Browning Reaction of Foods
2009-July
B.K.K.K.Jinadasa, GS/MSc/FOOD/3608/08, Department of food science and technology, University of Sri Jayawardanapura.
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1.0 Introduction: There are two major categories of browning that occur in foods: Non-enzymatic and Enzymatic. The major difference between these two categories of browning is that enzymatic browning requires the presence of an enzyme, such as polyphenol oxidase, whereas non-enzymatic browning does not. In general, enzymatic browning, which is responsible for the discoloration observed in some fresh cut fruits and vegetables, such as apples and potatoes, is undesirable. Normally, the cell structure separates the enzymes from the phenolic substrate; however, then the fruit or vegetable is cut or bruised the enzymes and phenols can now come together and react in the presence of oxygen to produce undesirable brown-color compounds. Non-enzymatic browning, on the other hand, is responsible for a number of pleasant and desirable aromas and flavors. Unfortunately, there are also some undesirable results of non-enzymatic browning. The two main types of non-enzymatic browning that occur in foods are the Maillard reaction and Carmelization. The Maillard reaction is a chemical reaction between (the carbonyl group) a reducing sugar and the free amino group of an amino acid or a protein. The rate of the Maillard reaction is accelerated by heat (e.g., baking temperatures of 300° F to 500° F), however, the reaction can readily proceed at lower temperatures, but at a slower reactions rate, which translates into a longer time. A reducing sugar is a type of sugar with an aldehyde or ketone group. Common reducing sugars include glucose, fructose, and lactose. It is important to note that sucrose, common table sugar, is a non-reducing sugar, however, high temperatures and low pH values can cause the sucrose molecule to hydrolyze that is break apart, into one unit of glucose and one unit of fructose. After hydrolysis, the component sugars of sucrose begin participating in the Maillard reaction. Once the initial Maillard reaction begins a cascade of additional reactions take place, eventually forming brown nitrogen containing polymers and copolymers called melanoidins. The Maillard reaction is responsible for many desirable colors and flavors in foods, such as caramel made from milk and sugar, the browning of bread into toast, the color of beer, chocolate, coffee, and maple syrup, and the flavor of roast meat. The Maillard reaction is responsible for literally hundreds of flavor compounds, and is even used by flavor chemists to create artificial flavors. The Maillard reaction is a chemical reaction between a reducing sugar and the free amino group of an amino acid or a protein. The rate of the Maillard Page 2 of 12
reaction is accelerated by heat, however, the reaction can readily proceed at lower temperatures, but at a slower reactions rate, which translates into a longer time. 2.0 Enzymatic browning: Normally, enzymatic browning that is responsible for the discoloration appeared in some fresh cut fruits and vegetable, such as apples and potatoes is undesirable. Enzymatic browning is a chemical process involving polyphenol oxidase or other enzymes that create melanin, resulting in a brown color. Enzymatic browning is an important color reaction in fruit, vegetable and seafood. The taste of fruits is influenced by phenoloic compounds. Phenolic compounds are widely distribute in the plant kingdom and are considered to be secondary metabolites. Structurally they contain an aromatic ring bearing one or more hydroxyl groups, together with a number of other substituent like simple phenolics, cinnamic acid derivatives and flavanoids. As well as phenolic compounds are substrate for polyphenol oxidases. These enzymes hydroxylate monophenols to o-diphenols and also oxidize o-diphenols to o-quinones. O-quinones can enter entre into number of reactions, thus giving the undesirable brown discoloration. An overview of known poly-phenols involved in browning. Polyphenoloxidase catalyzed reaction shown below. Table 1: An overview of known poly-phenols involved in browning Source
Phenolic substrates
Apple
chlorogenic acid (flesh), catechol, catechin (peel), caffeic acid, 3,4dihydroxyphenylalanine (DOPA), 3,4-dihydroxy benzoic acid, pcresol, 4-methyl catechol, leucocyanidin, p-coumaric acid, flavonol glycosides
Apricot
isochlorogenic acid, caffeic acid, 4-methyl catechol, chlorogenic acid, catechin, epicatechin, pyrogallol, catechol, flavonols, pcoumaric acid derivatives
Avocado
4-methyl catechol, dopamine, pyrogallol, catechol, chlorogenic acid, caffeic acid, DOPA
