymers and melanoidins
• Important intermediates are formed by rearrangements
and eliminations are 1-, 3- and 4-deoxydicarbonyl
compounds called 1-, 3-, and 4-deoxyosones. They
finally form 5- hydroxy methyl furfural
• In the process, hundreds of different flavor compounds
are created. These compounds in turn break down to
form yet more new flavor compounds, and so on. Each
type of food has a very distinctive set of flavor
compounds that are formed during the Maillard reaction.
It is these same compounds have been used over the
years to create artificial flavors.
Food products with Maillard reactions
• The Maillard reaction is responsible for many colors and
flavors in foods such as bread, biscuit, malted barley as
in malt whiskey or beer, roasted meat, dried or
condensed milk, roasted coffee etc 6-Acetyl-2,3,4,5tetrahydropyridine is responsible for the biscuit or
cracker-like flavor present in baked goods like bread and
popcorn.
• The structurally related compound 2-acetyl-1 pyrrpoline
has a similar smell and occurs also naturally without
heating and gives varieties of cooked rice their typical
smell.
• Maillard reaction may result in a reduction in nutritional properties
and the formation of potentially toxic and mutagenic compounds. In
a food system, the reactants are mostly amino acids (free forms or
peptide-bound) and reducing sugars.
• Since up to 50% of the food groups have been processed before
consumption, some of the amino acids and reducing sugars is lost
during processing.
• Maillard reactions affect protein bioavailability by derivatizing
protein-bound, dietary limiting amino acids such as lysine, arginine,
and histidine.
• Maillard reaction products also exhibit antinutritive effects by
mechanism involving complex formation with micronutrients,
destruction of vitamins, and by acting as inhibitors of digestive
enzymes
• High temperature, low moisture levels and alkaline
conditions promote the Maillard reaction.
• The rate of Maillard reactions increases as the water
activity increases, reaching a maximum at water
activities in the range of 0.6 to 0.7.
• However, as the Maillard reaction produces water,
further increases in water activity may inhibit Maillard
reactions.
• Pentose sugars react more than hexoses, which react
more than disaccharide.
• Different aminoacids produce different amounts of
browning
2) Browning reactions which occur in meat
• The browning reactions which occur when meat is roasted or seared
have often been referred to as Maillard reaction browning.
•
However, lean meat contains very few, if any, reducing sugars.
• Furthermore, red meat undergoes more extensive browning than
does white meat.
• The browning reactions in lean meat are most likely due to the
breakdown of the tetrapyrrole rings of the muscle protein,
myoglobin.
• Thus, the browning of meat is technically not a Maillard browning
since it does not involve the reaction with a reducing sugar
3) Caramelization
•
•
•
Caramelization is a browning reaction formed by heating carbohydrates like
sucrose or reducing sugars.
Reactions are facilitated by small quantity of acids, base and certain salts.
Caramelization is an entirely different process from Maillard browning,
though the results of the two processes are sometimes similar to the naked
eye (and taste buds).
•
The final product caramel contains a complex mixture of polymeric
compound, formed from unsaturated cyclic compounds.
•
Flavour and aroma compounds are also formed.
•
Heating causes the dehydration of sugar molecule with introduction of
double bonds or formation of anhydro rings. Intermediates such as 3-deoxy
osones and furans are formed.
•
The unsaturated rings may condense to form useful, conjugated doublebond containing, brown coloured polymers.
•
Catalysts increase the reaction rate and are used to direct the reaction to
specify types of caramel colour, solubility and acidities.
• To make caramel a carbohydrate is heated alone or in
the presence of acid, a base or salt.
•
The carbohydrates most often used are sucrose, but
fructose, glucose, invert sugar, malt syrups and
molasses may also be used.
• Acid used are food grade sulfuric, sulfurous, phosphoric,
acetic and citric acids.
• Bases that may be used are ammonium, sodium,
potassium and calcium hydroxides.
• Salts that may be used are ammonium, sodium,
potassium carbonates, bicarbonates, phosphates,
sulphates or bisulphates.
Classes of caramel
Class I caramel
:Prepared by heating a plain carbohydrate
Class II caramel
:Prepared by heating a carbohydrate in the
presence of a sulphite
Class III caramel :Prepared by heating a carbohydrate in the
presence of a source of ammonium ion.
Class IV caramel :Prepared by heating a carbohydrate in the
presence of a both sulphite and ammonium ions
• Caramelization may sometimes cause browning in the same foods
in which the Maillard reaction occurs, but the two processes are
distinct.
• They both are promoted by heating, but the Maillard reaction
involves amino acids, as discussed above, while caramelization is
simply the pyrolysis of certain sugars.