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 N-substituted 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-2,3,4,5-tetrahydropyridine 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 double-bond 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.