Banana
3,4-dihydroxyphenylethylamine (Dopamine), leucodelphinidin, leucocyanidin
Cacao
catechins, leucoanthocyanidins, anthocyanins, complex tannins
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Coffee beans
chlorogenic acid, caffeic acid
Eggplant
chlorogenic acid, caffeic acid, coumaric acid, cinnamic acid derivatives
Grape
catechin, chlorogenic acid, catechol, caffeic acid, DOPA, tannins, flavonols, protocatechuic acid, resorcinol, hydroquinone, phenol
Lettuce
tyrosine, caffeic acid, chlorogenic acid derivatives
Lobster
tyrosine
Mango
dopamine-HCl, 4-methyl catechol, caffeic acid, catechol, catechin, chlorogenic acid, tyrosine, DOPA, p-cresol
Mushroom
tyrosine, catechol, DOPA, dopamine, adrenaline, noradrenaline
Peach
chlorogenic acid, pyrogallol, 4-methyl catechol, catechol, caffeic acid, gallic acid, catechin, dopamine
Pear
chlorogenic acid, catechol, catechin, caffeic acid, DOPA, 3,4dihydroxy benzoic acid, p-cresol
Plum
chlorogenic acid, catechin, caffeic acid, catechol, DOPA
Potato
chlorogenic acid, caffeic acid, catechol, DOPA, p-cresol, phydroxyphenyl propionic acid, p-hydroxyphenyl pyruvic acid, mcresol
Shrimp
tyrosine
Sweet potato
chlorogenic acid, caffeic acid, caffeylamide
Tea
flavanols, catechins, tannins, cinnamic acid derivatives
2.1. Control of enzymatic browning: The control of browning is one of the most important issues in food industry. Enzymatic browning is usually controlled by different technique which is given below. Exclusion of oxygen – exclusion of oxygen is possible by immersion in water, syrup, brine or by vacuum treatment. Page 4 of 12
Application of heat – blanching is a short heat treatment to destroy or inactivate enzymes, blanching to destroy the enzymes is commonly used to preserve color in vegetable but has little effect on nutrient content or flavor as it is a relatively short process. Blanching temperature depends on the type of enzymes in foods. Table 2: inactivation temperature of some enzymes Enzyme
Effect
Inactivation temperature/0C
Lipolitye acyl hydrolase Rancidity
~ 75
Lipoxygenase
Rancidity
~ 80
Polyphenoloxidase
Browning
~ 100
Peroxidase
Deterioration ~ 135
Two types of blanching technique using in food industry which are blanching in steam/boiling water and microwave blanching. pH treatment- lowering the pH to 4 by using citric, malic, phosphoric or ascorbic acid Using chelators – mode of action is removal metals, because most polyphenoloxidase enzyme contains metals atoms. phosphates, EDTA, organic acid Reducing agents- Sulfites (formation of colorless addition products with quinines, change in protein conformation), ascorbic acid and analogs, cysteine, glutathione. Here action is removal of oxygen. Using enzyme inhibitors – enzyme inhibitors react with enzymes. E.g. Aromatic carboxylic acids, peptides, substituted resorcinol. Enzyme treatment to modify substrate like oxygenase, o-methyl transferase, proteases. Using complexing agents like cyclodextrins. Refrigeration and freezing – at temperature below 70C the polyphenoloxidase enzyme activity is inhibited, but enzyme is not inactivated. Page 5 of 12
Dehydration – at here remove the water from products, polyphenoloxidase enzyme need sufficient amount of water to be active, at here also enzyme is inhibited, but not destroyed. Ultra filtration- this is a membrane separation technique, which is remove larger molecule like polyphenoleoxidase. Ultrasonication – this is advance method which use sound waves to inactivate enzymes. Treatment with supercritical carbon dioxides (SC-CO2) at here also form carbonic acid and lowering pH helps to inactivate enzymes. Irradiation – at here product is espouse to the ionized radiation in order to kill the bacteria and reduce the enzyme activity. High pressure treatment (HPP) – at here uses 500-700 atmosphere pressure to achieve microbial and enzymatic inactivation. This technology is provide good beneficial for heat sensitive products, but still very expensive. Polyvalent phenols form colored complexes with metal ions, as example at pH > 4, Fe+3 forms complexes which are bluish grey or bluish black in color. Leucoanthocyanins when heated in the presence of an acid are converted into anthocyanins phenoloic compounds can also form complexes with proteins these complexes increase the turbidity of fruit juices, beer and wine. The red color of apple and pears form during cooking is derived from leucoanthocyanin. Normally the structure separate the enzyme from phenolic substrate, but after fruit or vegetable cut the enzymes and phenols can come together and react in the presence of oxygen to produce undesirable brown color compounds.
3.0 Non enzymatic browning: Two main type of non enzymatic browning reaction occur in the foods 9 Caramelization 9 Maillard reaction
3.1. Caramelization:
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It’s a non enzymatic reaction. Caramelization is the process of removal of water from a sugar (such as sucrose or glucose) followed by isomerization and polymerization steps. It is a complex series of chemical reactions. The process of caramelization starts with the melting of the sugar at high temperatures, followed by foaming (boiling). At this stage sugar decomposes into glucose and fructose resulting anhydro sugars. This is followed by a condensation step, in which the individual sugars lose water and react with each other to for example difructose-anhydride. The next step is the isomerization of aldoses to ketoses and further dehydration reactions. The last series of reactions include both fragmentation reactions (flavor production) and polymerization reactions (color production). Steps of caramelization of sucrose are given below; Sucrose 160 0C Melt in to glucose and fructose 200 0C for 35 min Isosaccharosan - wt loss 4.5% due to losses of one H2O Further heating for 55 min Caramelan (C24H36O18) - wt loss 9% Soluble in water & ethanol Melting point = 136 0C Further heating 55 min Caramelens (C36H50O25) - Soluble only in water Dark in color MP = 154 0C Further heating Caramelins (C125H188O80) - Very dark in color, insoluble Number of intermediate compounds form according to the Temperature & time combination. At the end of the process all compounds present in a mixture. If the reaction is controlled at a desirable stage, different concentration of intermediate compounds can be obtained. So the caramelization process can be used to produce different colors starting from light yellow color to dark brown color. E.g. beverage colors in beer. Apart from the color, it can be used to produce Page 7 of 12
different flavor compounds. E.g. caramel aroma in coffee. As well as caramilization can also lead undesirable flavors and colors also like “burnt-sugar smell”. Flavor is due to end products like diacetyl, acetic acid, formic acid etc. Apart from that degraded products of sugars like furanones, maltol, isomaltol, hydroxymaltol are important as flavor compounds.
3.2. Maillard reaction: Louis Camille Maillard discovered Maillard reaction in the early twentieth century. When he tried to figure out how amino acids linked up to form protein he found that when he heated sugar and amino acids, the mixture slowly turned brown. Scientist found that the unappetizing tastes were coming from the browning reaction after many studies done in laboratories. Many researches were done in the 1940s to 1950s preventing this reaction. Finally scientist discovered the role of the Maillard reaction plays in creating flavors and aromas. The Maillard reaction is a complex series of reactions between amino acids and reducing sugars at increased temperature. The main steps are Condensation – amine/carbonyl Rearrangement – enolization Fragmentation Strecker degradation Polymerization Into the initial step reaction start between a reducing sugar and primary amino acids, loss of water from this molecule produces an amine that is able to cyclic, resulting in the formation of an N glycoside (a sugar attached to an NR2 groups). Deoxyosones are reactive α-dicarbonyl compounds and give rise to other, secondary products. As example from 1-deoxyosone, the secondary products include furanose, pentose and hexoses. Secondary products also from 3deoxysones include pyrroles, pyridines and formylpyronoles. In the Amadori rearrangement step, instead of cyclisation of the ammonium ion, an Amadori rearrangement may take place. Deoxysones are reactive α-dicarbonyl compounds and give rise to other, secondary products. Pentoses may further react with amines to give orange dye products, influencing the colour of food. After the Amadori rearrangement three different main pathways can be distinguished. Page 8 of 12
Those are dehydration reactions, fission and Strecker degradation. These three main pathways finally results in very complex mixture, including flavour, brown high molecular weight pigments like melanoidins. Strecker reaction do between α-dicarbonyl compounds, such as the deoxyosones formed in the Maillard reaction, and amines. The reaction involves transamination and yields aminoketones, aldehyde and carbon dioxide. The Strecker aldehyde and aminoketone gives rise to strong odours. Common Strecker aldehyde includes ethanal (fruity, sweet aroma), metylpropanol (malty) and 2-phenylethanal (flowery honey like aroma). Condensation of two aminoketones may yield pyrazine derivatives that are also powerful aroma compounds. Into the polymerization step, formation of brown nitrogen containing pigments (melanoidins) by alsol condensation and carbonyl amine polymerization like N, O, S-heterocyclic compounds. This melanoidins have variable structure, molecular weights and nitrogen content, little is known about the structure of these compounds. There are many desirable and undesirable results. ¾ Flavour formation, volatile compounds which are often potent aroma substances. The Maillard reaction is important for the desired aroma formation accompanying cooking, baking, roasting or frying. It is equally significant for the generation of off flavour in food during storage, especially in the dehydrated state, or on heat treatment for the purpose of pasteurization, sterilization and roasting. ¾ Colour- browning is desired in baking, roasting, but not in foods which have a typical weak or other colour of their own (bread crust, syrup, meat). ¾ Flavouring matter, especially bitter substances, which are partially desired (coffee like product), but can also cause an off taste like grilled meat or fish. ¾ Antioxidants and anticarcinogens- compounds with highly reductive properties which can contribute to the stabilization of food against oxidative deterioration. ¾ Nutritional losses – losses of essential amino acids (lysine, arginine, cysteine, methionine) and digestibility and palatability of vitamin C. ¾ Compounds with potential mutagenic properties (IQs). ¾ Compounds that can cause cross-linkage of proteins. Reactions of this type apparently also play role in vivo, diabetes. There are so many factors influencing the browning reaction. Page 9 of 12
1. Ratio and type of amino acids and sugar 2. Sugar to amine ratio 3. pH- influences the ratio of product formed, the rate of color formation can be reduced by decreasing the pH. Under alkaline conditions, the 2,3-enolization pathway is favored. 4. Solvent state 5. Temperature
3.2.1. Control of Mailllard reaction: Water activity (aw) Water is produced during Maillard reaction, thus the reaction occurs less readily in foods with a high aw values while, at low aw, the mobility of reactants is limited, despite their presence at increased concentrations. In practice the Maillard reaction occurs most rapidly at intermediate aw values (0.5-0.8), and aw is of most significance to the reaction in dried and intermediatemoisture foods (IMFs), which have aw values in this range. However, aw values for maximum browning are affected by other components of the system: humectants, such as glycerol, can lower the aw value for maximum browning. pH Since the reaction itself has a strong influence on pH it is hard to evaluate the pH influence. However, reactions take place by all pathways, with the pH of the system influencing the ratio of products formed and the rate of color formation can be reduced by decreasing the pH. The most desirable meaty and pot-roasted aroma was obtained at pH 4.7, but pH had a less dramatic effect on aroma than did temperature, time or water content. The left figure is “effect of pH on Maillard browning of L-lysin, L-alanine and L-arginine heated with D-glucose at 121 C for 10 min”. Temperature The temperature dependence of chemical reaction is often expressed as the activation energy (Ea). Activation temperature data for the Maillard reaction have been reported within a wide range 10-160KJ/mole, depending on, what effect of the reaction has been measured. The Page 10 of 12
activation energy is also highly dependent on pH. The temperature dependence of the Maillard reaction is also influenced by the participating reactants. So it is difficult to isolate the effect of temperature as a single variable.
3.2.2. Maillard Reaction & Nutritional Effect: 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 our 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 lysin, arginine, and histidine. Maillard reaction products also exhibit antinutritive effects by mechanism involving complexation with micronutrients, destruction of vitamins, and by acting as inhibitors of digestive enzymes. In addition, the product of the maillard reaction, melanoidin, present in the incubation medium was found that there was negative effect on the uptake of sucrose through the averted sac of small intestine. Results from recent studies also showed that the consumption of heterocyclic amines arise from cooked meat might be associated with cancer development in human. Moreover, evidence has shown that spray-drying of infant formulae which contains more lactose, may give up to 10% lysine blockage as does high-temperature shot time (HTST) sterilization of liquid milk. Specialized formulae, such as lactose-hydrolysed milks or formulae containing glucose intolerant infants, give the greatest problems during drying. With the combination of lactose and lysine losses during heat treatment in milks, Maillard reaction products are formed. The brown pigment (melanodin) is responsible for the increased specific activities of small intenstinal disaccharidases which eventually yields lower sugar absorption. Therefore, the food processing does influence the food quality in terms of the nutritional aspect.
4.0 References and Resources: Science of cooking http://www.exploratorium.edu/cooking/ Page 11 of 12
Chemistry of Food Systems (410), http://www.landfood.ubc.ca/courses/fnh/410/colour/3_82.htm Food Resource, http://food.oregonstate.edu/ & http://food.oregonstate.edu/faq/color/brown79.html
Wikipedia http://en.wikipedia.org/wiki/Caramelization Kitchen Chemistry movie – V10 Maillard Browning http://www.chemsoc.org/networks/learnnet/kitchenchemistry/00_video.htm FN 301 Science of Foods, Dr. Jim Fitzgerald, Mississippi University for Women http://www2.muw.edu/~jfitzger/page91.html The Complete Idiot's Guide to Cooking Techniques and Science, 2003, Alpha Books
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